Stirring steel - a feasibility study into the friction stir welding of ferrous materials.

Friction stir welding (FSW) is a process for joining workpieces in the solid phase, using an intermediate non-consumable tool. As Wayne Thomas reports it has most recently been adopted to join ferrous materials.

According to the invention the method comprises a FSW tool of harder material than the workpiece being welded. ^[1] 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, generates 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 ( Fig.1).

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 a dissimilar 12% chromium steel/low carbon steel combination. An economic comparison with MMA, MIG and sub-arc fusion welding processes is also made.

Background

As with all friction processes, the heat for FSW is produced by direct conversion of mechanical energy to thermal energy at the welding interfaces. The resultant interfacial temperature can reach values just below the melting point.

The characteristics of the FSW technique can be compared with other friction process variants. ^[2] For example, when continuous drive rotary, inertia, linear, orbital and arcuate friction welding variants are used to join two bars of the same material and same diameter or aligned cross-section, axial shortening (consumption of the bars) occurs equally from each bar to form a common plasticised 'third-body'. However, differences in diameter or section lead to preferential consumption of the smaller component. Welding different materials also leads to preferential consumption of the comparatively softer material. ^[3] The unequal consumption and temperature distribution in rotary friction welding between different diameter bars has also been studied. ^[4,5] This preferential consumption and reprocessing of one component in a friction system has been put to good use in the development of friction surfacing, friction hydro pillar processing and friction pillaring, radial friction welding and friction plunge welding. Friction stir welding is a further development in that only the workpiece weld region is processed to form a solid-phase welded joint.

The lateral movement in friction surfacing ^[6-8] and FSW, created by introducing new workpiece material at nominally ambient temperature, modifies the already unequal temperature distribution between a comparatively small diameter rotating consumable bar in friction surfacing and the rotating tool in FSW. Both these techniques rely on producing suitable temperature and shear conditions within the 'third-body' transient region that exists in friction surfacing between the consumable bar and the substrate, and between the tool and the workpiece in FSW.

In friction surfacing any increase in temperature differential (by the intrusion of cold substrate material) enhances the deposition mechanism and allows comparatively harder materials to be deposited onto nominally softer materials. The inherent temperature gradient leads to minimal dilution. However, in FSW the intrusion of cold workpiece material can, in some cases, hinder the welding performance.

Previous work on the FSW of steel

At the time of publishing only four references in the open literature concern the FSW of steel. ^[9-12] All describe preliminary work to demonstrate the capability of FSW of short lengths of steel of up to 25mm thickness.

Materials and procedure

The following describes recent work conducted to demonstrate further the feasibility of FSW of 12mm thick low carbon 12% chromium steel plate (EN 10088-2, 1.4003, X2CrNi12), and of low carbon steel EN 10083-1, 1.1151 (BS 970, 07M20) 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 (0.1-0.24 m/min). More than four metres of weld were made before tool changes were necessary when welding the 12% chromium steel.

Results

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 occurs 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 travels 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 the tool. The tool shoulder maintained its bright orange colour throughout a one 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 is also dependent on rotational speed, increasing with higher speed and falling with lower 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) which was visually not unlike that of a steel friction surfacing deposit. However, unlike a deposit that clads the top of the workpiece, the friction stir weld appeared essentially flush with the surface as shown in Figs 2 and 3. Apart from being a little coarser, the almost semicircular ripple in the weld tracks for steel was essentially the same as those for aluminium FSW welds.

Transverse macrosections reveal HAZ profiles that correspond with the shoulder and probe geometry and reflect the degree of working. Frictional contact 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. Typical overall HAZ profiles for double sided welds are shown in Figs 3 and 4.

