| Valve is an important part of automobile parts. Due to suffer cyclical changes of high stress, friction and heat stress, mold surface especially the R root surface area of lower die is prone to wear and tear, injury and even cracking, which lead to lower the die life, increase the production cost and reduce the production efficiency during the valve forming process under bad conditions. So it is necessary to study the distribution of stress and temperature during the valve forming process, and then find out the main reasons for the die failure. At the same time to seek a more convenient and effective way to predict the die life accurately, which can provide guidance for practical production.The paper is based on the analysis of statistical data for failure valve lower die. The forming processes under different forming time and cooling time are simulated through the finite element software MSC.Marc, and then analysis the low cycle fatigue life of lower die according to the results file, which realize the goal to replace the specific experiments by simulation methods on the analysis of die stress and fatigue failure. Finally put forward the reasonable improvement measures to improve the service life of die and verified through simulation analysis.The specific research contents include the followings:1) The statistic of failure die was done in factory. Results show that:the main failure forms of lower die including wrinkle, crack, strain and deformation, and the failure almost occurs in the R root surface area of lower die; Die life expectancy is about 1100 times; The analysis results are collected by microscopic electron microscope and SEM analysis of samples which show that the main reasons of the failure are the contact fatigue、thermal fatigue、fatigue crack and abrasion etc. are produced in the R root area of lower die.2) The changing circumstance of stress, strain and temperature of die in the forming process was studied by the thermal-mechanical coupling simulated analysis of MSC.Marc, and further research was done to analysis the effect on die stress and temperature of different forming time and cooling time. Results show that:high stress and high temperature are concentrate on R parts of the lower die, and the maximum value are about 1300MPa and 500℃ respectively; Forming time has less effect on the temperature, and stress value is falling as the forming time becomes longer, and gets the minimum value under 1s. The influence on internal stress of cooling time is small while the temperature is reduced along with the cooling time, but the decline is not obvious after 10s.3) Through the finite element software MSC.Fatigue, the lower die fatigue life under different forming time and cooling time was studied by the thermal- mechanical compound fatigue life analysis. Results show that:low life is concentrate on the area of R root, and the lowest life of die is only 1151 times. The die fatigue life is rising as the longer forming time, but the change is not obvious until 1s. The life value is similar when the cooling time is within 10s, but decline when beyond 10s.4) In order to improve the die longevity, a new set of combination lower dies was designed according to the unified strength theory, and analyzed that through the finite element simulation software. Results show that:reasonable interference fit between combination lower die can effectively improve the fatigue life of the mold, the fatigue life of combined die is about 1889 times, which is increased about 60% comparing with the analysis results of the whole mold. |
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