【作者】 赵大志；

【导师】 路贵民；

【作者基本信息】 东北大学 ， 材料加工工程， 2009， 博士

【摘要】 本研究采用近液相线半连续铸造技术制备的ZL116、6063合金半固态坯料,在进行坯料试样的二次加热实验,合金压缩热模拟的基础上,进行了合金的半固态挤压成形,以期采用具有组织细小均匀,具有非枝晶组织的,适合于半固态触变成形的铝合金坯料,通过高固相率条件下的半固态挤压成形,获得成形性好,力学性能优良,高效率,低成本的成形件。本文通过近液相线半连续铸造技术制备ZL116及6063铝合金半固态坯料,研究了铸造速度与冷却强度对坯料组织的影响,分析了半固态坯料微观组织形成机理。由于过热度小,近液相线半连续铸造使熔体内的温度场趋于均匀分布,形核数量明显增多;成分过冷的减小,使晶粒在生长过程中互相接触后,枝晶的熟化程度加大,有利于蔷薇状晶粒组织的形成;铸造速度决定一、二次冷却时间和冷却效果,将直接影响到坯料横截面上微观组织的形貌及其分布,适宜的冷却强度和铸造速度有利于坯料截面上获得分布均匀的非枝晶组织。当冷却强度一定时,ZL116合金铸造速度控制在150～200mm/min,6063合金铸造速度控制在100～170mm/min时,可获得表面质量光洁,横截面上分布均匀、细小、呈近球状或蔷薇状组织的半固态坯料。对半固态坯料进行了二次加热实验研究及理论计算,考察分析了半固态坯料二次加热组织随加热温度以及保温时间的变化及演变机理。合金半固态坯料在二次加热过程中,随着加热温度的升高,晶粒球化进程加快,所需要的保温时间缩短;保温温度相同时,随着保温时间延长,球化的晶粒将发生粗化;在共晶合金二次加热过程中,共晶硅熔断发生粒状化,在重熔液相内形成分布均匀、细小的硅颗粒,硅颗粒将随着保温时间的延长不断长大,并逐渐减少,最终熔化消失。ZL116合金二次加热温度为575℃,保温10～15min时,6063合金二次加热温度为620℃,保温10～15min时,重熔组织演变比较稳定,可以获得尺寸较小,球化效果比较好,适于进行半固态触变成形的组织。ZL116、6063合金坯料在固态以及半固态条件下的压缩变形研究表明:在高固相率半固态压缩变形中,相同的温度条件下,变形应力随着应变速率的增加而增大;随着变形温度的提高,变形应力增加幅度逐渐减小。与固态压缩变形相比,半固态条件下应力变化受应变速率的影响较小,采用较低的应变速率可获得比较稳定的变形效果。根据实验分析及热模拟分析结果,对近液相线半连续铸造制备的ZL116、6063合金坯料进行半固态及固态挤压成形,分析研究了成形件的组织及性能,对半固态挤压成形件的热处理工艺进行了优化。实验表明:合金进行半固态挤压成形时,由于变形抗力很小,变形过程中各方向受力比较均匀,坯料截面上流速分布一致,受成形件壁厚影响很小,有利于获得分布均匀的变形组织;与固态挤压成形相比,半固态挤压成形温度高,固相颗粒内固溶大量的溶质原子,随着快速冷却,以析出相的形式弥散分布在基体组织上,不仅阻碍了再结晶晶粒的长大,而且起到了强化作用。成形中,由于晶间液相的缓解作用,使变形固相颗粒内位错堆积而形成的应力集中明显减小,变形颗粒仅发生了部分回复与再结晶,起到了细晶强化和位错强化的作用。由于半固态挤压是在固、液两相区内进行成形,不需要对铸态坯料进行均匀化处理,可以有效缩短合金的成形周期。ZL116合金在575℃进行高固相率半固态挤压成形,成形件表面光洁,横截面组织分布均匀,在成形组织上弥散分布着大量细小的硅颗粒以及沉淀相,起到一定的强化作用。部分变形的固相颗粒由于再结晶使晶粒更加细小,与固态挤压工艺相比,具有良好的半固态挤压成形性与力学性能。进行半固态挤压成形件热处理优化,确定出最佳工艺方案为:545℃固溶4h,178℃时效10h,抗拉强度为325MPa,伸长率为14.6%,与铸态成形件相比,塑性提高了192%,固溶时间明显缩短,降低了能耗。6063合金半固态挤压成形件经在线固溶及自然时效,抗拉强度达到240MPa以上,伸长率达到19%以上,完全达到合金挤压成形所要求的T6处理的性能指标。利用正交实验确定出成形件热处理优化工艺为:520℃固溶1h、175℃时效6h,测得的抗拉强度为275MPa,伸长率为14.23%。更多还原

