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Production process for automotive components notes

  1. 1. STRETCH FORMINGTraditional double action drawing permits a controlled amount of the blankto draw into cavity. In the stretch forming technique, the blank is clampedso tightly all around that it can not draw in. Rather, it is stretched over thepunch or lower die and set by the upper die. A lower blank holding-ring ismounted on a nitrogen pressure pad. It maintains a high load (about 100tons) against the blank and upper ring while traversing downward with theupper ring. It thus prevents the blank from slipping between the drawbeads. As shown in Fig. 5.8, the upper blanking ring drops to the lowerholding ring, and locks the perimeter of the blank in the draw bead.Thereafter, the ring lower together to a dwell position, stretching the blankover the lower die. At this point, the upper die descends, completing theoperation.Stretch forming is being used for automotive panels providing advantagessuch as 15 to 20% smaller blank size, and elimination of turnover operationafter the draw. The better quality results from the uniform stretch over theblank’s entire surface. For example, in conventional stamping of a hood,stretching occurs in the corners but very little, if at all, in the centre area.During stretch forming, measurable deformation occurs over the entiresurface.
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  3. 3. Sheetmetal Forming - Stretch Forming Stretch forming is a very accurate and precise method for forming metal shapes, economically. The level of precision is so high that even intricate multi-components and snap-together curtainwall components can be formed without loss of section properties or original design function. Stretch forming capabilities include portions of circles, ellipses, parabolas and arched shapes. These shapes can be formed with straight leg sections at one or both ends of the curve. This eliminates severalconventional fabrication steps and welding.The stretch forming process involves stretch forming a metal piece over a malestretch form block (STFB) using a pneumatic and hydraulic stretch press. Stretchforming is widely used in producing automotive body panels. Unlike deepdrawing, the sheet is gripped by a blank holder to prevent it from being drawn intothe die. It is important that the sheet can deform by elongation and uniformthinning.The variety of shapes and cross sections that can be stretch formed is almostunlimited. Window systems, skylights, store fronts, signs, flashings, curtainwalls,walkway enclosures, and hand railings can be accurately and precisely formed tothe desired profiles. Close and consistent tolerances, no surface marring, nodistortion or ripples, and no surface misalignment of complex profiles areimportant benefits inherent in stretch forming. A smooth and even surface resultsfrom the stretch forming process.This process is ideally suited for the manufacture of large parts made fromaluminum, but does just as well with stainless steel and commercially puretitanium. It is quick, efficient, and has a high degree of repeatability.
  4. 4. Stretch Forming:• Elongation control for unparalleled accuracy and consistency• Bend precision and consistency is excellent with high to medium per–bend cost• Non-symmetric profiles are readily formed without twist• Maximum bend radius is unlimited
  5. 5. • Can bend, twist and lift simultaneously • Minimal surface distortion and traffic marking • Maximum diameter 180° bend at full tonnage = 70” • Minimum diameter 180° bend at full tonnage = 15” • 300” part length capability • Tooling typically is higher than other forming/bending methods • Arms can swing only to 180° • Minimum bend radius is generally 2-3 times greater than other forming/bending methods • Bend radius cannot be modified without additional tooling charge The bending, shaping, and forming of aluminum is best accomplished with input from our engineers during the design phase of your projects. Alexandria Extrusion Company offers an in-depth aluminum extrusion seminar at your site where we can educate designers and engineers about aluminum extrusion designing options, alloys and cost drivers. If you have questions or need assistance with your designs, please contact your sales specialist. SHEET METAL STAMPING IN AUTOMOTIVE INDUSTRYSTEEL PANELS IN CAR BODY STRUCTUREEver increasing competition in automotive industry demands productivityimprovements and unit cost reduction. The manufacturing engineers andproduction managers of car body panels are changing their strategy ofoperation. The days of ‘a simple washer to a very complicated fender, all inplant stamping facility’, are gone. In-house manufacturing facilitiespreferably produce only limited number of major car panels, Fig. 5.1. Fig. 5.1 Major Panels of Car BodyAn automotive plant today produces some 40~50 critical panels per modelof car in-house, that require some 100~150 dies.. Criteria for takingdecision about the panels to be manufactured in-house vary from companyto company. Very lately, the stamping plant of the automobilemanufacturers includes the types of panels as given below in-house:1. External (skin) panels, such as fenders, bonnet, decklid, roof, sidepanels, doors, etc. Some of these are two panels in a set as left hand andright hand2. Internal mating panels, such as bonnet inner, decklid inner or door innerdeciding subassembly quality
  6. 6. 3. Dimensionally critical inner panels that are complicated either because oftheir complex shape or severe draw condition, such as, floor pans, dashpanel, etc.Automanufacturers prefer to procure the medium and small size panelsfrom vendors depending on the availability (nearer facilities are preferred)and their capability to meet demanded specifications. Some are evenfarming out the major subassemblies such as doors to specialised vendors.Trends are for farming out as much as possible.
  7. 7. MATERIALS FOR BODY PANELSMaterials for car body panels require certain specific characteristics to meetthe industry’s challenges: rationalisation of specifications for leanerinventory, improved formability for reduced rejection rate and better quality.Higher Strength Low Alloy (HSLA) steels of thinner gauges, are gettingpreference for weight reduction and the resulting better fuel economy.Other quality characteristics under demand are higher yield stress(strength), toughness, fatigue strength, improved dent resistance as well ascorrosion resistance in materials used for body panels for improveddurability and reliability.To obtain consistent quality of autobody skin panels without failures duringstamping, the formability/ductility specifications of strip steels are the basicrequirements. The numerical values of the strain hardening exponent (n-value), the plastic anisotropy (r-value), and the forming limit diagrams forthe sheet steels provide the index of formability of the panels. Strainhardening to some extent improves the dent resistance. Strain gradients inpressings are not to be unduly severe causing splitting and other relatedproblems. To maintain the shape after the forming operation, minimal‘spring back’ and high ‘shape fixability’ are also essential. As the panels arewelded to shape the body structure with various arc/resistance weldingoperations, the weldabilty of the materials in use is very important. Finally,the specific roughness levels (textures) of the steel used for skin panelsmust be consistent and reproducible. It will be essential for the goodadhesion of the various combinations of primers and paints used onautobody pressings to obtain high quality paint finishes (clarity of imageand gloss).Most of the steels used in automotive application are aluminium-killedsteels of about 0.7 to 0.9 mm thickness. For inner automotive parts,drawing quality steels, such as SPCD (JIS G 3141), A619 (ASTM), CR3(BS1449), and Sr13 (DIN 1623), while for outer panels requiring deepdrawing such as fenders, hoods, oil pans, etc. Non-aging extra deepdrawing steels such as SPCEN (JIS G 3141), A620 (ASTM), CR1 (Bs1449), and St 14 (DIN 1623), are used. Aluminium-killed steels show littleor no stretcher strains for a period of time sufficient to eliminate the needfor roller-leveling. Thinner High Strength Low Alloy (HSLA) steels are beingincreasingly used for certain autobody components including skin panels. Itmust combine its high strength with a good level of formability, as astrength increase is always accompanied by a fall in formability. The
  8. 8. improved bake hardening steels used specially for the external panelspossesses sufficiently high formability and provides an increase in strengthafter the paint baking. A consequence of strength increase obtained duringpaint baking, is the improved dent resistance of the surface. Difficultautobody pressings of complex geometry have necessitated the use ofsteel grades with lower strengths too. Vacuum degassed microalloyedsteels containing Ti and/or Nb additions are classed as Interstitial-freesteels (IF-steels). IF-steels are being used with advantages of extremelyhigh value of maximum drawing ratio, and the absence of the strainingeffect for difficult-to-form panels. Fig. 5.2 shows panels of High StrengthLow Alloy steel, and Table 1 provides a list of special steels for differentautomobile panels. Table 5.1 Special Steels for Different Automotive Panels Steels for Auto Yield Strength, Application Panels N/m2 conditions4 A. High Strength 220~260 Autobody structural parts- Steels door, roof, trunklid, hood, REPHOSPHORISED pillar outer, rear floor, etc. STEELS with additions of P upto 0.08 % GRAIN REFINED 300~400 Formability relatively modest, STEELS so used for components with appropriate alloy relatively less demanding additions which forms forming typically NbCN, TiC DUAL PHASE 400~500 High strength, with good STEELS formability. Suitable for door, appropriate alloy roof, trunklid, hoods, etc. additions (Mn, Mo, Cr, V) and processing BAKE HARDENING 200~250 Slightly stronger, but 40N/m2 STEEL strength increase after baking. Suitable for doors, fenders, hoods, pillars.
  9. 9. B. Low Strength 130~150 For difficult autobody panels Ultra-soft Steel of complex geometry. INTERSTITIAL FREE Suitable for automobile outer STEEL panels, oil pan, high roof  Ti and/or Nb panel, etc. additions combined with interstitial C and N to form stable TiC, TiN or NbCN precipitatesLaser textured steels, and new coatings such as nickel zinc are ensuringbetter paint finish and corrosion resistance respectively. Galvanised steelpanels that provide better corrosion resistance are used to the extent ofabout 40% or more in a modern car body. Surface texture and coatingprovided by steel manufacturers demand stricter quality assurance atstamping stage. Dents and damage caused in stamping requiring repair bygrinding or any surface deteriorating methods, may take away the basicadvantages of special texturing. Fig. 5.3 shows the typical panelsmanufactured out of galvanised steels.An intensive research and development are going on for alternatematerials, manufacturing processes and stamping tools for sheet-metalcomponents with the main objectives of cutting down the weight and unitcost of the vehicle. Simultaneously, the steel content of the car is fallingwith the use of aluminium and new materials, such as plastics. Aluminiummay provide the most sought after solution to reduce the weight of thevehicles. A reduction of 30% in weight is achievable if the same strength,stiffness, and stability of the component are to be realised by substitutingsteel with aluminium. Possibility of significant reduction in die cost will beanother advantage with aluminium. However, problems related to strength,serviceability, manufacturability, and above all the cost, require effectivesolutions before the acceptance of aluminium as a substitute to steel forbody panels. Plastics for bumpers, Fig. 5.2 High Strength Low Alloy Steel Panels in a Car Body Fig. 5.3 Galvanised Steel Panels in a Car Bodyfacia, radiator grilles and even fuel tanks have become almost universallyacceptable. Other applications will be commercially possible in yearsahead.
