PSI - Issue 18

Benjamin Möller et al. / Procedia Structural Integrity 18 (2019) 556–569 Author name / Structural Integrity Procedia 00 (2019) 000–000

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3. Conclusions and outlook Fundamental research and basic fatigue investigations with respect to optimisation of the laser beam welding process parameters have been conducted on lap joints with the following main findings: - Optimised process parameters including an energy per unit length of E = 300 kJ/m have been found for single and triple welds, as documented in Möller et al. (2017). A failure initiating from the root of the weld seam and propagating through the aluminium sheet is less critical compared to shearing of the weld seam, which is due to pre-existing defects in the weld. - The fatigue results for lap joints showed a good correlation with the weld width and penetration depth as well as their ratio. The weld width should be as high as possible, while the penetration depth should be comparably low, so that the loaded cross section is not increased. For lap joints, it has been found that a ratio between width and depth of approximately 2 is a convenient value. The shape of the weld seam (in the aluminium) should be rounded and not tapered, so that the notch effect is reduced. The shape of the weld seam has been influenced by welding process parameters and their optimisation. In contradiction to the tensile strength, the number of parallel weld seams has no major effect on the fatigue strength of steel-aluminium lap joints. - For optimised process parameters, the fatigue strengths of the process without control and the process with a spectroscopic control of the penetration depth are at the same level. However, the process control is suited to compensating for effects in the industrial environment, e.g. differences in the sheet thickness. - Corrosion fatigue of lap joints under sodium chloride solution according to DIN EN ISO 9227 (2017) shows a reductions of 10 % to 25 % in fatigue strength over the entire Wöhler curve, i.e. from the high cycle to the long life fatigue, and indicates that there might be a knee point. Even though, lap joints have a comparably low fatigue strength, which can be traced back to the asymmetric specimen type, the first step towards laser beam welding of steel to aluminium for maritime applications has been made. The fatigue strength is increased by a laser beam welded adapter ‘V1’ having six weld seams, which has been improved to a compact variant 2, which has been joined by the controlled laser beam welding process. Main findings for the fatigue of steel-aluminium adapters ‘V2’ are: - Different fatigue failure locations have been found for CAL and VAL, which can be traced back to different load levels. At lower force amplitudes, as applied under CAL, failure occurs at the first weld seam (from the steel) resulting from stress concentrations in a system with mainly metallic continuity. Fracturing of the intermetallic joint between steel and aluminium at the weld seam due to higher loading results in a domination of a form closure, which changes the stress field and concentration and affects the failure location and behaviour. A linear damage accumulation on the basis of the experimental Wöhler curve is not applicable, due to the change in the failure behaviour from CAL to VAL and the investigation of welded adapters without control of the penetration depth. Therefore, further investigations under VAL should be performed in the future. - The influence of corrosion fatigue is evidenced through a fatigue strength reduction for adapters ‘V2’ of 10 % to 20 % in the high cycle fatigue regime and increases in the long life fatigue regime. Wöhler curves under corrosion fatigue are nearly parallel to those in air. The gives a first idea of the impact of a corrosive impact on the fatigue of laser beam welded steel-aluminium joints. However, further research would lead to information about the effect of testing frequency and analysis of corrosion fatigue mechanisms. - A final step for the fatigue assessment of laser beam welded steel-aluminium joints will be a stress-based evaluation. The stress-based assessment using nominal stress definitions and finite element simulations for the application of local notch stress approaches will therefore be described in the final report of the LaSAAS project (2019). Among other things, the aim is the comparability with explosive welded adapters and integration in fatigue assessment standards, such as the IIW recommendations as released by Hobbacher (2016) or, rather, DNV-GL standards for the fatigue assessment of ship structures. However, comparison between explosive welded and LBW adapters is difficult due to the different loading situations and specimen types. - The typical fatigue investigation of explosive welded adapters shows a fatigue failure by necking of the unalloyed aluminium, but gives a remarkable fatigue strength. Unalloyed aluminium is needed to ensure