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硕士论文-镁合金锻造成形性研究及数值模拟
摘要
作为工业轻金属材料,镁合金具有低的密度,高的比强度和比刚度,优良的阻尼减震性和电磁
屏蔽性。但由于其具有密排六方的晶体结构,室温下难以发生塑性变形,高温下又易发生晶粒粗大
和表面氧化,当前的镁合金产品以铸件,特别是压铸件居多,塑性加工产品极少,但铸件的力学性
能较差、易产生缺陷,而变形镁合金组织细小均匀、综合机械性能好。目前国内外对镁合金塑性成
形技术的研究,主要是集中在挤压、轧制和超塑性变形等,由于镁合金锻造的难度较大,对温度和
应变速率敏感,镁合金锻造成形的研究很少。为了研究复杂形状镁合金零件的可锻性,本文从镁合
金材料的高温成形性能的基础性研究入手,通过实验建立了镁合金 AZ31B 的高温流变应力模型和微
观组织演化模型,为评价镁合金热变形的稳定性,提出并构造了包含应变的三维加工图。通过将这
些基础性研究结果与大变形热力耦合有限元方法相集成,形成了预报镁合金复杂零件热成形可成形
性的模拟方法,并以此为基础研究了镁合金直齿锥齿轮的锻造成形工艺参数范围。通过直齿锥齿轮
锻造实验,证明了所选择的成形工艺参数是合适的,同时也表明这种研究方法获得了成功。
通过镁合金 AZ31B 的 Gleeble-1500 热模拟试验,在蠕变方程的基础上,提出了一个新的镁合金
动态再结晶型流变应力模型,考虑了动态再结晶的软化效应。模型认为由于变形温度决定了原子的
扩散能力和位错移动的驱动力,而应变速率决定了位错密度和晶界能的累积速度,因而峰值应力仅
取决于变形温度和应变速率。由于动态再结晶过程是热激活过程,动态再结晶体积分数可通过唯象
理论模型表示成应变的函数,而由峰值应力和再结晶分数可确定由于动态再结晶软化作用引起的应
力的下降,因此任意时刻的应力可以认为取决于峰值应力和应变。该模型表示了温度、应变速率和
应变对应力的影响,不仅适用于镁合金 AZ31B,而且也适用于其它具有动态再结晶特性的材料。模
型参数相对较少且较易确定、适用于工程计算。
通过对经典动态再结晶动力学模型的分析,本文提出了一种新的具有单参数的、反映了动态再
结晶过程缓慢-快速-缓慢特点的动态再结晶动力学模型。基于 Avrami 形式的动态再结晶动力学模
型虽然反映了动态再结晶过程,由于采用指数函数的形式,无法准确表示动态再结晶过程的慢-快
-慢的过程。基于此分析,本文提出了一种新的具有单参数的动态再结晶动力学模型,模型参数少
且物理意义明确,反映了动态再结晶过程的特点,并可得到在一定变形条件下获得细小晶粒所需的
经济合适的应变。采用定量金相分析的方法,对镁合金 AZ31B 的微观组织演化进行试验研究并建立
了新的动态再结晶动力学模型与晶粒尺寸模型,模型得到的预测结果与试验结果相一致。
基于 Prasad 的二维加工图,首次建立了新的包含应变的三维加工图(三维功率耗散图和三维失
稳图),描述功率耗散系数和流变失稳区域随应变速率、温度和应变的变化。传统的基于动态材料模
型的加工图是二维温度和应变速率空间上功率耗散图和流变失稳图的叠加,没有考虑应变的影响。
对于复杂形状零件的热变形,应变与温度和应变速率一样是影响金属流动特性的因素,加工图作为
工艺设计和优化的工具,由于没有包含应变,就不能完备地分析复杂零件热变形的可加工性。新的
包含应变的三维加工图(三维功率耗散图和三维失稳图),解决了具有明显应变软化效应的合金(如
镁合金)热变形时可加工性对应变的敏感性问题,是一个完备地反映材料可加工性的、进行工艺设
计和优化的工具。进一步针对几种不同的失稳判据绘制三维流变失稳图,分析得到适合镁合金的失
稳准则。
本文首次提出并实现了将三维加工图与有限元模拟两个过程集成起来、通过进行包含三维加工
图的数值模拟、完整分析材料内禀可加工性和应力状态可加工性的方法。可加工性包括应力状态可
加工性和材料的内禀可加工性,传统的基于动态材料模型的加工图仅说明了材料的内禀可加工性。
通过进行包含三维加工图的数值模拟,可以得到材料在特定工艺条件下温度、应力、应变、应变速
率和流变失稳区域的分布,建立了一种分析金属材料热成形全过程可加工性的方法,对可加工性的
理论和应用进行了扩展和完善。通过二维圆柱压缩的可加工性模拟,验证了加工图与有限元相结合
的研究方法及程序开发的正确性。
本文成功地完成了镁合金 AZ31B 直齿锥齿轮的锻造成形工艺设计和实验。为了研究复杂形状锻
件的可加工性,选取直齿锥齿轮为理论的验证对象,采用无齿形预锻和终锻两步等温锻造的成形工
艺。将三维加工图与三维复杂锻造模拟相结合,建立镁合金热锻成形的三维变形-温度-组织演化
-失稳模拟的耦合分析系统,研究不同的锻造温度、锻压设备的工作速度、预锻模的形状参数和摩
擦对镁合金锥齿轮锻造成形的影响,确定了最优的预锻件形状以及锻造温度、锻造速度等工艺参数。
在此基础上,本文实现了镁合金 AZ31B 直齿锥齿轮的锻造成形实验,并运用定量金相分析技术研究
了变形体内微观组织。实验结果和数值模拟结果基本上一致,证实了本文所建立的流变应力模型和
微观组织演化模型以及本文提出的通过包含三维加工图的数值模拟分析材料热变形可加工性的方法
的正确性。本文所制定的镁合金热锻成形工艺对实际生产具有指导意义。
关键词:镁合金 流变应力 动态再结晶 微观组织演化 三维加工图 数值模拟 热锻成形

ABSTRACT
Magnesium alloys are currently the lightest among structural materials with low density, high specific
strength and stiffness, superior damping capacity, high thermal conductivity, and good electromagnetic shielding
characteristics. Due to hexagonal crystal structure, magnesium alloys have relatively low workability at room
temperature and the workability has been greatly increased at elevated temperature. At present, the vast majority
of magnesium products are in the form of die castings. In contrast with cast magnesium alloys, wrought
magnesium alloys have been widely applied in aeronautical and astronautical, automotive and electronic
industries as a result of fine and uniform grains and better mechanical properties. Presently, for the plastic
forming processing of magnesium alloys, the studies are focused on extrusion, rolling and superplastic forming.
