【作者】 吕书林；

【导师】 吴树森；

【作者基本信息】 华中科技大学 ， 材料加工工程， 2012， 博士

【摘要】 半固态成形作为一种新型的材料加工技术能够生产出形状复杂且力学性能优异的近净形零件。半固态成形包括流变成形和触变成形,流变成形以其低成本和高生产率等优点成为近年来半固态成形领域研究的热点,流变成形的关键在于开发高效的制浆工艺,超声振动制浆工艺就属于其中的一种。5000系列变形铝镁合金广泛应用于飞机、汽车、舰船、家电等领域,而传统的塑性成形工艺难以制造壁厚差别大的零件,半固态成形工艺有望解决这一问题并扩大该类合金的应用范围。本文利用直接超声振动法制备出5052铝合金半固态浆料,并开发出一种新型的间接超声振动制浆技术,将制得的铝合金半固态浆料直接压铸或挤压铸造成形。首次探索了制备5052变形铝镁合金半固态浆料的可行性及其充型能力。利用直接超声振动法制备半固态浆料,系统地研究了启振温度、超声作用时间和后续保温时间对半固态浆料组织的影响。结果表明,在665～675℃这一温度区间开始对5052铝合金熔体进行超声处理,90s后可获得初生α-Al晶粒直径在110μm左右、形状系数在0.4以上的非枝晶半固态浆料。当浇注温度为637℃、压射比压为80MPa、压射速度为3.5m/s时,浆料具有最佳的充型能力。开发出一种新型的间接超声振动制浆装置,通过对样杯底部施加超声振动以间接的方式处理杯内铝合金熔体。借助统计能量分析法(SEA)计算了样杯内铝液的声压分布情况以及样杯各处的均方振速。结果表明,超声波主要通过样杯底部发生的共振传入铝液,并在铝液内产生一自杯底向液面递减的声压梯度,杯底附近铝液的声压达到了4.86MPa,样杯与变幅杆共振时杯底均方振速为5.393m/s。物理模拟试验证明间接超声振动能使液体内部产生声流效应、空化效应和热效应。采用间接超声振动法制备出高质量的铝合金半固态浆料。与直接超声振动法相比,间接超声振动法制浆效率更高,制浆效果更好,且避免了铝液对钛合金变幅杆的腐蚀。系统地研究了间接超声振动启振温度和超声作用时间对5052、5083和ZL101铝合金半固态浆料组织的影响。在一定的制浆时间内,为避免浆料中出现大量的玫瑰状组织,启振温度设置在合金的液相线为宜。在液相线温度开启振动30s后,初生α-Al相的平均晶粒直径和平均形状系数均随超声作用时间的延长而不断增大,在50s左右能获得晶粒直径和形状系数匹配良好的半固态浆料,此时三种铝合金半固态浆料的初生α-Al相平均晶粒直径在75μm以下,平均形状系数高于0.52。间接超声振动对半固态浆料还兼具除气作用。结合高固相分数半固态浆料中非枝晶组织的演化过程,探讨了间接超声振动的制浆机理。样杯底部产生的空化效应促进熔体非均质形核,同时,对熔体有冷却作用的样杯底也是形核的重要场所。附着在杯底的细小晶粒在高频机械振动作用下于杯底-晶粒界面处产生微裂纹,随后被空化泡和液流冲击脱落进入熔体内部而成为异质核心。在声流效应和空化效应的搅拌下,熔体中的晶核直接生长为球晶。研究了间接超声振动时间对半固态流变压铸件的组织和力学性能的影响。由超声处理50s的浆料压铸而得的铸件具有最佳的力学性能,该条件下的ZL101和5083铝合金的力学性能优于传统液态压铸件。开发出5052、5083和ZL101铝合金的流变挤压铸造工艺,获得了力学性能优异的半固态成形件。重点研究了间接超声振动时间、挤压压力和模具预热温度对5052铝合金流变挤压铸件的组织与力学性能的影响规律。当超声处理时间为50s、挤压压力为100MPa、模具预热温度为300℃时铸件具有最佳的力学性能,铸件的抗拉强度和伸长率分别为225MPa和17.8%,与液态挤压铸件相比,分别提高了9.7%和42.4%。在此工艺条件下,5083铝合金流变挤压铸件的抗拉强度和伸长率分别为274MPa和13.5%；经T6热处理的ZL101铝合金流变挤压铸件的抗拉强度达到338MPa,伸长率为8%。更多还原

