Microstructural and Mechanical Properties of Novel Co-Free Maraging Steel M789 Prepared by Additive Manufacturing

Zbigniew Brytan; Mariusz Król; Marcin Benedyk; Wojciech Pakieła; Tomasz Tański; Mengistu Jemberu Dagnaw; Przemysław Snopiński; Marek Pagáč; Adam Czech

doi:10.3390/ma15051734

Microstructural and Mechanical Properties of Novel Co-Free Maraging Steel M789 Prepared by Additive Manufacturing

Materials (Basel). 2022 Feb 25;15(5):1734. doi: 10.3390/ma15051734.

Authors

Zbigniew Brytan¹, Mariusz Król¹, Marcin Benedyk², Wojciech Pakieła¹, Tomasz Tański¹, Mengistu Jemberu Dagnaw³, Przemysław Snopiński¹, Marek Pagáč⁴, Adam Czech⁵

Affiliations

¹ Department of Engineering Materials and Biomaterials, Faculty of Mechanical Engineering, Silesian, University of Technology, 44-100 Gliwice, Poland.
² Paks'D Sp Zoo, Strzelecka 74, 43-100 Tychy, Poland.
³ Department of Mechanical Engineering, Institute of Technology, Wollega University, Nekemte P.O. Box 395, Ethiopia.
⁴ Center of 3D Printing Protolab, Department of Machining, Assembly, and Engineering Technology, Faculty of Mechanical Engineering, Technical University of Ostrava, 17 Listopadu 2172/15, Poruba, 708 00 Ostrava, Czech Republic.
⁵ Department of Lightweight Structures and Polymer Technology, Chemnitz University of Technology, 09111 Chemnitz, Germany.

Abstract

This research aims to characterize and examine the microstructure and mechanical properties of the newly developed M789 steel, applied in additive manufacturing. The data presented herein will bring about a broader understanding of the processing−microstructure−property−performance relationships in this material based on its chemical composition and heat treatment. Samples were printed using the laser powder bed fusion (LPBF) process and then the solution was annealed at 1000 °C for 1 h, followed by aging at 500 °C for soaking times of 3, 6 and 9 h. The AM components showed a relative density of 99.1%, which arose from processing with the following parameters: laser power of 200 W, laser speed of 340 mm/s, and hatch distance of 120 µm. Optical and electron microscopy observations revealed microstructural defects, typical for LPBF processes, like voids appearing between the melted pools of different sizes with round or creviced geometries, nonmelted powder particle formation inside such cavities, and small spherical porosity that was preferentially located between the molten pools. In addition, in heat-treated conditions, AM maraging steel has combined oxide inclusions of Ti and Al (TiO2:Al2O3) that reside along the grain boundaries and secondary porosities; these may act as preferential zones for crack initiation and may increase the brittleness of the AM steel under aged conditions. Consequently, the elongation of the AM alloy was low (<3%) for both annealed and aged solution conditions. The tensile strength of AM M789 increased from 968 MPa (solution annealed) to 1500−1600 MPa after the aging process due to precipitation within the intermetallic η-phase. A tensile strength and yield point of 1607 ± 26 and 1617 ± 45 MPa were obtained, respectively, after a full heat treatment at 500 °C/6 h. The results show that 3 h aging of solution annealed AM M789 steel achieves satisfactory material properties in industrial practice. Extending the aging time of printed parts to 6 h yields slightly improved properties but may not be worth the effort, while long-term aging (9 h) was shown to even reduce quality.

Keywords: LPBF; M789 steel; SLM; heat treatment; mechanical properties; microstructure; oxide inclusions.