Key Engineering Materials Vol. 921

Abstract: Mechanical strengths and electrical conductivity are the very important engineering properties of lightweight aluminum (Al) alloys used in automobiles, especially for battery-powered electric vehicles (BEV). However, the main issue is that the mechanical properties and the electrical conductivity of Al alloys are mutually exclusive. This study aims to simultaneously improve both the tensile properties and the electrical conductivity of the squeeze as-cast Al-6wt% Si-3wt% Cu by modifying its microstructure with the addition of nickel (Ni) and strontium (Sr). In comparison to those of the alloy free of Sr and Ni, the additions of 0.03 wt.% Sr and 0.5 wt.% Ni in the Al-6Si-3Cu alloy significantly improved the ultimate tensile strength, yield strength and electrical conductivity. This was because the addition of Ni element, as a transition element, collaborated with Cu to form fine intermetallic Al-Cu-Ni phases for dispersion strengthening. Also, the modification of the Si morphology from micron needles to nanoparticles by the Sr addition enhanced both the strengths and electrical conductivity of the developed alloy.

Abstract: Fatigue crack propagation is closely associated to the chemical composition of Al-Zn-Mg-Cu alloys. In this work, two Al-Zn-Mg-Cu alloys with a variation of zinc content was investigated and multiple aging treatments were exerted on them and a same regime was selected for further fatigue analysis by tensile property tests. The corresponding precipitation characteristics and fracture surface were observed. The results showed that the alloy with lower zinc content (LZ alloy) possessed an inferior strength value compared with the alloy with higher zinc content (HZ alloy) under the same three stage aging treatments while the elongation had no obvious difference. In contrast, the LZ alloy had a higher fatigue crack propagation rate than the HZ alloy. The observation of fracture surface also proved it. The precipitation observation demonstrated that both had GP zones and η' phase, which possessed the majority. Quantitative analysis of precipitates exhibited that the HZ alloy had a larger proportion of large size precipitates than the LZ alloy. The mechanism of the interaction between dislocation and precipitate was employed to elaborate the difference.

Abstract: In the actual industrial production process, it is usually appropriate to reduce the cooling rate and control the residual stress. In this study, the Time-Temperature-Property curve of Al-Mg-Si alloy sheets was measured by interruption quenching and subsequent artificial aging method. The microstructure evolution of Al-Mg-Si alloy was carefully characterized using optical microscopy (OM) and transmission electron microscopy (TEM). It was found that the nose temperature of the TTP curve drawn by experiment was ~360°C, closing to the nose temperature of ~365°C obtained from the simulated TTT curves. The number of equilibrium phase rapidly increased with the increasing of holding time, while no obvious equilibrium phase formation at the low temperature region and high temperature region. The critical cooling rate is 14.3°C/s, the determination of the critical cooling rate has important reference value for the control of alloy sheet during quenching process in the actual industrial production. The quenching sensitive region of the Al-Mg-Si alloy sheet is between 290°C and 440°C.

Abstract: The effect of the inhomogeneity of the material properties on the quenching residual stress in the aluminum alloy was studied by using the finite element method simulation and the evolution of the quenching residual stress during the pre-stretching process was discussed in this paper. The results show that both the x-component and y-component of the quenching residual stresses in the non-uniform model are nearly overlapped with that in the uniform model. The effect of the inhomogeneity of the material properties on the quenching residual stress is negligible. After pre-stretching, the surface stress of the non-uniform model decreases from the middle line to the edge while the center stress increases. Contrastingly, both the surface and center stress of the uniform model are nearly stable along the same path, and the stress magnitudes are lower than that in the non-uniform model. The inhomogeneity of material properties mainly affects the pre-stretching residual stress distribution, while it has little effect on the quenching residual stress distribution.

Abstract: Laser selective melting (SLM) is a manufacturing process that uses laser to melt metal powder layer by layer to form parts, which can realize the integrated manufacturing of complex parts. In this paper, SLM technology and topology optimization technology are combined to carry out the integrated design of the aviation aluminum alloy support . After optimization, the weight of the support is reduced by 24%, the maximum displacement is reduced by 82%, and the maximum stress is reduced by 65%. The process simulation analysis of the whole SLM forming process is carried out by using Simufact Additive software. On this basis, AlSi10Mg powder is selected for laser selective melting of the support, The forming of the whole structure meets the expectation, is in good agreement with the process simulation results, and there is no visible cracking failure.

