Thermal fatigue of forging die for driven spiral bevel gear blank

(1) Aiming at the fatigue failure of the blank forging die of driven spiral bevel gear, the equivalent stress field and temperature field of the die are simulated and analyzed by software. From the stress concentration part, it can be seen that there is serious stress concentration in the upper and lower molds, and the high stress area is mainly concentrated in the fillet of the upper and lower molds, and there are both tensile stress and compressive stress near the fillet, which is easy to cause the cracking of the fillet. The change of die temperature is mainly due to the effect of heat conduction. The high-temperature part mainly lies in the part in direct contact between the die and the forming blank. In addition, the high-temperature area of this part is concentrated on the surface of the inner cavity of the die, which will produce compressive stress due to thermal expansion, while the internal material temperature is low, which will produce tensile stress. Due to the effect of internal and external temperature difference, the final result will lead to local fatigue cracks in the die.

(2) The effects of cavity geometry parameters (fillet size and skin thickness) on the temperature field and stress field of the mold were analyzed. From the simulation results, it is concluded that the mold temperature stroke curves obtained by different inner cavity fillet size and different skin thickness are similar. With the progress of the stroke, the temperature of the upper mold has been in the trend of temperature rise. The temperature rise in the early stage is fast, and the temperature rise in the later stage is slow and gradually stabilized. The lower die temperature first rises sharply, and then decreases and then rises again. This is because the sum of conduction heat and friction heat of the lower die temperature in the middle of forming does not reach the diffusion heat. In general, the shape process parameters have little effect on the temperature, and the final temperature difference under the same condition is within 100 ℃.

(3) The effects of forming process parameters (strain rate, friction factor and heat transfer coefficient) on die temperature field and stress field are analyzed. The best die equivalent stress can be obtained by higher strain rate, smaller friction factor and appropriate heat transfer coefficient. The optimal scheme of upper and lower mold temperature is also the same. Lower die temperature can be obtained by appropriate strain rate, smaller friction factor and lower heat transfer coefficient. This shows that the lower die equivalent stress and temperature require a smaller friction factor.

(4) According to the previous research results and one of the above simulation results, the die life is estimated by using the local stress-strain method. The fatigue life of the upper die is 7082 times and that of the lower die is 8904 times. In the actual factory, the die life is generally about 8000 times, which shows that the result of die simulation and calculation is credible.

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