ROLE OF MICROSTRUCTURE ON MECHANICAL PROPERTIES OF 3-D PRINTED γ-TiAl

Abstract

Gas atomized γ-TiAl powder with nominal composition of Ti-48Al-2Cr-2Nb (at%) was used as a precursor material to produce solid blocks in a sophisticated central composite design to study the influence of beam processing parameters on density and microstructure by Electron Beam Melting (EBM). Fifty specimens were built in EBM with different melt theme settings. The parameters varied were surface temperature, beam current, beam focus offset, line offset and beam speed. Density was selected as the prime factor to further study the microstructure and internal flaws present in the samples. These findings were investigated and correlated to the process settings using the area energy input index. Important ones among the parameters are: beam current, line offset and beam speed. By jointly deciding the total amount of energy input for each layer, these three parameters played a critical role in internal flaw generation as well as the microstructure evolution. 8 samples were divided on the basis of the level of energy input supplied by the process and the effect of beam focus offset was studied in different cases. Fully dense samples above 3.91g/cm3 were fabricated successfully which had a higher area energy input supplied or were fabricated with a minimum line offset with considerable beam power. The distribution of process related porosity was correlated to the processing parameter used. It primarily affected the dimensions of melt pool esp. the higher scan speed samples had narrow melt pool width leading to improper interlayer melting. Many of the process parameters weren’t optimized and hence the process defect evolution was understood for each of these samples to suggest the steps for its proper optimization. XRD analysis showed the prominent presence of γ-TiAl and α2-Ti3Al phases with low intensity β/B2 phases as well. A detailed study of microstructural homogeneity was analyzed within different regions of as-fabricated EBM samples for different energy input samples. A Fe-Ti intermetallic region in the bottom region was present in all the samples and a full/near lamellar microstructure towards the top. At the core of samples a homogeneous microstructure exhibited a lamellar γ/α2 colony structure and very fine γ-TiAl equiaxed grains as well resulting in duplex type of microstructure. The volume fractions of it however were dependent on the process parameters and a uniquely layered microstructure was observed for most of the samples consisting of duplex-like region and a coarse equiaxed γ grains organized in bands. Mechanical behavior of the analyzed samples were estimated from the micro-hardness values of the samples by measuring the yield strength as one-third of the hardness value. Hardness at the core of the samples was very much dependent on the microstructure hence depending on the processing parameters. The hardness values were fairly consistent throughout the sample region suggesting a structural stability. Nano-indentation hardness in the dual region of a uniquely layered microstructure proved that the duplex region is the strength providing component and the coarse γ band the ductile component. Macrotexture measurements through X-Ray Diffraction (XRD) didn’t yield any strong texture for either of the prominent phases present in the samples. However, the micro-texture results did provide some insight into the type of solidification process and showed a remarkable difference in the distribution of axis of misorientation other than <111> for a low energy sample compared to the higher energy one.