| High speed, energy saving, environmental protection, safety and comfort are themes of development of automobile industry. While light weight is the most effective way to achieve the objectives listed above. Engine is the heart of a car, which holds20～30%of the car’s whole weight. As the biggest casting in the engine, cylinder body takes up25～35%of engine’s weight. Aluminum alloy, which is a new kind of material, has recently gained extensive application in the automobile weight lightening process. Not only engine block can be lightened by30%when traditional cast iron engine block is replaced by aluminum alloy engine block, thermal conductivity can also be enhanced, which prevent steering deviation at high speed driving. Thus, producing cylinder body with aluminum alloy has significance in both reducing engine even the whole automobile weight and pushing on the process of energy conservation and emission reduction in China automobile industry. At present, there is a big gap between domestic high performance aluminum engine block research and international level. Three aspects of problems must be solved to achieve industrialize production of high performance aluminum alloy engine block:new type material of cast aluminum alloy for engine block, precise and mature casting techniques and complete sets of advanced production equipments, the key of the above is to solve the problem of material and casting techniques.For reasons such as noted above, with the help of Japan’s Mazda automobile casting aluminum alloy production experience, the casting process and application of new type casting aluminum alloy for Mazda series sedan engine cylinder block of Changchun First Automobile Work shop are investigated, which is cooperative developed with Suzhou Mingzhi Technology Co. Ltd. To enhance the ability of domestic aluminum engine cylinder production and to speed up China’s auto industry in the development of weight lightening are very important. The production technology of high performance aluminum engine cylinder with independent intellectual property rights is also very important. In this paper, the main research contents and results are as follows:1. Effects of grain refining and modification on microstructure and mechanical properties of multielement Al-Si-Cu alloy were investigated. Three of the most commonly used refiners and modifiers in the industrial field were chosen:Al-5Ti-B, Al-10Sr and RE. With the help of orthogonal test, optimized combination refinement and modification formula was as follows:Al-10Sr=0.1wt%, RE=0.3wt%, Al-5Ti-B=0.8wt%. It was also clear that the combined addition of grain refiner and modifier to multielement Al-Si-Cu cast alloy had resulted in maximum improvement in microstructure and mechanical properties as compared to the individual addition of grain refiners, modifier and in an untreated as cast condition. Its mechanical properties were σb=252MPa, σs=191MPa,δ=3.0%and brinell hardness was90.6HB. In the microstructure, a-Al phase was finer and had a clear outline. The component of eutectic silicon phase, Al-Cu phase, Al-Si phase and iron phase became diversified, and their sizes became smaller, their shape became more rounded and they distributed more uniform. All of these demonstrated that the three refiners and modifiers had mutual promotion to each other. At last, two parameters used to describe the modification effect were proposed:mean area and aspect ratio. The most optimal effect of modification was the alloy with grain refining and modification.2. Effects of mould sand type and casting wall thickness on microstructure and mechanical properties of multielement Al-Si-Cu alloy were investigated. Multistep moulds which wall thickness was from8mm to40mm were made by quartz sand, chromite sand and alumina sand. The mechanical property, secondary dendrite arm space and the effect of grain refining and modification of multielement Al-Si-Cu alloy were directly effected by different provided cooling rates. Alloy made by chromite sand core had the best tensile strength and elongation, while the alloy made by quartz sand core had the best hardness. With the increase of wall thickness, tensile strength of Al-Si-Cu alloy made by the three kinds of mould sands decreased from360MPa to280MPa, elongation from8%to3%, while the hardness did not appear evident trend with the different wall thicknesses. When the thickness was40mm, flake and massive Si phase appeared in the alloy made by quartz sand and alumina sand which meaned that the modification effect became worse. At last, the fitting equation, which could be used to guide practical production, was made between mechanical properties and secondary dendrite arm space of Al-Si-Cu alloy made by the three kinds of mould sands.3. The thermal fatigue property of multielement Al-Si-Cu alloy was investigated. Through the observation and analysis of thermal fatigue crack initiation and propagation of Al-Si-Cu alloy under different heat treatment conditions and temperature ranges, the alloy treated by T6heat treatment was found to have the longest thermal fatigue life because of its lowest temperature sensitivity and highest antioxidant property. Multielement Al-Si-Cu alloy experienced three periods during the thermal fatigue initiation process:the formation of micro oxidation layer, the formation of micro pit inside the oxide layer and micro craters grow to the crack source. The early stage of fatigue crack expansion was along the grain boundary, which was caused by the passivation-sharpen of crack tip. The later stage of crack propagation was mixed by the growth along the grain and through the grain, which was propagated in a complex way by passivation and sharpening on the crack tip and siamesed voids on front edge of crack tip. At last, two ways that effected the crack propagation way by the shape and position of Si phase were found:"bypass-wall expansion" and "through-wall expansion". Two methods of the new characterization of thermal fatigue properties were proposed:the tortuosity of the crack and the ratio of crack length and width.4. The friction behavior and wear mechanism of multielement Al-Si-Cu alloy were investigated. The results were as follows:the alloy treated by T6heat treatment had the lowest wear rate of weight and friction coefficient under different loads and wear times, and its abrasion morphology was in the state of minor wear mechanism. When the load was less than500N and wear time was less than3h, the alloy after T6condition and cast-quenching+aging had the similar wear property, which mainly because they had the similar hardness. When the load and wear time increased, broken Si phase was found in the subsurface in three kinds of alloy. The broken Si phase increased surface hardness indirectly and changed the lubrication effect. Excessive load might cause microcrack and even severe tear shaped plastic deformation around Si phase and even Al2Cu phase, what severely effected the wear property of Al-Si-Cu alloy. In the process of multielement Al-Si-Cu alloy’s friction and wear to failure, the evolution of wear mechanism is:abrasive wear→abrasive wear+adhesive wear→adhesive wear→adhesive wear+delamination wear→delamination wear+oxidative wear. According to the plastic deformation and hardness reduction on the worn surfaces, two kinds of oxidative wear were firstly announced, that was slight oxidative wear and failure oxidative wear. |
PDF Full Text Request