Research On High Pressure Die Casting Processing Of Particle Reinforced Aluminum Matrix Composites

Particle reinforced aluminum matrix composites(PAMCs)have been identified for achieving high specific strength and great stiffness,good electric and thermal conductivity,superior wear resistance and dimensional stability,and also have been applied extensively in automobile,aerospace,mechanical and electronic industry.However,the bad wettability between the reinforced particles and liquid Al as well as reaction tendency between most types of reinforcing particles and result in the generation of detrimental reaction products on the interface,and hence the composite's properties were significantly reduced.Therefore the large scale preparation of PAMCs with high quality is needed urgently.Furthermore,it is difficult for PAMCs to be mechanically processed because of the high hardness and strength.Therefore,high efficiency and precision processing methods for PACMs were desired.High pressure die casting(HPDC),a efficient and near-net shape forming technique,is the mainly precision forming of aluminum and magnesium alloys.While the study of PAMCs fabricated via the HPDC process was very limited.In this study,the effect of manufacturing processes on the microstructures and mechanical properties of PAMCs was studied,and the equipment for the large scale fabrication of the PAMCs slurry was developed.The experiments of PAMCs fabricated by the HPDC process were carried out.The HPDC process parameters,types of particle and matrix alloy affecting the microstructures,mechanical properties of PAMCs were investigated and compared.This study is very helpful to the development of PAMCs forming theory,and can promote the application of PAMCs.In this study,the liquid and semi-solid state stirring processes were used to fabricate the Al/A356-B4 C composites.The effects of process parameters and Ti element on the particle distribution,particle/matrix interfacial reaction and mechanical properties of the prepared composites were investigated.In addition,the semi-solid state stirring process was used to fabricate the A356/6061-SiC composites.The effect of solid fraction on the particle distribution and the effect of Ti element on the particle/matrix interfacial reaction were studied,and the mechanical properties of these composites were investigated and compared.The results showed that a continuous TiB2 layer was formed on the interface,then B4 C particles were prevented from further reacting with liquid Al,and good interfacial bonding between them was found,with 1.0 wt.% Ti addition during the stirring preparation.The distribution and volume fraction of particles can be significantly improved by increasing the solid fraction in the semi-solid stir casting process.A specific equipment was developed to prepare large scale PAMCs slurry for the die casting process.And the die casting mold was also designed and manufactured for this investigation.Then a series of die casting experiments with different process parameters were carried out to study the influence of pouring temperature,vacuum assistance,and intensification pressure on the formability of the A356-B4 C composite fabricated by the HPDC process.A multiphase flow mathematical model to simulate the PAMCs mold filling process was established,and verified later by indirect squeeze casting experiment.The influence of flow field on particle motion and distribution were discussed.The results indicated that the formability of PAMCs can be improved by using higher pouring temperature,larger intensification pressure and vacuum assistance.In regard to the distribution of reinforcing particles,the results of simulation show that the particles' trajectory has significant effect on its distribution,which is consistent with the experiment.The HPDC experiments of the A356-B4 C composite with three pouring temperatures(710,720 and 730°C)were conducted.The microstructures,particle distribution and porosity of the samples of different thickness were studied.The effect of flow field on the B4 C particle distribution was analysed by the mold filling simulation of the A356-B4 C composite with the established model.The influence of the particle/matrix interface and the particle distribution morphology on the tensile fracture mode was discussed further.It is concluded that as the pouring temperature was increased,the ESCs(the externally solidified crystals)became larger,and the porosity also went up;the aggregated B4 C particles tended to induce brittle fracture leading to inferior tensile properties.The HPDC experiments of the A356-SiC composite with three different intensification pressures(65,80 and 95MPa)were also carried out.The microstructures,the SiC particle distribution and the particle/matrix interface of the samples with different thicknesses were analyzed.And then the effect of SiC particles on the shrinkage pores' formation in the center of the samples was discussed.Followed that,the X-ray tomography technology was used to analyze the three-dimensional distribution and characteristic of pores in selected samples of different thicknesses.The effect of intensification pressure and wall thicknesses on the tensile properties of the A356-SiC composite was analyzed.A preliminary analysis of the effects of particle types on the microstructure and tensile properties of PAMCs fabricated by the HPDC process was carried out by comparing the A356-SiC with the A356-B4 C composite.The results showed that with the increase of intensification pressure,the number of ESCs was reduced and the amount and volume of shrinkage pores also decreased,so that the samples turned to fracture in a mixed ductile-brittle instead of a brittle mode resulting in the improvement of the tensile properties.Finally,the 6061-SiC composite was fabricated by the HPDC process with three different fast shot speeds(1,2 and 3m/s).And the microstructure,porosity and particle/matrix interface of samples with different thicknesses were analyzed.The influence of fast shot speed on SiC particle's distribution was discussed.The comparisons on the microstructure and tensile properties of the 6061-SiC with the A356-SiC composite were conducted.The results indicated that with the increase of the fast shot speed,a more uniform distribution of SiC particles was obtained with the fluid shear increased,and the thickness of surface layer and the porosity was significantly reduced,thus the tensile properties of the HPDC 6061-SiC composite were improved.