Developments in friction and MIAB welding
TWI Bulletin, November/December 1989
A member of the Institute's staff since 1977, Phil Hone is a Senior Research Engineer in the Forge and Resistance Processes Department. He has worked on MIAB welding for the past five years, concentrating - successfully - on establishing the process for joining thin-walled aluminium tube. He has published a number of reports and papers on the process, Phil's work is now focused on the application of MIAB to metal matrix composites.
Head of the newly created Forge and Resistance Processes Department, Dave Nicholas' interests centre on friction welding under water, radial friction welding, other motions to develop friction heating, and properties of solid phase welds. He gained an honours degree in metallurgy from the University of Wales, and has been at the Institute since 1967.
Dave's TWI career began in the Resistance Welding Section, where he worked on spot welding of refractory alloys. He soon transferred to Friction Welding, where he has been actively involved in this solid phase joining method for some 20 years and has published a number of papers. He is a Chartered Engineer, a Member of the Institute of Metals, and a Fellow of The Welding Institute.
Dave Nicholas and Phil Hone look at the current capabilities and industrial applications of two solid phase joining processes.
Both friction and magnetically impelled arc butt (MIAB) welding are automated processes which produce solid phase welds, i.e. no fusion. Friction welding is capable of joining tubular and solid parts and its production application range can be extended, especially when the process arrangements of friction surfacing, radial friction welding (pipe joining) and orbital, linear and angular systems are taken into account. These latter systems open the way for non-circular parts to be joined, while friction surfacing provides a metal deposition method that lays deposits in the solid phase with zero dilution.
MIAB welding is used in joining both circular and non-circular hollow section parts in butt configuration and, to a limited extent, for joining tubes to plates. The process requires only axial movement of components, and its major advantages are realised on parts whose wall thickness is less than 6mm.
Friction welding
In friction welding, heat is generated by rubbing one component against another under pressure, Once enough heat has been generated, the rubbing action is terminated and the pressure is maintained (or increased) to promote the solid phase bond. The most common form of friction welding uses rotating motion, but there are several variations which allow the process to be applied to a range of part geometries and configurations.
Friction welding equipment is now highly developed to meet the production engineer's desires of reliability, quality and high production. Machines are available from commercial suppliers to accommodate a size range from 1mm solid to 250mm OD x 70mm ID tubes. Automatic component handling systems can increase production, and methods of upset material removal can be incorporated into the machine design.
Radial friction welding
The Institute devised this method particularly for joining pipes. The technique (Figure 1), involves rotation and radial compression of a solid bevelled ring into a V preparation provided by the pipe ends to be joined. To prevent collapse of the pipe ends and metal penetration into the bore, in internal support mandrel is provided at the weld location. Experimental equipment is available at TWI to weld pipes in the size range 50 to 270mm OD. Trials have established that this technique can also be successfully applied to attach driving bands to artillery shells. In this application, a solid ring of copper alloy (gilding metal) is rotated and radially compressed on to the surface of the steel shell body, thus generating a complete circumferential bond. Trials on 105 and 155mm diameter shell bodies showed that good quality welds could be made in 10-15s. Firing trials showed that the bond integrity was equal to that of the conventional method of band attachment: cold crimping into complex machined recesses.
Fig. 1. Radial friction welding - schematic arrangement
Friction surfacing
In this process, a rotating consumable bar of the material to be deposited is pressed on to the surface of the base material (Figure 2). At the same time the base material is traversed so that a strip of material is deposited. The joint between the deposited layer and the base material is a solid phase bond; there is therefore no significant dilution between the two materials. This factor allows materials which may be considered unsuitable for fusion cladding to be combined successfully.
Fig. 2. Friction surfacing - the principle
TWI has been developing mechanisms, and evaluating material combinations and alternative uses of frictional heating for four years, and has established that the technique can be used for several applications as listed in Table 1.
