| Based on the lightweight, high strength, easy to recycle and other excellent characteristics, magnesium alloy has already been well known as the prospective green engineering structure materials in the21st century, The development of structural materials must depend on the welding technique. Vacuum electron beam welding (EBW) technology is very suitable for welding magnesium alloy material which have a lot of advantages such as high energy density, high penetration ability, large welding speed, vacuum state, etc. The correlative data indicates that70%to90%of failure accidents involving welding structures are caused by the fatigue fracture of welded joints. The crack initiation and propagation have an improvement affect on fatigue properties and life of welding structure. Therefore, fatigue property and fatigue crack propagation rate of magnesium alloy electron beam welding joints has important theoretical significance and practical value.In this paper, the microstructure, fatigue performance and fatigue crack propagation rate of the AZ31B magnesium alloy electron beam welding joint were studied.The results of EDS analysis shows that the content of Mg and Zn reduced and the relative content of Al and Mn increased. The analysis of X-RD indicate that fusion zone consists of single-phase of α-Mg, there is no obvious Mg17Al12phase. The microstructure of EBW seam is mainly composed of fine equal-axed grains and dendritic crystals, besides, the grains in HAZ growed slightly, but the width of the HAZ is very narrow.The microhardness results show that the average hardness of the welded joints was59.5HV0.1, and the hardness of heat affected zone was53.7HV0.1, the hardness of the base metal was54.6HV0.1, the microhardness values of weld cross-section almost no change from the top to the bottom. Tensile testing of welded joints showed that the yield strength and tensile strength of EBW joints are144MPa and242MPa respectively, which are slightly higher than that of the base metal. The joint of strength, plasticity and toughness are improved. Tensile fracture morphology mainly for dimple, shows toughness fracture characteristics.It is concluded that the fatigue strengths of AZ31B magnesium alloy and EBW joint are116MPa and91MPa, respectively under2×106cycles, it reaches78.4%of the fatigue strength of base metal, which show superior mechanical property.The fatigue crack propagation path initiated fatigue crack indicate at the specimen surface concave, which located in the weld toe; the direction of crack propagation is vertical to the load direction. Fatigue crack propagated through the heat affected zone and extended to base metal in the cross-section of the weld, in the weld direction, crack propagated along the HAZ. The crack tip plastic deformation by the way of slip, the crack propagated by transgranularly.The analysis of results fatigue fracture show that the fatigue fracture of magnesium alloy exhibits river pattern morphology, cleavage step, tear ridges and the secondary crack, showing quasi-cleavage brittle fracture. Three morphology features including quasi-cleavage, cleavage and dimple can be observed in fatigue fracture of EBW joints, which confirmed that it was a mixed ductile-brittle fracture but brittle was main.The fatigue crack propagation rate of the AZ31B magnesium alloy and EBW joints piecewise meet Parise formula. The da/dN-AK curves of notch located in the base metal, weld-seam, HAZ are comparative analysis. The fatigue crack propagation threshold value(1g△K) of base metal, weld-seam, HAZ are2.12MPa·m1/2,2.52MPa·m1/2,1.78MPa·m1/2, respectively. The threshold value of the notch located in weld-seam is higher than base metal and HAZ. The fatigue crack propagation rate of notch located in weld-seam is less than base metal and HAZ, under the same△K value.The fracture of the fatigue crack propagation of AZ31B base metal and HAZ are rivers shape pattern, which are quasi-cleavage brittle fracture. There are many torn edges and plastic dimple in the weld-seam fracture of fatigue crack propagation, which shows a certain degree of toughness, was a mixed ductile-brittle fracture but brittle was main. With the fatigue crack propagation rate increases, the cleavage steps becomes bulky in the three fracture, and the fatigue stripes widens. |
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