| A novel Al-Si-Ge-Zn aluminum alloy brazing filler metal was proposed and developed.Effects of Ge and Zn on the brazing process performance,micro structure and processability of Al-Si-Ge-Zn filler metals were investigated systematically.The novel Al-Si-Ge-Zn filler metal was used for brazing 6061 aluminum alloy,and the comparative study was carried out with the traditional Al-Si-Ge and Al-Si-Zn filler metals for brazing 6061 aluminum alloy.The novel Al-Si-Ge-Zn filler metal was modified by adding Sr,and the effect of Sr on the microstructure and properties of the Al-Si-Ge-Zn filler metal and brazed joint was studied.Finally,the mechanism of Sr modification was revealed deeply.Compared with Al-Si filler metal,the addition of Ge and Zn significantly reduced the melting point of the filler metal.However,the high content of Ge and Zn could affect the wettability of the filler metal.When 10wt.%Ge and 15wt.%Zn were added,the Al-Si-Ge-Zn filler metal had the most narrow melting temperature range(505.2～545.1℃),which was only 39.9℃.In this case,the Al-Si-Ge-Zn filler metal had the maximum spreading area,and the spreading coefficient of relative Al-Si filler metal was up to 192%.The as-cast micro structure of Al-Si-Ge-Zn filler metal was composed of a-Al,β-GeSi andη-Zn phases.The increase of Ge content promoted the formation of coarse β-GeSi phases,while the proper amount of Zn could refine the brittle β-GeSi phases.Compression test results showed that Al-9.45Si-10Ge-15Zn filler metal has the highest compression ratio(63.5%),far more than 50%,which showed the excellent processability of Al-9.45Si-10Ge-15Zn filler metal.The comparative study with the traditional Al-Si-Ge and Al-Si-Zn filler metals for brazing 6061 aluminum alloy showed that the novel Al-Si-Ge-Zn filler metal has more excellent brazing process performance and processability.The novel Al-Si-Ge-Zn filler metal was used to braze 6061 aluminum alloy,and the brazed joint was mainly composed of diffusion layer and brazing seam area.The diffusion layer was composed of the solid solution of Zn in Al,and the existence of diffusion layer was beneficial to improve the mechanical properties of the brazed joint.The brazing seam area was composed of a-Al,r)-Zn andβ-GeSi phases.The brazing parameters were designed and optimized.When the brazing temperature was 570℃ and the holding time was 2min,the shear strength of 6061 aluminum alloy brazed joint was maximum(138.4MPa).Compared with the traditional Al-Si-Ge and Al-Si-Zn filler metals for brazing 6061 aluminum alloy,the shear strength of joint brazed with the novel Al-Si-Ge-Zn filler metal was increased by 57.8%and 32.3%,respectively.The addition of Sr element had slight effect on the melting temperature of Al-Si-Ge-Zn filler metal but remarkable effect on the spreading area.When the Sr element content was 0.7%,the brazing filler metal has the maximum spreading area,and the spreading coefficient of relative Al-Si filler metal was up to 227%.There was no significant change in the microstructure of the filler metal when 0.1%Sr was added.When the content of Sr was increased to 0.4%,the microstructure of the filler metal was changed obviously,and the β-GeSi phase was changed from the needle flake to the strip shape.When the Sr content was increased to 0.7%,the β-GeSi phase was changed into fine fibrous shape.However,when the Sr content reached 1.0%,the microstructure of the filler metal was tended to grow up due to the effect of the over metamorphism.When 0.7wt.%Sr was added into Al-Si-Ge-Zn filler metal,compression ratio was increased by 9.4%.The addition of Sr element could also significantly improve the microstructure and properties of 6061 aluminum alloy brazed joints.Compared with Al-9.45Si-10Ge-15Zn filler metal,the width of diffusion layer was increased from 16～18μm to 25～27μm when brazed 6061 aluminum alloy with Al-9.45Si-10Ge-15Zn-0.7Sr filler metal,and the needle flake β-GeSi phase was transformed into fine fibrous shape.The shear strength of the joint brazed with Al-9.45Si-10Ge-15Zn-0.7Sr filler metal was increased by more than 10%,reached 152.4MPa.The fracture mode of the brazed joint was changed from ductile and brittle mixed fracture to obvious ductile fracture.The fine fiber structure in Al-Si-Ge-Zn-Sr filler metal was studied deeply by TEM,and the mechanism of Sr modification was revealed.The Sr modification on the Al-Si-Ge-Zn filler metal was achieved by changing the growth mode of Si.The Sr element was adsorbed on the growth steps of Si to prevent the growth of Si in the form of the ledge growth mechanism.Meanwhile,the adsorption of Sr on Si phase surface also changed the stacking sequence of the Si atoms.As a result,a large number of twins were produced in Si crystal,and the Si phase grew into fine fibrous shape according to the mechanism of twin plane reentrant edge(TPRE). |
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