Laser-based powder bed fusion of non-weldable low-alloy steels

ABSTRACT

This study focuses on the processability of four low-alloy steels (AISI 4130, 4140, 4340 and 8620) via laser-based powder bed fusion (LB-PBF). In the as-built condition, the alloys consisted of tempered martensite that was the result of an intrinsic heat treatment (IHT) during LB-PBF. In terms of defects, a distinct transition in porosity was observed that correlated to the volumetric energy density (VED). At low VED, specimens contained a lack of fusion porosity, while at high VED, they contained keyhole porosity. Additionally, cold cracking was observed in 4140 and 4340 specimens produced at low/intermediate VEDs. This cracking could be mitigated by increasing the VED or laser power, as both enhance the IHT. This enhanced IHT lowered the material hardness below specific thresholds (<500HV 4340 and <460 4140), increasing ductility and allowing the specimens to avoid cracking. From these findings, crack-free, high-density (>99.8%) low-alloy steel specimens were produced without the requirement of build plate preheating.

KEYWORDS:

• Laser-based powder bed fusion
• low-alloy steels
• lack of fusion porosity
• keyhole porosity
• cold cracking
• martensite tempering
• intrinsic heat treatment

Acknowledgements

The authors would like to acknowledge Peter Harlin (Sandvik Additive Manufacturing) and Sven Bengtsson (Höganäs AB) for providing the powder feedstock that was used in this study and would like to thank Anton Dahl-Jendelin (RISE-IVF) for helping characterise the powder properties.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The work was performed in the framework of the Centre for Additive Manufacturing – Metal (CAM²) that is supported by the Swedish Governmental Agency of Innovation Systems (Vinnova).

Notes on contributors

William Hearn

William Hearn is currently a PhD student at the Centre for Additive Manufacturing – Metal (CAM²) hosted by the Department of Industrial and Materials Science at Chalmers University of Technology, Gothenburg, Sweden. His work focuses on powder material development for powder-based additive manufacturing technologies.

Robert Steinlechner

Robert Steinlechner is currently a PhD student at the Institute of Chemical Technologies and Analytics at Vienna University of Technology (TU Wien), Vienna, Austria. This paper is based on the work performed by Robert Steinlechner during his MSc research visit hosted by the CAM² centre at Chalmers University of Technology.

Eduard Hryha

Eduard Hryha is a professor in Powder Metallurgy and Additive Manufacturing at the Department of Industrial and Materials Science, Chalmers University of Technology, Gothenburg, Sweden. He is also the director of the Competence Centre for Additive Manufacturing – Metal (CAM²), hosted by the same department. His research focuses on powder metallurgy and powder-based metal additive manufacturing.