Gradient microstructure in the bonding zone of explosively welded sheets

Abstract

Microstructure evolution in the bonding zones of explosively welded tantalum/stainless steel (304L) and tantalum/copper/stainless steel (304L) clads was examined by scanning electron microscopy (SEM) equipped with electron backscattered diffraction (EBSD) facility. To support the microstructural findings mechanical properties of the interfacial layers were evaluated using microhardness measurements. SEM/EBSD analyses showed that the interfacial layers undergo severe plastic deformation which produces fine equiaxed and/or thin flattened grains. Concurrently, other deformation mechanisms such as twinning, deformation banding and shear banding are developed. It has been established that severely deformed areas can easily undergo recovery and recrystallization already during clad processing. This leads to the formation of new stress-free grains near the interface. During these processes, the strain-hardened structure of flattened grains is replaced by small columnar or equiaxed ones free of dislocations. The microhardness of welded sheets increases significantly as the distance from the joining interface decreases, but aside from the volumes directly adhering to large melted zones, where a drop of microhardness is observed.