【作者】 姬峰；

【导师】 薛松柏；

【作者基本信息】 南京航空航天大学 ， 材料加工工程， 2013， 博士

【摘要】 Cu/Al异种金属钎焊接头已经逐渐在装备制造过程中得以推广应用。相对于其他组分用于铜铝钎焊的Zn-Al合金钎料，Zn-22Al的熔化区间与CsF-AlF3钎剂具有更好的工艺适配性，因此更适合用于Cu/Al异种金属的自动火焰钎焊过程。然而目前应用于Cu/Al火焰钎焊的Zn-22Al钎料仍存在极易氧化、在Cu母材上润湿性较差、特别是易与Cu生成脆性金属间化合物等问题。另外，目前仍缺乏对Zn-22Al钎料抗蠕变性能、Cu/Al钎焊接头服役时界面化合物演变规律、Cu/Al钎焊接头耐腐蚀性能等方面的研究数据。本研究针对Zn-22Al钎料和Cu/Al自动火焰钎焊接头的不足，系统地研究了微量Ti、Ce的添加对Zn-22Al钎料组织、性能以及Cu/Al钎焊接头性能的影响，并对其在钎料和接头中的作用机理进行了探讨。通过对不同钎料进行电阻率和熔化特性测试发现，钎料电阻率随Ti含量增加升高，随Ce含量增加先降低然后逐渐趋于稳定；Ti的添加可以提高钎料的固液相线并增大其熔化区间，而Ce对Zn-22Al钎料的熔化特性影响甚微。钎料的热重分析结果则表明，0.03wt.%的Ti或0.05wt.%的Ce可以提高钎料的抗氧化能力，分析是因为两种元素在钎料表面形成了比ZnO更致密的氧化膜所致，而当Ti、Ce的添加量分别达到1wt.%和0.5wt.%时则会显著恶化钎料的抗氧化能力。微量Ti、Ce的添加可以显著细化Zn-22Al钎料显微组织，分析认为Ti会与钎料中的Al原子结合形成TiAl3化合物颗粒，该颗粒在钎料凝固过程中优先析出为η-Zn提供形核质点从而使其由树枝状转变为“雪花状”；Ce同样会与Al、Zn原子结合形成Ce-(Al, Zn)化合物相，该相不仅可以为钎料提供形核质点，还会聚集在η-Zn枝晶臂间阻碍其生长，从而达到细化钎料组织的目的，但是当Ce含量超过0.15wt.%时，钎料中会形成大量含Ce硬质相，该相的大量出现会降低钎料的力学性能。另外，0.01~1wt.%的Ti或0.03~0.15wt.%的Ce的添加可以显著增大钎料在Cu和Al母材上的铺展面积。通过纳米压痕技术对Zn-22Al、Zn-22Al-0.03Ti、Zn-22Al-1Ti、Zn-22Al-0.05Ce、Zn-22Al-0.5Ce五种钎料的弹性模量、压痕硬度和室温蠕变应力指数进行了测量和计算。测量结果显示，钎料的弹性模量和压痕硬度均随Ti、Ce含量的增加而提高，从而使该钎料更适合应用于火焰钎焊的自动添丝设备。当加载载荷为50mN、加载速率分别为0.5、1、2.5、5、10mN/s时，五种钎料的室温蠕变应力指数n的变化范围分别为：24.72~27.10、33.11~38.14、28.17~33.21、32.63~36.70和28.31~32.32；而当加载速率为2.5mN/s，载荷分别为50、80、100、200、300mN时，五种钎料的蠕变应力指数n分别为：25.69~28.35、32.45~38.45、29.47~34.64、34.07~37.09和29.67~32.09，表明Ti或Ce的添加可以增强钎料在室温时的抗蠕变能力。分析认为，钎料弹性模量、压痕硬度及室温抗蠕变能力的提高主要是由于Ti、Ce的添加使钎料中出现了TiAl3或Ce-(Al, Zn)等化合物颗粒，这些强化相的出现提高了基体的抗变形能力。钎料中Ti、Ce的添加可以显著改善Cu/Al火焰钎焊接头力学性能和显微组织，Zn-22Al-0.03Ti和Zn-22Al-0.05Ce所得Cu/Al接头剪切强度较Zn-22Al接头分别提高了17.4%和23.6%；钎焊过程中Cu侧界面处先后发生两个反应： Cu Zn CuZn和9Cu+4Al Cu9Al4，Ti、Ce的添加促使钎缝中条块状Cu9Al4相转变为颗粒状，从而提高了Cu/Al接头的剪切强度；Cu/Al钎焊接头断裂形式为韧性断裂，接头断面处有明显的韧窝和化合物颗粒，Ti、Ce的添加可以使韧窝的分布更细小均匀，并促使断裂位置由靠近界面化合物处转移至钎缝处。采用高温时效试验模拟了Cu/Al钎焊接头的老化过程，研究了该过程中接头力学性能和显微组织的演化过程。时效过程中，接头剪切强度随时效时间的延长逐渐降低，但Zn-22Al-0.03Ti和Zn-22Al-0.05Ce接头的剪切强度始终高于Zn-22Al接头；期间还伴随着Cu侧界面化合物厚度的逐渐增加，其结构逐渐由Cu9Al4/CuZn转变为Cu9Al4/CuAl/CuZn，并最终转变为ε/Cu9Al4/CuAl/CuZn；Ti、Ce的添加降低了界面化合物的生长速率，并使Cu9Al4相的扩散激活能由76.9kJ/mol分别升高至83.9kJ/mol和87.6kJ/mol，同时还降低了其粗化通量，有利于接头性能的保持；时效时，钎缝处Cu9Al4相颗粒逐渐长大，分析认为是由于颗粒体积不同所导致的Cu原子浓度差异所致；时效后期，接头的断裂形式逐渐由韧性断裂转变为脆性断裂。研究了Ti、Ce的添加对Zn-22Al钎料在中性3.5wt.%NaCl溶液中腐蚀速率的影响，采用动电位扫描法和交流阻抗法对不同钎料的电化学腐蚀行为进行了研究。结果表明，Ti、Ce的添加有效降低了Zn-22Al钎料的腐蚀速率；当Ti、Ce的含量分别为1wt.%和0.5wt.%时，钎料的腐蚀电流密度由28.82μA cm-2分别减少至1.09μA cm-2和9.06μA cm-2，其交流阻抗也有明显增加；Ti、Ce的添加可以提高钎料表面腐蚀形貌的完整程度，而Cu/Al钎焊接头的中性盐雾试验则表明Ti、Ce的添加可以有效延缓其力学性能的衰减速率。更多还原

【Abstract】 The Cu/Al brazed joint is increasingly used to the equipment manufacturing. Zn-22Al fillermetal is more suitable for the Cu/Al automatic torch brazing than some other kinds of Zn-Al fillermetals, because the melt range of Zn-22Al is compatible with CsF-AlF3flux. However, there aresome problems when Zn-22Al alloy is applied to Cu/Al aotumatic torch brazing, such as the poorspreadability on the Cu substrate and the great