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Advancing FSW of Thick Section Aluminium

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Project Code: 30146

Objectives

  • Enhanced tools with greater strength, more efficient mixing and improved longevity
  • Improved processes to allow faster welding of thick, high strength aluminium
  • Improved knowledge of the in-service performance of thick section aluminium FSW 

Project Outline

It is now over 10 years since TWI last undertook a significant programme of tool development work (MR801, 2004). That programme did not look at materials greater than 25mm in thickness, nor did it cover simultaneous double sided, bobbin or stationary shoulder welding. The state of the art has progressed significantly since 2004 - to the extent that we are being informed by customers that competitors have tools and process parameters that enable them to produce welds of high quality at greater tool traverse speeds than we can attain

Customers now require FSW in the much stronger 2xxx and 7xxx series alloys, and often dissimilar welds between the two alloy classes. The current state of the art within TWI in these materials is welding 40mm thick material at around 50 mm per minute. Customers are demanding the capability to weld greater thicknesses and higher welding speeds. As examples, consider two typical demands from the aerospace sector.

  • XXXXX wish to make 50mm thick dissimilar welds between 2xxx and 7xxx alloys at rates in excess of 100mm minute in order to fabricate four wing sets of 27m long wing spars per day
  • XXXXX wish to weld 75mm thick AA2219 blade containment rings ‘as fast as possible’, ideally in excess of 150mm/minute 

In general, increasing the speed of welding requires that the material be softer such that it will flow around the tool more easily. However, increasing the welding temperature to enhance material plasticisation does not of itself effectively address the problems faced and presents a number of additional challenges that must be overcome: In particular,

  • The effects of the increased welding temperature on the in service performance of the joint need to be quantified (strength, fatigue, corrosion)
  • The effects upon tool life, already a limiting factor at lower speeds and lesser thicknesses, need to be understood and improvements made 

To meet these very demanding customer requirements, we propose a programme of research into the welding techniques and resultant joint properties to ensure that we regain our lead in the FSW of thick section, high strength aluminium alloys and capitalise on our unique capabilities. This work will include:

  • Improved tool designs and treatments.
  • Improved processes, in particular conventional and simultaneous dual head welding
  • A study of the effects of high speed welding and heating / cooling effects upon weld properties, covering:
    • Joint strength
    • Corrosion performance
    • Fatigue performance 

Relevant Industry Sectors

Industry Need

A review of existing customer activity, consultations with industry sector managers and the SYF technology theme mapping process have all illustrated a growing demand for friction stir welding thick section aluminium. This demand is seen from member companies operating in several sectors, these being:

All of these are safety critical applications in high value components that require high joint efficiency, high production rates and long term structural integrity.  These are all areas that require further fundamental research to support our existing programmes and those of our customers.

Welding thick section aluminium at industrially relevant speeds, quickly, requires advances in:

  • Tool design (for example, area ruled tools, thermomechanical treatments for improved longevity),
  • Process improvements, (supported bobbin, stationary shoulder, simultaneous dual head welding),
  • Metallurgical understanding of the welds produced (long term corrosion and fatigue performance) 

TWI has some unique capabilities (PowerStir FSW machine, simultaneous dual head welding), and expertise that are relevant to thick section welding and should be exploited for the benefit of our members and to provide TWI with a technological lead over our competitors to whom we are beginning to lose work on thick section aluminium welding.

For more information please email:


contactus@twi.co.uk