【作者】 陈洪美；

【导师】 于化顺； 姜石峰；

【作者基本信息】 山东大学 ， 材料加工工程， 2009， 博士

【摘要】 镁合金具有高的比强度、比刚度,良好的阻尼减震性,优良的机加工性能和易于回收利用,被誉为21世纪最具发展前途的金属结构材料,是近年来国内外材料界研究的热点之一。进一步提高镁合金的综合性能,满足某些结构件“以镁代铝”甚至“代钢”的性能要求;改善镁合金的塑性、减小各向异性,显著改善其塑性成形能力;简化镁合金制品的制备工艺,降低其生产成本,扩大镁合金的应用范围,是国内外研究的主要方向。本论文主要针对镁合金薄板制备难度大、性能较低且工艺稳定性较差、塑性较低且各向异性较大、塑性加工能力较差的问题,采用双辊铸轧法制备镁合金条带坯料,利用轧辊的急冷作用及半固态条件下的塑性变形细化坯料组织及第二相,减少偏析,从而改善其塑性变形能力,并缩短镁合金薄板制备工艺流程;通过轧制工艺参数及热处理工艺参数的优化,改善其综合力学性能、减小各向异性;并对铸轧过程中镁合金组织形成机理、温轧及热处理过程中组织演变及影响因素,以及轧制镁合金的强化机理进行了分析,为镁合金薄板的制备、镁合金组织及性能控制奠定了实验及理论基础。本文采用双辊铸轧(Twin Roll Casting,简记为TRC)法成功制备了厚度为3-3.5mm的ZK60和Mg-4.5Al-1.0Zn(简记AZ41)镁合金条带,确定了制备工艺参数:铸轧温度、轧辊转速及预留辊缝宽度分别为640～680℃、5～6rpm、2mm(ZK60)及630～650℃、5～6rpm、2mm(AZ41),并分析了其显微组织及形成机理。研究结果表明,双辊铸轧镁合金的组织呈枝晶状,枝晶间分布着细小的第二相,条带近表面有激冷区。铸轧过程中轧辊的急冷作用及半固态条件下的塑性变形导致铸轧镁合金的晶粒组织比传统铸锭法的组织显著细化,且晶界第二相尺寸也显著减小,分布均匀性明显改善,从而显著改善镁合金的塑性变形能力。通过讨论温轧工艺参数对铸轧ZK60镁合金组织和性能的影响,确定了0.5mm厚ZK60镁合金薄带制备的最佳轧制工艺,即先将3.5mm厚的铸轧条带在350℃轧制到1mm,1mm厚薄带在350℃退火30min,然后再在300℃下轧制到0.5mm,道次间压下量为30%,道次间退火温度为轧制温度,保温时间为5min。温轧变形改变了TRC镁合金的组织形态,由枝晶状变为纤维状组织,晶粒沿轧制方向被拉长,且薄带内部有剪切带、位错及孪晶产生。在轧制温度为350℃以上时,发生动态再结晶。随着轧制温度的降低、道次间压下量及总变形量的增加,镁合金的组织细化且剪切带的密度增加,镁合金的强度及硬度增加。在优化轧制工艺条件下,ZK60镁合金薄带的拉伸强度、屈服强度和伸长率分别为510MPa、441MPa和11.3%。通过分析退火、固溶、时效等热处理过程中温轧ZK60镁合金薄带的组织转变及其对性能的影响,确定了ZK60镁合金薄带最佳热处理工艺参数:退火热处理—375℃×10~3s;T6热处—375℃×3hrs+175℃×10hrs。在300℃及以上温度退火过程中有静态再结晶发生,随着退火温度升高和保温时间的延长,再结晶晶粒比例增加,得到均匀细小的等轴晶,继续提高温度或延长保温时间,晶粒明显长大。退火处理使硬度及强度有所降低,但塑性明显提高,在最佳退火工艺条件下,ZK60镁合金薄带的拉伸强度、屈服强度及伸长率分别为388MPa,301MPa和22.9%。适当的T6处理可获得均匀细小的等轴晶组织(平均晶粒尺寸为6.7μm),且显著减小了ZK60镁合金薄带力学性能的各向异性,其三个方向的拉伸强度、屈服强度和伸长率的偏差分别为23MPa、10MPa和1.0%。轧制态ZK60镁合金薄带表现出强的(0001)基面织构,由于试样内部存在高密度剪切带,使其织构分布向垂直于轧制方向分散。随着轧制温度降低、轧制变形量和道次间压下量增大,(0001)基面织构的最大极密度增加。温轧变形过程中产生的(0001)基面变形织构对合金起到强化作用。研究热处理过程中ZK60镁合金的相析出行为得知,退火处理后晶粒内部只有圆盘状的颗粒析出,而适当工艺参数的T6处理后薄带内部既有圆盘状的颗粒也有杆状的颗粒析出。这些强化相的析出有利于提高材料的组织均匀性和抗高温软化能力。无论是退火过程的圆盘状析出相还是T6处理过程的圆盘状析出相,均含有Zn元素和Zr元素,而杆状的析出相仅含有Zn元素不含Zr元素。另外,Zr还有一部分均匀分布在基体中,起到细化晶粒的作用。温轧变形ZK60镁合金变形过程中存在三种强化机制:细晶强化、第二相粒子强化和位错强化,三种强化机制分别在温轧变形的不同阶段起主导作用,在共同的强化作用下提高镁合金的强度。在三种不同强化机制共同作用的同时,温轧变形过程中产生的(0001)基面变形织构对镁合金起到强化作用。更多还原

【Abstract】 Magnesium alloys are considered to have the most promising development outlook as metallic structural materials in 21^st century,for their high specific strength and specific stiffness,good damping capacity,excellent machinability and high recycling rate.Magnesium alloy has become one of the research hotspots in materials research field.The principal direction to improve the integrated properties of magnesium alloy is to satisfy the requirements of some structural materials,such as aluminum,or even steel.Researches which aim to increase the elongation,decrease the anisotropy and improve the plastic deformation capability,simplify fabrication technics,reduce manufacturing cost and expand the applications of magnesium alloy are also received global attention.It is difficult to fabricate magnesium alloy sheet for its poor technologic stability, low elongation,severe anisotropy and poor plastic deformation capability.Twin roll casting（designated as TRC in short） technology has fast cooling rate and plastic deformation effect under semi-solid states.It combines casting