Thermal Fatigue Resistance of Alloy 718 for Aluminum Die Casting Dies

Abstract

Hot-work die steels have enjoyed extensive use as casting dies for over 70 years. Casting dies are typically made of H-13 when the die temperature remains below about 800°F (427°C). When the die temperature exceeds 800°F (427C), the selection of a die material must be made from a relatively limited range of high-strength maraging steels or modified H-13 alloys. The performance of aluminum casting dies is particularly troublesome because the casting temperatures often exceed the capability of current die materials. As a result, dies invariably fail by thermal fatigue. Nickel-base superalloys have never received serious consideration as die materials because of high raw material costs and poor thermal properties. The increased awareness of die life makes alloys, such as 718, attractive from a life-cycle cost point of view. Recent investigations at Allvac, an Allegheny Technologies company, have shown that over a range of conditions, 7 18 offers significantly better thermal fatigue resistance compared to H13. Further, 718 appears better able to tolerate high temperature exposures during aluminum die casting. The excellent high temperature strength and oxidation resistance of 718 cofibine for excellent thermal fatigue resistance. Thermal fatigue test results and microstructural evaluations of test specimens are summarized and discussed in this paper.