The present invention relates to the field of
fabricating parts by melting powder by means of a high
energy beam (laser beam, electron beam, ...)„
The invention relates more particularly to a method
5 comprising the following steps:
a) supplying a material in the form of powder particles
forming a powder beam;
b) heating a first quantity of the powder to a
temperature higher than the melting temperature TF of the
10 powder with the help of a high energy beam, and forming, at.
the surface of a support, a first pool comprising this
melted powder and a portion of the support;
c) heating a second quantity of the powder to a
temperature higher than its melting temperature TF with the
15 help of the high energy beam, and forming, at the surface of
the support, a second pool comprising this melted powder and
a portion of the support downstream from the first pool;
d) repeating step c) until a first layer of the part is
formed on the support;
20 e) heating an [n]th quantity of the powder to a
temperature higher than its melting temperature TF with the
help of a high energy beam, and forming an [n]th pool
comprising in part this melted powder above a portion of the
first layer;
25 f) heating an [n+l]th quantity of the powder to a
temperature higher than its melting temperature TF with the
help of the high energy beam, and forming an [n+l]th pool
comprising in part this melted powder downstream from, said
[n]th pool above a portion of said first layer;
30 g) repeating step f) so as to form a second layer of
the part above said first layer; and
h) repeating steps e) to g) for each layer situated
above an already-formed layer until the part is
substantially in its final shape.
35 In the above method, [n-1] quantities of powder are
needed to form the first layer.
2
Methods are known that make it possible to obtain
mechanical parts that are of complex three-dimensional (3D)
shape. Those methods build up a part layer by layer until
the shape desired for the part has been reconstituted.
5 Advantageously, the part may be reconstituted directly from
a computer-aided design and manufacturing (CADM) file
deduced from processing the data of a 3D computer assisted
design (CAD) graphics file, with a computer controlling the
machine that thus forms successive layers of material that
10 is melted and then solidified, one layer on another, with
each layer being constituted by juxtaposed fillets of size
and shape defined from the CADM file.
By way of example, the particles constituting the
powder may be metallic, intermetallic, ceramic, or
15 polymeric.
In the present application, when the powder is a metal
alloy, the melting temperature TF is a temperature lying
between the liquidus temperature and the solidus temperature
for the given composition of the alloy.
20 The build support may be a portion of some other part
on which it is desired to add an additional function. Its
composition may be different from that of the projected
powder particles, and it may thus have a different melting
temperature.
25 These methods include in particular projection by laser
or "direct metal deposition" (DMD), "selective layer
melting" (SLM), and "electron beam melting" (EBM).
The operation of the DMD method is explained below with
reference to Figures 2, 4, and 5.
30 A first layer 10 of material is formed, under local
protection or within an enclosure at a regulated high or low
pressure of inert gas, by projecting powder particles
through a nozzle 190 onto the material on a support 80.
Simultaneously with projecting particles 60 of powder, the
35 nozzle 190 emits a laser beam 95 coming from a generator 90.
The first orifice 191 of the nozzle 190 through which the
powder is projected onto the support 80 is coaxial around
3
the second orifice 192 through which the laser beam 95 is
emitted, such that the powder is projected into the laser
beam 95. The powder forms a cone of particles, the cone
being hollow and presenting a certain thickness (powder beam
5 94 in Figure 4), and the laser beam 95 is conical.
The working plane P is defined as being the plane
containing the surface on which the layer is being built
and/or formed.
In order to build the first layer, this surface is the
10 top (free) face S0 of the support 80. In order to build the
[n+l]th layer, this surface is the top (free) face of the
[n]th layer (with integer n, n>l).
The laser beam 95 forms a pool 102 on the support 80 by
melting the region of the support 80 that is exposed to the
15 laser beam. The powder feeds the pool 102 in which it
arrives already in the molten state, the powder being melted
on its path in the laser beam prior to reaching the pool.
Alternatively, and by way of example, the nozzle 190
and the focal point of the laser may be adjusted and/or
20 positioned in such a manner that the powder of given size
distribution does not pass sufficient time in the laser beam
95 for all of its particles of different sizes to melt
completely, so that they melt on reaching the pool 102 that
has previously been formed on the surface of the support 80
25 by melting the region of the support 80 that is exposed to
the laser beam 95.
The working distance WD is defined as being the
distance between the nozzle 190 and the working plane P.
