Temperature-dependent microstructural evolution and surface integrity in machining of directed energy deposition-316L

In the additive-subtractive hybrid manufacturing (ASHM) process, residual heat from the additive stage exists during the subtractive stage, and only dry cutting can be used. This results in relatively poor surface quality of the machined parts. Therefore, how to make use of this residual heat to enhance the surface quality is of vital importance. However, research in this area remains unclear and insufficiently thorough. To address this issue, 316 L stainless steel specimens manufactured by directed energy deposition (DED) are used to investigate the effects of elevated workpiece temperature on surface quality and tool wear during the milling process. The surface energy storage, and the strengthening mechanisms of the material's microstructure are systematically analyzed to reveal the underlying mechanism of the results. Experimental results demonstrate a non-monotonic relationship between workpiece temperature (20—400 °C) and machining outcomes: As the temperature rises from 20 °C to 220 °C, the tool wear rate decreases and surface defects are reduced; however, this trend reverses in the temperature range of 260—400 °C. This transition is related to fundamental changes in deformation mechanisms: at elevated temperatures, the stored energy in the material decreases, resulting in reduced dislocation density and twin boundary fraction. This weakens the microstructural strengthening effect, lowers the surface hardness, and makes the material more prone to machining defects. When the temperature rises from 20 °C to 220 °C, the annihilation of dislocations and the migration of grain boundaries intensify, resulting in a thinner grain refinement layer. However, as the temperature continues to rise to 400 ℃, it accelerates the nucleation of new grains and expands the nucleation zone, controlling the deformation mechanism and resulting in a thicker grain remelting layer. As the first investigation linking workpiece temperature rise to stored energy in the machined workpiece, this work establishes a theoretical framework for optimizing inter-process temperature management in additive-subtractive hybrid manufacturing. The findings advance fundamental understanding of ASHM while providing theoretical guidelines for enhancing surface integrity and tool life through strategic temperature selection during practical subtractive processing.