The influence of microstructure and defects on the mechanical behaviour of stainless steel manufactured by laser powder bed fusion

Abstract

Laser powder bed fusion (LPBF) can enable unprecedented local control of material properties in 3D by manipulating material deposition in an iterative additive process. However, such control is currently limited by an incomplete understanding of how process variation affects manufactured material. Assessing the structural integrity of parts made by LPBF is therefore a challenge. This thesis seeks to improve the knowledge foundation for part certification by investigating relationships between microstructure, defects and mechanical behaviour of stainless steel grade 316L manufactured by LPBF.

Heat treatment was used as an investigative tool to isolate the influence of porosity and microstructure on mechanical properties. Microstructural features were selectively removed by heat treatment and tensile testing enabled the influence of individual features to be assessed. Material microstructure and process defects were extensively characterised by metallography. A ductile damage model was developed based on the evolution of effective elastic modulus with plastic deformation. The evolution of microstructure and porosity with deformation under uniaxial and multiaxial stress was characterised.

Microstructural features were found to substantially influence mechanical behaviour. As-built material was strengthened by its microstructure and the strengthening effect could be removed by heat treatment. Manufacturing defects had a strong and detrimental effect on material ductility, which was also dependent on stress state and microstructure. Material defects were primarily located on melt pool boundaries which guided void growth with deformation. The material exhibited elastic-plastic strain hardening and ductile failure behaviour. Anisotropy in yield behaviour was correlated with the orientation of microstructural features, while anisotropy in failure behaviour was related to the orientation and morphology of defects as well as microstructural features. Anisotropy in both yield and failure behaviour was reduced by annealing heat treatment, increasing ductility and reducing strength.