The need for gas shielding - positive advantages for two friction processes
TWI Bulletin, September/October 1997
Wayne is a Principal Research Engineer in the New Technology Unit, he joined TWI in 1983 and has been responsible for the conception of a number of emergent technologies.
Dave is Technology Manager of the Friction and Forge Processes Group and has been involved with development projects spanning a wide range of industries.
Similar to other friction welding variants, friction seam welding and friction hydro pillar processing (FHPP) techniques join workpieces in the solid-phase. Both friction seam and FHPP techniques, however, are characterised by having areas of hot highly plasticised material exposed to possible atmospheric attack. As Wayne Thomas, Richard Wictorowicz (Air Products) and Dave Nicholas report, the feasibility of using a shielding gas for these friction techniques is being studied.
Figures 1a and b illustrate the basic principle of the friction seam welding process. Friction heat is generated between the workpieces to be joined and the rotating consumable bar whilst axially pressed into a 'U' shaped butt joint. The consumable material is deposited into the groove while the substrate, with respect to the consumable, is laterally traversed underneath the rotating consumable. [1-3]
During the friction seam welding operation, plasticised material may become exposed to the atmosphere through the gap between the abuting plates of the joint, as shown in Figure 1a. The greater the gap the greater the risk of atmospheric contamination that can occur. Movement of exposed plasticised material by way of the rotating consumable may influence deposit and substrate interface integrity beyond the gap region. Plasticised material can also become exposed at the leading edge of the deposit directly underneath the rotating consumable ( Fig.1b). Moreover, the need to minimise the deposit surface oxidation before application of the next multi-layer deposit may sometimes be necessary in order to maintain interpass integrity.
Fig.1: a) Basic principle of friction seam welding; b) Showing exposed leading semi-circular edge of the deposit.
FHPP is a recently invented technique which provides an alternative method for crack repair, hole filling, cladding, material processing, and plate fabrication. [2] Variants of the FHPP technique provide enormous potential for new application areas. The salient features of the process are illustrated in Figure 2.
Fig.2. Basic principle of FHPP
The FHPP technique involves rotating a consumable rod co-axially in an essentially circular hole whilst under an applied load, to generate continuously a localised plasticised layer. The plasticised layer coalesces and comprises a very fine series of adiabatic, helical rotational shear interfaces, part spherical in shape. During FHPP the consumable member is fully plasticised across the bore of the hole and through the thickness of the workpiece. The plasticised material develops at a rate faster than the axial feed rate of the consumable rod, which means that the frictional rubbing surface rises along the consumable to form the dynamically recrystallised deposit material. The plasticised material at the rotational interface is maintained in a sufficiently viscous condition for hydrostatic forces to be transmitted, both axially and radially, to the inside of the hole enabling a metallurgical bond to be achieved. Since this material is being forced hydrostatically into the surrounding bore, the diameter of the deposit material is nominally greater than the feedstock material.
With respect to FHPP and its variants, it is expected that the most important need for gas shielding will be for butt welding and crack repair. Figure 3 illustrates the obvious sites for atmospheric attack. The gap between the consumable and hole is typical for all variants of the process, and in this respect will always allow air onto the top outer edge of the rising pillar of friction processed material. However, for FHPP butt joining, a gap between two abutting plates presents a much greater path for the ingress of air to contaminate the pillar material and the substrate interface, than the air which surrounds the consumable on top of the pillar of processed material. Since the joint gap runs through the thickness, it may be possible for some oxidised material to be smeared around the inside diameter of the hole, and reduce the adhesion between filler and plate materials.
Fig.3. Simulation FHPP butt joint showing regions liable to atmospheric attack
Welding details
Preliminary welding trials were carried out to determine the effect of gas shielding during friction seam welding and FHPP for butt welding and repair. Friction seam welding trials were undertaken to minimise atmospheric contamination of the deposit interface and the deposit surface subsequent to mufti-layer deposition. FHPP trials were undertaken to minimise atmospheric contamination of the core material and the bond interface. Typically, the temperature developed during the FHPP operation will more readily oxidise the internal bore, above the rising rotational plane, making adhesion between the core deposit and the substrate material more difficult to achieve.
