In particular, a method for refining the particle size of titanium and titanium alloys is provided, including thermal management of high strain rate multi-axis forging. During forging, the high strain rate heats the inner area of the workpiece adiabatically, and the thermal management system is used to heat the outer surface area to the workpiece forging temperature while allowing the inner area to cool to the workpiece forging temperature. Another method involves multiple upsetting and stretch forging of titanium or titanium alloys using strain rates smaller than those used in conventional open die forging of titanium and titanium alloys. Increasing workpiece rotation and tensile forging cause severe plastic deformation and grain refinement in the titanium or titanium alloy forging process.
The key to grain refinement in the MAF process is the ability to operate continuously under dynamic recrystallization conditions produced by the ultra-slow strain rate used, i.e. 0.001s-1 or slower. During dynamic recrystallization, particles nucleate, grow and accumulate displacement simultaneously. The generation of displacement within newly nucleated particles continuously reduces the driving force for particle growth, and particle nucleation is favorable in terms of energy. The ultra-slow strain rate MAF process uses dynamic recrystallization to continuously recrystallize particles during the forging process.
A method of refining the particle size of a workpiece containing a metallic material selected from titanium and titanium alloys involves heating the workpiece to the workpiece forging temperature in the α+β phase field of the metal. Then the workpiece is multi-axis forged. Multi-axis forging involves forging the workpiece by pressure at a strain rate sufficient to heat the internal region of the workpiece adiabatically in the direction of the first orthogonal axis of the workpiece at the forging temperature of the workpiece. After forging in the direction of the first orthogonal axis, adiabatic heating of the internal area of the workpiece is allowed to cool to the forging temperature of the workpiece, while the outer surface area of the workpiece is heated to the forging temperature of the workpiece. The workpiece is then forged under pressure at a strain rate sufficient to heat the internal region of the workpiece adiabatically in the direction of the second orthogonal axis of the workpiece at the forging temperature of the workpiece. After forging in the direction of the second orthogonal axis, adiabatic heating of the internal area of the workpiece is allowed to cool to the forging temperature of the workpiece, while the outer surface area of the workpiece is heated to the forging temperature of the workpiece. The workpiece is then forged under pressure at a strain rate sufficient to heat the internal region of the workpiece adiabatically in the direction of the third orthogonal axis of the workpiece at the forging temperature of the workpiece. After forging in the direction of the third orthogonal axis, adiabatic heating of the internal area of the workpiece is allowed to cool to the forging temperature of the workpiece, while the outer surface area of the workpiece is heated to the forging temperature of the workpiece. Pressure forging and allow the steps to be repeated until a strain of at least 3.5 is obtained in at least one area of the titanium alloy workpiece. In non-restrictive embodiments, strain rates used during pressure forging range from 0.2s-1 to 0.8s-1, including endpoints.
