| 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. |
PDF Full Text Request