Solution Behavior of Spray-Formed Hypereutectic AlSiCuMg Alloy

초록

Al-Si multicomponent alloys are commonly used in automotive and aerospace fields owing to their excellent castability, good wear resistance, and low coefficient of thermal expansion. After adding Cu and Mg followed by appropriate heating, Al2Cu, AlCuSiMg, or Mg2Si phases precipitate in an alpha-Al matrix. Hypereutectic AlSiCuMg alloys have been used in wear-resistant products, such as engine cylinders, pistons, and air-conditioning rotors, owing to the hardening effect of Si particles and the solid solution strengthening and precipitation strengthening effects of Cu and Mg. The evolution behaviors of secondary phases and the grains of the spray-formed Al25Si4Cu1Mg (mass fraction, %) alloy were examined via OM, XRD, SEM + EBSD, TEM, hardness tests, and phase diagram calculations. The results showed that the hot extrusion microstructure of spray-formed hypereutectic AlSi alloys comprises equiaxed alpha-Al, proeutectic Si, eutectic Si, eutectic AlCuSiMg, a eutectic Al2Cu phase, and a low volume fraction Fe-bearing phase at the micron level but without a lamellar morphology. alpha-Al contained both micro-nano and nano-sized Al2Cu phases precipitates. Over the temperature range of 475-495 degrees C, the precipitated Al2Cu phase and some of the eutectic Al2Cu phase redissolved, and the residual Al2Cu phase concentrated to a Fe-bearing phase and coarsened. The volume fraction and size of the AlCuSiMg phase increased. However, when solution-treated at less than 515 degrees C, the volume fraction of the AlCuSiMg phase began to decrease, and no overbuming structure was observed. When the solution temperature exceeded 515 degrees C, the incipient melting of the nonequilibrium eutectic phase increased with increasing solution temperature. The main characteristics of the overburning structure were the network eutectic and grain boundary broadening. The hardness of the spray-formed Al25Si4Cu1Mg alloy was dependent on five factors: the solid solubility of solute atoms, the volume fraction of the residual second phase, the grain size of alpha-Al, the scale of the Si phase, and the incipient melting of the nonequilibrium eutectic phase.

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