Relationship between heat checking and residual stress in aluminum die casting dies

Heat Checking is a very important problem limiting the service life of aluminum die casting dies. In this paper, the relationship between the surface residual stress, which varies with die casting operation, and the initiation and propagation of heat checking is described.Measurement of residual stress and observation of heat checking were carried out on a cover and an ejector production die made by SKD61 (AISI H13). The surface residual stress was measured by X-ray and the heat checking was observed in detail by SEM after every prescribed shots from the beginning to the end of the die service life. The mechanism of residual stress variation and the effects of the position and the cavity shape of dies on the residual stress variation and the initiation and propagation of heat checking were clarified.The following conclusions were drawn from this investigation.(1) The surface residual stress, which was compressive before die casting operation, changed immediately toward tensile side and saturated in the early stage of operation, say at only 50 shots. This fact is considered to result from the following process; the surface layer of dies is rapidly heated by molten aluminum. The thermal expansion due to this rapid heating is restrainted by the substratum and thus the surface layer is subjected plastically to compressive strain. After cooling, the surface residual stress changes toward tensile side and saturates at some tensile value in the early stage of die casting operation.(2) The value of saturated tensile residual stress depended on the position and the shape of die cavity. This experimental result is caused by the magnitude of thermal load and restraint.(3) On the gate side of die cavity, in which the saturated tensile residual stress is large because of large thermal load, microcracks initiated in the saturated period. But on the position apart from the gate, in which the saturated tensile residual stress is small because of small thermal load, the residual stress did not decrease and microcracks did not initiate even at the end of die service life.(4) In the later stage of crack propagation on the gate side of die cavity, heat checking was observed by the naked eye and the residual stress decreased to zero.