Dissimilar metal resistance spot welds, critical to the manufacture of medical devices, typically form brittle intermetallic compounds that are prone to failure. Here, a case study of biocompatible metals platinum and niobium using advanced analytical techniques is presented. It describes the variation of properties and microstructure using microresistance spot welding under four conditions, including a legacy process and processing conditions optimized by design of experiments. Adjustments to the electrode force, welding current, surface roughness, and pulse duration and exchanging the platinum anode contact for a cathode result in a joint with less porosity and greater uniformity in the thickness, chemistry, and microstructure of the fusion zone. The optimized microstructure contains fewer defects, with increased plasticity under deformation and a more uniform microstructure reducing the propensity for failure and variability between welds. Extensive analysis with optical, scanning electron, transmission electron microscopy coupled with nano- and micromechanical testing (such as micropillar compression) was used to characterize the weld zone.
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The authors would like to acknowledge the Med-tronic Restorative Therapies Group leadership for financial support of this research. D. Sorensen would like to thank Mr. Brandon Van Leer and Dr. Jan Ringnalda of Thermo Fisher Scientific for helpful sample preparation advice, Mr. Brian Leigh of Med-tronic for performing the ICP–OES analysis, Ms. Elizabeth Rentas of Medtronic for assistance in sample preparation, Dr. Wen Tan of Medtronic for helpful discussion related to process–properties relationships, and Ms. Margaret Flury of Medtronic for careful proofreading of this manuscript. We would also like to thank Dr. Ryan Wu formerly of the Mkhoyan Group at the University of Minnesota for invaluable assistance with the monochromated EELS experiments. Finally, WZ acknowledges the support from the Ohio State University Simulation Innovation and Modeling Center (SIMCenter) and Ms. Ying Lu for helpful discussion on finite element modeling. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program.
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