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Low force mobile friction stir welding system for on-site marine fabrication

Final Report Summary - MOBI-WELD (Low force mobile friction stir welding system for on-site marine fabrication)

Executive Summary:
Aluminium is used extensively in shipbuilding worldwide. It is an ideal metal being light, strong, and corrosion-resistant. Aluminium vessels do not require such extensive care as steel vessels, which has an impact on the cost of their maintenance. Sports vessels are made entirely of aluminium which provides a significant gain in speed. Hulls of higher-capacity vessels are made of steel, while superstructures and other auxiliary equipment are made of aluminium alloys, reducing the total weight of the vessel and thereby increasing its load carrying capacity. The use of aluminium in shipbuilding is therefore growing. Friction stir welding is ideally suited for joining aluminium alloys, however, due to limited machine flexibility FSW is currently only used for pre-fabrication of a relatively small range of simple shape parts and flat sheets/panels.

The Mobi-Weld project has developed a prototype, mobile Friction Stir Welding (FSW) system, which was designed and constructed for use in final fabrication/assembly of aluminium panels in a shipyard. Fundamental to the success of the project is an assessment of industrial requirements. Potential applications for the Mobi-Weld system and associated technical requirements were discussed and a questionnaire prepared to capture the relevant information to guide the development of the welding system. Information obtained from shipyards and the wider shipbuilding industry has guided the specification of the prototype.

The Mobi-Weld system includes modular construction to create the possibility of further exploitation in other industry sectors. A number of FSW systems have previously been developed for use in shipyards however these are reported to be transportable, not mobile. Thus, the Mobi-Weld project aimed to create a new mobile FSW machine, offering increased flexibility (suitable for final fabrication activities) and reducing the capital investment by an order of magnitude. This provides an alternative to most on-site fusion welding currently used in ship building, speed-up ship production through increased welding rates through eliminating the costs associated with production steps such as distortion control. Currently there is no commercially available system with the developed Mobi-Weld system capabilities for in- situ assembly in the shipyard environment.

A crawler system concept was developed based on the outcome of a requirements capture exercise. This exercise has considered the requirements for the major prototype sub-systems; welding head motion and control, crawler motion & control, device attachment/gripping methods and the user interface. This work has been supported by laboratory trials, numerical simulation (MATLAB) and 3D-CAD (Inventory & Simscape). This approach facilitates the production of simulation videos enabling visualisation of the physical device, early identification of conflicting requirements and assessment of critical components. This approach will continue to support the detailed design and build of the prototype welding machine.

The project demonstrated that 5083 aluminium with thickness of 6mm can be welded with a mobile, lightweight, self-moving FSW device. Two 3m long aluminium plates were welded together with a welding speed 240mm/m. The welding was done automatically according to a predefined welding profile, with the device automatically following the seam. The welding parameters developed in the laboratory did not take account of the different clamping situation and heat flux distribution between lab and on site condition. This resulted in unacceptable weld quality and distortion levels. It is expected that the weld quality issue these issues will be resolved through a short program of welding trials which will be undertaken by the SME group after the end of the project. More in-depth investigations are required to obtain an appropriate workholding solution which provides adequate support for large plates. The main technical features of the prototype are also suitable for a production version of the Mobi-Weld device.

The development of the Mobi-Weld system has required the creation and integration of a number of technical sub-systems. Reduction of process forces is one of the key requirements allowing the mobile application of FSW welding. A key integration challenge has been developing a mobile FSW system incorporating a crawler system, capable of resisting the welding forces and synchronising of the forward movement of the crawler as it repositions with the movement of the welding head within its cradle. To weld effectively, the FSW tool must advance at a uniform rate along the weld line and thus the crawler carrying the welding head must be capable of precise, smooth, relative movement in order to facilitate this.

Project Context and Objectives:
Aluminium is used extensively in shipbuilding worldwide. It is an ideal metal being light, strong, and corrosion-resistant. Aluminium vessels do not require such extensive care as steel vessels, which has an impact on the cost of their maintenance. Sports vessels are made entirely of aluminium which provides a significant gain in speed. Hulls of higher-capacity vessels are made of steel, while superstructures and other auxiliary equipment are made of aluminium alloys, reducing the total weight of the vessel and thereby increasing its load carrying capacity. The use of aluminium in shipbuilding is therefore growing. Friction stir welding is ideally suited for joining aluminium alloys, however, due to limited machine flexibility FSW is currently only used for pre-fabrication of a relatively small range of simple shape parts and flat sheets/panels.

The Mobi-Weld project has developed a prototype, mobile Friction Stir Welding (FSW) system for use in final fabrication/assembly in a shipyard. The Mobi-Weld system includes modular construction to create the possibility of further exploitation in other industry sectors. A number of FSW systems have previously been developed for use in shipyards however these are reported to be transportable, not mobile. Thus, the Mobi-Weld project aimed to create a new mobile FSW machine, offering increased flexibility (suitable for final fabrication activities) and reducing the capital investment by an order of magnitude. This provides an alternative to most on-site fusion welding currently used in ship building, speed-up ship production through increased welding rates through eliminating the costs associated with production steps such as distortion control. Currently there is no commercially available system with the developed Mobi-Weld system capabilities for in- situ assembly in the shipyard environment.

