Microstructure Characterization of Feedstock Ti-6Al-4V Powders and Their Evolution During Cold Spray Deposition and After Heat Treatment

Different Ti-6Al-4V feedstock powders (i.e., plasma atomized, gas atomized and hydride de-hydride) and corresponding cold spray depositions were characterized using electron back scatter diffraction and x ray diffraction. In addition, a low temperature heat treatment was applied to the coatings and characterized using the above techniques to understand the microstructural evolution after annealing. XRD did not show BCC phase in the three powders and coatings. The axial ratio for all the powders and coatings were similar to what has been reported for Ti-6Al-4V. Atomized powders and hydride de-hydride (HDH) powders were characterized by alpha' microstructure, and equiaxed alpha phase microstructure, respectively. Phase maps for HDH powders revealed beta phase microstructure distributed around equiaxed alpha grains. Microstructural evolution during cold spray deposition resulted in the atomized coatings retaining as-received powder microstructure at particle interiors and formation of nanograins near the interface. On the other hand, HDH coatings were characterized by slightly deformed as-received powder microstructure with no evidence for nanograin formation. Higher average grain size of HDH powders resulted in the higher percentage of low angle grain boundaries observed after cold spray deposition. Subsequent heat treatment resulted in recovery, some recrystallization and grain growth for all the coatings. Nonetheless, the extent to which this took place was higher for atomized cold sprayed coatings.