镁铝合金成分、半固态组织和等温处理工艺
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摘要
镁及其合金是所有结构用金属中密度最低的,具有比强度、比刚度高,减震性、导热性良好,易切削加工等优点,广泛应用于汽车、电子、航空航天等领域,被称为21世纪的绿色工程材料。随着节能减排和结构轻量化需求的日益迫切,镁及其合金材料的研究已经成为世界性的热点。
半固态加工成形技术是20世纪70年代美国麻省理工学院的M. C. Flemings教授等提出的一种新的金属成形方法,该方法结合了液态成形和固态成形的长处,即加工温度比液态低(镁合金可降低100℃左右)、变形抗力比固态小,非常适合镁合金的加工被称为21世纪最有前途的材料成形加工方法。该技术的核心是使处于固液共存温度区间的合金浆料具有非枝晶组织,其中的固相颗粒需要呈细小的等轴状,这样才能使半固态浆料表现出良好的成形性能。因此,制备合格的半固态浆料是半固态加工的基础。等温热处理法是一种较好的半固态浆料制备方法,它是把材料在半固态温度区间进行热处理,以获得具有非枝晶组织的半固态浆料。等温热处理法工艺简单、成本低,具有广阔的发展前景。
目前,商业上应用的半固态成形镁合金主要以Mg-Al系合金为主,例如AZ91D、AM60、AM50等,其合金中的含Al量最多不超过10wt.%,这些成分的镁合金未必就是最佳的半固态成形用合金。因此,应该开发新的半固态成形镁合金,以满足各种不同结构零件的需求,充分发挥半固态成形的优势。
本文首先通过显微组织观察研究了等温工艺参数对应变诱发法半固态AZ91D合金微观组织的影响。研究发现,长时间等温处理能够使半固态浆料的固相颗粒变得更加圆整,但是固相颗粒将发生长大；较高的等温温度能够降低半固态浆料的固相率,并且促进固相颗粒球化进程。其半固态组织演变可分为两个阶段：当等温时间较短或等温温度较低时,变形合金发生回复再结晶和局部熔化；随等温时间延长,液相含量不断增加,α-Mg晶粒相互分离,形成独立的固相颗粒,同时固相颗粒逐渐长大,Ostwald熟化和合并粗化为固相颗粒长大的主要机制。此外,为了降低系统能量,由于界面曲率作用,固相颗粒边缘凸出的尖角逐渐熔化,颗粒变得圆整。对于压缩变形量为36.84%的AZ91D合金,适宜的半固态等温参数为,等温温度570℃左右、等温时间10～20min。
为了进一步研究Al元素对Mg-Al合金铸态及半固态组织的影响,我们提高了合金中的Al含量,制备出Mg-10Al-Zn、Mg-20Al-Zn和Mg-44Al三种合金。通过组织对比可以发现,随铝含量增加,初生a-Mg枝晶数量减少并逐渐细化,β-Mg17Al12相含量不断增加,合金的主要组成相由a-Mg固溶体向共晶混合物(a-Mg+β-Mg17Al12)转变。当Al含量达到44wt.%时,通过重熔后快速凝固的方法,可以制备出单一的β-Mg17Al12相。通过显微硬度测试可知Mg-20Al-Zn合金的α-Mg固溶体和共晶混合物硬度都高于Mg-10Al-Zn,其原因主要是在非平衡凝固条件下,Mg-20Al-Zn合金的α-Mg中固溶了更多的Al,使其固溶强化效果更明显,另外,β-Mg17Al12相为硬脆相,显微硬度高达280HV,β-Mg17Al12相在共晶混合物中含量的增加提高了Mg-20Al-Zn合金共晶混合物的硬度。Mg-Al合金铸态组织中α-Mg枝晶的细化和β-Mg17Al12相的增加都有利于合金半固态转变,可见,在一定范围内提高镁合金的含铝量,能够开发出适于半固态成形的镁合金。
接下来的研究中,以Mg-Al合金为基,适当提高含Al量,并加入少量Mn改善合金性能,制备了Mg-14Al-0.5Mn合金。研究了该合金等温热处理法半固态组织演变过程,并讨论了合金原始铸态组织、等温温度和时间等因素对半固态组织的影响。由实验结果发现,在Mg-14Al-0.5Mn合金中,由于铝含量较高,导致β-Mg17Al12相和共晶混合物含量明显增加。这些低熔点的金属间化合物和共晶混合物分布在初生α-Mg晶界处。当合金在半固态温度保温时,低熔点相将迅速熔化。在α-Mg枝晶臂根部或弯曲处,由于曲率过大导致熔点降低,这些区域也将首先熔化,使初生α-Mg由树枝晶状向不规则颗粒状转变。在初始等温阶段,熔化对合金固相颗粒尺寸有很大影响。在等温处理的前几分钟,熔化作用使固相颗粒平均尺寸减小。随等温时间延长,合金中液相含量增加,同时固相颗粒逐渐长大并变得越发圆整。在这一阶段,球化和粗化机制决定了固相颗粒的形貌。合金原始铸态组织、等温温度和等温时间是决定等温热处理法半固态浆料质量的主要因素。原始铸态组织中,较多的低熔点相可以加快合金半固态转变,细小的枝晶结构有利于获得尺寸较小的固相颗粒。在可控范围内,应尽量选择高温短时间的等温参数以达到节约成本、提高生产效率的目的。对于Mg-14Al-0.5Mn合金,合适的等温温度在520℃左右,等温时间为5～8min。
