Note: Descriptions are shown in the official language in which they were submitted.
~.2~ 6
PRODUCTION OF METAL SPRAY DEPOSITS
This invention relates ~o the production of metal
or metal alloy spray deposits using an oscillating spray-
for forming products such as tubes of semi-continuous or
continuous length or for producing tubulaP, rol~. rin~
cone or other axi-symmetric shaped deposits of discrete
length. The invention also relates to the production of
coated products.
Methods and apparatus are known (our UK Patent Nos:
137926~, 1472939 and 1599392) for manufacturing spray-
deposited sbapes of metal or metal alloy. In these
known meth~ds a stream of molten meta1. or metal allo
which teems from a hole in the base of a tundisH. is
atomised by means of high velocity jets of relatively -
cold gas and the resultant spray of atomised particles
is directed onto a substrate or collectlng surface to
form a coherent deposit. In these prior methods it is
al60 disclosed that by extractlng a controlled amount of
heat from the atomised particles ln flight and on
depositiod, lt ls possible to produce a spray-deposit
which is non-particulate in natur~, over 95~ dense and
po~se~ses a substantially unlformly distrlbuted, closed
to atmosphere pore structure.
At pre~ent product~, such as tubes ~or exampl~, are
produced by the ga~ ~to~i~ation of ~ stream of molten
metal a~d by directing the reaultant 6pray onto a
~' ~
.k~ '
. . .
.: . . -
.. .
'. .
.
..
~s~
rotating tubular shaped substrate. The rotating substrate can
either traverse slowly through the spray to produce a long tube
in a single pass or may reciprocate under the spray along its
axis of rotation (as disclosed in our UK PatPnt No: 1599392)
to produce a tubular deposit of a descrete length. By means
of the first method (termed the single pass technique~ the
metal is deposited in one pass only. In the second method
(termed the reciprocation technique) the metal is deposited in
a series of layers which relate to the number o~
reciprocations under the spray of atomised metal. In both
these prior methods the spray is of fixed shape and is fixed in
position (i.e. the mass flux density distribution of particles
is e~ectively constant with respect to time) and this can
result in problems with respect to both production rate and
also metallurgical quality in the resulting spray deposits.
In the drawings:
Figure la is a schematic ~ront view o~ a deposition
profile on a tubular shaped substrate;
Figure lb is a schematic front view of a deposition
profile o~ another embodiment o~ Figure la;
Figure 2a is a schematic front view of a deposition
proPile on the transverse spray depQsit on a tubular
substrates; and
~ igure 2b is a schematic ~ront view Q~ a deposition
pro~ile o~ another embodiment o~ Fiyure 2a~
These problems with regard to the single pass technique
are best understood by re~erring to Figure 1 and Figure 2. The
shape o~ a spray o~ a~omised molten metal and the mass
distribution o~ metal particles in the spray are mainly a
r r~
- 2a -
function of the type and specific design of the atomiser used
and the gas pressure under which it operates. Typically,
however, a spray is conical in shape with a high density of
particles in ths centre i.e. towards the mean axis of the spray
X and a low density at its periphery. The "deposition profile"
r
' C~
~.~75.~`6
-- 3
of the deposit D which is produced on a tubular-shaped
substrate 1 which is rotating only under this type of
spray is shown iD Figure l(a)~ It can be seen that the
thickness of the resultlng deposit D (and consequently
the rate of metal depositlon) varies considerably from a
position corresponding to the central axis X of the
spray to its edge~ Figure 1(b) shows a section through
a tubular spray deposit D formed by traversing a
rotating tubular-shaped collector 1 through the same
spray as in Figure l(a) in a single pass in the
direction of the arrow to produce a tube of relatively
long length. Such a method has several major
dlsadvantages~ For exampl~, the inner and outer surfaca
of the spray-deposited tu~e are formed from particles at
the edge of the spray which are deposited at relatively
low rates of deposition. A low rate of deposition
allows the already deposited me~al to cool excessively
as the relatlvely col~ atomlsing gas flows over the
deposition ~urface~ Consequently, subsequently arriving
particles do not "bond" effectively with the already
deposlted metal reRulting in porous layers of
lnterconnected porosity at the inner and outer surfaces
of the depositP. Thls lnterconnected porosity whlch
connects to the surface of the deposit can suffer
internal oxidation on removal oP the deposlt Prom the
pro~ecti~e a~osphere in~ide the ~pray chamber. In
total these porous layerfl can account for up to 15~ of
.
