【作者】 张洪涛；

【导师】 冯吉才； 何鹏；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2008， 博士

【摘要】 随着铝及其合金在汽车工业中的广泛应用,铝和钢之间的连接便成为焊接领域的热点问题之一。通过外加回抽力来实现短路过渡的CMT方法可以成功地以熔-钎焊的方式连接铝和镀锌钢板的搭接接头,但是其连接机理尚不明确。因此,系统研究铝和镀锌钢板CMT熔-钎焊连接机理、接头组织演化特征、界面反应机理、电弧加热行为以及镀锌层行为,不但可以丰富熔-钎焊焊缝成形理论,而且为促进熔-钎焊在其他异种材料连接中的应用起到积极的借鉴作用。采用电弧形态、熔滴过渡视觉传感系统以及电流电压信号同步采集系统对CMT方法的能量输入特点和熔滴过渡行为进行了研究。结果表明,CMT方法通过特殊的波形控制减小了电弧能量输入,同时较低的短路电流也降低了短路阶段产生的电阻热,因此可以更精确地控制焊接热输入。CMT短路过渡过程中,将熔滴尺寸控制在一定范围内,防止滴状过渡的发生,实现稳定的短路过渡。同时,稳定的外加回抽力避免了大的电磁力和电爆炸,从而有效地消除了飞溅。铝/镀锌钢板CMT熔-钎焊搭接接头的界面组织分析表明,接头界面主要由两类不同化合物相组成,分别是靠近钢侧厚度均匀的Fe2Al5相和靠近铝侧针片状的FeAl3相。界面化合物层的厚度对焊接热输入非常敏感,当焊接电流从66安培增加到85安培,界面化合物层的厚度可以从10μm左右增加到40μm以上,但是热输入的增加并没有改变界面化合物层的构成。通过建立的电弧形态视觉传感系统研究了镀锌层对电弧加热行为的影响。结果发现,小电流焊接时,镀锌层能够起稳定电弧的作用;随着焊接电流的增加,镀锌层开始剧烈蒸发,从而使电弧边缘上翘,减小了电弧和工件的接触面积,进而减小了焊接热输入。通过拍摄到的电弧形态对焊接中的比热流分布形状进行拟合,然后采用拟合的热源模型模拟了焊接中的温度分布特点,发现相同焊接参数下与非镀锌的Q235钢板相比,在镀锌钢板上堆焊时镀锌层的蒸发可以使界面最高温度降低200℃,温度的降低也直接减小了界面脆性化合物层的厚度。CMT熔-钎焊连接铝板和镀锌钢板时,钢板并没有熔化,因此界面反应的实质是液态铝和固态铁之间的反应。根据热力学计算结果,铝钢界面上可以生成FeAl、Fe2Al5以及FeAl3三种化合物相。通过对建立的动力学模型进行求解,可以发现,在液态铝/固态铁的界面上,Fe2Al5化合物相的生长动力学系数远远大于FeAl相和FeAl3相。因此,当液态铝和固态铁接触的瞬间,Fe2Al5化合物相便快速生长,而靠近铝侧的针片状的FeAl3相则在冷却的过程中结晶析出。采用被动视觉传感和复合滤光技术建立的传感系统,获取了焊接过程中清晰的钢板表面特征图像。对比分析在Q235钢板表面堆焊和镀锌钢板表面堆焊时的表面特征差异,发现焊接过程中电弧下方的镀锌层并没有被完全蒸发,未蒸发的镀锌层可以熔化,从而在钢板表面形成瞬时的液态薄膜,液膜的存在改变了钎料的平衡润湿条件,使其在钢板表面充分铺展,提高了焊缝成形质量。提出了利用“表面瞬时液膜”改变钎料润湿条件的方法,在Q235钢板表面刷涂锌层后,实现了与铝板的连接,接头成形美观,抗剪强度达到了80MPa。更多还原

【Abstract】 Joining aluminium to steel is becoming the hot focus in automotive industry with the extensive use of the aluminium alloy in car body. The heat input was decreased by the special droplet transfer process in CMT welding process and the CMT process could be used to make a good lap joint between aluminium and zinc-coated steel by welding-brazing process. The aim of this paper is to study the microstructure characteristics and the interface reaction mechanism of the dissimilar metal joint. The heating behavior and the coating behavior were also studied. This study did not only enrich the formation of weld theory, but also establish a basis in joining dissimilar metals by welding-brazing process.The experiment systems to photograph the arc shape and the molten droplet images by using high-speed charge coupled device camera and to record welding current and arc voltage in synchronism were established. It found the enery input of the welding arc could be reduced by the special wave control and the low short circuiting current also reduced the corresponding energy input during short circuit phase. The heat input to the workpiece can be controlled accurately. During the metal transfer process, the size of the droplet was controlled within some range in order to avoid globular metal transfer. The spatter was eliminated by the adscititious force comparing with the conventional short circuiting metal transfer and the post welding cleaning became needless.The lap joint between aluminum and zinc-coated steel made by CMT welding-brazing process mainly consisted of two kinds of compounds. The homogeneous compact compound near the steel side was Fe2Al5 phase and the needle-like compound near the alumium side was FeAl3 phase. The thickness of the compound layer was sensitive to the welding heat input. When the welding current increased from 66A to 85A, the compound layer thickness could increase from about 10μm to more than 40μm. The interfacial composition was not changed by the enhancement of heat input.The paper also studied the effect of the zinc coating on the arc heating behavior by the arc shape visual sensing system. The zinc coating could stabilize the arc when welding with small current. The evaporization of the zinc coating became obvious with the raise of the welding current and this evaporization changed the arc shape. As a result, the welding heat input was reduced because the changed arc shape decreased the contact area between the arc and the workpiece. The heat flux distribution was fitted by the arc shape and was used to simulate the temperature field during welding. The highest interface temperature could be reduced 200℃owning to the zinc coating evaporization and the thickness of the interfacial compound layer also could be decreased.When joining aluminium to zinc-coated steel, the steel base metal was not melted. The interfacial reaction essentially occurred between liquid aluminum and solid iron. According to the thermodynamics calculation results, only FeAl, Fe2Al5 and FeAl3 phases could be generated at the aluminium-iron interface. The growth kinetics model was established and the growth rate of every phase was calculated. The results showed that the Fe2Al5 phase grew rapidly when the liquid aluminium contacted with the solid steel. The FeAl3 was formed during the cooling step.The clear image of the steel surface was obtained by the passive visual sensing method and the composite filtering technique. After comparing the steel surface features when overlay welding on Q235 steel with that overlay welding on zinc-coated steel, a instantaneous liquid thin film formed by the unevaporated zinc coating was found. During welding process, the surface instantaneous thin film promoted the filller metal spreading extent and guaranteed the quality of the lap joint.Then the“surface instantaneous liquid thin film”method was presented to change the wetting condition of the filler meta. The Q235 steel coated by the zinc powder was joined to aluminium successfully. The appearance of the lap joint obtained by this technology was very good and the tensile stength could reach 80Mpa.更多还原