【作者】 饶晓晓；

【导师】 胡树兵；

【作者基本信息】 华中科技大学 ， 材料学， 2007， 硕士

【摘要】 现代汽车正朝着轻量化、高速、安全舒适、低成本与节能的方向发展。而目前满足上述要求的最有效的途径就减轻汽车自重。铝及其合金加工材料由于具有密度小、比强度高等一系列优良特性成为实现汽车轻量化最理想的首选材料。本文以A356铸造铝合金轮毂为研究对象,利用金相显微技术、扫描电镜、差热分析及力学性能测试等手段对合金的各种不同工艺的微观组织与性能进行了全面的测试与分析。通过改变固溶处理工艺参数,研究了固溶处理工艺与合金的力学性能之间的关系。当温度为535℃时,随着保温时间的延长,抗拉强度、硬度及延伸率基本上都是先升后降。当保温时间为3.5～4.5h时,轮缘的强度、硬度及延伸率才能达到很好的匹配。A356铸造铝合金在545℃×3.5h下进行固溶处理具有较好的综合力学性能,轮缘抗拉强度为251MPa,硬度为82HV,延伸率为12%;轮幅抗拉强度为233MPa,延伸率为9%,故固溶处理的优先工艺为545℃×3.5h。研究了室温停留时间与力学性能之间的关系,结果表明室温停留时间选择在2～10h范围之内,但不应超过12h。如果从生产周期及成本方面考虑,室温停留2h,合金就能达到预时效的目的,达到强度、硬度与塑性的良好匹配。通过改变时效处理工艺参数,研究了轮缘的时效处理工艺与力学性能之间的关系,结果表明在设定的在三个温度上进行时效处理保温3h时,合金的综合力学性能均很好,因此对A356铝合金进行时效,能够使其达到一个强度、硬度与延伸率均为极值的平衡点。当固溶处理工艺采用535℃×4.5h时,时效温度为135℃时,时效时间至少为3h;当采用优选的固溶工艺545℃×3.5h,时效温度为135℃时,则当时效时间为0.5h时,即能达到使强度与塑性达到较高的水平。当时效温度升高至145℃时,则在保温1h时,即能达到较好的强度与延伸率的配合。比较了固溶处理工艺为535℃×4.5h及545×3.5h两种工艺下合金的力学性能,表明对于A356铸造铝合金,如果在应用时对塑性的要求较高时,则固溶处理可采用工艺535℃×4.5h;如果对其塑性要求不高,而希望缩短生产周期,提高生产效率时,则建议采用工艺545×3.5h。通过对金相组织观察,随着固溶温度和保温时间的增加,α(Al)长大,同时Mg、Si在固溶过程中溶入α(Al)基体中的越多,在随后的时效处理中,非平衡析出大量细小弥散、均匀分布的Mg2Si相,随着保温时间的增加,共晶明显减少, Si颗粒聚集粗化大部分分布在α相中,α相更加圆整,并进一步增大。从而使合金经过热处理后强度、硬度上升,而塑性下降。利用扫描电镜观察了拉伸试样的断口,断口微观形貌表现为典型韧窝断口特征,均为塑性断裂,当固溶时间一定时,保温时间适度,则其韧窝大且深,塑性较好,当保温时间过长,则韧窝较浅,塑性较差。DSC分析了A356合金的时效析出动力学,50～60℃内析出G.P区, G.P区的析出主要是由空位的团聚所造成的,而与Mg、Si元素的扩散关系不大。250～300℃范围内析出β’相, Mg、Si原子在G.P区富集并有序化,在晶界上或螺旋位错上直接沉淀出来的。当温度在300℃以上析出与基体完全失去共格的β相,使合金性能下降。更多还原

【Abstract】 At present, vehicles are developing to be lightweight, high speed, safety, comfort, energy-saving and environmental protecting. It is trend to increase the aluminumzation of vehicles because of its lower density, high strength-to-weight ratio and so on. In this paper, the influences of heat treatment on the microstructures and tensile properties of A356 aluminium wheel are investigated by lots of analyzing and testing methods such as Optical Microscope (OM), Scanning Electron Microscope (SEM), Differential Scanning Calorimeten (DSC) and tensile testing.The relations between the solution treatment technology and the mechanical properties are investigated by variation of solution treatment parameter. .When the solution temperature is 535℃,the the mechanical properties including the tensile strength, the hardness and the elongation of the alloy rise first, then descend. When the solution time reach to 3.5～4.5h,the comprehensive properties are well matched.The results show that the A356 cast Aluminum alloys have good comprehensive mechanical properties by solution treatment for 3.5 hours at 545℃,and for the wheel rim the tensile strength is 251MPa,the hardness is 82HV,and the elongation is 12%; The tensile strength is 233MPa, and the elongation is 9% for the wheel breadth.So the technology of 545℃×3.5h is recommended .The relations between the aging time at room temperature and mechanical properties show that the time is suitable within 0～10h, but it can`t be over 12 hours. when the time is 2 hours ,the alloys have good comprehensive mechanical properties.The relations between the aging heat treatment technology and the mechanical properties are discussed by changing aging technology parameters. The results show that the mechanical properties go beyond the standard by aging treatment for 3 hours at the three temperature being set. So for A356, the tensile strength ,the hardness and the elongation can reach the highest simultaneously by aging treatment .When the solution technology is 535℃×4.5h,the aging time is at least for 3h.If the solution technology is 545℃×3.5h and the aging temperature is 135℃,when the aging time is 0.5h,the tensile strength and the plastic properties are very good .The time must be prolonged to 2.5～3.5h.If the aging temperture is 145℃,when the aging time is 1h,the comprehensive properties are well matched .So the time will be 1.5h～2.5h.The mechanical properties are compared between solution threatment of 535℃×4.5h and 545℃×3.5h.If the aim is to be good plastical properties ,the solution technology for 535℃×4.5h is suitable; and the solution technology for 545℃×3.5h is available for shortening the production cycle.The microstructure is observed through metallography microscope. The higher of the solution temperature and the longer of the heat preservation time are, the bigger the particles ofα(Al)grow, and the more Mg and Si would dissolve inα(Al) during the solution process. And the eutectic particles reduce obviously while Si particles become bigger.Α(Al)phases become grounder, the strength and hardness grow while the plastic properties drop.The fracture of tension specimens is observed on SEM. And the micro-appearance is typical plastic fracture. If the solution temperature and time are suitable, the dimples are big and deep. The plastics properties also are good. Either them less or higher than the optimal process parameters would lead to poor properties.The aging kinetics is discussed by means of DSC. G..P area is formed within 50～60℃, which attributes to vacancy’s reuniting .β’-phase is formed within 250～300℃and deposited from the grain boundary or the screw dislocation because of Mg and Si atom enrichs in G.P area. The properties of the Al alloy will become bad whenβis formed over 300℃.更多还原