The semi-solid compressive deformation behavior of Ti-Cu alloys was investigated by Gleeble-3500 hot simulator at the deformation temperatures ranging from 1273 to 1473 K with strain rates ranging from 5 3 10 23 to 5 3 10 21 s 21. The relationship between Cu concentration and flow stress was analyzed, and the deformation apparent activation energy was also calculated. The results show that Cu concentration has significant influence on the flows' behavior of Ti-Cu alloys, especially at high semi-solid deformation temperatures. The Ti-14Cu exhibits the highest flow stress at 1273 and 1373 K, Ti-2.5Cu alloy exhibits the highest flow stress at 1473 K, and Ti-7Cu alloy shows the lowest flow stress at all tested temperatures, which corresponds to liquid fraction caused by varied Cu concentration and the deformation temperature. The difference in microstructure suggests that the shape and distribution of Ti 2 Cu precipitates are significantly affected by Cu concentration. The increase in Cu concentration leads to the growth and precipitation of acicular Ti 2 Cu along grain boundaries at high semi-solid deformation temperatures. The deformation apparent activation energy of Ti-14Cu alloy significantly decreases from solid deformation to semi-solid deformation owing to the change in main deformation mechanism from plastic deformation of solid particles to solid particles' slippage and rotation of grain boundaries.
The formability of Ti-Cu alloys and their mechanical properties after semi-solid forging were investigated. The formability was evaluated by upsetting and die forging tests. Tensile test was also performed to study the mechanical properties of Ti-Cu alloys after semi-solid forging. The results show that the semi-solid forging requires lower upsetting forces in the temperature range from 1000 °C to 1150 °C compared with conventional solid forging. Die forging tests show that the semi-solid forged Ti-Cu alloys have an excellent workability with a forging ratio of 75% in the temperature range from 1000 °C to 1050 °C. The Ti2Cu phase increases with the increase of Cu content, and more liquid precipitates during semi-solid deformation at higher temperatures and higher Cu content, which relaxes the stress concentration caused by solid deformation and improves the formability. Tensile test reveals that semi-solid forged Ti-Cu alloys exhibit higher strength and lower ductility than conventional forged alloys. The Ti-Cu alloys have lower ductility and higher strength with the increasing of Cu content. The difference in tensile properties is attributed to the change of Ti2Cu precipitates at different Cu contents and semi-solid forging temperatures.
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