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Development of welding technologies for light alloys aircraft structures

Final Report Summary - LIGHTWELD (Development of welding technologies for light alloys aircraft structures)

Executive Summary:
Reduction of air transport contribution to climate change is nowadays one of the most challenging objectives for aircraft manufacturers. Weight reduction, manufacture and inspection process improvement are one of the main keys in order to achieve it. Nowadays, riveting is the state of the art of joining technology for aircraft structures. The necessary overlap joint demands a large amount of extra material which restricts weight saving requirements. Furthermore, the riveting process is quite intensive in process time regarding the production chain compared to welding processes.
A considerable lightening is obtained employing newly developed Al-Li and Mg alloys and eliminating the rivets and the extra material needed to ensure a sufficient overlap in the mechanical joint. By means of welded structures it is possible to eliminate the rivets, increase productivity (short production time) and minimizing cost management. Due to those considerations, welded integral structures could be an attractive alternative to usual riveted structures in order to reduce weight and cost.
Within the LIGHTWELD project, all the efforts have been focused in developing welding technologies for Al-Li and Mg alloys. Process parameters have been optimized for laser beam, resistance and friction stir spot welding, in order to ensure joint soundness, achieving a load capability for welded joints, comparable to the reference riveted ones. Laser and friction stir spot technologies have been used on the fabrication of a full-scale curved fuselage demonstrator to joint different Al-Li and Mg components, which result on a component weight reduction up to 10% (estimation) due to rivet removal, lower joint overlap material and lower density of the alloy selected for it. It must be mentioned the operation cost reduction that could be achieved employing laser and friction stir welding technologies, both of them with a high degree of automation, and eliminating manufacturing steps, as the rivet hole drilling.

Project Context and Objectives:
The LIGHTWELD project aims at developing and adapting LBW, FSSW and RSW for newly developed Al, Al-Li and Mg alloys, more weldable, with the objective of:

• Reducing weight through replacing the use of vast amount of rivets
• Enabling the automation of joining and NDT processes for decreasing process time

For this, the development and adaptation of joining technologies for newly developed light alloys is necessary. The proof of concept is validated through the implementation in a part of a panel demonstrator, considering different materials combinations (dissimilar joints) and focusing the study in the following welding technologies:

• Laser Beam Welding (LBW)
• Resistance Spot Welding (RSW)
• Friction Stir Spot Welding (FSSW)

However, the objective of the LIGHTWELD project was not only to achieve the aimed goals (manufacturing of light alloy panel demonstrator and comparative study with riveting joint technology), but also to identify the critical factors that could give rise to a further manufacturing cost reduction, weight reduction and mechanical and corrosion performance improvement. Some of these factors are:

1. Working with newly developed Al-Li lightweight alloys and Mg alloys in order to obtain appropriate joints by means of welding technologies. Simultaneously, utilization of the proposed welding technologies will provide the possibility to integrate and simplify the structure and reduce the weight.
2. Optimization of laser beam and resistance spot welding processes in order to obtain crack free joints with no (or low) porosity resulting in high mechanical performance of the welded joint.
3. Corrosion resistance improvement with new joint design, sealant selection and welding technologies.
4. Dissimilar welding of Al-Li/Mg alloys by fusion or other suitable joining welding technologies.
5. Automation of joining processes and NDT technologies to achieve a reduction of production time, operation cost saving and quality control costs.
6. Analysis of needs / requirements for further industrialisation of developed joining processes, with the aim of achieving production time reduction, operation costs saving and quality control costs.

The following specific goals are expected after the development and implementation of welding technologies as alternative to conventional riveting process of metallic aero-structures:

A. Technical:
1. Reduction of component weight between 5 and 10%. The weight reduction is due to lower density alloys, reduced mass of sealing, and simplified stringer and partial reduction of rivets weight.
2. Reduction of operation cost until 20%. The operation cost saving is due to reduced mass of material, high degree of automation and reduced manufacturing steps.
3. Corrosion resistance improvement by means of new joint design, welding technology selection and sealant selection when it is necessary.
4. Fabrication cost saving compared to aero-structures made of composite materials.
5. NDT inspection time saving due to the automation of inspection process for laser beam welded joints and also for spot welds.
6. Optimization of laser beam and resistance spot welding processes in order to obtain crack free joints with no (or low) porosity resulting in high mechanical performance of the welded joint.

B. Scientific:
1. Knowledge generation in the field of dissimilar joining of Al-Li and Mg alloys: process parameters, welding metallurgy and interface strengthen control.
2. Process development for LB, RS and FS welding of Al-Li and Mg alloys.
3. Characterization of mechanical behaviour and corrosion characteristics of Al-Li/Al-Li similar and dissimilar joints and Al-Li/Mg dissimilar welds.

