Formation Mechanism of Surface Crack in Low Pressure Casting of A360 Alloy
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RECENTLY, aluminum castings are becoming increasingly commonplace in automotive and aerospace industries due to their high specific strength.[1,2] However, the aluminum melt oxidizes easily and forms an oxide film on the liquid surface when it is exposed in air.[3–5] The unreasonable filling process submerges the surface oxide film in the liquid metal during the mold filling process. For both deformability and flexibility, the oxide film will be compressed or bent by surface turbulence, and the submerged surface oxide films are often double-layered.[6] Generally, the double oxide film defect in aluminum castings has become a vital issue which has attracted much attention. The entrainment of oxide films into the bulk of the melt is usually attributed to several reasons. In top gating casting, the melt is poured from a high
SHAN-GUANG LIU is with the School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, P.R. China and also with the Beijing Institute of Aeronautical Materials, Beijing 100095, P.R. China. FU-YANG CAO, XIN-YI ZHAO, HONG-XIAN SHEN, SHU GUO, and JIAN-FEI SUN are with the School of Materials Science and Engineering, Harbin Institute of Technology. Contact e-mails: [email protected], [email protected] TAO YING is with the School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P.R. China. JINGSHUN LIU is with the School of Materials Science and Engineering, Inner Mongolia University of Technology, Hohhot 010051, China. Manuscript submitted December 3, 2016. Article published online October 30, 2017. 2826—VOLUME 48B, DECEMBER 2017
position with a high speed, and the surface oxide film tends to be entrained into the melt during pouring.[7] In the bottom gating system, Runyoro et al.[8] suggested that when the gate velocity is greater than 0.5 m/s, the aluminum alloy melt can rise as a jet up to a sufficient height. The action of the fall back may cause the oxide film to be folded over and incorporated into the melt. Campbell and Hsu[9] also reported another double oxide film defect occurring at lower than the critical gating velocity, which was called oxide lap defect or confluence-welded defect. When the filling streams flow and encounter at a very low velocity, the thick oxide films come together ‘‘dry side’’ to ‘‘dry side,’’ resulting in an unbonded interface. The oxide film defects act as a crack after solidification, giving rise to a negative effect on the mechanical property and reliability of the castings.[10–12] Although some methods, such as melt processing[13,14] and healing of the double oxide film defect,[15–17] have been used to eliminate the negative effect of the oxide film defect, there are still many limitations in applications. The low pressure casting technique is therefore regarded as an effective solution to avoid the entrainment of oxide film[18] and has been widely used in the manufacturing of aluminum structure components.[19–21] The flow characteristics and oxide film entrainment in low pressure casting have gained much attention. Zeng et
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