The Mobi-Weld system enables the joining and fabrication of large aluminium alloy panels and sections forming parts of superstructure modules in a shipyard or fabrication facility. The Mobi-Weld system is formatted such that it can be transported to, and around, the shipyard environment. To implement the system, attaining high performance in the areas of positional control, structural stiffness and seam/surface tracking is of paramount importance. The developed system consists predominantly of a welding head mounted in a cradle incorporating drive, control and measurement systems. This system is mounted on a crawler system which has sufficient structural stiffness to react the process forces and maintain the position of the FSW tool, mounted in the welding head, to the required accuracy. Once positioned on the panel or module on which the welds will be made, on board seam tracking and height sensors are used to define the absolute weld path relative to the joint line and material surface.

The development of the Mobi-Weld system has required the creation and integration of a number of technical sub-systems. Reduction of process forces is one of the key requirements allowing the mobile application of FSW welding. A key integration challenge has been developing a mobile FSW system incorporating a crawler system, capable of resisting the welding forces and synchronising of the forward movement of the crawler as it repositions with the movement of the welding head within its cradle. To weld effectively, the FSW tool must advance at a uniform rate along the weld line and thus the crawler carrying the welding head must be capable of precise, smooth, relative movement in order to facilitate this. The objectives of the Mobi-Weld project were to make a number of technological advances combined into a prototype mobile welding system capable of making a 3 metre long weld in a shipyard environment at typical industrial welding speeds (rate ≥400mm/min); with weld quality to ISO 25239 standard.

Project Results:
Fundamental to the success of the project is an assessment of industrial requirements. Potential applications for the Mobi-Weld system and associated technical requirements were discussed and a questionnaire prepared to capture the relevant information to guide the development of the welding system. Information obtained from shipyards and the wider shipbuilding industry has guided the specification of the prototype.

FSW tool designs and welding parameters have been developed for 4mm and 6mm thickness AA5083-H111 and AA6082-T6 aluminium alloys. Welding procedures, including process start and stop operations, were optimised to minimise welding force levels during peak and steady-state welding conditions while ensuring industrially viable weld rates. Trials were also undertaken to assess whether the forces generated by the vertical movement of the tool during welding had any significant impact on weld geometry and quality. Weld quality was assessed in accordance with ISO25239 Part 4.

Information from the welding trials has determined the specification of the welding head. It has proved challenging to balance the welding process requirements (low rotational speed, high torque and moderate radial loads) with the size and weight constraints of a mobile device. To meet this need some custom elements have been implemented to maximise design flexibility. A test bench is being constructed to support testing of the welding head and integration with the device control system and other prototype elements.

A crawler system concept has been developed based on the outcome of a requirements capture exercise. This exercise has considered the requirements for the major prototype sub-systems; welding head motion and control, crawler motion & control, device attachment/gripping methods and the user interface. This work has been supported by laboratory trials, numerical simulation (MATLAB) and 3D-CAD (Inventory & Simscape). This approach facilitates the production of simulation videos enabling visualisation of the physical device, early identification of conflicting requirements and assessment of critical components. This approach will continue to support the detailed design and build of the prototype welding machine.

A customized welding head was designed and commissioned that is able to hold the developed floating-bobbin FSW tools. The head specification was based on the steady state welding forces established in the floating-bobbin FSW tool development stage. There was no deviation of the measured forces on site compared with the lab trials.

Different types of sensors were required to find the weld joint line and also the component surface, e. g. positioning or seam tracking sensors. A new heading adjustment routine for holding the system on the seam track was developed to compensate the shift and heading offset caused by the tool rotation/forces.

The crawler system is attached to the plates by vacuum cups, which were placed on 4 “skis”. By releasing the vacuum cups on a pair of skis and sliding them forward, the crawler is able to “walk” and also copes with the process forces and torques. Ejectors with air saving functionality reduce the compressed air consumption and also significantly decrease noise. This approach was demonstrated successfully.

A lightweight cradle, an integral part of the crawler upon which the welding head (FSW spindle) is mounted, was designed to be sufficiently stiff in order to react all moment welding loads and tool torque generated by the FSW process. The cradle incorporates X and Y axes movement, Z-axis movement is not required for floating-bobbin tool FSW. Forward movement (feed) of the FSW tool is done according to a predefined welding profile. Constant feed rate is achieved by co-ordinated movement of the cradle X motion and crawler forward movement. The position of the FSW tool relative to the seam is maintained through y-axis movement controlled via a feedback system from seam tracking sensors located on the cradle.

