Laser powder-bed fusion additive manufacturing: Physics of complex melt flow and formation mechanisms of pores, spatter, and denudation zones

Abstract

This study demonstrates the significant effect of the recoil pressure and Marangoni convection in laser powder bed fusion (L-PBF) of 316L stainless steel. A three-dimensional high fidelity powder-scale model reveals how the strong dynamical melt flow generates pore defects, material spattering (sparking), and denudation zones. The melt track is divided into three sections: a topological depression, a transition and a tail region, each being the location of specific physical effects. The inclusion of laser ray-tracing energy deposition in the powder-scale model improves over traditional volumetric energy deposition. It enables partial particle melting, which impacts pore defects in the denudation zone. Different pore formation mechanisms are observed at the edge of a scan track, at the melt pool bottom (during collapse of the pool depression), and at the end of the melt track (during laser power ramp down). Remedies to these undesirable pores are discussed. The results are validated against the experiments and the sensitivity to laser absorptivity is discussed.2

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

“…The experimental and numerical work detailed in Matthews et al (2016) focused on the denudation phenomena occurring near SLM beads and concluded that inward lateral gas flow induced by the melt-pool vaporization was the main dragging force for powder depletion. Khairallah et al (2016) also considered key-hole closure as a possible explanation for porosity formation, and confirmed that melt-pool vaporization was a key factor for the formation of spatters and denudation. Wang et al (2017) has considered three main types of spatter ejections (droplet spatters coming from the melt-pool surface instability, spatters included in the metallic jet and coming from the recoil pressure zone, and non-melted powder spatters at the front of the melt-pool).…”

“…Investigations on single beads have been previously carried out by many authors such as Yadroitsev et al (2010), King et al (2014), Khairallah et al (2016) and Matthews et al (2016), for investigating the physics of SLM. In the following work, single beads are considered to be representative of the first stage of a SLM process, near the platform, where the average temperature has not reached a high and stabilized level yet.…”

Laser powder-bed fusion of Inconel 718 to manufacture turbine blades

“…A function of heat and mass transfer may be found [2,13], based on the main processing parameters such as the size of the laser beam, the laser power, the scanning speed, the layer thickness, and the hatch spacing [14,15]. Therefore, extensive research has been conducted to draw a correlation between densification and microstructure of IN178 and IN625 AMmade parts.…”

“…Namely, laser powder-bed fusion (LPBF) is a solid freeform fabrication technique based on laser irradiation of metal powder [2]; in this frame, selective laser sintering (SLS) and melting (SLM) are even referred to [3,4]. Assemblies are turned into single parts; hence, streamlining of manufacturing and potential elimination of tooling are benefited [5]; moreover, customization is allowed and the mechanical strength of the bulk is achieved upon proper setup.…”

Modeling of Ti-W Solidification Microstructures Under Additive Manufacturing Conditions

“…[14] Large temperature gradients, capillary forces, and Marangoni convection play important roles in its evolution as well as in the evolution of the corresponding temperature fields. [15,16] For example, the role of Marangoni convection in welding has been studied extensively [17][18][19] and several FEM-based models of coupled fluid and heat transport in weld pools that include Marangoni and buoyancy forces have been developed. [20][21][22] Such models typically used multi-layer meshes and solved the incompressible Navier-Stokes equations for fluid flow in the melt pool.…”