Table 1: Applications of friction surfacing
| Requirement | Deposit | Substrate material |
| Corrosion protection | BS 970 316S16 Stainless steel | Mild steel |
| Wear resistance | Co-based alloy (Stellite grade 6) | 316S16 stainless steel |
| Repair (non-ferrous) | BS 4300/5, 2011 aluminium alloy (4%Cu) | BS 1474, 2014A Aluminium alloy (4%Cu) |
| Surface modification | | BS 970: 817M40 Ni-Cr-Mo alloy steel |
| Wear and impact | Stellite 12 nitrogen atomised powder (between 45-125µm) | 316S16 stainless steel |
| Consolidation of granules | Machined brass swarf (untreated) | Mild steel |
Examples of geometric arrangements that can be considered for friction surfacing are shown in Figure 3, and Table 2 shows typical parameters for deposit/substrate combinations given earlier.
Fig. 3. Arrangements for friction surfacing
Table 2: Friction surfacing parameters
| Combination | Consumable diameter, mm | Rotation speed, rev/min | Applied force, kN | Traverse rate, mm/s |
| Austenitic stainless steel to mild steel | 25 | 550 | 50 | 5 |
| Stellite 6 to austenitic stainless steel | 20 | 300 | 50 | 2-5 |
| Aluminium alloy (4%Cu) to aluminium alloy (4%Cu) | 25 | 780 | 17 | 4 |
Orbital friction welding
This variation is suitable for both circular and non-circular parts when precise angular orientation is required. The motion generated exhibits uniform surface velocity, and it is possible to join complex parts and a number of separate parts in one friction weld sequence. Unfortunately, the mechanical arrangement to provide orbital movement of one part about the face of a stationary part has not been successfully developed. However, it is possible to generate orbital motion using the arrangement shown in Figure 4.
Fig. 4. Orbital friction welding (practical arrangement):
- Both components rotated at the same speed, small contact force
- Contact is maintained, axes offset
- Force established, friction results in heating metal flow
- Axes realigned, force maintained (or increased) to produce weld
Here both parts are rotated in the same direction at the same speed but with their axes offset by up to 3mm. These conditions are maintained during the friction heating period. To complete the weld cycle, relative movement is ended by returning both parts to the common axis of the machine and maintaining or increasing the welding force.
We undertook trials at Abington using such a machine to develop information on the influence of welding machine settings on the metallurgical and mechanical properties of welds made with 25mm bars in a range of carbon-manganese and low alloy steels in various finished conditions. We found it possible to obtain tensile strengths near those of the parent material, together with acceptable bond ductility and impact resistance for all the steels in the as-welded condition. Macro sections of selected welds are shown in Figure 5, which reveals nominally narrow parallel sided weld regions with full interfacial bonding.
Fig. 5. Orbital friction welds in steels:
- Mild steel
- 0.19% C-Mn steel
- 0.29% C-Ni-Mo steel
Linear friction welding
The process using this motion is commonly referred to as vibration welding. The heat needed to produce a bond is generated by pressing the parts together while one is moved at frequencies up to 100Hz through a small relative displacement in the plane of the joint. It is thought that at present this method is being used in production only for welding plastics. Parts as long as 560mm and as wide as 305mm are being welded on standard equipment. However, in certain instances total weld areas have exceeded 6500mm 2. The technique is quoted as being particularly useful for crystalline resins such as acetal copolymers, polyamides, thermoplastic polyesters, polythene, polyimides and polypropylene.
TWI has been developing the process to establish the feasibility of joining metallic materials. Preliminary welding trials with rectangular sections of 25 x 10mm and 22 x 6mm in mild steel, stainless steel (Figure 6), titanium alloy and aluminium alloy have been carried out with varying degrees of success. Welds in titanium alloy (6-4) and austenitic stainless steel exhibit quite different flash profiles, but both welds are characterised by full interfacial bond formation and excellent mechanical properties. Tensile properties equivalent to those of the parent materials have been achieved over a range of welding conditions. For example, it was possible to vary the reciprocating frequency from 16 to 40Hz and still maintain excellent weld properties. Such welds were made without atmospheric protection, which is remarkable in the case of titanium, as no oxide contamination was observed. Impact tests confirmed this, since ductile failures were obtained near the parent metal/HAZ boundary. Clearly, if oxygen had been introduced into the weld, severe embrittlement would have resulted.
