Ａｌダイカスト金型のヒートチェックの発生と進展

Abstract

This paper describes the initiation and propagation of heat checking and the variation of residual stress on aluminum die casting dies. Detailed observations by SEM were conducted repeatedly after every prescribed number of shots on the cavity surface near the gate of die casting dies made of steel, SKD61 (AISI H13), in actual service, and the origins of heat checking were searched. Using the newly defined crack density, the propagation of heat checking on the cavity surface was discussed quantitatively. The residual stress on the surface of dies, which was produced by cyclic thermal stress, was measured by X-ray, and the variation of residual stress in the early part of the service life was made clear.The following conclusions were drawn from this investigation.(1) Cracks, appearing in the early stage of heat checking, started from three types of origins: First, pits and wavy patterns, which initiated during the use of the die from the clean surface without any defect before use, second, various machining scratches and defects such as indentations, and third, strain concentrated parts of the die surface.(2) The crack density, obtained from the number of surface cracks longer than 0.2mm, increased rapidly in the first period, and then saturated in the latter period of crack propagation.(3) The residual stress of die surface before use, was compressive stress which had been created during the making of the dies, but it changed immediately toward the tensile side and satulated in the early stage of casting process before 50 shots.