Investigation On Microstructure And Properties Regulation Of Centrifugal Casting Mg-Gd-Y-Zn-Zr Alloy

Magnesium alloys have attracted extensive attention due to the remarkable growth of the demand for lightweight structural materials.Mg-RE-Zn alloys are one kind of high-strength magnesium alloys with Long-period Stacking ordered structure（LPSO structure）.This series of alloys can not only be strengthened and toughened by LPSO phase,but also have excellent aging strengthening effect.After aging treatment,the ultimate tensile strength of wrought Mg-RE-Zn alloys reaches more than 500MPa.Therefore,this series of alloys have a wide application prospect in aerospace,military industry and transportation industry.Under the condition of gravity casting,the grain of Mg-RE-Zn alloy is coarse and the chemical composition is non-homogeneous,and the precipitates present a coarse network structure,which deteriorates the mechanical properties and plastic processing ability of the alloy.It is not conducive to the excavation of its potential advantages.Centrifugal casting is one of the most direct and effective casting methods to improve the mechanical properties of metal materials,which has been widely used in practical production.The centrifugal force field,fluid field and periodic vibration can greatly regulate the microstructure and second phase,so as to prepare the casting alloy with high metallurgical cleanliness and uniform microstructure.The use of centrifugal casting is beneficial to the improvement of the extrudability of the alloy,so that high-strength rare-earth magnesium alloys can be prepared under high-speed extrusion.In addition,magnesium alloy rings can be easily prepared by centrifugal casting.On this basis,the rolling of centrifugal alloy is used to prepare high-strength rolled Mg-Gd-Y-Zn-Zr alloy,which lays a foundation for the subsequent preparation of high-strength magnesium alloy rings by ring rolling.In this paper,the Mg-8.5Gd-4Y-1Zn-0.4Zr（wt.%）alloy is used for research.The evolution of microstructure and mechanical properties of centrifugal cast alloys under deformation and aging conditions are studied.Meanwhile,high-strength rare-earth magnesium alloys were prepared.The strengthening mechanism of high-strength Mg-Gd-Y-Zn-Zr alloy is quantitatively analyzed.The main work and research results are as follows:（1）The effect of centrifugal rotation speed on the microstructure and mechanical properties of the alloy is studied.The experimental results are analyzed and the best centrifugal rotation speed was 900 r/min.On this basis,the optimal homogenization treatment is 510℃/12h.After homogenization treatment,the Mg3RE eutectic phase in the centrifugal cast alloy was dissolved,and the LPSO phase and rare earth rich phase are precipitated.Compared with gravity cast alloy,centrifugal cast alloy has finer grain size and more aging precipitates after peak aging,which has higher comprehensive mechanical properties.High magnesium alloy casting rings with UTS and YS reaching378MPa and 338MPa respectively can be prepared through centrifugal casting+T6treatment.（2）The effects of different extrusion speed,casting method of initial billet,extrusion ratio and extrusion temperature on the microstructure and mechanical properties of Mg-Gd-Y-Zn-Zr alloy are studied.High strength and toughness Mg-Gd-Y-Zn-Zr extruded alloy bars can be efficiently prepared by extruding the homogenized centrifugal cast alloy at the temperature of 450℃ with the extrusion ratio of 16 and extrusion speed of 8mm/s.The ultimate tensile strength,yield strength and elongation of extruded alloy are 379MPa,322MPa and 10.6%respectively.On this basis,super-strength magnesium alloy rods with UTS and YS of 516MPa and 437MPa can be prepared by peak aging treatment of extruded alloys.（3）The effects of different rolling temperatures and accumulative rolling reductions on the microstructure and mechanical properties of Mg-Gd-Y-Zn-Zr alloys are investigated.High strength and toughness Mg-Gd-Y-Zn-Zr rolled alloy can be prepared by rolling the homogenized centrifugal cast alloy at the temperature of 450℃ with the rolling reduction of 80%.The ultimate tensile strength,yield strength and elongation of rolled alloy are 390MPa,330MPa and 12.2%.On this basis,super-strength magnesium alloy with UTS and YS of 511 MPa and 435 MPa can be prepared by peak aging treatment of rolled alloys.（4）Five kinds of strengthening mechanisms exist in the peak aged alloy.Their contribution to the improvement of the yield strength of the alloy is ranked as follows:β’phase precipitation strengthening（29.7%）>DRX fine grain strengthening（20%）>solid solution strengthening（18.8%）>LPSO phase strengthening（11.1%）>γ’phase precipitation strengthening（8.5%）>layer misalignment strengthening（3.6%）>Mg₅RE phase dispersion strengthening（2.7%）.