【作者】 孙建波；

【导师】 李廷举；

【作者基本信息】 大连理工大学 ， 材料加工工程， 2011， 博士

【摘要】 目前双金属复合材料的制备技术包括轧制复合、铸造复合、爆炸复合、扩散复合、挤压复合以及喷射沉积等。其中,连续铸造是一种理想的生产方法。采用连续铸造方法生产双金属复合材料使得两种不同成分的合金在液态下直接接触,避免了两种合金的接触界面产生氧化、夹杂、油污等而影响界面结合强度的问题,因而能够获得良好的冶金结合。本文的研究目的是以3003/4004铝合金复层铸坯为对象,研究新型的复层铝合金连续铸造技术。揭示复层铝合金连续铸造过程的基本规律以及复层铝合金组织、成分和性能的特点。本文主要研究内容包括：研究复层铝合金铸坯电磁连续铸造新技术。联合采用ANSYS和FLUENT软件模拟了水平直流电磁场对铸坯凝固过程的影响。数值模拟结果表明,施加0.15T的磁场后,金属液流向铸坯窄面和挡板下沿处的冲击速度和距离显著减小,抑制了金属液对铸坯液穴的冲击作用。金属液高温区由铸坯窄面向靠近铸坯中心处转移,且高温区远离挡板,分布更加均匀,挡板附近半固态区变宽,有利于两种铝合金的复合。当磁感应强度为0.25T时,达到最好的制动效果。当3003合金浇注温度为680℃,磁感应强度为0.2T,拉坯速度为100mm·min-1时,挡板恰好处于液穴的半固态区内,有利于两种铝合金的复合。当拉坯速度提高到140mm·min-1或浇注温度提高到720℃时,液穴深度增加,与挡板之间产生间隙,两种合金液可以自由流动而发生混流。在数值模拟的基础上优化设计出了适合复层铝合金铸坯生产的直流磁场发生装置、结晶器、挡板及其附属装置。实验中,当磁感应强度为0.25T,拉坯速度为130mm2min-1,浇注温度为680℃,结晶器冷却水流量为2.5m3·h-1时,实验获得的复层铸坯界面清晰、平直,没有氧化、夹杂和孔洞等缺陷,实现了牢固的冶金结合。证明水平直流电磁场抑制了两种合金的混流。成分分析结果表明,Si、Mg和Mn等合金元素含量在垂直复合界面的方向上呈梯度变化,Si和Mg元素向3003合金中扩散的距离比Mn元素向4004合金中扩散距离远。研究了复层铝合金铸坯直接冷却连续铸造新技术,研制了冷凝板、金属液流量控制和分配装置、牵引底模等一系列相关装置。用FLUENT软件模拟了工艺参数对复层铸坯凝固过程的影响。模拟结果表明,冷凝板冷却强度、拉坯速度和冷凝板冷却段高度对半固态支撑层离开冷凝板时的有效厚度及其分布均匀性具有较大影响。而3003合金浇注温度对铸坯凝固过程影响不大。在实验中,冷凝板冷却水流量分别采用180、200、225和250L·h-1；拉坯速度分别采用70和80mm·min-1,3003合金浇注温度为710℃,4004合金浇注温度为670℃时均获得界面清晰平直、结合牢固的复层铸坯。成分分析结果表明,不同铸造条件下的复层铸坯中,Si和Mg元素均通过界面向3003合金内部扩散,其扩散层厚度小于20μm。而Mn元素向4004合金的扩散距离则小于10μm。另外还分析了4004合金的浇注温度、底模的牵引力、浇口的数量和分布以及结晶器冷却水流量工艺参数对复层铸坯界面结合的影响。建立了半固态支撑层一维稳态传热模型,得出了工艺参数(冷凝板冷却水流量qw、拉坯速度vc、3003合金浇注温度Tc和冷凝板冷却段高度Hd)与冷凝板冷却水进出水温度差ATw及半固态支撑层离开冷凝板时的有效厚度Ee之间的函数关系。该传热数学模型的计算结果与实验结果基本吻合。复层铸坯界面结合分为半固态支撑层形成和冶金结合两个阶段。半固态支撑层形成阶段是在冷凝板上完成的。在冶金结合阶段,4004合金以完全共格或部分共格的形式将半固态支撑层表面作为非均匀形核基底,在其上形核、生长。生长的过程中随着凝固前沿液相中成分过冷区的出现和扩大,4004合金的结晶形态按照平面晶→胞状晶→树枝晶的顺序依次变化。其中,大部分树枝晶都沿着垂直于结合界面的方向生长。均质化退火结果表明,退火后4004合金一侧的布氏硬度和抗拉强度显著降低。当退火温度为490℃时,退火时间从7h延长到14h；或退火时间为7h时,退火温度从490℃提高到510℃,界面两侧Si和Mn元素含量的分布变化很小。而更多的Mg元素扩散到了3003合金一侧。采用热轧和冷轧相结合的方法,先将厚度为29mm的复层铸坯热轧至厚度为5mm,轧制温度为510℃,再进一步将其冷轧至厚度为1mm,微观组织观察和成分分析表明复层铸坯具有良好的可轧制性,轧板界面处保持了良好的冶金结合。更多还原

