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Thermo-electric temperature measurements in friction stir welding: Towards feedback control of temperature
University West, Department of Engineering Science, Division of Production System. (PTW)ORCID iD: 0000-0003-3261-9097
2016 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Friction Stir Welding has seen a fast uptake in many industry segments. Mechanical properties superior to fusion welding, the ability to weld "unweldable" aluminium alloys and low distortion are often described as the main reasons for the fast industrial implementation of FSW. Most existing applications consist of long straight welding joints. Applications with complex weld geometries, however, are rarely produced by FSW. These geometries can induce thermal variations during the welding process, thus making it challenging to maintain a consistent weld quality. In-process adaptation of weld parameters to respond to geometrical variations and other environmental variants allow new design opportunities for FSW. Weld quality has been shown to be reliant on the welding temperature. However, the optimal methodology to control the temperature is still under development.The research work presented in this thesis focuses on some steps to take in order to reach the improvement of the FSW temperature controller, thus reach a better and consistent weld quality. In the present work different temperature methods were evaluated. Temperature measurements acquired by the tool-workpiece thermocouple (TWT) method were accurate and fast, and thereby enhanced suitable for the controller. Different environmental conditions influencing the material heat dissipation were imposed in order to verify the controller effect on the joint quality. In comparison with no controlled weld, the use of the controller enabled a fast optimization of welding parameters for the different conditions, leading to an improvement of the mechanical properties of the joint.For short weld lengths, such as stitch welds, the initial plunge and dwell stages occupy a large part of the total process time. In this work temperature control was applied during these stages. This approach makes the plunge and dwell stages more robust by preventing local material overheating, which could lead to a tool meltdown. The TWT method was demonstrated to allow a good process control during plunging and continuous welding. The approach proposed for control offers weld quality consistency and improvement. Also, it allows a reduction of the time required for the development of optimal parameters, providing a fast adaptation to disturbances during welding and, by decreasing the plunge time, provides a significant decrease on the process time for short welds.

Place, publisher, year, edition, pages
Trollhättan: University West , 2016. , 62 p.
Keyword [en]
FSW, Robotic, Temperature, TWT, Plunge, Control
National Category
Robotics
Research subject
Production Technology
Identifiers
URN: urn:nbn:se:hv:diva-9982ISBN: 978-91-87531-43-9 (Printed version)ISBN: 978-91-87531-42-2 (E-version)OAI: oai:DiVA.org:hv-9982DiVA: diva2:1037275
Supervisors
Available from: 2016-11-09 Created: 2016-10-12 Last updated: 2016-11-10Bibliographically approved
List of papers
1. TWT method for temperature measurement during FSW process
Open this publication in new window or tab >>TWT method for temperature measurement during FSW process
2015 (English)In: The 4th international Conference on scientific and technical advances on friction stir welding & processing, San Sebastian, Spain, 2015, 95-98 p.Conference paper (Refereed)
Abstract [en]

Friction stir weld (FSW) has generated a high interest in many industry segments in the past 20 years. Along with new industrial challenges, more complex geometries and high quality demands, a better control of the welding process is required. New approaches using temperature controlled welding have been proposed and revealed good results. However, few temperature measurement methods exist which are accurate, fast and industrially suitable. A new and simple sensor solution, the Tool-Workpiece Thermocouple (TWT) method, based on the thermoelectric effect was recently developed.This paper presents a calibration solution for the TWT method where the TWT temperature is compared to calibrated thermocouples inside the tool. The correspondence between both methods is shown. Furthermore, a calibration strategy in different aluminium alloys is proposed, which is based on plunge iterations. This allows accurate temperature monitoring during welding, without the need for thermocouples inside the tool.

Place, publisher, year, edition, pages
San Sebastian, Spain: , 2015
Keyword
Friction stir welding, TWT method, Temperature measurements, Aluminium alloys
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering; Production Technology
Identifiers
urn:nbn:se:hv:diva-9122 (URN)
Conference
The 4th international Conference on scientific and technical advances on friction stir welding & processing October 1-2, 2015
Available from: 2016-02-25 Created: 2016-02-25 Last updated: 2016-10-14Bibliographically approved
2. Temperature measurements during friction stir welding
Open this publication in new window or tab >>Temperature measurements during friction stir welding
2016 (English)In: The International Journal of Advanced Manufacturing Technology, ISSN 0268-3768, E-ISSN 1433-3015, 1-10 p.Article in journal (Other academic) Published
Abstract [en]

The increasing industrial demand for lighter, more complex and multi-material components supports the development of novel joining processes with increased automation and process control. Friction stir welding (FSW) is such a process and has seen a fast development in several industries.This welding technique gives the opportunity of automation and online feedback control, allowing automatic adaptation to environmental and geometrical variations of the component.Weld temperature is related to the weld quality and therefore proposed to be used for feedback control. For this purpose, accurate temperature measurements are required. This paper presents an overview of temperature measurement methods applied to the FSW process. Three methods were evaluated in this work: thermocouples embedded in the tool, thermocouples embedded in the workpiece and the tool-workpiece thermocouple(TWT) method. The results show that TWT is an accurate and fast method suitable for feedback control of FSW.

