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Friction stir welding conquers austenitic stainless steels (November 2000)

   
Richard Johnson

Originally published in Welding and Metal Fabrication, 2000, Vol. 68, No. 10,
November/December, pp 16-17 by DMG World Media
http://www.dmgworldmedia.com/

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Dr Richard Johnson, FIM, FWeldI, is a Senior Project Leader - Business Development Friction & Forge Processes,TWI, Granta Park, Great Abington, Cambridge CB1 6AL, UK. He is currently working on friction welding and friction stir welding projects for member companies and on the internal TWI Core Research Programme.





Friction stir welding (FSW) is now nearing the end of its first decade in the world. Initially the process was taken up by the major aluminium fabricators, but extending its capability to weld higher temperature materials has been a continuing aim of TWI. This further development has now recorded another first, by the successful welding of austenitic stainless steels.

For the uninitiated, the concept of the process is deceptively simple. A rotating steel tool with a centre pin and a shoulder is inserted into the workpiece until the shoulder is slightly digging into the surface. In doing so the material around the pin is heated up and plasticised, and is swept around by the tool rotation. When the tool is traversed along the joint line material in front of the tool is plasticised and swept around to complete a weld behind the tool - in effect an internal extrusion process operating below the tool shoulder. Apart from assisting by generating friction heating, the shoulder being pressed onto the surface of the workpiece acts as a containment for the hot plasticised material, which would otherwise expand out of the joint region.

Fig. 1. A schematic of the FSW process
Fig. 1. A schematic of the FSW process

 

The process patent was applied for in 1991, and the ability to make high quality welds without melting the materials was rapidly taken on board by industry. Firms across the aluminium fabrication spectrum are using the technology:in shipbuilding - for high speed ferries and cruise liners; in railways - for the Japanese shinkansen or bullet trains; a wide variety of components in the automotive industry, including impact absorbing bumpers; and in the aerospace industry. The last-named not only covers aircraft fabrication, but in July of last year the first FSW processed component flew into space as part of the Delta launch rocket courtesy of the Boeing company.

More recent developments at TWI have concentrated on assessing alternative materials, such as magnesium and titanium alloys, copper and steels. The much higher processing temperatures required in most of these alloys means that the use of steel FSW tools is no longer practicable, and much of the research centres around the tool material and its geometry in order to weld these alloys efficiently. TWI has assessed a variety of options, and was able to announce at the end of 1998 that the first steel welds had been made. The steel used was a 12% chromium 410 steel, in the fusion welding of which there are some difficulties; however, it was found possible to FSW process 12mm thick plates in two passes. Since then successful welds have also been made in other steels, including some of the higher strength C-Mn steels.

It was known from steel producers that the austenitic steels would be more difficult to work than the ferritic or transformable steels, because of their higher hot strength. However, the culmination of further work came in August2000 when the first welds by the FSW process were made in 316L stainless steel. These were welded in plate thicknesses of 5 and 10mm, the latter being welded from both sides. With some optimisation of the processing parameters the surface finish was progressively improved, and it can be seen from the weld section that there is full consolidation of the material in the weld nugget. In order to effect the weld the tool is operating at a red heat, and the austenitic steels require much higher forces to keep the shoulder on the surface of the plate in comparison to the 12% chromium steels. These are obviously early days in the technology, and comparisons need to be made between these welds and those made by fusion techniques, for example to assess the weld zone properties in detail. It is already known that there is, as in aluminium, a very fine grain size produced in the weld nugget rather than the large grains that can be produced in some fusion welds, so the weld strength will be good. The HAZ region also needs to be characterised more fully - because of the temperature at which the tool is operating, there will still be heat-affected zones formed on each side of the weld.

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Figs. 2 and 2a. 316L weld in 5mm plate using a 30mm diameter FSW tool


Fig. 3. Weld section of 316L 5mm plate
Fig. 3. Weld section of 316L 5mm plate
 

This latest development will be of particular interest to fabricators of vessels and components for the food and chemical industry, where great attention is paid to ensuring there are no crevices left on weld surfaces that require absolute cleanliness from a hygiene or purity requirement. The very smooth weld surface finish which can be obtained will be of paramount importance in this respect. The technique is still being developed into a production process in conjunction with TWI industrial Member companies but, as they say, the first signs are extremely promising.

Further information on the development project and the process in general, contact Richard Johnson or Phil Threadgill at TWI, Cambridge CB1 6AL or +44 (0)1223 899000 or e-mail richard.johnson@twi.co.uk, phil.threadgill@twi.co.uk

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