Corrosion fatigue behaviour of 5083-H111 and 6061-T651 aluminium alloy welds

Abstract

In addition to being one of the highest strength non-heat treatable aluminium alloys, magnesium-alloyed wrought aluminium 5083 displays excellent corrosion resistance and good weldability. Aluminium alloy 6061, alloyed with magnesium and silicon, displays high strength, excellent formability, adequate weldability and good corrosion resistance. These aluminium alloys find application in the ship building and transport industries where 5083 is often joined to 6061 to produce welded structures such as complex I-beams and semi-hollow or hollow channels. This project aimed at characterizing the hardness, tensile properties, corrosion behaviour and fatigue properties (in air and in a 3.5% NaCl solution) of aluminium 5083 and 6061 in the as-received and welded conditions. Plates of 5083-H111 and 6061-T651 aluminium, prepared with double-V or square butt joint preparations, were joined using semi-automatic or fully automatic pulsed gas metal arc welding (GMAW). The pulsed GMAW process allows close control over the welding arc and facilitates the use of lower average heat inputs, thereby improving the bead appearance and mechanical properties. During this investigation, three filler wires were evaluated, namely magnesium-alloyed ER5183 and ER5356 aluminium, and silicon-alloyed ER4043. Hardness measurements revealed a decrease in hardness in the weld metal of the 5083-H111 welds. Dressed welds failed in the weld metal during transverse tensile testing, whereas undressed (as-welded) specimens failed at the weld toe or weld root due to the stress concentration introduced by the weld geometry. Significant softening, attributed to the partial dissolution and coarsening of strengthening precipitates and recrystallization during welding, was observed in the heat-affected zones of the 6061-T651 welds. During tensile testing, failure occurred in the heat-affected zone of all 6061 welds. Welding reduced the room temperature fatigue life of all specimens tested. In the 5083 welds, fatigue cracks initiated preferentially at gas pores, lack-of-fusion type defects and second phase particles in dressed welds, and at the stress concentration presented by the weld toes or the weld root in undressed welds. In 6061 welds, failure occurred preferentially in the softened heat-affected zone of the welds. As a result of improved control over the weld profile and a lower incidence of weld defects, fully automatic welds consistently outperformed semi-automatic welds during fatigue testing. The presence of a corrosive environment (a 3.5% NaCl solution in this investigation) during fatigue testing reduced the fatigue properties of all the samples tested. Corrosion pits formed preferentially at second phase particles or weld defects, and reduced the overall fatigue life by accelerating fatigue crack initiation. Copyright