Study On Microstructures And Properties Of Mg₂Si/Al Composites

Aluminium is a kind of low density light metal with atomic weight of26.98154. Aluminium products have excellent corrosion resistance andrecuperability. Thus, the consumption of Aluminium products increases day by dayin many kind of industry including transportation and seafaring. Also, the researchof aluminium metal matrix composites (MMCs) has become a hotspot in materialsand engineering fields. In comparison with steel and iron, particulate-reinforcedaluminum MMCs have a great potential to be applied in brake system inautomobile and high speed train because of their high specific tensile strength,rigidity as well as their high thermostability and wear resistance to meet thedemand of weight reduction.However, the present MMCs brake disks are made of SiC particulatereinforced aluminium MMCS and it is difficulty to use for the car because of itscomplicated fabrication technology, expensive cost and difficult machiningproperty. It is well known that Mg₂Si intermetallic compound forms easily duringaluminium alloys cast process and the compound exhibits a high meltingtemperature, low density, high hardness, a low thermal expansion coefficient and areasonably high elastic modulus, accordingly, Mg₂Si intermetallic compound canbe used as reinforced phase of Al MMC. Due to the reinforced phase of Mg₂Si isproduced in solidification process, Mg₂Si/Al composites exhibit a simple preparetechnology, low cost, uniform distributing reinforced phase, optimized interfaceand excellent thermodynamically stable systems in comparison with ex-situ process, such as SiC, TiC, TiB2 and Al2O3 reinforced Al composites. In addition, Mg₂Si/Alcomposites have a good machinability in comparison with ceramic particulatereinforced Al composite. Therefore, the Mg₂Si/Al composites have high potentialand applied prospects as automobile brake disc materials.However, the primary Mg₂Si phases in normal in situ Mg₂Si/Al compositesare usually very coarse, and thus, lead to poor properties to these composites.Accordingly, the poor properties restrict the material application in much field. Soit is a principal topic to modify the morphology of reinforced phase, size anddistribution as well as enhancing the poor properties. Rapid solidificationtechnology can refine microstructure modifying the materials performance.However, the technology is too expensive, which restricts its application anddevelopment. Therefore, it will have important theoretical and practical value tostudy nucleation and growth on primary Mg₂Si phase, and microstructuralmodification and phase distribution on Mg₂Si/Al composite, accordingly controlsthe microstructure character and toughness of composite, promoting its applieddevelopment in Engineering and practical process.In the present thesis, the crystalline and growth characters as well as the effectof cooling rate on growth manner of primary Mg₂Si crystal in Al melt are studied.The microstructure of Mg₂Si/Al composites are modified effectively via theaddition of alloy elements, semisolid processing technology and remelting method,discussing the scientific problems of that process. The mechanics and wearproperties are studied on Mg₂Si/Al composite. The major research efforts of thepresent study are as follows:(1) According to phase diagram and probative experiment, the solidificationprocess of Mg₂Si/Al was obtained: L-L1+Mg₂Sip-L2+(Al+Mg₂Si)e+Mg₂Sip-(Al+Si+Mg₂Si)e+ (Al+Mg₂Si)e+ Mg₂Sip. It was found that the growthmanner of primary Mg₂Si crystal transformed from facet growth manner tonon-facet manner under the rapid solidified condition and according to heatconducting model, it was calculated that the growth manner of primary Mg₂Sicrystal transformed as non-facet manner when the solidified rate exceed thevalue of 1.169*106C/s. (2) It was found that the coarse primary Mg₂Si could be refined as fined polygonalmorphology by P, Sr and rare earth addition as well as melt superheatingtreatment technology, modifying the microstructural character effectively.(3) The modification mechanism of P was obtained, namely Mg3(PO4)2 in meltcould be as the nucleus of crystallization of primary Mg₂Si, furthermore, themanner of Mg₂Si could be changed by P, forming tetrakaidecahedron Mg₂Sicrystal. It was concluded that the modification mechanism of Sr was adsorptionpoisoning effect. The volume fraction of primary Mg₂Si phase decrease after Sraddition, and the growth manner was changed, forming hexahedronmorphology. The modification mechanism of Ce was concluded, i.e. thesolid-liquid interface energy was changed. The Al-Mg₂Si-Si ternary eutecticphase changed to Al-Mg₂Si and Al-Si binary eutectic phase, at the same time,the Mg₂Si was refined, forming Al7Si3.5Cu3.5Ce1 intermetallic compound. Theresearch result showed that melt superheating treatment could change meltstructure and eliminate melt heredity, accordingly changed primary Mg₂Simorphology to polygon, refining eutectic phase.(4) Mg₂Si/Al semisolid composites were fabricated by strain-induced meltactivated, cooling slope and isothermal heat treatment technology, formingspherical reinforced phase and matrix structure.(5) The result showed that the semisolid Mg₂Si/Al composite with sphericalreinforced phase and matrix structure could be fabricated by strain-inducedmelt activated. With the increasing of compression ratio, spherical a-Al becamefiner and eutectic phase transformed from binary eutectic to ternary eutectic. Inaddition, the spheroidization mechanism was obtained. It was found that, aftersolidification by cooling slope on Mg₂Si/Al composite, the morphology of a-Alchanged to fined spherical particle with break primary Mg₂Si dendrite in as-castmicrostructure. After the cooling slope solidified sample was heat treated, thespherical reinforced phase and matrix structure formed. It was calculated thatthe cubic coarsening rate constants on a-Al is 1.02*10-16m3s-1 according to thestatistical data. Also, it was found that the spherical reinforced phase and matrixstructure formed by isothermal heat treatment technology, and the modified spherical reinforced phase and matrix became finer in comparison withunmodified composite.(6) It was found that the microstructure of Mg₂Si/Al composite can be changedgraded structure after remelted by electric arc with certain orientations from topto bottom on primary Mg₂Si, and those dendrite crystals in the same array arealmost parallel because of the difference of heat dissipation conditions. Due tothe Mg and Si atoms dissolve into Al matrix, forming supersaturated solidsolution at the bottom, its microhardness of graded material along the gradedmicrostructure gradual increased with the increasing distance from the top part.(7) Using a directional remelting and quenching technology, five layer Mg₂Si/Alfunctionally graded composites could be obtained and the difference ofremelting temperature promotes the muti-layer graded materials formation. Themeasuring result showed that, with the rising of distance, the primary Mg₂Siparticles exhibited a continuous increase in size from 20mm at layer (II) to140mm at layer (V).(8) After the composite was T6 heat treated for eight hour, its hardness reachedmaximum value, and at these heat treatment condition, the tensile strength andelongation rate of modified composite enhanced 25% and 31.1% in comparisonwith unmodified composite, respectively. The wear test results showed that thewear resistance of modified composites was higher than unmodified composites,because the fined, dense and uniformly distributed primary Mg₂Si reinforcedphase could restrain the occurring of plastic deformation, enhancing the abilityof plastic flow resistance. In addition, the wear resistance of composite washigher than pure aluminum because of the existence of reinforced particles.In a word, the present study not only changed the character and structure ofmicrostructure on Mg₂Si/Al composite, but also enhanced the mechanicalproperties and wear resistance, which have instructing significance for industrialapplication of the composite.