tendency to form intermetllic compounds (IMCs) withCu atom. Moreover, there are few reports about the creep deformation resistance of the Zn-22Al fillermetal, the IMC evolution law of the aged Cu/Al brazed joint as well as the corrosion resistance of theCu/Al joint. In order to improve the brazability of Zn-22Al filler metal and the properties of Cu/Albrazed joint, trace element Ti or Ce was added into the Zn-22Al alloy. The effect of adding elementson the properties and microstructures of Zn-22Al filler metal and Cu/Al brazed joints was studied inthis dissertation. The action mechanisms of two elements were also discussed in this study.The reisistance test indicated that the resistivity of Zn-22Al filler metal increased with the Ticontent increased but decreased with the addition of Ce. The differential scanning calorimetry testrevealed that adding Ti increased the solidus and liquidus temperature as well as the melting range.The antioxidant capacity of Zn-22Al alloy increased significantly with0.03wt.%Ti or0.05wt.%Ceaddition, but the excessive addition of Ti or Ce deteriorated the oxidation resistance of filler metal.The Zn-22Al alloy showed finer and more uniform microstructure with trace element Ti or Ceaddition. Ti atoms preferentially reacted with Al atoms to form TiAl3compound during thesolidification. The primary TiAl3compounds played a role as the nucleation sites of η-Zn phases,which translated the dendritic η-Zn to snowflake; Ce combined with Al, Zn to form Ce-(Al, Zn)compounds, which not only played a role as the nucleation sites but also existed in the dendrite arm torestrain the growth of η-Zn. However, excessive Ce-(Al, Zn) compound was found in the filler metalwhen Ce was added up to0.15wt.%. The spread area of Zn-22Al filler metal on Cu and Al substratescan be significantly improved when the addition amount of Ti or Ce was0.01~1wt.%and0.03~0.15wt.%respectively.The elastic modulus, indentation hardness, and creep stress exponent n of Zn-22Al,Zn-22Al-0.03Ti, Zn-22Al-1Ti, Zn-22Al-0.05Ce, and Zn-22Al-0.5Ce filler metals were measured bynanoindentation at room temperature. The results indicated that the elastic modulus and indentationhardness of alloys increased when increased the content of Ti or Ce. The creep stress exponent n of Zn-22Al, Zn-22Al-0.03Ti, Zn-22Al-1Ti, Zn-22Al-0.05Ce, and Zn-22Al-0.5Ce is in the range of24.72~27.10,33.11~38.14,28.17~33.21,32.63~36.70, and28.31~32.32respectively when theloading rate is0.5,1,2.5,5,10mN/s with a constant load50mN. Moreover, the creep stress exponentn is in the range of25.69~28.35,32.45~38.45,29.47~34.64,34.07~37.09, and29.67~32.09when theload is50,80,100,200,300mN with a constant loading rate2.5mN/s. These reinforced propertieswere attributed to the strengthen effect of TiAl3or Ce-(Al, Zn) IMC particles.The Cu/Al brazed joints