C. Environmental:
1. A 15% CO2 emission reduction and a 10% fuel saving due to the reduction of component weight.
2. Facilitate structure recycling compared to aero-structures made of composite materials.
3. Reduce environmental impact of the manufacture and inspection processes increasing the productivity more than 50% compared to manual riveting.

Project Results:
1. Development of welding parameters for LBW, RSW, FSSW and Delta Spot. Al-Li Weld characterization:

Welding process parameters have been optimized for FSSW, RSW and Delta Spot technologies for AA2198 Al-Li alloy with the purpose of obtaining sound welds:

a) For FSSW, the best welding properties has been achieved with a specific theaded triflute pin welding tool, with a diameter of 15 mm and length of the pin of 2 mm, and for 1000rpm rotational speed, 8kN force and 8s Dwell time.
b) In the case of RSW, different welding process iterations have been carried out in order to optimize the resistance spot welding process.
c) For Delta Spot welding technology, taking into account the different properties measured, metallographic characteristics and shear strength, the best results have been achieved on series 15 and 16 of the second round.
d) For the case of laser welding, the aim is to laser weld Al-Li aluminium alloys (AA2198-AA2050 and AA2099), 2 mm thickness sheets in a fillet configuration by a Disk Laser 6 kW power in a 400 microns core fibre connected to a focusing head of 200/200 mm, collimating / focusing lens, with a focus spot size of 400µ. The feed material is 1 mm diameters wire of AlSi5 alloy. It has concluded in LIGHTWELD project, that one of the most influence variables of the process is the surface preparation of the joint. Taking into account this phenomenon, LORTEK has carried out welding trials removing 0.3 mm from both sides of the stringer sheet and 0.3 mm from the skin sheet. Transversal metallographic inspection for LBW welded sample with 0.3 mm machining in all surfaces in contact with weld seam show no relevant porosity.

In the present project, for dissimilar AA2198-Mg WE43 material combination none of the samples welded in different trials, with the different welding technologies analysed, present a sound weld. It has been selected an alternative joining technology for this combination, adhesive bonding.

2. NDT inspection of Al-Li welded joints:

NDT inspection (surface and volumetric) of joints has been done in order to evaluate the soundness of the weld performed by different technologies. A new inspection technique, named active thermography, has been applied to “spot like” welded specimens. By means of this technique, spot welded samples are thermally excited using a heat or cold source (laser, flash, halogen lamps, hot air gun,...) and the thermal response of the material is analysed. After developing different trials, the obtained results show that the thermography can be a feasible technique to detect the welded areas but in most of the cases neither pores nor internal defects are detected due to the aluminium high thermal conductivity.

For continuous LBW samples, despite performing trials with different NDT techniques (Eddy current, FPI,...) X ray inspection (conventional) seems to be the most suitable to detect internal porosity trough weld bead in those specimens.

3. Static mechanical characterization for Al-Li LBW, RSW, FSSW and Delta Spot joints. Comparison with reference riveted one:

Static mechanical testing has been performed to provide additional information for the evaluation of welded samples. Different load cases and specimen geometry for the different technologies to evaluate have been defined. The mechanical test has been performed according to the following standards: “ASTM E8 / E8M - 11 - Standard Test Methods for Tension Testing of Metallic Materials” and “ISO 14273: 2000 – Specimen dimensions and procedure for shear testing resistance spot, seam and embossed projection welds”

Amongst the "spot like” welding technologies and riveted samples for similar AA2198-AA2198 joint, FSSW welded sample is the most resistant one. Taking into account the different joined area for each one of technologies, it has been defined a new parameter, denominated effective area, for doing a comparison in the same conditions with the objective to define an equivalent stress. Besides, it is straightforward that whereas Rivet joints are discrete joints presenting discontinuity in the force transmission, “continuous” type welds, such as LBW, the force transfer is smoother as the discontinuity does not exist. This fact implies that the corresponding test results are not fully comparable.

4. Corrosion tests:

Three types of corrosion tests have been performed: Salt Spray Test (SST) according to “ASTM B117-11: Standard Practice for Operating Salt Spray (Fog) Apparatus”, Humidity test (HT) performed according to “ASTM D 2247: Standard practice for testing water resistance of coatings at 100% relative humidity” and Galvanic Test (GT) tested according to “ASTM G71. Standard Guide for Conducting and Evaluating Galvanic Corrosion Tests in Electrolytes”.

The anodizing process causes mass loss in all the samples, being the high porosity in the grown oxide coating the predominant reason of the weight loss in the samples. On the contrary, almost all the samples have gained some weight during the corrosion tests. The exceptions are adhesively bonded and riveted samples during Galvanic corrosion tests which present highly degraded Mg plates, therefore, losing weight.

