| Among the commercial cast aluminum alloys, Al-Si-Cu-Mg cast alloy is extensively used mainly because of its good castability and excellent mechanical properties in the heat-treated condition. The heat treatment behavior of the alloy during heat treatment was investigated in this paper through adjusting heat treatment processing parameters, and inspecting the influence of main alloying elements and trace elements on heat treatment process, and corresponding strengthening mechanism was also carefully analyzed.It is found that double aging peaks are present in the age-hardening curves of Al-Si-Cu-Mg alloy aged above certain temperature and the first peak is higher than the second one, whereas an aging hardening plateau and single aging peak is present in age-hardening curves of Al-Si-Mg and Al-Si-Cu alloys respectively. The results of DSC analysis and TEM observation reveal that the age hardening behavior is related to the precipitation sequence of alloys.It is shown that two precipitation sequences, namely asss@GP zones and asss- Q'- Q, are mainly responsible for the precipitation hardening of Al-Si-Cu-Mg alloy. The Q' phase homogeneously precipitates in a matrix, regardless of dislocations and grain boundaries, and has excellent structural stability during aging treatment. On the contrary, ' phase preferentially precipitates on the dislocations, and structure transforms dramatically during aging treatment, which should be responsible for the double aging peak phenomenon of the Al-Si-Cu-Mg alloy. In ageing microstructure of the Al-Si-Cu-Mg alloy, the first aging peak is corresponding to the high-density GP zones (especially GP II zones), while the second one is due to the formation of metastable phases. Obvious interval during the transition from the GP zones to metastable phase in DSC curve, caused by the dissolution of GP zones and the nucleation of metastable phases on dislocation, probably is the main reason of the appearance of double aging peaks. The aging plateau of the Al-Si-Mg alloy is corresponding to the continuous transition from the GP zones to metastable phases, while single aging peak of the Al-Si-Cu alloy is caused by the formation of metastable phases.Through the morphological observation on silicon particles of Al-Si-Cu-Mg cast alloy, it is found that during solution heat treatment the evolution of eutectic silicon morphology and their effect on mechanical property can be classified into three stages. In the initial stage, necking, stubbing and fragmentation of silicon particles results in an improvement in plasticity of the alloy. In the intermediate stage, the mechanical properties of the alloy attain peak values due to the spheroidization of silicon particles. In the final stage, the drop of hardness and strength is related to the deterioration of silicon morphology. The facets and lap are occurred in siliconparticles and the coarsening process of silicon follows LSW model. On the other hand, a lot of dislocations are introduced by eutectic silicon after quench, which stimulates the formation of but suppresses the GP zone. At the same time, the excess Si atoms affect the process of aging precipitation and lead to a fine and highly dense distribution of GP zones/ phase.During ageing treatment, the addition of element Ti forms compounds such as Al3Ti, Al-Si-Cu-Ti, which have dispersion hardening effect for the alloy with other elements. The particles of a-Al15(MnFe)3Si2 are uniformly dispersed in the matrix, and can pin-up dislocations effectively.The addition of Cd and Sn remarkably increases the peak aging hardness and reduces the time to reach aging peak in the Al-Si-Cu-Mg alloy through suppressing the formation of GP zone and stimulating the formation of ' and phases. Due to the suppression of GP zone formation and the stimulation of formation, trace elements Cd and Sn cause the shortening of the interval between GP II and metastable phase. In the peak microstructure, phases coexist with phase and then cause effectively hardening of the alloy and eliminate the double aging pe...
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