Investigation On Microstructure And Mechanical Properties Of High-Al Content Magnesium Alloy Sheets Produced By Twin Roll Casting And Warm Rolling

Magnesium alloys is widely applied in lightweight structural material owing to their low density, good machinability, excellent specific strength, good electromagnetic shielding effect and recyclable. In recent years, with more and more attention for light weight in automobile and 3C industry, the demand for magnesium alloy has increased rapidly. However, due to magnesium alloy poor strength, corrosion resistance, plastic forming ability, which restricts the development of magnesium alloy. The strength and corrosion resistance can be improved significantly with adding more than 9 % Al in magnesium, but the plasticity decreases remarkably. In order to improve the ability of plastic deformation, simplify the product process, reduce production cost and improve microstructure, the deformation process by twin roll casting and sequential warm rolling was used to product high Al content magnesium alloy sheets.The twin roll cast(TRC) Mg-11.1 wt.% Al-1.1 wt.% Zn(AZ111) alloy strip with 3~4 mm thick can be produced by vertical twin-roll caster. The casting temperature 680 ℃ and roll speed 16 rpm was chose in manufacturing formation process. The ingot cast(IC) Mg-8.6 wt.% Al-0.2 wt.% Zn(AZ90) alloy strip with 5 mm thick was produced. The casting temperature was 680 ℃ and then air cooling. The TRC and IC strip by homogenizing treatment was warm rolled with multi-pass small per reduction and subsequent annealed.The present research is aimed at studying the feasibility of twin-roll casting and warm rolling for the production of new high Al content magnesium alloy sheets. The microstructural evolution and mechanical properties of twin-roll casting high Al content magnesium alloy at different homogenizing treatment time, multiple-pass warm rolling and subsequent annealing has been studied. The results of the study are as follows:(1) The surface microstructure of TRC strip is composed of dendrite crystal, crushed dendrite arm and dendrite tip, but the center microstructure is mainly composed of dendrite crystal and equiaxed grains. The dendrite arm spacing(DAS) of as-cast strip improves gradually from surface to center and the maximum difference is 1.54 μm. The maximum solidification rate different is 207.1 K/s.(2) With homogenizing treatment at 400 ℃ for 5 h, the TRC strip(AZ111) tensile strength increases by 12 %, to highest value 115 MPa, and the elongation increases by 11 %, also to highest value 2.03 %. The second phase β-Mg17Al12 eliminates and microstructure is uniform. Therefore, the optimal homogenizing annealing process is 400℃×5 h. The grain of IC strip grows and the second phase is not dissolved completely and distributed in the grain boundary with 400 ℃×12 h homogenizing treatment.(3) The TRC strips(AZ111) was warm rolled between 240 ℃ and 300 ℃ after homogenizing treatment at 400 ℃×5 h. The multi-pass small per reduction was used to manufacture the sheet and the per pass maximum reduction was 5%. When the total rolling reduction reached 90 %, the microstructure of surface and center was uniform, meanwhile, the thick of sheet was 0.35 mm.The tensile strength of twin roll cast sheets after rolling is 370 MPa, while its elongation is 1.5 %. The surface basal texture intensity of TRC sheet is larger than center in rolling process.(4) When the annealing time was less than 60 min, dynamic recrystallization did not take place at 350 ℃, but toke place at 400 ℃. The tensile strength of twin roll cast sheets after warm rolling and annealing with 400 ℃×30 min decreases from 370 MPa to 331 MPa, while its elongation increases from 1.5 % to 6 %. The suitable annealing process was 400 ℃×30 min to warm rolled twin roll cast sheets.