Study On Process And Property Of Low-pressure Die Casting Magnesium Alloy Steering Knuckle

Automobile lightweight is one of the important development paths in the field of automobile industry.Relying on its superior properties,rare earth(RE)magnesium alloys have become the focus of research in the development of automobile lightweight and have been gradually applied in production practice.Steering knuckle is one of the most critical automotive structural components.At present,no magnesium alloy can be used as its manufacturing material,so it needs further investigation.In this paper,the material selection,low pressure casting process optimization,heat treatment process optimization and steering knuckle performance of magnesium alloy automobile steering knuckle were systematically studied.The purpose is to achieve the application of Mg-GdY-Zr alloy in automotive steering knuckle.And we hope the comprehensive and systematic research process can provide theoretical and practical guidance for the follow-up application of magnesium alloys.Firstly,the properties of commercial aluminium alloys currently used in automotive industry are reviewed and compared with common commercial magnesium alloys.It is found that GW,GN and WE series of Mg-RE alloys have obvious advantages in mechanical properties compared with AZ,AM,AS and other common series of non-rare earth magnesium alloys.GW63 K magnesium alloy in T6 state has 198 MPa yield strength and 7.1% elongation,which is suitable for material performance requirements of automobile steering knuckles.AnyCasting software was used to simulate the low pressure die casting process.The optimum technological parameters are as follows: adding chiller to strengthen the performance of local area,peak pressure set to be 1.1 times of standard atmospheric pressure can achieve smooth,rapid and sequential filling,745? casting temperature can make the castings have ideal solidification sequence and solidification rate.Also,the grain sizes at different parts of the castings were predicted.The optimum heat treatment parameters of steering knuckle are determined,which are as follows: solid solution treatment at 500 C for 8 hours and aging treatment at 200 C for 24 hours.The yield strength and elongation of GW63 K magnesium alloy after T6 treatment are 175 MPa and 10%,which can meet the performance requirements of steering knuckle.Eutectic compounds distributed at grain boundaries can be observed in the steering knuckle at as-cast condintion.The morphology of eutectic compounds is strip-like,and the content of the second phase is small.The grain size is between 50 and 90 microns.Most of the second phase eutectic compounds have been dissolved into the magnesium matrix after T4 treatment,only a few of them have not been dissolved,and the grain size has not increased significantly.A small amount of pitted second phase can be observed in the grain under T6 state.The size of the precipitated phase is less than 1 micron,and the distribution of the precipitated phase is strip-like.The precipitated phase with nano-scale and relatively fine grain size are the main factors for the material to obtain high strength and toughness.The yield strength of all parts of the castings decreased to 130 MPa after T4 treatment,and the elongation increased to more than 10%.The mechanical properties of the castings after T6 treatment were obviously better than those of the as-cast ones.The yield strength was more than 170 MPa and the elongation was more than 5%.The main reason for the low elongation at center of castings is that the grain size is larger(>90 um)due to the slow solidification rate in this area during the actual casting process.By comparing the measured and predicted grain sizes of 12 parts of the castings,the deviations of 6 parts are less than 5%,5 parts are 5-10%,and only one part is more than 20%.The predicted and measured values are in good agreement.It proves that the optimization of low pressure die casting process by simulation has excellent guiding significance.The steering knuckle has passed the braking fatigue test,emergency braking fatigue test and bending fatigue test.The steering knuckle fails in the J-turn bending test which exerts the greatest stress.Comparing the mechanical properties of the steering knuckles sampled from different parts,the crack location belongs to the weaker part of the key stress parts of the steering knuckles,so the failure occurs.