Discrete element simulation of powder layer thickness in laser additive manufacturing

Abstract

The optimisation of the laser additive manufacturing (AM) process is a challenging task when a new material is considered. Compared to the selection of other process parameters such as laser power, scanning speed and hatch spacing, optimisation of powder layer thickness is much more time-consuming and costly because a new run is normally needed when the layer thickness value is changed. In practice, the layer thickness is fixed to a value that is slightly higher than the average particle size. This paper introduces a systematic approach to layer thickness optimisation based on a theoretical model of the interactions between the particles, the wiper and the build plate during the powder deposition. The focus is on a systematic theoretical and experimental investigation of the effect of powder layer thickness on various powder bed characteristics during single-layer and multi-layer powder deposition. The theoretical model was tested experimentally using Hastelloy X (HX) with an average particle size of 34.4 µm. The experimental results validated the simulation model, which predicted a uniform powder bed deposition when employing a 40 µm layer thickness value. Lower (30 µm) and higher (50 µm) layer thickness values resulted in large voids and short-feed defects, respectively. The subsequent optimisation of the scanning speed and hatch spacing parameters was executed using a 40 µm layer thickness. The optimum process parameters were then used to examine the microstructure and tensile performance of the as-fabricated HX. This study provides an improved understanding of the powder deposition process and offers insights into the selection of suitable powder layer thicknesses in laser AM.

Effect of process parameters on the microstructure and mechanical properties of AA2024 fabricated using selective laser melting

“…The hatch spacing values were chosen over a range that creates both laser spot overlapping (40 and 60 μm) and creates a gap between laser spots (80 and 100 μm), owing to the laser spot size (75 μm), in order to see the effect of the distance between spots on the microstructure of the as-fabricated specimens. The layer thickness was kept constant across both experiments due to the fact that using thinner layers can create voids on the deposited layer and can cause a high percentage of the powder to be pushed out of the building zone; however, using thicker layers may create short feeds, which can dramatically decrease the quality of the built part [20]. Similarly, the scanning strategy was kept constant across both experiments due to the small dimensions of the specimens.…”

Additive manufacturing of high-strength crack-free Ni-based Hastelloy X superalloy

“…Nickel-based superalloys such as Hastelloy X (HX) exhibit an extraordinary combination of oxidation resistance, formability, and mechanical properties in the temperature range of 540-1000°C. For these reasons, such materials are widely applied in gas turbine engine components [8][9] [10].…”

Cohesive Powder Flow: Trends and Challenges in Characterisation and Analysis

“…Across the community, every possible characteristic of bulk mechanical properties is being analysed in an attempt to characterise features of powder properties which give uniform spreading (e.g. Nguyen et al, 2017;Carrozza, 2017;Han et al, 2019). However, the recent work of Nan et al (2018) shows that uniform spreading is affected by transient jamming and arching.…”

“…Haeri (2017) identified the optimum blade tip shape to produce a spread particle layer with volume fraction and surface roughness comparable to a roller at the actual operation conditions. Geer et al (2018) and Han et al (2019) measured the repose angle and pile bulk density of metal powders, and then used DEM simulations to calibrate and characterise the sliding and rolling friction coefficient and surface energy of the powder to be used in their simulations of the spreading process. Desai et al (2019) also developed a DEM calibration method based on angle of repose testing and powder rheometry for AM, which was designed around multiple characterization experiments applicable to the spreading step.…”