Lisa Larsson: Additive Manufacturing of Biodegradable Magnesium Alloy WE43: Linking Process Parameters to Microstructure and Mechanical Performance

Abstract

Powder bed fusion – laser beam (PBF-LB) of magnesium (Mg) alloys, particularly WE43 (Mg-4wt%Y-3wt%RE-Zr), offers promising potential for biodegradable medical implants. This thesis investigates the influence of key process parameters in PBF-LB on the microstructure, residual stress, texture, and mechanical properties of alloy WE43 (Mg-4wt%Y-3wt%RE-Zr). This knowledge is intended to support the continued development and implementation of PBF-LB processed WE43 for applications in biodegradable medical implants. The effects of laser power, hatch distance, build size and orientation, as well as laser scan rotation, were systematically investigated. 

Increased energy input through higher laser power promoted equiaxed dendritic grain formation, which enhanced tensile strength. Hatch distance could be optimized to maintain tensile properties even at lower laser powers, and influenced grain size, texture and distribution of secondary phases. Build direction had a large impact on the magnitude of the residual stresses, with larger builds in the vertical direction giving larger stress gradients throughout the sample. Tensile residual stresses were observed at the sample edges, correlating with reduced hardness in those regions compared to the bulk.

Horizontally built specimens showed approximately 40% higher tensile strength (215 MPa vs 150 MPa) and about 20% higher elastic modulus (44 GPa vs 37 GPa) than vertically built ones, primarily due to the development of a strong basal texture along the build direction. This anisotropy implies that part orientation during PBF-LB has a significant impact on performance in service. It was demonstrated that laser scan rotation significantly influences the crystallographic texture, which has the potential to affect the mechanical response of the printed parts. Rotations of 67° and 90° maintain high densification and mechanical integrity while modifying texture. Rotations of 60° and 120° further demonstrate texture control, and a segmented chessboard strategy enhances compressive strength despite weaker texture, due to favourable pore distribution and dendritic grain formation. Conversely, limiting scan rotation to 0° or 180° results in poor densification (<99% relative density), compromising structural integrity. Together, the work included in the thesis provides a comprehensive foundation for PBF-LB considerations to achieve desirable microstructural and mechanical outcomes in WE43, supporting its potential use in biomedical applications.