Grain morphology evolution behavior of titanium alloy components during laser melting deposition additive manufacturing

Grain morphology control is a challenging issue for laser melting deposition (LMD) additive manufacturing of large metallic components. In this paper, the grain morphology evolution behaviors of laser deposited titanium alloy components were investigated via basic study on solidification nucleation and growth mechanisms of the local melt pool during the layer-by-layer deposition process. Results indicate that the heterogeneous nucleation on partially melted powders for equiaxed grains and the epitaxial growth from the pool-bottom for columnar grains are the two dominant solidification mechanisms. The competition between the above two solidification mechanisms within the melt pool dominates the grain morphological selection process and determines the as-deposited grain structures for the layer-by-layer deposited components. Low specific mass deposition rate leads to high superheating of the melt pool, large melting penetration to the underlying layer and high temperature gradient in front of solidification interface, making the bottom epitaxial growth mechanism prevail and promoting the formation of large full-columnar prior grain structures. While high specific mass deposition rate results in insufficient powder melting and low melt superheating, making superficial and endogenous heterogeneous nucleation within the melt pool prevail, favoring the production of fine near equiaxed prior grains. Other than the full-columnar and equiaxed as-deposited grain structures, a unique "steel-bar reinforced concrete-like" mixed grain structure consisting of coarse grain pillars and fine inter-pillar equiaxed grains was fabricated and the corresponding grain morphology selection mechanisms for the three representative as-deposited grain morphologies were established.