The Mobi-Weld project has developed a prototype, mobile Friction Stir Welding (FSW) system, which was designed and constructed for use in final fabrication/assembly of aluminium panels in a shipyard. The project demonstrated that 5083 aluminium with thickness of 6mm can be welded with a mobile, lightweight, self-moving FSW device. Two 3m long aluminium plates were welded together with a welding speed 240mm/m. The welding was done automatically according to a predefined welding profile, with the device automatically following the seam. The main technical features of the prototype are also suitable for a production version of the Mobi-Weld device.

The suite of floating-bobbin FSW tools developed to weld the materials and thicknesses of interest were first tested on gantry machines in the labs of TWI and VTT. The welded plates had outstanding welding quality and the distortion of the plates was very low. However the different clamping situation and heat flux distribution between lab (WP1) and on site condition (WP6) were not known during initial process development thus not taken into account. This resulted in unacceptable weld quality and distortion levels. It is expected that the weld quality issue these issues will be resolved through a short program of welding trials which will be undertaken by the SME group after the end of the project. More in-depth investigations are required to obtain an appropriate workholding solution which provides adequate support for large plates.

Potential Impact:
The Mobi-Weld project has delivered a novel flexible FSW machine that allows on-site joining of aluminium alloy sheet and panel sections within shipyards. The main objective is to provide a competitive edge to European shipyards and this will be achieved by facilitating their faster adoption of aluminium which in turn will be giving them the means to create higher performance (and added value) ships and thus grow their world-wide market share to the benefit of Europe.

The use of aluminium in shipbuilding has reached significant levels in the last 10 years, with a reported 12.4 million tonnes being used to deliver over 30 major vessels (~5% of 595 total ships manufactured) in 2006 with a total value of more than 16 billion Euros. The lightweight, superior mechanical properties and corrosion resistance of aluminium alloys present significant benefits to ship owners. However, despite the potential benefits, the use of FSW at the dockside is unfeasible due to the lack of availability of any mobile (or cost-effective) FSW system. Hence, the majority of dockside parts are fabricated and assembled using traditional Arc welding.

The Mobi-Weld project has developed a novel, mobile and low capital cost FSW system using a new low force FSW tool with a focus on applications in the shipyard. This allows the replacement of some on-site fusion welding and could significantly lower cost (~25% reduction) and speed up production (~50% reduction). The Mobi-Weld development will improve the competitiveness of the European shipbuilding and fabrication market against low-labour rate competitors. While Mobi-Weld is initially targeted towards shipbuilding, the consortium is looking to maximise the return by identifying potential partners in other industries. With transport manufacturers (road, rail & sea) finding themselves under more and more pressure to replace traditional iron and steel with aluminium alloys because of fuel consumption considerations and the need for this material is expected to keep growing over the foreseeable future.

Increased use of FSW will bring about significant economic advantages for its users but also major environmental and safety benefits. Use of the Mobi-Weld system in place of conventional fusion welding techniques will reduce power consumption, pollution (in terms of particulate fume, gases and noise) and improve safety for the operator who will no longer be exposed to dangerous UV radiation, extreme high temperatures and intense noise at close quarters.

The results of the project have provided useful information required for further understanding and industrial take-up of the FSW process. Results assist in the determination of parameters and set-up of machines for FSW. Although there are many standards in existence that control aspects of welding, these are predominantly associated with, and appropriate to, more traditional welding techniques, in particular fusion welding. There is an ISO standard, ISO 25239, which deals with all aspects of FSW for the welding of aluminium alloys using conventional tool designs. To date no agreed European Standards or Codes of Practice have been issued for FSW using bobbin tools. This project has provided an important element in moving toward other such codes of practice, in that it will develop information and data on application of the floating-bobbin tool FSW process. Such data will be made available where appropriate to those bodies able to use it in this way.

Exploitation & Dissemination
Promotion of the project and dissemination of project results began during the project and will continue as part of the business development activities. A project brochure has been professionally produced and all partners have been provided with copies to distribute to interested parties. A project video is also available through the project website. SME partners have promoted the project at various trade events. RTD partners have published technical articles & presented at conferences. Additionally all partners have held informal discussions with business associates and potential clients. All IP has been identified and appropriate protection measures have been implemented. A final plan for the use and dissemination has been developed giving a clear indication of past and future dissemination and exploitation activities.

List of Websites:
A project website was set up to act as a communication port between the partners and disseminate the project http://www.mobi-weld.eu.

Project
For general enquiries regarding the Mobi-Weld project please contact Mr Piotr Bistron, pbistron@abis.krakow.pl at ABIS Sp. z o. o., Spolka Komandytowa, ul. Kamienna 17, 30-199 Rzaska, Krakow, Poland. Website: www.abis.krakow.pl.

System
All enquiries regarding the Mobi-Weld system should be addressed to Dr Ginter Figner, gunter.figner@stirzone.at at STIRZONE GmbH, Golfstraße 5a, A- 8077 Gössendorf, Graz, Austria. Website: www.stirzone.at