Welding lectures 11 13


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Welding lectures 11 13

  1. 1. 10/17/2012Casting, Forming & WeldingCasting Forming & Welding (ME31007)  J u au Jinu Paul Dept. of Mechanical Engineering 1 Welding ecture Welding Lecture – 11 11 Oct 2012, Thursday 8.30 am‐9.30 am Design of Weld joints 2 1
  2. 2. 10/17/2012 Design of Weld joints(Refer class notes) 3 4 2
  3. 3. 10/17/2012 5Example No: 3 6 3
  4. 4. 10/17/2012 Example No: 4 7Welding ectureWelding Lecture ‐ 1212 October 2012, Friday 11.30 am ‐12.30 pm Welding Processes‐ Other fusion  Oh f i welding processes 8 4
  5. 5. 10/17/2012 Thermite mixture Metallic fuel + Oxidiser  Energy Thermite ReactionMetal oxide + Aluminum Metal + Aluminum oxide + Heat• Bimolecular reactions and reaction rates are controlled bydiffusion times between reactants.• Thermite mixtures of nano-sized reactants reduce the criticaldiffusion length thus increasing the overall reaction rate 5
  6. 6. 10/17/2012 Thermite Reaction stages (1/2)Fe3O4 + Al  Fe + (1/2)FeAl2O4 2FeO + Al  (3/2)Fe + (1/2)FeAl2O4 (1/2)FeAl2O4 + (1/3)Al  (1/2)Fe + (2/3)Al2O3 Thermite types Fuels OxidisersAluminium, Boron(III) oxide,Magnesium, Silicon(IV) oxide,Titanium, Chromium(III) oxide,Zinc, Manganese(IV) oxide,Silicon, Iron(III) oxide,Boron Iron(II,III) oxide, Copper(II) oxide oxide, Lead(II,III,IV) oxide, 6
  7. 7. 10/17/2012 Thermite welding (TW)• Heat for coalescence is produced by superheated molten metal from the chemical reaction of Thermite• Example: 2Al + Fe2O3  2Fe + Al2O3 + heat• Filler metal is obtained from the liquid metal• More in common with casting than it does with welding• Applications in joining of railroad rails and repair of cracks in large steel castings and forgings such as ingot moulds, large diameter shafts, frames for machinery, and ship rudders 13 Thermit welding (TW) Fe2O3 + Al  2Fe + Al2O3 + ~850kJ 14 7
  8. 8. 10/17/2012 High-Energy-Density Beam Welding Processes• Electron beam and Electron-beam• Laser-beam welding• Focussed beam of electromagnetic energy – IR welding – Imaged arc welding – Microwave welding 15Comparison of Conventional and E/Laser-Beam Welding 16 8
  9. 9. 10/17/2012 Electron-beam welding (EBW) • Uses kinetic energy of dense focused electrons • Electrons emitted by cathode, accelerated by ring shaped anode, focused by electromagnetic field • High energy density 10 MW/mm2 • Heat focus on few micrometers • Vacuum chamber 17Electron speed VsAccelerating voltage 18 9
  10. 10. 10/17/2012E-Beam interaction with work piece 19 Electron-beam penetration Vs operating pressure 20 10
  11. 11. 10/17/2012 EBW or LBW of a butt joint MeltingButt joint A key The keyhole occurs at the The weldprior t i to hole and it molten d its lt point of forms uponwelding forms envelope impingement solidification penetrates of the E-beam workpiece 21 Laser-beam welding (LBW) 22 11
  12. 12. 10/17/2012 Laser-beam welding• Coalescence is achieved by the energy of a highly concentrated, coherent light beam focused on the joint to be welded• LBW is normally performed with shielding gases (e.g., helium, argon, nitrogen, and carbon dioxide) to prevent oxidation• No vacuum chamber is required, no X-rays are emitted• Laser beams can be focused and directed by optical lenses and mirrors.• LBW does not possess the capability for the deep welds and high depth-to-width ratios of EBW 23 Example‐1A carbon dioxide laser with a power output of 1 kW operates inthe continuous wave mode. (For CO2 laser, wavelength = 10micron = 0.01 mm). Focal length f and diameter of the lensused is 100 mm and 8 mm respectively. The diameter of laserbeam is 6 mm.The laser-beam welding operation will join two pieces of steelplate together as shown in figure. The plates are 25 mm thick.The unit melting energy is 10 J/mm3. The heat transfer factor is0.70 and the melting factor is 0.55. Find the velocity of thelaser beam movement if the beam penetrates the full thicknessof the plates? 12