  10. 10. Plasma coating of engine block cylinder bores...SituationWeight savings in the automotive industry can result in lower fuelconsumption and therefore reduced pollution. Constructive designmeasures as well as the right material choice for the engineblocks create a lighter and cheaper solution in the building ofengines. Less friction and high wear resistance on the cylinderbore surfaces are required.The solutionThe solution is based on an optimum material selection betweenthe piston rings and cylinder bores. In a continous process, theengine blocks are grit blasted, cleaned and plasma sprayed. Theplasma sprayed coatings are thin (140-200 microns), uniform (±10% of the nominal coating thickness) and smooth (Ra = 8-12microns). The system is designed for a prototype series and can spray up to 200 inline four-cylinder engine blocks per day. Customer benefits The plasma coating of the cylinder bores is able to replace the pressing, shrinking, casting-in of cylinder sleeves or the use of expensive, nickel containing galvanic plating processes. By replacing sleeves the wall thickness between the individual bores of the engine blocks can be made much
  11. 11. thinner. This results in a more compact construction, considerableweight savings and lower production costs of engine blocks.The potential for lower friction values (compared to cast iron) andfor excellent wear properties of the plasma coating result in therequired life time of the engine blocks.In some cases the plasma coating is able to replace a galvanicprocess, which results in an environmentaly friendly solutionbecause nickel based materials are no longer being used.Depending on the coating, finishing costs of the plasma sprayedcoating may be lower because the coating can be honed faster,and with less wear to the honing tools.The coatingIn cooperation with the customer, a molybdenum based materialwas developed and supplied by Sulzer Metco which fulfils therequested friction behaviour and wear properties. This results in acoating solution meeting the economics and performance goalsset by the customer in this demanding industry.Method for Coating Combustion Engine Cylinders by PlasmaTransferred Wire Arc Thermal Spray1 BackgroundThe internal combustion engine generates power by igniting a combustiblesubstance in a closedchamber, with the addition of an oxidizer, and harnessing the resultingexpanding gases to push apiston and turn a crankshaft. The piston slides inside a carefullyconstructed cylinder, which servesas the combustion chamber. The piston is collared by several rings whichprovide a gas-tight seal
  12. 12. and lubricate the inner surface of the cylinder.Engine blocks have traditionally been made of gray cast-iron because ofthe ease of casting (due toabsence of appreciable volume shrinkage), machinability, wear resistance,and vibrational damping.Recently, engine manufacturers have been going to aluminum to saveweight. With aluminum castengine blocks (now accounting for more than 60% of automotive engines),silicon is usually addedto improve uidity during pouring and to increase hardness.The cylinder bore must withstand rapid temperature cycling, repeatedshear loading, extremepressure, and impingement of hot gases, all on the scale of fractions of asecond. This demandsextremely tight dimensional tolerances and superior wear resistance. Forlight metal engine blocks,this is traditionally achieved by manufacturing a separate cylinder sleeve,usually made of steel orgray iron. Cylinder liners generally come in two types, wet and dry,characterized by whether ornot they directly contact the cylinder coolant. The sleeve can either bepress-_t into the engineblock or suspended in the cylinder block sand mold prior to pouring (Kuhn4). Clearly, the meltingtemperature of the liner material must be well above that of the engineblock alloy.1.1 Sleeveless CylindersIn order to save weight and enhance heat transfer characteristics it isdesirable to reduce thethickness of the cylinder lining. In an aluminum engine block, every extramillimeter of castiron or steel lining adds to the overall weight and reduces fuel economy.Previous attempts at1MSE 121 Spring 2010 April 30, 2010manufacturing a sleeveless cylinder using a nickel-based liner depositedonto an aluminum substratewere employed by BMW and Jaguar. The lining material, known as NikasilR, has better wear
  13. 13. resistance than aluminum and replaces the heavy cast-iron lining.However, nickel is extremelyvulnerable to in_ltration by sulfur. Sulfur readily di_uses down grainboundaries into the nickelmatrix and forms brittle nickel sul_des which are extremely susceptible towear. Sulfur presentin low quality fuels led to disastrous wear, and Jaguar and BMW bothreplaced their Nikasillinings with steel or cast-iron (US Auto Parts). Nikasil continues to be usedin smaller engines, orapplications where fuel quality is not a concern (Wikipedia).Another approach is to cast the engine from a hypereutectic Al-Si alloy,having greater than 12.6wt% Si (ASM Handbook). The presence of proeutectic silicon increaseswear resistance and hardnesswhile decreasing the thermal expansion coe_cient. However, hypereutecticalloys also su_erfrom low toughness and di_culty of casting (Jorstad). Addition of graphiteincreases hardness andwear resistance of Al-Si alloys, but also has a disastrous e_ect ontoughness (Gibson, et al.).1.2 Development of Fords PTWA Cylinder Lining ProcessIn the early 1990s, Ford began developing an alternative process forcoating aluminum cylindersusing a plasma spray technique. Fords R&D team sought to addressseveral problems with previouscylinder manufacturing techniques:_ Weight: Steel or cast-iron liners are signi_cantly denser than theiraluminum housing._ Manufacturing time: Vapor deposition techniques require 10-60 hours(McCune, et al. 3)_ Cost: Cast-in-place methods and oxy-fuel thermal spray coatings aretedious and expensive._ Wear resistance: Machinable aluminum is too soft for aggressiveenvironments_ Heat transfer: Since gray iron has low thermal conductivity compared toaluminum, a thinnerlining is preferred to enhance heat extraction from the cylinder.Throughout the 1990s and early 2000s, Matthew Zaluzec, the chiefinvestigator and Fords manager
  14. 14. of Materials Science and Nanotechnology, made numerous re_nements tothe process (Wojdyla).Through 1997, signi_cant energy was focused on controlling or eliminatingoxide formationon both the substrate and the _nished lining (Zaluzec, et al.). From 1998-2001 the team perfecteda few more implementation details, including a process to improveevenness, increase depositionrate by controlling gas ow, and protect other engine components duringspraying. The re_nedprocess was used in the 2008 Nissan GT-R and the new 2011 ShelbyMustang GT500 (Goodwin).2MSE 121 Spring 2010 April 30, 20102 Detailed Description of the PTWA ProcessThe process of applying a thermal spray coating is achieved in severalsteps. First, the as-castcylinder surface is bored to a rough diameter. Second, the surface must becleaned and uxed.Next, a Ni-5Al bonding coat is applied. Then, the surface is coated with alow-carbon steel withhighly controlled quantities of FexO wustite using a plasma transferred wirearc process. The partis then inspected for defects and honed to its _nal shape.2.1 Cleaning and FluxingAfter the cylinders are bored to the approximate diameter, the surface isimmersed in a 0.5 Mpotassium-uoride bath. The KF solution etches away the oxide layer andthen reacts with thealuminum to form K3AlF6 and KAlF4. If present in a eutectic composition(approx. 45 molar %AlF3) the ux agent will melt well below metallic aluminum (Popoola et al. 8).An initial thermal spray coating of Ni-5Al is applied to the uxed surface tothermally activate theux (melt the salt and dissolve surface oxides) and e_ect a metallurgicalbond to the aluminumsurface. Aluminum is readily soluble in nickel up to quantities of 10 wt%without forming intermetallicphases (ASM Handbook). Ford initially experimented with feeding a ux-cored wire of
  15. 15. similar composition as mentioned above directly into the plasma sprayapparatus described below.They eventually abandoned this idea due to its cost and complexity.Several other roughing and preparation methods were compared byengineers at Flame-Spray andFord including water-jet roughing, grit blasting, and laser roughing. Water-jet roughing was foundto damage the aluminum substrate leaving surface pores even aftercoating. Grit blasting wasabandoned due to concerns of industrial contamination (E. Lugscheider, etal. 5).(a) Comparison of bond strengths for di_erent surfacepreparations(b) Microstructural damage due to water-jetrougheningFigure 13MSE 121 Spring 2010 April 30, 20102.2 Application of Final CoatOnce the ux and bonding agent have been applied, the _nal compositecoating is applied viaplasma transferred wire arc thermal spraying to a thickness of less than amilimeter. A 0.1 wt% Clow carbon steel wire is continuously fed into the nozzle apparatus anddeposited on the cylinderwall.Immediately after application, a thermal imaging camera inspects thetemperature pro_le due tothe heat of application of the plasam spray. The system looks for hot spotsthat may indicate adefect.2.3 Post Processing and FinishingAfter coating, the bore is honed to its _nal size and _nish. First, a coursegrained honing stoneis passed through, to remove any large deviations or rough patches. Next,"large" volume of thematerial is removed, exposing micorpores, enabling oil retention forlubrication (Schwenk, et al. 7).As sprayed, the typical thickness is in the range of 120-250 _m, which isreduced down to the _nal
  16. 16. thickness of 70-170 _m (Barbezat 50). Finally, a _nishing pass iscompleted by a _ne grained stoneor a diamond tipped tool to the _nal _nished surface (Schwenk, et al. 7).The diamond honing canachieve a surface roughness of Ra less than 0.3 _m (Barbezat 49).3 PTWA ApparatusPlasma spray techniques generally operate by striking an electric arc in aninert gas and then usingthe extreme temperature, high velocity plasma stream caused by the rapidexpansion of the gas toatomize and accelerate a feedstock of some kind towards a target. Thefeedstock may be a powderor wire, and may even include uxing agents in the wire itself or mixed inwith the powder.In Fords patented PTWA process, an arc is _rst struck from a tungstencathode (described in detailin US patent 6,559,407) to the copper nozzle, acting as the anode. The arcis maintained with a100-120V DC power source supplying 60-100 amps, and then "transferred"to the wire feedstock,which is negatively biased to provide a conducting path for the arc. Thewire is fed perpendicularto the direction of spraying. The ionizing gas is fed through radial ports inthe cathode assemblyto produce a vortex. A secondary high velocity gas with carefully controlledoxygen content isintroduced just beyond where the plasma stream intersects the wire arc.4MSE 121 Spring 2010 April 30, 2010
