Research On Crack Propagation Behavior Of 7B05-T5 Aluminum Alloy Welded Joints For High-speed Train

Aluminum alloy has been extensively used in high-speed trains because of its excellent properties.The welding quality and efficiency of high-strengh aluminum alloys is one of the key technologies in the manufacture of high-speed trains.With the increase in train speed and the operating life of high-speed trains,more stringent requirements have been put on the service safety and reliability of aluminum alloy welding structures.When the high-speed train is operated for a long time under the combined action of corrosive environments and random loads,the possibility of corrosion failure and fatigue failure of aluminum alloy welded structure increases.Therefore,it has important practical engineering application value to research the fracture behavior of aluminum alloy welded joints under the action of corrosion and load.In present work,the stress corrosion and corrosion fatigue crack growth behavior of the hybrid laser-MIG welded joint of 7B05-T5 aluminum alloy was studied.Set up a thermostatic water circulation device to examine the stress corrosion cracking behavior of the joints.Pre-cracked wedge-open loading?WOL?specimens were submitted to stress corrosion cracking tests.Stress corrosion cracking critical stress intensity factor K_Iscc of base metal?BM?is less than 13.9 MPa.m^1/2,and the K_Iscc of heat affected zone?HAZ?is between 15.21 and 17.49MPa.m^1/2,the weld metal?WM?is not sensitive to stress corrosion cracking.Stress corrosion cracking of aluminum alloy shows a mixed fracture of transgranular and intergranular.The anodic dissolution and hydrogen embrittlement contribute to stress corrosion crack propagation.The?phase distributed on grain boundary of the BM is large number and small space,but the number of?phase decreases in the HAZ after weld thermal cycle,the amount of?'phase in the crystal is also significantly reduced,making the BM easier to form an etched channel than HAZ.In addition,In the absence of precipitation free zone near grain boundary of the BM enhances electrochemical heterogeneity between grain boundary and adjacent areas,causing the stress corrosion crack propafation rate of the BM is faster than that of the HAZ.The results of fatigue crack propagation test in each area of welded joints show that the impurities phase rich in Fe,Mn and other elements in aluminum alloy has little influence on crack growth rate.Compared with laboratory air environment,the fatigue crack growth rate of the BM,HAZ and WM in 3.5%NaCl solution is obviously improved.The interaction between 3.5%NaCl solution and mechanical fatigue caused accelerated damage to the aluminum alloy,which reduced the threshold value and resulted in accelerating crack rate.The mechanism of corrosion fatigue crack propagation of aluminum alloy is discussed.It is considered that the combined effects of anodic dissolution and hydrogen embrittlement lead to accelerated propagation of corrosion fatigue cracks,hydrogen embrittlement is dominant.Considering the existence of hydrogen embrittlement during stress corrosion and corrosion fatigue crack propagation,the effects of hydrogen on mechanical properties and crack growth rate of the aluminum alloy was investigated.Rresults showed that the strength,hardness and plasticity of aluminum alloy induced by hydrogen.The effect of hydrogen on fatigue crack growth of aluminum alloy is that crack rate increases in the near-threshold and steady-state crack region,and the threshold value decreases.Reduction of the resultant force in atomic bonds triggered by the enrichment of hydrogen in the grain boundary and second phase of hydrogen trap in the aluminum alloy after charging hydrogen is the major reason for lower mechanical performance of the aluminum alloy and increasing crack propagation rate.