showed higher shear strength and more refined microstructure with theappropriate addition of Ti or Ce. The shear strength of Cu/Al joints brazed with Zn-22Al-0.03Ti,Zn-22A-0.05Ce were81.0MPa and85.3MPa, which improved17.4%and23.6%respectivelycompared with the joint brazed with Zn-22Al alloy. During brazing, Cu substrate reacted with fillermetal as Cu Zn CuZnand followed by9Cu+4Al Cu9Al4at the interface. Themorphology of Cu9Al4phase changed from bulk to granule due to the addition of Ti or Ce. Thefracture type of Cu/Al brazed joint was ductile fracture, and some IMC particles were found at thebottom of dimples. These dimples became narrower and without contained any IMC particles with theTi or Ce addition, which implied that the fracture sites changed from the interface layer to brazingseam.Accelerated aging test was carried out at200℃, the mechanical properties and microstructures ofthe Cu/Al joints at isothermal aging were studied in this research. The shear strength of all Cu/Albrazed joints decreased with increasing the aging time, the joints brazed with Zn-22Al-0.03Ti andZn-22Al-0.05Ce constantly possessed higher shear strength than those joints brazed with Zn-22Alfiller metal throughout the aging treatment. The thickness of the intermetallic compounds layerincreased as the aging time, and the interface structure changed from Cu9Al4/CuZn toCu9Al4/CuAl/CuZn and finally to ε/Cu9Al4/CuAl/CuZn. The addition of Ti or Ce can reduce thegrowth rate of IMC layer as well as the ripening flux of Cu9Al4phase. Moreover, the diffusionactivation energy of Cu9Al4phase increased from76.9kJ/mol to83.9kJ/mol and87.6kJ/mol with the0.03wt.%Ti and0.05wt.%Ce addition respectively. It is found that the coarsening of Cu9Al4existedin the brazing seam was dominated by the concentration gradient of Cu atom which caused by thevolume difference among Cu9Al4particles. The fracture type changed from ductile fracture to brittlefracture during later aging.The effect of Ti or Ce on the corrosion rate of Zn-22Al in the neutral3.5wt.%NaCl solution wasstudied in this research. Potentiodynamic method and electrochemical impedance spectroscopy werecarried out to study the electrochemical corrosion behaviors of filler metals in3.5wt.%NaCl solution. The results showed that the corrosion rate of Zn-22Al alloy significantly reduced with the Ti or Ceaddition. The corrosion current density decreased from28.82μA cm-2to1.09μA cm-2and9.06μA cm-2when the content of Ti or Ce increased from0wt.%to1wt.%and0.5wt.%respectively.The electrochemical impedance spectroscopy and the integrity of corrosion morphology of Zn-22Alfiller metal significantly increased with the Ti or Ce addition. The salt spray test indicated that thedecay rate of Cu/Al joints shear strength reduced when the Zn-22Al-xTi or Zn-22Al-xCe was appliedto brazing Cu and Al.更多还原