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 / Dissertation (MSc)--University of Pretoria, 2012. / Materials Science and Metallurgical Engineering / unrestricted

The mechanical properties achieved with high performance wrought aluminium alloys are superior to cast aluminum alloys. To obtain an intricate shaped component, wrought alloys are commonly subjected to forging followed by subsequent machining operation in the automobile industry. As machining of such high strength wrought aluminium alloys adds to cost, productivity gets affected. Shortening the process by near net shaped casting would tremendously enhance productivity. However, casting of such alloys frequently encounter hot tear defect. Therefore, circumventing hot tear to successfully die cast near net shaped wrought alloy components is industrially relevant. A recent advanced casting process, namely ‘Semisolid Die casting’, is proposed as a likely solution. Hot tearing originates due to lack of liquid flow in the inter-dendritic region. To reduce hot tear susceptibility, fine and non-dendritic grain structure is targeted, amenable for processing by semisolid route. For semisolid processing an adequate freezing range for processing is required. Accordingly A6061 wrought alloy whose composition is tuned with higher silicon and magnesium content within the grade limits, is chosen for the study. With the objective of obtaining fine and non-dendritic microstructured billets, electromagnetic stirring (EMS) and cooling slope (CS) methods are employed. On conducting a parametric study with EMS, a finest possible primary α-Al grain size of about 70 μm is obtained at low stirring time at stirring current levels of 175 A and 350 A, with the addition of grain refiner. CS, on the other hand, rendered a grain of 60 μm at a slope length of 300 mm at a slope angle of 45° with grain refiner addition. Of the two methods, CS billets are chosen for subsequent induction heating. A 3-step induction heating cycle has been devised to attain a temperature of 641°C in the billet on the basis of factors including coherency point, viscosity of the slurry and solid fraction sensitivity with temperature. The billet microstructure is found to be homogenous throughout after quenching in water. The characterization of phase along primary α-Al grain boundary and its composition analysis is done by SEM and EPMA respectively, after billet casting as well as induction heating. In addition, the bulk hardness is determined in BHN. The induction heated billets are semisolid die cast to produce an engine connecting rod used in automobiles. The microstructure is characterized at various locations, and is found to consist of smooth α-Al grains in a background matrix of fine grains formed due to secondary solidification. The component hardness is found to be 66 BHN comparable with A6061 alloy under T4 heat treated condition. X-ray radiography does not confirm presence of surface hot tear, which is the normal defect associated with casting of wrought aluminium alloys. No defects are observed along the constant cross-sectional area of the connecting rod, suggesting that the processing could be suitable for semisolid extrusion.

The main goal of this diploma thesis is the examination of the influence of structure directionality on fatigue properties of formed aluminium alloy 6082/T6. The main attention is focused on the study of the influence of structure directionality on kinetics of short fatigue cracks growth. The measurement of short fatigue cracks growth was performed on cylindrical samples. The samples were made in two different directions; one parallel with the forming direction and second perpendicular to the forming direction. Servo hydraulic machine MTS 880 was used for the cyclic loading. The samples were cycled at two different constant stress amplitudes. Cyclic loading was systematically interrupted in order to measure the length of short cracks by a light microscope. Tensile tests, measuring of cycling hardening-softening curves, observation of microstructure, observation of surface relief, measuring of microhardness and fractographical analysis of fracture surfaces were used for further examination of the influence of the structure directionality. Some of these measured characteristics did not show any influence of the structure directionality (microhardness, fatigue life curve, Young modulus). In other cases is this influence measurable, however insignificant (yield stress, ultimate stress, cyclic hardening-softening curves and kinetics of short fatigue cracks growth). It can be summarised that the material, though the directionality of its microstructure is apparent, shows relatively isotropic mechanical behaviour.