Joining two or more elements to make a single part is termed as afabrication process.Various fabrication process are1. Mechanical joints- bolts, screws & rivets2. Adhesive bonding- synthetic glue3. Welding, Brazing & SolderingSelection depends on….1. Type of assemble-permanent, semi-permanent or temporary2. Material being joined3. Economy4. Service conditions
Advantages and disadvantages of welding compared to other types of assembly operations Advantages:(1) It provides a permanent joint,(2) Joint strength is as high as the strength of base metals,(3) It is most economical in terms of material usage, and(4) It is versatile in terms of where it can be accomplished. Disadvantages:1) It is usually performed manually, so labor cost is high and the skilled labor to perform it is sometimes scarce,2) Welding is inherently dangerous,3) A welded joint is difficult to disassemble, and4) Quality defects are sometimes difficult to detect.
Introduction - Welding● Welded Joints are permanent joints.● Welding joints are obtained by localized heating and are based on molecular attraction.● The properties of the welded components are closest to those of a solid member.● Members of unlimited size can be fabricated.● Strength of welded joins in terms of static and impact loads is equal to the base metal.● All Structural steels including high alloy grades, non ferrous alloys can be efficiently welded.
ARC WELDING Name AWS Characteristics ApplicationsCarbon arc welding CAW Carbon electrode, historical Copper, repair (limited)Flux cored arc FCAW Continuous consumable electrode filled with Industry, constructionwelding FCAW-S fluxGas metal arc Continuous consumable electrode and GMAW Industrywelding shielding gasGas tungsten arc Nonconsumable electrode, slow, high quality Aerospace,Construction( GTAWwelding welds piping),Tool and DiePlasma arc welding PAW Nonconsumable electrode, constricted arc Tubing, instrumentation Consumable electrode covered in flux, canShielded metal arc Construction, outdoors, SMAW weld any metal as long as they have the rightwelding maintenance electrodeSubmerged arc SAW Automatic, arc submerged in granular fluxwelding
Oxy fuel gas weldingName AWS Characteristics Applications Combustion of acetyleneOxy with oxygen produces high- Maintenance,acetylene OAW temperature flame, repairwelding inexpensive equipment
Resistance weldingName AWS Characteristics Applications Two pointed electrodes apply pressureResistance spot Automobile industry, RSW and current to two or more thinwelding Aerospace industry workpieces Two wheel-shaped electrodes roll alongResistance seam Aerospace industry, ERW workpieces, applying pressure andwelding steel drums, tubing currentProjection welding PWFlash welding FW Butt joint surfaces heated and broughtUpset welding RSEW together by force
Solid-state weldingName AWS Characteristics Applications Transition joints for chemical Joining of dissimilar materials, e.g. corrosion resistantExplosion welding EXW industry and shipbuilding. alloys to structural steels Bimetal pipelinesElectromagnetic pulse Tubes or sheets are accelerated by electromagnetic forces. Automotive industry, pressurewelding Oxides are expelled during impact vessels, dissimilar material joints The oldest welding process in the world. Oxides must beForge welding FOW Damascus steel removed by flux or flames. Thin heat affected zone, oxides disrupted by friction, needs Aerospace industry, railway, landFriction welding FRW sufficient pressure transport Shipbuilding, aerospace, railwayFriction stir welding FSW A rotating consumable tool is traversed along the joint line rolling stock, automotive industry Metals are pressed together at elevated temperaturesHot pressure welding HPW below the melting point in vacuum or an inert gas Aerospace components atmosphereHot isostatic pressure A hot inert gas applies the pressure inside a pressure HPW Aerospace componentswelding vessel, i.e. an autoclave Bimetallic materials are joined by forcing them betweenRoll welding ROW Dissimilar materials two rotating wheels High-frequency vibratory energy is applied to foils, thin Solar industry. Electronics. RearUltrasonic welding USW metal sheets or plastics. lights of cars.
