Materials Science Forum Vols. 519-521

Abstract: The influence of variable amplitude loading on fatigue crack propagation was investigated for two high purity versions of the alloy Al 2024 in sheet form, one with fine equiaxed grains, and the other with coarse elongated grains. Fatigue tests on center cracked specimens were conducted in vacuum at constant amplitude (R-ratio of 0.1) and with periodically applied single tensile overloads with an overload ratio of 1.5. The number of intermittent baseline cycles between consecutive overloads was varied (n=100 and n=10.000). Detailed fractographic investigations were carried out for the identification of changes in the fracture surfaces due to the overloads. Crack closure measurements were performed in all cases. The results revealed a strong influence of the overloads on the crack propagation rate. Whether overloads are retarding or accelerating the fatigue crack propagation depends on the crack propagation mechanism at constant amplitude loading and the number of intermittent baseline cycles. For n=100 retardation occurred for the fine grained alloy exhibiting homogeneous slip at constant amplitude while acceleration was observed for the alloy with coarse elongated grains showing pronounced slip band fracture at constant amplitude. For n=10.000, the formation of steps parallel to the direction of crack propagation by overloads is assumed to be the reason for the observed increase in fatigue crack propagation resistance resulting in retardation for both alloys compared to constant amplitude and n=100. The influence of crack closure on the overload effects was minor. This was verified by additional tests at R=0.5.

Abstract: Microscale fatigue damage mechanisms in various Al-Sn-Si based bearing alloys used as linings of plain automotive bearings are reported. Extensive work on previously developed alloys has concluded that secondary phase particles such as Sn and Si are potential fatigue initiation sites with a complex combination of various particle geometry parameters. A newly developed alloy contains a number of complex widely scattered intermetallics with much finer and fewer Sn and Si particles. This alloy system appears to be more resistant to initiate microscale fatigue damage compared to the previous systems.

Abstract: The effects of alloy production method on microstructure and hence fatigue crack growth rate and fracture mechanism have been examined for a variety of fine-grained/high dispersoid Al-Li-Mg-Zr and Al-Li-Cu-Mg-Zr alloys. Microstructures have been assessed by scanning and transmission electron microcopy, together with electron back scattered diffraction pattern assessment. In these fine-grained/high dispersoid materials, high crack growth rates are seen in comparison to the traditional DC cast alloys, excepting a material with high volume fraction of shearable precipitate. The effects of fracture surface roughness and closure levels in determining crack growth rates has been assessed.

Abstract: Fatigue initiation behaviour in three multi-component Al-Si casting alloys with varying Si content is compared using a range of microscopy and analytical techniques. A higher proportion of stiffer secondary phases leads to load transfer effects reducing particle cracking and particle/matrix debonding. Si appears stronger than the Al9FeNi phase, which cracks and debonds to form initiation sites preferentially over Si. Reducing Si content results in clusters of intermetallics forming, and increased porosity. The effect of porosity, combined with mesoscopic load transfer effects to the high volume fraction intermetallic regions make these potent crack initiation sites in low silicon alloys.

Abstract: Residual stress fields may be rather frequently observed in several mechanical and structural parts, usually as a result of the manufacturing and joining techniques adopted. Their effects on material performances can be quite different, going from highly detrimental to beneficial, according to their distribution and to the acting material damage mechanisms (fatigue fracture, brittle or ductile failure, corrosion,…). Residual stresses are produced in welded structures by thermal expansion, plastic deformation and shrinkage during cooling. The amount of constraints determines the level of residual stress. Friction Stir Welding (FSW) is a quite new joining technique particularly for aluminium alloys difficult to fusion weld. Since conventional FSW showed its limits, Bobbin Tool technology has been developed. These both types of welds produce low-distortion of high quality (even no postweld distortion) and relevant residual stresses. The residual stresses of the aerospace material 6056-T78 aluminium alloy Friction Stir Welded sheets have been analysed on various tempers in accordance with these two different technologies. The effects have been investigated parallel and orthogonal to the weld direction. By means of local or global heat treatments, the residual stress state can be generally reduced or changed from tensile to compressive stresses on surface; in that way, the different heat aging conditions T78 as-welded and post-welded are checked in this study. This paper presents the method used, the measurements of this fatigue damage and their consequences on the fatigue life performance of structural materials.

Abstract: Two models to predict the microstructural evolution and post-weld properties of friction stir welds in aerospace aluminium alloys are presented. The first model is a develop- ment of an existing semi-empirical method for the prediction of hardness profiles after welding, calibrated using isothermal hardness data. Post-weld natural ageing is accounted for, and a new method that predicts natural ageing kinetics is introduced. Once calibrated, the model is shown to accurately predict weld hardness profiles. However, this model does not explicitly predict the microstructure and therefore cannot readily be extended to model other properties. It can also only be applied to alloys welded in peak or overaged conditions. The second model aims to explicitly predict the heterogeneous precipitate distributions obtained after welding for any initial condition. It is based on classical kinetic theory and the numerical framework of Kampmann and Wagner. Multiple nucleation sites and multiple phases are accounted for. This model provides detailed microstructural information required for prediction of complex properties.

Abstract: Sheet materials of the alloys 6056 and 2024A were joined using the friction stir welding technique. Similar and dissimilar butt welds were produced. Strength and corrosion behaviour of the joints were investigated in different heat treatment conditions. Hardness profiles across the weld revealed minima in the heat affected zones, being very pronounced at the advancing side of the joints. The hardness of the nugget zone increased after post-weld artificial aging. Strengths of postweld heat treated similar 6056 and dissimilar 2024A/6056 joints approached 90 and 85 %, respectively, of the ultimate tensile strength of 6056-T7X. As found by electrochemical measurements, alloy 2024A-T3 exhibited the most noble corrosion potential and artificially aged alloy 6056 the most active potential. Besides microstructural changes caused by the welding process, galvanic coupling affected the corrosion behaviour of friction stir welds, in particular with joints of dissimilar aluminium alloys, as indicated by accelerated corrosion of alloy 6056 in the nugget region. Intensified corrosion attack was also observed in the heat affected zones.

Abstract: Lap joining of A6111 alloy and steel (SPCC: Steel Plate Cold-rolled C) plates was performed using a defocused YAG laser beam. A detailed investigation was performed on the intermetallic compound (IMC) layer formed at the weld interface. Two representative joints fabricated under different welding conditions were selected and the effect of the welding conditions on the kind and morphology of the IMC was investigated using a transmission electron microscope (TEM). An electron diffraction pattern method was used to identify IMC. It was found that the morphology and kind of IMC formed at the weld interface were strongly affected by the welding conditions, in particular, by the amount of heat input during welding. The thickness of the IMC layer formed at the weld interface was about 1 μm and the average grain size of the IMC in the layer was less than 300 nm when the joining was carried out with a small amount of heat input. The IMC layer was composed of Fe3Al, FeAl, Al2Fe, Al5Fe2 and Al13Fe4 in this case. However, the thickness of the IMC layer was around 6 μm when the joining was carried out under high heat input conditions. In this case, the IMC layer was composed of coarse Al5Fe2 (5 μm) and Al13Fe4 (1 μm). Therefore, it is considered that the reduced bonding strength of the joint with a thick IMC layer is due not only to the overall morphology of the IMC layer but also to the formation of coarse Al-rich IMCs in the layer.