Cold supersonic deposition
It may soon be possible to both make and repair components using an entirely new method – that appears to achieve the impossible. Tom Shelley reports
Fabrications can be built up and repaired by cold particles hurled at a surface at supersonic speeds. Using the process, metals which cannot be fusion welded can be bonded to, and microstructures, impossible to produce by other methods, can be developed.
Cold Gas Dynamic Spraying was first demonstrated by Dr Anatoli Papyrin in the mid-1980s at the Institute of Theoretical and Applied Mechanics in Russia. The present development, designated Cold Gas Dynamic Manufacturing (CDGM), is currently being researched by a team at the University of Liverpool’s School of Engineering.
The key breakthrough is the ability to cause ballistic impacts that heat up particles to temperatures at which they deform plastically, bonding the particles together and to the substrate. However, the thermal stresses associated with molten metal spraying do not occur. The process has applications in near net shape fully dense forming of novel components as well as restoring the original profile of certain high value components which have been subject to wear.
The process involves the feeding of solid powders into a converging section within a gas nozzle, followed by entrainment through a gently diverging barrel. The gas stream and entrained powder is accelerated to a speed of 600 to 1,500m/s (the speed of sound in air being around 333m/s). The speed is controlled in such a way that the resultant impacts create enough energy to deform the particles plastically, but not so high as to cause melting. The impacts are nonetheless sufficiently violent enough to rupture any surface films, generating direct interfaces between the underlying materials. The bonding mechanism may be compared to that achieved by explosive or friction welding, but without the high cost and hazard of the former, or the expensive equipment required by the latter.
The temperatures experienced by the materials are much lower than those encountered in processes such as High Velocity Oxygen Fuel (HVOF) and arc and plasma spraying, blown powder laser cladding or arc welding processes. Thermal spraying technologies are widely used to deposit coatings, but have so far shown relatively little application for near net shape forming, apart from the Novarc injection and press tool mould manufacturing process described in Eureka’s January issue. Such high temperature processes have also been applied to the repair of worn turbine blades, but this type of work is fraught with all kinds of problems. Laser cladding, on the other hand, is more suited to automation, with better geometrical control of the deposited material, and significantly improved accuracy and resolution.
However, it has been discovered that the extremely high thermal gradients involved in laser processing can lead to a number of problems. These include: component distortion and cracking, due to thermally generated residual stresses; loss of control over the geometry of the deposited material, due to variable substrate temperature; and loss of control over the geometry over the microstructure of the deposited material, due to complex thermal cycling.
CDGM, on the other hand, runs at a much lower temperature, reducing thermal problems, and allows the bonding of powders to substrates which would normally be totally incompatible, if processed by thermal means and building up composite coatings from powders of different materials. Structures so constructed retain their original form, and powder compositions can be varied or changed entirely in different layers, so as to produce properties that change with depth. A typical objective might be to build up a structure with a tough inside and a hard outside, with a continuous gradation of properties from tough to hard. Such a construction would overcome one of the main drawbacks of hard coating conventional materials, which is to risk delamination or spalling of the hard coating as a result of thermal mismatch.
CDGM requires the use of large volumes of gas, generally nitrogen, helium or air. Given the high speed of sound in helium (around 948m/s) it is this gas which is preferred. Helium is presently obtained as a by-product of natural gas production from certain wells. These have a limited life expectancy and, although the United States has stockpiled about 20 years worth of supply, the price is eventually expected to rise for non-US users, since the gas is very expensive to recover from other sources. Hence, one of the aims of the research project is to develop and integrate a helium recovery and recycling unit, provided by BOC Gases.
The primary test metal is aluminium, since this is particularly difficult to process with lasers and other high temperature techniques. Other materials considered suitable for the process include copper, aluminium metal matrix composites, titanium alloys and 316 stainless steel. Metallic coatings produced have a porosity of 1 to 5% and are very hard. While the deposition rate for conventional processes ranges from 0.5 to 2.0kg/hr, the new process is expected to achieve deposition rates of 10 to 30kg/hr, depending on the materials involved.
There are very specific commercial goals, which we have been asked not to reveal. The road map of development, from a single spraying system to a complete manufacturing system, has three stages and is expected to last eight years. Stage one involves the fundamental process development and materials engineering relevant to the generic technology; stage two comprises research programmes focussed on application-specific process development and optimisation; and stage three represents the post R&D industrial activity required to commercialise the manufacturing systems developed and stimulate industrial exploitation.
The process is being developed with substantial support from the Engineering and Physical Research Council, under its Next Generation Manufacturing initiative, BOC Gases, BAE Systems and QinetiQ.
Eureka says: For once it is nice to see a UK institution taking an idea from abroad and developing it to commercial reality, rather than the other way round which is all too often the case these days. Having been party to a great number of important material breakthroughs, Eureka considers this one to offer tremendous commercial potential and will once again reinforce the UK’s standing in manufacturing process research.
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