Study On Fatigue Behavior Of AZ31B Magnesium Alloy And Its TIG Welded Joint Based On Thermographic Method

Magnesium alloys possess many advantages which make them very attractive as structural materials in a wide variety of applications, and its widely use cannot be separated from the support of welding technology. However, the fatigue problems of welded joint usually become the bottleneck of total performance. The previous fatigue research methods usually cost much time, resulting in low test efficiency. Infrared thermography is a fast, real time and accuracy fatigue research method, which can do deeply analysis for the nature of fatigue from the energy aspect, has broad application prospects in fatigue research.In this paper, the fatigue behavior of AZ31B magnesium alloy and its welded joints during high cycle tension-tension fatigue tests were investigated by means of infrared thermography. The mean temperature decrease under cyclic load caused by thermoelastic effect was discussed when the maximum stresses were below30MPa. The influence of weld reinforcement on fatigue performance and behavior was also investigated. At last, the fatigue strength and fatigue life were quick predicted by mean of infrared thermography. Result shows that when the stresses were above the fatigue limit and below the yield limit, five obvious stages of superficial temperature evolution were observed during a fatigue test:initial temperature increase stage, temperature decline stage, temperature equilibrium stage, abrupt temperature increase stage and temperature drop after failure, which is caused by the interaction of cyclic hardening and softening effect. When the stresses were below the fatigue limit, the viscous-elastic effect is in dominant state and no evident temperature increase was observed. When the specimens were under cyclic elastic deformation range, a relatively temperature decrease were observed, which may be caused by thermoelastic hysteresis phenomenon.The fatigue strength of AZ31B magnesium alloy under1×107cycles was87.6MPa, the fatigue strength of its TIG welding as-welded joint and ground flush joint under5×106cycles was59MPa and59.9MPa, respectively. The fatigue strength of welded joint was increased by only1.5%because of the removing of reinforcement. The fatigue limit of base matal, as-welded joint and ground flush joint obtained by thermographic method was95.9MPa,63.7MPa and65.6MPa, respectively, and the percentage error compared with experiment result was9.5%,8%and9.6%, respectively. The fatigue life prediction result obtained by infrared thermography was in good agreement with fatigue test result as a whole.The crack initiation location and fracture location of as-welded joints were all in the weld toe, which due to the stress concentration in the toe. The crack initiation location and fracture location of ground flush joints were all disabled in the middle of weld seams, which due to the low fatigue performance of welded seam. Infrared thermography can directly observe the process of crack initiation and propagation and timely find the dangerous points in the of fatigue process, thus sudden fatigue accidents can be prevented.In the present study, the fatigue limit and fatigue life of AZ31B magnesium alloy welded joints can be predicted rapidly through the maximum initial temperature increment measured within2minutes. The specimen would fail when the maximum initial temperature increment is higher than3.4K (base metal),2.15K (as-welded joint) and3.77K (ground flush joint) with reference to the specimens’ geometry and testing conditions adopted in the present paper.