Molten pool free surface solidification behavior in laser remelted near- titanium alloy

Free surface solidification (FSS) is a common physical phenomenon that has been found by various studies in metallurgy field while targeted studies are rare. This work investigated special structure and formation mechanism of surface grains generated by free surface solidification through laser remelting a near-beta titanium alloy Ti5Al-5Mo-5V-1Cr-1Fe (Ti-55,511). A thin layer crystal film covers molten pool surface through two-dimensional constrained solidification, which can be classified into flake-like crystal film located at molten pool edge and lotusleaf-like crystal film at molten pool center. Numerous cells grow from overflowing melt chill zone around molten pool edge and undergo two-dimensional constrained growth on surface, forming flake-like crystal films. TiNx particles formed from surface melt and N2 reaction can act as heterogeneous crystal nuclei, forming lotusleaf-like crystal films, whose secondary dendrite arm growth direction transforms from (100) of primary dendrite arm to (110). Equiaxed lotusleaf-like crystal films and columnar flake-like crystal films have a competitive growth relationship. Inward growth cells growing from two kinds crystal films form flake-like surface grains and central surface grains, respectively. This study can provide FSS theoretical guidance for laser additive manufacturing and surface modification.