and hot rolling into a single step,which can refine grain and second-phase size,reduce segregation, improve plastic deformation capability,and also shorten the fabrication process of magnesium alloy.This paper aims to improve the integrated properties,reduce anisotropy of magnesium alloy by optimizing the rolling and heat treatment parameters.The forming mechanism of TRC magnesium alloy’s microstructure during strip casting process,the microstructure evolution of ZK60 alloy sheet during rolling and heat treatment process,and factors which influence the microstructure were studied systematically.In addition,the strengthening mechanism of warm rolled magnesium alloy was also discussed in this paper.With the above research,the experimental and academic foundations of helping improve the formability, microstructural uniformity and integrated properties of magnesium alloy sheet were obtained.ZK60 and Mg-4.5Al-1.0Zn（designated as AZ41 in short） strip with 3～3.5mm thickness were fabricated by TRC technology in this paper,and the fabrication parameters of TRC temperature,rolling speed and roller gap were also determined （those of ZK60 and AZ41 strip are 640～680℃,5～6rpm,2mm and 630～650℃, 5～6rpm,2mm,respectively）.The forming mechanism of TRC magnesium alloy’s microstructure during strip casting process was studied.The results showed that, chilled grain structure appeared just below the surface of the TRC strip,and dendrite structure developed from surface to the center of TRC strip along the thermal gradient. Fine eutectics and intermetallic compounds were seen in the interdendritic region.The grain size of TRC alloy was much smaller than that of CC alloy due to higher cooling rate and the plastic deformation effect under semi-solid states during TRC process. The intermetallic compounds were also refined and distributed uniformly,which could improve the deformation capability of magnesium alloy.The optimized rolling parameters were obtained by discussing the effect of rolling parameters on microstructure and mechanical properties of ZK60 alloy sheet. It should be separated into two steps to get ZK60 alloy sheet with 0.5mm thickness which possessed good properties.The TRC strip was rolled at 350℃to reduce the thickness from 3.5mm to 1mm in the first step.And then the warm rolled sheet was annealed at 350℃for 30min,after that changed the rolling temperature to 300℃to reduce the sheet thickness to 0.5mm.The rolling reduction ratio of 30%per pass was made for warm rolling.The sheet was annealed at the rolling temperature for 5min between the two passes of the rolling process.Warm rolling changed the shape of grains from dentrite to fibrous structure, which consisted of elongated grains along the rolling direction.High density shear bands,dislocation and twinning were formed during the rolling process.Dynamic recrystallization took place during the rolling process at and above 350℃.With the decreasing of the rolling temperature,increasing of the per pass rolling reduction and total rolling reduction,the grain size decreased,while the density of shear bands increased,strength