Other welding ProcessesName AWS Characteristics ApplicationsElectron beam EBW Deep penetration, fast, high equipment costwelding Heavy plate Welds thick workpieces quickly, vertical fabrication,Electroslag ESW position, steel only, constructionwelding continuous consumable electrode. Construction, shipbuilding.Induction welding IWLaser beam Automotive LBW Deep penetration, fast, high equipment costwelding industry Combines LBW with GMAW in the same Automotive,Laser-hybrid welding head, able to bridge gaps up to Shipbuilding,welding 2mm (between plates), previously not Steelwork possible with LBW alone. industriesPercussion Following an electrical discharge, pressure is Components of PEWwelding applied which forges the materials together switch gear devices Exothermic reaction between alumniumThermite welding TW Railway tracks powder and iron oxide powder
Oxy Acetylene welding• OAW is a manual process in which the welder must personally control the torch movement and filler rod.• Cylinders contain oxygen and acetylene gas at extremely high pressure.• Acetylene is stored in the form of dissolved acetone under a pressure of 16 to 22 atm gauges.
Regulator Hoses • Hoses are fabricated from rubber • Oxygen hoses are green in color and have right hand thread. • Acetylene hoses are red in color with left hand thread. • Left hand threads can be identified by a grove in the body of the nut and it may have “ACET” stamped on it
Flame Settings• There are three distinct types of oxy-acetylene flames, usually termed: – Neutral – Carburizing (or “excess acetylene”) – Oxidizing (or “excess oxygen” )• The type of flame produced depends upon the ratio of oxygen to acetylene in the gas mixture which leaves the torch tip.
• The neutral flame is produced when the ratio of oxygen to acetylene, in the mixture leaving the torch, is almost exactly one- to-one. It’s termed ”neutral” because it will usually have no chemical effect on the metal being welded. It will not oxidize the weld metal; it will not cause an increase in the carbon content of the weld metal.• The excess acetylene flame, as its name implies, is created when the proportion of acetylene in the mixture is higher than that required to produce the neutral flame. Used on steel, it will cause an increase in the carbon content of the weld metal.• The oxidizing flame results from burning a mixture which contains more oxygen than required for a neutral flame. It will oxidize or ”burn” some of the metal being welded.
Type of flame Application1.Neutral flame Steel, Cast-Iron ,Copper, Aluminum2.Carburizing flame Low carbon steels, alloy steels, non ferrous metals3.Oxidizing flame Brass, Bronze
Filler rod• Used to supply additional metal to the weld zone during welding• Available as filler rods or wire and may be bare or coated with flux• Purpose of the flux is to retard oxidation of the surfaces of the parts being welded by generating a gaseous shield around the weld zone
Advantages of Gas welding1.Temp.of flame cane be controlled easily.2.The amount of filler metal deposits can be controlled easily.3.Flame can be used for welding and cutting.4.All types of metal can be welded.5.Cost of equipment is less.6.Portable equipment.7.Low maintenance cost. Limitations of Gas welding1.Not suitable for thick plates.2.Slow process.3.Handling and storing of gas cylinders needs more care.4.Strength of weld is not so good as arc welding.5.Gas flame takes up a longer time to heat up the metal than arc welding.
Gas cutting Ferrous metal is heated in to red hot condition and a jet of pure oxygen is projected onto the surface, which rapidly oxidizes Melt are blown away by the force of the jet, to make a cut Fast and efficient method of cutting steel to a high degree of accuracy Torch is different from welding Cutting torch has preheat orifice and one central orifice for oxygen jet
Flame CuttingFig. (a) Flame cutting of steel plate with an oxyacetylene torch, and across-section of the torch nozzle.(b) Cross-section of a flame-cut plate, showing drag lines.
Arc welding• Equipments:• A welding generator (D.C.) or Transformer (A.C.)• Two cables- one for work and one for electrode• Electrode holder• Electrode• Protective shield• Gloves• Wire brush• Chipping hammer• Goggles
Arc Shielding• At high temperatures in AW, metals are chemically reactive to oxygen, nitrogen, and hydrogen in air – Mechanical properties of joint can be degraded by these reactions – To protect operation, arc must be shielded from surrounding air in AW processes• Arc shielding is accomplished by: – Shielding gases, e.g., argon, helium, CO2 – Flux
FluxA substance that prevents formation of oxides and other contaminants in welding, or dissolves them and facilitates removalProvides protective atmosphere for weldingStabilizes arcReduces spattering
Power Source in Arc Welding DC arc welding is more expensive than AC welding. DC W is generally preferred because of the control of heat input offered by it. 70 % of heat is liberated near the anode ,30% cathode. If more heat is required at w/p ( thicker w/p, high thermal conductivity metals such as Al, Copper ) w/p can be connected to anode –Straight polarity or DCEN ( Direct current Electrode negative) It produces welds that are narrow and deep.
