| With the increasing demand for light weight,energy saving and emission reduction of automobiles,automobile engine blocks,cylinder cover and other complex parts are mostly produced by die-casting.However,the turbulent flow during filling process is an inherent problem that can lead to internal defects of gas porosity in the castings due mainly to the entrapment of air during the molten metal injected in the die cavity.The presentence of gas porosity in castings affects adversely the mechanical properties,then heat treatment can not be applied.The application of vacuum device in HPDC process can significantly reduce not only the quantities of gas porosities,but also the pore sizes.However,the research on the influence of vacuum degree and heat treatment in the porosity and properties of castings is few.Cylinder block were produced by vacuum-assisted high pressure die casting(HPDC)process under three different absolute pressure.The influence of absolute pressure(50kPa,20 kPa,10kPa)in the die cavity on the porosity and mechanical properties of castings were investigated.Meanwhile,the influence of solution treatment time on the porosity and mechanical properties were investigated.The main contents and conclusions of the paper are as follows:(1)The pores in castings under three vacuum degrees were observed and counted.It shows that the degree of vacuum increases,the number and average size of pores in the casting increases.Besides,most of the pores are round or oval and uniformly distributed as the vacuum increases.As the absolute pressure increases from 50 kPa to 10 kPa,the average porosity and pores sizes reduce from 4.8% to 2.6%,8.65μm to 5.61μm,respectively.(2)The mechanical properties of castings increase with the increase of vacuum degree.The specimen exhibits the best performance in tensile strength and elongation under the absolute pressure of 10 kPa,which is 258 Mpa 和 2.25%,respectively.Comparing absolute pressure of 50 kPa,the tensile strength and elongation improve by 14%和 21.8%.(3)Comparative analysis of fracture morphology of castings under three vacuum degrees were conducted.The result detects the distribution of pores on the fracture surface of the specimen at 50 kPa is not uniform,and the pore morphology is complicated and the size is large.While the distribution of pores on the fracture surface of the specimen at 10 kPa is relatively uniform,and the pore morphology is sound.Compared with the results of mechanical property,the morphology and distribution of pores both are important factors affecting the mechanical properties of specimens.(4)After 500℃×0.5h +150℃ heat treatment,the value of tensile strength increased,and elongation decreased slightly.The fracture surface shows that most of the pores are round or oval and uniformly distributed While the stability of both tensile strength and elongation decreased significantly after 500℃×2h +150℃ heat treatment.The pores on the fracture surface are large and not evenly distributed.The composite mechanical property index Q was used to measure the mechanical properties of the specimens before and after heat treatment,it shows that the comprehensive mechanical properties of samples improve after 500℃×0.5h +150℃ heat treatment.(5)Through comparing the pore morphology and porosity of castings after two kinds of heat treatment(500℃×0.5h and 500℃×2h),the porosity of the specimen after heat treatment at 500°C for 0.5 h is 4.3%,and the pore shape is round,while the porosity of the specimen after heat treatment at 500°C for 2 h is 5.7%,the adjacent pores of the specimen were merged and the pore morphology changed from spherical to irregular long strips.Compared with as-cast samples,the porosity increased by 53% and 103%,respectively.With the solution time prolonging,more new internal pores formed,the average pore size got bigger and pore morphology was more complex. |
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