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Automotive and aerospace industries consume a significant amount of Al alloys in structures and framing. There is, however, a significant challenge to join the alloy components by laser welding. A key problem is the presence of large amount of porosity in the welds. This research work aimed to understand factors affecting porosity formation in laser welding of AA6014 Al alloy and identification and verification of a suitable method for the porosity reduction and elimination. AC-170PX (AA6014) Al alloy was welded, for the first time, using a 5 kW disk laser in two different configurations: fillet edge and flange couch joints using a number of different filler wires. The experimental results showed that laser power (2-5 kW) and welding speed (20-50 mm/s) had a significant influence on porosity generation. Also, the introduction of a 0.2 mm gap between the sheets significantly reduced porosity for the fillet edge joint while it had a marginal effect for the flange couch joint. The effect of the chemical composition of the filler wire on the AA6014 weld quality was also evaluated for the first time by using different filler wires (AA3004, AA4043, AA4047 and AA5083) over a range of laser powers and welding speeds. The increase in Mg and Mn content in the filler wire's composition was found to reduce porosity in comparison with high silicon content filler wires. Nanosecond pulsed Nd:YAG laser cleaning was investigated as a surface preparation method for laser welding for AA6061, and its effect on porosity at various welding parameters was examined. The effect of laser cleaning on porosity reduction during laser welding using a filler wire has not been reported before. The surface characteristics before and after laser cleaning were analysed. The results showed that laser cleaning played an essential role in significantly reducing porosity in both the fillet edge and flange couch joints at different levels of power and laser welding speed. The developed surface preparation technology as a method for porosity reduction in laser welding has been successfully implemented in one of the largest UK/international car manufacturers. To study the laser cleaning process, a novel Smoothed particle hydrodynamics (SPH) meshless model has been implemented using a new 3-D multi-phase transient model. For the first time, a study was conducted to validate the temperature field distribution predicted in SPH method under nanosecond pulsed laser heating. The need for special surface treatment of the kernel truncation was also investigated. The proposed model accurately predicted the laser ablation depth and the crater shape and was validated using a significant number of experimental and numerical data reported in the literature. Moreover, a primitive laser welding model has been created to predict the material flow inside the welding pool. The research work has resulted in four publications in peer-reviewed journals. The research highlighted that future work should include the development of a more advanced SPH model for the prediction of porosity in laser welding and to fully describe the relationship between laser cleaning and porosity reduction in laser welding.

Le soudage par faisceau d’électrons d’alliages à durcissement structural de la série7xxx induit des modifications importantes de leur microstructure, et par conséquent de leur comportement mécanique. Cette étude visait à formuler une loi de comportement incluant l’endommagement de structures soudées par une approche locale de la rupture. Pour cela, la microstructure de la soudure et les propriétés mécaniques résultantes sont caractérisées. Le lien est fait entre la distribution de précipités fins et les propriétés plastiques, ainsi qu’entre la précipitation grossière et les micromécanismes d’endommagement. Le modèle aux éléments finis proposé prend en compte le comportement plastique des différentes zones et reproduit le comportement en endommagement de la zone fondue, qui constitue le maillon faible de la structure à cause de l’évaporation d’éléments d’alliages durant le soudage. Ce modèle a été validé dans une gamme étendue de sollicitations mécaniques (géométrie,entaille). / The electron beam welding of 7xxx series alloys leads to modification of their microstructureand consequently of their mechanical behavior. This study aimed toformulate a constitutive law including damage for welded structures by a local approachof fracture. For this purpose, the weld microstructure and the resulting mechanicalproperties have been characterized. The relationship between fine scaleprecipitate distribution and plastic properties has been established, as well as therelationship between coarse precipitation and micro-mechanisms of damage. Thefinite elements model takes into account the plastic behavior of the different zonesand reproduces the damage behavior of the fusion zone, which is the weakest linkof the structure due to evaporation of alloying elements during welding. This modelhas been validated in a large variety of stress conditions (sample geometry, notch).

When joining steel to aluminium there is a reaction between iron and aluminium which results in the formation of brittle intermetallic compounds (IMC). These compounds are usually the reason for the poor mechanical strength of the dissimilar metallic joints. The research on dissimilar metal joining is vast but is mainly focused on the automotive industry and therefore, the material in use is very thin, usually less than 1 mm. For materials with thicker sections the present solution is a transition joint made by explosion welding which permits joining of steel to aluminium and avoids the formation of IMCs. However, this solution brings additional costs and extra processing time to join the materials. The main goals of this project are to understand the mechanism of formation of the IMCs, control the formation of the IMCs, and understand their effects on the mechanical properties of the dissimilar Fe-Al joints during laser welding. Laser welding permits accurate and precise control of the welding thermal cycle and thereby the underpinning mechanism of IMC formation can be easily understood along with the factors which control the strength of the joints. The further goal of this project is to find an appropriate interlayer to restrict the Fe-Al reaction. The first stage of the work was focused on the formation and growth of the Fe-Al IMCs during laser welding. The understanding of how the processing conditions affect the IMC growth provides an opportunity to act and avoid its formation and thereby, to optimize the strength of the dissimilar metal joints. The results showed that even with a negligible amount of energy it was not possible to prevent the IMC formation which was composed of both Fe2Al5 and FeAl3 phases. The IMC growth increases exponentially with the applied specific point energy. However, for higher power densities the growth is more accentuated. The strength of the Fe-Al lap-joints was found to be not only dependent on the IMC layer thickness but also on the bonding area. In order to obtain sound joints it is necessary to achieve a balance between these two factors. The thermal model developed for the laser welding process in this joint configuration showed that for the same level of energy it is more efficient to use higher power densities than longer interaction iv times. Even though a thicker IMC layer is formed under this condition due to higher temperature there is also more melting of aluminium which creates a larger bonding area between the steel and the aluminium. The joint strength is thus improved ... [cont.].

Object of this dissertation work is increase of productivity capacity and labour productivity by welding process. This increase can be achieved by substitution of manual welding MIG and TIG for automatic welding line. So we can achieve the optimization of time schedule for the production of one piece and reduction of laboriousness of each production operation. The object of welding process are aluminium alloys - EN AW 6060 and EN AW 6063.