| Magnesium alloys, with a series of advantages, such as light-weight and high-strength, abundant storage volume and easily recycled, is titled as"the green structure materials for the 21st century". The welding technology is one of the most important interconnection techniques for the development of large-scale magnesium alloy structures, which may break through the bottle neck of the widely using of magnesium alloys. It is reported that 70%~90% of the welded structure's failure dues to the fatigue crack in welded joints. The research on fatigue property and improving methods of magnesium alloys welded joints is necessary, showing important theoretical and practical significance.In this thesis,the fatigue property of butt joint and cross joint of AZ31B magnesium alloy was investigated and the S-N curves were drawn by fitting fatigue test datas. For the butt joint specimens, the median S-N curve was lgN=13.78-4.70lgσ, the characteristic S-N curve was lgN=13.50-4.70lgσ;When the slope m was 3, which recommended by the IIW, the median S-N curve was lgN=10.97-3.00lgσ, the characteristic S-N curve was lgN=10.48-3.00lgσ. For the cross joint specimens, the median S-N curve was lgN=11.27-3.58lgσ, the characteristic S-N curve was lgN=10.74-3.58lgσ;When the slope m was 3, which recommended by the IIW, the median S-N curve was lgN=10.39-3.00lgσ, the characteristic S-N curve was lgN=9.91-3.00lgσ. Base on the S-N curves, the fatigue strengths of the butt joint and cross joint were respectively 39.00 MPa and 24.38 MPa at 2×106 cycles. There were no contiguous and clear fatigue striations on the fatigue fracture of magnesium alloy welded joints. But the fatigue fracture was consisted of dense cleavage surface, sector pattern fracture and river pattern fracture. There were lots of bigger or smaller secondary crack on the fatigue fracture, which showed that fatigue failure of magnesium alloy welded joints was a sort of brittle cleavage fracture.The ultrasonic peening treatment (UPT) was used to improve the fatigue property of the welded joints and the improving mechanism was studied. The fatigue strength of the cross joint treated by the UPT was 24.7 MPa, which had been improved by 43.6%, compared with the 17.2 MPa of the as-welded specimens. After the UPT process, the compressive stress was formed at the surface of the impact region. The increasing of the curvature radius at transition zone leaded to the decreasing of the concentration of stresses, which was another reason for the improving of the fatigue property. Meanwhile, the grains at the weld toe became refined, the micro-slag and micro-crimping at weld toe disappeared, which may retard the fatigue cracking and served a very good purpose for the improving of the fatigue property.The TIG dressing was used to improve the fatigue property of the welded joints and the improving mechanism was studied. After the TIG dressing process, the grains at the welding seam became refined, the micro-slag and micro-crimping at weld toe disappeared, which may retard the fatigue cracking and served a very good purpose for the improving of the fatigue property. The fatigue strengths of the butt joint and the cross joint specimens treated by the TIG dressing were 41.68 MPa and 34.13 MPa, which had been improved by 6.9% and 39.9%, respectively, compared with the 39.00 MPa and 24.38 MPa of the as-welded specimens. Because the degree of the stress concentration for cross joint was more severe, the fatigue strength of the cross joint increased more significantly than that of the butt joint.After the laser dressing process, the grains of the laser melted layer became refined highly. The microhardness of the laser melted layer was 50~80HV and that of the base metal was only 40~45HV. When the laser acted on the weld toe, the grains of the welded joints became refined. Because of the complex reactivity between the laser and the magnesium alloy, the quality of the surface forming of the welded joint was bad, represented lots of indented scar in the surface, which may lead to the stress concentration, stepping up the pace of fatigue failure. The fatigue strengths of the butt joint and the cross joint specimens treated by the laser dressing were respectively 25.03 MPa and 21.74 MPa, which had been decreased, compared with those of the as-welded specimens. It indicated that the laser dressing required stricter technology.With the comparison of these three kinds of improving methods, we can learn that both the TIG dressing and the UIT had improved the fatigue property of the welded joints in some degree. The TIG dressing was chiefly characterized by its simplicity of operation and reduced cost, but it may be affected by the operating personnel. Featuring compact and delicate structure, controllability, low cost and energy-saving, etc, the UIT, favorable for all kinds of welded joints, seemed to be a realizable methods of improving the fatigue property for welded joints.
|
PDF Full Text Request