고압 다이캐스팅과 중력 금형 주조로 제작된 AlSi10MnMg 합금의 초기 결정립 크기가 잔류응력에 미치는 영향

Abstract

Recently, the proportion of automobile parts manufactured using an aluminum casting process to reduce the weight of vehicles is increasing. Aluminum alloys are recognized as representative lightweight materials that can replace steel materials with about 1/3 of the weight of steel materials, and are used in engine-related parts such as bodies, tire wheels, cylinder heads, and cylinder blocks. Casting is a manufacturing process in which a melt is injected into a mold and then solidified to produce a product of the desired shape. The casting process repeats the process of putting molten material in one frame and solidifying it, and products produced in the same mold have the same shape and dimension. Therefore, the casting process has the advantage of being able to mass-produce items with the same shape and dimensions, and it is possible to easily manufacture products with internal shapes that are difficult to manufacture by other processing methods. However, solidification and cooling during the casting process inevitably cause uneven temperature distribution in the product, and residual stress occurs due to differences in molten solidification rates during the aluminum casting process. The residual stress of the casting part is the stress remaining in the casting after the casting has been removed from the mold. Residual stress caused by uneven expansion and contraction within the casting degrades the mechanical properties of the parts, such as fatigue life, dimensional stability, and breaking strength, and causes unexpected destruction during the casting process. When a product with a complex shape such as a cylinder head is made through a casting process, the rate and time of solidification vary for each part, resulting in residual stress. Residual stress arising from aluminum castings is a defect that can never be ignored and requires control of residual stress for the production of high-quality castings. Therefore, in order to analyze the effect of the grain size that changes due to heating and contraction during the casting process on residual stress, an experiment was conducted using specimens manufactured by High-Pressure Die Casting (HPDC) and Gravity Die Casting (GDC). AlSi10MnMg alloys with different grain size were heat treated at 500°C for 2 hours and then furnace-cooled, air-cooled and water-quenched to control variables that could affect residual stress in addition to microstructural changes. The average grain size of an as-cast HPDC specimen without heat treatment was about 30 times smaller than that of an as-cast GDC specimen, and the residual stress value of the gravity die specimen having a large grain size was about 20 MPa larger. The grain size of the furnace-cooled GDC specimen was about 50 times larger than the average grain size of the furnace-cooled HPDC specimen, and the residual stress value was also the largest difference of about 40 MPa. The average grain size of the air-cooled GDC specimen was about 400μm larger than that of the air-cooled HPDC specimen, and the residual stress value was also about 40 MPa larger in air-cooled GDC. The average grain size of the water-quenched gravity die casting specimen and the HPDC specimen was about 10 times different, and the residual stress value of the water-quenched gravity die casting specimen was about 8 MPa higher. The larger the grain size, the more low-angle grain boundaries (LAGB) that are easily penetrated by the dislocation are distributed. Therefore, the larger the grain size, the more LAGB is distributed, so it can be seen that many dislocation are distributed inside and around the grain. It can be seen that the distribution of dislocation affected the residual stress.