| Friction stir welding technology is widely used in the welding of aluminum alloy,magnesium alloy and other materials with low melting point due to its advantages of low cost,good joint properties and environmental protection.With the increasing demand for high melting point materials in automotive,military and aerospace fields,scholars began to extend this technology to high melting point material connection.However,the welding force of several tonnage levels in the welding process intensifies the wear and shortens the service life of the friction stir tool.At the same time,the large flow stress limits the good forming of high melting point material and affects the joint quality.Therefore,the welding procedure test and numerical simulation study of Q235 B low carbon steel with high melting point under auxiliary heat source were carried out.(1)Firstly,according to the characteristics of Q235 B,the test scheme under conventional welding process is designed and formulated.Then,according to the extrusion and friction state between the friction stir tool and the workpiece in the welding process,as well as the vertical temperature gradient distribution in the welding area of the workpiece,the tool surface force considering the temperature variation effect of the material yield strength was analyzed.On this basis,the force model of the tool based on micro integral was established and used to design and check the structure of the tool.The calculation results show that this model has higher welding force prediction accuracy than ignoring temperature distribution.Finally,the Q235 B conventional welding process test was carried out,that is,the test and comparison of joint morphology,microstructure and mechanical properties under different spindle speeds,as well as the test and comparison of the tool wear morphology and wear quantity.The welding characteristics and process parameter window of gu25 uf tool welding high melting point material Q235 B under conventional welding process are preliminarily obtained.(2)Firstly,the friction stir welding test equipment for Q235 B with back-heating was built,and the influence of back-heating on the temperature of Q235 B test plate was tested,so as to make the heat input provided by the built device controllable and effective.Then,the welding process test of Q235 B test plate with auxiliary heat source was carried out,and the joint morphology,microstructure and mechanical properties were tested and compared by changing the input intensity of auxiliary heat source.The results show that the microstructure of Q235 B joint material in the stir zone increases with the increase of the intensity of auxiliary heat source,while the microstructure in the heat affected zone is the opposite.With the increase of auxiliary heat source intensity,the micropores in the joint gradually disappear.Once the increase of grain size is the dominant factor,the joint properties will decrease again.Finally,the wear morphology and wear quantity of the tool were detected and compared.The results show that the auxiliary heat source can effectively improve the wear of the tool,but the oxidation effect gradually increases with the increase of the auxiliary heat intensity.Therefore,when welding Q235 B with the auxiliary heat source,it is necessary to take the gas protection.(3)Firstly,the finite element model of friction stir welding with Q235 B auxiliary heat source considering the temperature variation effect of material thermophysical parameters was established by using the numerical simulation software DEFORM,so as to further improve the reliability of the numerical simulation results.Then,based on the finite element model,the evolution process of welding force and welding temperature field with or without auxiliary heat source in the process of friction stir welding Q235 B was carried out,The interaction mechanism between flow stress and welding heat was used to explain the mechanism of auxiliary heat source to effectively improve welding force.Finally,the numerical simulation of Q235 B joint material flow with or without auxiliary heat source was carried out,and the influence of auxiliary heat source on material flow performance and void filling ability was deeply analyzed.Combined with the change of grain size,the micro mechanism of joint properties change was effectively revealed from the perspective of simulation.At the same time,the interfacial pressure distribution between the surface of the tool and the joint material obtained by numerical simulation was analyzed,which reveals the action mechanism of the auxiliary heat source to improve the wear of the tool head by reducing the interfacial pressure. |
PDF Full Text Request