The programme of work involved a series of friction seam, like to like test welds in 316L grade stainless steel and a series of FHPP test welds with (BS 970 Part 1, 15M19 grade steel) consumable material, and (BS 970 Part 1, 070M20 grade steel) substrate material. (All materials were in the normalised condition). Gas shielding comprised: Hytec 5 (Ar-5% H 2) and Hytec 35 (Ar-35% H 2).
All the friction seam welding trials were carried out on TWI's machine designated FW1. This machine was adapted for friction seam welding by incorporating a hydraulic slide mechanism to enable the substrate plate to be traversed, in a controlled manner, (at lateral travel rates which range between 1.5 and 11 mm/sec) across the rotating consumable bar, while under an axial load. Transmission power was provided by a 15kW motor. This enabled infinitely variable rotational speeds between 330 and 2500 rev/min to be used. Axial force up to 110kN was provided by an hydraulic cylinder.
The FHPP trials were carried out on TWI's machine designated FW 18. Transmission power is provided by a two speed reversing motor which develops 22kW at 1430 rev/min and 15kW at 960 rev/min. This provided ample power for rotation at 64 stepped spindle speeds between 200 and 1400 rev/min.
These machines are fitted with sensors to obtain dynamic readings of substrate traverse rate (for friction seam welding) and consumable burnoff rate (displacement rate), axial applied force, and rotational speed.
Experimental procedure
Friction seam welding
Prior to friction seam butt welding trials, the substrate materials were machined to form a concave weld preparation, and the consumable rod tips were machined to an included angle of 120°. The plates were clamped onto a vertical slide table and the consumable rod was clamped into a three jaw chuck. All materials were cleaned with petether prior to welding. Rotation of the consumable was started and was followed by semi-automatic initiation of the seam welding process, which was continued until the desired seam length had been achieved. For single run and multi-layer test deposits, welds were carried out with and without active shielding gas.
Considerable effort was needed to construct a suitable sliding gas shielding chamber using top and side gas diffusers. Shown as a complete gas shield in Figure 4.
Fig.4. Gas shielded assembly complete
Friction hydro pillar processing
For FHPP the consumable rod contact end was machined to an included angle of 70°, while in all cases a 23.9mm diameter hole was drilled to a depth, nominally 0.25mm from the bottom of a 55mm thick substrate specimen. These specimens were produced with plain holes, and some with a 4mm gap on one side, machined in the through thickness direction to simulate a butt joint. The gas shield chamber is shown in Figure 5.
Fig.5. Gas shield chamber for FHPP trials under positive gas pressure
In all cases when gas shielding was used (friction seam welding and FHPP) the system was purged for ten seconds. The gas was delivered at a pressure of two bar.
Results and discussion
Friction seam welding
Process variables were based on those developed in previous Core Research work, with rotational speeds of 700 rev/min for 16mm diameter 316L grade stainless steel consumables, substrate traverse rate of 5 mm/sec, consumable burnoff rates between 1.3 and 2.3 mm/sec, and axial applied forces of between 23 and 37kN. These conditions, in ambient atmosphere, resisted separation by chisel testing.
The friction seam welded deposits were of reasonable appearance and patterned with a regular series of part circular ripples ( Fig.6). During ambient in atmosphere conditions, the oxidised surface was revealed mainly as a grey/purple temper colour with lighter temper colours at the deposit edge. With gas shielding a less oxidised surface was apparent.
Fig.6. Friction seam deposit (in air) test weld 1, showing uniform appearance
The parameters which had already provided reasonable appearance test specimens and which had shown good mechanical integrity, were selected for multilayer deposition, with and without gas shielding.
Even with gas shielding, there was no significant visible change in the surface appearance; temper colours, indicative of some atmospheric contamination of the deposit surface as it cools outside the gas chamber, were still noticeable.
Metallurgical examination
Although the surface appearance showed only a nominal improvement, with gas shielding, metallurgical examination did show marked differences between welds produced with and without an active shielding gas. Figures 7a and b show a rash of folds, believed to contain oxides; these transverse sections were prepared from welds produced without gas shielding. This is in contrast to the transverse sections taken from gas shielded deposits ( Fig.8a and b), where the volume of oxide film would appear to have been reduced.
