【作者】 刘光磊；

【导师】 司乃潮；

【作者基本信息】 江苏大学 ， 材料学， 2013， 博士

【摘要】 高速、节能、环保、安全、舒适是汽车工业发展的主题,而轻量化是实现上诉目标最直接最有效的途径。发动机是汽车的“心脏”,质量占整车重量的20-30%,而缸体作为发动机中最大的铸件,其质量占发动机重量的25-35%。近年来,铝合金作为一种新型材料,在汽车轻量化建设中得到了广泛应用。用铝合金代替传统的铸铁材料生产发动机缸体后,不但可使缸体减重30%左右,还可以提高导热性能,防止汽车高速行驶过程中出现转向偏差。因此,采用铝合金生产缸体对降低汽车发动机乃至整车重量,推进我国汽车工业节能减排进程都具有重要意义。目前,国内在高性能铝合金缸体的研发应用上还与国际水平存在较大的差距。要想实现高性能铝合金缸体的工业化生产,必须解决三个方面的问题：新型的铸造铝合金缸体材料、精密成熟的铸造工艺和先进成套的工业化生产装备,其关键和根本是要解决材料和铸造工艺的问题。基于此,借助日本马自达品牌汽车发动机缸体用铸造铝合金的生产经验,与苏州明志科技有限公司合作开发了长春一汽马自达系列轿车发动机缸体用的新型铸造铝硅合金,并对其铸造工艺及其应用进行了研究。力争拥有自主知识产权的高性能铝制发动机缸体的生产技术,提升国内生产铝制发动机缸体的能力,加快我国汽车行业在轻量化道路的发展速度。本文主要研究内容和研究成果如下：1、研究了细化变质处理对多元Al-Si-Cu合金组织和力学性能的影响。选取目前工厂生产中最常用的三种细化变质剂：Al-5Ti-B、Al-10Sr和RE,借助正交试验优化得到复合细化变质配方为：Al-10Sr=0.1wt%、RE=0.3wt%、Al-5Ti-B=0.8wt%。与单一细化变质处理的合金以及母合金相比,该配方制备合金的力学性能和显微组织均得到极大的提升,其力学性能为：σb=252MPa、σs=191MPa、δ=3.0%、布氏硬度=90.6HB,其组织中a-Al相更加细小且轮廓清晰,共晶硅相、Al-Cu相、Al-Si相以及含铁相的成分变得多元化,且尺寸更小、形状更圆整、分布更均匀,说明三种细化变质剂起到相互促进的作用。最后提出了两个描述变质效果的参数：平均面积和长宽比,细化变质合金的变质效果最优。2、研究了不同原砂制砂芯及其壁厚对多元Al-Si-Cu合金组织和力学性能的影响。选用石英砂、铬铁矿砂和宝珠砂分别制成厚度范围在8mm～40mm的阶梯状砂芯片,由于提供的冷却速度不同,直接影响制备的多元Al-Si-Cu合金的力学性能、二次枝晶间距和细化变质效果。铬铁矿砂砂芯制合金的抗拉强度和伸长率性能最好,硬度是石英砂的最好。随壁厚增加,三种砂芯制合金的抗拉强度由360MPa变为280MPa,伸长率由8%变为3%,而壁厚对硬度性能的影响并没有明显规律。当壁厚为40mm时,石英砂和宝珠砂制合金组织中出现片状和块状Si相,变质效果恶化。最后,在三种砂芯制合金的力学性能与二次枝晶间距之间建立了拟合方程,用于指导工厂实际生产。3、研究了多元Al-Si-Cu合金的热疲劳性能。通过对不同热处理试样在不同温度幅下热疲劳裂纹萌生与扩展的观察和分析,发现由于T6态合金的温度敏感性最弱、抗氧化性能最强,其热疲劳寿命最长。多元Al-Si-Cu合金在热疲劳裂纹萌生期要经历三个阶段：形成微观氧化层、氧化层内生成微坑、微坑生长或微坑内部生成裂纹源。裂纹扩展前期为沿晶生长,主要靠裂尖钝化-尖锐化引起裂纹扩展；而后期为沿晶和穿晶混合生长,以裂尖钝化-尖锐化和裂尖前沿空洞连体复合方式扩展。最后发现了Si相形状和位向影响裂纹扩展行径的两种方式：“绕墙”扩展和“穿墙”扩展。提出了两种新的表征合金热疲劳性能的方法：裂纹曲折度和裂纹的长宽比。4、研究了多元Al-Si-Cu合金的摩擦行为和磨损机理。结果表明：T6态合金在不同载荷和磨损时间下的质量磨损率和摩擦系数最小,磨面磨损形貌处于较轻微的磨损机理状态。当载荷小于500N且磨损时间小于3h时,T6态和铸淬时效态合金的耐磨性能接近,主要是因为二者硬度性能相近；增大载荷延长磨损时间,发现三种状态合金的亚表面内Si相发生破碎,间接提高了合金的表面硬度,改善了润滑效果。载荷过大则导致Si相甚至Al2Cu相周围出现微裂纹和撕裂状塑性变形,严重影响合金的耐磨性能。多元Al-Si-Cu合金摩擦磨损至失效过程中,磨损机理的衍变过程是：磨粒磨损→磨粒磨损+粘着磨损→粘着磨损→粘着磨损+剥层磨损→剥层磨损+氧化磨损。依据材料磨面塑性变形程度及其硬度缩减情况,率先提出将氧化磨损分为氧化轻微磨损和氧化失效磨损两类。更多还原

【Abstract】 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.更多还原