Plastic flow and microstructure evolution during thermomechanical processing of laser-deposited Ti-6/Al-4V preforms

Plastic flow behavior and microstructure evolution during hot working and h eat treatment of Ti-6Al-4V synthesized via a laser-deposition, Laser Engine ered Net Shaping (LENS(TM)), process were established. To this end, isother mal, hot compression tests were conducted on samples in either a deposited + stress relieved condition or a deposited + hot isostatically pressed (hip ped) condition. The starting microstructures consisted of columnar grains w ith fine or coarse Widmanstatten (basketweave) alpha platelets. At subtrans us temperatures, the flow curves of both microstructural conditions exhibit ed a peak stress at low strains followed by extensive flow softening; these curves were almost identical to previous measurements on ingot-metallurgy (IM) Ti-6Al-4V with similar transformed microstructures. In addition, the k inetics of globularization of the alpha phase during subtransus deformation or subsequent static heat treatment were found to be the same as for IM Ti -6Al-4V with comparable alpha-platelet thicknesses. During supertransus hea t treatment, moderately fine beta-grain microstructures were developed in s amples that had been predeformed below the beta transus. Such a heat treatm ent for samples previously deformed above the transus gave rise to a nonuni form distribution of coarse beta grains, an effect attributed to critical g rain growth.