Fig. 6. A linear friction weld in type 316 austenitic stainless steel
Angular friction welding
Arcuate reciprocating motion has been exploited in joining plastics.
It is used for joining thermostat housings and petrol tanks where good angular alignment is required. This particular variation of friction welding has not been applied to metallic components in volume production, However, it is TWI's intention to examine this motion more fully for joining metals. To expedite this, we have made a special attachment for the rotary friction welding machine.
MIAB welding
MIAB welding is an automated hot forge welding process in which heat is generated by an arc drawn between tube ends. The arc is impelled to move round the joint line by the interaction of the arc current and an externally applied radial magnetic field. After a suitable heating time, the arc is terminated by the parts being forged together, which expels any molten material present and forms a solid phase bond. This principle is shown in Figure 7.
Fig. 7 Principle of MIAB welding
The magnetic field required can be produced in several ways but the two most commonly used configurations are shown in Figure 8. The fields are produced using electromagnets but recent developments in rare earth magnets make these a viable alternative. Their main advantage is reduced volume and weight compared with electromagnets.
Fig. 8. Coil arrangements for MIAB welding
The major benefits of MIAB welding are:
- No rotation of either component
- Short weld times (3.5s for 3mm wall thickness low carbon steel tube)
- Low material loss
- Low fume and spatter emission
- Relatively low arc current used
Trials
Ferrous materials
TWI carried out a detailed evaluation of MIAB welding, using 51mm OD x 3.2mm wall thickness low carbon steel tube. Welding conditions were established which resulted in good quality welds satisfying the bend test requirements of BS 4204:1981 (flash welding of steel tube for pressure applications). This is a stringent weld quality assessment test and less searching tests are likely to be used in practice, depending on the application of the welded joint.
It was found that there was sufficient tolerance to variation of the welding parameters to enable welds to be made under shopfloor production conditions. Arcing in air and using a 6s schedule we established that a variation of ±10% current, ±30% forge force or ±25% arc duration could be tolerated - the welds still meeting bend test criteria. Variation in arc gap of up to 1mm (due to out-of-square tube end preparation) could be tolerated without adverse effect on weld quality. Further trials showed that, if CO 2 was used as a shielding gas, good quality welds could be made in an arc time of 2s. During a consistency trial, it was noted that there was a slight increase in exposed defect level, but this was still well within the 5% limit.
Other work in low carbon steel tube has shown that good quality welds can be made in 19mm OD x 0.9mm wall thickness tube, and in 12.5 and 25mm OD x 1.2mm wall thickness tubes. Good quality welds were made in approximately 0.6s and material loss was similar to the tube wall thickness. A tolerance to current variation of ±10% was observed and good axial alignment was achieved, even on the thinner wall tube. Trials have also been carried out on 75mm OD x 3mm wall thickness SAE4130 low alloy steel tube with good quality welds being made in an arc time of 7.5s (using CO 2 shielding). The forge pressure was 75 N/mm 2 which resulted in only 3mm material loss. This compared favourably with friction welding where, although weld time was only 6s, 230 N/mm 2 forge pressure was required, which resulted in almost 8mm material loss.
Experimental welds have been made in low carbon steel tubes of square, rectangular and octagonal section as well as the two tube sections shown in Figure 9. No alteration to the magnetic pole piece was required except for the section shown in Figure 9b, where a small tongue was aligned with the groove to introduce the magnetic field at the innermost point of the section.
