Research on microstructure and mechanical properties of die casting Mg-4Al-1Si-3RE (Ce, La) and AS41 alloys

Abstract

Mg-4Al-1Si-3Ce (ASCe), Mg-4Al-1Si-3La (ASLa), Mg-4Al-1Si-1.5Ce-1.5La (ASCeLa), and Mg-4Al-1Si (AS41) alloys were prepared by die-casting process, and the microstructure, tensile mechanical properties, friction and wear behavior and wear mechanism under different loads were systematically analyzed. It was found that die-casting Mg-4Al-1Si-3RE (Ce, La) (ASRE) alloys mainly contain three kinds of rare earth phases: lamellar REMg2Si2, rod-shaped Al11RE3, and granular Al2RE. Besides, a finer CeLaMg4Si4 phase exists in the ASCeLa alloy, which has a semi-coherent interface relationship with Mg2Si. Moreover, the Al11Ce3 phase in the ASCe alloy was not only attached to the end of the Mg2Si phase, but also perpendicular to the Mg2Si, thus hindering the growth of the Mg2Si phase. Among three rare earth alloys, the ASCeLa alloy possesses the smallest grain size of about 9 µm. In addition, its hardness, ultimate tensile strength, yield strength and elongation reach 85.55HV, 263 MPa, 167 MPa, and 12.03%, respectively, which are increased by 22.9%, 23.7% and 64.6% compared with typical die-casting AS41 alloy, respectively. At the same time, under the materials and test conditions given in this paper, compared with AS41 alloy, ASRE alloys have better friction and wear properties. Under the low load, the presence of rare earth elements improves wear resistance and prevents a drastic increase in local frictional heat. As the load increases, the amount of the wear debris increases, while the coefficient of friction decreases. When the load increases from 40 N to 120 N, the wear mechanism of ASRE alloys changes to delamination wear. Among three rare earth alloys, the ASCeLa alloy has the best friction and wear performance, which is attributed to the dispersion of the second phases with high hardness and high melting point along the grain boundaries and within the grains, solid solution strengthening effect of mixed rare earth elements and grain refinement. This research helps reveal the strengthening mechanism of Ce and La elements, and provides a reference for the design of high-strength die-casting magnesium alloys.