The Research Of Multielement Al-Si Alloy Composition Optimization And Thermal Fatigue Properties

In this paper,the experiment was carried out on the basic of Al-Si alloys.The mechanical properties,microstructure observation of the alloy with different percentage of Mn has been investigated by Optical Microscope(OM),Scanning Electron Microscope(SEM),Energy Dispersive X-ray Detector(EDS)and the basic mechanical properties test methods.After the alloy has been T6 treatment,the testing methods also used to analysis the effect of aging time on the mechanical properties,microstructure and the secondary phases of the optimized alloy.As well as the analysis of the strengthening mechanism.The thermal fatigue experiment of the alloy was carried out by the self-constraint thermal fatigue tester,and the research of fatigue crack initiation and propagation under different treatment condition was also been studied.The result show that,with the increase of Mn content in Al-Si alloy,the volume fraction of coarse dispersed network phase in the ?-Al matrix has been increased,and elevated the tensile strength through the dispersion strengthening with fine precipitates phase dispersion-precipitation from the matrix.The tensile strength of the alloy has been reached to 285 MPa with Mn content of 0.4%.After Mn atom solution into the?-Al matrix,the grain was refined and elevated the hardness of the alloy through lattice deformation.Also,the content of excess Mn in the alloy will break the structure of network and resulting in coarse dendrite.When the content of Mn is more than 0.55%,the coarse and hard brittle Al6 Mn phase will deteriorated the tensile strength and decreased the elongation simultaneously.The T6 treatment of Al-Si alloy will promote the supersaturated solid solution decomposed and precipitated into Al2 Cu,Mg2Si phase.These changes impede the movement of solute atoms among the grain boundary and elevate the mechanical properties of the alloy at the same time.Combining the mechanical properties test,after6 h aging treatment,the ultimate tensile strength improved from 285MPa(as-cast)to368MPa(T6 treated)and increased the mechanical properties significantly.When aging time is more than 6h,with the coarse of precipitated phase,the mechanical properties of the alloy decreased gradually.The fatigue properties of Al-Si alloy are mainly related to the material ductility,microstructure,the size and distribution of the precipitate phase.After T6 treatment,with the eutectic Si spheroidization,the fragmentation of the brittle Si particles on the?-Al matrix was disappeared and the resistance of fatigue crack propagation was enhanced significantly.With the homogenization distribution of matrix structure and second phase morphology after aging treatment for 6h,the driving force for crack propagation was increased and impede the propagation of the crack around the grain boundary.That decreased the fatigue crack growth rate and improved the fatigue resistance of the alloy at the same time.In the fatigue crack growth stage,with the cyclic thermal stress going,the brittle phase particles enriched at the grain boundary and fall off from the ?-Al matrix boundary.The binding force around the interfaces can be weaken by the cyclic stress.Fatigue crack usually grows up around the weak district between the matrix and the secondary phase,the micro-voids gradually formed and evolved into fatigue cracks.At the same time,the oxygen element can often concentrate around the gaps and large number of intergranular oxide reduced the binding force of interatomic.The oxygen spread in the fatigue crack propagation area under the thermal stress,and gradually evolved into a loose area,that accelerate the propagation of fatigue cracks.