Failure Analysis of A7N01S-T5 Aluminum Alloy Crossbeam for High Speed Train

The fracture failure reason of A7N01S-T5 aluminum alloy crossbeam for high speed train was studied. After chemical and mechanical property tests, observation of crack morphology by optical microscopye and analysis of cross-section by SEM, the fracture failure of the crossbeam was judged to be stress corrosion cracking (SCC). The alloy matrix of bolt hole on crossbeam was easily to be exposed for stress concentration, and the reaction of the exposed alloy with H2O, Cl- and SO2 in environment resulted in the formation of corrosion pits which acted as the corrosion source. The improper heat treatment made grains coarsen and the coarsening grains made the second phase distributed along grain boundaries which acted as active corrosion route. Under tensile stress the corrosion cracks extended rapidly along grain boundaries which resulted in the fracture of crossbeam.