Study On The Fatigue Fracture Behavior And Assessment Of AZ31B Magnesium Alloy And Its TIG Welded Joints

Magnesium alloys have excellent mechanical properties such as low density and high specific strength, easiness of recycling, etc; they are prospective green engineering structure materials of the 21st century. This makes them ideal candidates in the traffic vehicle, automotive industry, aerospace industry etc. Welding technology is one of the most important methods for developing complicated and large-scale magnesium alloy structure. Fatigue fracture is a major failure pattern in metal structures, and 70% to 90% of failure accidents involving welding structures are caused by the fatigue fracture of welded joints. The crack initiation and micro-crack propagation have an improvement affect on fatigue properties and fatigue life of the magnesium alloy and its welded joints, the fatigue life are decided by the instability fracture critical crack size, investigations on the fatigue properties and fatigue crack propagation rate of the magnesium alloy welded joints have theoretical and practical significance in engineering applications.The fatigue properties, crack propagation rate and fracture toughness were researched for AZ31B magnesium alloy and its welded joints. The microstructure, crack propagation behavior and fracture were determined by OM and SEM; the fatigue life were forecasted by Parise crack propagation theory; the welded joints were assessed by critical distance method and the hot spot stress approach; the textures of AZ31B magnesium alloy and the effect on fatigue properties and crack propagation rate were analyzed.It is concluded that the fatigue strengths of AZ31B magnesium alloy and its butt joint (BJ), transverse cross joint (TJ), longitudinal cross joint (LJ), lateral connection joint (LCJ) (under 2×106 cycle times) are 66.72 MPa,39.00 MPa,24.38MPa,32.18 MPa and 24.4 MPa, respectively. The crack propagation of the AZ31B magnesium alloy base metal in a transgranular manner, the twin is the main format in the course of crack propagation. The crack propagates from the weld toe for the welded joints. Moreover, the fracture of the base metal and welded jointS are brittle and of cleavage-type, based on quasi-cleavage fracture. Secondary cracks between the base metal and welded joints fracture are also observed.The TIG dressing and UIT were used to improve the fatigue property of BJ and TJ. The fatigue property of BJ and TJ are 41.68 MPa and 34.13 MPa, respectively by TIG dressing, and 6.9% and 39.9% of the fatigue property are advanced. Under 2×106 cycles, the fatigue property of TJ improved to 24.7 MPa by UIT, and it have 43.6% advanced.Fatigue crack propagation rate on as-extruded AZ31B magnesium alloy is tested. C(T) specimens of the incision direction are parallel (T-L), vertical (L-T) and inclined 45°to extrusion direction. Results indicated that crack propagate along the extrusion direction for T-L and L-T specimens, Inclined 45°specimen has an angular deflection about 10°to extrusion direction. Fatigue crack begin to rapidly propagate when AK are about 5.5 MPa·m1/2,5.7 MPa·m1/2 and 4.1 MPa·m1/2,respectively for T-L, L-T and 45°specimen. Fatigue fracture consists of quasi-cleavage and it was brittle fracture. The fatigue crack is along the notch direction for L-T(W) and T-L(H) specimens, the crack fatigue propagation begin to slower propagation when AK is 6.12 MPa·m1/2 for L-T(W), the crack propagation rate of T-L(H) specimens is identical. Fatigue crack begin to slow propagate when AK is about 8.2 MPa·m1/2 for along welding direction. The crack tip propagates by both transgranular and intergranular fractures. The fracture is brittle fracture characterized by a cleavage plane, some secondary cracks and river pattern can be seen.Plane-strain fracture toughness KIC of a 10 mm-thick as-extruded AZ31B magnesium alloy and its welded joints are tested by three-point bending test. The result shows that fracture toughness KIC values of AZ31B magnesium alloy base metal in pallrel (0-sample),45 and vertical (90-sample) to the as-extruded direction is 9.36,9.40 and 9.89 MPa·m1/2, respectively, and the KIC of 90-sample is the highest. The KIC of notch in welding center and HAZ are 18.28 and 15.62 MPa·m1/2, respectively, and the fracture toughness of welded joints is well than base metal, that is, the ability resisting fracture of welding is well. The fracture appearance of AZ31B magnesium alloy and its welded joints consist of quasi-cleavage and are brittle type of fracture; the fracture appearance of notch in HAZ is an intergranular fracture.The critical distance method and the hot spot stress approach are used to assess the welded joints. The critical distance assess including PM and LM. The BJ and TJ fatigue strength are 79.55 MPa and 49.10 MPa for PM and 79.01 MPa and 44.38 MPa for LM. Hot spot stress approach evaluated result shows that the dispersity is reduced largely, the fatigue strength can be expressed by only one S-N curve for BJ and LJ, and fatigue level is FAT33. The tested fatigue level of two evaluation methods is higher than fatigue level obtained used 3.0 recommended byⅡW, which indicates there is greater safety factor in welded structures design if 3.0 applied.The texture is often formed in the course of molding and its existence effect on fatigue property and crack propagation rate. There are obvious basel texture{0002}<2110> and{0002}<1010> in the AZ31B magnesium alloy plate. The schmid factor average of non-basal texture in ED is greater than TD, so it is easy for non-basal texture slipping and crack source forming under the alternating loads, this causes the fatigue life difference between ED specimen and TD specimen.Polar diagram and ODF are analysized and the results indicate that the Schmid factors of non-basal slip systems are different in two directions: mED>mTD, and the threshold of fatigue crack propagation rateΔKth is different: the TD is greater than the ED. Plastic deformation will easier in ED specimen and the crack easy to propagate.