Size Effect On Mechanical Properties Of Resistance Spot Welded Joints

Resistance spot welding is one of the most important welding methods,which is widely used in various industrial sectors.For resistance spot welding,different organizations or industries have developed their own standards.By comparison,it is found that the size of resistance spot welding is not uniform.However,the size of the specimen has a very important effect on the experimental results.In order to study the effect of the size on the experimental results,the size effect of three kinds of common materials for low carbon steel(Q235),aluminum alloy(AA5052),high strength steel(DP780,TRIP980,22MnMoB)were studied.In order to ensure the size effect on the experimental results can be carried out smoothly,resistance spot welding process the materials used were studied.The optimum parameters of low carbon steel Q235,aluminum alloy AA5052,and high strength steel 22 MnMo B and TRIP980,DP780 were selected.The effect of spot welding parameters on mechanical properties of 22 MnMoB joint was studied.The microstructure,failure position,fracture morphology and hardness distribution of various materials were described.To ensure that the best spot welded joints are used as the prerequisite for the study of size effect.Then the size effect of the joint is studied.First,the shear tensile test is carried out on the different width and length of the specimen.The shear tensile test results of different width and lap length are obtained.The results showed that in the same welding parameters of spot welding joints,failure absorbed energy(E),peak load(Fmax)and the corresponding extensometer failure displacement(Xmax)displacement measurement results with the test size changes regularly.When the width or the length of the specimen increases to a certain critical value,the failure energy(E),the peak load(Fmax),and the failure displacement(Xmax)of the test results will not change with the size and remain stable.When the specimen width of less than,the base material is not enough to resist tensile force,deformation and the positive drawing force generated by the rotation of the nugget,leading to measurement results is less than the actual tensile strength of the joints.According to the experimental results,using the generalized least squares method to fit multivariate function,establish the critical width is calculated by the formula: W=20.4t+ 0.0158?y+13.4.In the shear tensile test,the mechanical properties of the joints can be accurately measured when the specimen width is greater than the critical width.Otherwise,the measurement results will be small.The lap length is also affected by the test results of the mechanical properties of the joints,but the influence of the overlap length is weaker than that of the specimen width.