提高Mg-Al合金中的Al含量可以使合金具有良好的半固态成形性能,但是β-Mg17Al12相的增加也使合金的韧性及高温性能有所下降。因此在增加Al含量的同时,考虑添加其他合金元素来抑制p-Mg17Al12相形成,提高合金性能,开发新型半固态成形三元镁合金。稀土Ce元素成为首先研究的对象,按Ce不同加入量制备了Mg-10Al-xCe (x=0,0.25,0.5,1.0wt.%)四种合金,通过对比发现,Mg-10Al合金中加入Ce元素后,其铸态组织晶粒变得粗大、并出现发达的α-Mg枝晶。主要原因是,加入Ce后α-Mg枝晶前沿液相中的过冷度降低,抑制了异质核心形核,促进α-Mg枝晶生长,而过冷度降低主要由于Ce元素加入降低了合金的固液界面能。在含Ce的三种合金中,随Ce加入量升高,合金铸态组织晶粒尺寸又出现减小趋势,这主要是由于晶界处Al-Ce化合物数量增加,阻碍枝晶生长造成的。Ce元素加入对Mg-10Al合金铸态组织的改变极大影响了等温处理后得到的半固态组织,在含Ce合金中,Mg-10Al-0.5Ce合金的半固态组织最佳,该合金适宜的等温温度为560℃,等温时间在20min左右。
由于Ce元素加入使Mg-Al合金铸态组织粗化,同时稀土元素成本较高,接下来的研究中考虑使用Ca来替代稀土,制备出不同含Ca量的Mg-13Al-xCa (x=0,0.3,0.6,1.0,3.0wt.%)合金,研究了Ca含量对合金在等温处理过程中组织演变的影响。结果发现,Ca元素加入能够细化Mg-13Al合金的铸态枝晶组织,但是,当Ca加入量小于1wt.%时,由于枝晶臂的搭接使等温处理后得到的固相颗粒大小不一、形状各异；当Ca含量增加到3wt.%后,二次枝晶臂断裂使枝晶进一步细化,Mg-13A1-3Ca合金的半固态组织较为理想。Ca元素加入还使半固态温度区间发生变化,含Ca量小于1wt.%时,Ca主要固溶于β-Mg17Al12相中,提高了β-Mg17Al12相熔化温度,从而使固相线温度升高；当Ca含量较多时,A12Ca相也将成为半固态液相重要组成部分,使半固态等温区间缩小。通过热分析可以判断,Mg-13Al-3Ca合金有效半固态温度区间大约为520～550℃。
最后,通过对以上实验数据的分析和热力学计算,我们进一步确定了半固态Mg-Al合金合适的含Al范围、及其它合金元素对半固态组织转变的影响。计算得出,对于Mg-Al二元合金,在基本保证性能的情况下,适宜合金半固态成形的含Al范围为3～15wt.%。当Al含量小于9wt.%时,合金只适于进行固相率较高的触变成形加工；当Al含量大于9wt.%时,合金既可以进行触变成形加工,也可以进行流变成形加工。合金元素对Mg-Al合金半固态组织的影响主要分为以下几方面：一、对原始铸态组织的影响,加入合金元素后铸态组织中晶粒尺寸越小、形态越趋近等轴状,得到的半固态固相颗粒越细小圆整；二、形成新相对半固态组织转变的影响,为了降低Mg-Al合金中β-Mg17Al12相含量,尽量选取与Al形成化合物的合金元素,等温处理过程中,化合物态存在的合金元素对半固态组织演变影响不大,游离态合金元素可能通过改变固液界面能而影响固相颗粒粗化速率；三、合金元素加入还能够改变合金半固态凝固区间(△TS-SS)、固相率的温度敏感性(dfs/dT|)等流变参数。本论文研究结果为半固态成形用镁合金成分设计提供了一定的实验和理论依据,希望借此工作推进半固态成形镁合金的发展。
Magnesium and its alloys are the metal structure materials with the lowest density, which have a lot of excellent properties, such as high specific strength and stiffness, excellent damping capacity, better thermal conductivity, and good machinability. Magnesium alloys are widely used in automobile, electronic, aviation and other fields, which are considered as green engineering materials during 21st. With the requirements of saving energy, reducing emissions and structural lightweight, research on magnesium and its alloy materials has become a global hot spot.
