Effect of layer band and heterogeneity of microstructure on electrochemical dissolution of laser solid formed Ti-6Al-4V alloy

Laser solid forming (LSF) is a newly advanced additive manufacturing technique which offers a huge potential in efficiency and cost savings for fabricating the components of aerospace titanium alloys with high performance. In this research, the macrostructure/microstructure and electrochemical dissolution features of the Ti-6Al-4V alloy produced by LSF and forging were investigated. The LSFed Ti-6Al-4V that included as-deposited and annealing heat treatment (HT) conditions contains three periodically distributed microstructural morphologies: fine microstructure zone (FMZ), coarse microstructure zone (CMZ), and layer band. All zones of the different microstructural morphology in an entire deposit are constituents of α-laths and a small amount remained in the β phase. An alternating structure of coarse and fine microstructures was observed along the deposition direction in the deposit. Layer bands are generally spaced above the CMZ. The CMZ is a constituent of a slightly coarser basket weave structure than that of the FMZ. However, the layer bands present the microstructure of colony α. Additionally, the α-laths are slightly coarsened by HT, and this simultaneously results in a more homogeneous microstructure compared with the as-deposited microstructure. Moreover, the forged Ti-6Al-4V possesses the most homogeneous microstructure and the largest size of the α phase. Simultaneously, a variety of dissolved morphology features were observed in the three types of samples after potentiodynamic polarization tests. The order of the surface macrosmooth level of these samples is forged > HT > as-deposited. For LSFed Ti-6Al-4V, the FMZs and the layer bands present a faster electrochemical dissolution rate than that of the CMZs. Consequently, the uneven surface is revealed by the distinct dissolution rate of different microstructural zones at the same LSFed sample. The forged Ti-6Al-4V alloys present a uniformly dissolved behavior, which is attributed to the homogeneity of the microstructure. The elimination of the layer bands and the homogenization of the microstructure of the LSFed Ti-6Al-4V are of great significance for improving the surface quality during electrochemical machining.