Effect of Zn Concentration on the Microstructure and Mechanical Properties of Al-Mg-Si-Zn Alloys Processed by Gravity Di
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INTRODUCTION
ALUMINUM alloys are widely used to produce castings with complex external geometries and internal cavities,[1,2] and they have the advantages of low density and good mechanical properties. This is attractive for transport because lightweight structures are beneficial for reduction of CO2 emissions.[3,4] Currently, Al-Si-Mg (Cu) alloys are the main materials used to produce casting components with good mechanical properties by either gravity die casting or low-pressure die casting.[5,6]
LONGFEI LI is with the Brunel Centre for Advanced Solidification Technology (BCAST), Institute of Materials and Manufacturing, Brunel University London, Uxbridge, Middlesex UB8 3PH and with the General Research Institute for Nonferrous Metals, Beijing 100088, China and also with the University of Science and Technology Beijing, Beijing 100083, China. SHOUXUN JI, YUN WANG, and XIXI DONG are with the Brunel Centre for Advanced Solidification Technology (BCAST), Institute of Materials and Manufacturing. Contact email: [email protected] QIANG ZHU is with the Southern University of Science and Technology, Shenzhen 518055, China. Contact e-mail: [email protected] WENCHAO YANG is with the State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China. STEPHAN MIDSON is with the General Research Institute for Nonferrous Metals and also with the Colorado School of Mines, Golden, CO 80401. YONGLIN KANG is with the University of Science and Technology Beijing. Manuscript submitted November 28, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Several strengthening phases have been reported for Al-Si-Mg (Cu) alloys. Ouellet and Samuel[7] found that the Al-Si-Cu alloy (319 type) is strengthened by h¢ and b¢ precipitates formed after heat treatment. Wiengmoon et al.[8] found that the h¢¢ and h¢-Al3Cu phases are responsible for the precipitate hardening of 319 alloys. However, Jahn et al.[9] found that the quaternary Q¢ (containing Al, Mg, Si, and Cu) and S¢ (containing Al, Mg, and Cu) phases are the key precipitates in the cast 319 alloys. Hwang et al.[10] identified h and Q precipitates in the 319-type Al-Si-Cu alloy. Mishra et al.[11] found that the ternary S¢ (Al2CuMg type) phase strengthens 339-type aluminum castings. On the other hand, Al-Zn-Mg alloys are used to manufacture high-strength components. Owing to fine-scale precipitation of metastable Zn- and Mg-rich phases, the strength of Al-Zn-Mg-Cu alloys can reach a level of 650 MPa for the yield strength and 700 MPa for the UTS after aging at 120 C for 24 hours.[12] Yang et al.[13] investigated an Al-Zn-Mg alloy with a Mg-to-Zn atomic ratio of 0.34 to improve the strength through formation of the coherent polyhedral s-phase [(Al, Zn)49Mg32]. Li et al.[14] investigated solution treatment of the microalloyed Al-Zn-Mg alloy system with Sc and Zr and found that both g¢-MgZn2 precipitates and fine Al3(Sc, Zr) particles are homogenously distributed throughout the a-Al grains under heat-treatment conditions. This alloy has a yield stren
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