Fig. 9. MIAB welds in unusual tube sections a)
The process can also be used for joining hollow sections to flat sheet. Trials with 19mm OD low carbon steel tube of 1.2mm wall thickness have established that good quality joints can be made in an arc time of 0.4s, However, the tolerance to parameter variation was smaller than when welding tube to tube joints of the same size, owing to the thermal imbalance of the parts being joined.
Aluminium alloys
To date there are no reported applications of MIAB welding for non-ferrous materials and early trials at Abington showed that these materials are particularly difficult to weld using conventional MIAB equipment. To establish the feasibility of welding aluminium alloys, a special research machine was designed and built at the Institute. The machine had a high forging speed and the ability to weld with the tube axis vertical.
When using 32 and 38mm OD x 1.6mm wall thickness 6063 aluminium alloy tubing, it was established that good quality welds could be made in 0.8s using a dual stage current cycle. A good quality weld is shown in Figure 10, and it can be seen that the upset is covered by material expelled by the forge impact. This is held by a thin ligament and can easily be removed by hand.
Fig. 10. MIAB weld in aluminium alloy (6063)
Production applications
The European automotive manufacturing industry has realised the benefits of the MIAB process and over 70 machines are engaged in manufacturing production components. These extend from safety critical joints on axles and suspension parts, to low stressed parts such as fuel tank vent pipes. Other examples are propeller shafts, shock absorbers and gas filled struts.
The rear axle of the Ford Fiesta is an example of a safety-critical joint application. The flanged spindles have been MIAB welded to a double-cranked cross tube since 1977. The joint is 60mm OD x 2.5mm wall thickness, requiring an arc heating time of about 2.5s and a current of around 600A.
MIAB welding has also been applied to coated sheet steel components in a production application. A terne metal coated inlet tube is welded to a 0.8mm sheet thickness petrol tank with virtually no disruption to the coating. The tank body is pierced and plunged to produce a circular upstand of the same diameter as the tube (41mm), and welding is carried out on a pneumatically operated machine in an arc time of less than 2s.
An example of tube-to-plate application is shown in Figure 11. Here, 44 x 15mm oval steel tubes of 2mm wall thickness are welded to plate for the tine of an agricultural beet elevator, with a weld time of about 1s. Four million welds a year are made in four automatic MIAB welding machines, replacing 25 CO 2 welders. Other examples of this configuration are mounting plates for wall brackets, end fittings for racking systems and bell cranks.
Fig. 11. Tube-to-plate MIAB welds
A variant of tube-to-plate welding is the tube-to-tube T configuration for non-circular tube. Again there are some production applications where such a joint configuration is MIAB welded, although the weld quality depends particularly on the ability of the tube to support the applied forge force.
MIAB welding has also been used for welding a town gas distribution network in Japan. In this case the welding head was suspended from a boom on a lightweight truck. The prime movers and the control equipment were housed within the truck so that welding on site, either above ground or in normal pipe width trenches, was possible. The pipes used were 60 and 89mm diameter, and good quality welds were produced consistently. Increased productivity was achieved, not only because of the greatly reduced weld time, but also because there was no need to enlarge the trench width when the joints were made below ground level. Further experimental work has shown that welds can be made in tubes of up to 300mm diameter. Many urban services are supplied in thin wall steel pipe and the concept of on-site MIAB welding has considerable potential in pipe distribution systems for both liquids and gases, and equipment is now being developed outside Japan.
In conclusion
Both MIAB and friction welding produce solid phase welds and are capable of being fully automated. MIAB welding is limited to welding tubular section components and most applications are within the automotive component manufacturing industry. There are also many automotive components which utilise friction welding, and this process is used in many other applications. Friction welding is capable of joining dissimilar materials and is an accepted technique in a range of applications where this is a requirement.
MIAB welding has advantages in high volume, thin-wall applications, and the potential of the process has yet to be fully exploited. The same is true for friction welding of heavier wall thickness tube and solid component applications and, when all the more recently developed variations of the process are taken into account, the possible applications are endless.
