Mechanical field control in electron beam powder bed fusion: Dual effect of heat accumulation and melting strategies based on equivalent infinite cooling time interval

Mechanical field control in powder bed fusion is critical in mitigating distortion and cracking by reducing residual stresses, and enhancing mechanical properties by regulating dislocation structures. However, achieving these while preserving optimal melt region dimensions for desired build quality and microstructures remains challenging due to the inherent positive correlations among input energy, melt region and heat-affected zone (HAZ) dimensions, and residual stresses and strains. This study establishes a framework for effectively manipulating residual stresses and strains in melt region and HAZ, on the premise of preserving melt depth with error margins <8%. Through thermomechanical analyses and experimental validations, we investigate the effects of melting strategies and heat accumulation on residual stress–strain constitutive behaviors. Although conventional strategies such as shortening beam path length or spot melting are commonly employed to reduce residual stresses, we reveal their limited effectiveness under typical conditions where heat accumulates. In contrast, we propose the concept of equivalent infinite cooling time interval, based on which the real-spot (RS) melting strategy can significantly reduce macroscopic residual stresses. The underlying mechanisms are elucidated by revealing the dual effect of heat accumulation on residual stresses and strains, which induces a trade-off between their peak levels and total affected areas. Moreover, we demonstrate that the RS melting strategy alleviates continuous compressive plastic deformation in melt region and HAZ by reshaping principal plastic strain orientations and alternating compressive and tensile plastic strain components. This enables reducing residual stresses while increasing cumulative plastic strains, thereby overcoming the limitations in mechanical field control imposed by the positive correlation between residual stresses and strains.