Site-specific microstructure, porosity and mechanical properties of LENS™ processed Ti–6Al–4V alloy

Abstract

In the present study, pre-alloyed Ti–6Al–4V powder is deposited on CP-Titanium substrate by laser engineered net shaping (LENS™) process using parameters optimized for best adhesion and densification. The optical montages from the three surfaces (front, side and top) show columnar β grains growing along the building direction due to conductive heat transfer through the substrate. The as-deposited microstructure contains thin lamellar (α+β)-colonies besides prior β grain boundaries and basket-weave (α+β)-structure inside prior β grains. Narrow band-like structure forms between consecutive layers due to re-melting of previously deposited layers, thereby creating additional interfaces in the microstructure. In addition, tiny isolated pores appears in negligible fraction throughout the LENSTM-processed specimen due to gas entrapment, shrinkage during cooling and un-melted or partially melted powder particles. Both the number and volume of the pores increase along the building direction. Hardness on different surfaces (front, side and top) differs considerably due to the presence of different heating/cooling zones, residual stresses and variations in the thermal cycles and consequent change in the α'-martensite phase fraction. Larger variation in the hardness between these surfaces is observed in nano-indentation technique signifying for inhomogeneity in nano-scale structure. These microstructural variations also resulted in measurable changes in the coefficient in friction (COF) during scratch testing from the substrate along the building direction due to presence of different heating/cooling zones (diffused vs. reheating/re-melting zones). The variation in hardness and COF along different directions can ultimately lessen the in-service performance of the as-deposited parts.