【作者】 郭强；

【导师】 严红革； 陈振华；

【作者基本信息】 湖南大学 ， 材料加工工程， 2007， 博士

【摘要】 多向锻造技术是一种强应变塑性变形技术,可以细化组织,提高性能。目前国际上有关镁合金多向锻造技术的研究报道很少,为此本文以两类代表性镁合金Mg-Al-Zn系(AZ31和AZ80合金)、Mg-Zn-Zr系(ZK60合金)为研究对象,探究了固溶处理工艺对镁合金变形性能的影响,进而系统研究了镁合金高温下单向压缩变形行为,在此基础上首次系统研究了镁合金铸锭多向锻造变形特性,为制备性能优异大尺寸镁合金锻坯提供了详细的实验数据和理论依据,主要研究内容和研究结论如下:(1)系统研究了固溶处理工艺对镁合金变形性能的影响,得到AZ80和ZK60两种合金的最佳固溶处理工艺参数,分别为390℃/16h和330℃/16h。实验结果表明合金内的粗大第二相粒子(共晶相)是材料在单向镦粗变形时开裂的根源,固溶处理能有效地提高合金变形能力,最大成形极限增幅可达20%~30%。(2)系统研究了Mg-Al-Zn系合金高温单向压缩变形行为。合金高温单向压缩流变应力行为呈现典型的动态再结晶特征,在温度T≥250℃时,流变应力行为对应变速率敏感;而温度T=200℃时,应变速率对流变应力影响不大。在高应变速率条件下变形,合金试样温度将有较大幅度升高,约为30~50℃,其温升变化量是应变量的函数。初始晶粒度差异和基面择优取向存在是造成挤压态AZ31合金与另两种铸态合金(AZ31和AZ80合金)流变应力和动态再结晶行为不同的主要原因。挤压态AZ31合金热压缩变形流变应力行为强烈地受到变形温度的影响,当变形温度低于300℃时,流变应力呈现幂指数关系;当变形温度高于350℃时,流变应力呈现指数关系;而铸态AZ31和AZ80合金高温压缩下的流变应力均符合幂指数函数关系。同时,合金元素铝含量的增加将降低Mg-Al-Zn系合金的堆垛层错能,阻碍位错束集,增加位错高温攀移倾向。随着合金化程度的增加,Mg-Al-Zn系合金的变形激活能呈现增大的趋势,与铸态AZ31合金变形激活能为167KJ/mol相比,相同条件下铸态AZ80合金变形激活能增大到220KJ/mol。(3)系统研究了变形条件(变形温度、应变速率和应变量等)对ZK60合金高温单向压缩变形过程中流变应力行为和显微组织演变规律的影响,并对ZK60合金在高温压缩道次间软化规律进行了研究。结果表明合金高温压缩道次间的静态软化程度较低,静态软化行为以亚动态再结晶为主,建立了亚动态再结晶动力学模型,相应的亚动态再结晶激活能约为50.12KJ/mol,远小于动态再结晶激活能189 KJ/mol。变形温度对亚动态再结晶过程影响较大,高于应变速率对其影响。(4)首次系统研究了Mg-Al-Zn系合金高温多向压缩变形行为。粗晶镁合金多向压缩大变形晶粒细化机制为变形带诱发晶粒破碎,同时伴随动态再结晶发生。形变中合金晶粒细化存在一定程度,继续变形晶粒度基本稳定;在一定范围内增大道次压下量和降低变形温度均有助于组织细化;重加热时再结晶组织得到一定程度的回复和软化,晶粒尺寸增大。与连续单向变形相比,多向变形时合金的变形带取向随外加载荷方向变化而改变,在晶粒内部相互交错,有利于组织细化。AZ80合金在多向压缩变形过程中,在晶粒细化的同时,合金中残留的少量大尺寸的Mg17Al12相颗粒在外加载荷作用下经历了破碎—回溶—析出—重溶过程,形成的细小点状析出相钉扎在再结晶晶界上,阻碍位错运动,限制晶粒的长大。(5)系统研究了合金化对Mg-Al-Zn系合金多向压缩变形过程中力学行为的影响。AZ31和AZ80两种合金具有相似的变化规律,硬度和抗拉强度随着变形道次的增加先升高而后逐渐降低,而伸长率则一直增加,经过7个道次的变形,两种合金的伸长率达到最大。在高温下反复加热导致合金回复软化是AZ31合金硬度值降低的主要原因,对于AZ80合金而言,析出相重溶是导致合金硬度值下降的另一因素。在相同变形条件下,适当地降低变形温度能有效提高合金的强度和伸长率。与多向压缩变形相比,连续单向锻压有助于合金的伸长率的提高,而强度则有所降低。(6)系统研究了多向锻造过程中镁合金的晶粒转动与取向行为。在对铸态AZ80合金研究中发现:多向锻造变形对合金晶粒转动和取向的定向流动是材料塑性变形过程发生择优取向的主要原因。在无新晶粒产生的小变形情况下即可通过滑移和孪生使晶粒发生转动形成较强的基面择优取向。本实验条件下,除第4道次外,其余前9道次压缩面均出现明显的基面取向。10道次后合金开始呈现随机取向分布。变形温度和变形方式对合金晶粒取向影响较大,适当降低变形温度和采用多向变形均能促使压缩面基面取向增强,而道次应变量在本实验中对晶粒取向影响不大。(7)研究了连续多向锻造工艺对大尺寸AZ80镁合金铸锭(110mm×70 mm×60 mm)组织和性能的影响,深入探究了合金在多向锻造过程中的开裂行为,存在两种方式:边部周向裂纹和芯部微裂纹。多向锻造工艺特点有利于阻碍合金内部微裂纹的扩展。变形温度和应变量的精确控制是保证合金形变中不发生开裂的关键。更多还原

