On the microstructure and mechanical properties of Al-Mg-Si（Mn, Zr）

Aluminum alloy materials can meet the needs of lightweight, large-scale and complex structures, so their applications are second only to steel in modern industrial production. Aluminum alloys have characteristics of low density, high specific strength, good plasticity, strong conductivity for heat and electricity, excellent processing and forming performance and good corrosion resistance, thus they have been widely used in aero- space, military equipment, transportation, construction industry, metallurgical chemical industry, energy industry and other fields. 6XXX aluminum alloy is one of the most important alloys in Al-Mg-Si (6XXX) series, which is a kind of medium strength aluminum alloy and can be heat-treated by aging. It has excellent corrosion resistance, weldability and coloring property, leading to its wide use. Although 80% of aluminum extrusion products in the world are made of 6XXX aluminum alloy, yet its application is restricted for its strength. To expand the application field of 6XXX series aluminum alloy, its strength needs to be further improved.

In this work, experimental alloys were designed, with 6XXX alloys and 6XXX alloys with additional Mn and Zr as the research object. By means of inductively coupled plasma direct reading spectrometer (ICP), differential scanning calorimeter (DSC), X-ray diffractometer (XRD), scanning electron microscope (SEM) with energy spectrum analysis (EDS), etc., properties such as hardness, tensile property, corrosion property and microstructure of the alloy were characterized. The effects of both addition of Mn and Zr and different casting solidification methods on mechanical properties and microstructure of the alloy were studied systematically. The mechanism of influence of aging precipitation and rolling deformation on material strength was studied. By XRD analysis, the lattice constants of Al and the solid solubility of main alloy elements in 6XXX alloys were calculated, and the apparent activation energies of β" in 6XXX alloy were calculated. In the meantime, their kinetic equations were determined.

By studying the microstructure and solution calculation of the original casting state, the effects of both addition of Mn and Zr and casting in water-cooling copper mold on the microstructure and mechanical properties of the alloy was analyzed. Casting in water-cooling copper mold reduces the segregation of elements and increases the solubility of the main alloy elements: the solubility of Mg, Si, Mn is increased by 7%~12%, 10%~14%, 10% respectively. At the same time, by way of EDS energy spectrum analysis and other means, the precipitations of the original cast alloy was determined.

For the sake of eliminating the inhomogeneity and nonequilibrium microstructure in the alloy, the as-cast alloy was treated by homogenization. The overburning temperature of the alloy was determined by DSC analysis, and the heat treatment method of the alloys was determined to be 560℃×3h according to metallographic observation. After homogenization treatment, the precipitation phase of the alloy was changed. The dispersion distribution of the crystalline phase improved the mechanical properties of the alloy. The solution degree of Mg in homogenized alloy is increased by about 30% than that in as-cast alloy, together with the solution degree of Mn and Si increasing by more than 50%, which explained the effect of homogenization treatment theoretically. At the same time, the effects of homogenization and casting in copper water-cooled mould casting on the mechanical properties of the alloy were analyzed by tensile test. The results show that the mechanical properties of homogenized alloy are better than that of as-cast alloy, and that of alloy cast in copper water-cooled mould casting is better than that of alloy cast in iron mould. However, 6XXX-xMn-yZr alloy homogenized and cast in water-cooling copper mold has the best mechanical properties with tensile strength of 286MPa, yield strength of 127MPa and elongation of 18%.

The main strengthening phase of the alloy is β" phase, whose precipitate activation energy is calculated. At the same time, the aging kinetics equations of the alloys were fitted, which provides theoretical basis for further study of aging precipitation behavior.

The tensile strength, yield strength and elongation of 6XXX-xMn-yZr alloy cast by water-cooling copper mould after aging are 356MPa, 230MPa, 21%, respectively. The tensile strength, yield strength and elongation of 6XXX-xMn-yZr alloy cast by copper water-cooled mould casting after rolling are 363MPa, 217MPa and 9%, respectively. Both methods improved the mechanical properties of the alloy and greatly improved the strength of it.

The corrosion properties of rolled 6XXX and 6XXX-xMn-yZr alloys in 3.5% (mass fraction) NaCl solution were studied. The results of immersion weightlessness test show that the corrosion rate of water-cooling copper mold casting alloy is lower than that of iron mould casting alloy, and the corrosion resistance of 6XXX alloy added with Mn, Zr is better than that of 6XXX alloy.

College of Applied Chemistry Shen Hua