摘要
镁合金和铝合金是目前工业生产中应用最为广泛的两种轻金属。镁合金和铝合金的连接问题一直受到国内外学者的广泛关注,实现二者之间的良好连接将极大地推进这两种轻金属在更广阔领域内的应用。本文从镁铝异种金属熔化焊接技术存在的问题入手,率先采用将激光焊接与胶接相复合的激光胶接焊技术,初步实现了镁铝异种金属的良好连接。激光胶接焊技术将冶金连接与机械连接有机复合,充分利用激光焊接与胶接的各自优势,使激光胶接焊试件在一定的范围内具有更高的连接性能。在此基础上本论文主要针对镁铝异质金属激光胶接焊工艺、焊接接头内部组织、元素分布以及晶体结构等方面开展了如下研究：
1.本文以AZ31B镁合金和6061铝合金为研究对象,对镁铝异种金属激光焊接和激光胶接焊工艺开展研究。首次采用激光胶接焊实现了上述镁铝异种金属的良好连接。在本实验条件下,铝合金上激光焊接模式为热导焊模式时,熔池内部形成相对较少的金属间化合物；而在铝合金上为小孔焊接模式时,激光焊接接头和激光胶接焊接头内部都形成了大量的金属间化合物,严重影响了焊接接头性能,因此应采用热导焊模式下的参数进行镁铝异种金属的激光焊接及激光胶接焊。在最佳参数下得到的激光胶接焊(焊缝涂胶)试件具有比单独激光焊接试件更加优良的拉伸和剥离性能,因而胶粘剂的加入将会对镁铝异种金属熔池组织结构产生有益的影响。
2.本文针对热导焊条件下激光胶接焊组织进行全面分析发现,在铝合金熔深小于0.3mm时,由于此时激光功率较小,焊缝中的胶粘剂不能完全分解并逸出熔池,进而在熔池底部形成胶粘剂的少量残留,对接头性能产生了不良影响；当铝合金上熔深为0.5mm时,此时在镁铝异质界面形成了相对较少的金属间化合物获得了具有最佳性能的焊接接头；当铝合金上熔深大于0.8mm,熔池底部形成了相对较多的金属间化合物,故性能略有降低。
3.通过与镁铝异种金属激光焊接接头比较分析发现：在激光胶接焊中,胶粘剂的加入改变了铝合金的表面状态,提高了铝合金激光吸收率,在相同的焊接参数下增加了铝合金的焊接熔深。通过对比激光焊接接头和激光胶接焊接头中的金属间化合物种类和分布特点发现：在激光焊接接头中在熔池底部形成了连续分布的带状金属间化合物,因为熔池底部最先凝固,受到后凝固金属的收缩应力作用,金属间化合物之间的界面极易形成裂纹,因而降低了接头的性能。在镁铝异种金属激光胶接焊接头中,胶粘剂的加入改变了镁铝异种金属在熔池中分布的特点,使其主要处在镁铝异质熔池中间区域,金属间化合物厚度略有减薄,在熔池中主要以金属间化合物Mg17Al12和非常薄的Mg2Al3形式存在,这样则减小了镁铝异种金属间化合物的不良影响,在一定程度上提高了焊接接头的性能。
4.通过对镁铝异种金属激光焊接接头和激光胶接焊接头内部裂纹的比较分析发现：由于激光胶接焊中的金属间化合物分布相对分散、成分相对简单,部分区域的金属间化合物厚度也小于激光焊接接头,进而减少了熔池内部形成裂纹的趋势。通过对镁合金和铝合金熔池温度的测量发现,激光胶接焊内部镁合金熔池与铝合金熔池温度差小于激光焊接镁合金熔池与铝合金熔池的温度差,在镁铝异种金属激光胶接焊镁铝异质熔池界面区域由于温度差异引起的热应力也会小于激光焊接熔池,因而镁铝异种金属激光胶接焊接头具有相对更小的裂纹形成趋势。
5.使用透射电镜对镁铝异种金属激光胶接焊接头熔池内部晶体结构进行分析,通过对金属间化合物的透射电镜形貌和晶体结构分析发现,Mg17Al12具有与镁合金晶体更加接近的结构,因而能够与熔化的镁合金相互混合形成相对稳定的共晶类组织,而Mg2Al3具有与铝合金更加接近的晶体结构,所以在本实验中Mg2Al3的出现在一定程度上会降低焊接接头的性能。通过对铝合金熔池的晶体结构特征进行分析发现,晶粒上出现了大量的位错滑移现象,这主要是受到激光焊接产生的金属蒸汽的反冲作用以及上层后凝固金属收缩压力作用在熔池底部形成的,这也证明了熔池底部存在着比较明显的冲击应力。因此在镁铝异种金属激光焊接接头中,在熔池底部形成的金属间化合物,更容易出现裂纹。
Mg and Al are two kinds of light metals widely used in the industry. The joining technology has attracted much attention of abroad and home reaserchers. The well joining of Mg and Al alloys could improve the using area of these kinds of light metals. In order to solve the problem in melting welding Mg to Al process, laser weld bonding as a new kind of hybrid welding technology is used to join Mg to Al sucessfully. Laser weld bonding technology is a synergy of metallurgy joining and mechnical joining, and develops in which the tensile and peel performance of the weld-bonded structure exceeds that of either technology alone. In this thesis, a study would be done on the process laser weld bonding technoloy, microstructure in the fusion zone, microcracks in the joint and the crystal structure in the fusion zone.