【Abstract】 The semi-solid billets of ZL116 and 6063 aluminium alloys were prepared by near-liquidus semi-continuous casting process. Based on the reheating test and thermal simulating of the sampling billets, the aluminium billets of ZL116 and 6063 with the homogeneous, fine non-dentritic microstructures, which is suitable to thixoforming in semi-solid state, were extruded in the semi-solid state of high percentage of solid phase in order to obtain the bars with quality, good feasibility, high properties and efficiency, cost saved.The effect of casting velocity, the mechanism and effect of processing parameters on the microstructure of ZL116 and 6063 aluminium alloys billets cast by near-liquidus semi-continuous casting process was studied. Because of small superheat in the melt during the near-liquidus semi-continuous casting process, the temperature distribution in the melt is trend to be uniform, and the quantities of nucleating is obviously increased. The ripening trend of dendritic grain would be increased, and the rosette-shaped grains could be obtained while the growing grains contacted each other because of the decrease of the composition undercooling. The acting time and cooling effect of first and secondary cooling on the melt would be changed with the casting velocity. And the microstructure morphology and distribution in the cross-section of the billets would be affected. The billet with the homogeneous, fine non-dentritic microstructures could be obtained under the condition of the suitable casting velocity and cooling intensity. Under the certain cooling intensity during near-liquidus semi-continuous casting, the semi-solid billets with the smooth surface and homogeneous, fine rosette-shaped or spherical grains could be obtained at 150～200mm/min for ZL116 alloy and 100～170mm/min for 6063 alloy.The reheating test and analyzed calculation of the semi-solid billets were studied. And the evolution mechanism of reheated microstructures with the change of reheating temperature and holding time was studied. The results showed that the spheroidizing process of the grains is fastened, and the holding time is shortened due to the increase of reheating temperature. At the same reheating temperature, the spheroidized grains would be coarse with the increase of holding time. During the eutectic alloy is reheated at the eutectic temperature, eutectic Si would be fused and become the homogeneous, fine granules distributed in the remelted liquid. The Si granules grow and are decreased with the increase of holding time, then disappeared finally. The evolution of remelted microstructure is stable, and the small-sized and spherical microstructure which is suitable for semi-solid thixoforming could be obtained while the sample of ZL116 alloy was reheated at 575℃for 10～15min and the sample of 6063 alloy was reheated at 620℃for 10～15min.The deformation-compressed mechanisms of the ZL116、6063 alloy samples in solid and semi-solid states were studied by the thermal simulation technology. The results showed that: In the semi-solid state of high percentage of solid phase, the deforming stress would be increased with the strain rate at the same temperature, and the increasing extent of the stress is decreased with the increase of the temperature. Compared with the compression in solid state, the effect of strain rate on the stress in the semi-solid state is less. The stable compressing deformation of the ZL116、6063 alloy billets, which were cast by near-liquidus semi-continuous casting could be obtained in semi-solid state at the low strain rateAccording to the analyzed results of reheating test and thermal simulation, the billets of ZL116 and 6063 alloys cast by near-liquidus semi-continuous casting, were extruded in solid and semi-solid state. The microstructures and properties of the extruded bars were analysed. The processing parameters of heat treatment for the semi-solid extruded bars were optimized with the orthogonal design method and artificial neural network method. The results showed that: during the extrusion in semi-solid state, the deformation of billets is mainly composed of sliding of solid particles and its plastic deformation. The energy produced for deforming is little because of the small deforming resistance. The distribution of the stress and the flow velocity in the each part of the billet is uniform during the deformation. And the effect of the bar thick is less. The uniform deformed microstructure could be obtained. During the semi-solid forming, most of solutes are dissolved into the matrix, and distributed uniformly in the matrix as the precipitated phase while the fast cooling. The growth of the recrystallized grains is damped and the matrix is strengthened. The stress concentration in the deformed solid phase for the dislocations packing is decreased because of the liquid remission between the grains. Just some of deformed grains were recovered and recrystallized, and the fine-grain strengthening and dislocations strengthening are promoted. The effect of composition segregation in as-cast microstructure on extruded forming could be avoided due to forming between the solid and liquid zone. So the as-cast billets needn’t to be homogenized.Extruded at 575℃in semi-solid state with high percentage of solid phase, the bar surface of ZL116 alloy is finished and the microstructure in the cross-section is uniform. A large number of Si particles and precipitated phases are distributed in the matrix, and the matrix is strengthened. Some of deformed solid particles became more finer because of recrystallizing. Compared with the solid extruded bars, the bar extruded in semi-solid state is more feasible and has the more noticeable properties. The optimized parameters of heat treatment for the semi-solid extruded bar of ZL116 alloy are: solid solution at 545℃for 4h and aging at 178℃for 10h. The tensile strength of the bar is 325Mpa, and the elongation is 14.6%. The plasticity is increased 192% than one of as cast part. And the time of solution is shortened. The energy consumption is decreased. The properties of the semi-solid extruded bars of 6063 alloy are improved due to the solid solution and natural aging on line. The tensile strength of the bar is above 240Mpa, and the elongation is above 19%. The properties index has been approached those of T6 heat treatment for the parts. The parameters of heat treatment for the semi-solid extruded bar of ZL116 alloy optimized by the orthogonal design method are: solid solution at 520℃for 1h and aging at 175℃for 6h. The tensile strength of the bar is 275Mpa, and the elongation is 14.23%.更多还原