Thermal Behavior and Microstructural Evolution during Laser Deposition with Laser-Engineered Net Shaping: Part I. Numerical Calculations

Abstract

Laser-engineered net shaping (LENS) is a rapid direct manufacturing process. The LENS process can be analyzed as a sequence of discrete events, given that it is a layer-by-layer process. The thermal history associated with the LENS process involves numerous reheating cycles. In this article, the thermal behavior during laser deposition with LENS is simulated numerically by using the alternate-direction explicit (ADE) finite difference method (FDM). The simulation results showed that deposited material experiences a significant rapid quenching effect during the initial stages of deposition and can attain a very high cooling rate. With an increase in deposit thickness, the rapid quenching effect decreases and eventually disappears. The effects of the processing parameters on the thermal behavior of deposited materials were also simulated and analyzed. The objective of this study is to provide insight into the thermal history during the LENS process, where the ability to correlate process parameters to microstructural evolution is a motivating force.

Microstructures and Grain Refinement of Additive-Manufactured Ti-xW Alloys

“…These calculated cooling rates are in good agreement with the specific range of 10 3 to 10 4 K/s for rapid solidification in LENSä process reported by Zheng et al from experimental [39] and computational results. [40,41] The corresponding average minimum grain size was plotted against these three calculated cooling rates, and the results are shown in Figure 11(b). There is a very clear relationship between the cooling rate and the grain size, which is expected.…”

Processing and Behavior of Fe-Based Metallic Glass Components via Laser-Engineered Net Shaping

“…The magnitude of the cooling rate during the first-layer deposition can be estimated to be 10 3 to 10 4 K/s on the basis of numerical simulations. [32,33] Figure 3 shows a micrograph, imaged with SEM BSEs, of an initially deposited layer obtained with a laser output power of 280 W and a travel speed of 12.7 mm/s. The features of the melt pool shape and the overlap between subsequent deposited lines are clearly visible.…”

“…The presence of light-colored phases in the second layer suggests that melting and resolidification occurred in this region presumably under a low cooling rate, e.g., approximately 10 2 to 10 3 K/s. [33,35] In order to investigate any possible reactions of the deposited layer with the substrate, an identical powder was deposited on an amorphous substrate using the same parameters as were used for deposition on the 304 SS substrate. Similar to the case of a crystalline substrate, laser deposition on the amorphous substrate made of Fe-based Fe-Cr-Mo-W-C-Mn-Si-Zr-B thick amorphous coatings (400 lm) on a 304 SS plate via high-velocity oxygen fuel thermal spraying [36] also resulted in overlapping, amorphous melt zones.…”

“…An additional factor that contributes to the observed difference in microstructure between the shell and cubic samples is the fact that the time interval between two sequential layers is shorter for the shell sample than for the cubic sample. According to the experimental and numerical simulation results, [24,25,33] the temperature at the end of each deposition cycle increases with a decreasing interval time, which corresponds to a decrease in the cooling rate during deposition. In addition, the accumulation of thermal energy that is likely to develop during the deposition of multiple layers is also likely to promote crystallization of the microstructure.…”

Fabrication of Fe–Cr–Al Oxide Dispersion Strengthened PM2000 Alloy Using Selective Laser Melting

“…Columnar rather than equiaxed microstructures are often formed as the result of laser melting where high scan speeds and small laser spot sizes are used, resulting in rapid cooling rates. [21,22] Sharp changes in grain direction observed near the centre of the 50W, 0.3ms -1 wall (Figure 3b), were where grain growth inclined from the outer part of the wall appeared to change direction and align with the vertical columnar structure. Again, this change in grain morphology was believed to be the result of thermal gradients, these being higher at the centre of the wall, where most of the heat imparted by the laser was conducted towards the substrate.…”