车用先进高强钢表面分析及胶接性能研究

目的 明确不同先进高强钢表面氧化物形貌、成分及含量对胶接性能的影响规律。方法 本研究采用白光干涉仪、扫描电子显微镜(SEM)及透射电子显微镜(TEM)观察高强钢表面形貌，通过金相显微镜及X射线衍射仪(XRD)分析其显微组织及物相，采用直读光谱仪、辉光光谱仪(GDS)以及X射线光电子能谱仪(XPS)表征高强钢表面特征元素及氧化物种类。结合接触角测试对比不同高强钢表面润湿性，并分析总结高强钢表面成分对胶接性能的影响规律。结果 DH780表面以膜层状Al基氧化物为主，DP980、DP1180以及CP1180表面以Si、Mn、Cr颗粒状氧化物为主。其中DH780的表面润湿性最差，自由能最小。CP1180钢表面具有最少的氧化物含量及最小的颗粒尺寸，其表面润湿性最好，表面自由能最高。DP钢随强度提高，表面氧化物含量增加，润湿性降低。对于同种焊装胶，DH780胶接剪切强度最小，CP1180剪切强度最大。结论 高强钢表面氧化物富集对表面润湿性及表面自由能具有重要影响，尤其Al基氧化物的存在会明显降低胶接接头的剪切强度。一方面是氧化物导致表面自由能降低，黏附功下降，另一方面氧化物会阻止胶中极性基团与基体表面形成强化学键，而更多地形成氢键。Al基氧化物由于呈薄膜状覆盖于高强钢表面，故对胶接性能的影响最大。

Surface Analysis of Advanced High Strength Steel for Vehicle and Their Bonding Performance

To investigate the effect of oxide morphology, composition and content on bonding properties of advanced high strength steels for automotive applications. In this study, a white light interferometer, a scanning electron microscope (SEM) and a transmission electron microscope (TEM) were used to observe the surface morphologies of different high-strength steels (DH780, DP980, DP1180, CP1180). The microstructure and phase structure of high-strength steels were analyzed with a metallography microscope and an X-ray diffractometer (XRD). The variation of surface characteristic elements content with depth and the types of surface oxides were analyzed with a direct reading spectrometer, a glow spectrometer (GDS) and an X-ray photoelectron spectroscopy (XPS). The surface wettability of different high strength steels was compared by the contact angle test of different systems, and the surface free energy was calculated. Finally, the bonding properties of high-strength steel and three different automobile welding adhesives were tested, and the influence of surface composition of high-strength steel on bonding properties was analyzed and summarized. The results indicated that there were oxides enrichment on the surface of high strength steels. The morphologies, types and content of oxides on the surface of different high strength steels were different. The surface of DH780 was mainly composed of membrane-layered Al based oxides, while the surface of DP980, DP1180 and CP1180 was mainly composed of Si, Mn and Cr granular oxides. The surface oxides content of DP steels increased with the increase of strength. CP1180 steel had the lowest oxide content on the surface and the smallest oxides particle size. The results of contact angle test and calculations showed that DH780 had the worst surface wettability and the lowest free energy (29.2 mn.m–1). Compared with DP1180, the wettability and surface free energy of DP980 decreased slightly with the increase of the steel strength. CP1180 showed the best surface wettability and the highest surface free energy (40.5 mN.m–1). For the same kind of adhesive, DH780 showed the lowest shear strength and CP1180 showed the highest shear strength. In conclusion, the surface oxides enrichment of high strength steel has an important effect on surface wettability and surface free energy. The presence of Al based oxides on the surface of high strength steel significantly reduces the shear strength of the bonded joint. On the one hand, it is due to the decrease of surface free energy, which leads to the decrease of adhesion work. On the other hand, the main compositions of welding adhesive are epoxy resin and dicyandiamide curing agent. Oxides will prevent the polar groups such as epoxy and amino groups in the adhesive from forming strong interactions (covalent or ionic bonds) with the surface of the steel matrix, and more often form weakly interacting hydrogen bonds with oxides. Compared with Si, Mn and Cr, Al has the smallest diffusion activation energy in Fe, and it is easier for Al to diffuse to the steel surface at the same temperature. Moreover, the Al based oxides cover the entire surface of the high strength steel in a thin film. Thus, DH780 adhesive joint has the lowest shear strength and the worst bonding performance.