Semi-solid forming (SSF) technology is a new metal forming method proposed by Professor M.C. Flemings, which combines the advantages of both liquid forming and solid forming. The processing temperature is lower than the liquid forming (Mg alloy can be reduced about 100℃), and the deformation resistance is lower than the solid forming. SSF is very suitable for the processing of magnesium alloys, and is known as the most promising materials processing methods during 21st. The core of the technology is to obtain the alloy slurry with non-dendritic structure between the solidus and liquidus temperature. The solid particles in the semisolid slurry should be small and equiaxed, in order to make the slurry have good formability. Therefore, the preparation of eligible semisolid microstructure is the basic for semisolid processing. Isothermal heat treatment is a good method for the preparation of semisolid slurry, which only needs to hold the alloy in the semisolid temperature range for a while. Isothermal heat treatment process is simple, low cost, and has broad prospects for development.
Currently, the magnesium alloy for SSF in commercial application is mainly dominated by Mg-Al system, such as AZ91D, AM60 and AM50. The content of Al in the alloy is up to a maximum 10wt.%, the magnesium alloys with these composition may be not the best alloys for SSF forming. Therefore, we should design new SSF alloys which can meet the requirements of different parts, and fully displayed the advantages of semisolid forming.
First of all, the effects of isothermal process parameters on the microstructure evolution of semisolid AZ91D alloy produced by strain-induced melt activation (SIMA) were investigated. The results showed that long isothermal time could make the semisolid particles more globular, but the size of the particles would grow larger; high semisolid isothermal temperature would reduce the solid volume fraction and accelerate the spherical evolution of the solid particles. The mechanism of the particles formation can be divided into two stages. First are recovery, recrystallization and partial melting when the holding time is short or the holding temperature is lower. With the extension of holding time, the amount of liquid increases, the grains separate from each other forming the solid particles and the particles grow large. During this stage coalescence ripening and Ostwald ripening are the dominant mechanisms for structural coarsening. Furthermore, due to the effect of interface curvature, the particle spheroidization takes place in order to decrease the free energy. It was found that the optimal isothermal process parameters should be 570℃and 10～20 min of isothermal temperature and time respectively for the AZ91D alloys with 36.84% compression.
In order to study the effects of Al element on the microstructures of as-cast and semisolid Mg-Al alloys, three types of Mg-Al alloys were prepared and characterized by Mg-10Al-Zn, Mg-20Al-Zn and Mg-44A1, respectively. With the increasing of Al, the content of primary a-Mg phases decrease, and the a-Mg dendrites are refined. Furthermore, the content of P-Mg17Al12 phases increasing, and the eutectic mixtures of a-Mg+β-Mg17Al12 become the main phases of the alloy. When the Al content increases to 44wt.%, the phase in the alloy would be onlyβ-Mg17Al12 through the rapid solidification after remelting. The results of micro-hardness indicate that the micro-hardness of the a-Mg solid solution and the eutectic mixture (a-Mg+β-Mg17Al12) in the Mg-20Al-Zn alloy are higher than in the Mg-10Al-Zn alloy. The Al dissolved in the a-Mg has an effect of solid solution strengthening. When the content of Al in the Mg-Al alloys increases, the Al dissolved in the a-Mg solid solution also increases. Therefore, the micro-hardness of the a-Mg solid solution improves. The intermetallic phaseβ-Mg17Al12 is a hard and brittle phase with the micro-hardness about 280 (HV), which is much higher than the a-Mg. So when the content ofβ-Mg17Al12 increases in the eutectic mixture, the micro-hardness of the eutectic mixture increases obviously. The a-Mg dendrites refinement and the increasing of P-Mg17Al12 phases are both good for SSF, so increasing the content of Al in a certain range could help to develop suitable SSF magnesium alloys.
During the next study, a new alloy of Mg-14Al-0.5Mn was prepared. And the microstructure evolution of semi-solid Mg-14Al-0.5Mn alloy during isothermal heat treatment was investigated. Furthermore, the effects of original as-cast microstructure, isothermal temperature and time on the semisolid microstructure were discussed. In the Mg-14Al-0.5Mn alloy, as a result of increasing Al content, the amounts of P-Mg17Al12 and eutectic mixtures increase obviously. These intermetallic compound and mixtures with low melting point distribute at the grain boundaries around the primary a-Mg. When the alloy is held at the semisolid temperature, theβ-Mg17Al12 and eutectic mixtures begin to melt immediately. At the bend of a-Mg dendrite arm, the melting point lowers down because of the large curvature. These areas are first melted, making the morphology of a-Mg change from dendrites to irregular solid particles. At the first few minutes, melting plays a major role on the size of the solid particles. With increasing of isothermal time, the amount of liquid increases and the solid particles grow large and become globular. At this stage, spheroidizing and particle coarsening determine the morphology of the solid particles. In the original as-cast microstructure, adequate amount of the second phase with low melting point is needed, so that the semi-solid alloy will achieve an ideal solid fraction in a short isothermal time. The finer the size of the initial grain is, the smaller the size of the semi-solid particles will be. Select high isothermal temperature between the solid-liquid range and short isothermal time to achieve cost savings and improve production efficiency. Mg-14Al-0.5Mn alloy held at 520℃for 5-8 min can obtain good semisolid microstructures.
