Development of Structural Steels for Powder Bed Fusion - Laser Beam

Doctoral thesis, 2023

Over the past decade, powder bed fusion – laser beam (PBF-LB) has attracted noticeable attention from both academia and industry. However, there remains a scarcity of approved material for the process, as fewer than 40 alloys are commercially available. Although structural steels are some of the most commonly used materials in traditional manufacturing, they have yet to be developed for PBF-LB as their high carbon content makes them susceptible to cracking. The objective of this thesis was to develop structural steels for PBF-LB by determining the impact of various process parameters on part quality, microstructure and mechanical properties. This involved the production and analysis of various carbon (0.06 to 1.1 wt.% C) and low-alloy steels (AISI 4130, 4140, 4340 and 8620).

In terms of part quality, specimen density was related to the volumetric energy density (VED) and the carbon content of the alloy. Regarding the VED, specimens produced at low VED formed lack of fusion porosity, while specimens produced at high VED formed keyhole porosity. As for the carbon content, increasing the carbon content would reduce lack of fusion porosity at low VED, while lowering the required VED to form keyhole porosity. As for cold cracking, this occurred in structural steels with ≥ 0.38 wt.% C as elevated carbon contents would increase specimen hardness. However, cracking could be mitigated by increasing the VED, laser power or build plate preheating temperature, as each enhanced the level of in situ tempering during PBF-LB. From these findings, process windows were established for each structural steel that produced defect-free and high-density specimens (> 99.8%).

In terms of the microstructure, the as-built specimens were primarily composed of tempered martensite, with retained austenite also observed in alloys with ≥ 0.75 wt.% C. During PBF-LB, martensite formed during layer melting and was initially in a quenched-like state, with carbon atoms segregating to dislocations and martensite lath boundaries. Subsequent tempering of this martensite was due to micro-tempering within the heat affected zone and macro-tempering within the previously solidified material. Although both influenced martensite tempering, micro-tempering had the most significant effect as it reduced martensite hardness by up to ~380 HV. This noticeable reduction in hardness was due to the precipitation of nano-sized carbides at the previously carbon enriched regions of martensite.

Lastly, mechanical testing found that structural steels produced by PBF-LB achieved a high ultimate tensile strength (4140: ∼1400 MPa, 4340: ∼1500 MPa, 8620: ∼1100 MPa), impact toughness (4140:∼90–100 J, 4340:∼60–70 J, 8620:∼150–175 J) and elongation (4140:∼14%, 4340:∼14%, 8620:∼14–15%) that met or exceeded the ASTM standards. Additionally, these specimens displayed limited directional anisotropy due to small grains with weak crystallographic texture, a homogenous microstructure and low levels of internal defects. These findings are meant to highlight that these alloys are not only suitable but actively take advantage of PBF-LB to achieve properties that meet or exceed those of conventionally produced alloys.