Research On Characteristics And Fracture Mechanisms Of Laser Spot Weld Bonding

In the aerospace and automobile manufacturing, laser spot weld bonding process(LSWB) had great potential for application, because of its strong structure adaptablility and good complex mechanical properties. But the adhesive would gaisfy in relatively low temeperature because of its low vaporizaiton temperature, then the adhesive gas would impact on the welding pool, which made the welding process instability. At present, some supplementary measures were used in the weld-bonding process to control the adhesive gasification process. In addition, the reasearch on fracture mechanism of laser spot weld-bonded joint fails to provide a satisfactory explanation of the strength lifted of weld-bonded joint. Thus, with Q195 and three different adhesives(J-11, AD-1301, Terokal-5089) as experimental materials, laser spot weld bonding process using combined pulse laser beam was introduced, and the technology and fracture mechanism were systematic studied.Firstly, the controlling of combined pulse laser beam for adhesive gas were investigated. The results showed that the pre-gasification pulse with low power and long pulse time would gasify the adhesive in the welding zone on the premise that the upper metal sheet partially melted, and formed an area without adhesive layer, which made the welding pulse work without adhesive. The feasibility of controlling the adhesive gasification with pre-gaisfication pulse laser was proved by a series of experiments. Three requirements were presented to guarantee the succes s of pre-gasification: 1, the upper metal wouldn’t be penetrated by the pre-gaisfication laser beam, 2, the pressure of adhesvie gas cound’t expel the molten pool, when the adhesive layer was in force balance, 3, the size of the area without adhesive shoul d be bigger than that of the fusion area of spot joint, when the adhesive layer was in force balance. A process model based on these three requirements was set up, then the parameters of pre-gasification pulse were optimized, and the maimum viscosity of adhesive was confirmed.Secondly, the fracture mechanism of LSWB joint was studied. The reason that the changing of properties of LSWB joint was explained and the main controlling factor of the properties of LSWB joint were achieved. By the comparison of adhesive bonded joint, laser spot welded joint and LSWB joint, the LSWB joint had good complex mechanical properties by mechanical property test. In static loading test, the LSWB joint got 140% more tensile-shear load, 17% more peel load and 33% more bend load than the laser spot welded joint, also the LSWB joint got 12% more tensile-shear load, 336% more peel load and 47% more bend load than the adhesive bonded joint. In dynamical loading test, the LSWB joint did as the adhesive bonded joint did. And the fatigue fracture showed that the fatigue performance of LSWB depended on the adhesive bonded part in LSWB joint.Based on tensile-shear test results, the fracture finite element model of LSWB joint was bulit. The fracture process of adhesive bonded joint and LSWB joint were analyzed, the end faces of LSWB joint were the key postion in the mechanical test. The reason that LSWB joint got higher load than that of other joints was explained by the fracture finite element model. The spot welded joint increased the e lastic modulus of LSWB joint, decreased the interface displacement of adhesive bonded interface, enhanced the ability of crack resistance adhesive bonded interface of LSWB joint. After that, the influence of the elastic modulus of adhesive, the fracure interface displacement of adhesive bonded, the fracture stress of adhesive bonded on the tensile-shear properties of the LSWB joint were also analysed, proved that the the fracture stress of adhesive bonded is major infactor. The influence of the arrangement of spot welded joint on the fracture process of the LSWB joint. The results show that the spot welded joint would block the expending of flaw in the adhesive bonded area. When the spot welded joint was in serial arrangement, the fracture process of LSWB joint would produce sequence fracture in adhesive bonded area.