Microstructures And Mechanical Properties Of Al-13Si-5Cu-2Ni Alloy And Its Composites

Confronted with the dual pressure of energy and environment, developing highperformance, light weight materials has been widely concerned. Al-Si alloys possessexcellent light weight, high strength, good heat and electricity conductivity, good wearresistance and low coefficient of thermal expansion. These properties result in the wideapplication of Al-Si alloys in automobile industry, aerospace industry and manufactureindustry. As for the traditional Al-Si binary alloy, the stress concentration caused by theprimary or eutectic silicon particles and the lack of precipitate phases in the microstructurelimit the widely use of the alloy. In recent years, the requirement of high-speed andhigh-power engine brings a great challenge to the elevated properties of Al-Si alloy. In orderto further improve the room and elevated strength of Al-Si alloy, Al-13Si-5Cu-2Ni alloy(Designed compositions (wt.%): Si13.0, Cu5.0, Ni2.0, Al balanced) has been studied in thethesis. The microstructures and mechanical properties of phosphorus modified alloy as wellas the nano-sized TiC master alloy reinforced Al-13Si-5Cu-2Ni alloy composites have beenstudied and the strengthening mechanism has been revealed.Conclusions are as follows:(1) It’s revealed that the optimized T6heat treatment process is solution heat treatment for8hours at510℃and aging for10hours at165℃.i) After solution treated for8hours at510℃, Al2Cu and Al-Cu-Ni phase at the grainboundries are completely dissolved into the α-Al grains, the sharp endge primarysilicon particles turn dull and the strip eutectic silicon particles become relativelyfine. Elevating the temperature of solution heat treatment will cause oversintering.Extending the solution time will lead to the coarseness of the eutectic silionparicles.ii) Aging for10hours at165℃, the size of θ precipitates are relatively small, the number increases, and the distribution turns more uniform. The hardness of thealloy reaches maximum. Aging for6h,10h and14h, the diameter of the θ precipitates is76.99nm,89.21nm and102.68nm, respectively; the width is8.62nm,16.91nm and17.30nm, respectively; and the hardness is160HV,164HV and152HV, respectively.iii) Through solution heat treatment for8hours at510℃and aging for10hours at165℃, Al-13Si-5Cu-2Ni alloy yields the best mechanical property with385MPa intensile stress and6.0%in fracture strain.(2) It is revealed that phosphorus can refine the size of primary and eutectic silicon particlessimultaneously. The optimized content of phosphorus is0.1wt.%with the averageprimary silicon size decreasing from the unmodified about24.09μm to about12.02μm.As to eutectic silicon particles, when the phosphorus content is0.005wt.%, therefinement is ideal with the average eutectic silicon size decreasing from the unmodifiedabout4.16μm to about2.55μm.(3) It is revealed that phosphorus modification can improve the tensile properties ofAl-13Si-5Cu-2Ni alloy at room temperature and the fracture strain at both room andelevated temperature:i) When the P content is0.1wt.%, the alloy possess the best tensile stress and fracturestrain with the tensile property411MPa and the fracture strain8.4%. Compared to383MPa and5.8%of the unmodified alloy, it is improved by7.3%and44.8%,respectively.ii)0.005wt.%P modification can improve the elevated fracture strain of the alloysignificantly. The elevated fracture strain almost improved by about1.8times fromthe unmodified8.6%to24.4%. Compared to the unmodified alloy, the elevatedtensile strength of modified alloys yield a small decrease.(4) It discover that the nano-sized TiCpin the TiCp/Al-13Si-5Cu-2Ni composites can refinethe eutectic silicon particles, the θ precipitates and the strip Al-Cu-Ni phase. With theaddition of1.0wt.%TiCp, the refinement of eutectic silicon particles and θ precipitatesis excellent with the size of eutectic silion from about4.16μm in the base alloy to about 2.80μm and the θ precipitates from about89.21nm in diameter,16.91nm in length toabout64.17nm in diameter,13.30nm in width. The strip Al-Cu-Ni phases in thecomposites transforms from the strip morphology to fine particles. When the TiCpaddtion excess1.0wt.%, Si element will promote the decomposition of TiCp, whichleads to the formation of needle-like Al3TiSi phase and block-shaped Ti-Si-C phase anddeteriorates the tensile property of the composites.(5) It is revealed that when the content of nano-sized TiCpis1.0wt.%, theTiCp/Al-13Si-5Cu-2Ni composites yields the the best tensile properties with408MPa intensile strength and7.5%in fracture strain at room temperature. Compared to the matrixalloy, the properties of the composites improved by6.5%and29.3%, respectively. Theelevated fracture strain of the composites increases from8.6%of the matrix alloy to10.1%of the composites, but the elevated tensile strength changes slightly.(6) It is revealed that the main strengthening mechanism of the nano-sizedTiCp/Al-13Si-5Cu-2Ni composites at room temperature is the grain refinement ofeutectic silicon particles, the dispersion strengthening of nano-sized TiCpand therefinement of θ precipitates.