.
'
,
'7S.X~
the total deposit thickness~ The machlning off of these
porous layers can adversely affect the economics of the
spray deposition process~ The central portion oP the
deposit is formed at much higher rates of particle
deposition with much smaller time intervals between the
deposition of successive particles. Consequently~ the
deposition surface is cooled less and the density of the
deposit is increased, any porosity that does exist is in
the form of isolated pores and is not interconnected.
The maximum overall rate of metal deposition (i~e~.
production rate) that can be achieved (for a given
atomiser snd atomising gas consumption) in the single
pass techn~que is related to the maximum rate o~
deposition at the centre of the spray. If ehls exceeds
a certain critical level insufficient heat is extracted
by the atomislng gas from the particles in flight and on
depositiod, resulting in an excessively high liquid
metal content at the surface of the already depasited
metal~ If this occurs the liquid metal is dePormed by
the atomislng gas as it impln~es on the deposltion
surface and cfln also be e~ected from the surface of the
pre~orm by the centrlfugal Porce generated from the
rotation of the collector~ Furthermor~, castlng eype
detect~ ~e~g. shrinka~e poro~lty~ hot tearln~ etc.) can
occur in the deposlt.
.
: ' ' '
~7~
A further problem with the single pass technique of
the ptior art ~s that the depos~tion sur~ace has a low
angle of incllnation relative to the dlrection of the
impinging particles (as shown in Figure l(b)) i.e. the
particles ~mpinge the deposition surface at an oblique
angle. Such a low i~pingement angle is not desirable
and can lead to porosity in the spray deposit~ This is
caused by the top parts of the deposition surface acting
as a screen or a barrier preventing particles from being
deposited lower down. As the deposlt increases in
thickness particularly as the angle of impingement
becomes less than 45 degree~, the problem becomes
progressiv~ely worse. This phenomenon is well known from
conventional metallising theory where an angle of
impingement of particles relative to the deposition
surface of less than 45 degrees is very undesirable and
can re6ult in porous zones in the spray deposit~
Consequently~ using the slngle pass technique there ls a
llmit on the thickness of deposit that can be
successully produced. Typically~ this is approximately
50mm wall thlckness for a tubular ~haped deposlt.
The three major problems associated with the single
pass technique; namely, surface poroslty. limited metal
deposition rate and llmited wall thickness can be partly
overcome ~y using the reciprocation technlque where the
m~al 18 dep~it~d in a series a~ layer~ by traverslng
the rotating collec~or backwards and orward~ under the
~, .
~,
'
.
- . ,
, ' ~'- ' ' ~' '
7~
spray~ Howeve~, where reciprocation movements are
required there is a practical limit to the speed of
movement particularly with large tubular ~haped deposits~
(e.g. 500kg) due to the deceleration and acceleration
forces generated at the end of each reciprocation
stroke. There iB also a limit to the length of tube
that can be produced as a result of an increasing time
interval (and therefore increased cooling of the
deposited metal) between the deposition of each
successive layer of metal with increasing tube length~
Moreove~, the microstructure of the spray deposit often
exhibits "reciprocation bands or llnes" which correspond
to each reclprocatlon pass under the spray~ Depending
on the coaditions of deposition the reciprocation bands
can consi6t of fine porosity and/or microstructural
variations in the sprayed deposit corresponding to the
boundary of two successively deposited layers of metal;
i.e. where the already deposited metal hafi cooled
excesslvely malnly by the atomising gas flowlng over lts
surface prior to returning to the fipray on the next
reciprocatlon of the substrate~ Typlcally the
reciprocatlon cycle would be of the order of 1-10
~econds dependin on the slze o~ the 6pray-deposlted
artlcle.
