A New Method For Gas Modification Of Mg-Al Magnesium Alloy

A novel modification method of CO2 and CO gas by virtue of process automation and adjustable modification parameters for the aerospace casting magnesium alloy was proposed in this study,based on the issues in traditional metamorphic methods for the casting magnesium alloy used in aerospace,such as manual control,heavily polluted magnesium melt,terrible working environment,the poor reproducibility of the modification effect,and the failure to achieve continuous online modification.The AM60 B casting magnesium alloy was used in this article as experimental material,the modification experiments were conducted by following the normal production process.The modification effect of modificators such as MgCO3,CO2 and CO with same dosage and process,as well as the recession of this effect with increasing holding time were investigated by the optical microscope(OM),scanning electron microscopy(SEM),energy dispersive spectrometer(EDS),X ray diffractometer(XRD),tensile testing machine and hardness tester.Finally,the mechanism of these modifiers was analyzed by calculating the thermodynamic of modification reaction and Heterogeneous nucleation theory.The main conclusions are as follows:(1)The gas modifier can be modified and operated conveniently.The CO2 had the best modification effect on the AM60 B magnesium alloy after modification treatment at 760 oC and standing for 10 min,then the CO followed and traditional MgCO3 was the worst.The grain size of the AM60 B magnesium alloy decreased from 212.9 ?m to 90.7 ?m(CO2),104.4 ?m(CO)and 112.8 ?m(MgCO3)respectively.(2)The hardness of AM60 B alloy was increased from 56.6 HV to 63.1 HV,61.3 HV and 63.6HV respectively after treated by MgCO3,CO2 and CO.It could be inferred that these three kinds of modifiers have little effect on it.The tensile strength of AM60 B alloy were 145 MPa,179 MPa,190 MPa and 183 MPa,respectively,before and after treated by MgCO3,CO2 and CO.And the yield strength were 85 MPa,90 MPa,102 MPa and 95 MPa,respectively.(3)The macro fracture model of AM60 B after modification treatment by MgCO3 was brittle fracture.The micro fracture showed a lot of cleavage planes and a few dimples,then it could be concluded that the micro fracture was dominated by cleavage fracture.The macro fracture model after modification treatment by CO2 showed obvious plastic deformation area,and the number of dimples in the micro fracture surface was increased.It was shown that the micro fracture model of it were dimple type and cleavage fracture.The macro fracture model after modification treatment by CO was brittle fracture,and the microstructure of it was mostly in the dissociation plane.So it was mainly characterized by cleavage fracture.(4)With the standing time prolonging,the modification effect of AM60 B after treated by MgCO3 had the worst recession,the grain began to grow after standing 10 min.The modification effect of AM60 B after treated by CO2 had the better recession,the grain began to grow slowly after standing 30 min.The modification effect decline most slowly after treated by CO,and grain growth was not obvious until stand for 60 min.(5)The phases of AM60 B magnesium alloy without any modification treatment were composed by ?-Mg and Mg17Al12.Al4C3 was found after modification treatment by CO2 and MgCO3.There was no Al4C3 but Al2 CO in AM60 B magnesium alloy.The refining mechanism of the three metamorphic methods is mainly considered as Al4C3 heterogeneous nucleation theory.