Effect Of Zr On The Microstructure,mechanical Properties And Corrosion Resistance Of Al-Mn-Zn Brazing Core Sheets For Automotive Radiator

With the development of automobile lightweight,the thickness of automobile radiator fins was gradually reduced from 0.2 mm to 0.07 mm,and their materials have also evolved from Al-Mn(AA3003)alloys to Al-Mn-Zn alloys.Al-Mn-Zn alloy foils for the brazing core sheets of the automotive radiator fins were produced by high throughput continuous casting and rolling(HTCR)method with short process,high quality,high efficiency,low energy consumption and low cost,but the mechanical properties and brazing properties of the HTCRed products were relatively low.Therefore,the improvement on the mechanical properties and brazing properties of Al-Mn-Zn alloy foil produced by HTCR through the microalloying.could a great significance for the production of radiator fins.Based on OM,SEM,EDS,EBSD,TEM,DSC analysis,mechanical properties text,electrochemical analysis,salt spray corrosion and other methods,the effect of trace element Zr on the microstructure,mechanical properties and corrosion resistance of the HTCRed Al-Mn-Zn brazing core sheets was systemically studied in this paper.The main results were as follows:(1).The grain size distribution of Al-Mn-Zn HTCRed sheets was not uniform.There were several coarse grains and the average grain size is 92.65 ?m.A small amount of Zr(0.03 wt.%and 0.09 wt.%)addition eliminates the abnormal coarse grains and makes the grains become uniform,but the grain refinement effect is not obvious When the Zr content was increased into 0.13 wt.%,the grain size was decreased significantly to 53.84 ?m.which was decreased by 41.9%as compared with the Al-Mn-Zn sheets.(2).The mechanical properties of Al-Mn-Zn sheet gradually improved with the increase of the Zr addition.When the Zr addition increased to 0.13 wt.%,the ultimate tensile strength increased from 226.3 MPa to 260 MPa,which was increased by 14.9%as compared to the Al-Mn-Zn sheets.In addition,the hardness from 73.3 HV to 83.6 HV as compared to the Al-Mn-Zn sheets.(3).During the simulated brazing process at 600? for 10min,the sagging process could be divided into two stages:(a)rapid sagging stage(0-210 s);(b)Slow sagging stage(210?600 s).The addition of 0.03 wt.%Zr could slightly improve the sagging resistance of Al-Mn-Zn brazing sheets.When the Zr addition was increased to 0.09 wt.%,the sagging resistance of Al-Mn-Zn sheets was significantly improved.According to EBSD analysis,this was mainly because of the uniform distribution of coarse equiaxed grains,the uniform grains and the reduction of the overall grain boundary in the sheets,and reduced solder elements diffusion channels and slip actuation interfaces in the simulated brazing process.When the Zr addition was increased to 0.13 wt.%,the sagging resistance of Al-Mn-Zn sheets was damaged,although the nano-second phase and nano-sized Al3Zr particles in the microstructure increased obviously by TEM analysis,the fine grains had a great damage to the sagging resistance of Al-Mn-Zn sheets.(4).The logarithmic relationship between the sagging distance and mechanical properties was studied and established.According to the values of sagging distance(d),ultimate tensile strength(UTS)and uniform elongation(UE)of the brazing sheets at different sagging time,the logarithmic relationship between sagging distance(d)and tensile properties values(including UTS(?)and UE(eF))was d=??U+?·Ln(eF)+?.The fitting formulas for the Al-Mn-Zn sheets with 0,0.03 and 0.09 wt.%Zr addition and the sheet with 0.13wt.%Zr addition were d=-0.06?U+0.84Ln(eF)+16.93(S1,S2 and S3 alloys)and d=-0.06?U+0.84Ln(eF)+14.66,respectively.(5).In the salt spray corrosion experiment,the corrosion rate of the brazing sheets in the 5 wt.%NaCl environment increased with the increase of the Zr addition.With the increase of the Zr addition,the number of the second phases in the Al-Mn-Zn brazing sheets increasesd significantly.In the electrochemical corrosion experiment,the second phases and the surrounding matrix form a micro-battery,which first induced pitting corrosion.As the corrosion continues,the corrosion form of the surface gradually changed from local corrosion to general corrosion,and the corrosion potential continues to increase.The corrosion resistance of the brazing sheets with 0.09 wt.%Zr and without Zr was still at the same corrosion level.(6).The hydrochloric acid immersion weight loss experiment shows showed that the corrosion rate of Al-Mn-Zn sheets increases with the increase of Zr addition in In HCl medium,because the surface of the Al-Mn-Zn sheets could not form a dense passivation film.The Zr addition could not form passivation effect,thus,the corrosion rate of Al-Mn-Zn sheets was significantly accelerated,which was obviously different from the electrochemical corrosion and salt spray corrosion.