Increasing the content of Al in magnesium alloys in a certain extent could make the alloy have good semisolid formability. But the increasing ofβ-Mg17Al12 phases also decreases the toughness and high temperature performance of the alloy. Therefore, a small quantity of Ce was added in order to suppress the formation of Mg17Al12 and increase the comprehensive performance of the alloys. Based on the above ideas, new Mg-10Al-xCe (x=0, 0.25,0.5,1.0wt.%) alloys for SSF were designed. Found by the comparison, the a-Mg dendrites in as-cast Mg-10Al-0.25Ce, Mg-10Al-0.5Ce and Mg-10Al-1.0Ce alloys are larger and more developed than those in Mg-10Al alloy. It is mainly caused by the decreasing of undercooling in front of the a-Mg tip with Ce addition. Besides, among the Mg-10Al-0.25Ce, Mg-10Al-0.5Ce and Mg-10Al-1.0Ce alloys, the grain sizes decrease gradually with the increasing of Ce content, which is the result of the Al-Ce compounds at grain boundaries restricting grain growth. The initial as-cast microstructures greatly influence the semisolid microstructures after the isothermal heat treatment. Among the Mg-10Al alloys with Ce addition, the Mg-10Al-0.5Ce alloy has the best semisolid rheological parameters. And the suitable isothermal temperature and time for Mg-10Al-0.5Ce alloy are about 560℃and 20min, respectively.
The addition of Ce element will coarse the as-cast structure of Mg-Al alloy, while the cost of rare earth elements is high, so in the next study, we consider to use Ca elements replace the rare earth, and prepare Mg-13Al-xCa (x=0,0.3,0.6,1.0,3.0wt.%) alloys. The effects of Ca content on the microstructure evolution during isothermal heat treatment were investigated. It is found that Ca elements addition could refine the microstructure of as-cast Mg-13A1 alloy. However, when the content of Ca is less than lwt.%, due to the overlap of a-Mg dendrite, the size of solid particles is nonuniform, and the shape of the particles is irregular; when the Ca content increased to 3wt.%, the secondary dendrite arm fracture makes the a-Mg dendrite further refined, which make the semisolid microstructure of Mg-13Al-3Ca alloy more desirable. Ca elements addition can also change the semisolid temperature range, when the content of Ca is less than lwt.%, Ca element mainly dissolved in theβ-Mg17Al12 phase, which increased melting temperature ofβ-Mg17Al12 phase, so that the solidus temperature; when the Ca content is more than 3wt.%, Al2Ca phases will also become an important component of the liquid phases in the semisolid slurry, which narrow the range of semisolid temperature. Judging from the thermal analysis, the effective range of semisolid temperature of Mg-13Al-3Ca alloy is about 520～550℃.
Finally, from the analysis of above experimental data and thermodynamic calculations, we can further identify the appropriate range of Al content in the SSF Mg-Al alloy, and the effect of other alloying elements on semisolid microstructure evolution. For the Mg-Al binary alloy, in the case of basic performance guarantee, the appropriate range of Al content in Mg-Al alloy for SSF is 3-15wt.%. When the Al content is less than 9wt.%, the alloy is only suitable for thixotropic forming with high solid fraction; when the Al content is more than 9wt.%, the alloy can be processed by either thixotropic forming or rheologic forming. The effects of alloying elements on the semisolid microstructure of Mg-Al alloy can be divided into the following aspects:First, the impact of the original as-cast microstructure, the solid particles in the semisolid slurry will be fine and globular, if the grain of as-cast alloy is small and equiaxed after the addition of alloying elements; Second, the formation of new phases with the addition of alloying elements, in order to reduce the content ofβ-Mg17Al12 phase, we should select the elements which can form compounds with Al, during the isothermal heat treatment, the alloy elements present as the compound state do little effect on the semisolid microstructure evolution, free state alloying elements may affect the solid-liquid interface energy which impact the coarsening rate of solid particles; finally, alloy elements addition could change the semisolid solidification range (△TS-SS), the temperature sensitivity of solid fraction (|df/dT|) and other rheological parameters. The results of this research provide some experimental data and theoretical basis for the composition design of SSF magnesium alloys. It is expected that the preliminary work could be significant in prompting the development of new semisolid magnesium alloys.
