Like isothermal forging, hot die forging is also a promising precision forging process. The difference is that the die temperature of hot die forging is higher than that of ordinary forging, but lower than that of isothermal forging. The temperature of a typical hot die forging die is 110 to 225°C lower than the billet temperature. Compared with isothermal forging, the reduction of die temperature can be used more widely, but the ability to form thin and complex shaped forgings is slightly worse.
Compared with conventional forging, hot die forging has the following advantages: (1) Reduced material consumption of forgings. During hot die forging, the chilling of the mold contacting the blank and the work hardening of the material are reduced, and the forgeability of the material is improved, thus allowing Forgings have a smaller fillet radius, a smaller draft angle and a smaller forging margin, which greatly reduces the quality of the forgings. For example, a Ti-6Al-4V alloy structural part has a mass of 28 kg, a mass produced by a conventional forging process has a mass of 154 kg, and a mass produced by a hot die forging process has a mass of 109 kg. The difference between the two methods is 45 kg. (2) Reduce the number of forging operations and improve the working capacity of the press. When hot die forging, the die temperature is higher and the billet temperature drop is less. Forgings that require two, three or more fires for conventional forging, hot die forging It only takes one time, and it can be completed with up to two fires. Due to the hot die forging, the deformation resistance of the metal is low, which relatively increases the working capacity of the equipment. (3) Reduce the amount of machining of forgings. Because the forgings produced are close to the weight and outline dimensions of the parts, compared with the forgings produced by conventional forgings, the amount of material removed in machining is reduced. (4) The uniformity of the product is good. During the forging process, the temperature gradient is greatly reduced, and the uneven deformation caused by the temperature gradient is easy to reduce, so the uniformity and consistency of the product’s organization and performance are better than conventional forging forgings. , But not as forgings produced by isothermal forging.
During hot die forging, although the billet has a temperature drop, it is still in 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 0.2s-1. If the strain rate is too low, the billet temperature may decrease.
In hot die forging of titanium alloys, forging heating temperature, strain rate, microstructure and holding time of preforms are extremely important factors, which play a decisive role in the dimensional accuracy and microstructure of shaped parts. Usually lower strain rate and longer dwell time increase the possibility of precision forming. The microstructure of the preform has a direct impact on the flow stress and superplasticity of the material, especially on the microstructure after forging. It is impossible to attempt to completely eliminate the defects and grain unevenness in the raw materials by isothermal forging or hot die forging.
At present, whether titanium alloys and high-temperature alloys adopt hot die forging process mainly depends on the total cost of forgings or the need of product uniformity and consistency. The development trend of this process is to use conventional forging preforms, and finally carry out isothermal or hot die final forging.