Fundamental Understanding of Bond Formation During Solid State Welding of Dissimilar Metals

Dissimilar metal welds are used in a wide range of applications to effect light weighting and for corrosion resistance. While fusion welding techniques are limited in their ability to fabricate dissimilar metal welds, solid state welding techniques are limited in their ability to fabricate complex geometries with dissimilar metal combinations. Hence alternative techniques need to be explored to fabricate complex geometries with dissimilar metals welds in the solid state. Ultrasonic additive manufacturing in a solid state additive manufacturing process that combines ultrasonic welding with mechanized tape layering to fabricate dissimilar metal welds in the solid state. Though extensive feasibility studies have been performed to fabricate dissimilar metal welds using ultrasonic additive manufacturing, the fundamental mechanisms related to the bond formation mechanism are not fully understood. In this work multi scale characterization using scanning electron microscopy, electron backscatter diffraction, nano indentation and atom probe tomography was performed to rationalize the mechanism of bond formation in dissimilar metal welds. The fundamental questions that needed to be answered were

1. Is possible for a solid state bond to form with extensive plastic deformation occurring only on one metal in a dissimilar metal combination
2. The effect of plastic deformation on the oxide layer at the interface of dissimilar metal welds.

To answer the above questions various dissimilar metal combinations (Steel-Ta) BCC-BCC, (Al-Ti) FCC-HCP, (Al-Steel) FCC-BCC were fabricated using ultrasonic additive manufacturing and characterized using the above-mentioned techniques. Bonded regions were characterized to study the role of plastic deformation by analyzing the micro texture developed at the interface. The general conclusion is the presence of a strong shear texture in the softer metal while the harder metal did not show any evidence of change in texture. To understand the effect of plastic deformation on the oxide dispersion atom probe tomography analysis was performed and the results indicate the possibility of oxide breakdown resulting in oxygen super saturation in the lattice. The bond formation is hypothesized to occur as a result of plastic deformation localized in the softer metal alone.