W M Thomas, P Woollin, and K I Johnson
(Published in Steel World, Vol.4, No.2, 1999, pp.55-59, by IOM Communications Ltd - www.materials.org.uk )
Friction stir welding (FSW) is a process for joining workpieces in the solid phase, using an intermediate non-consumable tool see Fig.1
Fig.1. Friction stir welding with a rotating tool: salient features.
According to the invention, the method comprises a FSW tool of harder material than the workpiece material being welded  . The marked difference between the elevated temperature properties of the tool and the workpiece, together with a suitable cyclic movement between the tool and the workpiece, generate sufficient frictional heat to cause plasticised (third-body) conditions in the workpiece material. Thus, friction stir welding is a continuous hot shear process whereby the weld material is heavily deformed and oxide layers disrupted. The process involves slowly plunging a portion of a specially shaped rotating tool between and then along the abutting faces of the joint. The contacting surface of the shoulder of the tool and the length of the probe below the shoulder essentially allow the probe to maintain penetration to the required depth.
Initially, FSW was confined to relatively soft workpiece materials such as lead, zinc, magnesium and a range of aluminum alloys. More recently, copper, titanium, low carbon ferritic steel and low carbon chromium alloy steels have been welded. This range of harder workpiece materials has been made possible by maintaining a suitable differential between the hardness and the elevated temperature properties of the tool and the workpiece material.
Continuing investigations suggest that the FSW of steel will become commercially attractive for such applications as ships, pipe fabrication, trucks, railway wagons, and hot plate fabrication.
This paper describes some FSW results on 12% chromium alloy steel and on dissimilar 12% chromium steel/low carbon steel combinations. An economic comparison with MMA, MIG & sub-arc fusion welding processes is made.
2. Materials and procedure
The following describes recent work conducted to further demonstrate the feasibility of FSW of 12mm thick low carbon 12% chromium steel plate (DIN 1.4003, X2CrNi12), and of low carbon steel (BS 970, 070M20) to the same 12% chromium plate.
Detailed weld parameters, tool dimensions and tool material are currently confidential. Welds were made in two passes, one on each side of the plate and welding speeds used varied from 1.7 to 4 mm/sec (0.1-0.24m/min). More than four metres of weld were made before a tool change was necessary when welding the 12% chromium steel.
3.1 Welding trials
Unlike aluminium and most non-ferrous materials, which show little or no visible change during FSW owing to increase in temperature, a colour change occurred when welding steel. The tool shoulder reached a bright orange colour within a few seconds of making contact with the plate, which indicated an approximate temperature of over 1000°C. Also, as the tool travelled along the seam, the weld track behind the trailing edge of the rotating tool appeared orange/bright red (900-1000°C). This colour changed to a darker cherry red (about 600°C) 25mm from behind the tool. The tool shoulder maintained its bright orange colour throughout a 1-metre length of weld. Thermal imaging measurements when welding the 12% chromium steel gave a maximum welding temperature close to the tool of around 1090°C. The temperature was also dependent on rotational speed, increasing with increasing speed.
The surface of the steel welds showed a uniform surface ripple (caused by the final sweep of the trailing edge of the rotating tool). The weld appeared essentially flush with the plate surface as shown in Figs 2 & 3. Apart from being a little coarser, the almost semicircular ripples in the weld tracks for steel was essentially the same as that for aluminium FSW welds.
Fig.2. 1m long 12mm thick 12%Cr alloy steel weld
Fig.3. Transverse section of 12mm thick 12%Cr alloy steel FSW weld made in two passes
Transverse macrosections revealed HAZ profiles that correspond with the shoulder and probe geometry. Frictional heating at the shoulder produces a wide but relatively shallow HAZ, which deepens in the central region and extends through-the-thickness to a depth and breadth governed by the probe. A typical overall HAZ profile for double-sided welds is shown in Fig 3.
A marked difference was found in the welding speed possible for the 12% chromium steel and for the dissimilar steel joints. Acceptable welds could be produced at up to 4 mm/sec (0.24m/min) traverse rate for the 12% chromium steel, but only at a slower 1.7 mm/sec (0.1m/min) for the dissimilar joint.
The cyclical nature of the rotary FSW process is illustrated in the macrophotograph of a 20° tapered transverse section taken from a dissimilar 12% chromium/carbon steel weld ( Fig 4). This shows the substantial stirring occurring in the thermomechanically-affected zone during FSW.