The results show that most HT samples gain less weight (negative mass variation) than the riveted samples, with exception of adhesively bonded ones. The values for the samples in the group of SST corrosion test behave more randomly, and present a larger scattering, especially FSSW samples

5. Fatigue tests:

The fatigue tests have been done according to the standard ASTM E466-07 as far as possible, including at least, 4 load conditions and 6 samples per load condition, with a total of 175 samples.
In order to compare the fatigue behaviour, the average values of the number of cycles under the same load conditions have been plotted, for different analysed load conditions in all joining technologies. It is important to note that the tested load conditions are different for each technology (because these load conditions are based on the UTS values of each technologies), but it is possible to make a comparison based on the shape of the curves. This comparison has been made just for spot-like joining technologies, leaving aside the LBW samples (different sample geometry, making the results not comparable).

It can be concluded that, in general the RSW samples have endured more than FSSW samples, showing a better fatigue behaviour. But anyway, it is important also to keep in mind that in both welding technologies the nature of the joining area is not the same (Hooking effect in FSSW).

6. Plan for panel demonstrator manufacturing:

The joining technologies selected for panel demonstrator manufacturing have been the following ones:

• LBW for extrusion longerons (AA2099), machined frames (AA2050) and formed frames (AA2198) to the skin (AA2198).
• FSSW for sheet metal intercostals (AA2198) to the skin (AA2198).
• Adhesive bonding for sheet metal intercostals (Mg WE43) to the skin (AA2198) and for shear clips (Mg WE43).

The panel demonstrator geometry and the different components considered for manufacturing it during demonstrator activities have been selected. Some of the sub-components need to be modified in order to be joined by selected welding technologies. Taking into account the jig requirements and accessibility problems between the welding tools of the selected technologies, a welding sequence has been defined. Welding jig design has been performed taking into account different components characteristics and selected welding process requirements.

Once welding trials start, some local distortions have been observed just under the frames. In order to avoid this phenomenon, the welding jig has been modified with reinforcement plates just under the frames.

Panel demonstrator is a real scale part from an airplane fuselage. The assembly process, considering non-conventional joining technologies, like laser and FSS welding, involves big technological challenges due to size and geometry differences from initial sample testing. In order to implement at industrial scale the developments carried out at coupon level for laser and FSS welding processes some modifications have done in laser and friction stir workshops.

Friction Stir Spot welding technology will be applied to the panel demonstrator components in a KUKA robotized arm. A tool holder has been designed (see Figure 10), taking into account the accessibility problems, the geometry of the components and the requirements of the spindle of the KUKA robotic arm.

7. LIGHTWELD project demonstrator manufacturing:

A KUKA robotic arm equipped with a FSW spindle has been used to perform the FSSW joints between the sheet metal intercostals (AA2198) and the skin (AA2198). A flexible clamping jig system mounted on a flat table and a specifically designed FSSW tool and tool-holder have been used. Each sheet metal intercostal has been joined by 10 FSSW welds.

There has no happened any remarkable incidence during FSSW operations for panel demonstrator.

A laser welding head integrating a laser source guided by optical fibre, a wire feeding system and a shielding gas supply system mounted in a FANUC robotic arm has been used to perform all LBW operations involved in the manufacturing of the panel demonstrator.

There happens different type of incidence or problems during laser welding of panel demonstrator components:

• Zones not welded due to laser head accessibility problems. The laser welding head dimensions, and the selected welding sequence, are the cause for these accessibility problems. FSS welded components difficult the access of the laser welding head to some zones to weld. The solution would be to perform the FSS welding of the intercostal components, after finishing all the laser welding sequence.
• Lack of welding due to a gap generation between the components due to process distortions. The solution to this problem has been to modify the welding jig to avoid relative motions.

A SIKADUR-33 2-part structural epoxy adhesive has been applied manually to join the sheet metal intercostals (Mg WE43) and the skin (AA2198) as well as all the shear clips. A total number of 28 shear clips have been implemented, one in each intersection between stringers present at the panel demonstrator.

8. Towards Industrialisation – Improvement Plan:

An improvement plan has been elaborated thinking on further process industrialisation, including different kind of suggestions. The aim of the improvement plan is to define or identify further suggestions to industrialise the joining and welding processes performed for LIGHTWELD panel demonstrator (in a lab scale), for project results application. The improvement plan has focused in the following topics:

• list of relevant variables having a clear impact on productivity;
• which are the best commercial available welding equipment (depending on the chosen technology), and if applicable, alternatives to them;
• the best ways to automate the process: robotic solutions, welding gantries, etc.;
• the implementation of on-line control systems to ensure “zero defects policy”;
• process evaluation through best-of-breed NDT (automated if possible);
• welding process standardisation: requirements, specifications to consider,...;
• needed training and required experience for technician and facilities operators;
• environmental aspects: energy efficiency, material savings, wastes production, etc. → Life cycle demonstration: Comparison of obtained results with information from other projects;
• work safety aspects to consider before process industrialisation.

