An experimental study on effects of temperature gradient on microstructure of a 308L stainless steel manufactured by directed energy deposition

The effect of spatial temperature gradient on the microstructural evolution of a 308L stainless steel during the directed energy deposition (DED) process was experimentally investigated. A novel cooling system was designed and incorporated to a DED system in order to control the temperature gradient along the deposition direction during solidification. During deposition, the workpiece was placed on a lifting platform, and as the deposition process proceeded, the platform and workpiece were gradually lowered into cooling water so that the temperature gradient along the deposition direction could be controlled and maintained stable during the deposition process. The microstructure characterization results indicated that a deposition strategy with higher G and G/R values (where G is temperature gradient and R is solidification rate) produced finer cellular grains that were better aligned with the deposition direction, while a deposition strategy with lower G and G/R values produced columnar grains with larger primary arm spacing and less aligned with the deposition direction.