Influence of feedstock powder and cold spray processing parameters on microstructure and mechanical properties of Ti-6Al-4V cold spray depositions

A high pressure cold spray system was used to deposit three Ti-6Al-4V feedstock powders (i.e., hydride de hydride, plasma atomized, and gas atomized) on Ti-6Al-4V substrates while varying gas temperature and nozzle length. Particle impact temperature and particle velocity were calculated using a 1-D axial model. The microstructure of the feedstock powders and the cold spray depositions were characterized via optical and scanning electron microscopy. The hardness of the as-received powders was determined using nanoindentation. To assess deposition quality, coatings were characterized in terms of porosity, microhardness, and adhesion strength. Results showed that hydride de-hydride powders were characterized by an equiaxed alpha grain structure with intergranular beta phase regions while atomized powders were characterized by martensitic alpha phase structures. Cold sprayed coatings revealed two distinct microstructures. Regions that experienced low/moderate plastic strain retained the as-received powder microstructure while regions that experienced significant plastic strain were characterized either by a featureless microstructure (atomized coatings) or the presence of fine, elongated beta precipitates (hydride de-hydride coatings). Depositions performed using a long nozzle resulted in the best deposition quality, with porosity as low as 0.3% and adhesion strengths > 69 MPa. While atomized powders resulted in comparatively higher quality coatings for all process conditions, hydride de-hydride coatings of excellent quality (average porosity approximate to 0.6%, adhesion strength > 65 MPa) were achieved under optimal conditions. Thus, hydride de-hydride powders may hold promise as a cost effective alternative to atomized powders for Ti-6Al-4V cold spray depositions.