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Grain-level response of additively manufactured metals to hot isostatic pressing; a dislocation dynamics study

Esosa Agbongiator, MS Defense, Advised by Professor Hesam Askari

Tuesday, August 10, 2021
10 a.m.

Hot Isostatic Pressing (HIP) is performed on additively manufactured metal parts to improve their mechanical properties. The process involves subjecting the specimen to a high temperature then successively applying a high pressure to cure defects including pores/bubbles and homogenize the crystal microstructure. Previous studies have characterized HIP’s effects as positive or neutral on mechanical strength depending on the specimens as built condition. Despite this, HIP has been demonstrated to consistently shrink and close most pores but the aggregate effect of the pores internal pressure post-HIP is poorly understood. It is observed that the size of the pores shrink after a HIP process results in a considerable increase in internal pressure. Majority of the lingering pores are expected to persist below the 5μm detection limit of most X-ray tomography scans, making this problem extremely difficult for experimental techniques.

Therefore, to understand the mechanical effect of the HIP process on these pores at the grain-level, dislocation dynamics (DD) was employed to understand why these pores do not contribute to an increase in yield stress as observed in cavity pores. This was executed by implementing continuum-level formulations of stress contributions of gas pores into a DD code with randomized pore and dislocation configurations within a representative volume element.

DD results show that the stress-strain response of these various structures were not significantly affect by post-HIP macroscopic stress. Further analysis using closed form solutions showed that the peak stress generated by these pores post-HIP was comparable to Pre-HIP conditions but featured a much quicker decay. This highly localized stress field around the pore quantitatively indicates pore size to be the limiting factor to stress concentration despite the dramatically increased contribution from internal pressure.