Abstract
Under the typical hot isostatic pressing (HIP) processing conditions, plastic deformation by dislocation slip is considered the primary mechanism for pore shrinkage, according to experimental observations and deformation mechanism maps. In the present work, a crystal plasticity model has been used to investigate the influence of applied pressure and holding time on porosity reduction in a nickel-base single crystal superalloy. The influence of trapped gas on pore shrinkage is modeled by coupling mechanical deformation with pore–gas interaction. In qualitative agreement with experimental investigations, we observe that increasing the applied pressure or the holding time can effectively reduce porosity. Furthermore, the effect of pore shape on the shrinkage is observed to depend on a combination of elastic anisotropy and the complex distribution of stresses around the pore. Simulation results also reveal that, for pores of the same shape, smaller pores (radius < 0.1 μm) have a higher shrinkage rate in comparison to larger pores (radius ≥ 0.1 μm), which is attributed to the increasing pore surface energies with decreasing pore sizes. It is also found that, for smaller initial gas-filled pores (radius < 0.1 μm), HIP can result in very high gas pressures (on the order of GPa). Such high pressures either act as a driving force for argon to diffuse into the surrounding metal during HIP itself, or it can result in pore re-opening during subsequent annealing or mechanical loading. These results demonstrate that the micromechanical model can quantitatively evaluate the individual influences of HIP processing conditions and pore characteristics on pore annihilation, which can help optimize the HIP process parameters in the future.
Highlights
Modern gas turbine blades in aero-engines are usually made of nickel-base superalloys, solidified as single crystals to obtain excellent resistance to extreme working conditions such as temperatures up to 1100 ◦ C as well as static and cyclic loading
These results demonstrate that the micromechanical model can quantitatively evaluate the individual influences of hot isostatic pressing (HIP) processing conditions and pore characteristics on pore annihilation, which can help optimize the HIP process parameters in the future
As the HIP processing temperature is above the γ0 -solvus, we assume that the whole material contains only the soft γ phase
Summary
Modern gas turbine blades in aero-engines are usually made of nickel-base superalloys, solidified as single crystals to obtain excellent resistance to extreme working conditions such as temperatures up to 1100 ◦ C as well as static and cyclic loading. In recent manufacturing techniques such as additive manufacturing (AM) [11], parts are fabricated in an inert atmosphere, and the formation of pores filled with gas (argon) is a real possibility [12,13] As microstructural defects such as porosity can be very detrimental to the effective mechanical property of the fabricated material [14,15], more research has been focused on alleviating these defects [13,16]. The current work investigates the influence of pore characteristics on pore shirnkage and the effect of HIP parameters such as isostatic pressures and holding times on the pore annihilation Throughout this manuscript, a distinction between voids and pores is made unless mentioned otherwise; pores that do not contain trapped gas are referred to as voids
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