  13. 13. 10/17/2012 Laser beam‐ Depth of  penetration 25 Focussed IR welding• Infrared radiation from the sun or an artificial light source can be used• Radiation is focused into an intense, high-density spot directed onto the work 13
  14. 14. 10/17/2012 Imaging arc welding• High energy density due to focussing• Advantage is freedom from the electromotive Lorentz forcesassociated with conventional arc welding Comparison of Electron‐Beam and Laser‐ Beam Welding  EBW LBW1.1 Deep penetration in all materials 1. 1 Deep penetration in many materials but materials, not in metals that reflect laser light/or of specific wavelengths2. Very narrow welds 2. Can be narrow (in keyhole mode)3. High energy density/low linear 3. Same4. Best in vacuum, to permit electrons 4. Can operate in air, inert gas, or vacuum5. Usually requires tight-fitting joints 5. Same6. Difficult to add filler for deep welds 6. Same p7. Equipment is expensive 7. Same8. Very efficient electrically (99%) 8. Very inefficient electrically (- 12%)9. Generates x-ray radiation 9. No x-rays generated 28 14
  15. 15. 10/17/2012 Welding ecture 3 Welding Lecture ‐ 13 17 October 2012 9.30 am ‐10.30 am Solid state welding  processes 29 Solid state/Nonfusion welding• Accomplish welding by bringing the atoms (or ions or  molecules) to equilibrium spacing  through plastic molecules) to equilibrium spacing through plastic  deformation  application of pressure at  temperatures below the melting point of the base  material • Without the addition of any filler• Chemical bonds are formed and a weld is produced  as a direct result of the continuity obtained,  di l f h i i b i d always with the added assistance of solid‐state  diffusion 30 15
  16. 16. 10/17/2012 Solid state/Nonfusion welding1. Pressure Welding  By pressure and gross  deformation2. Friction welding  By friction and  microscopic deformation3. Diffusion welding  By diffusion, without or  with some deformation4. Deposition welding  Solid‐state deposition  welding 31 Pressure WeldingCold welding• Pressure is used at room temperature to produce  coalescence of metals with substantial plastic  l f t l ith b t ti l l ti deformation  No heat• The faying surfaces must be exceptionally clean• Cleaning is usually done by degreasing and wire brushing  immediately before joining  32 16
  17. 17. 10/17/2012 Pressure Welding Cold welding• At least one of the metals to be joined must be highly  ductile and not exhibit extreme work hardening and not exhibit extreme work hardening• FCC metals and alloys are best suited for CW. Example‐ Al,  Cu, and Pb• To a lesser degree, Ni and soft alloys of these metals such  as brasses, bronzes, babbitt metals (Sn, Cu, Sb, Pb), and  pewter (Sn, Cu, Sb, Bi) pewter (Sn Cu Sb Bi)• Precious metals, Au, Ag, Pd, and Pt, are also ideally suited  to cold welding, as they are face‐centered cubic (soft)  and are almost free of oxides 33 Pressure Welding  Cold welding• Ideal for joining of dissimilar metals  no  intermixing of the base metals is required • Allows inherent chemical incompatibilities that  make fusion welding difficult to be overcome• E.g.  Cold welding of relatively pure Al to  relatively pure Cu  Electrical connections• Formation of brittle intermetallics (e.g., AI,Cu)  either during postweld heat treatment or in  service, (resistance heating in the electrical  connector) 34 17
