【作者】 肖兵；

【导师】 王东坡； 王惜宝；

【作者基本信息】 天津大学 ， 材料加工工程， 2017， 博士

【摘要】 采用含铯盐氟化物钎剂辅助的中温钎焊技术,是实现含镁铝合金钎焊连接的具有广阔应用前景的方法。但含镁铝合金,尤其是含镁量大于2.0wt.%的铝合金,钎焊时表面易形成富镁氧化膜,铯盐钎剂由于活性不足,去膜性能较差,该类铝合金钎剂钎焊性能不佳。研究含镁铝合金表面氧化膜结构,分析铯盐钎剂去膜机制,从而开发适当的活性钎剂,对改善含镁铝合金钎焊性能、促进铝合金钎焊技术应用具有重要意义。本文主要研究工作和结论如下:（1）5052铝合金在中温钎焊时表面氧化膜的结构。铝合金的Mg元素极大的影响表面氧化膜的结构及其钎焊性能。为了解含镁铝合金氧化膜的成分和物相,本文选择含镁量大于2.0wt.%的铝合金中典型的5052铝合金,采用光电子能谱技术和透射电镜对其表面氧化膜结构进行试验研究,并对其氧化膜转变的热力学进行计算分析。研究结果表明,在室温下,抛光后的5052铝合金试样表面氧化膜主要是Al₂O₃。在中温钎焊条件下加热后,5052铝合金试样表面出现了Mg元素的富集和氧化,表面氧化膜主要由两层组成,表层为MgO,内层为MgO加上少量弥散分布的MgAl₂O₄。5052铝合金钎焊时需要去除的氧化膜物相是MgO和MgAl₂O₄相。（2）CsF-AlF₃钎剂与含镁铝合金氧化膜之间的作用机理。利用密度泛函理论,建立钎剂中F离子在γ-Al₂O₃和MgO表面的吸附模型,计算分析了吸附能、电子态密度和差分电荷密度。研究结果表明,与F离子在γ-Al₂O₃氧化膜表面易于发生化学吸附和置换反应,导致晶格碎裂,从而实现溶解去膜的机制不同,当F离子在MgO表面吸附时,随着吸附覆盖度增加,吸附能反而增加,吸附强度逐渐减小,导致F离子难以置换O离子,限制了后续溶解反应的进行。因此,CsF-AlF₃钎剂难以通过反应、溶解的方式去除以MgO为主相的含镁铝合金氧化膜。（3）CsF-AlF₃活性钎剂的开发。从重金属离子、络合离子、稀土元素离子等活性剂中,选取ZnCl₂、ZnF₂、BiCl₃、SnCl₂、（NH4）₂GeF₆、ZrF₄、CeF₃等七种典型化合物做为添加活性物质,制备CsF-AlF₃活性钎剂,并分析了钎剂活性的影响规律。研究结果表明,上述活性物质中,ZnCl₂和Zr F₄是对含镁铝合金表面氧化膜有效的界面活性剂,其中ZrF₄具有较好的活性且无腐蚀性,它是通过松脱机制来去除以MgO为主相的含镁铝合金表面氧化膜的。活性物质添加量对钎剂活性有很大的影响,添加浓度为4-6 mol.%时钎剂活性达到最佳。（4）典型含镁铝合金钎焊试验。为验证开发活性钎剂在钎焊中的实际应用效果,对三种典型含镁铝合金5052、5083、6061铝合金进行了钎焊工艺试验。试验结果表明,在活性钎剂作用下,采用Zn-15Al钎料在中温钎焊温度下得到了成型良好、结合紧密的钎焊接头,钎焊接头强度均超过100MPa。钎焊得到的钎缝组织主要由α-Al相、β-Zn相、（α+β）共晶相和共析组织组成。更多还原