半固态加工成形技术是20世纪70年代美国麻省理工学院的M. C. Flemings教授等提出的一种新的金属成形方法,该方法结合了液态成形和固态成形的长处,即加工温度比液态低(镁合金可降低100℃左右)、变形抗力比固态小,非常适合镁合金的加工被称为21世纪最有前途的材料成形加工方法。该技术的核心是使处于固液共存温度区间的合金浆料具有非枝晶组织,其中的固相颗粒需要呈细小的等轴状,这样才能使半固态浆料表现出良好的成形性能。因此,制备合格的半固态浆料是半固态加工的基础。等温热处理法是一种较好的半固态浆料制备方法,它是把材料在半固态温度区间进行热处理,以获得具有非枝晶组织的半固态浆料。等温热处理法工艺简单、成本低,具有广阔的发展前景。
目前,商业上应用的半固态成形镁合金主要以Mg-Al系合金为主,例如AZ91D、AM60、AM50等,其合金中的含Al量最多不超过10wt.%,这些成分的镁合金未必就是最佳的半固态成形用合金。因此,应该开发新的半固态成形镁合金,以满足各种不同结构零件的需求,充分发挥半固态成形的优势。
本文首先通过显微组织观察研究了等温工艺参数对应变诱发法半固态AZ91D合金微观组织的影响。研究发现,长时间等温处理能够使半固态浆料的固相颗粒变得更加圆整,但是固相颗粒将发生长大；较高的等温温度能够降低半固态浆料的固相率,并且促进固相颗粒球化进程。其半固态组织演变可分为两个阶段：当等温时间较短或等温温度较低时,变形合金发生回复再结晶和局部熔化；随等温时间延长,液相含量不断增加,α-Mg晶粒相互分离,形成独立的固相颗粒,同时固相颗粒逐渐长大,Ostwald熟化和合并粗化为固相颗粒长大的主要机制。此外,为了降低系统能量,由于界面曲率作用,固相颗粒边缘凸出的尖角逐渐熔化,颗粒变得圆整。对于压缩变形量为36.84%的AZ91D合金,适宜的半固态等温参数为,等温温度570℃左右、等温时间10～20min。
为了进一步研究Al元素对Mg-Al合金铸态及半固态组织的影响,我们提高了合金中的Al含量,制备出Mg-10Al-Zn、Mg-20Al-Zn和Mg-44Al三种合金。通过组织对比可以发现,随铝含量增加,初生a-Mg枝晶数量减少并逐渐细化,β-Mg17Al12相含量不断增加,合金的主要组成相由a-Mg固溶体向共晶混合物(a-Mg+β-Mg17Al12)转变。当Al含量达到44wt.%时,通过重熔后快速凝固的方法,可以制备出单一的β-Mg17Al12相。通过显微硬度测试可知Mg-20Al-Zn合金的α-Mg固溶体和共晶混合物硬度都高于Mg-10Al-Zn,其原因主要是在非平衡凝固条件下,Mg-20Al-Zn合金的α-Mg中固溶了更多的Al,使其固溶强化效果更明显,另外,β-Mg17Al12相为硬脆相,显微硬度高达280HV,β-Mg17Al12相在共晶混合物中含量的增加提高了Mg-20Al-Zn合金共晶混合物的硬度。Mg-Al合金铸态组织中α-Mg枝晶的细化和β-Mg17Al12相的增加都有利于合金半固态转变,可见,在一定范围内提高镁合金的含铝量,能够开发出适于半固态成形的镁合金。
接下来的研究中,以Mg-Al合金为基,适当提高含Al量,并加入少量Mn改善合金性能,制备了Mg-14Al-0.5Mn合金。研究了该合金等温热处理法半固态组织演变过程,并讨论了合金原始铸态组织、等温温度和时间等因素对半固态组织的影响。由实验结果发现,在Mg-14Al-0.5Mn合金中,由于铝含量较高,导致β-Mg17Al12相和共晶混合物含量明显增加。这些低熔点的金属间化合物和共晶混合物分布在初生α-Mg晶界处。当合金在半固态温度保温时,低熔点相将迅速熔化。在α-Mg枝晶臂根部或弯曲处,由于曲率过大导致熔点降低,这些区域也将首先熔化,使初生α-Mg由树枝晶状向不规则颗粒状转变。在初始等温阶段,熔化对合金固相颗粒尺寸有很大影响。在等温处理的前几分钟,熔化作用使固相颗粒平均尺寸减小。随等温时间延长,合金中液相含量增加,同时固相颗粒逐渐长大并变得越发圆整。在这一阶段,球化和粗化机制决定了固相颗粒的形貌。合金原始铸态组织、等温温度和等温时间是决定等温热处理法半固态浆料质量的主要因素。原始铸态组织中,较多的低熔点相可以加快合金半固态转变,细小的枝晶结构有利于获得尺寸较小的固相颗粒。在可控范围内,应尽量选择高温短时间的等温参数以达到节约成本、提高生产效率的目的。对于Mg-14Al-0.5Mn合金,合适的等温温度在520℃左右,等温时间为5～8min。
