Research On The Fatigue Crack Growth Rate Of Welded Joints Of A6N01S And A7N01S Aluminium Alloy For High-speed Train

The development of modern railway vehicles is mostly heading towards high-speed aluminium alloy trains and most countries in the world are energetically developing high-speed aluminium alloy trains.Fabrication of high-speed aluminium alloy trains with advanced world levels requires not only reliable equipments and excellent structure designs but also perfect fabrication techniques and quality quarantee systems.And one of the most citical techniques among these is welding quantity controlling of the high-speed train carriages. A6N01S and A7N01S aluminium alloys possess excellent compositive properties after heat-treatment ,but it doesn,t mean its welded joints have such excellent properties,and most of the welded structures endure alternate load.So studying on the fatigue crack growth rate in the welded joints of A6N01S and A7N01S aluminium alloy using appropriate welding process parameters ,which is used to calculate the residual life of the welding structures,has significant value for the fabrication and exploiture of high-speed trains.Research on mechanical properties of welded joints shows that the welded joints of home-made A6N01S and A7N01S aluminium alloys with ER5356 filler are suitable for the international standard on the carriages of high-speed aluminium alloy trains . There is a softened zone of microstructures in the heat-affected zone of the joints welding A7N01S aluminium alloy ,as well as A6N01S aluminium alloy .And the weakest area of A7N01S joints lies in the center of the welds while the weakest area of A6N01S joints lies in the softened zone of the heat-affected zone. The microstructures of A6N01S and A7N01S joints were investigated ,and the results indicated that the coarening of the microstrcture in the softened zone was the main reason for weakening.The fatigue crack growth rate problem of the welded joints and base metal of A6N01S and A7N01S aluminium alloy was compared and studied through the fatigue crack growth rate experiment with the SE(B) specimen. For A6N01S aluminium alloy, the fatigue crack growth rate in the HAZ is the lowest ,but its value incrases very quickly because of the curve's big slope. With theΔK value increasing ,the distance of the two curves is minishing gradully.For two kinds of base metals,in the entire process of fatigue the fatigue crack growth rate of the first kind of base metal ,whose original incision is parallel to the extrusion processing direction,is obviously bigger than the other kind of base metal. It shows more obvious difference with high ΔK value. The difference between the weld and base metal ,whose original incision is vertical to the extrusion processing direction ,is slight.With th increasement ofΔK value,there are two points of intersection between the two curves ,but their distance is also very slight.For the welded joints of A7N01S alluminium alloy, the fatigue crack growth rate in the center of the welded joint is bigger than that in the heat-affected zone slightly where the value ofΔK is small.With the increasment ofΔK value ,the distance of the two curves is minishing gradully.By comparing the fatigue crack growth rate of A6N01S and A7N01S welded joints,the one of the A7N01S welded joint is bigger than that of the A6N01S welded joint,but the distance is slight.So as the new type of alluminium alloys for high-speed trains,their welded joints,with the help of MIG,possess excellent properties of fatigue crack growth resistance and fatigue resistance.