~ he proble~ a~ociatad with both the single pa~s
technique and the reciprQcation technique can b~
.~ ' .9
.
5~
- 7
substantially overcome by utilising the present
invention.
According to the present invention there i5
provided a method of forming a deposit on the surface of
a substrate comprising the steps of;
generating a spray of gas atomised molten meta~,
metal alloy or molten ceramic particles which are
directed at the substrat~,
rotating the sub6trate about an axis of the
substrat~.
extracting heat in flight and/or on deposition from
the atomlsed particles to produce a coherent deposi~.
and
oscillating the spray so that the spray is moved
over at least a part of the surface of the substrate~
The atomising gas is typlcally an inert gas such as
Nitrogen, Oxygen or Helium~ Other gase~. howeve~. can
also be used including mixed gases which may contaln
Hydroge~, Carbon Dioxid~. Carbon Monoxide or Oxygenr.
The atomising gas is normally relatlvely cold compared
to the stream of liquid metal.
The present invention ls particularly applicable to
the continous production of tube~ or coated tubes or
coated bar aad in this arrangement the substrate ls in
~he Porm o~ R tube or sQlld bar which is rotated and
traver~ed ln an axial direction ln a ~ingle pass under
the oscillatlng ~pray~, In this arrangement the
~'
~ ~ 7 ~ j ~d ~ ~ ~
-- 8
oscillatiod, in the direction of movement of the
substrate has several important advantages over the
existing method using a fixed spray. These can be
explained by ~efe~ence to Figures 2(a) and ~(b). The
"deposition profile" of the deposit which is produced on
a tubular shaped collector which ls rotating only under
the oscillating spray i6 shown in Figure 2(a)~ By
comparing with Figure l(a) which is produced from a
fixed spray (of the same basic shape as the oscillating
spray) it can be seen that the action of oscillatlng the
spray has produced a deposit which is more uniform in
thickness. 'Flgure 2(b) shows a section
through a 'tubular fiprayed deposit formed by traversing
in a single pass a rotating tubular shaped collector
through the oscillating spra~. The advantages of an
o~cillating spray are apparent and are as follows
(compare'Figures l and 2):
(i) Assuming that there i8 no varlation in the
~peed o$ movement of the 6pray within each oscillation
cycle the majority of metal will be deposited at the
same rate of deposition and therefore the conditlons of
deposition are relatively uniform. The maxlmum rate of
metal deposition is also lower when compared to the
fixed spray oE~Fl~ure l~a~ which means that the overall
d~po~ieion rate can be increased withou~ the deposltion
~urface becoming exce~ively hot (or cQntalning an
excesslvely hlgh liquid content~.
~,. ..
~ 7~
g
(ii) The percentage of metal at the leading and
trailing edge~ of the spray which is deposited at a low
rate of deposition is markedly reduced and therefore the
amount of interconnected poro~ity at the inner and outer
surface of the spray deposited tube is markedly reduced
or eliminated altogether.
(iii) :Por a given deposit thickness the angle of
lmpingement of the depositing partlcles relative to the
deposition surface i8 conslderably higher.
Consequently much thicker deposits can be successfully
produced using an 06cillating Qpray.
It 6hould be noted that slmply by increaslng the
amplltude of oscillatlon of the sprqy (within limits
e.gr. included angles of oscillation up to 90 can be
used) the angle of impingement of the particles at the
deposition surface can be favourably influenced and
therefore thicker deposits can be produced~ In
additiod, for a given depo6i~, an lncreased amplitude
also allows deposltion rates to be increased, ~or gas
conHumption to be decreased~ Therefor~, the economics
and the production output of the spray depo6iticn
process can be increased.