提高Mg-Al合金中的Al含量可以使合金具有良好的半固态成形性能,但是β-Mg17Al12相的增加也使合金的韧性及高温性能有所下降。因此在增加Al含量的同时,考虑添加其他合金元素来抑制p-Mg17Al12相形成,提高合金性能,开发新型半固态成形三元镁合金。稀土Ce元素成为首先研究的对象,按Ce不同加入量制备了Mg-10Al-xCe (x=0,0.25,0.5,1.0wt.%)四种合金,通过对比发现,Mg-10Al合金中加入Ce元素后,其铸态组织晶粒变得粗大、并出现发达的α-Mg枝晶。主要原因是,加入Ce后α-Mg枝晶前沿液相中的过冷度降低,抑制了异质核心形核,促进α-Mg枝晶生长,而过冷度降低主要由于Ce元素加入降低了合金的固液界面能。在含Ce的三种合金中,随Ce加入量升高,合金铸态组织晶粒尺寸又出现减小趋势,这主要是由于晶界处Al-Ce化合物数量增加,阻碍枝晶生长造成的。Ce元素加入对Mg-10Al合金铸态组织的改变极大影响了等温处理后得到的半固态组织,在含Ce合金中,Mg-10Al-0.5Ce合金的半固态组织最佳,该合金适宜的等温温度为560℃,等温时间在20min左右。
由于Ce元素加入使Mg-Al合金铸态组织粗化,同时稀土元素成本较高,接下来的研究中考虑使用Ca来替代稀土,制备出不同含Ca量的Mg-13Al-xCa (x=0,0.3,0.6,1.0,3.0wt.%)合金,研究了Ca含量对合金在等温处理过程中组织演变的影响。结果发现,Ca元素加入能够细化Mg-13Al合金的铸态枝晶组织,但是,当Ca加入量小于1wt.%时,由于枝晶臂的搭接使等温处理后得到的固相颗粒大小不一、形状各异；当Ca含量增加到3wt.%后,二次枝晶臂断裂使枝晶进一步细化,Mg-13A1-3Ca合金的半固态组织较为理想。Ca元素加入还使半固态温度区间发生变化,含Ca量小于1wt.%时,Ca主要固溶于β-Mg17Al12相中,提高了β-Mg17Al12相熔化温度,从而使固相线温度升高；当Ca含量较多时,A12Ca相也将成为半固态液相重要组成部分,使半固态等温区间缩小。通过热分析可以判断,Mg-13Al-3Ca合金有效半固态温度区间大约为520～550℃。
最后,通过对以上实验数据的分析和热力学计算,我们进一步确定了半固态Mg-Al合金合适的含Al范围、及其它合金元素对半固态组织转变的影响。计算得出,对于Mg-Al二元合金,在基本保证性能的情况下,适宜合金半固态成形的含Al范围为3～15wt.%。当Al含量小于9wt.%时,合金只适于进行固相率较高的触变成形加工；当Al含量大于9wt.%时,合金既可以进行触变成形加工,也可以进行流变成形加工。合金元素对Mg-Al合金半固态组织的影响主要分为以下几方面：一、对原始铸态组织的影响,加入合金元素后铸态组织中晶粒尺寸越小、形态越趋近等轴状,得到的半固态固相颗粒越细小圆整；二、形成新相对半固态组织转变的影响,为了降低Mg-Al合金中β-Mg17Al12相含量,尽量选取与Al形成化合物的合金元素,等温处理过程中,化合物态存在的合金元素对半固态组织演变影响不大,游离态合金元素可能通过改变固液界面能而影响固相颗粒粗化速率；三、合金元素加入还能够改变合金半固态凝固区间(△TS-SS)、固相率的温度敏感性(dfs/dT|)等流变参数。本论文研究结果为半固态成形用镁合金成分设计提供了一定的实验和理论依据,希望借此工作推进半固态成形镁合金的发展。
Magnesium and its alloys are the metal structure materials with the lowest density, which have a lot of excellent properties, such as high specific strength and stiffness, excellent damping capacity, better thermal conductivity, and good machinability. Magnesium alloys are widely used in automobile, electronic, aviation and other fields, which are considered as green engineering materials during 21st. With the requirements of saving energy, reducing emissions and structural lightweight, research on magnesium and its alloy materials has become a global hot spot.