Fig.7. Friction seam welding 316L stainless steel deposit on to 316L stainless steel. Microsections in air:
Fig.8. Friction seam welding 316L stainless steel deposit on to 316L stainless steel, with Hytec 5 gas shield:
Although the shielding gas proved difficult to contain around the external surface of the deposit, trials have shown that major oxidation of the weld interface between the deposit/substrate interface can be prevented. No significant improvement was observed between the deposit/deposit interface.
Hardness survey
Hardness measurements taken midway through each deposit, the interface between deposit and substrate region, and substrate gave the following average values:
Substrate material 185HV 5 Interface between deposit and substrate 202HV 5 Deposit material 200HV 5
Friction hydro pillar processing (FHPP)
Trials were carried out between (BS 970 Part 1, 15M19) consumable and (BS 970 Part 1, 070M20) substrate grade steels, where the rotational speed ranged between 240 and 1448 rev/min, for a range of consumable bars diameters between 20.4 and 22.9mm, and burnoff rates of between 1.1 and 5.2 mm/sec. From amongst these process variables, welds were produced that provided good mechanical integrity as defined by hammer bend testing to achieve failure, as shown in
Figure 9. Welds were characterised as good if the interface and HAZ region withstood the above test, with failure occurring in the substrate parent metal.
Fig.9. FHPP - hammer bend test
Welds which achieved good bend properties and showed acceptable metallurgical features were produced from the following conditions: rotational speed of 584, 745, and 938 rev/min, consumable/substrate hole gap of lmm (2mm across the diameter), and with burnoff rates which ranged between 2 and 2.7 mm/sec.
The use of (Hytec 5) gas shielding effectively enabled comparatively lower (1.5 mm/sec) burnoff rates to be used to produce sound welds. Conversely, low burnoff rate test welds without a gas shield failed to provide acceptable welds. Given similar rotational speeds, use of lower axial force directly relates to a lower burnoff rate. Although more work will be necessary to establish a complete tolerance envelope the preliminary results showed that the of the use of a gas shield could be used to produce sound welds.
To simulate the feature of a butt joint a 4mm gap was machined, in the through-thickness direction of the substrate. Welds were produced with and without gas shielding; welds of good quality were produced under both conditions. In this respect, use of a gas shield showed encouraging results. The welds produced with a slot also revealed that plasticised material had flowed along the gap; the uniformity of this feature demonstrated that hydrostatic conditions had been maintained throughout the weld cycle.
Manufacture of the experimental gas shielding chamber ( Fig.5) proved successful in preventing oxidation. Typically, the surplus flash material at the top of the hole was non-tarnished and only became oxidised just after the shielding gas was shut off, effectively allowing ambient atmosphere to gain access while still at a temperature that will readily oxidise. Figure 10 shows surplus flash, a splurge which was originally highly plasticised material, that became only partially oxidised on removal from the gas chamber ie after the weld was made.
Fig.10. Example of gas shielded FHPP weld
Metallurgical examination
Typically, metallurgical examination revealed that a less wide HAZ in the plate material was associated with the start (bottom of the weld), and that the HAZ increased in thickness towards the top of the weld. Heart shaped hot sheared regions within the deposit were also noticeable. These heart shaped, not fully transformed regions are associated with high rotational speed, and comparatively high feed rate. These features are indicative of an imbalance in the shear rate, and the rate of heat conduction along the consumable. The frictional heat generated conducts along the consumable bar quicker than consumption of the consumable bar. Essentially, frictional heat thermally softens and weakens the consumable material, so that a discreet region of not wholly transformed material shears away from the consumable.
Figure 11 shows that a very good metallurgical bond between the fill material and the substrate can be achieved with preferred pillar processing conditions. This microsection represents good welds produced with gas shielding.
Fig.11. Typical microsection of weld bonded pillar, HAZ and plate substrate
Hardness survey
Hardness measurements taken at mid plate thickness gave the following average values:
| Substrate material | 180HV 10 |
| HAZ | 250HV 10 |
| Pillar material | 182HV 10 |
FHPP process characteristics
Very good quality FHPP welds have been produced in steel and certain non-ferrous materials, using a parallel hole geometry, and these have been characterised by good impact, tensile, and bend results. In ideal conditions, the distance by which the rotational frictional interface moves should be very small, giving an almost continuous movement of a series of helical shear interfaces along the weld. On occasion, however, periodic changes in the microstructure were observed which are believed to be associated with changes in location of the rotational interface, ie when the torsional resistance of the frictional interface equals the torsional strength of a thermally softened region further along the consumable. These periodic changes represent regions within the deposit material where the consumable has not been fully transformed. This effect is particularly noticeable in parallel hole welds where a comparatively high rotation speed and a high consumable displacement (burnoff) rate was used. The onset of periodic changes are also effected by the consumable material properties. In addition, periodic changes can occur in parallel hole welds, when the hole depth to hole diameter exceeds a ratio of about 3.5:1.
