| Avionics chassis heat sink has a large welding area and high assembly precision,traditional welding technology has been difficult to meet the quality requirements.Diffusion welding,as a precision connection technology,has unique advantages in radiator welding.In order to further achieve the lightweight requirements of the heat sink,some of the components of the heat sink are chosen to be replaced by 7075 aluminum alloy with higher strength and stiffness than 6061,and the connection problem between the two is inevitable.In this thesis,diffusion welding tests without intermediate layer and with the aid of pure Al intermediate layer were conducted on 6061 and 7075 aluminum alloys.The study of the pre-treatment method before welding,welding process parameters,intermediate layer addition and alloying element gradient and other factors on the joint organization and properties of the influence of the law.The main findings are as follows:The use of chemical cleaning after the surface roughness Ra=0.697 μm,lower than sandpaper polishing(Ra=0.968 μm),conducive to the diffusion welding process.In the range of welding temperature 500～540℃,welding pressure 1～6MPa,holding time60～120min,with the increase of welding temperature,welding pressure and holding time,the joint welding rate,shear strength and joint deformation rate of 6061 aluminum alloy direct connection are significantly improved.At 520℃-4MPa-120 min,the joint weld rate of6061 aluminum alloy direct joint reached 81.5%,the shear strength reached 57.92 MPa,and the joint deformation rate was 6.36%.A large number of tough nests can be observed at the shear fracture morphology,indicating that the current fracture form is ductile fracture.When the welding temperature exceeds 520℃,welding pressure exceeds 4 MPa,further increase the welding temperature,welding pressure and holding time,the joint shear strength increase is limited,but the joint deformation rate surges.The pure Al interlayer can effectively improve the initial contact area at the weld interface by taking advantage of its low yield strength at the beginning of diffusion welding.It can also create a difference in the concentration of alloying elements at the joint interface to further improve the weldability of the welded joint.6061/Al/6061 diffusion welded joints at 520℃-4MPa-120 min shear strength of the joint was 61.08 MPa and the joint deformation rate was 5.32%.Joint fracture in the middle layer region,which is mainly due to the welding process in the middle layer is not completely in the base material "fusion".At the same time,the welding temperature is low,the Si element across the joint interface is slow,resulting in its enrichment at the interface to form the Si phase,the Si phase in the shear process is easy to become the weak area of the entire joint.6061 aluminum alloy in the diffusion process there is an abnormal growth of some of the grains,which is mainly due to the reinforcing phase elements occur to the middle layer diffusion enrichment,and back soluble,resulting in the weakening of the nailing effect on the grain boundaries.By improving the diffusion reaction of alloying elements can significantly promote the closure of holes at the weld interface and improve the joint quality of the weld interface.Under the same welding process parameters,the weld closure rate of 6061/Al/7075 aluminum alloy joints is higher than that of 6061/Al/6061 aluminum alloy joints.Interfacial alloy phases Al Zn Mg Cu appeared at the 6061/Al/7075 aluminum alloy joints at 500～520°C.These interfacial alloy phases deteriorated the shear properties of the joints.Increasing the welding temperature and extending the holding time can inhibit the generation of interfacial alloy phases.At 540 ℃-4MPa-120 min,6061/Al/7075 aluminum alloy joints at the alloy phase has basically disappeared,the joint weld rate reached 93.18%,the shear strength reached 85.28 MPa,the joint deformation rate of 7.9%,the shear fracture is ductile fracture characteristics. |
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