Place, publisher, year, edition, pages
Springer, 2016
Keyword
Friction stir welding, TWT method, temperature, aluminium, thermocouples
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-9531 (URN)10.1007/s00170-016-9007-4 (DOI)
Available from: 2016-07-08 Created: 2016-07-08 Last updated: 2016-10-14Bibliographically approved
3. Cooling rate effect on temperature controlled FSW process
Open this publication in new window or tab >>Cooling rate effect on temperature controlled FSW process
2015 (English)Conference paper, Presentation (Refereed)
Abstract [en]

A continuous trend towards more demanding jointgeometries is imposed across various manufacturingindustries. During Friction Stir Welding (FSW) of suchcomplex geometries, the surrounding environment playsan important role on the final weld quality, especially inthermal aspects. In order to guarantee a consistent weldquality for different conditions, in-process weldingparameter adaptation is needed.This paper studies the effect of the cooling rate onmechanical properties for temperature controlled FSW byusing different backing bar materials. A new temperaturesensor solution, the Tool-Workpiece Thermocouple(TWT) method [1], was applied to measure thetemperature during welding. A FSW-robot equipped withtemperature and force feedback control was used, whererotation speed was varied to maintain a constant weldingtemperature. AA7075-T6 lap joints were performed withand without temperature control. The cooling rate duringwelding was acquired and macrographs and mechanicalproperties were evaluated for each weld. The rotationspeed offered a fast response promoting the heat inputnecessary to weld at the set temperature. Temperaturecontrolled welds presented a better behaviour undertensile loads. The results prove that temperature controlusing the TWT method is suitable to achieve higher jointquality and provides a fast setup of optimal parameters fordifferent environments.The work presented is an important step in the processoptimization through feedback control which willconsider not only the operational parameters of theprocess as such but also the resulting quality of the joint.

Place, publisher, year, edition, pages
Helsinki, Finland: , 2015
Keyword
Friction Stir weld, Cooling rate, TWT method, Temperature control, Aluminium alloy
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering; Production Technology
Identifiers
urn:nbn:se:hv:diva-9120 (URN)
Conference
IIW International Conference High-Strength Materials - Challenges and Applications, 2-3 July 2015, Helsinki, Finland, Helsingfors, 2015
Available from: 2016-02-25 Created: 2016-02-25 Last updated: 2016-10-14Bibliographically approved
4. Analysis of Plunge and Dwell Parameters of Robotic FSW Using TWT Temperature Feedback Control
Open this publication in new window or tab >>Analysis of Plunge and Dwell Parameters of Robotic FSW Using TWT Temperature Feedback Control
2016 (English)In: Proceedings of 11th International Symposium on Friction Stir Welding, Cambridge, UK, 2016, 1-11 p.Conference paper (Other academic)
Abstract [en]

Friction stir welding (FSW) and variants of the process have generated high interest in many industries due to its several advantages such as low distortion, superior mechanical properties over arc welding and the possibility of joining dissimilar materials. Increased complexity of industrial applications require a better control of the welding process in order to guarantee a consistent weld quality. This can be achieved by implementing feedback control based on sensor measurements. Previous studies have demonstrated a direct effect of weld temperature on the mechanical properties of FSW joints, [1], and therefore, temperature is chosen as primary process variable in this study.A new method for temperature measurement in FSW referred to as the Tool-WorkpieceThermocouple (TWT) method has recently been developed by De Backer. The TWT method is based on thermoelectric effect and allows accurate, fast and industrially suitable temperature monitoring during welding, without the need for thermocouples inside the tool [2].This paper presents an application of the TWT method for optimisation of the initial weld phases, plunge and dwell, operation in conventional FSW, which can also be applied to friction stir spot welding (FSSW). An analysis of the operation parameters by using feedback temperature control is presented aiming to better control of the initial weld phases through temperature feedback.

The introduction of the TWT temperature sensor provides additional process information during welding. Fast data acquisition gives opportunity to differentiate different process phases: contact of probe tip with workpiece surface; plunge phase; dwell phase. This would be followed by tool retraction for FSSW or tool traverse phase for FSW.The effect of the plunge parameters on weld temperature and duration of each phase were studied for the purpose of optimising the process with respect to process (i) robustness, (ii)time, (iii) robot deflection and (iv) quality. By using temperature feedback, it is possible to control the plunge phase to reach a predefined weld temperature, avoiding overheating of the material, which is known to have a detrimental influence on mechanical properties. The work presented in this paper is an important step in the optimization of robotic FSSW and FSW.

Place, publisher, year, edition, pages
Cambridge, UK: , 2016
Keyword
Plunging, Friction Stir Spot Welding, Temperature, TWT, Robot
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-9530 (URN)
Conference
11th International Symposium on Friction Stir Welding, Cambridge, UK, 17-19 May, 2016
Available from: 2016-07-08 Created: 2016-07-08 Last updated: 2016-10-14Bibliographically approved

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