  17. 17. 4 Thermal Coat MicrostructureThe rotational velocity of the gas vortex described above was shown tohave a strong e_ect onparticle size, with higher velocities corresponding to a _ner mist (Kim, etal.). Presumably, thisresults in a _ner grain structure resulting in increased hardness. The _nalcoat can be controlledto have porosity less than 2%. The characteristic "splat morphology"resulting from high-velocityimpact of spherical globules is apparent in Figure 3 (a).Gas atomization of the advancing wire produces predictably sphericalmolten globules, for whichthe surface area to volume ratio can be readily predicted (Beddoes andBibby 176). Additionally,the feasibility of predicting oxygen content in a plasma arc torch withsecondary shielding gas
  18. 18. shroud using computational uid dynamics in a simpli_ed geometry wasdemonstrated by Jamais,et al. Under these conditions, the molten iron forms a surface layer ofwustite, FexO mid-ight,with x ranging from 0.5 to 1.5, mid-ight. The oxygen content can becarefully controlled duringdeposition to obtain a composition gradient between 10% and 30% wustite,with an oxide-richregion below and an oxide-depleted, easily machinable layer at the top(Zaluzec, et al. 6).Wustite is almost 70% harder than the steel matrix it inhabits (Bobzin, et al.5). Among traditionalliner materials, hardness generally correlates positively with wearresistance. Among thermal spraycoatings, on the other hand, oxide content may be a more signi_cant factorin wear resistance(Hart_eld-W• unsch and Tung 23). Although independent test results forwustite-rich surfaces islacking, it has been found that the presence of Fe2O3 in plasma sprayedcoatings greatly improveswear resistance (Kleyman, et al. 139). Wustite easily shears along thef001g plane (principle cubefaces shown in Figure 3 (b)) and acts as a solid lubricant.5MSE 121 Spring 2010 April 30, 2010(a) Cross section of cylinder lining showing aluminum sub-strate, steel matrix, and iron oxide(b) Cubic wustite crystal.Figure 35 Performance of PTWA lined enginesFord constructed a 32 vehicle eet to run at 9 di_erent North Americanlocales (with varied drivingand weather conditions) The eet accumulated well over 3 million total miles- including severalvehicles driving over 250,000 miles. In the laboratory, Ford also conductednumerous dynamometertests, including a 2000+ hour endurance test. After these millions of milesand thousands ofhours of testing, the PTWA process proved to be a success and hadmeasurable e_ects on engine
  19. 19. performance. At large, PTWA coated engines boast improved mileage, aswell as reduced wear,weight, and cost compared to their cast-iron lined counterparts. As aconsequence of the wustiteacting as a solid lubricant, friction is greatly reduced | on average 6.8%below the values oftraditional cast iron liners and 14.1% below cast-iron engines (Milliken).This aspect alone allowsan overall 4% potential reduction of fuel expenditure (assuming 40% ofmechanical power lost inpiston/cylinder component and a thermodynamic e_ciency of 35%).Plasma spray lined engines measured half the amount of wear of ironlinings in a 300 hour full powerendurance test (Barbezat 141). The reduced wear can be contributed toseveral things. First, thepresence of FeO increases the microhardness of the coating, whichreduces friction by decreasingthe amount of plastic deformation between contacting surfaces (Cook, etal.). Additionally, aftersubsequent machining, the smooth surface with microcavities (from theexposed 2% porosity) allowsfor oil retention providing excellent lubrication conditions (Barbezat 2031).The cost savings are equally attractive. The wire feedstock itself costsabout $0.50 to $0.75 perpound (Cook, et al.). It is estimated that the process, on average, saves$.50 to $1.50 per bore fora cost of $2 to $4 USD when scaled to high production volumes (Barbezat141). Due to the highdeposition e_ciency (over 80%) and high feed rates, the entire process canbe completed in under6MSE 121 Spring 2010 April 30, 201060 seconds making mass-production feasible via this process feasible.Automakers are targeting weight reduction as their main strategy toimprove fuel economy. Thereduced mass of the PTWA coatings o_ers a reduction in weight of 6 to 8pounds (Wojdyla).In fact, coupled with the transition from a cast-iron engine block toaluminum, the 2011 GT500
  20. 20. reduced the engine weight by a total of 102 pounds, noticeably improvingoverall fuel economy.Due to the high energy and CO2 investment required to cast new engineblocks, recycling andreuse of old engine blocks is becoming increasingly attractive. As engineblocks have reduced insize (thus shrinking space between bores), oversize boring of original liningand subsequent re-liningis no longer a possibility. Without PTWA process, the majority of theseengine blocks would bescrapped. However, remanufacturing engine blocks with the PTWAprocess allows for a 50%-80%total manufacturing energy saving as well as a 25-50% cost savings overre-manufacture (Schwenk,et al. 1-3)6 ConclusionsIn 2009, the engineers who perfected the PTWA deposition techniquereceived the National Inventorof the Year Award. In presenting the award, the Intellectual PropertyOwners EducationFoundation cited the energy savings and possible environmental impact ofthe process. The processhas already proven itself worthy through extensive testing and marketadoption. Researchers atGeneral Motors are investigating a similar process. Engineers from Flame-Spray Industries, LandRover, Jaguar Cars, and Caterpillar presented a joint paper at the 2007ASME Internal CombustionEngine conference touting the bene_ts of thermal spray coatings andurging exploration of furtherapplications. It is almost certain that thermal spray coatings will become anindustry standard inautomotive applications in the very near future.
  21. 21. ICSP9 : SHOT PEENINGSHOT PEENING OF GEAR COMPONENTSFOR THE AUTOMOTIVE INDUSTRYABSTRACTShot peening using blast wheels offers a high level of flexibility and processrespecially for larger series and larger workpieces. This paper describesshotsolutions for bevel gears and crown wheels for the automotive industry atBMW,Dingolfing. The significance of shot peening trials to determine the optimumsolutionis outlined and solutions found - satellite shot peening systems - arepresented. Theinfluence of a proper working dust removal plant and separation system onshotpeening results are shown, importance and control of process reliability areoutlined.SUBJECT INDEX1) Shot peening with blast wheels2) Automotive gear parts3) The significance of blast trials4) The machine concepts, automatic handling5) Dust removal and separation system6) Process reliability
  22. 22. SHOT PEENING USING BLAST WHEELSShot peening by means of the air blasting method is generally known. Inthe currenttechnology of surface treatment, shot peening with blast wheels hasbecome widelyaccepted as a reliable and economic processing method. The fields ofapplication arethus accordingly diverse. Blast wheel systems offer a high level of flexibilityandprocess reliability and can be applied for controlled treatment of a widerange ofworkpieces, especially larger series and parts. Shot peening is principallysuitable forparts which are subjected to bending or alternating torsional stress.1 Throwing blade2 Shot opening3 Control cage4 Accelerator (impeller)
  23. 23. APPLICATIONSSHOT PEENING GEAR PARTS FOR THE AUTOMOTIVE INDUSTRYShot peening leads to significant improvements of the mechanicalproperties of workpiecesand serves to extend the stress-load limit of components or permits designoflighter weight parts. At the Dingolfing plant near Munich, BMW ismanufacturing allgear components for passenger cars and motorbikes. Production can beboth - singleand mixed product runs (parts for passenger cars and motor bikes). Afterhardeningand annealing, all gear parts are shot peened to increase vibration strengthandresistance against stress cracks and vibration crack corrosion.Requirements are to acertain extent complex: On one hand root and tooth flanks of the parts haveto beshot peened, with other areas only requiring descaling or deburring.Following partfamilies are processed:BEVEL GEARSDimensions: min. height 160 mm; max. height 220 mm; max. unit weight3.5 kgRequirements: Shot peening of the tooth flanks to 0.28 - 0.32 mm Almen Awith acoverage of > 98 %, cleaning resp. removal of the cover paste on thethread withoutimpairing the thread function; descaling the remaining parts. An excessiveincrease inhardness at the shaft must be prevented. For certain types of bevel gears,it isnecessary to additionally ensure that the tip surface is not hardened, whichmeans
  24. 24. that the tooth flanks also have to be shot peened whereas further areasand theuntempered head parts, in particular the centre where the parts areclamped intoposition for concentricity measurements, must not be shot peened.CROWN WHEELSDimensions: max. diameter 240 mm; max. height 42 mm; max. unit weight4 kgRequirements: Shot peening of tooth flanks to 0.28 - 0.32 mrn Almen Awith acoverage of > 98 % and descaling of the remaining parts.Further requirements and specifications for both part families are asfollows:Abrasive of the size S230 (hardness 46-52 HRC) is used for all parts.Crown wheelshave to he shot peened individually (d 2), pair-wise (fig. 31, ormaking special carrier facilities nece ary. Production status:s are working in 3-shift opertion process. In this specificng and shot peening. In theprime importance that tole44ICSP9 : SHOT PEENINGrances for Almen values are always complied with to guarantee runningsmoothnessof the gear. If the programmed Almen values are under-run, excessivematerial isremoved during lapping, and if the values are exceeded a reduced amountofmaterial is removed. This would either lead to raised noise emission of thevehiclegearbox or result in an inhomogeneous tooth contact pattern. Runningsmoothness isevaluated in a special test whereby the tolerance zone is between 7 and 10on ascale of 10 units. It is therefore essentially important to ensure that thegiven Almenvalues are observed explicitly.
