Mechanical properties of heat-treated additively manufactured titanium alloys

The rapid growth of additive manufacturing (AM) technologies has made possible the fabrication of near net shape components in less time and at lower cost than traditional wrought processes. As a result, AM has been shown to reduce substantially buy-to-fly ratios for titanium and other non-ferrous structural aerospace alloys. However, the microstructure (grain size, shape, and crystallographic orientation) and its evolution (precipitation and coarsening), which is highly process dependent, determines the resulting mechanical properties and performance. If AM technologies are to be adopted more widely, with AM components to replace wrought components in critical structural applications, then it is essential to develop a fundamental understanding of the microstructure-property relationships of AM materials. AM components are rarely put into service in the as-built condition, and as such, heat treatments are typically required. Differences across the numerous AM variants (e.g., small-volume versus large-volume, wire versus powder, laser versus electron beam) and post-build heat treatments (e.g., hot isostatic pressing versus beta annealing versus stress relieving) often result in microstructural differences, and thus, variances in mechanical properties. In the first part of this work, a database of experimentally measured mechanical properties was compiled for 113 explicit Ti-6Al-4V samples. This is the first time that samples produced via wrought and a variety of AM variants (i.e., large-volume electron beam, large-volume laser hot wire, and small-volume laser powder bed fusion) have been aggregated and analyzed in an effort to develop a framework for the prediction of uniaxial tensile behavior beyond the yield point for Ti-6Al-4V across a wide range of manufacturing processes. But unlike the "workhorse" α + β titanium alloy Ti-6Al-4V, which has been studied heavily, and long been an alloy of interest to the AM community, the metastable β titanium alloy Ti-5Al-5V-Mo-3Cr has only recently become a candidate alloy for investigation. In the second part of this work, samples of Ti-5Al-5V-Mo-3Cr manufactured by laser power bed fusion (L-PBF) were subjected to 27 unique heat treatments and subsequently tested in uniaxial tension. Backscattered electron (BSE) micrographs were acquired using a scanning electron microscope (SEM) and microstructural features of interest were quantified using MIPAR image analysis software. A single equation was developed to predict the yield strength of heat-treated L-PBF Ti-5Al-5V-5Mo-3Cr, with predicted yield strength values having an average error of less than 3% and a maximum error of 8%. The experimentally measured yield strengths ranged from 925-1469 MPa and were shown to depend on only two microstructural variables: α phase fraction and α-to-α inter-lath spacing. Equally important as heat treatments for microstructural modification and tuning of mechanical properties are low temperature heat treatments for stress relief of AM materials. In the last part of this work, the microstructural evolution of L-PBF Ti-5Al-5V-5Mo-3Cr during low temperature aging was investigated. When quenched from above the β transus, the β phase decomposes into the athermal ω phase, and during isothermal aging below the ω solvus temperature, the athermal ω precipitates coarsen and become solute lean with respect to the β matrix. Due to the rapid cooling temperature inherent to the AM processes it is critical to understand the evolution of the ω phase and the potential effect of structure and composition in L-PBF Ti-5Al-5V-5Mo-3Cr. The results of this study are useful for understanding the mechanical properties of AM titanium across AM variants and titanium alloy classifications (i.e., α + β alloys, β alloys). This research fills critical knowledge gaps and provides powerful tools for the prediction of structure-property relationships for AM titanium alloys.