Fatigue Strength of Laser Beam Welded Steel-Aluminium Joints Considering Variable Amplitude Loading and Corrosive Environment

Abstract

Hybrid material combinations of steel and aluminium are currently being joined by explosive welding in ship design, in order to reduce fuel consumption and increase transport weight. As an alternative to the elaborate and expensive explosive welding, a replacement for steel-aluminium adapters by means of laser beam welding in a lap joint configuration is being aimed at. Therefore, a laser beam welding process has been developed and optimised with regard to the different melting points and coefficients of thermal expansion of the materials as well as the formation of intermetallic phases due to the limited solubility of the elements. The characteristic of the weld seam, which is the result of a melt pool through the steel into the aluminium material, has a major impact on the quasi-static and cyclic strength of the joint. Especially the weld width and penetration depth have been identified as main influencing parameters. In the end, the developed laser beam welding process under spectroscopic control of the penetration depth is validated by fatigue investigations of lap joints and compact designed adapters. In addition to the fatigue strength under constant amplitude loading (CAL), the fatigue performance under variable amplitude loading (VAL) is particularly promising, even though the scatter increases from CAL to VAL and a linear damage accumulation is not applicable. A change in the fracture behaviour is explained by the intensity of the loading. Finally, the corrosion fatigue behaviour is shown by constant amplitude loading in a corrosive environment with a sodium chloride solution of 5 %, which causes a strength reduction from high cycle to long life fatigue.