Effect of bed temperature on solute segregation and mechanical properties in Ti-6Al-4V produced by selective laser melting

Abstract

Advanced characterisation techniques were used on LPBF Ti-6Al-4V samples produced on a heated base plate. When the substrate temperature is 100{\deg}C the elongation is 6\%, which increases and peaks at 10\% at 570{\deg}C, then sharply decreases to zero ductility at 770{\deg}C. At 100{\deg}C, a heavily strained and twinned microstructure, primarily composed of {\alpha}+{\alpha}', was observed and it was comparable to asbuilt microstructures obtained by conventional LPBF methods. At higher temperatures, twins are no longer present and instead nano-scale {\beta} precipitates are observed within {\alpha}' and {\alpha}, as well as dislocation networks (570{\deg}C) and tangles (770{\deg}C). Solute segregation at crystal defects was observed in all pre-heating conditions. Al and V segregation at microtwins was observed in the 100{\deg}C sample, reporting for the first time `selective' and mutually exclusive Al- and V-rich regions forming in adjacent twins. V segregation at dislocations was observed in the 570{\deg}C and 770{\deg}C samples, consistent with the higher preheating temperatures. High O contents were measured in all samples but with apparent opposing effects. At 100{\deg}C and 570{\deg}C was estimated to be below the critical threshold for O embrittlement and locally aids in maintaining a strength high by solid solution strengthening, whereas at 770{\deg}C it was above the threshold, therefore failing in a brittle fashion. Based on these observations, the initial increase in ductility from 100{\deg}C to 570{\deg}C is attributed to a reduction in microtwins and the dislocation networks acting as `soft barriers' for slip within a coarser microstructure. The lack of ductility at 770{\deg}C was attributed to local solute redistribution causing dislocation pinning and an increase of O content in this sample.