Double-pulse laser micro sintering of iron powder in multiple overlapping tracks: Experimental study and material characterizations

Abstract

Laser micro sintering (LMS) can be defined as a process producing micro features via laser-induced coalition of particles. In LMS, using short-pulse lasers and small particle sizes (a few μm or less) have potential advantages such as small theoretically achievable feature sizes. However, in this case it is often more challenging to obtain a highly densified material than conventional laser sintering. To help address this challenge, the corresponding author previously proposed a novel patented process named “double-pulse laser micro sintering” (DP-LMS), which typically utilizes “sintering laser pulses” to melt the powder and “pressing laser pulses” to induce in-situ transient high pressures on the powder surface to enhance densification. In this paper, DP-LMS of an iron powder layer with laser spot scanning multiple overlapping tracks has been studied, with sintered samples characterized via SEM, XRD, EDS and nanoindentation. The powder has a small original particle size of ~1.98 μm. Under the conditions studied, the surface porosity (surface pore area percentage) of material sintered by DP-LMS is ~3.4 %, much lower than ~19.0 %, the surface porosity of laser-sintered material without using DP-LMS. The material sintered with DP-LMS appears to have roughly equiaxed grains with typical sizes of ~1 to ~3 μm in the observed cross section region, and the small grain sizes should be related to the high cooling rate in re-solidification. The nanoindentation measurements show a high nanohardness of ~7.0 GPa for the material sintered with DP-LMS, which should be related to the small grain size and/or the existence of martensite. It has been found that to obtain a good DP-LMS result, an overlong melt pool lifetime induced by each laser pulse group should be avoided.