On the Microstructural Evolution and Cracking Behavior of a Titanium Aluminide Alloy During Selective Laser Melting.

ABSTRACT

Selective laser melting of Ti-48Al-2Cr-2Nb usually ends up with serious cracking. The cracking mechanism, however, remains elusive. In this study, both bulk samples and samples containing only several layers were prepared and investigated. It is shown that a freshly built layer is dominated by single α2 phase. γ started to form from α2 during subsequent thermal cycling due to reheating effects and its volume fraction increased continuously with increased thermal cycles. The γ phase contains higher geometrically necessary dislocation (GND) density than α2. This could be due to its relatively lower hardness and higher thermal expansion coefficient, which made it easier to deform under stresses. With higher GND and thus probably higher distortion energy, the γ experienced more extensive recrystallization than α2 during reheating. Cracks are more liable to initiate from the interior of α2 or the γ/α2 interfaces, which could be due to incompatible deformation between the two phases.