  18. 18. 10/17/2012 Micro‐patterning of Organic Electronic  Devices by Cold‐Welding Calculated normal stress at the interface (yy) normalized to the applied pressure (P) as a function of distance (x) normalized to the stamp half-width (a) Figure 1 35 Micro‐patterning of Organic Electronic  Devices by Cold‐Welding• A prepatterned, metal‐coated stamp composed of a rigid material (Si) is  pressed onto an unpatterned film consisting of the organic device layers  coated with the same metal contact layer as that used to coat the stamp.  coated with the same metal contact layer as that used to coat the stamp.• Organic layer thickness ~ 100 nm, same thickness for the metal cathode• When a sufficiently high pressure is applied, an intimate metallic junction is  formed between the metal layers on the stamp and the film, leading to a  cold‐welded bond (Fig 1, top). • To induce selective lift‐off, additional pressure is applied to weaken the  metal film at the edge of the stamp (Fig. 1 , middle).• This additional pressure leads to substrate deformation, which is expected to  enhance the local weakening of the metal film. • When the stamp and film are separated, the metal cathode breaks sharply,  forming a well‐defined patterned electrode (Fig. 1, bottom). 36 18
  19. 19. 10/17/2012 Fabrication of OLEDs(A) Optical micrograph of an array of 230-mm-diameter Mg-Ag alloycontacts patterned by cold-welding followed by cathode lift-off. (B) Scanningelectron micrograph (SEM) of the edge of a 12-mm-wide stripe showing aclearly defined nearly featureless layer pattern. 37 Cold welding of ultrathin gold nanowires Singlecrystalline gold nanowires with diameters between 3 and 10 nm can be cold-welded together within seconds by mechanical contact alone 38 19
  20. 20. 10/17/2012 Head‐to‐head welding of two Au‐ nano rodsa,b, One nanorod (right)is caused to approachanother (left) until theirfront surfaces come intocontact.contactc–e, The weldingprocess is completedwithin 1.5 s (c,d)followed by structurerelaxation (d,e).f–i, After withdrawal ofthe STM probe (f–i), theas-welded nanowire isleft in the free-standingstate (Triangles indicate the front edges of the two nanorods. Arrows indicate the 39 withdrawing direction of the STM probe. Scale bars, 5 nm) Pressure WeldingHot Pressure  Welding HEAT + PRESSURE Vacuum or shielding MACROSCOPIC DEFORMATIONExamples: COALESCENCE1) Pressure gas welding2) Forge welding 20
  21. 21. 10/17/2012Pressure Welding Forge welding (FOW)• Earliest form of welding  still used today by  blacksmiths• Produces the weld by heating  work pieces to hot working  temperatures and applying  blows sufficient to cause  deformation at the faying  deformation at the faying surfaces• Low‐carbon steels (most  commonly forge‐welded  metal), high‐carbon steel 41 Pressure Welding Forge welding Schematic of typical joint designs for (a) manual and (b) automated forge welding ldi 42 21
  22. 22. 10/17/2012 Pressure WeldingRoll Welding• Pressure applied by rollers  Performed hot or cold li d b ll f dh ld• Applications  cladding stainless steel to mild or low alloy steel  for corrosion resistance• Making bimetallic strips• Producing ‘‘sandwich’’ coins for the U.S. mint 43 Pressure WeldingExplosion welding• Coalescence of two metallic surfaces is caused by the  energy of a detonated explosive energy of a detonated explosive• Commonly used to bond two dissimilar metals• E.g.  To clad one metal on top of a base metal over  large areas 44 22
  23. 23. 10/17/2012 Pressure WeldingExplosion welding:  Applications• Applications include production of corrosion‐ resistant sheet and making processing equipment  it t h t d ki i i t in the chemical and petroleum industries• E.g. Commercially pure titanium clad to mild steel• Often performed under water to enhance the  shock wave to move and deform material shock wave to move and deform material 45Compatible materials for Explosion welding 46 23
  24. 24. 10/17/2012 2.1 Friction welding (FRW)• Solid state welding  Coalescence is achieved by  frictional heat combined with pressure frictional heat combined with pressure• Friction is induced by mechanical rubbing  between two surfaces  usually by rotation of  one part relative to the other  raises the  temperature at the joint interface to the hot  working range  Parts are driven toward each working range  Parts are driven toward each  other with sufficient force to form a metallurgical  bond 47 2.1 Friction welding (FRW)Mechanical Rubbing FRICTION HEAT + MICROSCOPIC DEFORMATION PRESSURE No melting occurs at the faying surfaces COALESCENCE No filler metal, flux, or shielding gases 24
