| This subject takes ADC12 aluminium alloy automobile engine cylinder block die casting as the research object,carries on its composition optimization design and rare earth modification treatment,the aim is to improve the microstructures and casting defects of alloy castings and improve their comprehensive mechanical properties,makes it have high strength,toughness and low thermal expansion coefficient at high temperature,thus meets the market requirements for the performance of aluminium alloy for cylinder block and expands its application scope.In this study,the single factor and orthogonal experimental methods were used to optimize the composition of the alloy,and the final content of the main elements was determined.Then,three rare earth elements,Ce,La and Sm,were added to the optimized alloy for modification.The effects of various rare earth elements on the mechanical properties and microstructure of the alloy were analyzed and the optimization results were obtained.The specific conclusions of this study are as follows:(1)After optimization of the composition of the aluminum alloy of ADC12,the content of main elements in the alloy was finally determined to be 11 wt% Si,2.5wt% Cu,0.5 wt% Mn,0.3 wt% Mg,and the tensile strength and elongation were measured to be 285 MPa and 2.23%,respectively.Compared with the alloy before unoptimization,it increased by 15.38% and 20.5% respectively.(2)Studies have shown that the porosity defects in the alloy after the modification of the rare earth have been significantly improved,and the eutectic Si phase and the?-Al phase were also significantly refined and the size was reduced.With the increase of the rare earth content,the eutectic Si phase and ?-The size of the Al phase first decreases and then increases.When the addition amount of Sm is 0.2wt.%,the refining effect of the alloy structure is the best,the elongation,tensile strength and microhardness of the alloy are optimized,and the thermal expansion coefficient of the sample does not change much.Under normal temperature conditions,the effect of rare earth metamorphism on the fracture morphology is not significant.The fracture modes of the samples are coexisting ductile fracture and brittle fracture,but the ductile fracture is the main.Comparing the data and micrographs,it is found that the order of influence of three rare earth elements on the tensile strength of the alloy at room temperature is Sm>Ce>La,and the order of influence on elongation and microhardness is Sm>La>Ce,the order of influence on coefficient of thermal expansion is Ce>La>Sm.(3)The specimen was subjected to a high temperature tensile test and the microstructure change after the test was observed.The results show that the microstructures of the alloys are coarser than those of the alloys at room temperature.Many second phases are coarser and easier to aggregate,but some equiaxed grains also appear.When 0.4wt.%Ce is added,the refining effect of ?-Al phase and eutectic Si in the alloy structure is the most significant and uniform distribution,and the elongation is optimal.When the content of Sm is 0.2 wt.%,the strength of the alloy is optimized.The order of influence of three kinds of rare earth elements on the high temperature tensile strength of the alloy is Sm>Ce>La,and the order of influence on the elongation is Ce>La>Sm;At the same time,at high temperature,the fracture mode of the alloy can be completely changed from the coexistence of ductile fracture and brittle fracture to ductile fracture.(4)Considering the comprehensive data,when the addition amount of Sm is0.2wt.%,the deterioration effect is the best,and the comprehensive mechanical properties of the alloy are remarkably improved.The tensile strength and elongation at room temperature reach 331 MPa and 2.56%,respectively,compared with the base alloy increased by 16.14% and 14.80% respectively.The tensile strength and elongation were 21.48 MPa and 3.23% respectively at 250?,which were 79.02% and30.78% higher than those of the base alloy. |
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