Research Of Bonding And Forming Process For Dissimilar Laminated Sheets Based On Double Sided Froming System

For the increasing demanding for the density,strength-to-weight ratio,conductivity along the gradient direction.Dissimilar laminated sheet plays a more and more important role in aerospace,ship-building and high-speed train manufacturing industry.However,the current manufacturing process of dissimilar laminated sheet needs combine bonding,stamping and other processes,which is complex and time consuming.To overcome these drawbacks,under the support of National Natural Science Foundation of China(No.51675332),bonding and forming process was integrated on the double sided incremental forming system.Strategies for spot bonding,line bonding and the forming technology of the bonding sheets were studied.Fracture load,sheet thinning,surface oxidation,microstructure and microhardness were analyzed.Based on the analysis,incremental forming process of the laminated sheets were studied.The main achievements of the thesis are shown as follows:1.The current double sided incremental forming system was improved.High speed spindle motor and back pressure was added for the friction stir bonding process.Resistances spot welding of aluminum/steel sheets were carried out,the strength of the joints are below 0.5kN.The strength is closely related with the back pressure.When the back pressure is less than 0.2MPa,the joint strength tends to be high(0.3kN),but the tool wearing is severe.2.Nickel-based tools were used for the dissimilar steel/aluminum friction stir spot welding by three tool rotation plans.The fracture load,sheet thickness,microstructure,intermetallic compounds and microhardness were analyzed.It is proved that the fracture load of double sided friction stir welding are always higher than single sided friction stir welding.Fracture load increases as the increase of back pressure in all the plans.However,when the back pressure is too high,the fracture load of plan A and B will turn to plug fracture.3.In different rotational plans,under high plunge pressure(more than 0.5MPa),plan A has higher indentation,welding area and intermetallic compounds thickness than plan B,which contributes to the higher fracture load achieved by plan A.While under low plunge pressure(less than 0.5MPa),plan B had lower welding area but thicker IMC layers thus the fracture load was slightly higher than plan A.It proved that IMC thickness can strengthen the welding joints in double sided FSSW.On steel side,microhardness increased remarkably(35%)due to the rotation of the left tool,especially near the sheet surface.4.In friction stir line welding,when the feed rate varies between 2mm/s and 10mm/s in constant tool indentation welding.The joint strength changes little along the tool indentation,within 0.5kN.However,feed rate plays an important role in joint strength.When the feed rate varies between 5mm/s and 10mm/s in constant back pressure welding.The joint strength is not sensitive to the feed rate.In the experiments of the paper,the welding lines are not bonding completely,which influences average strength by unit length.5.In the forming experiments after bonding process,the sheet deformation could be synchronized.However,when the forming angle is too big,the bonding area is separated because of the shear between two sheets.Furthermore,when the feed rate is too low in bonding process,fracture occurs on the aluminum sheet because the sheet thinning is severe.Optical feed rate(8mm/s~10mm/s)in bonding process could ensure stable incremental forming process of laminated sheets.