  25. 25. 10/17/2012 2.1 Friction welding (FRW) 49Drive parameter characteristics in FRW 50 25
  26. 26. 10/17/20122.2 Friction stir welding (FSW), 51 FSW Tool 26
  27. 27. 10/17/2012 2.2 Friction stir welding (FSW),• A rotating tool is fed along the joint  p line between two work pieces  Generates friction heat • Mechanically stirring of the metal to  form the weld seam• The process derives its name from  this stirring or mixing action• FSW is distinguished from FSW is distinguished from  conventional FRW   Friction heat is  generated by a separate wear‐ resistant tool rather than by the  parts themselves 53 2.2 Friction stir welding (FSW),• The rotating tool is stepped,  consisting of a cylindrical shoulder consisting of a cylindrical shoulder and a smaller probe projecting  beneath it• The probe has a geometry  designed to facilitate the mixing  action• Th h ld The shoulder serves to constrain  i the plasticized metal flowing  around the probe 54 27
  28. 28. 10/17/2012 2.2 Friction stir welding (FSW),• During welding, the shoulder rubs  against the top surfaces of the two  parts, developing much of the friction  heat• While the probe generates additional  heat by mechanically mixing the metal  along the butt surfaces• The heat produced by the combination  of friction and mixing does not melt the  metal but softens it to a highly plastic  condition 55 Heat generated in FSW (Refer Class notes) 28
  29. 29. 10/17/2012 2.2 Friction stir welding (FSW),• Typical applications  butt joints on large aluminium parts• Other metals, include steel, copper, and titanium, as well as  polymers and composites l d i• Advantages of FSW – Good mechanical properties of the weld joint, – Avoidance of toxic fumes, warping, shielding issues, and other  problems associated with arc welding, – Little distortion or shrinkage – Good weld appearance• Disadvantages include  – An exit hole is produced when the tool is withdrawn from the work,  and  – Heavy‐duty clamping of the parts is required 57 Key benefits of friction stir welding Metallurgical benefits Environmental Energy benefits benefits1. Solid phase process 1. No shielding gas 1. Improved materials2. Low di t ti of work2 L distortion f k required use (e g joining (e.g., piece 2. No surface cleaning different thickness)3. Good dimensional required allows reduction in stability and 3. Eliminate grinding weight repeatability wastes 2. Only 2.5% of the4. No loss of alloying 4. Eliminate solvents energy needed for a elements required for laser weld5.5 Excellent metallurgical degreasing 3. Decreased fuel properties in the joint 5. Consumable consumption in light area materials saving, weight aircraft, such as rugs, wire automotive and ship6. Fine microstructure applications7. Absence of cracking or any other gases8. Replace multiple parts joined by fasteners 29
  30. 30. 10/17/2012 2.3 Ultrasonic welding (USW) 59 2.3 Ultrasonic welding (USW)• Two components are held together under  modest clamping force d t l i f• Oscillatory shear stresses of ultrasonic  frequency are applied to the interface to  cause coalescence• Oscillatory motion between the two parts Oscillatory motion between the two parts  breaks down any surface films  allows  intimate contact and strong metallurgical  bonding between the surfaces 60 30