Potential Impact:
Industrial impact:
The LIGHTWELD project has determined the weldability of newly developed light metallic alloys, Al-Li alloys, with different joining technologies, LBW, FSSW and RSW, making possible to define methods for laser and friction stir spot welding of similar and dissimilar Al-Li alloys. These methods include inspection and defect diagnosis of non-conventional welds using NDT’s, making it cost efficient and feasible for industry manufacturers of different markets. Those welding methods have been implemented in the manufacturing of a complex part (fuselage panel) which results in the definition of an improvement plan for future industrialisation considering aspects like, productivity, on-line control system, process standardisation, experience, work safety aspects, etc. The welding methods have been developed based on an aeronautic application but it would be also possible to customize for any other use case where lightening aspects are important, like in railway, automotive and space. All the entities related to those sectors could improve their manufacturing process using the knowledge generated on LIGHTWELD project, what would mean a huge industrial impact for European competitiveness, considering also their suppliers and value-chain (driving or boosting force).This will make a new business available for the studied alloys, but mainly it will open new manufacturing possibilities for European industry.

Environmental impact:

Principal improvement will be the raw material amount decrease. If the demonstrator panel is joined using welding processes, instead of a riveting process, a lot of material will not be needed.
Considering the total quantity of stringers in an airplane, total material saved is relevant not only by the direct material saving (cost) but because of the consequences in the airplane’s weight, fuel saving and consequent NOx and CO2 decrease.

Socio-economic impact:

Another consequence, not so linked with European competitiveness but nevertheless with huge relevance, is that LIGHTWELD outputs are expected to have essential societal impact. More than 3.0 million people are employed in the European Aircraft and Airlines industry, thus strengthening the competitiveness of this industry and related SME’s through the development of state-of-the-art technologies is vital to Europe’s economic future:

1. Due to saves and increased safety in manufacturing process and fuel consumption as a consequence of the replacement of several functionalities by a unique lighter multifunctional systems.
2. Technologies and tools that will result from this project are essential to increase the European market from the current level in the next 10 years, and they will provide opportunities for the employment of highly skilled professionals. This would contribute in solving of heavy societal
problems interconnected with the high unemployment in Europe derived from the economic crisis.
3. Jobs will primarily be created at subcontractors and suppliers, due to additional constituents of the material used, and therefore suppliers will need to increase production.
4. The new technology has broad potential applications in many other industries (automotive and general transportation, etc.) creating opportunities for further employment.
Summarizing, a successful LIGHTWELD project will ensure a strong strategic impact and will have clear Socio-Economic benefits within the next five to ten years by contributing to:

# Enhance European aeronautic industry competitiveness
# Enhance European employment
# Meet societal needs for more environmental friendly, safer and efficient air transport.
# Meet societal needs for more environmental friendly, safer and efficient manufacturing

Dissemination activities and exploitation of results:

The overall objective of the LIGHTWELD project was not only to manufacture light alloy panel demonstrator and comparative study with riveting joint technology, but also to identify the critical factors that could give rise to a further manufacturing cost reduction, weight reduction and mechanical and corrosion performance improvement.

LIGHTWELD project has not been until now too active disseminating the results, because it has been carried out in just 23 months, (after two extensions of 4 and 3 months, in total 7 months) and the time has been spent performing research and development activities. Nevertheless, after completion of project some dissemination activities are programmed, presenting the project and the results / developments on various conferences and scientific journals including at least one publication in a peer-reviewed journal, to be submitted and published in 2016.

Apart from that, a fiche of LIGHTWELD project has been uploaded in LORTEK’s website with similar information than the one included in CORDIS.

Potential results with exploitation possibility have been identified both in terms of general advancements in knowledge and results that can be commercially exploitable. In the latter group, we identify the following key exploitable results:

• Method for laser welding Al-Li dissimilar components
• Method for welding similar Al-Li by means of FSSW
• Method for welding similar Al-Li by means of robotic FSSW
• Method for spot welding (by RSW) Al-Li components

List of Websites:
Website address:
http://cordis.europa.eu/project/rcn/110757_es.html

Contact details:
Dr. Maria San Sebastian (Project coordinator)
Principal researcher at IK4-Lortek
Email: msansebastian@lortek.es
Tel: +34 943882303