The pre~ent lnvention is also applicable to the
production of a 6prayed depoHlt of dlscrete length whe~e
there i~ no axlal movement of the subs~rat~, i.e~ the
~ubHtrate ro~ate~ onl~. A "discrqte len~th deposit" 18
typically a slngle product of relatively Qhort
,~
.
. . .
' ~ . '
~ ~ 7 ~J~
-- 10
length~ lae. typically less than 2 metres long, Por a
given 6pray height (the distance from the atomising ~one
to the deposition surface) ths length of the depo~it
formed will be a function of the amplitude of
oscillation of the ~pray The discrete deposit may be a
tub~, ring, cone or any other axi-symmetric shape~ For
exampl~, in the formation of a tubular depoæit the spray
is oscillated relative to a rotating tubular shaped
collector 80 that by rapidly oscillating the spray along
the longltudinal axis of the collector being the axis of
rotatio~, a deposit is built up whose micro~tructure
snd properties are substantially uniform.
The reasoh'for this is that a spray. because of its low
inertl~, can be oscillated very rapidly (typically in
excess of 10 cycles per second i_e. at least 10-100
times greater thsn the practical limit for reciprocating
the collector) and consequently reciprocation line~
which are formed ln the reciprocatlon technlque using a
fixed spray are effectively eliminated or markedly
reduced using this new method~.
By controlllng the rate and amplltude of
osclllatlon and the instantaneous speed of movement o~
the spray throughout each osclllatlon cycle it 18
possible to form the deposit under whatever condltions
are required to ensure uniform depoQitlon cQnditlons and
thorcfore a uni~orm micro~ructure and a controlled
Qhape. A slmple depo~ltion pro~ile 18 ~hown ln
i, , ,~ .
"
-, -
~.~7~
-- 11
`Figure 2(a) but this can be varied to suit the alloy and
the product~. In Figure 2(a) most of the metal ha~ been
deposited at the same rate of deposition.
The invention can also be applied to the production
of spray-coated tube or bar for either single pass or
discrete length production~ In this case the ~ubstrate
(a bar or tube) ls not removed after the deposition
operation but remains part of the final product~ It
should be noted that the bar need not necessarily be
cylindrical ln section and could for example be squar~
rectangula~, or oval etc,
The inventlon will now be further described by way
of examplé'with reference to the ~ccompanying
diagrammatic drawings in Figures 3-~;
Figure 3 illustrates the continuous formation of a
tubular deposit in accordance with the present
lnventlon;
'Figure 4 is a photomlcrograph of the mlcrostructure
of a nlckel-based superalloy IN625 spray deposited in
conventional manner wlth a fixed spray on to a mild
steel collector;
~ F~gure 5 18 a photomicrograph of the microstructure
of IN625 spray deposited by a slngle pass tec~nique in
accordance with the invention onto a mild steel
collqct~ r ;
'Figure fi illu~trate~ dia~rammatically the ~ormation
of a discrete tubular depos1t.
- -
Figure 7 illustrates the formation of a dlscrete
tubular deposit of ~ubstantially frusto-conical shape;
'Figure 8 lllustrates diagrammatically a method for
osclllating the spray; and
Figure 9 is a diagrammatic view of the deposit
formed in accordance with the example discussed later.
In the apparatus ~hown in Figure 3 a collector 1 is
rotated about an axis of rotation 2 and is withdrawn in
a direction indicated by arrow A beneath a gas atomised
spray 4 of molten metal or metal allo~. The spray 4 is
oscilliated to either side of a mean spray axis S in the
direction of the axis of rotation of the substrate 1 -
which in fact coincideæ with the direction of
withdrawal.