  31. 31. 10/17/2012 2.3 Ultrasonic welding (USW)• Although heating of the contacting surfaces occurs due to  interfacial rubbing and plastic deformation, the resulting  temperatures are well below the melting point temperatures are well below the melting point• No filler metals, fluxes, or shielding gases are required in  USW.• The oscillatory motion is transmitted to the upper work  part by means of a sonotrode, which is coupled to an  ultrasonic transducer.  This device converts electrical power  ultrasonic transducer. This device converts electrical power into high‐frequency vibratory motion. Typical frequencies  used in USW are 15 to 75 kHz, with amplitudes of 0.018 to  0.13mm  61 2.3 Ultrasonic welding (USW)• Clamping pressures are well below those used in  cold welding and produce no significant plastic  ld ldi d d i ifi t l ti deformation between the surfaces.• Welding times under these conditions are less  than 1 sec.• USW operations are generally limited to lap joints USW operations are generally limited to lap joints  on soft materials such as aluminum and copper. 62 31
  32. 32. 10/17/2012 3. Diffusion welding (DFW)• Welding process results from the application of heat and  p pressure, usually in a controlled atmosphere, with  y p sufficient time allowed for diffusion and coalescence to  occur• Temperatures are well below the melting points of the  metals (about 0.5 Tm)• Plastic deformation at the surfaces is minimal• The primary mechanism of coalescence is solid state  diffusion, which involves migration of atoms across the  interface between contacting surfaces  63 3. Diffusion welding (DFW) 64 32
  33. 33. 10/17/2012 3. Diffusion welding (DFW)• Applications of DFW include the joining of high‐strength  and refractory metals in the aerospace and nuclear  industries.  industries• The process is used to join both similar and dissimilar  metals, and in the latter case a filler layer of a different  metal is often sandwiched between the two base metals  to promote diffusion.• The time for diffusion to occur between the faying The time for diffusion to occur between the faying  surfaces can be significant, requiring more than an hour in some applications• Key parameters of the process‐ temperature, time, and  pressure 65 3. Diffusion welding (DFW)• Diffusion occurs by an Arrhenius relationship, that is, exponentially  with temperature: • Where,  – D is diffusion coefficient (of the diffusing species) at temperature T,  – Do is a constant of proportionality (dependent on the particular diffusing species and  host),  – Q is the activation energy for diffusion to occur,  – k is Boltzmann’s constant, and  – T is the temperature on an absolute scale T is the temperature on an absolute scale• In general, diffusion welding begins to take place at a reasonable  rate when the temperature exceeds half the absolute melting point  of the base or host material(s), and, as a rule‐of‐thumb, the rate of  diffusion doubles every time the temperature is raised  approximately 30°C 66 33
  34. 34. 10/17/2012 3. Diffusion welding (DFW) • Time is important because diffusion takes time to  occur, since for atoms to jump from site to site takes  occur, since for atoms to jump from site to site takes time. Thus, the distance over which diffusion occurs  depends on time: • Where  – x is the diffusion distance x is the diffusion distance,  – D is the diffusion coefficient (as above),  – t is time, and C is a constant for the system. 67 3. Diffusion welding (DFW)‐ Features1. Metals as well as ceramics can be joined directly to form a  completely solid state weld2. Filler can be used to permit increased micro deformation to  ll b f provide more contact for bond formation and/or promote more  rapid diffusion by providing a faster diffusing species3. Dissimilar materials either by class or type, including metal‐to‐ ceramic joints, can be joined directly or with the aid of a  compatible filler or intermediate4. Large areas can be bonded or welded, provided uniform  intimate contact can be obtained and sustained5. No heat‐affected zone as such, since the entire assembly in  which the diffusion weld is being made is virtually always heated  to the same temperature. 68 34