Computer modeling and prediction of thermal fatigue cracking in die-casting tooling

Thermal fatigue cracking is a major mode of failure of hot working tool steel dies in die-casting and forging applications. This results in huge loss owing to high cost of dies, downtime and die repair and replacement. The cracking initiates due to the large thermal shock experienced by the die surface when it is rapidly heated to 700 degreesC and then quenched to 150 degreesC by the lubricant spray. This thermal fatigue loading propagates the crack until gross failure occurs or- die becomes unusable. Most of the previous efforts have been focused on the improvement of die steels and heat treatment techniques for high fracture toughness. This paper investigates the cracking mechanism using laboratory experiments. The thermal fatigue loading is applied by dipping the test coupons in molten liquid aluminum bath and quenching in water at room temperature. The dip times in aluminum and water are changed to represent different conditions. Commercial FEM software is used to simulate the test conditions and to analyze the temperature and stress profile. The general thermal fatigue equations are modified to model the cracking in die-casting and predict the number of cycles and regions more susceptible to such cracking. It also presents a methodology to use a computer model to predict failure in actual dies and to make design changes like placement of cooling lines, thermal cycles, etc. to minimize cracking and increase die life. (C) 2003 Published by Elsevier Science B.V.