Abstract: The deformation parameters of aluminum alloys during thermal deformation have a significant impact on the alloy's properties. The industrial free forging of the Al-Zn-Mg-Cu alloy was carried out at deformation rates of 10 mm/s and 20 mm/s, respectively, at a deformation temperature of 430°C and a deformation degree of 60% in this study. The microstructure was determined using EBSD, and the mechanical characteristics were examined. According to EBSD observations, the recrystallization fraction of the alloy is nearly identical under both deformation rates; however, the average grain size of the alloy with a deformation rate of 10 mm/s is 10.8 μm larger than that of the alloy with a deformation rate of 20 mm/s. As the deformation rate increased from 10 mm/s to 20 mm/s, the alloy's yield strength and fracture toughness increased. The resistance to fatigue crack propagation, on the other hand, displayed the reverse pattern. That is, the alloy with a 20 mm/s deformation rate had a higher FCP rate than the alloy with a 10 mm/s deformation rate. In summary, the influence of deformation rate on the microstructure and mechanical properties of a high alloy Al-Zn-Mg-Cu alloy was investigated.

Abstract: In order to analyze the effect of insoluble Fe-rich phases on the fracture toughness of high-strength Al-Zn-Mg-Cu alloys, plane-strain fracture toughness test, scanning electron microscopy, finite element analysis and quantitative modeling were used to investigate the fracture toughness of three commercial high-strength Al-Zn-Mg-Cu alloys. The results show that the fracture mode of the three experimental alloys is strongly related to the Fe-rich phases, which are prone to initiate intergranular fracture of the experimental alloys. Finite element analysis proved that the larger the shape factor of the Fe-rich phases, the more sensitive it is to external stress and more prone to secondary cracking, which is consistent with the fracture morphology observation. Quantitative analysis shows that the size, volume fraction and morphology of the Fe-rich phases have a greater influence on the fracture toughness of the high-strength Al-Zn-Mg-Cu alloy. The fracture toughness of the alloy will be improved when the shape factor of the Fe-rich phases is small and close to circular and the volume fraction of the Fe-rich phases is small. The fracture toughness value of the Al-Zn-Mg-Cu alloys can be better predicted when the deviation of the shape and distribution of the Fe-rich phases is considered.

Abstract: The microstructures of as-cast and homogenized Al-Cu-Mg-Ag alloys with 1.98, 3.66 and 5.16 wt.% Cu contents (Alloy 1, Alloy 2, Alloy 3, respectively) were investigated by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and calculation of phase diagram. The results indicate that the second phases in the as-cast alloys consist of θ (Al₂Cu), ternary α (Al)-θ (Al₂Cu)-S(Al₂CuMg) eutectic and Fe-enriched phase. The invariant reactions are L→α (Al) + θ (Al₂Cu) at 542°C and L→α (Al) + θ (Al₂Cu) + S(Al₂CuMg) at 505°C during solidification. An increase of Cu content promotes the formation of θ (Al₂Cu) phase, therefore leading to the total second phases increasing in the as-cast alloys. The ternary eutectic completely dissolves into the matrix after the first-step homogenization at 490°C, and θ (Al₂Cu) phase dissolves subsequently after the second-step homogenization at 510°C. It is suggested that the proper homogenization treatments are 490°C/24 h for Alloy 1, 490°C/24 h + 510°C/24 h for Alloy 2 and 490°C/24 h + 510°C/48 h for Alloy 3.

Abstract: The effect of Zn/Mg ratio on the as-cast microstructure and its evolution during homogenization of Al-Zn-Mg-Cu alloys was investigated by optical microscopy (OM), differential scanning calorimetry (DSC), scanning electron microscope (SEM) and X-ray diffraction (XRD). Experimental results showed that serious dendritic segregation existed in the as-cast microstructures while the second phases were mainly AlZnMgCu phase and Al₂Cu phase. With the Zn/Mg ratio increasing from 1.5 to 2.0, the area fraction of AlZnMgCu phase decreased from 2.85% to 2.53%, which was attributed to the content of Mg element. Non-equilibrium eutectic phases dissolved into the matrix during homogenization and phase transformation from AlZnMgCu phase to Al₂CuMg phase (S phase) was observed in low-Zn/Mg ratio alloy and mid-Zn/Mg ratio alloy. In the high Zn/Mg ratio alloy, the eutectic AlZnMgCu phase directly dissolved into the matrix during the homogenization, and no transformation from AlZnMgCu phase to S phase was found. A higher number of S phases appeared in low-Zn/Mg ratio alloy during homogenization treatment compared with mid-Zn/Mg ratio alloy with a regime of 465°C/24h. It could be inferred that low-Zn/Mg ratio alloys had a stronger phase transformation tendency from AlZnMgCu phase to S phase. Increasing the homogenization treatment temperature could impair the transition tendency from AlZnMgCu phase to S phase.