Research On Microstructure And Friction Performance Of Mg₂Si/Al Gradient Composite Materials By Centrifugal Casting

Along with the development of society and natural science and technology progress,the demands are constantly improving for the diversity of the material in the actualenvironment. Among them, the aluminum matrix composites has been the research focus inengineering and material, especially at the present time when the automotive industry isdeveloping rapidly. The advantages of energy saving and weight loss have considerablevalue in the practical application.Al-Si alloy has the good wear resistance performance and corrosion resistanceperformance,so that it has been widely used in the aerospace, automotive and other keyindustries. But there are some Si with larger sizes in the alloy, so that the tools arise morewear and tear on the processing of alloy . Therefore, on the basis of ensureing mechanicalproperties about Al-Si alloys,finding the way to improve the processability performance ofalloy becomes an important research direction. Among the many ways to improveproperties of the Al-Si alloy, the method that adds reinforcement in the Al matrix isparticularly convenient and effective.Mg2Si particles could be generated by in situ reaction in the Mg2Si/Al compositematerial.With the advantages of low density, high hardness and high melting point, Mg2Si isthe thermodynamic stable phase generated in the process of in situ growth, and has highcompatibility with aluminum matrix. The techniques method to prepare aluminum matrixcomposites reinforced with Mg2Si is more simple and costs less consumption than that withplus ceramic particle. Thus, it would gradually reveal its potential advantages and broadprospects in the development of the particle reinforced aluminum matrix composites,especially in the path of lightweight in the automotive industry.The substance of the Mg2Si/Al gradient composite materials is to make the Mg2Siparticles gathered to the side as far as possible, where forms a function layer of gradient with better physical properties than the matrix.compared with other Mg2Si/Al gradientcomposite material preparation technology, using the method of centrifugal casting isconvenient, simple and has advantages of high efficiency and good formability. But thetraditional centrifugal casting method has many factors influencing the gradient of thematerials. Accordingly, the investigation is very essential about the effects of variousparameters on the material gradient and function, in the process of centrifugal casting.The Mg2Si particles reinforced the aluminum matrix composites with wear-resistingfunction and gradient have been prepared using centrifugal casting method.In the presentthesis, the form of organization and gradient distribution has been investigated underdifferent control parameters. The relationship between the formation causes and change ofparameters has been . According to the actual experiment result analysis, the experimentalconditions have been seeked in the process for preparation of reasonable structure and goodperformance gradient. Furthermore, semi-solid squeeze method was used on the castingsprepared by the centrifugal casting. Characteristics of the semi-solid squeeze method havebeen analyzed through the research on the morphology, gradient distribution and the wearresistance of the inner function layer. The major research conclusions are as follows:1) Bi-directional solidification is a characteristic of centrifugal casting so that the gradientof the primary Mg2Si content appear concave distribution in the radial direction of thecomposites materials reinforced by in-situ particles.2) By centrifugal casting,the greater the centrifugal speed, the better the primary crystalparticles migrate to the inner layer of castings, where the Mg2Si particle deposition istighter and denser. While improving mould temperature, can also enhance the effect ofthe primary particles migration. the different gradient change of the castings could becontrolled under the condition of the two factors above. In the preparation process ofMg2Si/Al composites material via horizontal centrifugal casting, when the centrifugalspeed reaches 1000r/min and the mould temperature ranges between 300 and 450degrees centigrade, Mg2Si content reached a maximum value somewhere in the innerwall of the casting. Furthermore, the dense layer becomes thicker and closer to theinner wall with mold temperature increasing. 3) After the mould temperature reaches 300 degrees centigrade and centrifugal speedreaches 1000 r/min, wear resistance performance have little change, and the changesand trends of the abrasion loss keep basicly stable with the change of abrasionparameter. In addition, there would be easily involved in air because of the shock andvibration Under the high-speed condition. And improving mould temperature wouldmake the cooling speed slow, nucleation rate reduced and the grain easy to grow up. So,by contrast, it is the best choice of parameters when the mould preheating temperatureis about 300 degrees centigrade and centrifugal speed reaches 1000 r/min.4) Heated until the temperature of the sample reached between solidus and liquidus,thematerials would be kept in the semi-solid state,so the eutectic Si can adequatelymelt.Then stop heating and extrude in this state. With the cooling ,there would appearlots of small and round grain Si. And after extrusion, gradient of grain Mg2Si becomesmore obvious, and the volume fraction and density of the particulate will increase.5) After semi-solid squeezing, the enhancement mechanism of the casting turns compositestrengthening of the Mg2Si and Si particles, so that the wear resistance of the internallayer are better than before. Furthermore, the overall performance of the materialimprove via the precipitation of Si particles.6) The results got from the dry sliding wear experiment with different loads and frictionvelocity reflect that the wear performance of inner layer on the Mg2Si/Al gradientcomposite materials is bettle than that of TH200,and among them ,the one prepared bythe semi-solid extrusion is the best. Therefore, because of the light weight and lowmiscibility relative to the steel, the Mg2Si/Al gradient composite materials have thepossibility to replace the traditional steel components in certain practical applications.