Research On Microstructure And Mechanical Properties Of Variable-cavity Liquid Forging Mg-Zn-Y-Zr Alloy

Due to low density,high specific strength,large reserves,magnesium alloy receives extensive attention.At present,the main forming process of magnesium alloy is die casting process,but mechanical properties of die casting parts are poor,it is difficult to meet the performance requirements of structural parts.The plastic processing can improve the mechanical property,but it is difficult to form complex shape parts because of typical hexagonal-close-packed?hcp?.Liquid forging technology based on die casting and forging technology combines the advantages of two processes,which can not only improve the the mechanical property,but also form complex shape parts.However,because of tiny plastic deformation,the mechanical property of liquid forging part is lower than forging part.In order to further strengthen the performance of the liquid forging parts,the variable-cavity has been used in the technology on the basis of liquid forging technology,which is called variable-cavity liquid-forging technology.This technology can effectively increase the plastic deformation and improve the microstructure and mechanical property.In this dissertation,the influence of Y on the microstructure and mechanical properties of liquid forming Mg-Zn-Y-Zr magnesium alloys has been researched.The experiment data indicates that Y elements can cause undercooling and refine grain.When the content of Y is 1%,the mechanical property is best—tensile strength is 247.31MPa and elongation is 15.13%.The magnesium alloy smelt-filling-forging delivery equipment has been designed and manufactured.The quantitative delivery and filling of liquid magnesium alloy are controlled by PLC program,the PLC is a part of electronic control system.Numerical simulation of lifting process and filling process of magnesium alloy are carried out.The results show that the liquid metal rises steadily during lifting process and there is no obvious gas involved.The temperature of molten metal decreases quickly and the solid metal may block the equipment.The results of numerical simulation of filling process indicate that when the molten metal fills too fast,flow cracks and gas-holes defects appear in the parts.On the other hand,when the molten metal fills slowly,the temperature of metal liquid is too low,and it is harmful to the next experiment.On the whole,the best filling speed is 20mm/s.The variable-cavity liquid forging experiment has been performed,which is basis on liquid forging process and integrated delivery mechanism.The effects of the deformation temperature and variable-cavity liquid forging process on the microstructure and mechanical property of Mg-Zn-1%Y-Zr magnesium alloy are described,based on the experimental results and analysis.The research indicates that after variable-cavity liquid forging,there are obvious deformation textures,the dendrites which formed during liquid forging process have been elongated along deformation direction and it is hard to be distinguished.There are still obvious dendritic structures along the cross section due to less deformation.The second phase consists of I phase?W phase and trace amount of Mg-Zn binary phase.Most of second phase is broken to fine particles-zone during variable-cavity forging process.There are some striated I phase remained at deformation temperature of 350?.The mechanical property is significantly affected by the morphology and distribution of precipitates.The highest tensile strength is 351.51MPa at a deformation temperature of 350°C,and the elongation of part is 3.92%.The best elongation is 7.15%at a deformation temperature of450°C.The effects of heat treatment on the microstructure and mechanical property of variable-cavity liquid forging parts at deformation temperature 350?are described,based on T4 and T6 heat treatment experimental results and analysis.After T4 solution heat treatment,the alloy clearly recrystallizes and the crystalline grain is refined.I phase dissolves completely and most of W phase resolves from thick bulk to tiny granule.After T6 heat treatment,the bulk W phase resolves completely.There are new binary phase—Mg₂Y and massive rodlike Mg-Zn phase precipitated.These tiny precipitated phases dispersed distribute in the matrix,and greatly improve the strength and plasticity.On the whole,after T6 heat treatment,the strength of alloy decreases and the elongation increases obviously.For this experiment,T6 heat treatment is suitable heat treatment method and the best parameter is 475?×3.5h+200?×48h.The variable-cavity liquid forging experiment with different contents of Y has been performed.The change rule of morphology,type and distribution of second phase and the internal relation between second phase and mechanical property are researched.With the increase of Y,the changing process of main second phase is MgZn2—I phase—W phase—LPSO phase and the volume of second phase gradually increases from 5.30%to 22.32%?vol.%?.The influence of three kinds of second phases is different during variable-cavity liquid forging process.The interface bonding between I phase and Mg matrix is fine,there is little microcrack nearby I phase after deformation.W phase usually forms net-like microstructure which usually causes stress concentration and generates microcrack.LPSO phase is plate-like shape.LPSO phase consists of the periodical stacking of close-packed planes,which is laminar.LPSO phase cracks into tiny phase during deformation.On the whole,The highest tensile strength is 351.51 MPa at a deformation temperature of 350°C,and the elongation of part is 3.92%.;When the content of Y is 4%,the tensile strength and elongation of part are all good,which are350.65MPa and 5.71%respectively.