ABSTRACT
An essentially fully acicular alpha-prime martensite within an equiaxed grain structure was produced in an Electron Beam Melting (EBM)-fabricated Ti-6Al-2Sn-4Zr-2Mo (Ti6242) alloy using two different Arcam EBM machines: An A2X system employing tungsten filament thermionic electron emission, and a Q20 system employing LaB6 thermionic electron emission. Post-process Hot Isostatic Pressing (HIP) treatment for 2 h at 850, 950, and 1050 °C resulted in grain refinement and equiaxed grain growth along with alpha-prime martensite decomposition to form an intragranular mixture of acicular martensite and alpha at 850 °C, and acicular alpha phase at 950 and 150 °C, often exhibiting a Widmanstätten (basketweave) structure. The corresponding tensile yield stress and ultimate tensile strength (UTS) associated with the grain growth and acicular alpha evolution decreased from ~1 and ~1.1 GPa, respectively, for the as-fabricated Ti6242 alloy to ~0.8 and 0.9 GPa, respectively, for HIP at 1050 °C. The optimum elongation of ~15-16% occurred for HIP at 850 °C; for both EBM systems. Because of the interactive role played by equiaxed grain growth and the intragrain, acicular alpha microstructures, the hardness varied only by ~7% between 41 and 38 HRC.
ABSTRACT
Hot isostatic pressing (HIP) treatments are traditionally used to seal internal porosity, because defects exist in as-built Ti-6Al-4V parts produced by electron-beam melting powder-bed fusion. Standard HIP treatment of Ti-6Al-4V parts results in decreased strength due to coarsening of the microstructure. We present a new HIP strategy with the following steps: hold above the ß-transus, rapid quenching, and tempering. This new HIP treatment seals internal porosity, causes a columnar-to-equiaxed transition in morphology of prior-ß grains, changes the α lath aspect ratio, removes microstructural heterogeneities and matches the yield and ultimate tensile strength of the as-built condition.
