It is expected that components will soon be made and
repaired by an entirely new method that achieves the impossible.
Tom Shelley reports
Fabrications can be built up from cold particles hurled at a
surface at supersonic speeds.
Thermal stress effects arising from molten metal spraying methods
do not occur and it is possible to bond to metals impossible to
fusion weld to and develop microstructures impossible to produce
by other methods.
The process has applications in near net shape fully dense
forming of novel components as well as restoring the profile of
certain high value worn components.
Cold Gas Dynamic Spraying was first demonstrated by Dr Anatolii
Papyrin in the mid 1980s at the Institute of Theoretical and
Applied Mechanics in Russia. The present development, designated
Cold Gas Dynamic Manufacturing, is 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.
The process involves the feeding of solid powders into a
converging section within a gas nozzle, followed by a gently
diverging barrel. The gas stream and entrained powder is
accelerated to a speed of 600 m/s to 1,500 m/s. (The speed of
sound in air is normally around 333 m/s.) The speed is controlled
so that the resulting impacts cause the particles to deform
plastically, but are not sufficient to cause melting. The impacts
are nonetheless sufficiently violent to rupture 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 edition. Such high
temperature processes have also been applied to 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 control of the geometry 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 led to a number of
problems. These include: component distortion and cracking 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 with which they 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 to 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 2.0 kg/hour, the
new process is expected to achieve deposition rates of 10 to 30
kg/hr, depending on the materials involved.
The process has very specific commercial goals, which we have,
unfortunately 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.
University of Liverpool
Manufacturing Science and Engineering Research Centre
Jim Pattison email
Pointers
Process allows the deposition by firing cold particles at a
substrate at supersonic speed
Impact temperatures are sufficient to cause plastic deformation
and ruptures of surface films but insufficient to cause melting
It is thus possible to bond particles to substrates and particles
to each other in metallurgical combinations normally considered
impossible to weld
For more technical
developments see www.eurekamagazine.co.uk