Over the working distance WD under consideration, the
30 powder may likewise not be melted by the laser beam 95 or it
may melt in part only because the sizes of some or all of
the particles making up the powder are too great for them to
be melted. As can be seen in Figure 3, the smaller the mean
diameter Dp of the powder particles, the greater the speed
35 with which they heat up, but the shorter the time they are
maintained at the melting temperature and the faster their
cooling. Furthermore, Figure 3 shows that the narrower the
4
distribution of sizes, the greater the extent to which all
of the particles of the powder are molten when they reach
the pool for a given working configuration.
Under all circumstances, the powder particles are
5 heated by passing through the laser beam 95 prior to feeding
the pool.
While the laser beam 95 (or the support 80) moves
downstream, the pool 102 is maintained and solidifies
progressively to form a fillet of solidified material 105 on
10 the support 80. The process is continued so as to form
another solidified fillet on the support 80, this other
fillet being juxtaposed with the first fillet, for example.
Thus, by moving the nozzle 190 or the support 80 in a plane
parallel to the above-mentioned working plane P, a first
15 layer 10 of material is deposited on the support 80, which
layer forms by solidifying a first element 15 in a single
piece of shape that complies with the shape defined in the
CADM file.
Thereafter, the nozzle 190 and the laser beam 95 are
20 caused to perform a second scan together so as to form in
similar manner a second layer 20 of material on top of the
first element 15. This second layer 20 forms a second
consolidated element 25, and together these two elements 15
and 25 form a single-piece block. The pools 102 formed on
25 the first element 15 during building of the second layer 20
generally comprise at least a portion of the first element
15 that has melted by being exposed to the laser beam 95,
together with the particles of the powder feeding the pools
102.
30 Consideration is given to a reference frame constituted
by the vertical axis Z0 perpendicular to the top surface S0
of the support, and by the surface S0 of the support. This
reference frame is tied to the support 80, or more exactly
to the part being built for which the reference plane P is
35 defined by the surface S0 of the support while depositing the
first layer of material, or by the top surface of the most
recently deposited layer.
5
For a layer in general, the working plane P is not
necessarily parallel to the surface S0. The axis Z defined
as being perpendicular to the working plane P is thus not
necessarily parallel to the axis Z0.
5 Between two successive layers, the nozzle moves along
the axis Z by a value AZ that is theoretically equal to the
height of material Happ that has actually been deposited and
that should be constant (independently of the path of the
nozzle) and that is sufficiently large when building is
10 optimized and stable (Figures 4 and 5) . Figure 5 is a
cross-section of the liquid pool formed in part in the
support, and it shows the shape of the pool.
The surface S0 of the support 80 is the plane at height
zero. Thus, while building the first layer, a plane
15 parallel to S0 and having a portion contained in the support
or below the support (relative to the axis Z0) is at negative
height, and a plane parallel to S0 with a portion above the
surface S0 of the support (relative to the axis Z0) is at
positive height.
20 A given working plane P relating to building an [n]th
layer is above another working plane attached to a lower
layer if it has a height that is positive, greater than the
height of that other plane.
In this reference frame tied to the support 80 and to
25 the part, the second layer 20 is constructed on a working
plane P that is situated above the working plane of the
first layer 10, these two planes being spaced apart by a
distance AZ measured along the axis Z perpendicular to the
working plane P.
30 In general, the working plane of a higher layer need
not be parallel to the working plane of the preceding lower
layer, in which the axis Z of the higher layer is at a nonzero
angle relative to the axis Z of the working plane of
the lower layer, and the distance AZ measured along the
35 latter axis Z above each point of the lower layer is a mean
value.
6
This process of preparing the part layer by layer is
then continued by adding additional layers over the assembly
that has already been formed.
Figure 4, which shows the prior art, shows in greater
5 detail the configuration of the laser beam 95 and of the
powder beam 94, The laser beam 95 leaves the nozzle 190
diverging at an angle 2p from its focal point FL (situated in
the bottom portion of the nozzle 190) and it illuminates a
region of the support 80, contributing to creating a pool
10 102 therein.
The powder beam 94 leaves the nozzle 190 while
converging at an angle 28 towards its focal point Fp, which
lies inside the laser beam 95 and immediately over (or
above) the surface of the support 80 (working plane P) , in
15 such a manner that the powder particles 60 spend a maximum
length of time in the laser beam 95 in order to be heated.
The advantage of a large amount of interaction between the
laser and the powder upstream from the pool is to generate
both a high deposition rate and low dilution as are
20 frequently desired when building up the surfaces of worn
parts in order to repair them and when depositing hard
coatings.
The theoretical efficiency of melting is defined as
being the ratio of the diameter 0L of the laser beam 95
25 divided by the diameter 0P of the powder beam 94, these two
diameters being determined in the working plane P.
