Friction and flash welding - mechanical properties reviewed

TWI Bulletin, February 1985

Steve Westgate, BSc, is Principal Research Welding Engineer in the Sheet and Precision Processes Department, and Sue Dunkerton, BSc, MWeldI, is a senior Research Metallurgist in the Advanced Heavy Section Processes Department.

A number of research programmes at The Welding Institute have concentrated on determination of mechanical properties of 'non-fusion' welds. The data collected have allowed guidelines on choice of welding parameters and material compositional effects to be determined. The results obtained over the last few years are summarised in this article, which compares friction and flash welding.

The mechanised, non-fusion, forge butt welding processes are becoming increasingly used in industry, not only in place of arc welding processes but also as an alternative to other metal forming procedures such as casting, forging and machining. Bend and tensile testing are the normal means of assessing forge butt welds and the majority of published literature covers these techniques. In the past there have been only a few applications where toughness is an important consideration, e.g. flash welded chain and friction welded drill pipe. As a result, limited data are available on toughness properties ^[1,2] and existing data are often difficult to interrelate because of the widely different criteria and test methods used.

An increase in the application of forge butt welds in areas where toughness properties are specified, such as oil and gas pipelines ^[3-6] together with an increasing awareness of the need to specify toughness properties on existing applications, has indicated the need for more fundamental data.

Published work indicates that the as-welded properties of friction and particularly flash welds are poor and that post weld heat treatment can be beneficial in improving toughness. However, little information is available to recommend welding schedules for improved toughness and it was considered that this aspect deserved deeper study.

Research projects are being carried out at The Welding Institute on factors affecting toughness in non-fusion welds, particularly in flash and friction welded joints. Effects of post-weld heat treatment and welding parameters have been studied in detail. Use of AC and DC power supplies has been compared for flash welded joints, and various forms of friction welding continuous drive, inertia and orbital welding have been studied. Work has been carried out principally on C/Mn steel bar but comparisons have been made with tubular material and a range of C, C/Mn and low alloy steels for friction welding.

Work on other non-fusion techniques, i.e. magnetically impelled arc butt (MIAB) welding and diffusion bonding, has been aimed largely at process optimisation. Some assessment of impact properties has, however, been carried out on diffusion bonds in a medium C steel.

The Welding Institute's work on mechanical properties of friction and flash welds covering the influence of the welding process, the materials being welded and their subsequent heat treatment is summarised below.

Friction welding

Process type

Continuous drive friction welding, which uses a constant rotational speed during frictional heating, is favoured in the UK and Europe, so much of the process work has concentrated on this variant of the process. However, inertia welding, which employs the energy stored in a rotating flywheel and is a widely used production technique in the USA, and orbital welding, which is little used, have also been investigated. (Orbital welding provides an orbital rather than circumferential relative motion of the weldments during frictional heating.) Continuous drive friction welds gave joints with various heat affected zone (HAZ) profiles depending on the welding parameters set (Fig. 1). Narrow HAZs were associated with low burnoff and lower rotational speed conditions. Welds made using inertia welding were found to have narrower HAZs than conventional friction welds, but as the mechanics of the two processes vary considerably, welding conditions cannot be strictly compared. Orbital welds, which were produced at just one set of welding conditions, gave similar weld zone structure to the continuous drive friction welds at comparable burnoff distances.

Early work indicated that the absorbed energies obtained on Charpy testing friction welds in the as-welded condition were low even at temperatures as high as + 60°C for all but orbital welds (Fig. 2). The types of weld microstructures produced by friction welding are not conducive to higher impact energies, and at temperatures above the transition it was found that failures become dominated by inclusions which are reorientated during the weld cycle. In orbital welding, a modified distribution of inclusions is obtained which is thought to account for the better impact properties. The radial distribution of inclusions produced using friction welding is compared with the spiral distribution typical of continuous drive and inertia welds in Fig.3.

Welding parameters

Variations in welding parameters which led to decreased weld times were found to give finer microstrutctures as a result of restricted austenite grain growth during welding and thus could improve weld toughness. Such conditions included reduced burnoffs, increased friction pressures and reduced rotation speeds (the last two increasing the burnoff rate). It was found that a minimum burnoff distance was required, however, to ensure freedom from weakly bonded areas, particularly at the centre of solid bar weldments.

