Magnesium alloys are attractive for lightweight structural applications in the transportation industry because of their low density and high specific strength and stiffness . AZ31 magnesium alloy is one of the important magnesium alloys as structural materials and offers good mechanical properties. However, the symmetry of the hexagonal close-packed crystal structure has the limited number of independent slip systems, resulting in poor formability and ductility near room temperature. Unfortunately, the alloy exhibits very limited ductility accompanied by brittle-like behavior at room temperature. This is one of the main limitations for expanding the application of magnesium alloys. Consequently, it is necessary to improve the formability of magnesium alloy. Recently, improved ductility and formability can be achieved by refining and homogenizing the grain structure. FSP, which is a novel grain refinement technique for light metals, has the potential to become an effective tool for microstructural modification of sheet metals. Using FSP, a cast microstructure with coarse grain size was refined to equiaxial fine grain through dynamic recrystallization; second phase particles were finely dispersed b FSP. Moreover, FSP is effective to eliminate cast defects such as microshrinkages or porosities.
The material for this study was commercial AZ31 Mg alloy with the chemical composition Mg–3.1Al– 1.3Zn–0.5Mn (in wt.%). This material was cast using conventional casting technique. A mixture of Argon and carbon dioxide with ratio 2:1 was introduced as a protective gas during melting. The obtained alloy was homogenized. After homogenization, the metal sheet with 5 mm thickness was subjected to stir friction process using a cylindrical tool without thread with 14 mm shoulder diameter, 6 mm pin diameter and 3 mm pin length with rotational speed of 885 rpm, transverse speed of 2.5 mm/min. during friction stirring, directional cooling by liquid nitrogen was carried out. This process was applied in a double pass mode of 100% overlap. The pin was rotated clockwise in the first pass and counter clockwise in the return pass. Microstructure examination using optical and scanning electron microscopes was carried out. Also, mechanical properties through HV hardness and tensile data for both AZ31 cast and stir friction alloys were also calculated.
The results revealed that, very fine microstructure through dynamic recrystallization obtained in the present study, figures 1,2, has clearer grain boundaries and more uniform very fine grain sizes of about 7 ?m without abnormal local grain growth. Such fine grains appear to be fully recrystallized and do not belong to subgrain structures with a ‘‘diffused’’ boundary nature. Also, fine precipitates homogenously distributed are also recognized. On the other hand, the hardness, yield and ultimate tensile strength are increased as a result of fine structure, figure 3. Since there is no twin observed in such fine grains, the hardening is postulated to be a result of the fine grain microstructure plus the retained matrix dislocations. So, the ductility, also, increased.
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