Analysis of laser–melt pool–powder bed interaction during the selective laser melting of a stainless steel

Abstract

The laser powder bed fusion (LPBF) or powder-bed additive layer manufacturing process is now recognized as a high-potential manufacturing process for complex metallic parts. However, many technical issues are still to overcome for making LPBF a fully viable manufacturing process. This is the case of surface finish and the systematic occurrence of porosities, which require postmachining steps. Up till now, the porosity origin remains unclear but is expected to be related to the stability of the process. As a LPBF part is made by the accumulation of hundreds of meters of small weld beads, it also appears to be important to understand all the phenomena that occur during the laser-powder-melt pool (MP) interaction for each single track. For this reason, in the first part of our study, using an instrumented LPBF setup and a fast camera analysis (>10 000 image/s), single tracks were fabricated and analyzed in real time and postmortem. Spatters ejections and powder denudation phenomena were observed together with variations of melt pool dimensions and meltpool instabilities. In turn, the physical origin of this powder denudation and the dynamics of the MP were investigated and discussed.

Experimental analysis of spatter generation and melt-pool behavior during the powder bed laser beam melting process

“…In the light of our recent work (Gunenthiram et al, 2016) and the present paper, we can first make a distinction between balling and humping as major unstable SLM regimes, and second correlate SLM regimes with spattering effect.…”

“…for SLM, we also tried to correlate normalized bead penetrations (e/D) for both materials with a normalized enthalpy value ΔH/h s incorporating the thermo-physical properties of irradiated materials (diffusivity α). In a logarithmic scale, and considering recent reflectivity data obtained in Gunenthiram et al (2016), bead penetrations obtained on 316L and A4047 can be rather correctly represented on a single curve (Fig. 14).…”

“…Such a pressure can be roughly estimated using the Clapeyron's equation. The melt-pool surface deformation clearly reveals at least the initiation or the full formation of a key-hole regime which affects the hydrodynamic behavior of a SLM melt-pool already addressed by Gunenthiram et al (2016). In turn, comparisons can be made with the key-hole/melt-pool interactions in laser welding widely investigated by Fabbro (2010).…”

“…This results in a lower humping tendency because such Rayleigh-Plateau like instabilities, obtained in Gunenthiram et al (2016) for high velocity, high power conditions, are observed for length over width ratio > > π. This is confirmed on cross-sections where undercuts and humping are visible only above 1520 W (Fig.…”

“…In Gunenthiram et al (2016), instability regimes in SLM were also evidenced either in the form of balling or humping issues. The first one (Balling) has been widely reported to be due to a lack of melting of the substrate and resulting spheroïdisation effects whereas humping (periodic formation of humps and valleys) was attributed to large (length/ width) melt pool ratios generating Plateau -Rayleigh instabilities.…”