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Friction stir welding UNiversal HEAD

Periodic Reporting for period 3 - FUN HEAD (Friction stir welding UNiversal HEAD)

Reporting period: 2021-04-01 to 2021-10-31

Friction Stir Welding (FSW) is a solid-state joining process with significant advantages over conventional fusion welding methods or classic assembly methods such as fasteners joining. The implementation of FSW process seems to be particularly suitable for welding aero-structures of high strength aluminium alloys because FSW assembly maintains the excellent properties in the weld seams. Compared to conventional riveting, the use of FSW technology can reduce or eliminate fuselage overlap areas and rivets mass, leading to substantial weight reduction. Weight savings are a major issue in aeronautics and contribute to reduce the environmental footprint of aviation.

The aim of FUN HEAD project is to design and develop a universal head for Friction Stir Welding (FSW) of aluminium alloys in particular series 2xxx and 7xxx that should fit existing Computer Numerical Control (CNC) machines. FUN HEAD stands for FSW UNiversal HEAD.

This special head reaches the following 5 main objectives:
> Providing integration of the FSW head with most commercially available CNC machines (independently from machine control system).
> The head is able to automatically adjust the retraction of the pin in and out of the tool shoulder.
> The developed head integrates a constant welding force control system following the FSW tool axis.
> The proposed solution has the capacity of butt and lap joining of sheet metal plate having thicknesses of 0.5 mm to 3.0 mm.
> Welding of flat and concave-convex geometries (R > 200 mm).
> The head includes a temperature measurement tool. This objective is a consortium add on to strengthen the project ambitions.

The FSW head developed in FUN HEAD project reduces investment costs of FSW technology with a performance on par with specialised FSW machines or robotic workstations. The advantages include higher quality welds (mechanical and fatigue properties, corrosion resistance), absence of welding consumables, low energy costs, environment-friendly, no joint preparation or post treatment because of the low distortion and shrinkage.
1) Analysis of current solutions

Several analyses have been carried out in order to make a state of the art, know about the tools and machines currently available on the market and intended for FSW process and eventually select the CNC machine:
- Analysis of tools available related to an adjustable pin and a stationary shoulder configuration,
- Analysis of dedicated CNC machines and industrial robots available for FSW.

2) The design of special welding head for FSW process compatible with CNC machines whose dimensions are sufficient to fit the head in and whose nozzle has the necessary surfaces to be interfaced with it has been completed. The head as designed allows automatic adjustment of the pin, force control system modifications and concave-convex geometries welds.

3) The manufacture of the special welding head is achieved and set up on a 5-axis CNC machine.

4) Optimization of FSW process parameters

FSW Process parameters (welding tools, forging force, advance and rotation speeds...) have been determined on butt and lap joint welds of 2024 T3 and 7075 T6 aluminium alloys with thicknesses ranging from 0.5 to 3 mm.

Weld qualification tests have been performed such as:
- Tensile tests
- Microstructural analyses
- Microhardness measurement

The results show that mechanical properties reached for 2024-T3 welds with thicknesses of 0.5 to 3 mm are between 75% and 95% of the base metal. For 7075-T6 alloy welds, a change of behaviour with the base metal was observed. Yet, 7075-T6 welds remain ductile with mechanical properties of 70% of base metal.

Microstructural observations reveal the failure through the weld and allow to identify different parts of the weld such as nugget, heat affected zone (HAZ) and thermo-mechanical affected zone (TMAZ). A scientific paper had been submitted on the subject.

At the same time, numerical simulations of the temperature during the welding process based on the FSW parameters were performed to help find optimized process parameters. Numerical calculations of the Heat Flux and temperature with different FSW parameters were carried out on 2024-T3 alloy sheet with 1 & 3 mm thicknesses. Results of numerical approach according to process parameters were presented in a FSW comission.

5) Performance tests of the retractable pin and force control have been carried out. For example, exit hole closure was performed.

6) A temperature measurement tool has been added on the FSW head. Tests of the tool and a comparison of the results with the thermal numerical model have been realized.

7) Final tests on concave and convex welds have been performed.

Capacities of this FSW head will increase joint design possibilities and better weld quality for applications such as cold plates or stiffened panels.
The FSW head includes a retractable pin system and the capacity of welding of concave and convex geometries. To reach these objectives the FSW head has to be adapted on a 5-axis CNC machine leading to several challenges:
- The head is compact to avoid collision between FSW head and the workpiece or the tool holder. Moreover, the head allows to weld short radius convex geometries especially.
- To be compatible with the torque limit of rotational axis CNC machine the weight has to be limited to 30 kg.
- The height of the FSW head has to be limited to 315 mm to be compatible with the CNC machine size because most of the CNC machines have a Z capacity of 500-600 mm, rarely higher.

Results:

The performance tests of the FSW head demonstrate that the 5 objectives of the project are reached.

Potential impacts:

- ECONOMIC IMPROVEMENTS & COMPETITIVENESS:
• Reduction of manufacturing and assembly costs
• Possibility of higher automation of joining process
• Reduction of investment for FSW technology
• Optimization of FSW technologies for structural parts
• Integral structure concept

- IMPROVING INNOVATION CAPACITY &INTEGRATION OF NEW KNOWLEDGE:
• Universal head means an easier access to FSW technology => new possibilities for design and assembly
• Providing performance tables and better knowledge of FSW techniques
• Facilitate the implementation of FSW process in aeronautic industries

- ENVIRONMENTAL IMPACTS => GREENER AVIATION:
• Weight saving & drag coefficient reduction => Fuel consumption reduction
• Raw material saving
• Material recyclability
• Clean & eco-friendly process => FSW
Set-up of the special FSW head on AVANTIS CNC machine
Automatic adjustable pin & force control system tests
Optimization of FSW process parameters & temperature measurements
Example of CNC machine
Types of weld geometries & sheets thicknesses
Stirring lines tests
Welds characterization & qualification tests
FUN HEAD Project Logo
FSW process Explanation