Understanding the Effect of Substrate Preheating Temperature and Track Spacing on Laser Assisted Cold Spraying of Ti6Al4V

In this study, laser-assisted cold spray (LACS) of titanium alloy Ti6Al4V onto Ti6Al4V substrates has been investigated in two phases: (i) single-track deposits on substrates preheated to 400 degrees C, 600 degrees C, and 800 degrees C, respectively, and (ii) single-layer (multi-track) deposits on substrates preheated to 600 degrees C with three different track spacings (1 mm, 2 mm, and 3 mm). Cross-sectional microstructures of the single-track deposits showed intimate contact at the interfaces, especially extensive interfacial mixing for specimens with substrate preheating at 600 degrees C and 800 degrees C. Cross-sectional area porosity content in single layer LACS coatings was found to be around 0.4%, which is significantly lower than the standard or conventional cold spray (CS) process having similar to 2.3% porosity. The microstructure reveals that the LACS process has improved the adhesion and cohesion of the deposits, in addition to the other advantages of the CS process. The average microhardness values of LACS deposits were found to be in the range of 388-403 HV (the highest hardness with the lowest track spacing), which is approximately 6-10% lower than that of the CS deposits without laser substrate preheating. Tensile residual stresses were found in all three LACS coatings, which was due to elevated process gas temperature along with high heat input during laser preheating of the substrate. It was observed that the higher the track spacing, the higher the stress magnitude, i.e., 31 MPa, 135 MPa, and 191 MPa in the longitudinal direction when deposited with 1 mm, 2 mm, and 3 mm track spacings, respectively. Heat treatments induced varied microstructures in LACS coatings, encompassing fully equiaxed or lamellar alpha-phase within the beta-phase, or a bimodal microstructure, with characteristics linked to track spacing variations. Key contributions of this study include enhanced coating-substrate adhesion through extensive interfacial mixing, a substantial reduction in cross-sectional area porosity compared to CS, insights into the effects of residual stresses, and, ultimately, advancing the comprehension of LACS and its potential advantages over conventional CS process.