| Magnesium alloy, one lightweight structural metal, is regarded as green materials in twenty-first Century for the better properties of strength, impact toughness, damping performance, casting and easy recycling. Magnesium alloy is widely used in the field of automobile, computer, communication, aviation, aerospace and household electric appliances. Because metal mineral resources are exhausted largely nowadays, the alloy draws attention for its abundant resources. China has the world’s largest number of Magnesium resources, moreover, China is the world’s largest magnesium production and export country. Magnesium alloy is produced mainly by casting. Large and complex workpieces may be difficult to manufacture by die or traditional casting, meanwhile, Welding technology shows better processing features than casting. But magnesium alloy welding performance is poor. It is difficult to obtain reliable welding joints, which has become an important problem to restrict its wide application.Outside magnetic field can influence the flowing molten pool and microstructure and mechanical properties of the joint during the welding. Heat treatment received attention in the field of improve the comprehensive properties of magnesium alloy. In the research, the technology of outside magnetic field welding and heat treatment is integrated to obtain good quality. Processing parameters were analyzed to know the suitable value and its relationship.This paper mainly studied the TIG welding of AZ91 magnesium alloy under AC longitudinal magnetic field, by changing the welding current, the magnetic current and the magnetic field frequency. Heat treatment of Solid solution and aging was carried out after welding respectively, moreover, the mingle function was also analyzed. The experiments of metallographic microscope, scanning electron microscope observation and XRD analysis were carried out to illustrate the microstructure and alloy element distribution of the joint. The experiments of tensile test, hardness test and electrochemical corrosion test were done to know mechanical properties of the joint. By the test, the influence of magnetic field and heat treatment on the welding joint were considered.The experiment results show that mechanical properties of the joint is good when the welding current is 100 A without magnetic field action. Microstructure of the joint is fine. Tensile strength is 222 MPa,reduction of cross sectional area is 9.23%, microhardness is 83.8HV, free corrosion potential is-1488 m V when corrosion current density is 72.2μA /cm2.The mechanical properties and electrochemical corrosion resistance properties are the best when the welding current of 100 A, magnetic field current of 1.5A and magnetic field action frequency of 15 Hz are selected. Tensile strength is 249 MPa, reduction of cross sectional area is 10.86%, microhardness is 92.3HV, free corrosion potential is-1445 m V when corrosion current density is 62.2 μA /cm2.After heat treatment of Solid solution, the mechanical properties and corrosion resistance of joint showed the trend of first increasing and then decreasing with the increase of magnetic field current and the magnetic field frequency. After heat treatment of Solid solution and aging, the mechanical properties of joint showed the trend of first increasing and then decreasing while the corrosion resistance of joint showed the opposite trend. After heat treatment of Solid solution, the properties of reduction of cross sectional area and corrosion resistance are the optimum when magnetic field current of 1.5A and magnetic field action frequency of 15 Hz are selected. Reduction of cross sectional area is 13.25%, free corrosion potential is-1412 m V and corrosion current density is 42.4μA /cm2.After heat treatment of Solid solution and aging, the properties of tensile strength and microhardness are the optimum when magnetic field curren of 1.5A and magnetic field action frequency of 15 Hz are selected. Tensile strength is 309 MPa, microhardness is 123.9HV.The function of electromagnetic stirring can raise the nucleation rate when the magnetic field is added during welding. Electromagnetic stirring can make the grain of the joint finer than usual process. The mechanical properties of the joint can be improved by the help of the function. Furthermore, The function of electromagnetic stirring can take effect on the heat treatment. |
PDF Full Text Request