Study On Microstructure, Defects And Mechanical Properties Of High Pressure Die-Cast Al-10Si-0.3Mg Alloy For Automotive Applications

Aural 2(Al-10Si-0.3Mg)alloy can obtain a good ductility(elongation up to 14%)with the help of heat treatment and vacuum assisted Minimum Fill Time die cast technology(MFT),but a good understand of the correlation between the microstructure and mechanical properties is required before the wide application in automotive industry,while a lack of systematic study exists.Moreover,a large deviation of elongation has been noticed in heat treated Aural 2 die castings,which indicates a terrible instability of ductility and is supposed to be in relation to casting defects according to researches about other alloys.Therefore,in this paper,the correlations between microstructure and mechanical properties before and after heat treatment of HPDC Aural 2 alloy have been studied,using optical microscope,scanning electron microscope and Digital Image Correlation technology(DIC).Effects of defects on ductility and deformation behavior have been conducted,using X-ray Computed Tomography(CT)and quantitative fractography.The main conclusions are summarized as follows:Typical as-cast microstructure of HPDC Aural 2 alloy consists of ?-Al matrix,brittle eutectic structure(eutectic Si and Al),blocky AlSiFeMn and needle-like AlMgSi intermetallics.?-Al matrix includes large dentritic Externally Solidified Crystals(ESCs)and equiaxed fine cells,which nucleates in shot sleeve and cavity respectively;Supersaturated Si in as-cast ?-Al matrix can precipitate in the form of Si dispersoids and continuous coral-like eutectic silicon can be fractured and spheroidized after heat treatment.Reduction of yield strength caused by loss of Si solution strengthening and eutectic structure strengthening has been offset by Si dispersion and aging strengthening.Ultimate tensile strength decreases dramatically due to the drop-off of strain hardening capacity after heat treatment;Eutectic Si and AlSiFeMn intermetallic particles can serve as the main crack source during tensile process.The change of geometry of eutectic silicon occurs and the resistance to dislocation movement decreases due to the loss of Si solution strengthening after heat treatment,which both improve ductility;The size of defects in fracture surface has an obvious correlation with ductility.If large defects(shrinkage or cold flake with an area faction higher than 2%)exits,elongation could be reduced dramatically and lower than 8%.The elongation will decrease with the increase of size of those defects;if no large defects exists,fine defects distribute homogenenously and can be much less deleterious to ductility.However,other factors such as clusters of fine pores,distribution of second phases and ESC could show much effect on the ductility and fracture behavior.Thus the elongation could be higher than 8%,but still have a large deviation.