The Experimental Research On Low-rare-earth High-Strength Magrnesium Alloy

Magnesium alloy is the lightest metal structural material in engineering application,with lower density,greater specific strength,good damping vibration performance,but there exist some shortcomings,such as lower strength,poor room-temperature plasticity and hot forming ability.Although the present high-rare-earth magnesium upgrade mechanical properties,the higher price of rare earth restrict its general use.Therefore,it demands prompt development of magnesium alloy which owns higher comprehensive mechanical properties and lower price.This paper replacing rare-earth with Zn,designs a new type low-rare-earth and high-strength Mg-7Gd-5Zn-0.5Ca-0.2Sr(wt.%)magnesium alloy,aiming to strengthen the alloy through precipitated phase which is formed by the process of heat treatment.It studies the mechanical properties and microstructure under the casting state,the homogenization annealing state and the aging state through chemical component tests,mechanical properties and microstructural analysis.It studies high temperature mechanical behavior and the microstructural evolution through hot compress test under the condition of different temperatures and strain rate,and drafts thermoplasticity constitutive equation and hot processing map,laying the foundation of engineering application.Main conclusion as follows:(1)It demonstrates that the central structure be next to balanced state because the boundary of the casting state alloy appears the feature of branch-like crystal and the central microstructures are equiaxed Mg grain.The main component of the phase of alloy is a-Mg matrix and MgZn2 eutectic structure.The alloy tensile strength is 85 MPa,yield strength is 84 MPa.fracture strain is 1.1%.(2)The best homogenization annealing process of the alloy is at 480 ?/6h/air cooling.Casting alloy microstructure take places many apparent changes after being handled by homogenization annealing,precipitating W-phase(Mg3Zn3Gd2).Its mechanical property is obviously superior to casting alloy's,whose average tensile strength is 141.5 MPa,mean yield strength is 102.5 MPa,and mean fracture strain is 2.45%.(3)When the deformation temperature is 573?673 K,strain rate of 0.001?1 s-1,the temperature is higher at the same strain rate,and the lower the flow stress of the alloy is,the dynamic recrystallization is more likely to cccur.At the same temperature,the higher the strain rate is,the higher the flow stress is.The sample fracture during lower temperature and higher strain rate.It is shown that the alloy has obvious strain rate and deformation temperature sensitivity during the process of the thermal compression.The plastic deformation activation energy of the alloy is 395.39 KJ/mol through fitting.It can be seen from the thermoplastic deformation processing map of that the alloys in the range of 606?723K temperature,0.001?0.002 s-1 strain rate have excellent thermal processing properties.The microstructure is significantly refined after thermal compression.(4)The best aging process of the alloy is 200 0C/40 h/air cooling,alloy hardness after aging treatment is increased significantly,but the tensile mechanical properties have no obvious improvement,showing that the alloy can be put into engineering application after homogenizing annealing treatment.