【Abstract】 Medium-temperature brazing with the aid of Cs-salt-containing fluoride fluxes is a promising joining technology for Mg-containing aluminium alloys.However,for such alloys containing more than 2.0 wt.%Mg,a Mg-enriched oxide film is readily formed on the surface that is difficult for pure Cs-salt-containing fluoride fluxes to remove,thus making the flux brazeability of Mg-containing aluminium alloys poor.Therefore,understanding the oxide-film structure,analyzing the oxide-film removal mechanism,and developing appropriate fluxes to remove such oxide films are essential for improving the brazeability of Mg-containing aluminium alloys.The main research works and conclusions are as follows:（1）The oxide-film structure on the surface of the 5052 aluminum alloy under medium-temperature brazing conditions.Elemental Mg in the aluminium alloy drastically affected the compositions and phases of the surface oxide-film.Theoretical and experimental studies were carried out to understand the structure of the oxide film on the surface of aluminium alloys contain more than 2.0 wt.%Mg.In this paper,a typical 5052 aluminium alloy was selected and investigated by X-ray photoelectron spectroscopy（XPS）and high-resolution transmission electron microscopy（HRTEM）.Thereafter,the interface thermodynamic behavior of oxide overgrowths was analyzed.The results showed that the oxide film was mainly composed of Al₂O₃ on the surface of the polished 5052 aluminium-alloy sample at room temperature.After being heated for medium-temperature brazing,the activated Mg（segregated from the alloy）was significantly enriched and oxidized at the surface of the sample.The oxide film on the5052 aluminium-alloy sample was divided into two layers:the outer oxide surface was composed of an amorphous MgO-like phase,while the interior of the oxide film was composed of both the amorphous MgO-like phase and small distributions of MgAl₂O₄.Therefore,MgO and MgAl₂O₄,the main phases of the oxide film,should be removed for the successful brazing of the 5052 aluminium alloy.（2）Interaction of CsF-AlF₃ flux with the surface oxide film of Mg-containing aluminium alloys.An adsorption model of F ions,which comes from the CsF-AlF₃flux,on the surface ofγ-Al₂O₃ and MgO was built using density functional theory,and adsorption energies,charge density differences and density of states were calculated and analyzed.Based on the model,chemical absorption of F ions on the surface ofγ-Al₂O₃ oxide film readily occurred,which promoted the breaking and dissolution ofγ-Al₂O₃ in the flux.When F ions absorb on the surface of MgO oxide film however,the adsorption energy increased and the adsorption decreased as the coverage increased,limiting the displacement between F ions and O ions in MgO and the subsequent dissolution.Therefore,the pure CsF-AlF₃ flux not ideal for removing MgO oxide film from the surface Mg-containing aluminium alloys by way of dissolution.（3）The development of activated CsF-AlF₃ flux.Of the activating agents of containing heavy-metal ions,complex ions and rare earth ions,ZnCl₂,ZnF₂,BiCl₃,SnCl₂,（NH4）₂GeF₆,ZrF₄,CeF₃ were selected and added to the CsF-AlF₃ flux,and the interfacial activities of the developed fluxes were analyzed.ZnCl₂ and ZrF₄ were effective activating agents for removing the oxide film from Mg-containing aluminium alloys.The ZrF₄-containing CsF-AlF₃ flux was active and non-corrosive,and removed MgO oxide film from the surface of the Mg-containing aluminium alloys by way of loosening.The contents of the activating agents greatly influenced the activity of the flux,and the highest activity was obtained when the concentration of activating agents in the CsF-Al F₃ flux was 4-6 mol.%.（4）Brazing of typical Mg-containing aluminium alloys.To evaluate the effects of the developed activated fluxes,brazing experiments were conducted with three typical Mg-containing aluminium alloys—5052,5083 and 6061.Experimental results indicated that sound joints were obtained with the aid of the developed activated flux.The strengths of the brazing joints of these three aluminium alloys were more than100 MPa.Microstructural observations of the brazing seams showed that they were composed ofα-Al phase,β-Zn phase,as well as eutectoid structure and（α+β）eutectic structures.更多还原