and hardness increased.The tensile strength,yield strength and elongation of as rolled ZK60 alloy sheet produced by the optimized rolling parameters are 510MPa,441MPa and 11.3%,respectively.The optimized heat treatment parameters were also determined by analyzing the effect of annealing,solid solution and artificial aging heat treatment on microstructures and mechanical properties of warm rolled ZK60 alloy sheets.The optimized annealing parameter is annealed at 375℃for 10³s.And the optimized T6 treatment parameter is solid solution treatment at 375℃for 3hrs,and then artificial aging at 175℃for 10hrs.Static recrystallization was observed during the annealing process of ZK60 alloy sheet at and above 300℃.Fine equiaxed structure was obtained after annealing treatment.Fraction of recrystallized grain increased with the increasing of annealing temperature and holding time,but too high annealing temperature and too long holding time would induce grain growth.The strength and hardness of ZK60 alloy sheet decreased a little after annealing heat treatment,but the elongation increased obviously.The tensile strength,yield strength and elongation of ZK60 alloy sheet annealed at optimized annealing parameter are 388MPa,301MPa and 22.9%, respectively.It can be noted that proper T6 treatment can obtain fine equiaxed structure（the mean grain size is 6.7μm）,so the anisotropy of ZK60 alloy sheet’s tensile properties was obviously decreased.The deviation of tensile strength,yield strength and elongation along different angle to rolling direction are 23MPa,10MPa and 1.0%,respectively.Warm rolled ZK60 alloy sheet exhibited high intensity of（0001） basal pole texture,and the formation of shear bands tends to cause the basal pole texture tilt slightly to the transverse direction.The maximum pole intensity of（0001） pole figure increased with the decreasing of rolling temperature,increasing of per pass rolling and total rolling reduction.ZK60 alloy was strengthened by（0001） basal pole texture.Disc-shaped precipitates could be observed after annealed at different temperatures,and both disc-shaped and rod-like precipitates could be found after proper T6 treatment in the warm rolled ZK60 alloy sheet.The existence of the dispersed strengthening particles improved the uniformity of structure and the resistance to softening at elevated temperatures of ZK60 alloy sheet.Zn could be found both in disc-shaped particles and rod-like particles.Zr was found in disc-shaped or irregular shaped particles,but was not found in the rod-like precipitates.In addition, the uniformly distributed Zr in the matrix plays a role of refining the grain size.Three strengthening mechanisms existed during the rolling process of ZK60 alloy—grain refining strengthening,second-phase particle strengthening and dislocation strengthening.The three strengthening mechanisms dominate the strength effect at different moment during rolling process separately,and the strength of ZK60 alloy was improved by their corporated effects.In addition,the formation of high density（0001） basal pole texture can also strengthen ZK60 alloy.更多还原