【Abstract】 The semisolid metal processing, including thixoforming and rheocasting, offers the opportunity to manufacture net-shaped components with complicated shape and good mechanical properties. More and more researches have been focused on rheocasting in recent years because of its low cost and high productivity. The most critical task in rheocasting is to develop efficient processes for making of semisolid slurry, and the ultrasonic vibration process is one of these processes. The 5000 series Al-Mg alloys have been found a large variety of applications in aircraft, automobiles, ships and home appliances. Nevertheless, it is hard for traditional plastic forming technologies to produce components with great difference in thickness. Rheocasting process is expected to solve this problem and expand the application fields of these alloys. In this paper, semisolid slurry of 5052 Al alloy was prepared by direct ultrasonic vibration (DUV). A new type of indirect ultrasonic vibration (IUV) process was developed based on DUV. The semisolid slurry was formed by rheo-diecasting or rheo-squeeze casting.The feasibility of preparing 5052 Al alloy semisolid slurry and its filling capability were firstly explored. The effects of vibration starting-temperature, treatment time and isothermal holding time on the microstructure of semisolid slurry produced by DUV were systematically investigated. The results indicate that non-dendritic 5052 Al alloy semisolid slurry with average particle diameters about 110μm and average shape coefficients above 0.4 could be obtained when the melt is treated by DUV for 90s with starting-temperatures between 665℃and 675℃. The slurry has the best filling ability when the pouring temperature, specific pressure of injection and injection speed are 637℃,80MPa and 3.5m/s, respectively.A new IUV apparatus, in which the horn was vibrated under the bottom of the metallic cup containing alloy melt, has been developed in this research. Statistical energy analysis was employed to calculate the acoustic pressure distribution of the melt and the mean-square vibration velocity of the cup. The results indicate that the ultrasound is mainly introduced into the melt through the resonance of the cup bottom, and an acoustic pressure gradient is induced by IUV. The acoustic pressure of the melt has a maximum value of 4.86MPa near the bottom and decreases from the bottom to the melt surface. The mean-square vibration velocity of the cup bottom reaches 5.93m/s. Physical simulation experiments confirm that acoustic stream, cavitation and thermal effect can be induced by IUV.The IUV process can produce excellent Al alloy semisolid slurry. IUV process not only has a higher efficiency and better effect for preparing slurry but also avoid the erosion problem of titanium alloy horn by the melt, compared to DUV process. The effects of processing parameters such as IUV starting-temperature and treatment time on the microstructures of semisolid slurry of ZL101,5052 and 5083 Al alloys were investigated. In order to eliminate rosettes in the slurry, the optimum vibration starting-temperature is the liquidus temperature of an alloy in a specified time. When the alloys are treated by IUV for more than 30s, the average diameters and shape coefficients of primary a-Al particles increase with the time. Semisolid slurry with good combination of grain size and shape coefficient could be obtained by IUV treatment for 50s. In this case, the average diameters of the ZL101,5052 and 5083 Al alloys are less than 75μm, and shape coefficients above 0.52. Apart from preparing semisolid slurry with fine microstructure, IUV had degassing effect on semisolid slurry.The mechanism of preparing semisolid slurry with IUV was discussed by analyzing the evolution of non-dendritic grain in the semisolid slurry with high solid fraction. Cavitation generated by IUV enhances heterogeneous nucleation in the melt, and the cup bottom plays an important role in producing nuclei because of its cooling effect for the melt. High frequency vibration causes micro-cracks on the interface between the cup and grains, and then cavitation and acoustic streaming infiltrate liquid into the cracks and lead to the separation of the grains which act as heterogeneous nuclei. The nuclei directly grow into spherical particles under stirring by cavitation and acoustic streaming.The effects of IUV treatment time on the microstructures and mechanical properties of rheo-diecasting samples were studied. The rheo-diecasting samples of ZL101 and 5083 Al alloys treated by IUV for 50s have the maximum mechanical properties which are superior to liquid diecasting samples.The rheo-squeeze casting of 5052,5083 and ZL101 alloys was studied, and semisolid components with good mechanical properties were obtained. The emphasis was to study the effects of processing parameters such as IUV treatment time, squeeze pressure and mould preheating temperature on the micro structure and mechanical properties of 5052 Al alloy. The best mechanical properties of rheo-squeeze casting 5052 Al alloy were achieved under a squeeze pressure of 100MPa, IUV treatment time of 50s and mould preheated temperature of 300℃. The tensile strength and elongation were 225MPa and 17.8% respectively, which were improved by 9.7%and 42.4%respectively compared to conventional squeeze casting sample. Under this processing conditions, the tensile strength and elongation of 5083 Al alloy were 274MPa and 13.5%respectively, and the T6 heat treated ZL101 Al alloy with a tensile strength of 338MPa and an elongation of 8%.更多还原