:Figures 4 and 5 contrast the microstructures of an
IN625 deposit formed on a mild steel collector in the
conventlonal manner (Figure 4) and in accordance with
the invention (Figure S) on a s~ngle continuous pass
under an oscillating spray. The darker portion at the
bottom of each photomicrograph is the mild steel
collecto~, and the lighter portion towards the top of
each photomicrograph i~ the spray deposlted IN62S~ In
Figure 4 there are ~ub~tantial areas in the spray
deposited IN625 which are black and wbich are areas of
poro~ity. In Flgure S using the o~cillating spray
. i .
~ ~75X(~~
- 13
technique of the invention the porosity i6 6ubstantially
eliminated~.
In Figure 6 a ~pray of ato~ised metal or metal
alloy droplets 11 is directed onto a collector 12 which-
is ~otatable about an axis of rotation 13. The spray
deposit 14 builds up on the collector 12 and uniformity
is achieved by oscillating the spray 11 in the direction
of the axis of rotation 1~. The speed of oscillation
~hould be sufficiently rapid and the heat extraction
controlled so that a thin layer of semi-solid/semi-
liquid metal i8 malntained at the surface of the deposit
over its complete length~ ~or exampl~, the 06cillation
is typically 5 to 30 cycles per 6econd.
As seen from Figure 7 the shape of the depos~t may
be altered by varying the speed of movement of the spray
within each cycle of oscillatiod~ Accordingly, where
the deposit is thicker at 15 the ~peed of movement of
the spray at that point may be slowed 80 that more
~etal is depo6ited a8 opposed to the thinner end where
the speed of movement is increased~. In a simllar manner
6hapes can also be generated by spraying onto a
collector surface that itself is conc1cal in shape-.
More complicated shapes can also be generated by careful
control of the osclllating amplltude and in~tantaneous
Qpecd o~ mov~ment within each cycle o~ oscillation~ I~
18 also po~sible to ~ary the ~a~ to metal ratio during
each cycle o~ oscillatlQn in order to accurately control
/ ~. b.
" ' .
'
,
~ ~7~5~
the cooling conditions of the atomlsed particles
deposited on di~ferent part of the collector.
Furthermore the axis of rotation of the substrate need
not necessarlly be at right angles to the mean axis of ---
the oscillating spray and can be tilted relative to the
spray~
In one method of the invention the oscillation of
the spray is su1tably achieved by the use of apparatus
disclosed diagrammatically in Figure 8~ In Figure 8 a
liquid stream 21 of molten metal or metal alloy is
teemed through an atomising device 22~ The devlce 22 is
generally annular in shape and is supported by
diametrica~}ly prajecting supports 23~. The supports 23
also serve to supply atomising gas to the atomising
device in order to atomise ehe stream 2l into a spray
24~ In order to lmpart movement to the spray 24 the
pro~ecting supports 23 are ~ounted in bearlngs (not
shown) so that the whole atomisln~ device 22 ls able to
tilt about the axis defined by the prajecting supports
23~. The control of the tilting of the atomlsing devlce
22 comprises an eccentric cam 25 and a cam follower 26
connected to one of the supports 23~ ~y altering the
speed of rotatlon of the cam 25 the rate of oscillat10n
of the atomising device 22 can be varled~. In additio~,
by changing the surPace profile of the cam 29, the speed
o~ moveMent of th~ ~pray at any ln6tant du~ing the cyle
oP o~cillaton can be varled~ In a prePerred method oP
- 15
:
the invention the movement of the atomlser is controlled
by electro-mechanical means such as a programme
controlled stepper mo~o~. or hydraulic means 6uch as a
programme controlled electro-hydraulic servo mechanism.
In the atomisation of metal`in accordance with the
inventlon the collector or the atomiser could be tilted.
The important aspect of the invention is that the spray
is moved over at least a part of the length of the
collector so that the hlgh density part of the spray is
moved too and fro across the deposition surfa~e.
Preferably~ the oscillation is such that the spray
actually moves along the length of the collecto~. which
(as shown~'is preferably perpendicular to the spray at
the centre of its cycle of osclllatlonr. The spray need
not oscillate about the central axis of the atomise~, -
thls will depend upon the nature and shape of the
deposit being formed.