Semi-solid forming (SSF) technology is a new metal forming method proposed by Professor M.C. Flemings, which combines the advantages of both liquid forming and solid forming. The processing temperature is lower than the liquid forming (Mg alloy can be reduced about 100℃), and the deformation resistance is lower than the solid forming. SSF is very suitable for the processing of magnesium alloys, and is known as the most promising materials processing methods during 21st. The core of the technology is to obtain the alloy slurry with non-dendritic structure between the solidus and liquidus temperature. The solid particles in the semisolid slurry should be small and equiaxed, in order to make the slurry have good formability. Therefore, the preparation of eligible semisolid microstructure is the basic for semisolid processing. Isothermal heat treatment is a good method for the preparation of semisolid slurry, which only needs to hold the alloy in the semisolid temperature range for a while. Isothermal heat treatment process is simple, low cost, and has broad prospects for development.
Currently, the magnesium alloy for SSF in commercial application is mainly dominated by Mg-Al system, such as AZ91D, AM60 and AM50. The content of Al in the alloy is up to a maximum 10wt.%, the magnesium alloys with these composition may be not the best alloys for SSF forming. Therefore, we should design new SSF alloys which can meet the requirements of different parts, and fully displayed the advantages of semisolid forming.
First of all, the effects of isothermal process parameters on the microstructure evolution of semisolid AZ91D alloy produced by strain-induced melt activation (SIMA) were investigated. The results showed that long isothermal time could make the semisolid particles more globular, but the size of the particles would grow larger; high semisolid isothermal temperature would reduce the solid volume fraction and accelerate the spherical evolution of the solid particles. The mechanism of the particles formation can be divided into two stages. First are recovery, recrystallization and partial melting when the holding time is short or the holding temperature is lower. With the extension of holding time, the amount of liquid increases, the grains separate from each other forming the solid particles and the particles grow large. During this stage coalescence ripening and Ostwald ripening are the dominant mechanisms for structural coarsening. Furthermore, due to the effect of interface curvature, the particle spheroidization takes place in order to decrease the free energy. It was found that the optimal isothermal process parameters should be 570℃and 10～20 min of isothermal temperature and time respectively for the AZ91D alloys with 36.84% compression.
In order to study the effects of Al element on the microstructures of as-cast and semisolid Mg-Al alloys, three types of Mg-Al alloys were prepared and characterized by Mg-10Al-Zn, Mg-20Al-Zn and Mg-44A1, respectively. With the increasing of Al, the content of primary a-Mg phases decrease, and the a-Mg dendrites are refined. Furthermore, the content of P-Mg17Al12 phases increasing, and the eutectic mixtures of a-Mg+β-Mg17Al12 become the main phases of the alloy. When the Al content increases to 44wt.%, the phase in the alloy would be onlyβ-Mg17Al12 through the rapid solidification after remelting. The results of micro-hardness indicate that the micro-hardness of the a-Mg solid solution and the eutectic mixture (a-Mg+β-Mg17Al12) in the Mg-20Al-Zn alloy are higher than in the Mg-10Al-Zn alloy. The Al dissolved in the a-Mg has an effect of solid solution strengthening. When the content of Al in the Mg-Al alloys increases, the Al dissolved in the a-Mg solid solution also increases. Therefore, the micro-hardness of the a-Mg solid solution improves. The intermetallic phaseβ-Mg17Al12 is a hard and brittle phase with the micro-hardness about 280 (HV), which is much higher than the a-Mg. So when the content ofβ-Mg17Al12 increases in the eutectic mixture, the micro-hardness of the eutectic mixture increases obviously. The a-Mg dendrites refinement and the increasing of P-Mg17Al12 phases are both good for SSF, so increasing the content of Al in a certain range could help to develop suitable SSF magnesium alloys.
During the next study, a new alloy of Mg-14Al-0.5Mn was prepared. And the microstructure evolution of semi-solid Mg-14Al-0.5Mn alloy during isothermal heat treatment was investigated. Furthermore, the effects of original as-cast microstructure, isothermal temperature and time on the semisolid microstructure were discussed. In the Mg-14Al-0.5Mn alloy, as a result of increasing Al content, the amounts of P-Mg17Al12 and eutectic mixtures increase obviously. These intermetallic compound and mixtures with low melting point distribute at the grain boundaries around the primary a-Mg. When the alloy is held at the semisolid temperature, theβ-Mg17Al12 and eutectic mixtures begin to melt immediately. At the bend of a-Mg dendrite arm, the melting point lowers down because of the large curvature. These areas are first melted, making the morphology of a-Mg change from dendrites to irregular solid particles. At the first few minutes, melting plays a major role on the size of the solid particles. With increasing of isothermal time, the amount of liquid increases and the solid particles grow large and become globular. At this stage, spheroidizing and particle coarsening determine the morphology of the solid particles. In the original as-cast microstructure, adequate amount of the second phase with low melting point is needed, so that the semi-solid alloy will achieve an ideal solid fraction in a short isothermal time. The finer the size of the initial grain is, the smaller the size of the semi-solid particles will be. Select high isothermal temperature between the solid-liquid range and short isothermal time to achieve cost savings and improve production efficiency. Mg-14Al-0.5Mn alloy held at 520℃for 5-8 min can obtain good semisolid microstructures.