If less heat is required at w/p, (thinner w/p) w/p can be connected to negative. This is referred as reverse polarity, or DCEP(direct current Electrode positive ) The weld zone is shallower and widerDCEN DCEPFig. The effect of polarity on weld beads:(a) dc current straight polarity; (b) dc current reverse polarity; (c) accurrent.
Comparison of AC & DC welding machines AC machine ( Transformer) DC machine (Generator)1.Efficency is more (80 to 85 %) Efficiency is less (30 to 60 %)2.Power consumption is less Power consumption is more3.Cost of equipment is less Cost of equipment is more 4. Any terminal can be connected Polarity is significantto work or electrode5.Voltage is higher, not safe Voltage is low, safer operation6.Not suitable for welding Very much suitable for bothnonferrous metals ferrous & nonferrous metals7.Not preferred for welding thin Preferred for welding thinsections sections
Arc weldingAdvantages Limitations – Most efficient way to join • Manually applied, therefore metals high labor cost. – Lowest-cost joining • Need high energy causing method danger – Affords lighter weight • Not convenient for through better utilization disassembly. of materials • Defects are hard to detect at – Joins all commercial joints. metals – Provides design flexibility
Heat transfer in arc welding H VI H = heat input l = weld length• Heat input is l e v V = voltage applied I = current v = welding speed e = efficiency• Heat input to melt a certain volume of material is u = specific energy required for melting H uV m uAl Vm = volume of material melted A = cross section of the weld• Welding speed is VI v e uA
Consider the situation where a welding operation is beingperformed with V = 20volts, I = 200A and the cross-sectional areaof the weld bead is 30 mm2. Estimate the welding speed if the workpiece and electrode are made of (a) aluminium, (b) carbon steel,and (c) titanium. Use an efficiency of 75%.Solution VI 20 200 v e 0 . 75 34 . 5 mm/sa)For aluminium, uA 2 . 9 30b)For carbon steel, v 8 . 1 mm/sc)For titanium, v 7 . 0 mm/s
Consumable Electrode Non consumable AW Processes Electrode Processes• Shielded Metal Arc • Gas Tungsten Arc Welding Welding (TIG)• Gas Metal Arc • Plasma Arc Welding Welding(MIG) • Carbon Arc Welding• Flux-Cored Arc Welding • Stud Welding• Electrogas Welding• Submerged Arc Welding
Weld materials:Carbon steels, low alloy steels, stainless steels, most aluminum alloys, zinc based copper alloys
MIG or GMAW Consumable electrode is in the form of a wire reel which is fed at constant rate. Weld area is shielded by an external source of gas. Deoxidizers are present in the electrode to prevent oxidation. Process is rapid, versatile and economical. Shielding gas : Argon Nitrogen Helium
TIG (Tungsten Inert Gas) or Gas Tungsten Arc Welding ( GTAW) As the tungsten electrode is not consumed, a constant and stable arc gap is maintained at a constant current level GTAW process is used for applications with aluminium, magnesium, titanium and the refractory metals Cost of the inert gas is more expensive but provides high quality welds and surface finish
Gas Tungsten Arc WeldingAdvantages:• High quality welds for suitable applications• No spatter because no filler metal through arc• Little or no post-weld cleaning because no fluxDisadvantages:• Generally slower and more costly than consumable electrode AW processes
Shielded Metal Arc Weld (SMAW):Most popular welding technique (stick welding).The electrode coating performs the following: A – Produce gaseous shield to exclude oxygen. B – Introduce dioxider material to improve grain. C – Produce a blanket of slag to retard cooling and prevent oxidation.The SMAW process isdesignated by AWS as“E6OXX” or “E7OXX”e.g. (E 6013)
Submerged Arc Weld (SAW): In this process the automatically fed arc (spool) isprotected by a blanket of granular material called “flux”.This flux material acts to improve weld quality and to protect itfrom the air. Submerged arc welding (SAW)
Welded Joints Fig. Examples of welded joints and their terminology.