A marked difference was found in the welding speed possible for the 12% chromium and for the dissimilar steel joints. Acceptable welds could be produced at up to 4 mm/sec (0.24 m/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 dissimilar 12% chromium/carbon steel weld specimens differed from normal in that certain regions of the weld profile protruded above the plate surface. Some undercut was noticeable but essentially the surface appearance was marked with a shallow bulge 0.6mm above the plate, which ran along the entire length of the weld ( Fig 5). This bulge lay on the retreating side and mainly comprised 12% chromium material that came from the original plate of the joint, as shown in Fig 6.

The cyclical nature of the rotary FSW process is revealed in detail in the macrophotograph of a 20° tapered transverse section ( Fig 6). This shows the substantial stirring occuring in the weld due to FSW.

Integrity of low carbon 12% chromium alloy steel welds

Cross-weld tensile tests recorded an ultimate tensile stress of 539-541 N/mm ² with failure occurring in the parent metal well away from the joint or the HAZ region. Acceptable transverse face and root bends achieved 180°.

Metallographic examination of selected sections are shown in Fig 3 and indicate 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 weld thermomechanically affected zone (TMAZ), which had transformed with associated recrystallisation and grain growth. ^[13] On both sides of the TMAZ central weld zone, a HAZ region showed 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.

Discussion

Welding mechanisms

As the rotating tool moves along the joint, the 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. Evidence that hydrostatic pressure leads to displacement of plasticised material and recovery of the through-thickness dimension is shown in the dissimilar metal weld sections. Figures 5 and 6 show that even where the trailing edge (heel part of the shoulder) is sunk below the plate surface during welding, recovery in plate thickness is possible. In the case of dissimilar materials, preferential recovery occurs with the more plasticised material, especially when positioned on the retreating side of the weld. The presence of a shallow bulge above the plate surface, Fig 5 confirms this.

Defects can occur when welding because of the asymmetric nature of the process. Owing to rotation of the tool along the joint line, different hydrostatic pressures arise on the advancing and retreating * sides of the tool at non-optimised conditions. This pressure differential can lead to buried voids or a surface breaking defect along the weld line and can cause the tool to veer away from the tool retreating side. This can be avoided by secure fixturing and robust machine equipment.

* The advancing side of the tool is that where the direction of rotation opposes the movement of the weldments. The retreating side is that where both are aligned.

Thermal management and use of filler wire

Preheating of the tool is also recommended for certain tool materials which are brittle at room temperature, so that they become more ductile and thus better suited to carrying out the welding process. It is considered that any suitable heating process can be adopted for heating the workpiece including flame, coherent or incoherent radiation, friction, induction resistance or arc/plasma. High frequency induction heating and high frequency resistance heating may be of particular advantage since they can achieve heating through the thickness of the workpiece, rather than just the surface.

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 8. 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 to refine and improve the weld from the prior fusion process.

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.

The FSW process seems ideally suited to the welding of hot plate where the entire plate or product is raised to a high temperature eg hot plate welding in steel mills or hot strip tube manufacture in pipe mills.

Process costs

Table: Comparison of arc welding and FSW for joining 25mm thick low carbon 12% chromium steel plate

MMA		2.5	30	2	0.5	MMA electrodes	2.2kg	33	83
MMA		2.5	30	4	0.5	MMA electrodes	4.4kg	66	156
MIG		6.7	50	0.66	0.5	Flux-cored MIG wire Shielding gas	2.1kg 450 litre	15	39.2
MIG		6.7	50	1.2	0.5	Solid MIG wire Shielding gas	3.5kg 900 litre	30	66
SAW - twin wire		10	80	0.12	0.5	SAW wire Flux	1.5kg 2kg	10	26.4
Friction stir		2	80	0.18	0.17	-	-	-	7
*Assumes labour rate for welding and machine shop technicians is £20/hr

Conclusions

The above feasibility work has demonstrated that more than four metre lengths of weld can be made without tool change when double sided friction stir welding 12mm thick low carbon 12% chromium steel. The welds showed sound structures and static mechanical properties approaching parent metal strength. Further work is needed to extend further 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.

References