Fig.4. Transverse 20° taper section of dissimilar 12%Cr-low carbon steel FSW joint showing cyclic flow pattern
3.2 Integrity of low carbon 12% chromium alloy steel welds
Cross-weld tensile tests recorded an ultimate tensile stress of 539 to 541 N/mm 2
with failure occurring in the parent metal well away from the joint or the HAZ region. Transverse face and root bend tests achieved 180° deflection without tearing or revealing any welding defects, see Figs 5 & 6
Fig.5. Crossweld tensile samples (weld surfaces removed in sample on left)
Fig.6. Typical bend test results in 12mm thick alloy steel plate, showing parent metal, and first pass and second pass in tension
Metallographic examination of a section is shown in Fig 3 and indicates a reasonably uniform shaped double-sided weld profile with no evidence of buried defects. The weld region exhibited two distinct microstructural zones. One of these is the central thermomechanically affected zone (TMAZ), which had transformed with associated recrystallisation and grain growth. On both sides of the TMAZ, a HAZ region was present, showing some transformation close to the weld but with no evidence of grain growth. Further out, towards the parent material, the HAZ still showed a degree of tempering, but had not transformed. The HAZ zones on either side were similar in all features. Typical of this type of steel, the parent material showed a very fine ferritic/martensitic structure.
Within the TMAZ, a range of ferrite and martensite structures had developed, a typical example being shown in Fig 7. Some light etching bands were present towards the top of each weld pass. Energy dispersive x-ray microanalysis of these bands indicated the presence of some tool debris. Longitudinal weld sections, however, confirmed that no measurable reduction in weld depth had occurred after steady state welding conditions had been established. There was no evidence of buried defects within the weld region.
Fig.7. Microstructural banding in 12%Cr TMAZ weld region
The mechanisms which occur during FSW have been described previously  . Basically, heat is generated by friction on the tool probe and shoulder and the stirring action causes intimate mixing of the two abutting parent materials. As the rotating tool moves along the joint, hydrostatic pressure forces the plasticised weld material to flow around the tool. The plasticised weld material then coalesces behind the tool, to form a solid phase joint as the tool moves away.
4.1 Potential applications
Not only does the FSW process show promise for welding a range of ferrous materials at ambient temperature conditions for ships, power plant, truck and railway wagon fabrication, the process is also ideally suited to the welding of hot products. Material could be welded at the processing stage, e.g. the welding of hot strip, during the fabrication of beams in the steel mills, and hot strip tube manufacture in pipe mills, see Figs 8 and 9
Fig.8. Example of hot wire hybrid techniques for gap FSW components
Fig.9. Simultaneous double sided FSW of rolled or extruded components
4.2 Process costs
FSW can give an economic advantage compared to fusion welding, in terms of savings in weld preparation time, welding time and consumable costs. The indications are that cost savings will progressively increase with increase in plate thickness. Discounting the capital costs of both fusion welding and solid phase welding equipments, the weld costs per metre are given in Table 1 for 25mm thick 12% chromium steel. Based on the results achieved so far, FSW cost savings of a factor of three or more can be achieved. It should be noted that the comparison shown in Table 1 does not include the cost of back gouging and grinding, or cut back inspection often necessary for Double 'V' arc welding.
Table 1: Comparison of arc welding and FSW for joining 25mm thick low carbon 12% Cr steel plate
|Process||Joint design||Requirements per metre|
|Direct Labour||Associated labour - machining time, hrs||Consumables||Total cost/m £*|
|Travel speed, mm/s||Assumed duty cycle, %||Welding time, hrs||Type||Amount required||Approx. Cost, £|
||Flux cored MIG wire
|SAW (Fig.iii) - twin wire
||Solid MIG wire
|Friction Stir (Fig.v)
||Square Edge Butt
|* Assumes a labour rates for welding and machine shop technicians are £20/hr and £20/hr respectively
4.3 General comments
Work is continuing at TWI to investigate the use of hybrid processes to fill substantial gaps between the plates thereby accommodating poor fit-up. The use of hot and cold wire filler materials, as used in TIG, MIG, sub arc, etc, processes can be used to fill gaps between plates just in front of the FSW tool, as shown in Fig 10. Alternatively, an arc welding process (with or without filler) can be used in advance of the FSW operation. This latter hybrid approach effectively allows the FSW technique to become a gap filling and a post fusion welding process, which may help to refine and improve the weld from the prior fusion process.
Fig.10. Process concept for FSW pipe fabrication
In some cases, where the FSW process is used at high temperatures, a non-oxidising gaseous atmosphere may be needed to protect the joint from atmospheric contamination and to prevent certain tool and workpiece materials becoming oxidised.
5. Concluding remarks
The above feasibility work has demonstrated that double sided FSW weld lengths of more than 4m can be achieved without tool change in 12mm thick low carbon 12% chromium steel. The welds showed sound structures and static mechanical properties meeting parent metal strength. Further work is needed to extend further the tool life and to establish the weldability of other steel grades.
Significant economic advantage is expected as the technology for FSW of ferrous materials progresses.
The Authors thank Dr R E Dolby, Mr P B Fielding, Mr E D Nicholas, Mr I J Smith, and Mr P T Smith for their support and advice.
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