  25. 25. THE RELEVANCE OF SHOT PEENING TRIALSTo meet these requirements, extensive shot peening trials were performedon alaboratory machine using the customers workpieces at the DlSA TestCentre inSchaffhausen I Switzerland. In these tests the aim was to determine theposition ofthe blast wheel (angle) as well as blasting intensity, abrasive quantity andblastingtime. The shot peening programmes were to be tailored to the specificproductionprocess required and the process reliability was to be guaranteed. Theinformationderived from such tests indicates that this task can be optimally solved witha satelliteshot peening system, Due to the test results the machine construction wasadaptedto the customers requirements and thus to the specific applications.With SRS Shot Peening Systems, bevel gears and crown wheels, wheelhubs, gearshafis and similar components, as well as cup springs and clutch springsare shotpeened in cycled rotary operation. In this case 45 speed-controlledsatellites,integrated into an indexing tmrntable carry "Ehe wsrkpreces through thesystem in
  26. 26. AUTOMATIC HANDLINGThe excellent performance of the first two machines led BMW to investigatethepossibility of using a third systems including automatic handling for large-seriesproduction runs (monocultures) in fully automatic operation in anautonomousmanufacturing cell. In this application (fig 5) robots are responsible forautomaticfeeding and unloading. The profitability can be additionally enhancedconsiderably byreducing staff costs and increasing the output. The cell functions are:Fully automatic loading and unloading the workpiece palletsShot peeningAligningInductive starting (threaded pins)Grinding (front surfaces of thread)Ball callipers (for surveying the tooth gaps)Loading procedure: The robot gripper retrieves the workpiece from a pallet,graspingthe bevel drive pinion by its shaft, and then swivels to place the componentinto acentral position above the fixture on the satellite table. A further gripperdevice,activated by a pneumatic cylinder, descends vertically to secure the pinionby itstoothed head. The robot then releases and retracts. The grippersubsequently lowersthe workpiece into the fixture and then moves upwards, back to its originalposition.Unloading then takes place as follows: After the machine table has indexedoneposition, bringing a processed component into view, the gripper devicemovesdownwards. It grasps the gear drive-pinion and raises it sufficiently for therobot tomove in and secure the component. At this point, the gripper releases itsgrasp,
  27. 27. allowing the robot to withdraw the part from the table and place it in afurther pallet.Figure 5: DlSA Satellite Shot Peening System integrated inmanufacturing cell46 ICSP9 : SHOT PEENINGCell solutions require clear interfaces and clearly defined processes to thefollowingfunctions. The modularity and flexibility of the systems is a further keycriterionallowing adaptations to be made at short notice if there is a change inprocess steps.Cell solutions also permit a drastically improved exploitation of theproduction areaFor all applications, the shot peening programs themselves are assigned totherespective part families, ensuring that the given parameters are used inoperation
  28. 28. and that process reliability is guaranteed. The individual programs areactivated viathe appropriate settings at the operators panel.DUST REMOVAL AND SHOT RECONDITIONING SYSTEMA dust separation system (we principally recommend our customers to useseparatefilters for each individual shot peening system) with a capacity of 5,000Nm31h is incorporated,allowing the function of the pneumatic separator to be optimally adjustedby suction-cleaning the system. The separator fulfils the following functions:Separating and discharging scale and dust from the abrasivee Eliminating abrasive particles which are unfit for the shot peeningoperationAs the life of the wear parts depends primarily on the degree of purity of theabrasive,the following is applicable:e The better the separator, the higher its profitabilityThe purer the abrasive, the cleaner and more dust-free the workpieces willbe.Besides the wear and tear issue, the abrasive consistency in distribution ofgrainabrasive size - in particular in shot peening - is the key criterion in terms ofqualityorientedmanufacturing. Losses in quality or insufficient shot peening results(surfacehardening) can thus be excluded. New shot is automatically fed into thecycle via anelectronically controlled replenisher.In shot peening process reliability is the dece blasting intensity canDue is the fact thatt sizes and distribution)are defmed exactly, it 1s possible to adjust and examine the Almenintensity andAPPLICATIONS 4 7coverage. The process reliability is supervised in periodic intervals andmust beguaranteed at all times.
  29. 29. CONCLUSIONWe would once again like to underline that the requirements in terms of ashotpeening solution in comparison with the more familiar blast cleaningapplications, e.g.for cleaning workpieces in foundries and in forges, are highly complicated.Themachine manufacturer therefore plays a decisive role in designing thesystem andhas to make available the appropriate know-how.
  30. 30. Shot Peen Forming Material Temperature [°C (°F)]Magnesium 350-450 (650-850)Aluminium 350-500 (650-900)Copper 600-1100 (1200-2000)Steel 1200-1300 (2200–2400)Titanium 700-1200 (1300-2100)Nickel 1000-1200 (1900–2200)Refractory alloys up to 2000 (4000)Shot peen forming is a dieless processperformed at room temperature, wherebysmall round steel shot impact the surfaceof the work piece. Every piece of shot actsas a tiny peening hammer, producingelastic stretching of the upper surface andlocal plastic deformation that manifestsitself as a residual compressive stress.The combination of elastic stretching andcompressive stress generation causes thematerial to develop a compound, convexcurvature on the peened side.The shot peen forming process is ideal for forming large panel shapes where thebend radii are reasonably large and without abrupt changes in contour. Shot peenforming is best suited for forming curvatures where radii are within the metalselastic range. Although no dies are required for shot peen forming, for severeforming applications, stress peen fixtures are sometimes used. Shot peen formingis effective on all metals, even honeycomb skins and ISO grid panels.
  31. 31. Shot peen forming is often more effective in developing curvatures than rolling,stretching or twisting of metal. Saddle-back shapes also are achievable. Because itis a dieless process, shot peen forming reduces material allowance from trimmingand eliminates costly development and manufacturing time to fabricate hard dies.The shot peen forming process also is flexible to design changes, which may occurafter initial design. Metal Improvement Company can make curvature changes byadjusting the shot peen forming process.Parts formed by shot peen forming exhibit increased resistance to flexural bendingfatigue. Unlike most other forming methods, all surface stresses generated by shotpeen forming are of a compressive nature. Although shot peen formed piecesusually require shot peening on one side only, the result causes both sides to havecompressive stress. These compressive stresses serve to inhibit stress corrosioncracking and to improve fatigue resistance. Some work pieces should be shotpeened all over prior to or after shot peen forming to further improve fatigue andstress corrosion cracking resistance.Shot peening of parts that have been cold formed by other processes overcomesthe harmful surface tensile stresses set up by these other forming processes.Shot peening is a cold working process in which small spherical media called shotbombard the surface of a part. During the shot peening process, each piece ofshot that strikes the material acts as a tiny peening hammer, imparting to thesurface a small indentation or dimple. To create the dimple, the surface of thematerial must yield in tension. Below the surface, the material tries to restore itsoriginal shape, thereby producing below the dimple, a hemisphere of cold-workedmaterial highly stressed in compression.Nearly all fatigue and stress corrosion failures originate at the surface of a part, butcracks will not initiate or propagate in a compressively stressed zone. Because theoverlapping dimples from shot peening create a uniform layer of compressivestress at metal surfaces, shot peening provides considerable increases in part life.Compressive stresses are beneficial in increasing resistance to fatigue failures,corrosion fatigue, stress corrosion cracking, hydrogen assisted cracking, fretting,galling and erosion caused by cavitation. The maximum compressive residualstress produced just below the surface of a part by shot peening is at least asgreat as one-half the yield strength of the material being shot peened.
