Environment-sensitive fracture of aluminium alloy 6013

摘要

The aim of this paper is to contribute to a better understanding of the reasons of the fatigue behaviour in corrosive environment of aluminium alloys used in aircraft industry. Fatigue crack propagation experiments were carried out at various test frequencies on CCT specimen of Al 6013T6 in air and in 3.5% sodium chloride solution. The rate of crack propagation is accelerated in sodium chloride solution, and the da/dN-ΔK curve for Al6013T6 shows characteristics which differ from the curve in air. In order to clarify the influence of the corrosion medium the composition and the pH value of the electrolyte were varied. In this way the crack propagation was inhibited or accelerated during fatigue test. During the fatigue test in corrosive environment the dissolution of aluminium occurs and at the same time hydrogen evolves. Hydrogen embrittlement means a loss of ductility caused by the incorporation of hydrogen into the lattice. For the fatigue behaviour in corrosive medium it is decisive how atomic hydrogen is generated and which conditions allow the absorption of the hydrogen into the material. The results show that chloride ions and an acid pH value are necessary for the absorption of hydrogen. Besides the hydrogen embrittlement a second process of mechanical degradation occurs. Secondary cracks were determined after finishing the fatigue crack propagation test. These cracks could be observed as well in air as in corrosion medium. But in corrosion medium the occurrence of secondary cracks was more pronounced. The degradation mechanism in the presence of corrosion medium and cyclic stresses is discussed. It is shown that superposition of hydrogen embrittlement due to hydrogen absorption and formation of secondary cracks plays an important role in the fatigue behaviour of Al 6013T6 in corrosive medium.