Effect of hot isostatic pressing on the cast Ti6Al4V alloy with shrinkage cavities inside: Healing behavior, microstructure evolution and tensile property

Abstract

Healing behavior, microstructure evolution and tensile properties of cast Ti6Al4V alloy with the millimeter-scale shrinkage cavities inside during hot isostatic pressing (HIP) were investigated. X-ray computed tomography (CT) has been used to track pore closure process during a standard HIP cycle. The comparison of the CT scans before and after HIP indicates that HIP has the potential to close the millimeter-sized shrinkage cavities and the main closure mechanism of shrinkage cavity is plastic deformation which can be deduced by the rapid closure of the shrinkage cavity. The microstructure in the closure region was characterized in detail using scanning electron microscope (SEM) and electron backscattered diffraction (EBSD). It is inferred that closure interface can be partially healed by HIP-1 while can be totally healed by HIP-2. In addition, it was found that as the HIP duration became longer, the Burgers orientation relationship between α and β phases gradually deviated, and the misorientation along and across the single α colony gradually became larger, which could be attributed to the rotation of the crystalline lattice after hot deformation. In the individual α lamella, the rotation axis changed where the point-to-point misorientation was not continuous, thus the low-angle boundaries were formed. With the accumulation of strain energy, the low-angle boundaries transformed into high-angle boundaries, which resulted into the breakdown of α lamellae. The sub-division of α lamellae and recrystallized grain growth are determined to be the main spheroidization mechanisms of α lamellae during HIP. After HIP treatment and with the increase of HIP duration, the yield strength and tensile strength increased first and then slightly decreased, but the elongation was progressively increased due to the enhanced metallurgical bonding.