  32. 32. In most modes of long-term failure, the commondenominator is tensile stress. Tensile stressesattempt to stretch or pull the surface apart andmay eventually lead to crack initiation. Becausecrack growth is slowed significantly in acompressive layer, increasing the depth of thislayer increases crack resistance. Shot peening isthe most economical and practical method ofensuring surface residual compressive stresses.For applications that require deeper residualcompressive stresses than those provided by shot peening, Metal ImprovementCompanys laser peening process imparts a layer of beneficial compressive stressthat is four times deeper than that attainable from conventional shot peeningtreatments.Shot peening also can induce the aerodynamic curvature in metallic wing skinsused in advanced aircraft designs. Additional applications for shot peening includework hardening through cold work to improve wear characteristics, closing ofporosity, improving resistance to intergranular corrosion, straightening of distortedparts, surface texturing and testing the bond strength of coatings.Metal Improvement Company engineering specialists can help you best addressthe fatigue, forming and distortion correction challenges that manufacturers offabricated metal parts face every day. Metal Improvement Companys shot peeningfacilities employ the latest state-of-the-art processing capabilities for shot peening
  33. 33. components of diverse shapes, sizes and materials under rigidly controlledconditions.Metal Improvement facilities are capable of meeting most industry standard shotpeening specifications, including:  AMS-S-13165  AMS 2430  ABP 1-2028  BAC-5730  AMS 2432  MIL-P 81985(AS)  PWA-6  J2441  MIL-STD-852  RPS-428  MIL-S- 13165C  P11TF3EXTRUSIONExtrusion is a process used to create objects of a fixed cross-sectional profile. A material ispushed or drawn through a die of the desired cross-section. The two main advantages ofthis process over other manufacturing processes are its ability to create very complex cross-sections and work materials that are brittle, because the material only encounterscompressive and shear stresses. It also forms finished parts with an excellent surfacefinish.[1]Extrusion may be continuous (theoretically producing indefinitely long material) or semi-continuous (producing many pieces). The extrusion process can be done with the materialhot or cold.Commonly extruded materials include metals, polymers, ceramics, concrete and foodstuffs.Hollow cavities within extruded material cannot be produced using a simple flat extrusiondie, because there would be no way to support the center barrier of the die. Instead, the dieassumes the shape of a block with depth, beginning first with a shape profile that supportsthe center section. The die shape then internally changes along its length into the finalshape, with the suspended center pieces supported from the back of the die.Process
  34. 34. Extrusion of a round blank through a die.The process begins by heating the stock material (for hot or warm extrusion). It is thenloaded into the container in the press. A dummy block is placed behind it where the ramthen presses on the material to push it out of the die. Afterward the extrusion is stretched inorder to straighten it. If better properties are required then it may be heat treated or coldworked.[2]The extrusion ratio is defined as the starting cross-sectional area divided by the cross-sectional area of the final extrusion. One of the main advantages of the extrusion process isthat this ratio can be very large while still producing quality parts.edit] Hot extrusionHot extrusion is a hot working process, which means it is done above the materialsrecrystallization temperature to keep the material from work hardening and to make iteasier to push the material through the die. Most hot extrusions are done on horizontalhydraulic presses that range from 230 to 11,000 metric tons (250 to 12,000 short tons).Pressures range from 30 to 700 MPa (4,400 to 100,000 psi), therefore lubrication is
  35. 35. required, which can be oil or graphite for lower temperature extrusions, or glass powder forhigher temperature extrusions. The biggest disadvantage of this process is its cost formachinery and its upkeep.[1]
  36. 36. [edit] MetalMetals that are commonly extruded include:[9]  Aluminium is the most commonly extruded material. Aluminium can be hot or cold extruded. If it is hot extruded it is heated to 575 to 1100 °F (300 to 600 °C). Examples of products include profiles for tracks, frames, rails, mullions, and heat sinks.  Copper (1100 to 1825 °F (600 to 1000 °C)) pipe, wire, rods, bars, tubes, and welding electrodes. Often more than 100 ksi (690 MPa) is required to extrude copper.  Lead and tin (maximum 575 °F (300 °C)) pipes, wire, tubes, and cable sheathing. Molten lead may also be used in place of billets on vertical extrusion presses.  Magnesium (575 to 1100 °F (300 to 600 °C)) aircraft parts and nuclear industry parts. Magnesium is about as extrudable as aluminum.  Zinc (400 to 650 °F (200 to 350 °C)) rods, bar, tubes, hardware components, fitting, and handrails.  Steel (1825 to 2375 °F (1000 to 1300 °C)) rods and tracks. Usually plain carbon steel is extruded, but alloy steel and stainless steel can also be extruded.  Titanium (1100 to 1825 °F (600 to 1000 °C)) aircraft components including seat tracks, engine rings, and other structural parts.Magnesium and aluminium alloys usually have a 0.75 µm (30 μin) RMS or bettersurface finish. Titanium and steel can achieve a 3 micrometres (120 μin) RMS.[1]In 1950, Ugine Séjournet, of France, invented a process which uses glass as alubricant for extruding steel.[10] The Ugine-Sejournet, or Sejournet, process is nowused for other materials that have melting temperatures higher than steel or thatrequire a narrow range of temperatures to extrude. The process starts by heatingthe materials to the extruding temperature and then rolling it in glass powder. Theglass melts and forms a thin film, 20 to 30 mils (0.5 to 0.75 mm), in order toseparate it from chamber walls and allow it to act as a lubricant. A thick solid glassring that is 0.25 to 0.75 in (6 to 18 mm) thick is placed in the chamber on the die tolubricate the extrusion as it is forced through the die. A second advantage of thisglass ring is its ability to insulate the heat of the billet from the die. The extrusionwill have a 1 mil thick layer of glass, which can be easily removed once it cools.[3]
  37. 37. Another breakthrough in lubrication is the use of phosphate coatings. With thisprocess, in conjunction with glass lubrication, steel can be cold extruded. Thephosphate coat absorbs the liquid glass to offer even better lubricating properties.[3][edit] PlasticSectional view of a plastic extruder showing the componentsMain article: Plastics extrusionPlastics extrusion commonly uses plastic chips or pellets, which are usually driedin a hopper before going to the feed screw. The polymer resin is heated to moltenstate by a combination of heating elements and shear heating from the extrusionscrew. The screw forces the resin through a die, forming the resin into the desiredshape. The extrudate is cooled and solidified as it is pulled through the die or watertank. In some cases (such as fibre-reinforced tubes) the extrudate is pulled througha very long die, in a process called pultrusion.A multitude of polymers are used in the production of plastic tubing, pipes, rods,rails, seals, and sheets or films.[edit] CeramicCeramic can also be formed into shapes via extrusion. Terracotta extrusion is usedto produce pipes. Many modern bricks are also manufactured using a brickextrusion process.[11]
  38. 38. Extrusion is the process by which long straight metal parts canbe produced. The cross-sections that can be produced vary fromsolid round, rectangular, to L shapes, T shapes. Tubes and manyother different types. Extrusion is done by squeezing metal in aclosed cavity through a tool, known as a die using either amechanical or hydraulic press.Extrusion produces compressive and shear forces in the stock. Notensile is produced, which makes high deformation possiblewithout tearing the metal. The cavity in which the raw material iscontained is lined with a wear resistant material. This canwithstand the high radial loads that are created when thematerial is pushed the die.Extrusions, often minimize the need for secondary machining, butare not of the same dimensional accuracy or surface finish asmachined parts. Surface finish for steel is 3 µm; (125 µ in), andAluminum and Magnesium is 0.8 µm (30 µ in). However, thisprocess can produce a wide variety of cross-sections that arehard to produce cost-effectively using other methods. Minimumthickness of steel is about 3 mm (0.120 in), whereas Aluminumand Magnesium is about 1mm (0.040 in). Minimum cross sections
  39. 39. are 250 mm2 (0.4 in2) for steel and less than that for Aluminumand Magnesium. Minimum corner and fillet radii are 0.4 mm(0.015 in) for Aluminum and Magnesium, and for steel, theminimum corner radius is 0.8mm(0.030 in) and 4 mm (0.120 in)fillet radius.Cold Extrusion: Cold extrusion is the process done at roomtemperature or slightly elevated temperatures. This process canbe used for most materials-subject to designing robust enoughtooling that can withstand the stresses created by extrusion.Examples of the metals that can be extruded are lead, tin,aluminum alloys, copper, titanium, molybdenum, vanadium,steel. Examples of parts that are cold extruded are collapsibletubes, aluminum cans, cylinders, gear blanks. The advantages ofcold extrusion are: • No oxidation takes place. • Good mechanical properties due to severe cold working as long as the temperatures created are below the re- crystallization temperature. • Good surface finish with the use of proper lubricants.Hot Extrusion: Hot extrusion is done at fairly hightemperatures, approximately 50 to 75 % of the melting point ofthe metal. The pressures can range from 35-700 MPa (5076 -101,525 psi). Due to the high temperatures and pressures and itsdetrimental effect on the die life as well as other components,good lubrication is necessary. Oil and graphite work at lower
  40. 40. temperatures, whereas at higher temperatures glass powder isused.Typical parts produced by extrusions are trim parts used inautomotive and construction applications, window framemembers, railings, aircraft structural partsExtrusion is the process by which long straight metal partscan be produced. The cross-sections that can be producedvary from solid round, rectangular, to L shapes, T shapes.Tubes and many other different types. Extrusion is done bysqueezing metal in a closed cavity through a tool, knownas a die using either a mechanical or hydraulic press.Extrusion produces compressive and shear forces in thestock. No tensile is produced, which makes highdeformation possible without tearing the metal. The cavityin which the raw material is contained is lined with a wearresistant material. This can withstand the high radial loadsthat are created when the material is pushed the die.Extrusions, often minimize the need for secondarymachining, but are not of the same dimensional accuracyor surface finish as machined parts. Surface finish for steelis 3 µm; (125 µ in), and Aluminum and Magnesium is 0.8µm (30 µ in). However, this process can produce a widevariety of cross-sections that are hard to produce cost-effectively using other methods. Minimum thickness ofsteel is about 3 mm (0.120 in), whereas Aluminum andMagnesium is about 1mm (0.040 in). Minimum crosssections are 250 mm2 (0.4 in2) for steel and less than thatfor Aluminum and Magnesium. Minimum corner and filletradii are 0.4 mm (0.015 in) for Aluminum and Magnesium,and for steel, the minimum corner radius is 0.8mm(0.030in) and 4 mm (0.120 in) fillet radius.
  41. 41. Cold Extrusion: Cold extrusion is the process done at roomtemperature or slightly elevated temperatures. Thisprocess can be used for most materials-subject todesigning robust enough tooling that can withstand thestresses created by extrusion. Examples of the metals thatcan be extruded are lead, tin, aluminum alloys, copper,titanium, molybdenum, vanadium, steel. Examples of partsthat are cold extruded are collapsible tubes, aluminumcans, cylinders, gear blanks. The advantages of coldextrusion are: • No oxidation takes place. • Good mechanical properties due to severe cold working as long as the temperatures created are below the re- crystallization temperature. • Good surface finish with the use of proper lubricants.Hot Extrusion: Hot extrusion is done at fairly hightemperatures, approximately 50 to 75 % of the meltingpoint of the metal. The pressures can range from 35-700MPa (5076 - 101,525 psi). Due to the high temperaturesand pressures and its detrimental effect on the die life aswell as other components, good lubrication is necessary.Oil and graphite work at lower temperatures, whereas athigher temperatures glass powder is used.Typical parts produced by extrusions are trim parts used inautomotive and construction applications, window frame
  42. 42. members, railings, aircraft structural partsExtrusion press is a sophisticated machinery in the extrusion process thatis available in a huge variety of sizes ranging from 400 tonnes to 1600tonnes. From the common large extruded profiles to thin-wall extrudedprofiles, extrusion presses are geared to meet virtually any demand of theextrusion industry. Modern extrusion presses are equipped with all thelatest technology and innovative features, for example the infinitely variableextrusion speeds, PLC control, reducing an operators need etc. It is thepress size that determines the size of an extrusion, and therefore theselection of a proper extrusion press is of critical importance in theextrusion process. discussed.Aluminium Extrusion PressTo be in a position to understand an aluminium extrusion press completelyand thoroughly, we need a basic understanding of what are the main partsof an extrusion press, what do they look like, what functions do theyperform, etc. Going through the diagram below would be helpful inunderstanding the aluminium extrusion process step by step in detail.[edit] Hot extrusionHot extrusion is a hot working process, which means it is done above the materialsrecrystallization temperature to keep the material from work hardening and tomake it easier to push the material through the die. Most hot extrusions are done onhorizontal hydraulic presses that range from 230 to 11,000 metric tons (250 to12,000 short tons). Pressures range from 30 to 700 MPa (4,400 to 100,000 psi),therefore lubrication is required, which can be oil or graphite for lower temperatureextrusions, or glass powder for higher temperature extrusions. The biggestdisadvantage of this process is its cost for machinery and its upkeep.[1] Hot extrusion temperature for various metals[1] Material Temperature [°C (°F)]Magnesium 350-450 (650-850)
  43. 43. Aluminium 350-500 (650-900)Copper 600-1100 (1200-2000)Steel 1200-1300 (2200–2400)Titanium 700-1200 (1300-2100)Nickel 1000-1200 (1900–2200)Refractory alloys up to 2000 (4000)The extrusion process is generally economical when producing between severalkilograms (pounds) and many tons, depending on the material being extruded.There is a crossover point where roll forming becomes more economical. Forinstance, some steels become more economical to roll if producing more than20,000 kg (50,000 lb).[2]  Aluminium hot extrusion die  Front side of a four family die. For reference, the die is 228 mm (9.0 in) in diameter.  Close up of the shape cut into the die. Notice that the walls are drafted and that the back wall thickness varies.