Full detallGof the preferred apparatus may be
obtalned from our co-pendlng application filing herewith
to which rePerence is directed.
The speed of rotation of the substrate and the rate
of oscillation of the spray are important parameters
and lt 1~ essentlal that they are selected 80 that the
metal i0 deposited uni~ormly during each revolution of
the collector. Knowing the mass flu~ den~lty
di~tribu~ion o~ the ~pray tran~verse to the direction of
o~cillatioa it is po~ible to calculate the num~er of
' .
7~
- l6
spray oscillation per revolutlon of the substrate which
are required for uniformity~
One example of a discrete length tubular product is
now disclosed by ~ay of example: -
EXAMPLE OF DISCRETE LENGTH: TUBULAR PRODUCT
DEPOSITED MATERIAL - 2~5% Carbod. 4~3%
Chromlu~, 6~3%
Molybdenu~ 7~3%
Vanadiu~. 3~3X TuDgste~.
0~.75% Cobal~. a~8%
., Silico~ a~35% Nanganes~.
~alance IroD plus trace
elements
POURING TEMP~ - 1450 degrees C
METAL POURING NOZZLE - 4~8mm diameter orifice
SPRAY HEIGHT - 480mm (Dlstance from the
underslde of the
atomiser to the top
surface of the
collector)
OSCILLATING ANGLE - ~/- 9 de8rees about a
vertical axls
QSCILLATING SPEED - 12 cycles/sec
A~OMISTNG GAS - Nitr~gen at ambient
temperature
,~
.
: ." :
- , . : '
s~
- 17
COLLECTOR - 70mm outslde diameter by
lmm wall thickness
6tainless steel tube (at.
ambient temperature)
COLLECTOR ROTATION - 95 r.pX.m.
LIQUID METAL:FLOW RATE
INTO ATOMISER - 18kg/min
GAS/METAL RATIO - Q~5-Q~7 kg/kg
Note that this was deliberately varied throughout the
deposition cycle to compensate for excessive cooling by
the cold collector of the first metal to be deposited
and to maintain uniform deposition conditions as the
deposlt in~reases in th~ckness.
DEPOSIT SIZE - 90mm ID 170mm OD llOmm
long
The averaga density of the deposit in the above
example was 9~o8% wlth essentially a uniPQrm
microstructure and uniform distribution of porosity
throughout the thickness of the deposlt. A simi1ar tube
made under the same conditionfi except that the collector
was oscillated under a flxed spray at a rate of 1 cycle
per 2 seconds, showed an flverage den~ity of ga~7~. In
additlo~ the porosity Wfl8 mainly present of the
reclprocation line~ aad not un$formly dlstributed~ The
grain ~tructure flnd ~ize of carbide precipitate~ were
~ fllso varlable being condiderably finer in the
.~
. .
,
- , : ,
~. ~75~
- 18
reciprocation zones~ This was not the case wlth the
above example where the microstructure was uniform
throughout. `
There is now disclosed a second example of a
deposit made by the single pass technique and with
reference to Figures 4 and 5 discussed above:
EXAMPLE OF DEPOSIT MADE BY THE SINGLE PASS TECHNIQUE
'FIXED SPRAY OSCILLATING SPRAY
DEPOSITED MATERIAL IN625 IN625
POURING TEMPERATURE 1450C 1450C
METAL POURING NOZZLE
(ORIFICE DIAMETER) 6.8mm ~6mm
SPRAY HEIG~T 380mm 380mm
OSCILLATING ANGLE 0 3 about
vertical axis
OSCILLATING SPEED O 25 cycles per
fiecond
ATOMISIMG GAS Nltrogen Nitrogan
COLLECTOR 80mm dlameter stainless steel
by lmm wall thlckness
COLLECTOR ROTATION 3 r.p.s. 3 r~p.s.
