Investigating In-Situ Alloying During Blended Elemental (BE) and Master Alloy (MA) Sintering of Ti6Al4V

Abstract

Despite an increasing body of research, there exists a lack of understanding about the alloying behaviour of PM titanium alloys. Specifically, Ti6Al4V, which is the most common alloy, has not been investigated sufficiently to understand the behavior of alloying additions in the various forms that exist. The objective of this research was to investigate the role of alloying additions via in-situ analysis with differential scanning calorimetry (DSC). Mixtures of Ti6Al4V were prepared using master alloy (MA) and blended elemental (BE) additions, and analyzed during sintering profiles where the specimens were heated to 1200°C and held for various amounts of time. Of particular interest was the allotropic phase transformation that occurs during sintering, transforming from α-Ti to β-Ti on heating and then reversing on cooling. The reverse transformation was analyzed in detail using DSC in hopes of understanding how the nature of the alloying additions and the sintering profile affected various characterisitics of this transformation. Measurements of the onset, end, and peak temperature of the transformation were taken, along with the specific enthalpy and temperature span. A full compositional and microstructural analysis was performed on these DSC specimens as well in order to corroborate the findings of the DSC. Analysis of the binary BE mixtures of Ti6Al and Ti4V gave significant insight to the role of each alloying addition in the Ti matrix. Aluminum was found to reach a homogeneous state within a 1hr, but significant porosity formed as a result of highly dissimilar diffusion rates between Ti and Al, creating titanium aluminides. The binary Ti4V mixture required significantly more thermal exposure in order to reach homogeneity, but produced a denser product. The ternary BE Ti6Al4V mixture exhibited many of the characterisitics of the two binary systems. The measured enthalpy of transformation of the BE Ti6Al4V mixture was considerably lower than both MA mixtures, due to the porosity formed by the melting and spreading of the elemental Al additions. Results from the DSC suggested that the use of coarse and fine MA additions led to relatively homogeneous specimens after more that 2hrs at 1200°C. The smaller particle size of the fine MA led to faster homogenization than the coarse, however, in both cases the homogeneity of V in the matrix was the limiting factor. Unlike the BE mixutres, the addition of Al in MA particle did not result in the formation of porosity because it was introduced as an intermetallic. All DSC results were supported by both XRD and SEM/EDS analysis. Overall, a more complete understanding of the alloying behavior of PM Ti6Al4V has been developed along with a methodology for the use of DSC to analyze the sintering behavior in-situ.