Study On Fatigue Behavior Of Transverse Joints Of AZ31B Magnesium Alloys With TIG-Welding

With the development of science and technology, and the attention of energy and environmental protection, magnesium alloy which known as the 21st century 'green' engineering material, is paid more and more attentions. As to large-scale and complicated structures, the welding technique is the most important process technology, and the fatigue destroy is a main kind of failure form of welded structures. To the development and extensive using of magnesium alloy, it's very important that realizing the characteristic of the fatigue performance of magnesium alloy welded structures, and adopting the rational evaluation method to guide the fatigue design of magnesium alloy welded structures.Fatigue performances and corresponding S-N curve of transverse butt joints and transverse cross joints with TIG-welding of AZ31B magnesium alloy plates was analyzed. The experimental results were compared with the fatigue data of aluminum alloy and stainless steel welding joints with the same form. The fatigue evaluation with the hot stress method to transverse welding joints of magnesium alloy were researched. The relation between coefficient of stress concentration of magnesium alloy weld toe and geometrical measurement of welding seams were analyzed by finite element analysis software.The result shwoed that the S-N curve slope of AZ31B magnesium base metal and the transverse joints were 4.45, 4.43 and 3.05 respectively. Under 2×10~6 cycle, the nominal fatigue resistance are 57.81 MPa, 24.60MPa, 17.25MPa respectively, and the hot stress fatigue resistance of two kinds of joints were 29.5MPa and 20.7MPa respectively. The S-N curve slope and the fatigue resistance of the nominal stress and the hot stress were higher than that recommended by International Institute of Welding (IIW). That proved that to the magnesium alloy, using m=3.0 had certain safe allowance.Comparing experimental results, the fatigue resistance of transverse welding joints of AZ31B magnesium alloy were about 40% value of aluminum alloy joints with the same form, and 10% that of stainless steel joints. The FAT of magnesium alloy joints which dealing with experimental dates with m=3.0 were compared with that of aluminum alloy and structural steel joints with the same form that recommended by International Institute of Welding (IIW). The results indicated that the fatigue resistance of magnesium alloy joints which were evaluated by secure assessment was the half of that of aluminum alloy joint, and 10%-16% of the structural steel joint.Hot stress value of magnesium alloy joints were caculated. The coefficients of hot stress concentration of transverse butt joints and transverse cross joints were 1.2 and 1.6 respectively, which proved that the coefficients of hot stress concentration were related to the form of joints. The hot stress fatigue resistance of two kinds of transverse joints of AZ31B magnesium alloy could be expressed with one S-N curve. It showed that the fatigue grade of magnesium alloy joints could be reduced by evaluation with hot stress method. It was superior to evaluating with the nominal stress method.To butt-jointed seam and fillet weld of magnesium alloy, the angle of weld toeθand the radius of weld toeρwere the main factors which influenced the coefficient of stress concentration. With the increment ofθand the reduction ofρ, the coefficient of stress concentration of toe place should increment. The welded toe angleθis the most influential factor to the coefficient of stress concentration. When the change in the sectional appearance of welding seam was bigger, the coefficient was more sensitive to the other influence factors.