Influence of Print Strategy and Localized Thermal Post-Processing on the Microstructure, Residual Stresses and Fatigue Strength of AISI 4140 Manufactured by Laser Powder Bed Fusion

Laser-based powder bed fusion (PBF-LB) has become one of the standard processes for metal additive manufacturing (AM), especially for steels. The freedom of design allows relatively new approaches such as topology optimization to be fully exploited. In recent years, steels with higher carbon content have become more common, such as AISI 4140. These steels are more difficult to print due to the martensitic transformation during cooling. This leads to high local stresses associated with the volume change during transformation, resulting in residual stresses that are difficult to predict.

This study investigates the complex process-structure-property relationships in AISI 4140 produced by PBF-LB in combination with a localized thermal post-processing route. It focuses on the influence of different AM processing strategies and the influence of thermal post-processing on the resulting microstructure, residual stresses and fatigue strength. The effects of varying the base plate preheating temperature, as well as an induction hardening post-processing step were systematically investigated. Microstructural analysis, hardness testing, residual stress measurements and fatigue testing was performed to separate and identify the specific contributions of each state.

The results of this study provide valuable insights for optimizing the PBF-LB process strategy for AISI 4140 to improve fatigue properties of the printed components, thus enhancing their performance and reliability in industrial applications.