【作者】 李娜；

【导师】 李文亚；

【作者基本信息】 西北工业大学 ， 材料加工工程， 2019， 博士

【摘要】 搅拌摩擦焊（Friction Stir Welding,FSW）由于其典型的固相焊接特征,在工业领域应用越来越广泛,尤其是对于高强铝合金的焊接。然而,大量研究表明高强铝合金（2系列和7系列）FSW接头的抗腐蚀性能较差,严重制约了FSW技术的应用推广,在实际工业应用中必须对FSW接头进行腐蚀防护以提高其服役寿命。目前,对高强铝合金FSW接头腐蚀防护方法均不尽完善,冷喷涂技术制备的铝及铝基复合涂层具有良好的防腐蚀效果,但目前鲜有采用冷喷涂方法制备FSW接头防腐涂层的报道。因此,本文以3.2mm厚2024-T3高强铝合金FSW接头为研究对象,采用冷喷涂技术在接头表面制备纯铝、铝基复合涂层以及Cu、Ni涂层,分析了冷喷涂对接头显微组织、力学性能和腐蚀防护性能的影响,通过试验设计揭示了冷喷涂对力学性能改善的内在机制。本文的主要研究内容和结果如下:为了获得较优的接头性能,分析了旋转速度对接头显微组织和力学性能的影响规律,并分析了接头不同区域显微组织与全局力学性能的关系。结果表明:旋转速度对接头显微组织和力学性能影响显著,随着转速的增加,接头硬度值增加,低转速时接头硬度曲线呈“U”形,高转速时呈“W”形,归因于低转速有效地避免了铜、镁以及铜-镁原子团簇（Guinier-Preston-Bagaryatsky,GPB）的进一步溶解及焊后冷却过程S相的析出,高转速时促进了GPB区的析出。然而,只有在适当焊接热输入时,接头的拉伸性能才能最佳,最佳焊接参数为:600 rpm和200mm/min,接头具有最高的抗拉强度（约420 MPa）和延伸率（约8.7%）。选取上述最佳焊接参数的FSW接头作为冷喷涂基体,并选用适当的冷喷涂工艺参数在FSW接头上表面制备成形良好的Al-20 vol.%Al₂O₃铝基复合涂层。冷喷涂处理对接头近表面显微组织产生了显著影响,喷涂后接头表面GPB区相对含量增加,且焊核区（Stir Zone,SZ）表面晶粒尺寸得到细化,同时,大角度晶界比例增大,此外,经冷喷涂处理的接头力学性能得到显著提高,喷涂后接头表面硬度值均高于焊态,抗拉强度和延伸率分别提高了7%和9%,在200MPa水平下,喷涂态接头疲劳寿命是焊态寿命的4倍。为了揭示冷喷涂改善FSW接头强塑性的内在机理,采用试验设计的方法将冷喷涂过程中高温高速气流的“热效应”和粒子高速撞击的“喷丸效应”剥离。两种效应单独作用时均对接头近表面显微组织和整个接头的力学性能产生了与冷喷涂过程类似的改善作用,两种效应均增加了接头GPB区含量、细化了晶粒尺寸、增加了大角度晶界比例,“热效应”对接头的主要贡献在于延伸率的提高,“喷丸效应”的主要贡献在于强度的提高。“热效应”时,接头基体表面温度约200^oC～300^oC,促进了接头GPB区的溶解但又不会使S相粗化,“喷丸效应”时,接头残余应力得以部分释放,并细化晶粒且由于形变引起相变增加GPB区含量,进而相对分数较高的GPB区和细化的晶粒可以对接头起到强塑化的作用。对比冷喷涂后接头组织性能变化,其两种效应单独作用时对力学性能的变化并不是简单的相加,而是协同诱导实现对接头的强塑化作用。为了深入揭示冷喷涂影响机理,采用与冷喷涂Al-20 vol.%Al₂O₃复合涂层相同的喷涂工艺参数,在FSW接头表面制备了成形良好的纯Cu和纯Ni涂层,Cu涂层的厚度与铝基复合涂层厚度相当,Ni涂层没有获得有效沉积,对比不同密度粉末的冷喷涂层对接头拉伸性能的影响,发现Cu涂层和Ni涂层对接头的抗拉强度提高程度略高于铝基复合涂层,分别提高了8%和9%,而延伸率提高程度较铝基复合涂层明显提高,使焊态接头延伸率分别提高了44%和39%。究其原因,冷喷涂过程中Cu粒子和Ni粒子获得的动能远大于Al-20 vol%Al₂O₃粉末,对基板的“喷丸效应”增加。最后,研究了FSW接头在不同腐蚀溶液中的耐蚀性及旋转速度对接头耐蚀性的影响,揭示了FSW接头耐蚀性较差的原因,接头不同区域耐蚀性均低于母材,主要是由于显微组织差异尤其是第二相粒子分布差异所致。由于接头耐蚀性差,故采用冷喷涂层对接头进行腐蚀防护。采用电化学测试方法和摩擦磨损实验对比了冷喷涂纯铝涂层和Al-20 vol.%Al₂O₃复合涂层对FSW接头的腐蚀防护情况以及两种涂层的耐磨性。研究表明:两种涂层均可以对FSW接头提供腐蚀防护,铝基复合涂层的耐蚀性较纯铝涂层差,是由于Al₂O₃陶瓷相的加入使得涂层内部残余压应力增加进而加剧了腐蚀程度,且Al₂O₃陶瓷颗粒与纯铝颗粒边界处易受腐蚀介质攻击,使得铝基复合涂层的耐蚀性低于纯铝涂层。但是Al₂O₃陶瓷相硬度较大,增加了铝涂层的耐磨性。该研究将促进FSW接头抗腐蚀性能与力学性能一体化调控的实现,有利于FSW技术和冷喷涂技术的推广应用。更多还原

