RESISTANCE WELDING PDF
What is Resistance Welding? Resistance welding is one of many methods of fastening two or more pieces of metal together. Some of these methods are listed . GENERAL PRINCIPLES. Resistance welding is a thermo-electric process in which heat is generated at the in- terface of the parts to be joined by passing an. Lecture 11 & Resistance Welding. Resistance welding processes are pressure welding processes in which heavy current is passed for short time through the.
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Introduction. Electric resistance welding refers to a group of thermo-electric welding processes such as spot and seam welding. The weld is made by. RESISTANCE WELDING. Fundamentals and Applications. Hongyan Zhang. Jacek Senkara. Boca Raton London New York. A CRC title, part of the Taylor. INTRODUCTION. Resistance welding is a process used to join metallic parts with electric current. There are several forms of resistance welding, including spot.
Force is normally applied before, during and after the flow of current to avoid arcing between the surfaces and to forge the weld metal during post heating. The necessary pressure shall vary from 30 to 60 N mm -2 depending upon material to be welded and other welding conditions. For good quality welds these parameters may be properly selected which shall depend mainly on material of components, their thicknesses, type and size of electrodes.
Apart from proper setting of welding parameters, component should be properly cleaned so that surfaces to be welded are free from rust, dust, oil and grease. For this purpose components may be given pickling treatment i.
After that components may be dried through the jet of compressed air. If surfaces are rust free then pickling is not required but surface cleaning can be done through some solvent such as acetone to remove oil and grease. The current may be obtained from a single phase step down transformer supplying alternating current.
However, when high amperage is required then three phase rectifier may be used to obtain DC supply and to balance the load on three phase power lines. The material of electrode should have higher electrical and thermal conductivities with sufficient strength to sustain high pressure at elevated temperatures.
Commonly used electrode materials are pure copper and copper base alloys. Copper base alloys may consist of copper as base and alloying elements such as cadmium or silver or chromium or nickel or beryllium or cobalt or zirconium or tungsten.
A duty cycle is the percentage of one period in which a signal is
Pure tungsten or tungsten-silver or tungsten-copper or pure molybdenum may also be used as electrode material. To reduce wear, tear and deformation of electrodes, cooling through water circulation is required. Figure Fig Commonly used resistance welding processes are spot, seam and projection welding which produce lap joints except in case of production of welded tubes by seam welding where edges are in butting position.
In butt and flash welding, components are in butting position and butt joints are produced.
The welding cycle starts with the upper electrode moving and contacting the work pieces resting on lower electrode which is stationary.
The work pieces are held under pressure and only then heavy current is passed between the electrodes for preset time. The area of metals in contact shall be rapidly raised to welding temperature, due to the flow of current through the contacting surfaces of work pieces.
The pressure between electrodes, squeezes the hot metal together thus completing the weld. The weld nugget formed is allowed to cool under pressure and then pressure is released. This total cycle is known as resistance spot welding cycle and illustrated in Figure Spot welding electrodes of different shapes are used.
Resistance (spot) welding (fixed)
Domed electrodes are capable of withstanding heavier loads and severe heating without damage and are normally useful for welding of nonferrous metals. The radius of dome generally varies from mm. A flat tip electrode is used where minimum indentation or invisible welds are desired. Most of the industrial metal can be welded by spot welding, however, it is applicable only for limited thickness of components. Ease of mechanism, high speed of operation and dissimilar metal combination welding, has made is widely applicable and acceptable process.
It is widely being used in electronic, electrical, aircraft, automobile and home appliances industries. In seam welding overlapping sheets are gripped between two wheels or roller disc electrodes and current is passed to obtain either the continuous seam i. Welding current may be continuous or in pulses. The process of welding is illustrated in Figure Overlap welds are used for air or water tightness.
It is the method of welding which is completely mechanized and used for making petrol tanks for automobiles, seam welded tubes, drums and other components of domestic applications. Seam welding is relatively fast method of welding producing quality welds.
However, equipment is costly and maintenance is expensive. Further, the process is limited to components of thickness less than 3 mm.
Projections are little projected raised points which offer resistance during passage of current and thus generating heat at those points. Of the various Resistance-welding-tips one should remember that cooling water direction should be checked to make sure that the central cool water jet impinges on the internal back side of the electrode tip closest to the weld face to ensure maximum effective cooling. While troubleshooting for spot weld problems one should pay attention mostly to two areas.
One is the constant good condition of the electrode Resistance-welding-tips, that play a major ask in providing good welds. That is why severe maintenance schedules must be introduced and enforced.
The second is the good setup of adequate modern control devices, which may need updating and improvement, especially if weld results are not consistent. Portable spot welding machines may be required to perform spot welding on large and cumbersome objects that cannot be readily brought to a standing welding machine. They are usually held by a structure that bears the weight and eases manual manipulation.
While the machine carries the electrodes and the force application mechanisms to deploy the necessary compressive force, the required power source can be either separate or integral with the unit.
In the first case electrical cables are connected to fixed transformers. Because of large power losses due to the cables, a higher secondary voltage is needed than in a stationary machine of the same current output.
In the second case as recommended by Resistance-welding-tips the transformer is integral with the portable unit which is called a "transgun", with the advantage that there is no voltage drop on the secondary cable.
Although this configuration is used in manual units it can be easily mounted on robotic arms, especially after the adoption of Inverter technology since such transformers became much smaller and lighter. Resistance-welding-tips suggest that modern control devices based on feedback should be employed to permit supply of constant direct current to each weld, even in cases of primary alternating current AC line voltage fluctuations.
The other necessary functions are easily regulated with standard equipment.
Resistance-welding-tips recommend to use the latest technological developments available for feedback based process control. Force gauges are currently used for monitoring and controlling the applied force. Time control functions are now based on microprocessor digital technology. Obtaining consistently high quality welds is nowadays a much easier task than it was only a few decades ago.
Plated and coated steel sheets can be joined by resistance welding, following recommended Resistance-welding-tips. Welding procedures and schedules should preserve the coating presence and function, while providing welds of acceptable strength.
Machine settings remain quite similar to those used on bare steel of similar thickness, but adjustments are required. Compensation of coating type and thickness on contact resistance, effects on indentation, alloying of coating with electrode Resistance-welding-tips material, and coating pick-up should be cared for.
Coating thickness and uniformity is most important, for good weldability the thinnest the better. Resistance-welding-tips of coated steel call for higher welding current, greater electrode force and shorter welding time that that used for uncoated steel of same thickness. Good tip maintenance and effective electrode cooling are the best preventive actions.
Click on PWL to see it. Click on PWL to read it.The operation is performed on a press welding machine and components are put between water cooled copper platens under pressure.
Resistance Welding : Principle, Types, Application, Advantages and Disadvantages
Aluminium alloys can be spot welded, but their much higher thermal conductivity and electrical conductivity requires higher welding currents. For this purpose components may be given pickling treatment i.
Resistance Projection Welding Machine These projections can be generated by press working or machining on one part or by putting some external member between two parts. No consumables are required in projection welding.
To reduce wear, tear and deformation of electrodes, cooling through water circulation is required.