Titanium alloy forging - Hot die forging

Like isothermal forging, hot die forging is also a promising precision forging process. The difference is that the die temperature in hot die forging is higher than that in ordinary forging, but lower than that in isothermal forging. The typical hot die forging die temperature is 110 to 225℃ lower than the billet temperature. Compared with isothermal forging, the reduction of die temperature allows for the selection of a wider range of die materials, but the ability to form very thin and complex-shaped forgings is slightly inferior.

Compared with conventional forging, hot die forging has the following advantages: (1) Reducing material consumption of the forgings As in hot die forging, the contact between the die and the billet is lessened due to the reduced cooling of the die and the material's work hardening, improving the forgeability of the material, thus allowing forgings with smaller fillet radii, smaller draft angles, and smaller forging allowances, thereby significantly reducing the weight of the forgings. For example, for a Ti-6Al-4V alloy structural part with a mass of 28 kg, the forged part produced by conventional forging has a mass of 154 kg, while the one produced by hot die forging has a mass of 109 kg, a difference of 45 kg. (2) Reducing the number of forging operations and improving the working capacity of the press As in hot die forging, the die temperature is higher, and the temperature drop of the billet is less. For conventional forging, two, three or more heats are required to form the forging, while hot die forging can complete the process with just one or at most two heats. Moreover, due to hot die forging, the deformation resistance of the metal is lower, which relatively increases the working capacity of the equipment. (3) Reducing the mechanical processing volume of the forgings Because the forged parts produced by hot die forging are close to the weight and contour dimensions of the parts, the material removal during mechanical processing is reduced compared to the forged parts produced by conventional forging. (4) Better product uniformity As the temperature gradient during forging is greatly reduced, the deformation unevenness caused by the temperature gradient is alleviated, so the uniformity and consistency of the microstructure and properties of the products are better than those of the forged parts produced by conventional forging, but not as good as those produced by isothermal forging.

During hot die forging of titanium alloys, although the billet has a temperature drop, it remains within the forging temperature range, and the deformation resistance does not rise as sharply as in conventional forging. The strain rate used in hot die forging varies within the range of 0.05 to 0.2 s-1. If the strain rate is too low, the billet temperature may decrease.

In titanium alloy hot die forging, the forging heating temperature, strain rate, the microstructure of the preformed billet, and the holding time are extremely important factors that play a decisive role in the size accuracy and microstructure of the formed parts. Generally, a lower strain rate and longer holding time improve the possibility of precise forming. The microstructure of the preformed billet has a direct impact on the flow stress and superplasticity of the material, especially on the post-forging microstructure, and cannot attempt to completely eliminate the defects and grain inhomogeneity in the raw material through isothermal forging or hot die forging. Currently, whether titanium alloys and high-temperature alloys adopt the hot die forging process mainly depends on the cost of the forgings or the need for product uniformity and consistency. The trend of this process is to use the preformed billets from conventional forging and then perform isothermal or hot die final forging.