| The joining processes of aluminum and steel offer additional advantages over conventional materials by combining outstanding mechanical properties and relatively low density from steel and aluminum,respectively.The hybrid structures composed of aluminum and steel is receiving increased recognition.As a result,joining of these two metals has been a popular topic among many investigators.The benefit of using brazing technology for joining dissimilar materials has been regarded as a promising method,because of lower heat input needed during proesing.In this research,a series of Zn-Al filler metals were developed and used to braze 6061 aluminum alloy and 304 stainless steel using automic torch brazing.The influence factors of the filler metal spreading on the base metals were studied,and the wetting kinetics of the liquid filler metal spreading on the stainless steel were also investigated.Moreover,the effect of Al content and Zr addition on the microstructure and properties of the Zn-Al filler metal,as well as the brazed joints were evaluated.The spreading test and hot-stage microscopy with a real-time in situ observation system were used to investigate the wetting mechanism of Zn-Al filler metal on aluminum alloy and stainless steel.The results indicated that the oxide film on the surface of the aluminum alloy was removed by reacting and dissolving.While the mechanism of removing oxide film on the surface of stainless steel could be explained as the follows.The oxide film on the surface of stainless steel cracked when the flux reacted with MnO.Al atoms in the liquid filler metal permeated the oxide film through the cracks and then reacted with stainless steel.The chemical reaction burst the oxide film and then the liquid filler metal spread on stainless steel.The spreading test results showed that the spreadability of the filler metal increased with the increase of the wetting temperature,and the spreading area decreased with increased content of Al in the filler metal.In the non-reactive wetting system,the spreadability was improved with the increase of the roughness of aluminium alloy.However,in the reactive wetting system,the wetting process is dominated by the chemical reaction and the roughness of stainless steel has a minimal effect on the spreadability.The wetting process could be divided into three stage:initial stage,main stage,and stable stage.In the part 2?main stage?,the spreading radius vs.time was fitted with the power law Rnt,n=0.4.The effects of Al content on the properties of Zn-Al filler metal?Zn-2Al,Zn-5Al,Zn-15Al,Zn-22Al,and Zn-25Al?were investigated.The shear strength results showed that the maximum shear strength was up to 131 MPa when Zn-15Al fill metal was employed.The salt spray test indicated that the decay rate of the shear strength reduced with increased content of Al in the filler metal.Microstructure analysis showed that the thickness of intermetallic compound?IMC?layer increased with increased content of Al.Moreover,the interfacial layer between the brazing seam and stainless steel consisted of Zn-rich phase and?-Fe4Al13 phase in the joints brazed with Zn-2Al.With the increased content of Al,the Zn-rich phase disappeared and the interfical layer consisted of?-Fe4Al13 phase.The surface analysis for the fracture showed that the fracture was a typical cleavage fracture and the fracture occurred at the interface layer between the barzing seam and stainless steel.The effects of Zr addition on the properties of Zn-15Al filler metal were investigated.The microstructure analysis showed that adding an appropritate amount of Zr appeared to refine the?-Zn dendrite and improve the microstructure of the filler metal.During the solidification process of the filler metal,as the temperature was decreased,Zr began to segregate along the solid-liquid interface with limited solubility and the concentration gradient of Zr in the liquid alloy increased.As undercooling increased,the crystal growth of?-Zn changed from a bulk dendrite to a cellular dendrite.However,some Al2ZnZr phase particles formed with increased Zr content in the filler metal and as the concentration gradient correspondingly decreased,the modification of?-Zn dendrite stopped.The addition of trace amount Zr had little influence on the melting point of the filler metal,while the antioxidant capacity of the filler metal improved with the increase of Zr content.The electrochemical performance test and corrosion test showed that the corrosion rate of Zn-15Al filler metal was decreased by 30.2%with 0.3%Zr,and the corrosion current density was also decresed with Zr addition.The spreading area of Zn-15Al-0.2Zr fille metal on aluminum alloy and stainless steel was improved by15.9%and 10.2%respectively,when compared with that of Zn-15Al filler metal.The nanoindentation results showed that the creep stress exponents for Zn-15Al,Zn-15Al-0.2Zr,and Zn-15Al-0.3Zr were6.64,7.35,and 8.07,respectively.The Zr-bearing filler metals performed a better creep resistance than Zn-Al filler metal.The mechanical property test results showed that the Zr addition in Zn-15Al filler metal enhanced the shera strength of the brazed jonts after both brazing and aging at 200oC.The addition of 0.2%Zr increased the shear strength of a joint by 9.2%compared to virgin Zn-15Al brazed joint.With the aging time increased,the shear strength of the joint has decreased gradually,as expected.The average shear strengths of Zn-15Al-0.2Zr brazed joints after aging for 800 h improved by 14.3%compared to Zn-15Al.During the brazing process,Zr atoms preferentially reacted with Al atoms and Zn atoms to form Al2ZnZr phase,and the activity of Al was decreased.Consequently,the Gibbs free energy of Fe4Al13phase was increased and the IMC growth was inhibited.During the aging process,the growth rate constants of the IMC layer of Zn-15Al and Zn-15Al-0.2Zr filler metal were 2.9×10-7?m2·s-1 and2.9×10-7?m2·s-1,respectively.The dispersion of Zr and Zr-bearing particles enriched at the interface acted as the diffusion barrier for Fe atoms and Al atoms,what will restrict the growth of the IMC layer during aging treatment. |
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