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Delhi

TIG , PAW & Resistance Welding

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Published in: Mechanical
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Welding techniques and their applications 

Sandeep K / Kolkata

3 years of teaching experience

Qualification: M.Tech. (Production Engineering)

Teaches: Chemistry, English, Hindi, Physics, Drawing, Mechanical

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  1. RESISTANCE WELDING AND TYPES RESISTANCE WELDING Resistance Welding is a welding process, in which work pieces are welded due to a combination of a pressure applied to them and a localized heat generated by a high electric current flowing through the contact area of the weld. Resistance Welding Processes and Equipments - Resistance welding is a group of welding processes wherein coalescence is produced by the heat obtained from resistance of the work to the flow of electric current in a circuit of which the work is a part and by the applications of pressure. No filler metal is needed. Heat produced by the current is sufficient for local melting of the work piece at the contact point and formation of small weld pool ("nugget"). The molten metal is then solidifies under a pressure and joins the pieces. Time of the process and values of the pressure and flowing current, required for formation of reliable joint, are determined by dimensions of the electrodes and the work piece metal type. AC electric current (up to 100 000 A) is supplied through copper electrodes connected to the secondary coil of a welding transformer. The following metals may be welded by Resistance Welding: Low carbon steels - the widest application of Resistance Welding Aluminum alloys Medium carbon steels, high carbon steels and Alloy steels (may be welded, but the weld is brittle) Advantages of Resistance Welding - (i) Fast rate of production. (ii) No filler rod is needed. (iii) Semi automatic equipments. (iv) Less skilled workers can do the job. (v) Both similar and dissimilar metals can be welded. (vi) High reliability and reproducibility are obtained. (vii) More general elimination of warping or distortion of parts. Disadvantages of Resistance Welding - (i) The initial cost of equipment is high. (ii) Skilled persons are needed for the maintenance of equipment and its controls. (iii) In some materials, special surface preparation is required. (iv) Bigger job thicknesses cannot be welded. Applications of Resistance Welding -Resistance welding is used for (i) Joining sheets, bars, rods and tubes. (ii) Making tubes and metal furniture. (iii) Welding aircraft and automobile parts. (iv) Making cutting tools. (v) Making fuel tanks of cars, tractors etc. ER. SANDEEP KUMAR (DEPARTMENT OF MECHANICAL ENGINEERING) Page 1
  2. RESISTANCE WELDING AND TYPES (vi) Making wire fabric, grids, grills, mash weld, containers etc. The most popular methods of Resistance Welding are: spot Welding (RSW) Seam Welding (RSEW) Flash Welding (FW) Resistance Butt Welding (UW.) spot Welding (RSW) Spot welding came into use in the period 1900-1905. It is now the most widely used of resistance welding processes. Spot welding is employed for joining sheet to sheet, sheets to rolled sections or extrusions, wire to wire, etc. Spot welding is used for joining relatively light gauge parts (up to about 3 mm thick) superimposed on one another (as a lap joint). Definition Spot welding is a resistance welding process in which overlapping sheets are joined by local fusion at one or more spots by the heat generated, by resistance to the flow of electric current through workpieces that are held together under force by two electrodes, one above and the other below the two overlapping sheets. Spot Weldable Metals - (i) Low carbon steel (mild steel). Resistance Spot (RSst.V) FOree Ton g Electrode AC Supply E I ectrod e Fore e subs tech. (ii) Hardenable steels, which, after getting spot welded are treated in an annealing furnace. (iii) High speed steel bits are spot welded to tool shanks for use in lathes, shapers, etc. The tool, after A. Unequal thickness; Band C Multiple thickness getting welded, is annealed before final hardening. (iv) Stainless steels Ferritic stainless steels behave very much as mild steel, however, the pressure should be kept a little longer after welding. Martensitic (cutlery and similar qualities) stainless steel can be treated as hardenable steel as it has pronounced air hardening qualities. Non-ferrous Metals (i)Aluminium ER. SANDEEP KUMAR (DEPARTMENT OF MECHANICAL ENGINEERING) Page 2
  3. RESISTANCE WELDING AND TYPES (ii) Aluminium Magnesium Alloys (iii) Aluminium Manganese Alloys (i), (ii) and (iii) may be spot welded satisfactorily. Oxide film on them is removed and a high capacity machine is used as aluminium is a good conductor. (iv) Copper. For welding copper upto 1.5 mm thick, hardfaced or pure tungsten (welding) electrodes are necessary. For bigger thicknesses spot welding is not preferred. Copper and aluminium and their alloys being very good conductors of heat and electricity are difficult metals from the resistance welding stand point as compared to mild steel. (v) Nickel, Nickel alloys and Monel Metal require machine capacity and settings rather similar to those employed for spot welding stainless steels. Electrode Materials - Considering the above-mentioned requirements, the following materials are used for manufacturing spot (resistance) welding electrodes. (a) Group A 1. Copper 99%, Cadmium 1% alloy It has high strength and hardness coupled with high electrical and thermal conductivities. It is non-heat-treatable and is, therefore, hardened and strengthened by cold working. It is recommended for spot welding: (i) Low-carbon steel coated with tin, terne metal, chromium or zinc, (ii) Scaly hot-rolled low-carbon steel, (iii) Aluminium and Magnesium alloys. 