Due to the sensitivity to the temperature and strain rate, less research on forging of magnesium alloys was
conducted. In order to investigate the complicated shape parts forgeability of magnesium alloys, to begin with,
the hot formability of magnesium alloys was studied and the flow stress model and microstructural evolution
model were put forward. To evaluate the stability of magnesium alloys during hot deformation, the 3-D
processing maps were proposed. The numerical simulation of hot forging processing by integration 3-D
processing map and FEM was used to evaluate the forgeability of magnesium alloy AZ31B. Based on these, a
new hot forging technique was developed and hot forging experiment of spur bevel gear was carried out to
validate the accuracy of the new model and method.
Based on the Gleeble-1500 thermo-mechanical simulation tests of magnesium alloy AZ31B, a new model
of flow stress characterized by dynamic recrystallization (DRX) for magnesium alloy was put forward.
Theoretically, in the flow rule the atomic diffusibility and the driving force of dislocation migration are
dependent on the temperature, and the dislocation density and the cumulation of grain boundary energy are
dependent on the strain rate. So, the peak stress is taken as the function of the temperature and the strain rate
according to the creep equation. Since the DRX is a thermally activated process, the recrystallized volume
fraction can be regarded as the function of strain through Avrami equation. The descending of flow stress is
mainly dominated by the recrystallized volume fraction. On the basis of this idea the flow stress at different
strain is regarded as the function of the peak stress and the strain. This new model demonstrates the dependence
of the flow stress on various temperatures, strain rates and strains, which is fit for not only magnesium alloy
AZ31B but other materials characterized by DRX as well. Three are few parameters in the new model which are
easy to be determined and the new model is simple and accurate.
By analyzing Avrami equation, a new one-parameter DRX kinetics model exhibiting the characteristic of
DRX process, i.e. slow, fast and slow was put forward. Though the DRX kinetics models based on Avrami
equation is able to generally denote the process of DRX, it is not able to accurately denote the flow-fast-slow
process of DRX because of the exponent function form. The new DRX kinetics model not only demonstrates the
characteristic of DRX but also obtains the favorite and economic strain for fine grains. Through quantitative
metallography, the DRX kinetics model and grain size model of magnesium AZ31B were proposed. The
predicted values of model are in good agreement with the experimental results.
Based on the traditional two-dimensional (2-D) processing maps proposed by Prasad, the three-dimensional
(3-D) processing maps including strain were first put forward in this paper, which describe the variations of the
efficiency of power dissipation and flow instability domains with strain rate, temperature and strain. For the hot
deformation of complicated shape parts, the stain is regarded as an important factor like the temperature and the
strain rate, but 2-D processing maps is not able to entitely analyze the workability of hot deformation since they
do not include strain. For metals characterized by dynamic recrystallization (DRX) such as magnesium alloys,
the 3-D processing maps are able to deal with the sensitivity of the workability to strain at elevated temperatures.
Moreover, different flow instability criterions were discussed to obtain the most favorite flow instability criterion
suitable for magnesium alloys.
Through numerical simulation including 3-D processing maps, a new method to analyze the workability of
metal deformation was first proposed. Workability consists of two independent parts: State-of-Stress (SOS)
workability and intrinsic workability. By integration of the 3-D processing maps and finite element simulation
(FEM), the distributions and variations of stress, strain, strain rate, temperature and flow instability domains are
obtained under different hot deformation conditions. This new method extends and perfects the theory and
application of workability. The 2-D cylinder compression simulations were conducted and validated the accuracy
of the new method.
The hot forging process design and experiments of magnesium alloy AZ31B spur bevel gear were
successfully conducted. Taking the complicated shape spur bevel gear as an example, the two-step isothermal
forming process (pre-forging without gear form and finishing-forging) was carried out. By combination 3-D
processing maps with 3-D complex forging simulation, a 3-D numerical simulation system for analyzing
deformation, heat transfer, microstructural evolution and flow instability during complex hot forging process of
magnesium alloy was developed. By analyzing the influence of temperature, velocity, pre-forging die shape
parameters and friction on the forgeability of magnesium alloy, the optimum pre-forging part shape, velocity and
temperature etc. were determined. Based on the process design, the hot forging experiments of magnesium alloy
AZ31B spur bevel gear were completed, furthermore, the microstructural structures were revealed through
quantitative metallography. The coincidence of the predicted results with measured ones validates the accuracy
of the new flow stress model, new microstructural evolution model and the workability analysis method of
material deformations by numerical simulation including 3-D processing maps.
Key Words: Magnesium alloy, Flow stress, Dynamic recrystallization, Microstructural evolution,
3-D processing maps, Numerical simulation, Hot forging process.

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