Effects Of Melt Mixing And T6 Heat Treatment On Microstructures And Mechanical Properties Of Multicomponent Hypereutectic Al-18%Si Alloys

It is generally accepted that the solidified behaviors,microstructures and properties are markedly influenced by their melt states.Besides,our previous investigations disclosed that the primary Si crystals of binary Al-Si alloy can be dramatically ameliorated by mixing the high-temperature melt experienced temperature-induced liquid-liquid structure transition(TI-LLST)with low-temperature melt in semisolid state.In this work,taking multi-component hypereutectic Al-18Si-1.5Cu-0.5Mg-0.5Mn-0.2Ti alloy as the research object,in light of previous results,we report an optimized melt mixing manipulating which combined with Sr modification for synchronously optimizing the primary Si and eutectic silicon crystals.Furthermore,the influence mechanisms of melt mixing manipulating and Sr modification,as well as the T6 heat-treatment on microstructures and mechanical properties of multicomponent hypereutectic Al-18%Si alloys were systematically investigated,which can be used for guiding practices.The main conclusions in this paper are as follows:(1)For the melt smelted with the conventional casting,the solidified primary Si crystals present in irregular polyhedrons plate and octahedrons,which show the behavior of lateral growth.Conversely,for the melt treated with the melt mixing and Sr modification,the primary Si crystals become drastically refined(minimum to 10.27μm),and the eutectic silicon change into the coral-shaped,simultaneously.Moreover,it is worth noting that the surface of primary Si also gets passivated,suggesting the behavior of continuous growth.(2)The author argues that,the refinement of primary Si crystals mainly benefits from three aspects.Firstly,during the suitable melt mixing period,the ready-existing Si dendrites inherited form low-temperature melt are decomposed into large amounts of micro crystals or large clusters,which can act as active Si nuclei in later solidification.Secondly,as the high-temperature melt experienced TI-LLST,the dispersive atoms need a larger undercooling degree to nucleate,which strikingly increases the nucleation rate and the final number of nuclei.In addition,for the melt experienced TI-LLST,the relative homogeneous melts generate a decrease of free energy,ultimately impeding the growth of primary Si crystals.More intriguingly,the dispersive atoms also coarsen the growing interface of Si crystals,which finally turn the growth behavior from a lateral to a continuous growth.Hence,the primary Si crystals with melt mixing are markedly refined and their rounded morphology is presented.(3)Through the morphological observation on the Si crystals of Al-18%Si alloy,it is found that the evolution of eutectic silicon morphological significantly affect the hardness of alloys.In the initial stage,the fibrous eutectic Si crystals occurs necking,and fragmentation due to the thermal effect,leading to an increase in hardness of alloys.In the intermediate stage,the hardness of alloys attains the peak value owing to the suitable size,perfect morphology,and uniform distribution of eutectic silicon crystals.In the final stage,the eutectic Si crystals obviously grow up and coarsen in accordance with Ostwald ripening,which results to the drop of hardness.(5)The artificial ageing results illustrate that there is an apparent age-hardening phenomenon at various temperatures,and the peak condition achieves faster with the increase of ageing temperature,but the peak hardness is different for each ageing curve,with the maximum hardness is obtained at 175 oC for 8h.Interestingly,it can be observed that double aging peaks are present in the ageing curves of multicomponent Al-18%Si alloy when aged at 175 oC or 195 oC,while the first peak is higher than the second one.In addition,combined the DSC and ageing curves,it can be infered that the age-hardening peaks of alloys are correlated to their precipitation sequence,while the obvious interval during the transition from GP zone to metastable phase may be the main reason of the formation of double aging peaks.In virtue of above results,a solution treatment at 520 oC for 8h and an ageing treatment at 175 oC for 8h are adopted for the multicomponent Al-18%Si alloy.(6)Mechanical property testing indicates that the mechanical properties of multicomponent hypereutectic Al-18%Si alloy are markedly improved with the melt mixing and T6 heat-treatment.Compared to conventional casting,the melt mixing shows an 11.0-14.0%,15.9-19.8% and 55.7-58.9% improvements in Brinell hardness,tensile strength and elongation,and 8.8-12.1% decrease in mass loss,respectively.It is attributed to the grain strengthening and the increase of intrinsic fracture stress caused by the refinement of Si crystals with melt mixing.Analogously,for the alloys after T6 heat treatment,the enhancements of Brinell hardness and tensile strength are 32.4-36.0%,39.9-44.7%,and the decrease of mass loss is 21.6-25.4%,which ascribes to the increase of strength aroused by separated precipitation phase after heat treatment.