Multi-Laser Powder Bed Fusion Benchmarking—Initial Trials with Inconel 625

Abstract

Production rate is an increasingly important factor in the deployment of metal additive manufacturing (AM) throughout industry. To address the perceived low production rate of metal AM systems based on single-laser powder bed fusion (L-PBF), several companies now offer systems in which melting has been parallelised by the introduction of multiple, independently controlled laser beams. Nevertheless, a full set of studies is yet to be conducted to benchmark the efficiency of multi-laser systems and, at the same time, to verify if the mechanical properties of components are compromised due to the increase in build rate. This study addresses the described technology gaps and presents a 4-beam L-PBF system operating in "single multi" (SM) mode (SM-L-PBF) where each of the four lasers is controlled so that it melts all of a particular components' layers and produces specimens for comparison with standard L-PBF specimens from the same machine. That is all four lasers making all of some of the parts were compared to a single-laser manufacturing all of the parts. Build parameters were kept constant throughout the manufacturing process and the material used was Inconel 625 (IN625). Stress-relieving heat treatment was conducted on As-built (AB) specimens. Both AB and heat-treated (HT) specimen sets were tested for density, microstructure, tensile strength and hardness. Results indicate that the stress-relieving heat treatment increases specimen ductility without compromising other mechanical properties. SM-L-PBF has achieved a build rate of 14 cm 3 /h when four 200 W lasers were used to process IN625 at a layer thickness of 30 μm. An increase in the build rate of 2.74 times (build time reduction: 63%) has been demonstrated when compared to that of L-PBF, with little to no compromises in specimen mechanical properties. The observed tensile properties exceed the American Society for Testing Materials (ASTM) requirements for IN625 (by a margin of 22 to 26% in the 0.2% offset yield strength). Average specimen hardness and grain size are in the same order as that reported in literatures. The study has demonstrated that a multi-laser AM system opens up opportunities to tackle the impasse of low build rate in L-PBF in an industrial setting and that at least when operating in single mode there is no detectable degradation in the mechanical and crystallographic characteristics of the components produced.

A Review on Laser Powder Bed Fusion of Inconel 625 Nickel-Based Alloy

“…Above 1000 • C, the δ phase was dissolved in the Ni matrix, so the hardness decreased rapidly. Wong et al [61] printed IN625 alloy using the multi-laser method and found that the average hardness (284.25 HV 0.5 ) and grain size (10-100 µm) of the samples obtained were the same as those reported in the literature (280 HV 0.5 [73]) by a single laser. They also found that heat treatment could reduce the hardness (271.08 HV 0.5 ).…”

“…The macroscopic characteristics of the as-built IN625 samples are closely related to the process parameters. It is obvious that the chevron feature of the track can be seen in almost all previous studies [55][56][57][58][59][60][61][62].…”

“…Overall, IN625 has good LPBF forming ability with a wide limit range of process parameters [60]. High density (e.g., 99.99% [61] and 99.74% [62]) and smooth surfaces can be obtained by carefully controlling the processing parameters. The macroscopic characteristics of the as-built IN625 samples are closely related to the process parameters.…”

“…As can be seen from Table 2, the strength of LPBF IN625 alloy parts is generally higher than the wrought part. Although adopting the multi-laser method to increase the construction efficiency, the mechanical properties of the LPBF samples are still comparable with a single laser method and heat treatment at 1048 • C for 1 h can reduce the strength of the multi-laser specimen and improve the plasticity to a certain extent [61]. The average yield strength and tensile strength of as-built parts obtained by Wang et al [74] are lower than those by Yang et al [59], and the elongation after fracture is similar.…”

“…In summary, the typical textures that existed in LPBF IN625 alloy parts leads to the anisotropy of tensile strength and other mechanical properties. Heat treatments can significantly change the tensile strength and increase the ductility of LPBF IN625 alloy samples [59,61,74]. The significant strengthening effect of adding carbon nanotube into IN625 [74] indicates that adding second tiny strengthening phases could be a useful method to improve the mechanical properties of LPBF IN625.…”