Study On Ultra-high Cycle Fatigue Performance Of Aluminum Alloy Welded Joint For Rail Transit Vehicles

Aluminum alloy car-body is a development trend of modern rail transit vehicles,as an important form of Al-alloy welding connecting some parts of the body,the static strength and fatigue strength of weld directly affects the integrity of the body structure and operation safety,but also restricts the further acceleration of track vehicle.Rail vehicles will suffer various complicated and abrupt loads at high speed,the key components such as axle,bogie and car-body often suffer from high frequency and low stress cyclic loads,and the cycle load has exceeded 10~7 weeks during a whole service,the study of the traditional fatigue performance unable to meet the needs of modern structural design.Based on ultrasonic fatigue technology and scanning electron microscopy,the fatigue performance and fracture mechanism of 6005A-T6Al-alloy profile for rail vehicles and its welded joints under three different welding processes are studied in this paper,the results obtained are:All TIG welding,MIG welding and hybrid laser-MIG welding have good process adaptability for welding of 6005A-T6 Al-alloy profile.Hybrid laser-MIG welding technology can achieve better weld shaping effect and weld quality for 18mm thick plate welding.The base metal and welded joint specimens under different technological conditions will be fractured after 10~7 stress cycles,and the fatigue strength of welded joints is obviously decreased relative to the parent material,the traditional"fatigue limit"has not adapted to the modern fatigue design,the fatigue life of welded structure design must consider the difference in performance the weld and base material mechanics.The 6005A Al-alloy base metal and welded joint specimen fracture on the surface and sub-surface initiation mainly by scanning electron microscopy,part of the parent material sample symmetric tension compression fatigue crack exists shear extension form,welded joint crack propagation is more irregular,part of multi-source,adverse effects and the surface and near surface porosity than the inner buried deep hole on the fatigue strength is more obvious.Compare the fatigue properties of symmetry tensile-compressive and bending fatigue performance of the base mental and the welded joint,found that the bending fatigue performance is significantly better than the tension-compression fatigue performance in the low cycle and high cycle fatigue range,based on the conclusions discussed the influence mechanism of size effect on the bending fatigue properties of specimens.Synthesized ultrasonic fatigue test results of welded joints of three different processes,the welded joints in different life interval relative base metal fatigue strength reduction coefficient to evaluate the fatigue performance of welded processes:within the cycle of 10~5 to 10~9,MIG welding,hybrid laser-MIG welding process is obviously better than TIG welding process on the tension-compression fatigue performance and the bending fatigue performance.In the high cycle corresponding to symmetric tension and compression fatigue,the hybrid laser-MIG welding process is superior to the MIG welding process,The MIG welding process is superior to the hybrid laser-MIG welding technology at the super high cycle level.