Revealing melt pool dynamics during laser temporal shaping directed energy deposition of 316L stainless steel

A three-dimensional thermofluidic coupling transport model of laser temporal shaping directed energy deposition (LTS-DED) for 316L stainless steel is established to investigate the influences of three laser temporal modes on heat transport and fluid flow, and the mechanisms of remelting and macro/microstructural formation of the melt track are elucidated. In three laser temporal modes, an inward vortex flow pattern is observed in the melt pool due to a positive thermocapillary coefficient induced by the oxygen from the oxidation-deoxidation reaction. In a given continuous wave (CW) mode, the melt pool only undergoes the constraint solidification process; and the evolution processes of the heat transport, fluid flow, and pool morphology are stable. The as-deposited sample exhibits the coarsest microstructure dominated by columnar grains. In the given two pulsed wave (PW) modes, the melt pools alternately undergo constraint solidification and free solidification processes, as-deposited samples are predominated by free solidification zones. The periodic remelting phenomenon leads to the periodic fish-scale structures and discrete bands on the surface and interior of the melt track, respectively. The discrete bands are determined by the maximum contour of the regions scanned by the solidification interface during single-pulse remelting. The high-frequency PW mode produces relatively smooth fish-scale structures and low-angle discrete bands. Additionally, the high-frequency PW mode inhibits the constraint solidification zone and CET behavior and results in cross-band columnar grains at the discrete bands. However, in the low-frequency PW mode, constraint solidification zones dominated by coarse grains appear near the discrete zone.

针对316L不锈钢，构建了激光时域整形定向能量沉积（LTS-DED）的三维热流耦合输运模型，以探究三种激光时域模式对热传输与流体流动的影响，并阐明熔道重熔及宏/微观组织形成的机制。在三种激光时域模式中，由于氧化还原反应产生的氧气引发正热毛细系数，熔池内呈现向内的涡流流型。在指定的连续波（CW）模式下，熔池仅经历约束凝固过程，热传输、流体流动及熔池形貌的演化过程均较为稳定，沉积态试样的微观组织最为粗大，以柱状晶为主导。在指定的两种脉冲波（PW）模式中，熔池交替经历约束凝固与自由凝固过程，沉积态试样以自由凝固区为主。周期性重熔现象分别在熔道表面与内部形成周期性鱼鳞状结构与离散带。离散带由单脉冲重熔过程中凝固界面扫描区域的最大轮廓所决定。高频PW模式可产生相对平整的鱼鳞状结构与低角度离散带。此外，高频PW模式可抑制约束凝固区与柱状晶-等轴晶转变（CET）行为，并在离散带处形成穿带柱状晶。而在低频PW模式中，离散带附近会出现以粗大晶粒为主的约束凝固区。