TRAVERSE SPEED OF
CQLLECTOR a:39 m/mln 0~51 m/mln
LI~UID METAL FLOW
RATE INTO ATOMISER 32 kg/min 42 kg/mln
GAS/METAL RATIO ~S k&tkg 0~38 kg/kg
SIZE QF DEPOSIT 80mm ID by 130mm OD
,
' '- ' ' , :
:
. . :
-- 19
POROSITY See Fi~. 4 See Fig~ 5
It will be noted from Figure 5 that there is
reduced porosity for the Oscillating Spray~. Also a
higher flo~ rate of metal and a lower gas/metal ratio
has been achieYed.
In the method of the invention it i~ essential tha~,
on averag~ a controlled amount of heat is extracted
from the atomised particles in flight and on deposition
including the superheat and a signlficant proportion of
th0 latent heat.
The heat extraction from the atomised droplets
before and after deposition occurs ln 3 main stages:-
(i) ~n-flight coollng malnly by convectlve heat
transfer to the atomisfng gas~. Cooling will typically
be in the range 10-3 - 10-6 degC/sec depending mainly on
the size of the atomised particles. (Typically atomised
particles sizes are in the ~lze range 1-500 mlcrons);
(ii) on depositiod. cooling both by convection to
the atomislng gas as lt flows over the surface of the
spray deposlt and also by conductlon to the already
deposited metal and
(lii) after deposition coollng by conductlon to
the already deposited metal~
~ t la e~sential to carefully control the heat
e~traction in ~ach of the three abo~e stages~ It 15
al00 i~portant to enfiure that the ~urface oE the already
deposlted metal confilsts of a layer of seml-solld~aeml-
7~
- 20
liquid metal into which newly arriving Atomised
particles are deposited. This is achieved by extracting
heat from the atomised particles by supplying gas to the
atomis~ng device under carefully controlled conditions -
of flo~, pressur~. temperature and gas to metal mass
ratio and also by controlling the further extraction o$
heat after deposition~. By using this technique deposits
can be produced which have a non-particulate
microstructure (i.e~ the boundarle~ of atomised
particles do not show in the microstructure) and which
are free from macro-segregation.
If desired the rate of the conduction of heat on
and after deposition may be increased by applylng cold
injected particles as disclosed in our European Patent
published under No: 0198613
As indicated above the invention is not only
applicable to the formation of new product~ on a
substrate but the invention may be u~ed to form coated
product~ In such a case it is preferable that a
substrat~, ~hlch 18 to be coated is preheated in order
to promote a metallurglcal bond at the substrate~deposlt
interface. Moreove~. when forming discrete deposits,
the lnvention has the advantage that the atomising
conditions can be varied to give substantially unlform
depositlon canditlons as the deposit increases in
thickn~ss~ `For exampl~, any coollng of the flrst metal
particle8 to be deposited on the collector can be
s.
i
~.~7~
- 21
reduced by depositing the initial particles with a low
gas to metal mass ratio. Subseguent particles are
deposited with an increased gas to metal mass ratio to
maintain constant deposition conditions and therefo~, -
uniform solidification conditions with uniform
microstructure throughout the thickness of the deposit.
It will be understood thae, whilst the invention
has been described wlth reference to metal and metal
8110y depositio~. metal matrix composites can also be
produced by incorporating metallic and/or non-metallic
particles and/or fibres into the atomised spray~ In the
discrete method of production it is also possible to
produce graded microstructures by varying the amount of
particles and/or fibres injected throughout the
deposition cycle~ The alloy composition can also be
varled throughout the deposition cycle to produce a
graded mlcro~tructure~ This is particularly useful for
products where different propertles are required on the
outer surface of the deposit compared to the lnterior
(e.g~. an abraslon reslstant outer layer with a ductile
main body)~ In addltio~. the lnvention can also be
applied to the spray-deposition of non-metal~ e.g.
molten ceramics or refractQry materials~
. ,`, . ~53>
