The development of welded structures has been identified by airframe manufacturers as potentially leading to lighter airframes and low-cost manufacturing. Weight and cost efficiency are obtained on the 'Integral Structure' or 'Rivet-Free' Al-alloy airframes through the use of advanced welding technologies, such as laser beam (LBW) and friction stir welding (FSW), and the introduction of new aluminium alloys with improved performances. A318 and A380 aircraft are already flying having had their fuselage panels manufactured with large distance LBW skin-stringer joints. There was a need to extend the current level of technology to 'more critical and difficult-to-join' sections of metallic airframes with the replacement of conventional riveted sub-sections with a short distance welded integral structure, which exhibits light-weight and damage-tolerant features.
Within this context, the main scientific and technological objectives of the project were:
- to optimise and validate the most suitable short distance laser beam welding process parameters for various Al-alloy combinations for the joining of stiffener/clip-skin connections of airframes by understanding and controlling the basic mechanisms of hot tearing, crack initiation and crack growth at the run-in/out location;
- to develop a short distance friction stir welding process for suitable joints and non-laser weldable alloys and gain knowledge about these new applications;
- to develop repair schemes of short distance welds and define allowable damages to minimise the maintenance and operational costs;
- to conduct systematic damage tolerance analysis on short distance welded coupons to establish the mechanisms of initiation and spread of the damage at or around the run-outs;
- to establish structural safety provisions for the case of ageing and corrosion damage (long-term behaviour/durability) by understanding the micro-mechanism/metallurgy of the damage at the short distance welds.
The WEL-AIR project provided:
- a complete database, which is related to the manufacture and performance of innovative and improved welding concepts for stiffener-clip-skin connection including new joint design, laser beam welding and friction stir welding procedures and a selection of new high performance light alloys for both stiffeners and skin;
- run-in/out control and repair procedures for both laser beam welding and friction stir welding;
- recommendations on optimum material conditions (temper and surface) prior to welding to optimise the post-welding behaviour;
- damage tolerance data and fundamental rules for the integration of new welding on aircraft sections that are more critical.
The technical approach of the project followed a 'develop, test, check, make recommendations, validate on components' pattern, split into five technical Work Packages. The first three Work Packages deal with the establishment of databases relative to potential techniques for improving the fatigue and damage tolerance behaviour of short distance welds, using both friction stir welding and laser beam welding for stiffener-skin connections.
The first Work Package aimed to develop laser beam and friction stir welding procedures for control of the run-ins and run-outs of the welds, and to propose some repair techniques for non-allowable welding defects. Firstly, an overview related to cracking occurrence in laser beam welding and industrial conditions, previously tested non-allowable welding defects, run-out control and repair procedures will be presented by aircraft manufacturers with a background in this field (AIRBUS, EADS). Taking this overview into account, various run-in/out control and repair procedures will be tested by the LBW welder partners (EADS, GKSS, ALENIA, Institut de Soudure) using various laser beam welding equipment (YAG technology, various powers, fibre diameter, focal length), clamping equipments and various filler wires. The validation of the improved procedures was tested in industrial conditions. Concerning friction stir welding, the retractable pin tool technique for control of the run-outs of the short distance stiffener-skin joints and potential repair technique of the FSW hole located at the end of the welds was developed (EADS CRC, Institut de Soudure, GKSS).
The second Work Package dealt with design aspects of the stiffener-skin connection and especially the evaluation of new generation aluminium alloys with improved mechanical performances (fatigue and damage tolerance) and/or better weldability or hot cracking resistance.
Damage tolerance (fatigue and fracture) and durability of the laser beam and friction stir welded joints for various welded configurations were the main topics of the third Work Package (including alloys, processes, joint configuration, thermal temper and surface treatment).
Data bases were obtained from the three first work packages and the most relevant welding concepts for improved stiffened structures were proposed.
The last two Work Packages dealt with concept validation on technological specimens, especially flat and curved specimens, with welded stringers only or welded bi-directional stiffening. In the fourth Work Packa
The WEL-AIR consortium systematically investigated the basic mechanisms of hot tearing control, crack initiation and crack growth at the vicinity of the run-in and run-out of skin-stringer welds as well as short distance clip-skin welds. Therefore, the WEL-AIR project provided new knowledge for:
- innovative fabrication and design concepts for clip-skin configurations;
- design or welding recommendations to avoid hot tearing;
- development of new joint configurations using Laser Beam and Friction Stir Welding processes;
- development of improved understanding of the damage tolerance behaviours (fatigue, residual strength and corrosion) of short distance welds.
The weldability of the new candidates using laser beam process has been investigated through three different approaches. First one aimed at developing and using an analytical approach based on the RDG (Rappaz- Drezet-Gremaud) hot tearing criterion in steady state condition. This analytical tool has been developed by the Computational Materials Laboratory. It allowed to classify the WEL-AIR alloys according to their hot cracking susceptibility (HCS) without filler wire, to study the influence of the filler material and its dilution on their HCS and finally to investigate the potential of hybrid welding cases with dissimilar material joining.
The main achievement of this new stringer end design is the elimination of overlapping of both welding starts (or ends) and highest stress concentration locations. Hence, circular shape of the stringer ends and continuous welding process aim to prevent possible solidification cracking and later easy initiation of the fatigue crack.
The design suggested using two welding processes:
- current double-sided LBW for welding of stringers by starting and finishing of the welding process at the beginning of the curvature;
- use of Nd:YAG process with robot arm to complete the welding of stringer ends as one-sided welding by starting and finishing of the welding at the stringer sites away from stringer ends.
For strengthening the competitiveness of the European aeronautical manufacturing industry, knowledge based development of high performance and innovative metallic light-weight airframe concepts are important. For this purpose, weight and cost efficiency will be reached with the development of the 'Integral Structure' or 'Rivet-Free' Al-alloy airframes through the use of advanced welding technologies such as laser beam welding (LBW) and friction stir welding (FSW) together with the introduction of new aluminium alloys with improved weldability.