Research On Surface Activation-assisted Brazing Process And Mechanism Of 7075 Aluminum Alloy

Aluminum alloy is used widely in aerospace filed due to its good properties such as lightweight,excellent thermal conductivity and electrical conductivity,high strength to weight ratio and good corrosion resistance,as well as good capacity for materials processing.The surface oxide film of aluminum alloys can deteriorate the properties of brazed joints in the brazing process.In order to eliminate the surface oxide film and achieve the surface activation bonding,the technique of surface activation-assisted brazing process was put forward by ion bombardment and magnetron sputtering.The effect of surface activation on the surface oxide film and surface state of aluminum substrate were investigated in details.Moreover,comparative analysis of the surface activation was carried out,which was focused on the effect on the interfacial microstructure and mechanical properties of brazed joint.The effects of brazing temperature and thickness of the Cu layer deposited by magnetron sputtering on the interfacial microstructure and mechanical properties of brazed joint were studied.The formation mechanism of brazed joint was expounded.The interconnection of microstructure,grain size and mechanical properties of brazed joints was analyzed comprehensively.The post-weld heat treatment of brazed joints was carried out,which solid solution and aging.The effect of post-weld heat treatment on the interfacial microstructure and mechanical properties of brazed joint were analyzed,so did the morphology and size of grain in brazed joint.The surface oxide film of substrate was removed effectively by ion bombardment with argon ions.Meanwhile,both surface structure and stress distribution were changed.A compact Cu layer was coupled tightly with substrate and formed the diffusion layer in the interface,which achieved the surface activation of aluminum substrate.By comparison it can be found that the surface activation treatment can eliminate the residual oxide film in the brazed joints,and avoid the negative impact to the microstructure and atoms diffusion during the brazing process.The atoms diffusion was intensified and improved the interfacial microstructure of brazed joints,as well as the mechanical properties of joints,which resulted in the formation of brazed joints with well bonded and no defects.When the brazing temperature was raised to 600 °C,surface activation-assisted brazed joints with 20?m and 5?m thickness of the Cu layer all have the maximum shear strength of 42.2MPa and 50.7MPa respectively.Further increasing brazing temperature to 610 °C,the shear strength of brazed joints with different thickness of the Cu layer were decreased sharply due to the abnormal grain coarsening in the brazing seam.It can be consider that the increase of brazing temperature will enlarge the degree of composition homogenization of brazed joints and facilitate the shear strength of brazed joints.On the other,the increase of brazing temperature will lead to the grain growth and coarsening,which produced the negative impact on the shear strength of brazed joints.During the brazing process,the Al-Cu eutectic liquid phase would penetrate into the grain boundary along the depth direction of the substrates and reduce the bonding strength of the grain boundary,causing the grains of substrate spalling into the liquid phase and dissolving gradually.After post-weld heat treatment,the intermetallic compounds in brazing seam were reduced significantly and the degree of composition homogenization of brazed joints was enhanced.Moreover,recrystallization take place in the brazed joints and the grain morphology was transformed from strip-like crystals to bulk-like equiaxial crystals.The grains of the brazed joints relatively refined and improved the shear strength of brazing joints.The increase of shear strength of the brazed joints was up to 47%,and the fracture style of the brazed joints changed from brittle fracture to ductile fracture.