Modelling of beam energy absorbed locally in conduction mode laser metal fusion

Abstract

Fluid dynamics models for laser material processing with metal fusion in conduction mode generally assume a constant absorptivity. This parameter is known to govern the process. However, it used to be pre-set by extrapolating absorptance measurements made at different conditions or adjusted to reproduce experimental bead shapes. In this study a new approach is developed. It consists in predicting the absorptance as a function of local surface conditions, including the surface temperature. The proposed absorptance model is applied to the metal alloy Ti-6Al-4V. It is found that the absorptance of this alloy changes with surface temperature over a wide range of beam incidence angles. Thermo-fluid simulations with tracking of the free-surface deformation are performed for conduction mode beam welding test cases with a Yb fibre laser and different travel speeds. It is found that the absorptivity coefficient commonly used for this process clearly underestimates the absorptance and the melt pool geometry predicted for the process conditions of this study. The computational results are also compared against experimental results and good quantitative agreement of the melt pool depth, width, length, free surface contour geometry, and the curvature of the end depression left after re-solidification at the laser switch-off location is obtained. The results show that the absorptance field predicted depends on the melt pool development stage, on the spatial location within the beam spot, and on the process conditions.

CFD-Based Feasibility Study of Laser-Directed Energy Deposition With a Metal Wire for On-Orbit Manufacturing

“…A one-fluid approach is applied and the model is made of five partial differential equations governing mass, momentum, (thermal) energy, volume fraction of alloy, and electric potential. Presented in the same order as implemented, these equations are written as follows (Luo et al, 2019;Noori Rahim Abadi et al, 2021;Noori Rahim Abadi et al, 2022):…”

“…At the right hand side, the second term is the buoyancy force, in which β is the volume expansion coefficient of the alloy in liquid state, ρ m = ρ m (T m ) is the working material density at the melting temperature, and g is the gravitational acceleration. The last momentum source term includes surface tension forces that are the capillary and thermocapillary forces, as well as the vaporization (or recoil) pressure force applied on the free surface (Panwisawas et al, 2018;Noori Rahim Abadi et al, 2022). These forces depend on the surface tension coefficient, σ = σ(T) (see Table 1), the local unit vector normal to the free surface, n ∇α |∇α| , and the surface curvature, κ −∇ • n. The last coefficient of Eq.…”

“…Underlying assumptions and derivation steps are available in Noori Rahim Abadi et al (2022) and therefore not repeated here. The fraction of beam power deposited by the s-and p-polarization waves into the metal surface is given as follows (Noori Rahim Abadi et al, 2022): (Li et al, 2019;Boivineau et al, 2006;Mills, 2002;Fan, 2013;Rai et al, 2009;Wieting and Schriempf, 1976;Desai, 1987)…”

“…The refractive index, n ϵ ′, and the extinction coefficient, n ′′ ϵ , are defined as follows (Noori Rahim Abadi et al, 2022):…”

Laser light absorption of high-temperature metal surfaces

“…E.g. Abadi et al [ 1 ] developed a model predicting the absorptance based on surface conditions and temperature, finding that typical assumptions underestimate the absorption. Similarly, Ebrahimi et al [ 2 ] observed an increase in absorption values of 316L stainless steel at increasing temperatures below boiling temperature.…”