Residual stresses in CO2 laser beam welded AA6056 Al-sheets of 3.2 and 6.0 mm thicknesses were studied using neutron and high-energy X-ray diffraction. The influence of the temper T4 and T6 before welding, the effect of sheet thickness in T6 temper, and the effectiveness of post-weld heat treatments T6 and T78 for the reduction of residual stresses in the sheets welded initially in the T4 temper were examined. It was found that tensile longitudinal stresses were significantly higher when welded in T6 than in T4. With the increase in the sheet thickness from 3.2 to 6.0 mm, the residual stress profile was affected more than the stress level when welded in T6. For 3.2 mm sheet, the post-weld heat treatments T6 and T78 did not lead to a significant reduction in residual stresses when welded in T4.

Sheets of AA6056 and Ti6Al4V were butt-joined by inserting the Ti-sheet into the profiled Al-sheet and by melting the Al-alloy alone using a split beam Nd:YAG laser.To study microstructural effects on properties, the Al-alloy was used in two tempers; T4followed by post weld heat treatment T6, and in T6 followed by a defined duration of natural ageing at room temperature. As a basic step for fatigue and fracture investigations, local gradients in properties of this dissimilar joint are investigated using microscopy, hardness and tensile tests. Possible sites, from which fracture may initiate, have been then identified. All property changes are found to confine to the aluminiumside. An intermetallic layer, although very thin, is found to form on the interface. The changeovers, firstly between the fusion zone and the heat affected zone and secondly between the heat affected zone and the base material, are found to be associated with changes in microstructure, hardness and strength. These are identified as the possible critical sites in addition to the interface. Fig. 1 to follow) for further weight saving and economy, since a specific material can be chosen for a specific local requirement. However, dissimilar welds are a metallurgical challenge [4]. Local gradients in microstructure, chemistry and properties become inherent and need to be accounted for in design.Moreover, when a non-ferrous partner is involved, intermetallic phases formed tend to be brittle [4] and consequently properties of the weld are impaired. Residual stresses [5] at the interface and in the partners may complicate the matter further.With weight alone as the criterion, three base metals become interesting on the density basis; magnesium, aluminium and titanium. Magnesium is, however, yet to find acceptance for wide aerospace applications and so only Al-and Ti-alloys become relevant for dissimilar joints. Some physical properties collected from the literature [6] are shown in In the following, the terminology "welding" [25] has been retained, since at least one partner was melted (and thus it is not considered as brazing). The partners are also referred to synonymously by the respective side, e.g., AA6056 as the Al-side.Moreover, specimens from the laboratory coupons (and not the component in Fig. 1c) have been tested. Experimental procedureSheets of the precipitation hardenable AlMgSiCu type alloy, AA6056 (uniform thickness 2 mm) and Ti6Al4V (thickness 1.8 mm; mill annealed) were welded to coupons as shown in Figs. 1a,b using a split beam 4 kW Nd:YAG laser in the heat conduction mode at BIAS in Bremen, Germany, without using filler wire. The coupon had the dimensions 330 x 94 mm 2 . To study the effect of starting microstructure on properties two tempers were selected for AA6056:• welding in T4 followed by artificial ageing T6 (190 °C-4h/air); denoted also as "T4/T6" or "T4 followed by post weld heat treatment T6", and• welding in T6 and naturally aged (at least for 7 weeks) without any further artificial ageing; denoted also as "T6...

A B S T R A C T Laser beam butt welds in Al-alloys are very narrow and are accompanied by steep residual stress gradients. In such a case, how the initial crack orientation and the distance of the notch tip relative to the weld affect fatigue crack propagation has not been investigated. Therefore, this investigation was undertaken with two different crack orientations: along the mid-weld and perpendicular to the weld. Fatigue crack propagation 'along the midweld' was found to be faster in middle crack tension specimens than in compact tension specimens. For the crack orientation 'perpendicular to the weld', the relative distance between the notch tip and the weld was varied using compact tension specimens to generate either tensile or compressive residual stresses near the notch tip. When tensile residual stresses were generated near the notch tip, fatigue crack propagation was found to be faster than that in the base material, irrespective of the difference in the initial residual stress level and whether the crack propagated along the mid-weld or perpendicular to the weld. In contrast, when compressive weld residual stresses were generated near the notch tip, fatigue crack arrest, slow crack propagation, multiple crack branching and out of plane deviation occurred. The results are discussed by considering the superposition principle and possible practical implications are mentioned.

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