Although most welding conditions yielded as-welded Charpy values which would not meet the common pipeline specification requirements of 27J at -10°C, welds made in tubular material, with high burnoff rates, exceeded this level. However, the higher burnoff rates resulted in higher weld hardnesses and narrower, finer grained welds. Thus, a compromise has to be reached between acceptable impact energies and acceptable hardness values.

One other parameter found to have a pronounced effect on both microstructure and impact properties for continuous drive friction welds is the deceleration time ^[9] (controlled by braking effort) An increased deceleration time (i.e. decreased deceleration rate) introduces more plastic deformation in the weld region during cooling. This produces finer weld microstructures with improved Charpy values without an increase in hardness.

In view of the effectiveness of introducing deformation of the weld during cooling, a study was made of post rotational twist. This technique was initially developed for correcting radial alignment of parts as shown in Fig. 4 and was adapted in this work to give a controlled twist rate and angle for working the weld metal after the forging stage. ^[10] Figure 5 illustrates the improvement in tensile properties which was achieved when post rotational twist was applied to friction welds in a 0.2%C/ 1.5%Mn steel. Furthermore, maximum strengths arid satisfactory bend properties were obtained at lower welding forces than were required for conventional welds.

The impact properties of the friction welds made in this particular steel, which contained 0.024%S, both with and without a post rotational twist, were poor when compared with the as-received parent metal. However, work with a low sulphur content steel (<0.005%S) showed that a post rotational twist could improve the impact properties at ambient temperature from 16 to 52J.

Post weld heat treatment

Although the as-welded toughness properties of friction welds can be improved by choice of welding conditions or special techniques, Charpy values are still generally low relative to parent material levels. The greatest improvement in properties can be obtained by a post weld heat treatment which produces a tougher weld zone microstructure. In some work done using a 0.2%C/1.5%Mn steel, the best results were obtained using a furnace treatment, either normallsing or quench and tempering. An example of the effectiveness of such a treatment is shown in Fig. 6 where the Charpy value at -10°C was increased from about 25J to almost 200J for the low sulphur content steel.

Good results have also been achieved by using induction heating to provide a rapid, practical heat treatment.

Material composition

The work described above, which used a 0.2%C/1.5%Mn steel, highlights the importance of material cleanliness. As sulphur content increases, the reorientated inclusions influence toughness and largely override the effect of process parameters. Even post weld heat treatment is less effective for high sulphur content steels, as indicated in Fig. 6, where the results for 0.024 and <0.005%S steels are compared. Only orbital friction welding gave reasonable results for the higher sulphur content steels, because the orientation of inclusions is different from that found in continuous drive welds (Fig. 3). These results show that, in general, very low sulphur levels should be specified for applications where good weld toughness is a requirement.

Work on a wider range of steels including C, C/Mn, and low alloy steels ^[11] has shown that weld properties are related to carbon content and alloy content. In particular, increasing chromium, molybdenum and vanadium were found to reduce toughness. The work carried out has enabled guidelines to be produced for maximum acceptable compositional levels of carbon, sulphur, etc, to meet specified toughness properties.

Flash welding

Welding conditions

In general flash welds exhibit a wider HAZ but less steep an upset in comparison with friction welds (cf Fig. 1 and 7). The resultant microstructure is coarser and gives poor as-welded toughness properties (Fig. 2) although similar bend and tensile properties might be expected for the two processes. A range of weldiiig conditions has been examined to attempt to improve these properties ^[12] but none was effective. For heat treated (normalised) welds, however, two factors were found to be significant in influencing the toughness heat input and forge force.