【Abstract】 Clad metal is conventionally manufactured by roll bonding, cast bonding, explosive welding, diffusion bonding, extrusion cladding and spray deposition technique. Among them, continuous casting is an ideal method for producing clad metal because it realizes liquid state contact between two metals directly and avoids the problem of low bonding strength attribute to oxide, inclusion and greasy dirt.The main purpose of this dissertation is to develop novel continuous casting processes for preparing the 3003/4004 aluminum alloy bimetal slab and reveal their basic rules, the structure, composition and properties of the bimetal slab are also studied. The main contents of this dissertation include:A new electromagnetic continuous casting process for preparing clad slab was studied. The software ANSYS and FLUENT are combined to study the effect of magnetic flux density of level direct current electromagnetic field on the flow of the alloy melts. The results indicate while the magnetic flux density (B) is 0.15T, the impact velocity and distance of the flow to the narrow face and underside of the dividing plate decreased remarkably. So the flow impact to the liquid pool was depressed. At the same time, the high temperature zone of the melt moved away from the narrow face and the dividing plate, and distributed more uniform. The wider semi-solid area next to the dividing plate helped to the bonding of 3003 and 4004 aluminum alloys. While the B is 0.25T, the best braking effect was reached. While the casting temperature of 3003 alloy was 680℃, the B was 0.25T and the casting speed was 100 mm·min-1, then the dividing plate would just be positioned in the semi-solid area, that would be helpful to the bonding of 3003 and 4004 aluminum alloys. However, the casting speed was increased to 140mm·min-1 or the casting temperature was increased to 720℃, the gap between the dividing and liquid pool would increase and lead to mixing of these two aluminum alloys.On the basis of numerical simulation results, the direct current electromagnetic field generator, mold, dividing plate and other additional equipments which suited to produce clad aluminum alloy slab were designed. In the experiments, while the casting temperature of 3003 alloy was 680℃, the B was 0.25T, the casting speed was 130 mm·min-1, and the cooling water flow rate of mold was 2.5m3·h-1, the clad slab which had metallurgical excellent bonding and without any discontinuities along the interface was obtained. It was proved that the mixing of 3003 and 4004 alloys were depressed effectively during the continuous casting of clad slab under direct electromagnetic field. The experiment results also supported the validity of the simulation results. The results of composition analysis showed that Si, Mg and Mn element contents gradiently distributed transverse to the clad interface direction. The diffusion distance of Si and Mg element were greater than that of the Mn elements.A new direct chilled continuous casting process for preparing clad slab was presented. A series of related devices including the water-cooled dividing plate, the flow rate control and distribution device and the draw block were developed. The influence of process parameters on the solidification of clad slab was numerically studied with the engineering software FLUENT. The simulation results showed that the cooling intensity of the dividing plate, casting speed and height of the cooling part of the dividing plate had more influence on the effective thickness, which is the thickness of 3003 alloy solidification shell formed on the water-cooled dividing plate, and the distribution uniformity of the solidification shell. In the experiments, well clad slab could be obtained while the cooling water flow rate of dividing plate chose 180,200,225 and 250 L·h-1, respectively, the casting speed chose 70 and 80 mm·min-1, the casting temperature of 3003 and 4004 alloys were 710 and 670℃, respectively. The prepared clad slabs had metallurgical excellent bonding and straight bonding interface without any discontinuities. The results of composition analysis under different casting conditions showed that Si, Mg and Mn element contents gradiently distributed transverse to the clad interface direction. The diffusion distance of Si and Mg elements were less than 20μm and that of the Mn element was less than 10μm. In addition, the influence of other process parameters including casting temperature of 4004 alloy, the insert depth of dividing plate into the mold, the draw force of block, the numbers and distribution of sprues as well as the cooling water flow rate of the mold on the interface bonding of the clad slab were also analyzed.A one-dimensional steady heat transfer model about solidification of 3003 alloy on the dividing plate was built. The quantitative relationship of influence of casting parameters, including cooling water flow rate of dividing plate, the casting speed, casting temperature of 3003 alloy and the height of the cooling part of the dividing plate, on the water temperature difference in and out of the dividing plate as well as the effective thickness of 3003 solidification shell were obtained. The computation results showed good agreement with the experimental results. The bonding of the clad interface was composed of two phases, called formation phase of core layer and metallurgical bonding phase. The formation phase of core layer was completed on the cooling part of the dividing plate. At the metallurgical bonding phase,4004 alloy attached on surface of the core layer, which had coherent interface with 4004 alloy, to nucleate and to grow. As the emergence and enlargement of constitutional super cooling zone on solidification front, the morphologies of the crystals of 4004 alloy changed according to following order:planar, cellular and dendritic. Among them, most growing directions of the dendrites were perpendicular to the interface.The results of homogenization treatment showed that the Brinell hardness and tensile strength of the 4004 alloy side of the clad slab was remarkably decreased. The distribution of Si and Mn element contents changed little while the heat treatment temperature was 490℃, the treatment time varied from 7h to 14h or the heat treatment time was 7h, the treatment temperature varied from 490℃to 510℃. However, more Mg element diffused into the 3003 alloy side. The clad slab with thickness of 29 mm was rolled to 3mm by hot rolling at 510℃and then continued to roll till 1mm in thickness by cold rolling. The microstructure observation and composition analysis indicated that the clad slab had excellent rellability, and well metallurgical bonding was maintained at the bonding interface.更多还原