Grain morphologies and microstructures of laser melting deposited V-5Cr-5Ti alloys

Abstract

A laser melting deposition (LMD) technique has been applied to fabricate thin wall V-5Cr-5Ti samples. By applying a scanning speed of 400 mm/min, laser powers of 1200W, 1400W, 1600W, and 1800W, and scanning strategies of single directional scanning and dual directional scanning, full dense thin wall samples have been successfully prepared. Microstructures at the bottom region of walls mainly consist of coarse columnar grains, showing a 〈100〉 fiber texture. Microstructures in the middle and at the top regions of walls mainly consist of fine columnar grains or equiaxed grains, showing random textures. Columnar dendritic growth takes place for columnar grains. The columnar dendrites grow epitaxially from parent grains, following one [100] axis closest to the heat flux direction. However, the integral growth directions of columnar grains are opposite to the heat flux direction. Including a small deviation of one [100] axis from heat flux direction, the growth space of columnar dendrites also counts for the success in competitive growth for columnar grains. Deposition height, laser power, and scanning strategy show significant effects on the grain structure evolutions of thin wall samples, due to their effects on the temperature gradient. As the deposition height increases, columnar to equiaxed transitions (CETs) have been observed for all samples. The CETs happen faster for higher laser power, and for dual directional scanning compared to single directional scanning. Due to the differences in local composition caused by microsegregation, clusters containing lath-like precipitates are observed to delineate the dendrites. The effects of processing parameters on microstructures can be represented by a metric of volumetric energy density. A decrease of energy density will lead to CETs.