For those materials that are less viscous and have a lesser tendency to extrude, tapered holes and consumables with a correspondingly more acute taper are used, as shown in Figure 12. The use of tapered holes together with tapered consumables enables a reactive force as well as hydrodynamic forces to be exploited in making the joint. This variant of the technique enables FHPP welds to be made in materials regarded as difficult to extrude or flow at forging temperature. The geometry design is such that a nominally uniform gap is maintained between the changing cross-section of the consumable and the changing hole diameter during welding. If the angle of the taper is too obtuse, the deposited pillar material will not climb. [3] To some extent the angle of the taper will be material dependent, ie the relative ease with which the material can be conventionally extruded may be a feature worth noting. Current indications are that included angles of less than 30° for certain copper alloys, and angles of less than 25° for aluminium alloys would be preferred.
Fig.12. Friction hydro pillar processing with taper hole and more acute tapered consumable
Use of tapered holes and correspondingly more acute tapered consumables reduces the tendency for large periodic regions to form in the deposit material. The taper pillar welding technique allows comparatively higher rotational speeds and higher consumable displacement rates to be used than are possible with parallel holes and parallel consumables.
As well as providing a nominal amount of sidewall reactive support, tapered holes and tapered consumables give an increasing cross-sectional area. As the tapered bar is consumed the increasing cross-sectional area provides a corresponding increase in strength. In addition, the diverging gap above the interface tends to prevent the back extrusion of plasticised material, again reducing the tendency for large changes in frictional interface location.
With material deposition, the friction rotational interface preferentially travels toward the relatively smaller mass consumable bar that grows by a series of uniform helical shear regions. [4] Heat flow and thermal conduction cause the rubbing surface to be part spherical in shape.
Notwithstanding the need for development work, the combined use of gas shielding with taper holes and a more acute tapered consumable will help make the FHPP technique more robust.
Work at TWI has shown that good mechanical integrity can be achieved. Metallographic examination has shown that the FHPP deposit material is hot worked with very tine grained microstructure.
The process advantages can be summarised as follows:
- relatively deep penetration narrow gap welding and repair technique
- low cost (bar stock) consumables
- environmentally friendly process
- rapid - 100mm deep holes can be filled in less than 20 seconds
- suitable for magnetically hostile environment
The process is still in embryonic development, nevertheless, it is not unreasonable to consider its use for fabrication and repair for a wide range of materials.
Summary
The main features highlighted by this investigation into friction seam and FHPP butt welding were:
- active gas shielding (Hytec 5) can reduce atmospheric contamination of the friction seam welding deposit on to the substrate and minimise the deposit surface contamination, subsequent to multi-layer deposition.
- use of active gas shielding (Hytec 5) has enabled lower burnoff rates to be used where good quality FHPP welds can be produced.
Future work
Although the conditions used provided acceptable results, further work will be necessary to optimise the process parameters and consumable and substrate geometry.
References
| N° | Author | Title |
|
| 1 | Klopstock H and Neelands A R: | Patent specification: 'An improved method of joining of welding metals.' Application date 15 October 1941, ref 572789. | Return to text |
| 2 | Fukakusa K and Satoh T: | 'Travelling phenomena of rotational plane during friction welding - application of friction hardfacing.' International symposium Resistance welding and related welding processes, 10-12 July 1986, Osaka. | Return to text |
| 3 | Hasui Atsushi: | 'Effect of the relative difference of bar diameter on the friction welding phenomena - study of the friction welding of different diameter bars.' (report 1) Keio University IIW doc Comm 11-679-81. | Return to text |
| 4 | Thomas W M and Nicholas E D: | 'Friction hydro pillar processing.' Connect June 1992. | Return to text |
The friction department often works with single clients to investigate specific applications. Member companies are also welcome to join any of the multi-client projects being planned or in progress.
