12 May 2017

Resistance welding

Introduction :

All welding process are fusion welding process. The resistance welding process is also a fusion welding process where both heat and pressure are applied on the joint but no filler metal or flux is added. The heat necessary for the melting of the joint is obtained by the heating effect of the electrical resistance of the joint and hence, the name is resistance welding.

How it works?

In this process, a low voltage and very high current are passed through the joint for a very short time. This high heats the joint, due to the contact resistance at the joint and melts it. The pressure on the joint continuously maintained and the metal fuses together under this pressure. The heat generated in resistance welding can be expressed as 

H = k I2 R t
H = The total heat generated in the work ( J )
I = electric current ( A )
t = time for which the electric current is passing through the joint ( s )
R = Resistance of the joint ( ohms ) 
k = A constant to account for the heat losses from the welded joint 

In this process, the amount of heat released is directly proportional to the resistance. It is likely to be released at all of the mentioned points below :
  • The resistance of the electrodes
  • Contact resistance between the electrode and workpiece
  • Contact resistance between the two workpiece plates
  • The resistance of the workpiece plates
All of above a large amount of heat is to be generated to have an effective fusion is at the interface between the two workpiece plates. 
Because of the squaring in the above equation, the current I needs to be precisely controlled for any proper joint.

Resistance welding

At first, apply force and current through the electrodes contacted with metal parts to be welded and resistance heat is generated at the interface of metal parts and hence the metal parts melt and form the joint. Through a large current flows, there is no danger of an electric shock because the only low voltage is impressed.

Electrodes for resistance welding :

The electrodes in resistance welding carry the very high current required for fusion, as also transmit the mechanical force to keep the plates under pressure and in alignment during fusion. They also help to remove the heat from the weld zone thus preventing overheating and surface fusion of the work. For both of the purpose in this process, the electrode should have higher electrical conductivity as well as high hardness. 
Copper is alloyed form is generally used for making electrodes. Though pure copper has high electrical and thermal conductivity, it is poor in mechanical properties.

Copper-cadmium alloys have the highest electrical conductivity with moderate strengths and therefore are used for welding non-ferrous materials such as aluminium and magnesium alloys.

Copper-chromium alloys have slightly lower electrical conductivities than above but better mechanical strength. These are used for resistance welding of low-strength steels such as mild steels and low alloy steels. 
When cobalt and beryllium are added to copper, its conductivity is decreased to a great extent but the strength is increased. Hence, these are used for welding higher heat-resisting alloys such as stainless steels and steels with tungsten and other such alloying elements.  

Advantages of resistance welding :
  • High-speed welding.
  • Economical welding process.
  • Easily automated.
  • Easy operation so that suitable for high rate of production.
  • No flux require such as solder is necessary.
  • Electric facility is required in some cases due to use of large current.
  • Possible to weld dissimilar metals as well as metal plates of different thickness.
  • Very little skill is required to operate resistance welding machine.
Disadvantages of resistance welding :
  • Visual inspection is difficult because welded portion can't be checked from the outside.
  • Initial equipment cost is high.
  • Lower tensile and fatigue strength of welded joint that formed by using this process.
  • This welding process are limited only to lap joints.
The various resistance welding processes of interest are mentioned below :
  • Resistance spot welding 
  • Resistance seam welding 
  • Projection welding 
  • Upset welding 
  • Flash welding