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Friction stir welding UNiversal HEAD

Periodic Reporting for period 2 - FUN HEAD (Friction stir welding UNiversal HEAD)

Reporting period: 2020-04-01 to 2021-03-31

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. Indeed, FUN HEAD stands for FSW UNiversal HEAD.

This special head will reach the following 5 main objectives:
> Providing integration of the FSW head with most commercially available CNC machines (independently from machine control system).
> The head will be able to automatically adjust the retraction of the pin in and out of the tool shoulder.
> The developed head will integrate a constant welding force control system following the FSW tool axis.
> The proposed solution will have the capacity of butt and lap joining of sheet metal plate having thicknesses of 0.4mm to 3.0 mm.
> The developed head will allow welding of flat and concave-convex geometries (R > 200 mm).

The FSW head developed in FUN HEAD project reduces the 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 for FSW

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 on which the FSW head tests will be performed:
- Analysis of tools currently available related to an adjustable pin and a stationary shoulder configuration,
- Analysis of dedicated CNC machines and industrial robots currently available for FSW technology,
- Analysis of the use of conventional numeric machine tools,
- Analysis of necessary developments to conduct FSW process on the selected CNC machine.

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

Preliminary tests have been performed on a robotic FSW workstation. 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 1 to 3 mm.
Visual observation of welds quality allows to select some parameters sets for microstructural and strength tests.

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

The first results show that best mechanical properties are reached for the greatest thicknesses of the range with a tensile strength of 93% of the base metal.

The 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).

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.

5) The first performance tests of the retractable pin have been carried out. For example, the first exit hole closure was performed.

6) A change in the description of the work leads to the implementation of temperature measurement on the FSW head.
Progress beyond the state of the art:

The FSW head as designed integrates now 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 has to be compact to avoid collision between FSW head and the workpiece or the tool holder. Moreover, the head has been designed to allow 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.

Expected results:

The several performance tests of the FSW head must demonstrate that the 5 objectives of the project are reached even if the success of welding the thinnest thicknesses seems difficult to achieve.

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
Example of CNC machine
First stirring lines tests
Optical microscopies: identification of weld zones
FUN HEAD Project Logo
First exit hole closing out test
FSW process Explanation
Optical microscopies: fracture surfaces of a weld