  44. 44.  Back side of die. The wall thickness of the extrusion is 3 mm (0.12 in).[edit] Cold extrusionCold extrusion is done at room temperature or near room temperature. Theadvantages of this over hot extrusion are the lack of oxidation, higher strength dueto cold working, closer tolerances, good surface finish, and fast extrusion speeds ifthe material is subject to hot shortness.[1]Materials that are commonly cold extruded include: lead, tin, aluminum, copper,zirconium, titanium, molybdenum, beryllium, vanadium, niobium, and steel.Examples of products produced by this process are: collapsible tubes, fireextinguisher cases, shock absorber cylinders, automotive pistons, and gear blanks.[edit] Warm extrusionWarm extrusion is done above room temperature, but below the recrystallizationtemperature of the material the temperatures ranges from 800 to 1800 °F (424 to975 °C). It is usually used to achieve the proper balance of required forces,ductility and final extrusion properties.[3][edit] Equipment
  45. 45. A horizontal hydraulic press for hot aluminum extrusion (loose dies and scrapvisible in foreground)There are many different variations of extrusion equipment. They vary by fourmajor characteristics:[1] 1. Movement of the extrusion with relation to the ram. If the die is held stationary and the ram moves towards it then its called "direct extrusion". If the ram is held stationary and the die moves towards the ram its called "indirect extrusion". 2. The position of the press, either vertical or horizontal. 3. The type of drive, either hydraulic or mechanical. 4. The type of load applied, either conventional (variable) or hydrostatic.A single or twin screw auger, powered by an electric motor, or a ram, driven byhydraulic pressure (often used for steel and titanium alloys), oil pressure (foraluminum), or in other specialized processes such as rollers inside a perforateddrum for the production of many simultaneous streams of material.Typical extrusion presses cost more than $100,000, whereas dies can cost up to$2000.[edit] Forming internal cavitiesTwo-piece aluminum extrusion die set (parts shown separated.) The male part (atright) is for forming the internal cavity in the resulting round tube extrusion.There are several methods for forming internal cavities in extrusions. One way isto use a hollow billet and then use a fixed or floating mandrel. A fixed mandrel,also known as a German type, means it is integrated into the dummy block andstem. A floating mandrel, also known as a French type, floats in slots in thedummy block and aligns itself in the die when extruding. If a solid billet is used as
  46. 46. the feed material then it must first be pierced by the mandrel before extrudingthrough the die. A special press is used in order to control the mandrelindependently from the ram.[1] The solid billet could also be used with a spider die,porthole die or bridge die. All of these types of dies incorporate the mandrel in thedie and have "legs" that hold the mandrel in place. During extrusion the metaldivides and flows around the legs, leaving weld lines in the final product.[4][edit] Direct extrusionPlot of forces required by various extrusion processes.Direct extrusion, also known as forward extrusion, is the most common extrusionprocess. It works by placing the billet in a heavy walled container. The billet ispushed through the die by a ram or screw. There is a reusable dummy blockbetween the ram and the billet to keep them separated. The major disadvantage ofthis process is that the force required to extrude the billet is greater than that needin the indirect extrusion process because of the frictional forces introduced by theneed for the billet to travel the entire length of the container. Because of this thegreatest force required is at the beginning of process and slowly decreases as thebillet is used up. At the end of the billet the force greatly increases because thebillet is thin and the material must flow radially to exit the die. The end of the billet(called the butt end) is not used for this reason.[5]
  47. 47. [edit] Indirect extrusionIn indirect extrusion, also known as backwards extrusion, the billet and containermove together while the die is stationary. The die is held in place by a "stem"which has to be longer than the container length. The maximum length of theextrusion is ultimately dictated by the column strength of the stem. Because thebillet moves with the container the frictional forces are eliminated. This leads tothe following advantages:[6]  A 25 to 30% reduction of friction, which allows for extruding larger billets, increasing speed, and an increased ability to extrude smaller cross-sections  There is less of a tendency for extrusions to crack because there is no heat formed from friction  The container liner will last longer due to less wear  The billet is used more uniformly so extrusion defects and coarse grained peripherals zones are less likely.The disadvantages are:[6]  Impurities and defects on the surface of the billet affect the surface of the extrusion. These defects ruin the piece if it needs to be anodized or the aesthetics are important. In order to get around this the billets may be wire brushed, machined or chemically cleaned before being used.  This process isnt as versatile as direct extrusions because the cross- sectional area is limited by the maximum size of the stem.[edit] Hydrostatic extrusionIn the hydrostatic extrusion process the billet is completely surrounded by apressurized liquid, except where the billet contacts the die. This process can bedone hot, warm, or cold, however the temperature is limited by the stability of thefluid used. The process must be carried out in a sealed cylinder to contain thehydrostatic medium. The fluid can be pressurized two ways:[6] 1. Constant-rate extrusion: A ram or plunger is used to pressurize the fluid inside the container. 2. Constant-pressure extrusion: A pump is used, possibly with a pressure intensifier, to pressurize the fluid, which is then pumped to the container.
  48. 48. The advantages of this process include:[6]  No friction between the container and the billet reduces force requirements. This ultimately allows for faster speeds, higher reduction ratios, and lower billet temperatures.  Usually the ductility of the material increases when high pressures are applied.  An even flow of material.  Large billets and large cross-sections can be extruded.  No billet residue is left on the container walls.The disadvantages are:[6]  The billets must be prepared by tapering one end to match the die entry angle. This is needed to form a seal at the beginning of the cycle. Usually the entire billet needs to be machined to remove any surface defects.  Containing the fluid under high pressures can be difficult.Extrusion is the process by which long straight metal parts canbe produced. The cross-sections that can be produced vary fromsolid round, rectangular, to L shapes, T shapes. Tubes and manyother different types. Extrusion is done by squeezing metal in aclosed cavity through a tool, known as a die using either amechanical or hydraulic press.Extrusion produces compressive and shear forces in the stock. Notensile is produced, which makes high deformation possiblewithout tearing the metal. The cavity in which the raw material iscontained is lined with a wear resistant material. This canwithstand the high radial loads that are created when thematerial is pushed the die.Extrusions, often minimize the need for secondary machining, butare not of the same dimensional accuracy or surface finish asmachined parts. Surface finish for steel is 3 µm; (125 µ in), andAluminum and Magnesium is 0.8 µm (30 µ in). However, thisprocess can produce a wide variety of cross-sections that arehard to produce cost-effectively using other methods. Minimumthickness of steel is about 3 mm (0.120 in), whereas Aluminum
  49. 49. and Magnesium is about 1mm (0.040 in). Minimum cross sectionsare 250 mm2 (0.4 in2) for steel and less than that for Aluminumand Magnesium. Minimum corner and fillet radii are 0.4 mm(0.015 in) for Aluminum and Magnesium, and for steel, theminimum corner radius is 0.8mm(0.030 in) and 4 mm (0.120 in)fillet radius.Cold Extrusion: Cold extrusion is the process done at roomtemperature or slightly elevated temperatures. This process canbe used for most materials-subject to designing robust enoughtooling that can withstand the stresses created by extrusion.Examples of the metals that can be extruded are lead, tin,aluminum alloys, copper, titanium, molybdenum, vanadium,steel. Examples of parts that are cold extruded are collapsibletubes, aluminum cans, cylinders, gear blanks. The advantages ofcold extrusion are: • No oxidation takes place. • Good mechanical properties due to severe cold working as long as the temperatures created are below the re- crystallization temperature. • Good surface finish with the use of proper lubricants.Hot Extrusion: Hot extrusion is done at fairly hightemperatures, approximately 50 to 75 % of the melting point ofthe metal. The pressures can range from 35-700 MPa (5076 -101,525 psi). Due to the high temperatures and pressures and itsdetrimental effect on the die life as well as other components,
  50. 50. good lubrication is necessary. Oil and graphite work at lowertemperatures, whereas at higher temperatures glass powder isused.Typical parts produced by extrusions are trim parts used inautomotive and construction applications, window framemembers, railings, aircraft structural parts.
