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This work investigated non-standard HIP cycles for PBF-L Ti-6Al-4V and characterized microstructure and tensile properties to compare between material that originated from the same build. For 920°C, faster cooling rates (100°C/min, 2000°C/min) were found to promote bi-lamellar α microstructure, while the 2000°C/min cooling rate improved the strength. For HIP with lower temperature (800°C, 200 MPa), coarsening was minimized resulting in strength improvement. The slow cooling rate (12°C/min) showed the highest strength as faster rates increased the amount of orthorhombic martensite (α″). For HIP with higher temperature (1050°C), the as-built crystallographic texture was reduced and equiaxed prior-β grain morphology resulted, leading to more isotropic tensile properties. However, the cooling rate (2000°C/min) was not enough to prevent formation of grain boundary α, which reduced strength and elongation. Machine learning was carried out on the dataset via Principal Component Analysis (PCA) to reduce the dimensionality of the parameters and microstructural features.

In laser powder bed fusion (L-PBF), the mechanical performance and especially fatigue properties of fabricated parts are significantly improved by hot isostatic pressing (HIP) as the density increases (pores are closed) and the microstructure improves. HIP ensures consistent and defect-free material, and consequently, this high temperature and high pressure process is often a requirement for safety-critical aerospace applications. The use of HIP to directly consolidate intentionally-unmelted interior powder in a L-PBF part was recently demonstrated. By confining the laser melting to only the outer shell (contour) of the structure, L-PBF production times can be dramatically reduced. A subsequent HIP cycle, which may be mandatory for reliability reasons, and therefore does not add additional costs, is then used to densify the entire structure. Production rates and energy efficiencies can therefore be improved in this way. This study explores the effect of relying on the HIP process to consolidate interior sections of test coupons, for which micro computed tomography (microCT), process simulation and tensile tests were conducted. MicroCT of coupons with varying shell thicknesses identify the minimum shell thickness required; and provide indications of the shrinkage ratio as a function of powder content relative to shell thickness. Preliminary results are included in which the shrinkage can be compensated for during design, by way of a “bloated cube” which collapses to a nominal cube geometry after HIP. The mechanical evaluation of the consolidated shelled parts indicates that their tensile performance is equivalent to those observed on fully dense printed parts. Up to an order of magnitude faster build rates are possible and any resulting shrinkage or distortion can be offset at design.