Abstract
A new approach to modelling the microstructure evolution and yield strength in laser powder bed fusion components is introduced. Restoration mechanisms such as discontinuous dynamic recrystallization, continuous dynamic recrystallization, and dynamic recovery were found to be activated during laser powder bed fusion of austenitic stainless steels; these are modelled both via classical Zener-Hollomon and thermostatistical approaches. A mechanism is suggested for the formation of dislocation cells from solidification cells and dendrites, and their further transformation to low-angle grain boundaries to form subgrains. This occurs due to dynamic recovery during laser powder bed fusion. The yield strength is successfully modelled via a Hall–Petch-type relationship in terms of the subgrain size, instead of the actual grain size or the dislocation cell size. The validated Hall–Petch-type equation for austenitic stainless steels provides a guideline for the strengthening of laser powder bed fusion alloys with subgrain refinement, via increasing the low-angle grain boundary fraction (grain boundary engineering). To obtain higher strength, dynamic recovery should be promoted as the main mechanism to induce low-angle grain boundaries. The dependency of yield stress on process parameters and alloy composition is quantitatively described.
Highlights
A new approach to modelling the microstructure evolution and yield strength in laser powder bed fusion components is introduced
Bertsch et al [9] suggested that there could be four main possible variables that control the formation of dislocation cellular structures: (i) cooling rate/strain rate during cooling, which determine the dendritic arm spacing; (ii) temperature gradients, which can determine the localisation of stress/strain during processing; (iii) hatch distance and layer thickness of the process, which can determine the number of heating/cooling cycles that a layer experiences during Laser powder bed fusion (LPBF); (iv) the penetration of the melt pool to the substrate that can determine the characteristics of geometric constraints that are present around the newly deposited layers during processing
Austenitic stainless steel laser powder bed fusion microstructures were correlated to their yield strength
Summary
A new approach to modelling the microstructure evolution and yield strength in laser powder bed fusion components is introduced. A mechanism is suggested for the formation of dislocation cells from solidification cells and dendrites, and their further transformation to low-angle grain boundaries to form subgrains. This occurs due to dynamic recovery during laser powder bed fusion. It can be concluded that residual stresses exceeding the yield strength of the material generate a plastic residual strain This is imposed during LPBF due to repetitive thermal cycling, potentially having a substantial effect on the formation of dislocation cellular structures. It is generally accepted that alloys with low to medium stacking fault energy undergo DDRX during thermo-mechanical processing at all temperatures and strain rate regions, due to the large and complex thermal gradients involved in LPBF, this needs further clarification. The various mechanisms for the formation of cellular structures and new grains result in a complex microstructure; this is hierarchically arranged in solidification/dislocation cellular structures within subgrains that are formed in the actual grains
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