Wayne Thomas is Section Leader Surface Engineering within the Forge and Resistance Processes Department. The Section is currently concerned with development of friction deposition techniques for hardfacing, corrosion resistant, non-ferrous, and metal matrix composite materials. He joined The Welding Institute in 1983. Since that time, Wayne has been responsible for friction surfacing, friction seam welding and transformation hardening by friction. He is also currently project leader for TWI's advanced disc brake initiative. In parallel with his activities at TWI, Wayne is pursuing an external MPhil at Brunel University on 'Fundamental aspects of solid phase deposition techniques'.
Dave Nicholas is Head of the Forge and Resistance Processes Department which is concerned with research on, and development and application of, resistance welding (spot, seam, projection, flash etc), friction welding (rotary, linear, stud and surfacing), MIAB and MIAF welding, and explosive welding.
He joined TWI in 1967, after obtaining his degree in metallurgy, working for the first year with resistance welding then in friction welding where he led the Section for almost 20 years. He was responsible for the introduction of friction surfacing and the use of alternative motions such as orbital and linear for joining non-round components. During the last few years he has led a project to develop a linear motion friction welding machine, which was successfully commissioned in January 1990. Another milestone during his time in friction welding was the development of the world's first underwater friction stud welding machine capable of operation at water depths of around 300m.
In his TWI career he has been involved with development projects spanning a wide range of industries including nuclear, automotive, electrical, offshore, chemical and metal winning.
Advances in friction surfacing now make it possible to use the process to manufacture bimetallic industrial cutting machine blades and to deposit metal matrix composite materials. The Institute's demonstration machine has shown industry how friction surfacing can be applied to deposit coatings to withstand corrosive or abrasive service environments. Wayne Thomas and Dave Nicholas review the latest work on friction surfacing.
The design and manufacture of a demonstration friction surfacing machine at Abington ( Figure 1) has reawakened interest in friction surfacing. The process involves pressing a rotating consumable rod on to a moving substrate to leave a surfaced layer.
The machine has made it possible to introduce the technology to business and industry at international conferences and exhibitions in America, Germany and the UK. Not only does the machine demonstrate the friction surfacing process: it also shows the way forward for the manufacture of comparatively low-cost mechanically actuated force system friction welding and surfacing machines.
An advantage of a mechanically actuated force system is that the consumable feed rate (displacement rate) is maintained at a preset value; the optimum consumable feed rate is thus directly enforced, rather than the rate being a result of the applied welding conditions.
When not travelling the world, the machine is used in a Co-operative Research Programme investigation into use of small (nominally 4mm diameter) consumable bars in friction surfacing.
Metal matrix composite materials
Since the early seventies, there has been a need to join metal matrix composites (MMCS) to other materials, and their wider use has been hindered by the lack of suitable joining techniques. However, the solid-phase nature of friction surfacing offers particular advantages which will aid the application and development of such materials.
Some of the less-obvious advantages of friction surfacing have just been realised not only does it enable commercially-produced MMCS to be deposited ( Figure 2); it also allows the manufacture of composite materials during surfacing.
Figure 3 for example shows a clad layer with dispersed discontinuous alumina particulate in an aluminium metal matrix produced during friction surfacing using composite consumables.
Alumina (aluminium oxide - Al 2 O 3 ) particulate, depending on the deposition technique, has a hardness value of at least 1500 HV. Its ability to abrade the surface of other materials places alumina ninth on Mohs' scale of hardness, equal to sapphire (diamond equals 10). Improved mechanical properties are achieved when alumina is embedded in a confining matrix. Exceedingly good wear performance has been reported by TWI member companies, and it is expected that MMCS will replace conventional hardfacing materials in many applications.
Applications
Friction surfacing techniques are already helping to satisfy demands for longer product lives, under more arduous process conditions, at lower cost. Not only can friction-surface-clad layers improve the tribological behaviour of many engineering components; the technique can conserve strategic materials and in so doing reduce demands on the environment - for it can reduce the waste normally produced during manufacturing, and facilitate the reclamation and recycling of components at the end of their normal life.
Industrial machine blades are a typical example of applications which are benefiting from friction surfacing. Friction surfacing has improved the cutting edge quality of a number of cold-work, hot-work and high-speed tool steels by providing a deposit with a fine hot-forged microstructure. In addition, deposition of these materials on to tougher substrates has improved the safety of the blade by reducing the possibility of its shattering on contact with foreign bodies. Cladding of valve seats with cobalt base material and carrier rollers with nickel base alloy, and the refurbishment of certain engine components are typical of the many varied applications.
