| In this paper,it was based on ZMT613 magnesium alloy as the experimental alloy,which was treated by h omogenization,extrusion,solution,Single aging and Double aging.The effect of different heat treatment techniques on microstructure and properties of ZMT613 wrought magnesium alloys were studied systematically by u sing microstructure observation,scanning electron microscopy,DSC,EDS,hardness test and tensile test.The homogeneous annealing and solution treatment techniques of the alloy were optimized by observing and analyzing the microstructure of as-homogenized and as-solutionized alloy.The changes of the microstructure and properties of the alloy treated by Direct Single aging,"Solution + Single aging" and "Solution + Double aging" were compared and analyzed.The as-cast ZMT613 alloy consists of coarse dendrites and intergranular compounds.The second phases present in the as-cast ZMT613 alloy were ?-Mn,Mg7Zn3 and Mg2 Sn.Mg7Zn3 and Mg2 Sn phases were mainly distributed in interdendritic and ?-Mn mainly distributed in the dendrites.After the Double homogenization annealing,the dendrite segregation has been eliminated.Especially after homogenization annealing at 330??15h+430??4h,the coarse dendrites changed into small equiaxed crystals.Coarse Mg7Zn3 phase decomposed into the matrix and the dendrite segregation was eliminated.There were only ?-Mn in the alloy,which is difficult to dissolve into the matrix.The alloy can be extruded at 360 ?,and the surface of the extruded bar was bright and free from cracking.Dynamic recrystallization occurred and small recrystallized grains were formed in the alloy during extrusion process.The second phases present in the as-extruded ZMT613 alloy were ?-Mn,MgZn2 and Mg2 Sn.The solution temperature and time of ZMT613 alloy were optimized.It was found that the grain size of the alloy after 420?×2h solution treatment was uniform.The MgZn2 phase in as-extruded alloy was completely decomposed into the matrix,leaving only a small amount of fine ?-Mn and Mg2 Sn phase which were difficult to dissolve into the matrix.The solid solution is ideal.The 420?×2h was the best process for the alloy.The microstructure in the alloys after Direct Single aging and "Solution + Single aging" treatment was significantly different.Mg2 Sn and MgZn2 phase gradually precipitated in the alloy during Direct Single aging and "Solution + Single aging" treatment.However,the second phase precipitated in the alloy treated by Direct Single aging was distributed unevenly,and the second phase precipitated in the alloy treated by "Solution + Single aging" was evenly distributed.The mechanical properties of the two kinds of alloy were obtained by the testing of the hardness and tensile properties of the alloys.The hardness of the alloy after Direct Single aging treatment was 82.2HV,the yield strength,tensile strength and elongation are 277 M Pa,335 M Pa and 10.7%,respectively.The hardness of the alloy after "Solution + Single aging" treatment was 90.8HV,the yield strength,tensile strength and elongation are 302 MPa,351 MPa and 7.8%,respectively.By contrast,it was found that the comprehensive mechanical properties of the alloy after "Solution + Single aging" treatment were better than those after Direct Single aging treatment,which is due to the uneven distribution of the second phase precipitated in the alloy after Direct Single aging treatment,weakening the effect of age hardening.If the solution treatment was carried out before the aging process,the second phase can be evenly distributed in the process of precipitation,the effect of age hardening was better.Therefore,the solution treatment of ZMT613 alloy before the aging process was necessary.The second phase of the ZMT613 alloy after Double aging treatment were ?-Mn,Mg2 Sn and MgZn.The hardness of the alloy after 90?×24 h+180?×7h Double aging treatment was 94.9HV,the yield strength,tensile strength and elongation were 333MPa?386MPa and7.6%,respectively.The mechanical properties of the alloy were the best.The hardness of the alloy after 110?×6h+180?×6h Double aging treatment was 92.8HV,the yield strength,tensile strength and elongation were 323MPa?371MPa and 7.5%,respectively.The mechanical properties of the alloy decreased,but the aging time was reduced significantly.The dispersion of G.P.zones formed during different pre-aging processes was different,resulting in the difference in the dispersion of MgZn2 phase.The MgZn2 phase precipitated in the alloy after 90?×24 h+180?×7h Double aging treatment was more dispersed,so the alloy had the best mechanical properties. |
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