| SiC ceramic has become an important part in the field of materials owing to its excellent oxidation resistance,good abrasion resistance and good thermal stability.Al alloys have high strength,specific stiffness and good toughness and machinability.They play an important role in various fields,particularly in aerospace,military manufacturing and automobile production.SiC ceramics and Al alloys must be combined to maximise their use.Therefore,the connection technology of SiC ceramics and Al alloy bears importance.The coefficients of the linear expansion of SiC and Al alloys differ significantly.Therefore,their joint formation will produce a large residual stress,causing difficulties in brazing connection.Based on this condition,to relieve the residual stress,a composite intermediate layer with foamed aluminium and filler metal is prepared and SiC ceramics and Al alloys are brazed to achieve the gradient transition of the residual stress of the dissimilar material joint of SiC ceramic/Al alloy.Through theoretical analysis of the joint interface and finite element simulation,the strengthening mechanism of the SiC ceramic and Al alloy brazing joints is analysed.According to research,the filler metals Sn and Zn are used in the preparation of the middle layer and the aluminium foam with aluminium foam compound.Interface structures of the joint filler metal and aluminium are analysed.The effects of filler type,ultrasonic time and ultrasonic amplitude on the interface microstructure are studied.The formation mechanisms of the compound layer structure of the metallurgical bonding interface are summarised.Results show that it is difficult to wet Sn filler metal on the aluminium foam.Numerous defects were observed in the Sn filler metal + aluminium foam composite layer,whereas the Sn filler metal was able to achieve better wetting,which can combine the Zn filler metal + aluminium foam composite layer.Ultrasonic amplitude significantly influences the preparation of the composite layer.When ultrasonic amplitude was 6 ?m,the microstructure in the composite layer comprised zinc solid solution and eutectic.When the ultrasonic amplitude increased to 8 ?m,the number of zinc solid solution reduced,but the eutectic system increased.When the ultrasonic amplitude was 10 ?m,the filler metal filled with the spherical eutectoid structure contained small-sized bubbles.To analyse the stress field distribution of the Zn filler metal and the composite layer under different cooling rates,the brazing joint model with the same size was established and simulated by ANSYS.The effects of the Zn filler metal and the composite layer on relieving the residual stress and improving the performance of the brazing joint,respectively,were studied.The results of this revealed that in comparison with the use of Zn filler metal and composite layer,the two kinds of joint distribution trends of the residual stress in the joints were the same.The maximum residual stress appeared in the ceramic/filler at the corners.The composite layer reduced the equivalent residual stress at the sides of the ceramic material.The effects of different cooling rates on the Zn filler metal and composite layer in theSiC ceramic/Al alloy joints were studied.The reasons for mitigating the SiC/alloy joint residual stress in the middle layer of ceramic/Al composite were summarised and analysed based on the thermal expansion coefficient,elastic modulus,phase organisation and other factors.The findings revealed that a complete joint can be obtained using the Zn filler metal and the composite layer when the joint is cooled in a furnace.When the brazing joint is air cooled,a crack forms parallel to the brazing seam when using the Zn filler metal.The brazing joint can still be completed with the composite layer,indicating that the composite layer plays a role in relieving the residual stress.When the brazing joint is water cooled,cracks form parallel to the brazing seam in the joint of the Zn filler metal and the composite layer. |
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