Relationship Between The 3D Characteristics Of The Pore And The Mechanical Properties Of The Rear Trailing Arm Of The Automobile

Die casting is one of the most widely used forming methods in the automotive industry.The rear trailing arm is also an important part of the suspension system.The force requirement for the rear trailing arm is very strictly,so it is particularly important to study its mechanical properties.Based on the results of X-ray computerized tomography and three-dimensional reconstruction,the volume statistics of the pores in die castings under different high pressure injection velocities were carried out,the three-dimensional morphology characteristics and formation mechanism of the pores were studied,the size distribution of the pores was studied,and the morphology of the pores on the fracture surface was observed by scanning electron microscopy.The results show that the total number of pores in group P(1.5m/s)is significantly lower than that in group V(2.0m/s).The distribution of pores in group V is more dense,and there is little difference between the number of pores in group P and group V in the volume larger than 0.01mm~3.The equivalent spherical diameter distribution of P9 and V4 samples(larger than 0.01mm~3)conforms to the lognormal distribution of three parameters.The threshold values of P9 and V4 are 0.31533mm and 0.31076mm,respectively.This indicates that there are more small pores in sample V4.The individual gas pores look rounder and smaller with smooth surface.Sphericity of most gas pores obtained in our dataset were>0.58.On the other side,the shrinkage pores are larger with irregular shape.the sphericity of shrinkage pores was<0.40.The size of this gas-shrinkage pore is usually larger than that of the gas pore,which is composed of a relatively round pore body and a convex connection to some shrinkage.Thus,sphericity value of gas-shrinkage pores was 0.40-0.58.SEM analysis was performed after the tensile test,and typical SEM fractograph of a tensile fracture surface shows,the pre-existing shrinkage pore can be detected due to presence of unfractured dendrites below the pore.Thus,the regions of the fracture surface that contain intact dendrites are essentially the areas occupied by the shrinkage pore.Gas pore can be easily identified because it is very round.The gas-shrinkage pore is composed of a gas pore and a long-tail shaped shrinkage pore.The inner surface of the gas pores contain a smooth oxide film.The effect of pores on mechanical properties is also studied in this paper.Weibull distribution was used to analyze the stability of mechanical properties of specimens under different high pressure injection velocities,and the relationship between area fraction of porosity or bulk porosity with mechanical properties was analyzed.The maximum tensile strength and elongation of samples at different high pressure injection velocities were predicted by power function model.The results showed that the average tensile strength and elongation of group P were199.01549Mpa,3.58125%,and those of group V were 154.99415Mpa and 0.70708%,respectively.The average area fraction of porosity and bulk porosity of group P were6.18583%and 0.12125%,respectively,while those of group V were 11.60916%and0.48542%,respectively.The area fraction of porosity and bulk porosity of group V fluctuated greatly than group P.The tensile strength of P and V groups can well obey Weibull distribution with two parameters.The Weibull modulus is 17.04111 and12.63402,respectively.The stability of mechanical properties of P group is better than that of V group.The relationship between the area fraction of porosity or bulk porosity and the elongation of the P and V samples were fitted by the power function model,y(28)ax~b.The value of R~2 were 0.78294 and 0.81893 in area fraction of porosity.The value of R~2on bulk porosity were 0.48346 and 0.0029,indicating that the area fraction of porosity is the main reason for the elongation.Using this model,it is predicted that when the porosity is almost zero,the maximum elongations achievable in the P and V samples are 12.78%and 2.235%,respectively.At the same time,the model predicts that when the porosity is almost zero,The maximum tensile strengths achievable in the P and V samples were 239.760038 MPa and 194.57156MPa,respectively.At the end of this paper,the actual fracture location of the P9 sample is compared with the results of the three-dimensional reconstruction.The model at the fracture location is extracted,and a simulated tensile test is carried out through a series of pre-treatment work.The stress distribution around each pore was observed with HYPERWORKS software.The crack initiate from the shrinkage pore and gas-shrinkage pore,rather than from the gas pore.In the elastic stage,simulation results were very close to the experimental ones,because the pores did not undergo obvious deformations.At the plastic stage,simulated fracture strength was slightly higher than experimental fracture strength.However,our method provides a new way to study effect of 3D pore influence on mechanical properties of die castings.