Solid-state phase transformation of TC11 titanium during unstable thermal cycling in laser melting deposition process

Thermal cycling procedure during laser additive manufacturing (LAM) process causes the appearance of bright and dark patterns on the etched surface of TC11 alloy components. The formation mechanisms of these patterns and the solid-state transformation related to LAM process are systematically investigated with the predication of temperature fields using the finite element software ABAQUS. The results indicate that by increasing subsequent thermal cycles, the peak temperatures for every cycle decrease. When peak temperatures are above T_β (phase transition temperature of β phase), which is 1010 °C in TC11 alloy, no pattern is observed. Meanwhile, a decrease in peak temperature leads to appearance of an ultra-fine basket-weave α + β microstructure (dark contrast) with gradually increased amount of α colonies in the alloy. A special bimodal microstructure with ‘fork-like’ α lamella appears in the layer when the peak temperatures of thermal cycles firstly fall into α + β dual-phase region. And this special bimodal microstructure gives a bright contrast and only appears at the region where the peak temperatures are below 970 °C, leaving the rest region with a dark contrast. With the continuous increase in thermal cycles in α + β dual-phase region, α lamella gradually coarsens. After five thermal cycles in α + β two-phase region, no further changes in microstructure are observed, and the morphologies of α lamella in dark and bright regions are almost the same but with different amounts of α phase.