DEFINITION OF RESISTANCE WELDING
- 1 DEFINITION OF RESISTANCE WELDING
- 2 “Resistance welding process is a fusion welding process where heat and pressure are applies on the joint but no filler metal or flux is added”. In resistance welding a low voltage and very high current is pass through the join for very short time. This current heats the joint due to the contact resistance at the joint and melts it.
- 3 HEAT PRODUCED IN RESISTANCE WELDING
- 4 IMPORTANT FACTOR OF RESISTANCE WELDING
- 5 Advantage of resistance welding
- 6 TYPES OF RESISTANCE WELDING
“Resistance welding process is a fusion welding process where heat and pressure are applies on the joint but no filler metal or flux is added”. In resistance welding a low voltage and very high current is pass through the join for very short time. This current heats the joint due to the contact resistance at the joint and melts it.
Resistance of the joint composed of :
- Resistance of electrodes.
- Contact resistance between two work piece plates.
- Contact resistance between electrode and work piece.
- Resistance of work piece plates.
HEAT PRODUCED IN RESISTANCE WELDING
H = I 2 RT
H is heat generate indicate in joules.
I is the current in root-mean-square amperes.
R is the resistance in ohms
T is time of the current flow in seconds.
IMPORTANT FACTOR OF RESISTANCE WELDING
- The formula also indicates that in simple resistance welding, a high ampere current is necessary for an adequate weld. Heating metal to the temperature necessary for welding, therefore, is indicate a direct application of joule law to resistance welding.
- joule law, applied to a law of conduction, is that a poor a greater extent than a good conductor with the same amount of amperes passing through it.
heating takes place on the surface of the two piece of metal to be spot weld because of the poor conduction between the two. the spot welding machine passes a high current at low voltage through the electrode to the spot where welding is to take place.
- The second factor in the welding process, the forging factor , is the next step after the metal heated to a plastic state. Pressure must asserted on the piece being joined in the order for the forging process to take place.
- spot welding and butt welding are two major methods of applying heat energy and mechanical force in resistance welding.
- The time of electricity supply should closely controlled so that heat released is just enough equal to requirement.
- The transfer of heat to different parts being joined together differently so that proper fusion obtained even when the parts are dissimilar from the stand point of material, thickness.
- when join two parts one is more thicker than other so for proper heat balance electrode with smaller contact area should provide at thinner piece and a thicker electrode at thicker piece.
- Similarly in case of piece have different resistance electrode of high resistivity is used at piece of lower resistivity or electrode with large contact area at higher electrical conductivity.
Advantage of resistance welding
- These are very economical.
- These are suitable for high production as they give high production rate.
- Heating of work piece is subjected to very small part which results in less distortion.
- Very little skill is require to operate the resistance welding machine.
- Work piece of dissimilar metal or thickness can welded easily.
- It can weld thin(0.1 mm) as well as thick (20 mm) metals.
- High welding speed.
Limitations of resistance welding
- Welding machine have very complex operating system such as heavy transformer, electrode,force applying mechanism all these make it very expansive unless it is used for large scale production.
- Resistance welding process are limited only to lap joint.
TYPES OF RESISTANCE WELDING
- Resistance spot welding
- Resistance seam welding
- Projection welding
- Upset welding
Resistance spot welding: It is simplest and most commonly use for making lap welds in thin sheets up to maximum thickness 12.7 mm .
- Current- 3000 Amp to 40000 Amp
- Weld time-Time of current flow through the work piece till they are heat to required temperature.
- Hold time-Time when pressure is maintain on the molten metal without electric current during this time piece are forge weld.
Resistance seam welding: It is modify from of spot, welding in it disc electrodes are used instead of cylindrical electrodes. They are continuously rotating that work pieces get advanced underneath them and pressure also apply through the electrodes in series of pulse.
- Using a continuous A.C power supply. also called roll spot welding . High frequency current used for butt welded tubing.
Projection welding: It is another form of spot weld where one of sheets provided with a number of projection to help localize the current at predetermined spot projection is of order of 8 mm and obtained from punching.
Upset welding: This process is use to from electric welded pipes from a metal plate of suitable thickness the plates is first formed in to the shape of pipe with help of three rolls. the ends of the plate of should forming but joint.