Yuanhua Cai

Microstructural characteristics and aging response of Zn-containing Al-Mg-Si-Cu alloy

Al-Mg-Si-Cu alloys with and without Zn addition were fabricated by conventional ingot metallurgy method. The microstructures and properties were investigated using optical microscopy (OM), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), tensile test, hardness test, and electrical conductivity measurement. It is found that the as-cast Al-Mg-Si-Cu-Zn alloy is composed of coarse dendritic grains, long needle-like β/δ-AlFeSi white intermetallics, and Chinese script-like α-AlFeSi compounds. During high temperature homogenization treatment, only harmful needle-like β-AlFeSi phase undergoes fragmentation and spheroidizing at its tips, and the destructive needle-like δ-phase does not show any morphological and size changes. Phase transitions from β-AlFeSi to α-AlFeSi and from δ-AlFeSi to β-AlFeSi are also not found. Zn addition improves the aging hardening response during the former aging stage and postpones the peak-aged hardness to a long aging time. In T4 condition, Zn addition does not obviously increase the yield strength and decrease the elongation, but it markedly improves paint-bake hardening response during paint-bake cycle. The addition of 0.5wt% Zn can lead to an increment of 99 MPa in yield strength compared with the value of 69 MPa for the alloy without Zn after paint-bake cycle.

Microstructure of spray formed Al-Zn-Mg-Cu alloy with Mn addition

With the aim to improve the strength of Al-Zn-Mg-Cu alloy, the alloy billet containing Mn was produced by spray forming method, and the microstructural features were investigated using X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The results show that the billet mainly consists of fine equiaxial grains of MgZn2 and Al6Mn with size ranging from 5 μm to 25 μm. Nano-scaled MgZn2 is dispersed in the as-sprayed alloy, primary Al6Mn particles are precipitated at grain boundaries with an average size of 5 μm. A few CuAl2, Al3Zr and eutectic are also found in as-sprayed Al alloy. The volume fraction of the porosity is about 12%. DSC result indicates that most of the solutes are precipitated during spray forming process, and no obviously thermal effects occur below 450 °C. Both matrix grains and Al6Mn particles grow monotonously with the increase of annealing temperature, but the growth rate of Al6Mn particles is markedly lower than that of Al grains, and the matrix grains grow rapidly when the annealing temperature is above 375 °C.