Experimental Characterization and Processing of Additively Manufactured Ti-6Al-4V

Unencumbered by the limitations of traditional subtractive manufacturing, additive manufacturing (AM) has opened a whole new world of possibilities in complex part geometries that can be produced quickly. Design tools, such as topology optimization (TO), can fully utilize this newfound design freedom, but they require process accurate material properties that are often anisotropic and inhomogeneous. The motivation and goal of this study was the characterization and measurement of mechanical properties of laser powder bed fusion (LPBF) Ti-6Al-4V to inform TO methods for the design of lightweight structures. A LPBF build consisting of cylinders in multiple build orientations was designed and executed to produce a total of nearly 100 tension, compression, and shear mechanical test specimens. The experiments from these samples were used to form a robust description of the stiffness and yield surface associated with the printed material. There was no notable anisotropy measured with elastic moduli in the build direction 113±6 GPa as compared to in the build plane 116±7 GPa. A tension-compression asymmetry in the yield strength was measured to have a disparity of 892±15 MPa to 996±32 MPa, and this behavior was captured in an asymmetric yield criterion. The simple to machine compact forced simple-shear specimen geometry was investigated to provide measures of shear elastic and yielding properties. Through an investigation of wrought Ti-6Al-4V conducted with DIC and finite element simulations, it was determined that the shear geometry has an inhomogeneous yielding behavior that causes the stress-strain response to diverge from linear well before reaching the macroscopic yield strength. This led to an undervaluation of the strength when a traditional 0.2% strain offset was employed, it was determined that a strain offset of 2.8% was needed to accurately determine the macroscopic yield point. Using this new metric for measuring yield, the AM shear yield strengths were evaluated to be 560±30 MPa, which agreed well with the yield surface that contained the tension and compression results. Metal AM processes, such as LPBF, are prone to microstructures that contain defects and non-equilibrium phases that can be deleterious and are typically mitigated via post-processing. Hot isostatic pressing is the industry standard, but this requires specialized chambers and can be costly. Alternatively, thermo-hydrogen refinement of microstructure (THRM) has recently been introduced as a technique that uses hydrogen as a temporary alloying element to enable a novel phase transformation that offers recrystallization and homogenization of as-built Ti alloys into an ultra-fine microstructure. A study of the temperature used in the THRM process was conducted and showed significant gains in ductility over the as-built condition. The THRM temperatures investigated all exhibited tensile properties comparable to those achieved using HIP. Heating over the β transus temperature, resulted in a significant growth of the prior β grains resulted. This increase in grain size, coupled with the formation of a continuous layer of α phase along the boundaries, was found to present a preferable path for crack growth, leading to a reduction in ductility at higher temperatures (15% for 850 C and 11% for 1200 C). Taken as a whole the work was successful in generating a description of the full elastic and yielding behavior to inform TO models to enable the design of lightweight lattice structures. Additionally the THRM process was demonstrated to be an effective and cost efficient replacement for traditional post-processing of LPBF Ti-6Al-4V