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Material Selection

Selection of materials is the closely related to product evaluation but also depends on process.  All the product evaluation parameters including strength, application temperature, corrosion, and electrical properties have a direct impact on material selection.  If the weld is expected to be carry a load the weldment has to strong enough.  If the weld is under compression, material selection is not so complicated but if the weld is under tension/torque/shear, then material selection is difficult since not only the parent metal has to be strong enough but also the heat affected zone (HAZ), and the weldment (fusion zone) itself has to be strong.  If any fatigue/vibration requirements are present, then the weldment also has to tough.  Toughness is often a difficult criteria since it not only requires the weld to ductile enough, it also requires the weld geometry to be such that any stress risers or sharp notches do not become a source for crack initiation and growth.  Even though the parent metal is tough, the weldment may be brittle due to the rapid heating/cooling and also potentially due to mixing of dissimilar metals in the weld.  For example, when fusion welding a carbon steel to stainless steel, the individual components may not be crack sensitive, but the weldment could be very brittle due to the presence of martensite.  Some high carbon steels may also have a brittle weldment due to rapid cooling of the weld zone.

Corrosion is another important issue which can be affected by welding.  Prior to welding, an alloy may be homogenous enough to be corrosion resistant but the weldment or the HAZ may not be.  A good example is 304 SS, which has enough chromium to provide corrosion resistance under normal circumstances.  However, during welding, chromium can end up being bonded to carbon and is no longer available for providing corrosion resistance; this is especially significant in the HAZ where the corrosion can start along grain boundaries. Such loss of corrosiong protection in stainless steels is called sensitization.  A good solution is to use 304L, a low carbon variant of 304, which has less 0.03 Carbon thus signficantly reducing the amount of Chromium that bonds with carbon.

Electrical conductivity can also be an important issue where conductivity is an important function.  A good example would be welding nickel to copper in battery application.  Even though nickel is a reasonably good conductor, an alloy of nickel and copper is very resistive.  In such situations, it would be better to go for a solid state bond between the two instead of a fusion bond.  A solid-state bond can be produced by resistance welding, ultrasonic welding, explosion welding, and friction welding.

As always, there are multitude of choices and proper selection of materials will go a long way in success of your welding operation.  Feel free to give us a call if you have any questions.