Li Zhihui

Microstructure Evolution of a High Zinc Containing Al-Zn-Mg-Cu Alloy during Homogenization

Abstract The microstructural evolution of a high Zn-containing Al-Zn-Mg-Cu alloy during homogenization was investigated by optical microscopy, differential scanning calorimetry, scanning electron microscope and X-ray diffraction. A homogenization kinetic equation derived from a diffusion kinetic model was established to confirm the optimum homogenization parameter. Results indicate that severe segregation exists in the as-cast alloy. The non-equilibrium eutectics consist of α(Al), Mg(Zn, Cu, Al)2, S(Al2CuMg), θ(Al2Cu) and Fe-enriched phases. In the present work, no transformation from Mg(Zn, Cu, Al)2 to S(Al2CuMg) phases occurs during homogenization and Mg(Zn, Cu, Al)2 phases directly dissolve into the matrix. θ(Al2Cu) phases dissolve into the matrix while Fe-enriched phases still exist after homogenization. In addition, the contents of Zn and Mg elements in Fe-enriched phases are reduced or even disappear by prolonging the holding time of homogenization. The proper homogenization parameter is 440 °C/12 h + 468 °C/24 h, which is in consistent with the results of homogenization kinetic analysis.

Distribution and Evolution of Porous Defects in Spray Formed 7XXX Aluminum Alloy

Abstract Al-10.8Zn-2.8Mg-1.9Cu-0.16Zr alloy was manufactured by the spray forming technique. Optical microscopy (OM) and field emission scanning electron microscopy (FESEM) were used to investigate microstructures of the spray-formed alloy. Morphology characteristics of all kinds of pores in as-deposited billet were summarized, and their formation mechanism in the alloy was studied. The influences of hot isostatic pressure treatment (HIP) and homogenizing treatment on porous defects in spray formed 7XXX aluminum alloy were studied. The results indicate that the porous defects in the spray-formed alloy are interstices, interlinked pores and gas pores. The distribution and types of porous defects are related with positions of the preform. The main defects in the center of the alloy perform are interstices and gas pores. More interconnected pores are located at the edge of as-deposited billet. The walls inside gas pores are smooth and concave, and the morphology of gas pores remains nearly spherical. Other porous defects show irregular geographies. HIP treatment can get rid of porous defects except for gas pores. Gas pores reappear after heat treatment because they are close pores actually.

Microstructure and mechanical properties of an electron beam welds in a spray-deposited Al-Zn-Mg-Cu alloy

In this study, an electron beam welds produced in a spray-deposited Al-8.6Zn-2.6Mg-2.2Cu (wt,%) alloy were characterized by optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM) and tensile tests. It is found that the joint of the alloy contained three distinctive regions, i.e. fusion zone, heat affected zone and base metal region. Tensile properties of the joints were obtained by testing flat transverse tensile specimens, and the results indicated that tensile strength of these welds approached 82.3~85.3% of the base metal.