Study On Thermal-Force Action Of Permanent Die Caused By Alloy Melt

Die life was restricted link of popularization and application of Non-solid Near-net Forming Technology. Heat shock behavior, heat penetration behavior and thermal-force action of die, which were caused by alloy melt and studied by theory and simulation method systematically in this paper, were main reasons of die failures. Also criteria of parameters and die design were improved based on these study achievements.Heat shock caused by alloy melt was the main reason of die plastic deformation and early fracture. Based on theoretical analysis of heat shock phenomenon, two descriptive parameters of die damage caused by heat shock were proposed, which were heat shock plastic deformation index and heat shock fracture index. Affection of heat shock behavior on die plastic deformation and fracture behavior quantifiably were described by these two parameters. Heat shock plastic deformation index represented approach of heat shock stress to material yield stress. Plastic deformation or fracture would happen on a die if this index reached to 1. Heat shock fracture index represented approach of heat shock stress to material tensile strength, die fracture would happen if this index reached to 1. According to theoretical analysis of heat transfer, theoretical models of heat shock plastic deformation index and heat shock fracture index were established. It was found that, with greater of thermal contact resistance, higher of pouring temperature and die temperature before heat action, the greater of heat shock plastic deformation index and heat shock crack index, and which could be fitted as exponential curve, line and cubic curve.Another effect of alloy melt acting on die was heat penetration. During single forming process, when forming ended, the distance of transmitting distance of heat released by alloy melt in a die or die core was defined as single heat penetration of die or die core. Based on approximate integral solution of die temperature field and analysis of alloy solidification course, theoretical models for estimating single heat penetration depth of die and die core were established. It was found that, with greater of thermal contact resistance, higher of pouring temperature and die temperature before heat action, the greater of single heat penetration depth, which could be fitted as exponential curve, cubic curve and cubic curve respectively. With certain thermal contact resistance, pouring temperature and die temperature before heat action, heat penetration depth of die was greater with time, which could be fitted as exponential curve.By analysis on cold surface temperature of heat penetration depth of die with continuous-using and without cooling system, it was known that the temperature was greater with using times, which meant average temperature of the die would be higher gradually. When average temperature of a die reached to a certain value, the stiffness of the whole die would be lost, and the die reached to its maximum using times. So there must be cooling system when a die was used continuously. According to the above analysis, design principles of cooling system were as follows: cooling system location should be within heat penetration depth, heat quantity taken away by cooling system should equal to that released by alloy melt and the cooling ability should be equal to or greater than die heat conductivity ability.Formulas for calculating knockout force and core-pulling force were established. And then core failure criterion was brought out as: if shear stress caused by fiction during core pulling process was smaller than plastic rheological stress of the die core, there would be no failure; or, plastic rheological failure on circumference surface of the die core could not avoided. These calculation models and the failure criterion were proved to be valid in experiments and production process.