Institute of Fundamental Technological Research

In this paper, the effects of heating parameters, including temperature ranging from 900 ℃ to 1000 ℃, heating rates ranging from 2 ℃∙s-1 to 100 ℃∙s-1, and 120 s soaking on the non-equilibrium microstructure evolution of Ti-6Al-4V alloy were studied. Microstructures after heating were characterized to reveal the mechanism of non-equilibrium phase transformation. Uniaxial tensile tests at room temperature were carried out to evaluate the effects of non-equilibrium microstructure on the mechanical properties. Results show that higher heating rate and lower temperature lead to lower β phase volume fraction and finer β grains. A transition region with element gradient forms in the αp grain near the αp/β phase boundary and transfers into β phase gradually during the heating. Rapid heating could confine the movement of the transition region, and therefore reduce the α→β transition and growth of the β phase. When the Ti-6Al-4V alloy was heated to 1000 ℃ at a rate of 50 ℃/s and then quenched immediately, the tensile strength was improved by 19.5% and reached up to 1263.0 MPa with the elongation only decreasing from 13.6% to 9.6% compared with the initial material. The main strengthening mechanism is that the rapid heating in the single-phase region avoids the rapid growth of the β phase, which leads to fully fine martensite formation after water quenching.

Rapid heating,Non-equilibrium microstructure,Mechanical properties,Strengthening mechanisms

During hot stamping of titanium alloys, insufficient forming temperatures result in limited material formability, whereas temperatures approaching the β phase transus also result in reduced formability due to phase transformation, grain coarsening and oxidation during the long-time heating. To solve this problem, Fast light Alloys Stamping Technology (FAST) is proposed in this paper, where fast heating is employed. Effects of heating parameters on the formability and post-form strength were studied by tensile tests. Forming of a wing stiffener was performed to validate this new process. Results show that microstructure of TC4 alloy after fast heating was in nonequilibrium state, which could enhance ductility significantly compared with the equilibrium state. When TC4 alloy was first heated to 950 °C with heating rate of 100 °C/s and then cooled to 700 °C, the elongation at 700 °C was more than 3 times that of a slow heating rate with soaking. Nano-scaled martensite with high dislocation density transformed from β phase was observed under fast heating condition. A complex shaped wing stiffener was successfully formed from TC4 titanium alloy in less than 70 s including heating, transfer and forming, and the post-form strength was almost the same with the initial blank.

titanium alloys, fast heating, hot stamping, formability, post-form strength