Alternatively, diameter 0L may be replaced by the
diameter of the liquid pool 0BL (see Figure 4) in order to
evaluate the efficiency, which depends amongst other things
30 on the selected parameter settings, in particular the laser
power PL, the scanning speed of the laser beam V, and the
mass flow rate Dm of powder.
The laser diameter at its focal point (i.e. 0LO) is
often much smaller than the diameter 0PO of the powder beam
35 at the powder focal point so the working configuration in
the prior art logically requires the laser beam to be
unfocused (its focal point FL lies above the working plane P)
7
for a powder beam that is focused (its focal point Fp is
situated on the working plane P) , or a powder beam that is
unfocused with its focal point Fp lying above the working
plane P and below the laser focal point FL, since otherwise
5 the structure being built will be unstable and there is no
guarantee of acceptable melting efficiency. As mentioned
above, in general, the laser beam diameter 0L measured in
the plane P does not correspond to the liquid pool diameter
0BL which is approximately equal to the width (written e )
10 of the fillet after solidification (Figures 4 and 5).
This diameter 0BL of the liquid pool is assumed to be a
function of 0L and thus of 0LO and also of the settings
defined by the triplet (PL, V, Dm) and also the size Dp of
the various powder particles and their speeds Vp, in addition
15 to depending on their thermo-physical properties.
During the process of building the part layer by layer,
the nozzle 190 moves in particular vertically, and while
keeping constant the distance between the points FL and Fp
(i.e. DefocL-Defocp=constant) where DefocL and Defocp
20 represent respectively the laser defocus and the powder
defocus defined as follows:
DefocL = {distance from FL to the working plane P}
and
Defocp = {distance from Fp to the working plane P}
25 as can be seen in Figure 4.
Thus, the focal point Fp of the powder beam 94 remains
inside the laser beam 95 and immediately over (or above) the
surface of the previously constructed layer (working plane
P) .
30 There is thus a defocused laser beam (DefocL>0) and a
defocused powder beam (Defocp=0) on the plane P or defocused
(Defocp>0) above the plane P, and the two angles 2(3 and 25
need to be configured in such a manner that firstly the
working distance WD between the outlet from the nozzle and
35 the plane P is large enough to avoid damaging the bottom of
the nozzle by radiation from the pool, and secondly to
ensure that the aperture of the laser beam at the outlet
8
from the nozzle remains less than the diameter of the inside
cone.
Moving the support 80 or scanning the assembly
comprising the nozzle 190 and the laser beam 95 makes it
5 possible to give each layer a shape that is independent of
the adjacent layers. The lower layers of the part are
annealed and they cool progressively as the higher layers of
the part are formed.
Nevertheless, there exists a need to improve the
10 melting mass efficiency Rm (i.e. the ratio of the quantity of
material forming the finished part to the quantity of
material that is projected by the nozzle in order to form
the part), the recycled powder mass efficiency Crecy (i°e° the
ratio of the quantity of morphologically intact powder and
15 agglomerates, e.g. as obtained after screening, to the
quantity of material that is projected), the stability of
the pool formed at the surface of the part, and the material
soundness of the fabricated part, for a given non-exhaustive
set of parameter settings (size distribution Dp for the
20 powder particles, nature of the powder material, powder mass
flow rate Dm, travel speed V of the assembly comprising the
nozzle and the laser beam, power PL supplied by the laser,
distribution of power density on the working plane P, type
of laser source (solid or gas), mode (pulsed or continuous),
25 coaxial nozzle, nature of the gas carrying the powder
particles and its flow rate Dgp, nature of the protective gas
crossing the axis of the nozzle and its flow rate Dgl, the
angles 2p and 25, and also the above-defined diameters 0LO
and 0PO, etc. ) .
30 The invention seeks to propose a method and more
particularly a working configuration that are optimized
(defined by: DefocL, Defocp, WD) for the DMD method serving
firstly to improve the stability of the pool and secondly to
improve the melting mass efficiency, the recycled powder
35 mass efficiency, the material soundness, and the building
speed (maximizing the Z rise increment of the nozzle,
written AZ).
9
This object is achieved by the fact that the powder
particles reach each pool at a temperature that is cold
relative to the temperature of the pool.
