Thermal and microstructural analysis of laser-based directed energy deposition for Ti-6Al-4V and Inconel 625 deposits

Accurate temperature measurements based on careful experimentation and microstructural analysis were conducted for Ti-6Al-4V and Inconel 625 alloys deposited using the laser-based directed energy deposition process. In the case of the Ti-6Al-4V alloy, thermal measurements were made in the first layer during the first and four subsequent deposits to ascertain microstructural evolution during the heating and cooling cycles. Four energy densities were utilized during deposition of the Inconel 625 alloy to alter cooling rates and determine the impact of processing conditions on solidification morphology. The precise experimental measurements enabled a comprehensive analysis of the solid state reactions for Ti-6Al-4V, and the solidification phenomena to be elucidated for Inconel 625. The results for the Ti-6Al-4V alloy indicated that the measured thermal response could be used to anticipate initial microstructure based on cooling rates from the beta-transus, and subsequent thermal cycles could be utilized to define potential transformations between alpha, alpha', and beta. Analysis of the measured thermal cycles from the liquid through solidification for the Inconel 625 alloy showed that processing parameters could be linked to factors governing the solidification process and microstructural features. Using these relationships, an accurate processing map for laser-based directed energy deposition for Inconel 625 was constructed to enable the identification of solidification morphology and microstructural scale based on critical processing parameters.