Increasing the content of Al in magnesium alloys in a certain extent could make the alloy have good semisolid formability. But the increasing ofβ-Mg17Al12 phases also decreases the toughness and high temperature performance of the alloy. Therefore, a small quantity of Ce was added in order to suppress the formation of Mg17Al12 and increase the comprehensive performance of the alloys. Based on the above ideas, new Mg-10Al-xCe (x=0, 0.25,0.5,1.0wt.%) alloys for SSF were designed. Found by the comparison, the a-Mg dendrites in as-cast Mg-10Al-0.25Ce, Mg-10Al-0.5Ce and Mg-10Al-1.0Ce alloys are larger and more developed than those in Mg-10Al alloy. It is mainly caused by the decreasing of undercooling in front of the a-Mg tip with Ce addition. Besides, among the Mg-10Al-0.25Ce, Mg-10Al-0.5Ce and Mg-10Al-1.0Ce alloys, the grain sizes decrease gradually with the increasing of Ce content, which is the result of the Al-Ce compounds at grain boundaries restricting grain growth. The initial as-cast microstructures greatly influence the semisolid microstructures after the isothermal heat treatment. Among the Mg-10Al alloys with Ce addition, the Mg-10Al-0.5Ce alloy has the best semisolid rheological parameters. And the suitable isothermal temperature and time for Mg-10Al-0.5Ce alloy are about 560℃and 20min, respectively.
The addition of Ce element will coarse the as-cast structure of Mg-Al alloy, while the cost of rare earth elements is high, so in the next study, we consider to use Ca elements replace the rare earth, and prepare Mg-13Al-xCa (x=0,0.3,0.6,1.0,3.0wt.%) alloys. The effects of Ca content on the microstructure evolution during isothermal heat treatment were investigated. It is found that Ca elements addition could refine the microstructure of as-cast Mg-13A1 alloy. However, when the content of Ca is less than lwt.%, due to the overlap of a-Mg dendrite, the size of solid particles is nonuniform, and the shape of the particles is irregular; when the Ca content increased to 3wt.%, the secondary dendrite arm fracture makes the a-Mg dendrite further refined, which make the semisolid microstructure of Mg-13Al-3Ca alloy more desirable. Ca elements addition can also change the semisolid temperature range, when the content of Ca is less than lwt.%, Ca element mainly dissolved in theβ-Mg17Al12 phase, which increased melting temperature ofβ-Mg17Al12 phase, so that the solidus temperature; when the Ca content is more than 3wt.%, Al2Ca phases will also become an important component of the liquid phases in the semisolid slurry, which narrow the range of semisolid temperature. Judging from the thermal analysis, the effective range of semisolid temperature of Mg-13Al-3Ca alloy is about 520～550℃.
Finally, from the analysis of above experimental data and thermodynamic calculations, we can further identify the appropriate range of Al content in the SSF Mg-Al alloy, and the effect of other alloying elements on semisolid microstructure evolution. For the Mg-Al binary alloy, in the case of basic performance guarantee, the appropriate range of Al content in Mg-Al alloy for SSF is 3-15wt.%. When the Al content is less than 9wt.%, the alloy is only suitable for thixotropic forming with high solid fraction; when the Al content is more than 9wt.%, the alloy can be processed by either thixotropic forming or rheologic forming. The effects of alloying elements on the semisolid microstructure of Mg-Al alloy can be divided into the following aspects:First, the impact of the original as-cast microstructure, the solid particles in the semisolid slurry will be fine and globular, if the grain of as-cast alloy is small and equiaxed after the addition of alloying elements; Second, the formation of new phases with the addition of alloying elements, in order to reduce the content ofβ-Mg17Al12 phase, we should select the elements which can form compounds with Al, during the isothermal heat treatment, the alloy elements present as the compound state do little effect on the semisolid microstructure evolution, free state alloying elements may affect the solid-liquid interface energy which impact the coarsening rate of solid particles; finally, alloy elements addition could change the semisolid solidification range (△TS-SS), the temperature sensitivity of solid fraction (|df/dT|) and other rheological parameters. The results of this research provide some experimental data and theoretical basis for the composition design of SSF magnesium alloys. It is expected that the preliminary work could be significant in prompting the development of new semisolid magnesium alloys.
引文
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