Resistance Welding• RW- heat required is produced by electrical resistance across the two components to be joined H = Heat• Heat generated is H I Rt 2 I = Current R = Resistance t = Time of current flow• By including a factor K, which denotes energy losses through conduction and radiation, we have H I RtK 2
The desirable properties of a metal thatwould provide good weld ability for resistancewelding are High resistivity, Low electrical conductivity Thermal conductivity and Low melting point.
Resistance Welding• Total resistance is the sum of:1. Resistances of the electrodes2. Electrode–workpiece contact resistance3. Resistances of the individual parts to be welded4. Contact resistance between the two workpieces to be joined (faying surfaces)• Temperature rise at the joint depends on the specific heat and the thermal conductivity of the metals to be joined
Resistance Spot Welding• Tips of 2 opposing solid, cylindrical electrodes touch a lap joint of two sheet metals, and resistance heating produces a spot weld• To obtain a strong bond in the weld nugget, pressure is applied until the current is turned off and the weld has solidified• Surface of the spot weld has a slightly discolored indentation• Current level depends on the materials thicknesses
Heat generated = I2RtHeat required for melting uV mVolume of nugget Vm = (3.14*D2 )/4 *dWhere D =dia of nugget d= thickness of nuggetHeat distribution through conduction and radiation is =Heat generated-heat required for melting.
Resistance Spot Welding• Simplest and most commonly used• May be performed by means of single or multiple pairs of electrodes• Required pressure is supplied through mechanical or pneumatic means• Variety of electrode shapes are used to spot- weld areas that are difficult to reach
An RSW operation is used to make a series of spot welds between twopieces of aluminum, each 2.0 mm thick. The unit melting energy foraluminum = 2.90 J/mm3. Welding current = 6,000 amps, and time duration= 0.15 sec. Resistance = 75 micro-ohms. The resulting weld nuggetmeasures 5.0 mm in diameter by 2.5 mm thick. How much of the totalenergy generated is used to form the weld nugget? How much of heat isdissipated into the surroundings?Solution: H = I2Rt = (6000)2(75 x 10-6)(0.15) = 405 W-sec = 405 JWeld nugget volume V = πD2d/4 = π(5)2(2.5)/4 = 49.1 mm3Heat required for melting = UmV = (2.9 J/mm3)(49.1 mm3) =142.4 JProportion of heat for welding =142.4/405 = 0.351 = 35.1%The remaining heat 405 J-142.4 J= 262.6 J is dissipated into themetal surrounding through conduction and radiation.
Resistance Spot WeldingTesting Spot Welds• Spot-welded joints may be tested by:1. Tension-shear2. Cross-tension3. Twist4. Peel
Advantages &Drawbacks of RWAdvantages:• No filler metal required• High production rates possible• Lends itself to mechanization and automation• Lower operator skill level than for arc welding• Good repeatability and reliabilityDisadvantages:• High initial equipment cost• Limited to lap joints for most RW processes
Resistance Seam Welding• Electrodes are replaced by rotating wheels or rollers• Using a continuous AC power supply to rollers• In roll spot welding, current is applied intermittently and a series of spot welds at specified intervals• In mash seam welding, overlapping welds are about one to two times the sheet thickness
High-frequency RW• High-frequency current (up to 450 kHz) is used• Used for production of butt-welded tubing or pipe• For high-frequency induction welding (HFIW), the roll-formed tube is subjected to high- frequency induction heating
Resistance Projection Welding• High electrical resistance is developed by embossing one or more projections on one of the surfaces to be welded• Used for resistance projection welding by modifying the electrodes
RW:Flash Welding• Heat is generated from the arc as the ends of the two members begin to make contact and develop an electrical resistance at the joint• Quality of the weld is good• Suitable for end-to-end or edge-to-edge joining of sheets of similar or dissimilar metals• Can be automated• Can be used in operating rolling mills and feeding of wire-drawing equipment