【Abstract】 Multiple forging technology is a novel severe plastic deformation method to prepare structural materials with refined grain and high performance. Fever reports are available about the application of this technology in magnesium alloy. In the present dissertation, two typical types of magnesium alloys, i.e. AZ31 and AZ80 alloy of Mg-Al-Zn system, ZK60 alloy of Mg-Zn-Zr system are chosen as the samples to study the effects of homogenizing annealing process on the deformability and the deformation behaviors during uni-axial compression and multi-directional deformation at elevated temperature. The main conclusions are drawn as follows:(1) The effect of homogenizing annealing process on the microstructures and mechanical properties of AZ80 and ZK60 alloy are systematically investigated. The best homogenizing annealing regularities of the two alloys are annealing at 390℃for 16h and at 330℃for 16h, respectively. The initiation of the cracks in the as-cast AZ80 alloy or ZK60 alloy during upset test is related with the second phase or eutectic compound in the matrix. After solution treatment, the plastic forming ability of the alloys can be effectively improved and the maximum amplitude of increment reaches 20%~30%.(2) The deformation behaviors of Mg-Al-Zn alloys are studied by uni-axial compression (UC) testing under different deformation variables. The experimental results show that flow stress behavior, which is characteristic for hot working processes, strongly depends on the strain rate during high deformation temperatures(T≥250℃)except low temperature (T=200℃). The temperature rise is between 30~50℃and is a function of the strain when the specimens are compressed at high strain rate of 5s-1. Constitutive analysis suggests that the flow stress behaviors of the as-extruded AZ31 alloy strongly depend on the deformation temperature and the relationships between the flow stress and the deformation temperature as well as the strain rate can be represented by the exponential equation during deformation temperature below 350℃, and by the power equation when deformation temperature over 350℃, whereas the flow stress behaviors of as-cast AZ31 or AZ80 alloy can be represented by the exponent-type equation during the whole deformation temperatures (T≥250℃). Initial grain size and a distinctive basal texture lead to the great difference between as-extruded AZ31 alloy and the other two groups of cast alloys AZ31 and AZ80, including the variation in the flow stress equations and dynamic recrystallizaion behaviors. The increment of alloying element Al can decrease the stacking fault energy and enhance the process of dislocation climb, and thus reduce the tendency for dislocation pile-up to cross-slip. As a result, for cast AZ80 alloy, dynamic recrystallization (DRX) is delayed and the activation energy for the plastic deformation process sharply increases from 167 KJ/mol to 220 KJ/mol as compared with the cast AZ31 alloy under the same deformation condition.(3) Not only the effects of deformation variables, including deformation temperature, strain rate and strain, on the deformation behaviors of ZK60 alloy during UC are investigated, but also the static softening behaviors during multistage hot deformation of ZK60 alloy are studied by isothermal interrupted hot compression tests. The results show that the main static softening mechanism of the samples during the interrupted deformation is metadynamic recrystallization (MDRX). The MDRX model of ZK60 has also been established with the activation energy of approximately 50.12KJ/mol which is much lower than that of DRX, approximately 189 KJ/mol. In the present study, deformation temperature has a more important influence on the MDRX process than the strain rate.(4) The deformation behaviors of Mg-Al-Zn alloys under multi-compression (MC) at elevated temperature are systematically studied. A main characteristic of microstructure evolution is directly associated with grain splitting due to the formation of microbands that develop in various directions. Such microbands intersect with each other during hot MC processing, resulting in continuous subdivision of the coarse grains into misoriented fine domains. Further deformation leads to increase in the number and misorientation of these boundaries and finally development of fine equiaxed grains at high strain. During the deformation there exists a critical strainεc, above which a homogeneous microstructure with fine DRX grains can be attained, and after that the grains of the alloys can only be refined to a certain size. The second phase Mg17Al12 in the as-cast AZ80 alloy which remained after solution treatment experiences a series of change with the process of breaking up—dissolving—precipitating—re-dissolving. Fine precipitated phase locates at the DRX grain boundaries, restraining the slipping of the dislocations and the movement of the grain boundaries.(5) The effects of alloy composition on the mechanical properties of the Mg-Al-Zn alloys during hot MC process are systematically investigated. The results show that for AZ80 alloy and AZ31 alloy, both hardness and tensile strength firstly increase, then reduce gradually with increment of pass number. However, the elongation continuously increase with the pass number before 7-passes. Softening and the re-dissolving of the second phase Mg17Al12 during the re-heating process lead to the decrease of hardness of the AZ80 specimens. Decreasing deformation temperature in a certain range and deformation by multi-directional forging method can both refine the grains in the as-cast ingots and improve the mechanical properties. (6) The grain orientation of the as-cast AZ80 alloy during hot MC process is studied for the first time in the world. The results show that the preferred orientation during plastic flow is mainly resulted from grain rotation. For the samples deformed at small strain, slipping and twinning result in grain rotation and lead to the formation of relatively strong basal preferred orientation. Basal preferred orientation changes with the direction of the applied loading axis and is parallel to the compression direction during the first 9 passes except the 4th one. After 10 passes, no preferred orientation is observed by XRD. Forging temperature and deformation mode have an important influence on the evolution of grain orientation except for pass strain. Both decreasing deformation temperature in a certain range and multi-directional deformation mode can promote the formation of strong basal orientation.(7) Large sized AZ80 alloy samples with the dimensions of 110mm×70mm×60mm were produced by continuous multiple forging processing. The two fracture modes of the samples during forging are intensively investigated. Deformation temperature and pass strain are the key factors controlling the cracking of the samples during deformation.更多还原