铝钢异种材料激光焊接工艺与机理研究

发布时间：2018-04-24 19:26

本文选题：铝钢异种金属 + 金属间化合物　；参考：《南昌大学》2015年博士论文

【摘要】：在环境和能源问题日益严峻的今天,实现汽车轻量化,受到各大汽车厂商的关注；如何实现铝钢的可靠连接,加快轻质铝合金材料在汽车中的应用,已成为该领域的关键技术之一。然而,铝钢之间诸如熔点,密度和线胀系数等性质差异很大,焊后冷却过程中,容易开裂；而且,两者冶金反应生成的硬脆Fe-Al金属间化合物,将加剧接头的开裂,降低接头的力学性能。因此,在铝钢焊接的研究中,Fe-Al金属间化合物的生成机理和控制一直是研究重点之一。激光焊具有热输入量小、冷却速度快和可添加钎料等优点,一定程度上可以控制异种材料连接过程中的金属间化合物生成。然而,激光铝钢焊接中,Fe-Al金属间化合物的生成和形态分布还有待于进一步研究。为此,本文通过采用脉冲Nd:YAG激光深熔焊和连续二极管激光熔钎焊的方法进行了铝钢焊接试验。本文首先进行了脉冲Nd:YAG铝钢激光深熔焊试验。由于脉冲Nd:YAG热输入量很小,有利于控制金属间化合物形成和生长,因而着重分析了金属间化合物随工艺参数变化的机理。通过调整试验工艺参数,获得了两种典型的接头——深熔深和浅熔深接头。界面金属间化合物由深熔深接头的富Al相变化为浅熔深接头的富Fe相,接头承载力随之增加。通过SEM和EDS分析,揭示了界面反应的机理。Fe元素和Al元素随热输入增加在熔合区/Al界面的相互扩散作用加剧,改变了界面冶金反应,进而改变了金属间化合物的生成种类。由于在深熔焊模式下,尽管热输入量小,但钢和铝都发生了熔化,使得Fe-Al金属间化合物生成量增加。此外,深熔焊模式由于小孔形成后难以稳定存在,使得焊接过程不稳定,容易产生焊接缺陷。以上这些因素都将恶化接头力学性能。进一步,选用了焊接过程更为稳定的激光热传导模式进行焊接试验和研究。采用连续二极管激光进行了铝钢熔钎焊试验,并在试验中加入了含有不同合金元素的钎料。激光熔钎焊中所添加的合金元素,有利于控制金属间化合物的生成类型和分布。因此,除分析了金属间化合物随热输入变化的机制外,还着重分析了金属间化合随合金元素的变化的机理。试验结果表明,钎焊界面金属间化合物的形貌和种类随热输入变化而变化,并将影响接头的力学性能和断裂模式。采用中低功率激光时,钎焊界面皆由θ-Fe(Al,Si)3和τ5-Al7.2Fe1.8Si构成。而采用高功率激光时,钎焊界面由θ-Fe(Al,Si)3,τ5-Al7.2Fe1.8Si和η-Fe2(Al,Si)5组成,并伴有微裂纹产生。通过EDS线扫描分析,发现在熔钎焊方法过程中钎焊界面金属间化合物的形成和生长主要是依赖于固态钢中Fe元素向液态熔合区的扩散,在扩散初期只会引起金属间化合物形貌的变化,但随着扩散的进行,将最终引起金属间化合物类型的改变。Zn元素的加入使得钎焊界面金属间化合物的种类发生了本质的改变。钎焊界面由层状的Fe2Al5和针状的FeAl3变为层状的FeAl5-xZnx,弥散分布的FeZn10以及少量的富Al非晶结构。这一变化提升了接头承载力并改变了断裂模式。通过硬度测试揭示FeZn10对接头力学性能和断裂模式的影响,由于FeZn10性质软韧,能够通过塑性变形吸收断裂能,从而防止界面开裂,提高接头承载力。基于热力学理论计算,确定了金属间化合物的生成顺序,并分析了金属间化合物随激光热输入和合金元素的演化机制。最后,深入分析了金属间化合物厚度、种类和分布对断裂模式的影响机理。基于界面晶面间距失配率与界面能和界面强度的相关性理论,采用边对边晶体匹配模型,计算了各反应相界面上的晶面间距失配率,估算并比较了不同界面的结合强度,从而预测了接头界面断裂模式的机制。研究发现晶面间距失配率越高,界面能越高,界面结合强度越低,界面越容易开裂。
[Abstract]:With the increasingly severe environmental and energy problems, the realization of automobile lightweight is paid attention by all the major automobile manufacturers. How to realize the reliable connection of aluminum and steel and speed up the application of light aluminum alloy materials in the automobile has become one of the key technologies in this field. However, the differences in the properties of aluminum and steel, such as melting point, density and coefficient of linear expansion, are very different. In the process of post weld cooling, it is easy to crack, and the hard and brittle Fe-Al intermetallic compound produced by the two metallurgical reactions will aggravate the cracking of the joint and reduce the mechanical properties of the joint. Therefore, in the study of the welding of aluminum and steel, the formation mechanism and control of the Fe-Al intermetallic compound have always been one of the key points of study. The advantages of fast cooling rate and filler metal can control the formation of intermetallic compounds in the connection of heterogeneous materials to some extent. However, in the welding of laser aluminum steel, the formation and distribution of Fe-Al intermetallic compounds are still to be further studied. Therefore, this paper uses pulse Nd:YAG laser deep fusion welding and continuous diode. In this paper, the experiment of laser welding of aluminum steel was carried out by laser welding. First, the laser deep fusion welding test of pulsed Nd:YAG aluminum steel was carried out. Because the heat input of pulse Nd:YAG is very small, it is beneficial to control the formation and growth of intermetallic compounds. Two kinds of typical joints - deep penetration depth and shallow deep joint are obtained. The interface metal compound is rich in the rich Al phase from deep penetration joint to the rich Fe phase of the deep penetration joint. The bearing capacity of the joint is increased. Through the analysis of SEM and EDS, the mechanism of the interfacial reaction is revealed to increase the phase of the /Al interface of the.Fe and Al elements with the heat input