1. In this thesies, AZ31B Mg alloy and 6061 Al alloy are the subject investigated. Laser weld bonding (LWB) technology is used to join Mg to Al sucessfully firstly. By analyzing laser welding and LWB Mg to Al process, it is found that when the laser welding mode in Al alloy is in conductive mode, there is less Mg-Al intermetallic forming in the fusion zone. However, when the welding mode in Al alloy is in keyhole mode, there is more Mg-Al intermetallic forming in the fusion zone, which makes obviously harmful effect on the property of the joint. Therefore, conductive mode should be used in laser welding and LWB process. The LWB Mg to Al joint without cured has a better tensile shear and peel properties than that of laser welding joint. Thus the addition of the adhesive would make effect on the microstructure of Mg and Al joint.
2. By studying the microstuctures of LWB Mg to Al joint in conductive mode, it is found that when the welding depth in Al alloy of LWB joint is less than 0.3mm, there is some resudial adhesive in the Al fusion zone because of the low laser power, which makes harmful effect on the property of the joint; when the welding depth in Al alloy of LWB joint is about 0.5mm, there is relatively less Mg-Al intermetallic, and the best shear load is obtained in this kind of joint; when the welding depth in Al alloy in LWB joint is larger than 0.8mm, there is a little more Mg-Al intermetallics forming in the fusion zone, which makes the property of the joint decreased.
3. Through the comparative experiments of laser welding and LWB joint, it is found that the addition of adhesive could change the surface state of Al alloy, and increase the laser absorptive of Al alloy. In same welding parameters, the welding depth of LWB Mg to Al joint has a deeper penetration than that of laser welding joint. By analyzing the species and distributions of Mg-Al intermetallics in laser welding fusion zone, there is a continous Mg-Al intermetallics layer at the bottom of the fusion zone in laser welding Mg to Al joint, which should has a lowest the temperature and freeze the firstly in the fusion zone. As the temperature at the bottom of the Al fusion zone in laser welding joint was the lowest, the intermetallics would be freezing firstly, and then the melted Mg alloy would freeze. The shrink stress of Mg freezing and the laser welding recoil pressure would act on the interface between the Mg17Al12 and Mg2Al3. Therefore some microcracks could form at this interface, which make harmful effect on the property of the joint. In LWB Mg to Al process, the addition of adhesive makes effect on the distribution of Mg-Al intermetallics, which locates in the middle of the fusion zone. The thickness of Mg-Al intermetallics is decreased in extent, and the intermetallics in the fusion zone are composed by a mainly Mg17Al12 and few Mg2Al3, which makes less harmful effect on the joint.
4. The microcracks in laser welding and LWB joint are analyzed in comparison. In LWB Mg to Al joint, the intermetallics distribution is deconcentration, the content of intermetallics is singleness relatively, and the thickness of intermetallics is decreased. Therefore the tendency of microcracks formation in LWB Mg to Al joint is less than that in laser welding joint. Through the observation of the temperature in Mg and Al fusion zone, the temperature contrast between Mg and Al fusion zone in LWB joint is less than that in laser welding joint. Therefore the thermal stress acting on the Mg and Al interface in LWB joint should be less than that in laser welding joint. Therefore the microcracks forming tendency in LWB joint should be less than that in laser welding joint.
5. Transmission electron microscope (TEM) is used to analysis crystal strucuture in different parts of the LWB Mg to Al joint. By analysis the crystal of the intermetallics, the Mg17Al12 intermetallics has a similar structure with that of Mg alloy and could forming a steady mixing eutectic phase with Mg alloy. However the Mg2Al3 intermetallic has a similar structure with that of Al alloy, which would make bad effect on the LWB joint. A number of dislocations are found in Al grains, which is formed by the impact stress of the laser welding recoil pressure and the.shrink stress of the upper alloy freezing. Therefore, there is obviously pressure stress at the bottom of the fusion zone in laser welding Mg to Al joint, which makes the tendency of microcracks in intermetallics enlarged.
引文
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