  51. 51. The Hydroforming ProcessHydroforming was developed in the late 1940s and early 1950sto provide a cost effective means to produce relatively smallquantities of drawn parts or parts with asymmetrical or irregularcontours that do not lend themselves to stamping. Virtually allmetals capable of cold forming can be hydroformed, includingaluminum, brass, carbon and stainless steel, copper, and highstrength alloys.A hydroforming press operates like the upper or female dieelement. This consists of a pressurized forming chamber of oil, arubber diaphragm and a wear pad. The lower or male dieelement, is replaced by a punch and ring. The punch is attachedto a hydraulic piston, and the blank holder, or ring, whichsurrounds the punch.
  52. 52. The hydroforming process begins by placing a metal blank on thering. The press is closed bringing the chamber of oil down on topof the blank. The forming chamber is pressurized with oil whilethe punch is raised through the ring and into the the formingchamber. Since the female portion of this forming method isrubber, the blank is formed without the scratches associated withstamping.The diaphragm supports the entire surface of the blank. It formsthe blank around the rising punch, and the blank takes on theshape of the punch. When the hydroforming cycle is complete,
  53. 53. the pressure in the forming chamber is released and the punch isretracted from the finished part.Hydroforming Advantages  Inexpensive tooling costs and reduced set-up time.  Reduced development costs.  Shock lines, draw marks, wrinkling, and tearing associated with matched die forming are eliminated.  Material thinout is minimized.  Low Work-Hardening  Multiple conventional draw operations can be replaced by one cycle in a hydroforming press.  Ideal for complex shapes and irregular contours.  Materials and blank thickness specifications can be optimized to achieve cost savings.
  54. 54. Manufacturing Costs: Hydroforming versus Deep DrawStampingTooling - With low volume runs, tooling is often the mostimportant cost consideration. With hydroforming, a male die, orpunch, and a blank holding ring are the only tools required as therubber diaphragm and pressurized forming chamber act as thefemale die. As a result, hydroforming tooling is typically 50% lessexpensive than matched die tooling. With hydroforming, mostpunches are made from cast iron as opposed to the hardened toolsteels used for match die drawing punches. Finally, hydroformingtools are easily mounted and aligned, making set-ups fast andefficient.Development Costs - Proto-typing is often a necessary step inthe manufacturing process. Changes in material type or wallthickness specifications can typically be accommodated withhydroforming without creating a need for new tooling.Reduced Press Time - Complex parts requiring multiple presscycles in matched die operations can be drawn in a singlehydroforming cycle. Hydroforming presses frequently achievereductions of 60-70% compared to 35-45% for conventionalmatched die presses.Finishing Costs - Aerospace, medical and commercial cookwareapplications often demand parts with outstanding surfacefinishes. Unlike matched die metal forming, which can leavescratches and stretch lines, the flexible diaphragm utilized inhydroforming eliminates surface blemishes, reducing the need forcostly finishing processes like buffing.
  55. 55. LASER BEAMWELDING [ PRINT ] Laser welding is a high energy beam process and in thisregard is similar to electron beam. With that exception they are unlikeone another. The energy density of the laser is achieved by theconcentration of light waves not electrons. The laser output is notelectrical, does not require electrical continuity, is not influenced bymagnetism, is not limited to electrically conductive materials and in factcan interact with any material whether it be metal, plastic, wood, ceramic,etc. Finally its function does not require a vacuum nor are x-raysproduced.The focal spot (thousandths of an inch in diameter) is targeted on theweld joint surface or by focal length selection above or below it. At thesurface the enormous concentration of light energy is converted tothermal energy. Surface melting occurs and progresses through the weldjoint by thermal conductance. For welding, beam energy is maintainedbelow the vaporization temperature of the weld joint material. For holedrilling or cutting vaporization is required. Because weld joint penetrationis dependent on conducted heat the thickness of materials to be welded
  56. 56. is generally less than .080 inches if the ideal metallurgical and physicalcharacteristics of laser welding are to be realized. These benefits arenarrow welds, no distortion, minimal heat affected zones and excellentmetallurgical quality.As with electron beam the intense, concentrated energy produces meltingand coalescence before a substantial heat affected zone can develop.Because the welds are narrow and therefore are of correspondingly lowvolume there is a minimal reservoir of heat for conductance into theadjacent area. When materials to be welded are thick and particularly ifthey have high thermal conductance (aluminum for example) thisimportant metallurgical advantage of minimal heat affected zone can bedetrimentally affected. It is claimed that since the source of energy is lightof a specific wavelength contaminants in the weld pool or on the facingsurfaces of the joint may be preferentially vaporized by their particularlight absorbing characteristics resulting in a kind of weld purification. Theexcellent fatigue strength of laser welds is sometimes attributed to thispurifying phenomenon.
  57. 57. Energydistribution across the beam is generated by the design of the resonantcavity, including mirror curvatures or shape and their relativearrangement. This combination results in photon oscillation within thecavity producing specific output beam energy distributions or patterns.These patterns are labeled transverse energy modes (TEM) and havespecific identifying numbers. We cannot within this seminar describe theirvariety and effects. However, we will point out that the Gaussian modeTEM 00 is often preferred for welding inasmuch as its peak energy is inthe center of the beam feathering off to its periphery. It might be likenedto a pointer. The symmetry and profile of the Gaussian Beam isparticularly suited for welding.We have learned how light energy is amplified in the solid state lasercavity and how the laser beam and its unique characteristics are formed.It is important to note before proceeding that the function of all laserswhether they be gas (carbon dioxide, helium neon, etc.) or other lasing
  58. 58. sources is based on the principle of the excitation of atoms by means ofintense light, electricity, electron beam, chemicals, etc., and thespontaneous and stimulated emission of photons. Depending on thelasing source, output frequencies differ widely and are capable of a greatnumber of applications. These range from welding to critical surgery,resistance trimming, communication, etc. As a clear demonstration of the effect of light wavefrequency, the beam of a neodymium YAG laser (l.06 micron wavelength)will pass through quartz lenses, clear plastic or glass and othertransparent materials. However a carbon dioxide laser emitting a beamwith a wavelength of 10.6 microns will not pass through the quartz lensetc. but rather will be absorbed by those materials resulting in theirdestruction. Carbon dioxide lasers must achieve focusing either byconverging reflective optics or special salt based lens materials such aszinc selenide.We have discussed the role of the objective focusing lens and how itconcentrates the beam energy into a focal spot as small as .005 inches in
  59. 59. diameter or less. We have also reviewed how a laser weld is produced byconducted heat and the excellent quality of the weld. Because the energy density is so intense, infact second only to the electron beam, the laser is capable of vaporizingmetals such as tungsten or non-metallic materials such as ceramics. Infact, in conductance welding, care must be taken to prevent thisvaporizing action. However, as with electron beam, lasers can producedeep penetration welds by the keyholing technique. Laser keyholing islimited to perhaps 3/4 to 1 1/2 inch thickness and for these depths amulti-kilowatt laser, such as the carbon dioxide type, must be used.We need to mention that although there are many laser types, theNd:YAG and carbon dioxide lasers (CO2) are most common in productionmetal working. Carbon dioxide lasers utilize a combination of carbondioxide, the primary lasing source, helium and nitrogen. The gas mixturecirculates through a bank of electrodes, which is the energy source. Theoutput wavelength is 10.6 microns. Carbon dioxide lasers have beendeveloped with outputs exceeding 25 kilowatts. This high output of CO2lasers is possible since they can be efficiently cooled. In contrast, coolingthe solid state YAG laser crystal is difficult and critical. Considerable
  60. 60. design attention is directed towards cooling, excitation lamps, theirreflectors, cavity shape, materials, plating of reflectors, lens anti-reflection coatings, etc. This includes power supplies, which may bedesigned for continuous or for a pulsing output. Pulse repetition ratesand pulse shaping are programmable.We must now continue our initial discussion of the interaction of the laserbeam with metals. As stated, heat is generated by the conversion of lightenergy. All metals reflect light to some degree, with gold and silver highon the list and carbon steel low on the list. Gold, silver, copper, andaluminum are therefore difficult to weld requiring intense energy usuallyavailable from high energy peaking pulses or resorting to light absorbingcoatings such as graphite on the weld joint surfaces to reduce theirreflectivity. The 1.06 micron wavelength of the Nd:YAG laser is morereadily absorbed than the longer 10.6 micron wavelength of the COlasers, therefore, in this respect more suited for welding highly reflectivematerials. However though metallic reflectivity is a factor, once melted,the reflectivity essentially disappears at the curie temperature (about1425 degree F). Therefore most metals are readily welded. The intenseenergy of the beam quickly melts the surface, from which thermalconductance progresses to achieve penetration.Because the beam can be reflected from mirrored surfaces (reflective atthe laser wavelength) it follows that beam manipulation is almostunlimited. It is this feature that makes marking or engraving laserspossible. Holes can be drilled or cut as square, round, any geometricpattern, size or dimensional proportions by mirror manipulation. Beamenergy can be tailored to produce strategic pulse profiles. Energy can becontinuous, or weld seams may be produced by overlapping individualpulses which tend to reduce heat input by the brief cool cycle betweenpulses, an advantage for producing welds in heat sensitive materials. A
  61. 61. third arrangement is a continuous output with pulsing actionsuperimposed by an acousto-optic (Q) switch located in the cavity. Thisdevice is capable of generating pulse rates in the tens of thousands andcan increase cavity energy by interrupting the output thus causing a briefperiod of gain or storage in the laser crystal. Considering that photonoscillations within the cavity occur at the speed of light even a briefinterruption of the output is extremely effective for increasing the gain.The manipulative ability of the laser establishes it as ideal for automationand robotics. Fiber optics dramatically adds to this versatility. Utilizingthis capability, production assemblies on trays, fixtures or shuttles can beconveyorized while the laser focusing optics, incorporating the necessaryaxis of motion (x, y, z) including targeting and scanning, can track andfollow the weld joint. This flexibility combined with motion and parameterprogramming is seemingly unlimited. Inert gas shielding of the weld isusually incorporated coaxially with the laser. However, inert gas trailers,underbead coverage and other strategic, and beneficial inert gasapplications are easily adopted. If necessary assemblies can be placed ina vacuum chamber and the laser beam introduced through a quartzwindow. The raw beam can be focused through optics within thechamber or may be focused external to the chamber utilizing theappropriate focal distance. Alternately fiber optics may be utilized androuted through appropriate hardware in the chamber walls. The fiberoptics can be terminated with focusing optics internal to the chamber.Many arrangements are possible.
