A power to reduce the cost of forming

Tom Shelley reports on a novel process that could greatly expand the commercial use of metals currently considered too expensive and exotic

By passing high frequency pulses of electric current through metal as it is formed, it is possible to drastically increase the amount of deformation that can be achieved in a single pass, slashing costs and improving metallurgical properties. The process was developed for and is implemented in a Russian aerospace factory whose exact location is still a State secret. One of its most useful attributes is its ability to greatly reduce the cost of rolling and forming titanium. But the technique is of completely general application and is equally suitable for rolling, stamping and drawing stainless steel, aluminium alloys, tungsten, copper and precious metals and alloys. The process was invented and has been developed by Professor Oleg Troitsky, working at the Institute of Physical Chemistry in Moscow. It came out of a piece of pure research, when he and a colleague found that zinc crystals irradiated with accelerated electrons became easier to deform, a discovery which was originally published in 1963. Much of the early development work was undertaken with zinc, not because it has much use as a structural engineering material, but because it is well known to be difficult to form because of its hexagonal crystalline structure. The present process involves the passage of a brief, but intense, pulse of electric current at current densities of 10,000 to 100,000 A/square cm or more. It is well known that passing electric current through metal generates heat by ohmic losses and produces an electromotive pinch effect. However, while ohmic heating is important, it is evident that the new process depends primarily on a third and previously unknown phenomenon. Exactly what is happening is unclear, but it would appear that the brief passage of large numbers of electrons facilitates the movement of dislocations, the imperfections within metal crystals that allow them to be plastically deformed. Troitsky calls this the 'Electroplastic Effect' or EPE. The effect is strongly affected by the magnitude and direction of current flow, and can be shown to still occur even when the heating effect is negligible. To take one specific example, by the application of electric pulses, stainless steel, 100mm wide, and initially 2mm thick, can be rolled to thin foil suitable for razor blades in only 10 passes. The metal on the input side is made positive, and that on the input side, negative. Or, in the small scale apparatus shown at this year's Hannover Fair, current may be passed between the rolls. Applied voltage is 36V, and current pulses 500,000A/square cm, applied for periods of 50 microseconds at a time, 1,000 times per second. A suitably equipped rolling mill, 15m long, is said to be in operation at the Khrnitchev aircraft factory. Force requirements are reduced by 30%, and no annealing is required between reduction stages. The process is applicable to a wide range of metals and alloys, including some that are normally very difficult to form, such as the tungsten-rhenium alloys for very high temperature thermocouples used in gas turbines, which have to be rolled down from powder compacts. A published paper reveals that if ultrasound applied in addition to the electric current, it is possible to roll 0.41mm diameter tungsten wire to 0.06mm thick strip in a single pass. The manufactured strip resisted 40 bends around a 1mm radius. Without the application of electric current, no usable strip could be produced at all under the same rolling conditions. The article reports that the application of electric current produces what are described as, "profound structural changes in the metal," including a complete change in the crystallography of orientation of the grains in the final rolled structure. It is also possible to apply the process to facilitate drawing, stamping and plating processes, allowing alloys to be stamped and drawn which would, in some cases, defy conventional processing. Research on electrically-assisted rolling of titanium and other electrically-assisted forming processes is being undertaken in conjunction with Professor Hans Conrad, a Professor Emeritus at North Carolina State University. Another collaborating researcher is Professor Kenji Okazaki at the University of Kentucky. In both cases, research is being funded by the US Department of Defence. The aerospace industries are heavy users of titanium in both the US and Europe. Titanium is also an ideal material with which to make products from car components to bicycles. It is as strong as steel, half the weight and sufficiently corrosion resistant to render it a favoured material for surgical implants. However, its use on the road is presently restricted to expensive competition bicycles and Formula One racing cars, mainly because of the high forming costs. Fundamentally, titanium is an abundant metal. The base metal cost is expected to halve shortly, thanks to a new extraction process invented by Professor Derek Fray at Cambridge, England, currently being piloted by Qinetiq. The new electrically-assisted forming processes should cut forming costs by 50%, and the combination of the two could make titanium parts as ubiquitous as those presently made in aluminium alloys. Should anyone wish to try the efficacy of the method, Professor and his colleagues have set up a small company, HighT Technology, which is manufacturing electrically-assisted hand powered rolling units, suitable for producing products for jewellery and other small volume applications. The 'Mini Walzen,' shown at this year's Hanover Fair, is capable of rolling 70mm wide strip, 2mm thick, down to 50 microns thickness. Used to roll copper, silver and gold, it is said to achieve 50% reductions in a single pass. Electric power is applied via graphite contacts on the axle of the upper roll. The frame provides the return path. Price (ex-works) is $3,000 including electrics. For those interested in obtaining more technical information, Professor Troitsky has contributed substantially to an 843-page book on the subject (in Russian!). The firm is also looking for partnerships and investment to enable the commercial manufacture of larger units and further development of the process. Pointers Application of very short bursts of intense electric current greatly facilitate the rolling, stamping and forming of a wide range of metals and alloys Although the passage of electric current produces heat, the primary action is new to science The process is proving to be of particular interest to those engaged in finding better ways of forming materials presently regarded as expensive and difficult Eureka says: A process that could drastically cut the cost of forming components such as titanium could bring Formula 1 technologies into the mainstream automotive marketplace, as well as encouraging the manufacture of light weight bicycles at affordable prices HighT Technology Professor Oleg Troitsky HighT Technology by email