2. Copper 99.2%, Chromium 0.8% alloy It has high mechanical properties but lower thermal and electrical conductivities than Cu-Cd alloy. Optimum properties are developed by heat-treatment or by a combination of heat- treatment and cold work. It is used for spot welding. (i) Cold rolled low-carbon steels (ii) Hot rolled pickled low-carbon steels (iii) Nickel plated steel (iv)Stainless steel (v) Nickel alloys (vi)Copper-base alloys such as silicon bronze and nickel silver. 3. Beryllium 0.5% , Nickel 1 % Cobalt 1 % , and rest is copper It is hardenable alloy with higher mechanical properties, but lower electrical and thermal conductivities than Cu-Cd or Cu-Cr alloys. It is preferred where pressures and workpiece resistance are high. It is used for spot welding (i) Thick sections of low-carbon steel (ii) Stainless steel (iii) Monel and Inconel. (b) Group-B 4. Refractory-Metal Compositions. These materials are employed where high heat, long weld time, inadequate cooling or high ER. SANDEEP KUMAR (DEPARTMENT OF MECHANICAL ENGINEERING) Page 3
  4. RESISTANCE WELDING AND TYPES pressure would cause rapid deterioration of the copper-base alloys discussed above. A typical refractory-metal composition is given below: 42% Cu, 58% W (by volume): This is used for spot welding of stainless steel. When a copper alloy is being spot welded to steel, a group B electrode is used to contact the copper alloys and a group A electrode of type (1) or (2) is used to contact the steel. 5. Special alloy electrodes They are made up of copper-zirconium and copper-cadmium zirconium. They find applications similar to alloy (1) but where resistance to softening of the electrode face is a must. Resistance-Seam Welding (RSEW) Seam Welding is a Resistance Welding (R W) process of continuous joining of overlapping sheets by passing them between two rotating electrode wheels. Heat generated by the electric current flowing through the contact area and pressure provided by the wheels are sufficient to produce a leak-tight weld. Resistance seam welding (RSEW) is a resistance welding which process produces coalescence at the faying surfaces the heat obtained from resistance to electric current through the work parts held together under by electrodes. pressure The resulting weld is a series of overlapping resistance spot welds made progressively along a joint rotating the electrodes. When the spots are not overlapped enough to produce gaslight welds it is a roll variation known as resistance spot welding. This Seam Welding (RSEW) Sliding contact AC Power Supply Upper electrode wheel Welded metal sheets weld Lower electrode wheel www. su bstech. coin process differs from spot welding since the electrodes are wheels. Both the upper and lower electrode wheels are powered. Pressure is applied in the same manner as a press type welder. The wheels can be either in line with the throat of the machine or transverse. If they are in line it is normally called a longitudinal seam welding machine. Welding current is transferred through the bearing of the roller electrode wheels. Water cooling is not provided internally and therefore the weld area is flooded with cooling water to keep the electrode wheels cool. In seam welding a rather complex control system is required. This involves the travel speed as well as the sequence of current flow to provide for overlapping welds. The welding speed, the ER. SANDEEP KUMAR (DEPARTMENT OF MECHANICAL ENGINEERING) Page 4
  5. RESISTANCE WELDING AND TYPES spots per inch, and the timing schedule are dependent on each other. Welding schedules provide the pressure, the current, the speed, and the size of the electrode wheels. This process is quite common for making flange welds, for making watertight joints for tanks, etc. Another variation is the so-called mash seam welding where the lap is fairly narrow and the electrode wheel is at least twice as wide as used for standard seam welding. The pressure is increased to approximately 300 times normal pressure. The final weld mash seam thickness is only 25% greater than the original single sheet. Seam Welding is high speed and clean process, which is used when continuous tight weld is required (fuel tanks, drums, domestic radiators). Flash Welding (FW) Flash Welding is a Resistance Welding (R W) process, in which ends of rods (tubes, sheets) are heated and fused by an arc struck between them and then forged (brought into a contact under a pressure) producing a weld. The welded parts are held in electrode clamps, one of which is stationary and the second is movable. Flash Welding method permitts fast (about 1 min.) joining of large and complex parts. Welded part are often annealed for improvement of Toughnesstoughness of the weld. Steels, Aluminum alloys, Copper alloys, Magnesium alloys, Copper alloys and Nickel alloys may be welded by Flash Welding. Flash &Ve'ding N'lovable c 1 a nnp Stationary c la Inp Force Arc Supply »,relded parts s ubstec h - Thick pipes, ends of band saws, frames, aircraft landing gears are produced by Flash Welding. ER. SANDEEP KUMAR (DEPARTMENT OF MECHANICAL ENGINEERING) Page 5
  6. RESISTANCE WELDING AND TYPES Resistance Butt Welding (U W) Resistance Butt Welding is a Resistance Welding (R W) process, in which ends of wires or rods are held under a pressure and heated by an electric current passing through the contact area and producing a weld. The process is similar to Flash Welding, however in Butt Welding pressure and electric current are applied simultaneously in contrast to Flash Welding where electric current is followed by forging pressure application. Butt welding is used for welding small parts. The process is highly productive and clean. In contrast to Flash Welding, Force Force Butt Welding (Us«V) Stat i ort ary e I arnp M ovable W'elded parts AC Power S u p ply Weld Butt Welding provides joining with no loss of the welded materials. ER. SANDEEP KUMAR (DEPARTMENT OF MECHANICAL ENGINEERING) . su b st ech corn Page 6

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