【Abstract】 Friction stir welding（FSW）,as a high quality,efficient and environment-friendly solid-state joining technology,has prominent advantages in the welding of high strength aluminum alloys in aerospace,shipbuilding and other industrial fields.However,a large number of studies have shown that FSWed joints of high strength aluminum alloys.Hence,the FSWed joints must be protected for corrosion attack to improve the service life.At present,the methods of corrosion protection for FSWed joints are not perfect.The Al and Al composite coatings deposited by cold spraying（CS）have good corrosion resistance for FSWed joints,but there are few studies about CSed coatings for corrosion protection of FSWed joints.In this research,pure Al,Al-20 vol.%Al₂O₃ composite coatings,puer Cu and pure Ni coatings were CSed on the surface of the FSWed joints of 3.2 mm thick 2024-T3 Al alloy.The effect of the coating on the microstructure,mechanical properties and corrosion performance of FSWed joints was investigated.Furthermore,the mechanisms for improving mechanical properties of FSWed joints were revealed by experiments.The main conclusions are as follows:The effect of rotation speed on microstructure and mechanical properties of joints is analyzed.Results show that the hardness is improved with the rotation speeds;in case of lower rotation speed the hardness profile shows“U”shape,in case of higher rotation speed it shows“W”shape;the reasons are that lower rotation speed can avoid the dissolution of GPB（Guinier-Preston-Bagaryatsk）zones and the precipitation of S phase,but higher rotation speed can promote the dissolution of GPB zones.When in the proper welding heat input the best tensile properties can be achieved.The optimal welding parameters are:600 rpm and 200 mm/min,and the joint has the best ultimate tensile strength（420 MPa）and elongation rate（8.7%）.High-quality Al composite coating can be deposited on the optimal FSWed joint by CS using the proper process parameters.CS has a significant influence on the surface integrity of the FSWed joint.Results show that CS increases the GPB zone of the joint near the surface,makes the grain of the（Stir Zone）SZ refine and the high-angle grain boundaries（HAGBs）increase.Furthermore,the mechanical properties have been improved after CS.The hardness value of CSed joint is higher than the as-welded joint in the whole joint.The ultimate tensile strength and elongation are increased by 7%and 9%,respectively.The fatigue life of the CSed joint is four times as the as-welded joint in 200 MPa.In order to reveal the strengthening mechanism of the FSWed jont induced by CS,the project focuses on the heat flow effect（HFE）caused by the high speed and high temperature gas flow and the shot peening effect（SPE）resulting from the high velocity impact of spray particles during CS.The two effects make the surface integrity and mechanical properties have the similar effect with the CS.The two effects make the GPB zone increase,the grain size refine and the HAGBs increase.However,the“HFE”mainly contributes to the elongation and the“SPE”mainly contributes to the elongation tensile strength.The“HFE”during CS heats up the near-surface layer of the coated joint at a temperature between 200-300^oC,which improves dramatically the dissolution of the GPB zones both in SZ and HAZ.Moreover,the“SPE”during CS may release the residual stresses of the joints,which may also contribute to the hardness improvement.The higher hardness can significantly reduce the formation and growth rate of cracks,and thus improves the joint strength.The coated joint successfully produces superior strength-ductility synergy not due to the simple adding effect of the two effects but synergy work of the two effects.The pure Cu and Ni coatings are produced on the FSWed joints using the same CS process parameters as the Al-20 vol%Al₂O₃ composite coating.The Cu coating has the same thickness as the Al-20 vol%Al₂O₃ coating and the Ni coating has hardly be deposited on the surface of the joint.The influence of the coating with different powder density on the tensile properties of the FSWed joints is compared.The results show that the Cu and Ni coatings have the same effect on the ultimate tensile strength like Al-20 vol%Al₂O₃ coating and the elongation is significatively increased.The elongation of the FSWed joint deposited on Cu and Ni coatings are increased by 44%and 39%,respectively.The reason is that during the cold spraying the kinetic energy of Cu and Ni particle is greater than that of Al-20 vol%Al₂O₃ particle.Thus,the effect of"shot peening effect"on the substrate can be improved significantly,which will make the grain finer and the tensile properties better.The corrosion behavior of the FSWed joint under different solution and different rotation speed is investigated and the mechanism of lower corrosion resistance for the joint is revealed.The reason is due to the different microstructure of the different zones especially the second phase particle distribution of the different zones.Hence,the CSed Al coating can be deposited on the surface of the FSWed joint to supply corrosion protection.The corrosion and wear resistance of the pure Al coating and Al-20 vol%Al₂O₃ coating are tested by electrochemical and wear tests.Results show that the two coatings can both supply corrosion protection to FSWed joint.The corrosion resistance of the Al composite coating is lower than that of the Al coating.It is attributed to the presence of more active sites induced by the high severe plastic deformation in comparison with the unreinforced pure Al coating.Moreover,the interfaces between reinforcement and matrix particles are also active sites easily attacked by the electrolyte.However,The Al-Al₂O₃ coating has better wear resistance than pure Al coating due to the increasement of Al₂O₃ particles.The addition of the hard Al₂O₃ phase into Al coating has improved to a limited extent the wear performance at the expense of corrosion resistance.Based on this finding,what is required next is the combination of the improved corrosion resistance of pure Al with the better wear resistance of the Al-20 vol.%Al₂O₃ coating.The achievements of this project will help realize the integrated control of corrosion resistance and strengthening of FSW joints,to promote the applications of FSW in many important industries.更多还原