  62. 62. Thedisadvantages of lasers include its high capital cost, the need for cleanenvironment (to protect the optics), and the safety considerations. Thelatter two are most often resolved by the installation of the laser in a laserroom. Warning signs, sounds and/or flashing lights are employed tosignal when the laser is on. An additional disadvantage is thoughmaintenance is generally minimal, laser operation requires training andexperience. Maintenance and machine operator personnel must becomeaware of the subtleties that can influence the laser output. In this regard,the laser is somewhat unlike the electron beam, which has with ratherpositive switch controlled reactions.We will next discuss how a laser beam is generated. It is necessary tostart with atoms. Depending on the particular element there may be one,two or more electrons in single or multiple energy orbits circling thenucleus of the atom. In their normal quiescent or ground state the orbitsare at discrete energy levels or distances from the nucleus that arecharacteristic of the specific atom. All of the atoms of a given elementshare this identical behavior.
  63. 63. Spontaneous EmissionThe basis for laser action occurs when an atom (for this example,neodymium) is excited by an external energy source. The absorbedenergy will cause the atoms electrons to move from their ground state toone of the discrete and exact energy orbits, characteristic of the specificatom. Therefore when these orbiting electrons return spontaneously totheir ground state, they release the energy difference as a photon. Sinceall of the photons originate from electrons in the same energy orbit, theyhave the same wavelength. Stimulated EmissionEinstein theorized and proposed that a photon passing near an excitedelectron of the same energy would cause the approached electron toreturn to its ground state and in doing so release its photon of light. Twoidentical photons would now exist. The two photons would travel as acoherent pair and in the exact same direction. This phenomena would berepeated over and over again as each of the triggered photonsapproached other excited electrons in the same energy orbits. Groups ofphotons, depending on the emanated direction of their original source.
  64. 64. When an external source of energy, whether it be intense light, chemical,electrical, etc. is absorbed by the atom, its excited electrons will move toa new energy orbit but only after they have absorbed a specific amount ofenergy. They do not move half way or one and one-half times out butrather to a discrete new energy orbit characteristic of the atom. Ananalogy would be stepping from stone to stone across a stream. Tooshort or too long a step will result in a soaking. There is no margin forerror. Another analogy is throwing a ball through a distant basket or loop.It requires a specific amount of energy to succeed. Too little, too muchresult in misses. This relationship between atoms, their electrons andenergy is an atomic law. Therefore, the orbiting electron must absorb aspecific energy value before they move to that very discrete and distinctnew orbit. The electrons of every atom of a specific element excited by acommon energy source contain the same energy inasmuch as they are inthe same energy orbit and will release this energy as photons when theyrandomly drop back to their ground level, or normal state, an actioncalled spontaneous emission. The released photons, therefore contain thesame energy and, as a result, the same light wavelength.Based on this fundamental atomic action a laser crystal (ground rod)containing an element such as neodymium, which is capable of releasingphotons, when appropriately energized can become the basis for laseractivity. All of the neodymium (Nd) electrons in their identical energyorbits will randomly return to their ground state, collectively releasingenormous quantities of photons each containing the energy differencebetween the excited orbit and the ground state. Their light wavelength isequivalent to this energy. In the instance of neodymium, the lightwavelength is 1.06 microns. Other photon emitting elements can haveother wavelengths. It should be noted there are numerous elementscapable of emitting photons but many are ruled out for laser use because
  65. 65. of the difficulty in acquiring, their instability, etc. To return to theforegoing, a neodymium: yttrium, aluminum garnet (Nd:YAG) solid statelaser consists of the element neodymium dispersed in the host yttriumaluminum garnet (YAG) crystal.Lets proceed to the next important phase of laser beam generation. Infact, the term laser (light amplification by stimulated emission ofradiation) is the function we are about to discuss. As a matter of interestthis theory was postulated by A. Einstein contributing to thedevelopment, design and fabrication of the first research lasers.We have learned the atoms of certain elements when they absorb energymove their electrons to a new and discrete orbiting energy level. On theirrandom, spontaneous return to their ground or quiescent state they emitphotons of light. We may think of the photons as particles. The photonscontain the energy difference between the excited and ground stateorbits. Their specific wavelength is a result of and proportional to thisenergy.The photons move in random spatial directions at the speed of light andof a specific wavelength. However, Einstein postulated that when aphoton passes close to an excited electron of equal energy, it wouldtrigger the electron spontaneous emission of a photon. There would nowbe two. Interestingly, the second photon as we now know will contain thesame energy, therefore wavelength as the first triggering photon. Tocontinue the phenomena the second triggered photon will travel in theexact same spatial direction as the first. As these two paired photonscontinue on they will trigger other electrons by stimulated emissioncreating an enormous amplification of photons traveling in the exactsame direction dependent on their origin. Needless to say all of thephotons have the same light wavelength.
  66. 66. Characteristics of the laser beam MonochromaticAll of the photons which compose the beam are of the same energy andtherefore the same wavelength. If the laser beam was directed through anoptical prism it could not split up into the separate colors representingthe wavelengths of the optical spectrum.CoherentThe light waves are in phase (in step).CollimatedThe laser beam does not diverge. It can be projected great distanceswithout significant spreading. For example, it is used for topographicalsurveys where elevations miles away can be measured from a single,
  67. 67. central location. Collimation makes is possible for laser beams to targetsatellites, etc. at great distance. Because of these three characteristics thelaser beam can be precisely focused to very small diameters, resulting inan enormous increase in energy density.From here we will proceed to understand how all of this activity i.e. thetriggered release of photons by stimulated emission and the cascadingeffect resulting from the stimulating action of photons approaching otherexcited electrons of equal energy become the basis of the laser. Thequestion now is how to harness, organize, control and concentrate thespatial motion of photons into a controlled beam of light capable of beingprojected without significant divergence, for miles and to contain a levelof concentrated energy capable of vaporizing such high temperaturematerials as metals and ceramics.The atoms of reodymium are a stable source of lasing action. The "YAG"crystal which is grown as a boule is doped with the element neodymium.This crystal is precisely ground to a rod configuration. When assembled ina resonant cavity it becomes the basis for solid state laser action, emittinga laser beam having a light wavelength of 1.06 microns.The diagram shows a neodymium doped YAG crystal absorbing energyfrom an intense light source resulting in the release of photons in randomspatial directions by the combined mechanism of spontaneous andstimulated emission.
  68. 68. Next, another view of the crystal with mirrors added to each end toproduce a resonant cavity. By coincidence, the spatial direction of some ofthe photon groups will cause them to travel along the longitudinal axis ofthe cavity. The result is the impingement of these photons on the mirrorslocated at the ends of the crystal from where they will be returned to thecrystal by reflection and will continue to stimulate the emission of otherphotons. This activity creates an enormous amplification of photonstraveling back and forth between the mirrors, continually stimulating andaligning their travel direction. One of the mirrors designated the frontmirror is deliberately designed to allow a controlled leakage ortransmission of light -up to 60%. This transmission is the raw laser beam.The beam is pure light since it consists of a single wavelength(monochromatic) and in addition is both coherent (in phase) andcollimated (low divergence).The basic solid state neodymium "YAG" laser cavity consists of theND:YAG crystal, an energy source, a 100% reflective rear mirror and afront or output mirror which is up to 60% light transmissive. The cavity
  69. 69. may also contain accessories such as shutters, apertures and electrooptical mechanisms. A laserbeam is generated when photons traveling in a direction along thelongitudinal axis of the crystal are reflected and returned to the crystal bythe end mirrors where they continue to amplify the output through thephenomena of stimulated emission. Since the front mirror is up to 60%light transmissive a laser beam is emitted. This beam of light ismonochromatic, collimated and coherent. Because of these threecharacteristics and resultant low divergence, the beam is capable oftraveling great distances with minimal energy loss.These characteristics are extremely important. The ability to deliver thispure beam of light through an optical system and project it for miles or tofocus it to so small a diameter its energy density can vaporize ceramics, isdependent on these three characteristics. It is timely to mention there aremany lasers emitting pure light beams of different frequencies dependingon their atomic origin. Special applications require particular lightfrequencies to be efficiently absorbed by the characteristics of the targetmaterial. Eye surgery or measurement operations, etc. require differentlight frequencies than those we commonly use for metal working. We
  70. 70. must not neglect to add the cavity and optical system just describedincludes other items, apertures, collimators, safety shutters, beamsplitters, viewing optics etc. to fine tune and control the beam. After leaving the cavitythrough the partially transmissive front mirror, the beam continuesthrough a safety shutter followed by an up collimator. The latter, a kind ofreverse telescope, expands the beam, further improves the collimationand prepares it for the final optics. After leaving the up collimator, thebeam proceeds to a turning or beam bending mirror where it is usuallydirected vertically downward through the final, objective focusing lens.Between the turning mirror and final lens, other accessories such as atrepanning device may be introduced. Depending on the system thebeam, after leaving the up collimator, may enter a fiber optic couplerrather than the hard optics just described. Fiber optics terminated byfocusing optics provide complete mobility of beam direction.The final lens focuses and concentrates the raw laser beam into thedesired spot diameter. In addition, it establishes the focal distance
  71. 71. between the lens and workpiece and relative to this produces a specificdepth of field within which distance there are negligible changes in focalspot diameter. Short or long focal distances have their correspondingshort or long depth of fields. In all of the foregoing arrangements thereare numerous variations; lens design, lens combinations, beam splitting,trepanning heads, power sampling, apertures, up collimator ratios, etc.intended to provide particular performance and control characteristics.