Case Study 6: Finding hidden costs

A client had been manufacturing electric power devices for over 20 years, using the same set of materials and manufacturing processes during this time.  The device included a subassembly consisting of two metal components that were resistance welded as shown in the figure.  The top component was an electrical contact that contained a precious metal that added considerable cost to the component.  The challenge was to find a way to reduce the cost of the component without impacting the performance and reliability of the product.

Could the materials be engineered to optimize cost, performance, and reliability?

Our client viewed the subassembly as two materials.  But, actually it could be viewed as three  if the weld joint interface between the two pieces of metal was considered a material.  The interface was important because the electricity had to pass through the interface.  Furthermore, heat generated at the contact surface had to pass through the interface into the other piece of metal. 

Up to this point a mechanical shear test was used to evaluate the quality of the weld joint.  However, the actual interface between the two components was never studied directly.

An analysis of the interface using scanning acoustic microscopy and cross-section microscopy revealed that less than 50% of the interface between the two components was welded.  More than 50% of the interface was an air gap, which is not good for electrical or heat conduction. 

The welding process parameters were modified so that more of the interface was welded.  This resulted in an improvement in the performance of the subassembly during product testing.  This improvement enabled a 33% reduction in the size of the electrical contact, reducing its cost by 25%.