The influence of heat input was determined for a series of welds made using a preheat sequence, where the heat input was varied by using a long duration/moderate current preheat sequence and a short duration/higher current sequence. The latter, which produced a narrower weld and finer microstructure, led to improved toughness after post weld normalising. However, this improvement, achieved when welding a 25mm diameter C/Mn steel bar, could not be reproduced for a 50mm diameter bar. In the latter, little change in microstructure was achieved with different heat inputs. The potential for influencing weld toughness by choice of heating conditions is therefore fairly limited, especially as the initial requirement is to ensure freedom from weld defects. Only on thin sections is there scope for limiting the width and coarseness of the HAZ.

Forge pressure is an important parameter in the production of flash welds and a level of 70 N/mm ² is generally used for C and C/Mn steels. Heat input is normally adjusted so that the subsequent upset height is sufficient to eliminate defects from the weld region. Also, upset angle is normally limited to about 90° (included) to avoid excessive reorientation of inclusions in the HAZ.

Substantial benefit can be gained from a reduction of the forge pressure requirement, as a greater cross sectional area of material can be welded on a machine of a given capacity. Work on 50mm diameter 0.2%C/1.5%Mn steel bar showed that welds exhibiting good bend quality could be obtained at a forge pressure as low as 30 N/mm ². Impact properties deteriorated as upset pressure was reduced (Fig. 8) although the reduction in Charpy values was only slight between upset pressures of 70 and 40 N/mm ². Forge pressures higher than 70 N/mm ² are of no benefit since the wide softened zone suffers excessive upset and weld properties are increasingly influenced by reorientated inclusions. These results demonstrate the potential for valuable savings on machine capacity.

Post weld heat treatment

As-welded impact properties of flash welds are always low and consequently most of the impact testing work has been on normalised welds. A study of 50mm diameter C/Mn steel bar indicated that normalised welds achieved between 50-75% of the impact energy of the normalised parent material above the Charpy transition temperature. Transition temperatures as low as -30°C for an impact energy of 50J were obtained for normallsed flash welds, but this was still 50°C above that of the parent material. Although higher impact energy values can be achieved for friction welds than for flash welds, specified minimum properties, e.g. 27J at -10°C, can be readily exceeded for both processes.

Lower impact properties were obtained for quenched and tempered or tempered welds, compared with normalised welds and so the requirements of material strength and toughness must always be balanced in making a choice of heat treatment.

Flash welding offers the potential for short duration in-machine heat treatments and the alternative of induction heat treatment has been used for large gas pipeline welding applications. ^[3] Such treatments, although less effective than furnace heat treatment, offer cost and time savings and may be adequate to restore moderate toughness to welded joints.

Power supply type

The capability of providing high welding currents from secondary rectified DC power supplies for resistance welding has led to the application of such supplies for flash welding. The major advantages claimed ^[13] are a reduced and balanced demand on the three phase mains supply, more even heating of the weld, minimisation of inductive losses and a consequently greater freedom in machine design.

Weld properties have been compared over a range of conditions on AC and DC equipments. ^[14] It was found that Charpy values were similar for equivalent welding conditions. Despite the narrower HAZs produced on the DC machine, there was insufficient difference in microstructure after heat treatment to affect the Charpy values. Figure 8 gives an example of these results for post weld normalised samples.

Whilst it is unlikely that DC welding offers a significant advantage over AC in terms of weld quality, there may be benefits to be gained in welding speeds as well as in the electrical supply requirements.

Concluding remarks

Studies have been made of the factors affecting Charpy impact properties of flash and friction welds, in view of their increasing application in areas where such properties are important. The as-welded impact properties of these welds are generally poor and are most readily improved by use of a post weld heat treatment. However, significant improvements in the as-welded properties of friction welds may be achieved by appropriate selection of welding parameters and process type. The best results for continuous drive friction welds have been achieved when using high burnoff rates and low deceleration rates. A post rotational twist of the weld was found to be beneficial and good results have been obtained with the alternative orbital welding process. Data have also been generated to indicate the influence of steel composition on weld toughness.

Charpy values of as-welded flash welds are invariably low and post weld heat treatment is required to achieve acceptable levels. The impact properties are not greatly influenced by changes in welding parameters but good results can be achieved for normallsed welds, even when using much lower forge forces than would normally be recommended. Comparison of AC and DC power supplies for flash welding revealed no significant difference in weld quality.

References