in the fusion zone. The interaction of the intermetallic compound is changed by the interaction of the interface, which changes the formation of the intermetallic compound. In the deep fusion mode, the heat input is small, but the steel and aluminum have melted, which makes the Fe-Al intermetallic compound increase. In addition, the deep fusion welding mode is difficult to be stable due to the formation of the small hole. The process is unstable and easy to produce welding defects. All these factors will deteriorate the mechanical properties of the joint. Further, the laser heat conduction mode which is more stable in the welding process is used to carry out the welding test and study. The continuous diode laser is used for the welding of aluminum and steel, and the brazing with different alloy elements is added in the test. Material. The addition of alloy elements in laser welding is beneficial to control the formation and distribution of intermetallic compounds. Therefore, besides the analysis of the mechanism of intermetallic compound with the change of heat input, the mechanism of the change of the intermetallic compound with the alloying elements is emphatically analyzed. The experimental results show the morphology of the intermetallic compounds at the brazing interface. The mechanical properties and fracture modes of the joint will be changed with the change of heat input. The brazing interface is made up of theta -Fe (Al, Si) 3 and tau 5-Al7.2Fe1.8Si when using low power laser. The brazing interface is composed of theta -Fe (Al, Si) 3, tau 5-Al7.2Fe1.8Si and ETA -Fe2 (Al, Si) 5 with high power laser, and is accompanied by micro cracks. EDS line scanning analysis shows that in the process of brazing, the formation and growth of intermetallic compounds in brazing interface are mainly dependent on the diffusion of Fe elements in solid steel to the liquid melting zone. In the initial stage of diffusion, the morphology of intermetallic compounds can only be changed, but the type of intermetallic compounds will be changed with the diffusion. The addition of.Zn elements makes the types of intermetallic compounds in the brazing interface essentially change. The brazing interface changes from layered Fe2Al5 and needle like FeAl3 into layered FeAl5-xZnx, dispersed FeZn10 and a small amount of rich Al amorphous structure. This change improves the bearing capacity of the joint and changes the fracture mode. The hardness test is tested by the hardness test. The effect of FeZn10 on the mechanical properties and fracture mode of the joint is revealed. Because of the soft and flexible properties of FeZn10, the fracture energy can be absorbed through plastic deformation, thus preventing the interface cracking and improving the bearing capacity of the joint. Based on the thermodynamic theory, the formation order of the intermetallic compound is determined, and the intermetallic compound with the laser heat input and alloy is analyzed. Finally, the influence mechanism of intermetallic compound thickness, type and distribution on fracture mode is deeply analyzed. Based on the correlation theory of interface spacing mismatch rate and interface energy and interface strength, edge to side crystal matching model is used to calculate the mismatch rate of interplane spacing on each reaction phase interface, and the estimation and comparison are compared. The bonding strength of different interfaces is used to predict the mechanism of fracture mode at the interface. It is found that the higher the mismatch rate is, the higher the interfacial energy, the lower the interfacial bonding strength, and the easier the interface cracking.

【学位授予单位】：南昌大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG457.1

【相似文献】