Fatigue Behavior And Damage Mechanism Of Al-Si-Cu-Mg-(RE) Cast Aluminum Alloys

The Al-Si-Cu-Mg cast aluminum alloys have high mechanical properties and good cast performance. Due to their excellent comprehensive properties, the Al-Si-Cu-Mg cast aluminum alloys have wild application, and have become one of the most important structural materials applied in the equipment manufacturing industry. With the fast development of aviation and automobile industries, the higher properties are required for aluminum alloys, and rare earth micro-alloying has always been the significant method for property improvement and further development of new aluminum alloys. Actually, many key components in practical engineering application are often subjected to the alternating load, and thus the fatigue failure has become an important factor which concerns the safety and economy for those structures used in various engineering fields. Although some researches for the fatigue behavior of aluminum alloys have been performed, but mainly focus on the regularity understanding. Especially, the influences of rare earth elements and processing states on the low-cycle fatigue behavior of aluminum alloys have not been comprehensively revealed. Obviously, the investigation concerning the microstructure and fatigue property of the Al-Si-Cu-Mg cast aluminum alloys can not only provide the theoretical basis for the development of new type cast aluminum alloys but also the reliable theoretical foundation for the safety design and reasonable use of these alloys. Consequently, the cyclic stress response behavior, fatigue life behavior, fatigue deformation and fracture mechanisms for the Al-Si-Cu-Mg cast aluminum alloys with different processing states under low-cycle fatigue loading condition were investigated, and the effect of rare earth elements and processing states on the low-cycle fatigue properties of these alloys was determined.The microstructural observation shows that for the permanent-mold cast Al-Si-Cu-Mg alloy, the addition of rare earth elements Sc and Er can obviously refine the a-Al grains. Meanwhile, the rare earth elements Sc and Er have the favourable metamorphism, and can not only reduce the size of flake eutectic Si phase significantly, but also promote the transformation of a-Al grains from dendrite grains to equiaxed grains. After the solid solution treatment, the coarse flake eutectic Si phase in the permanent-mold cast Al-Si-Cu-Mg-(Sc, Er) alloys transforms into fine spherical or ovate shape. The addition of Sc and Er can promote the precipitation ofθ'(Al2Cu) phase which has half coherent interface with the matrix during the aging process. Compared with the permanent-mold cast process, die cast can greatly refine the microstructure of the Al-Si-Cu-Mg-(Sc, Er) alloys. The addition of Sc or Er can more significantly refine the microstructure of die-cast alloys, but has relatively less metamorphism for eutectic Si phase.Under low-cycle fatigue loading condition, the Al-Si-Cu-Mg-(Sc, Er) alloys can exhibit the continuous cyclic strain hardening or initial cyclic strain hardening followed by cyclic stability, which mainly depends on the type of alloys, processing states and imposed total strain amplitudes. The addition of rare earth elements Sc or Er can effectively enhance the cyclic deformation resistance and low-cycle fatigue life of the Al-Si-Cu-Mg cast aluminum alloys with different processing states. For the permanent-mold cast alloys with as-cast and solution plus aging treated states as well as the die-cast alloys, Sc has better improving effect in the cyclic deformation resistance than Er, while for the permanent-mold cast alloy with solution treated state, Er can better enhance its cyclic deformation resistance. The different processing states also have significantly effect on the low-cycle fatigue properties of the Al-Si-Cu-Mg-(Sc, Er) alloys. Compared with the permanent-mold cast alloys, both solution plus aging treatment and die cast can effectively increase the cyclic deformation resistance, while the solution treatment and solution plus aging treatment can prolong the low-cycle fatigue life. In addition, die cast can also improve the low-cycle fatigue life of the alloy at lower total strain amplitudes. For the permanent-mold cast Al-Si-Cu-Mg-(Sc, Er) alloys with as-cast and solution treated states as well as the die cast alloys, a single-slope linear relationship between elastic and plastic strain amplitudes as well as reversals to failure is noted. For the permanent-mold cast alloys with solution plus aging treated state, the relationship between elastic strain amplitude and reversals to failure is linear, while the relationship between plastic strain amplitude and reversals to failure is bilinear. At the lower total strain amplitudes, the cyclic plastic deformation mechanism of the Al-Si-Cu-Mg-(Sc, Er) alloys with different processing states exhibits the plane slip, while at the higher total strain amplitudes, the cyclic plastic deformation mechanism becomes the wavy slip.The observations on fracture surfaces reveal that under low-cycle fatigue loading condition, the fatigue cracks initiate transgranularly at the surface of fatigue samples and propagate transgranularly for the Al-Si-Cu-Mg-(Sc, Er) alloys with different processing states. In addition, the quasi-cleavage fracture characteristics can be noted for the die cast Al-Si-Cu-Mg-(Sc, Er) alloys.