By means of these provisions, the mass efficiency r\ of
5 the method defined as the sum of the melting mass efficiency
(Rm) plus the recycled powder efficiency (Crecy) ^s greater
than the mass efficiency of the method when the powder
particles reach the pool hot or even partially or totally
melted. In addition, on reaching the pool, the powder
10 particles serve to reduce the temperature of the liquid pool
TBL (because they are much colder than the pool, the
particles being substantially at ambient temperature prior
to penetrating into the pool), while increasing the volume
of the pool and in particular its volume above the plane P
15 without increasing the width and the height of the diluted
zone (volume of the pool that lies below the plane P) . This
leads inevitably to a rapid increase in the surface tension
between the liquid surface and the vapor of the pool, and
consequently gives rise to better stability of the pool.
20 Furthermore, encouraging a large amount of dilution in
this way in each deposited layer serves to minimize
fabrication defects.
Advantageously, the high energy beam focal point FL is
situated above the working plane P or in this plane, and the
25 powder beam focal point Fp is situated below the working
plane P, such that the powder particles do not at any time
cross the high energy beam between the outlet from the
nozzle and the working plane P. In particular, the powder
beam focal point Fp may be situated within the support, in
30 particular when depositing the initial layers. After a
certain number of layers have been deposited, the powder
beam focal point Fp may be situated within previously
deposited layers.
Thus, the majority of powder particles are cold when
35 they reach the pool previously formed on the already-built
portion of the part.
10
These particles then penetrate into a pool that is wide
enough (0BL>0P) and deep enough (HZR>Happ: see definitions
above given with reference to Figure 5) to ingest a maximum
quantity and a maximum fraction of all of the particles
5 projected by the nozzle during the laser/pool interaction
time, as defined by the ratio of 0L over V.
Furthermore, since the remaining powder particles are
intact, unheated by the high energy beam, they are entirely
suitable for recycling.
10 Furthermore, the power beam and the high energy beam
may be substantially coaxial, i.e. their axes may form
between them an angle of less than 30°, preferably less than
20°, more preferably less than 10°, still more preferably
less than 5°. The high energy beam can thus easily follow
15 the powder beam during fabrication of parts that are complex
in shape. It is much more difficult to track the shape of
the part for fabricating when projection or melting is
offset, i.e. when the powder beam and the high energy beam
are not substantially coaxial.
20 The invention can be well understood and its advantages
appear better on reading the following detailed description
of an implementation given by way of non-limiting example.
The description refers to the accompanying drawings, in
which:
25 • Figure 1 is a diagram showing one possibility for
positioning the high energy beam and the powder beam in the
method of the invention;
• Figure 2, described above, is a diagram for
explaining the prior art method and shows the device for the
30 DMD method;
• Figure 3, described above, shows the effect of the
diameter Dp of the particles of Ti-6A1~4V powder on their
temperature at the outlet from the nozzle when they reach
the liquid pool;
35 • Figure 4, described above, is a diagram showing the
positioning of the high energy beam and of the powder beam
in the prior art method; and
11
• Figure 5, described above, is a diagrammatic crosssection
of the liquid pool formed in the support.
In the invention, the powder particles are cold when
they reach the pool formed at the surface of the preceding
5 layer (or of the support) . The term "cold" means that the
temperature of the particles is much lower than the
temperature of the pool. Prior to penetrating into the
pool, the temperature of the particles is substantially
equal to ambient temperature, e.g. being about 20°C.
10 In comparison, the temperature of the liquid pool TBL is
higher than the melting temperature TF of the material
constituting the powder, but lower than the boiling
temperature Tevap of that material. This melting temperature
is higher than 550°C for aluminum alloys, higher than 1300°C
15 for nickel-based alloys, higher than 14 50°C for steels, and
higher than 1550°C for titanium alloys.
Figure 1 shows an implementation of the invention that
enables powder particles to be cold when they reach the pool
formed in the surface of the preceding layer (or of the
20 support). Such an implementation also presents the advantage
of making it easier to view the pool on the axis e.g. by
means of a charge-coupled device (CCD) camera so as to
monitor the method on line, which is useful for
industrializing the method.
25 Figure 1 is a section view of a support 80 together
with a first layer 10 of material that has already been
deposited on the support 80. A second layer 20 is then
deposited on the first layer 10. A fillet 105 of the second
layer 20 is shown while it is being built, with the fillet
30 105 advancing from left to right, and from upstream to
downstream (the forward travel direction of the fillet 105,
or in equivalent manner of the liquid pool 102) . The pool
102 is thus situated immediately downstream from the fillet
105 under the nozzle 190 from which there emerge the laser
35 beam 95 and the powder beam 94. The top surface of the
first layer 10 then constitutes the working plane P relative
to the second layer that is being built and from which the
12
following are measured: the laser defocus distance DefocL,
the powder defocus distance Defocp, the working distance WD,
the diameter 0L of the laser beam, and the diameter 0P of
the powder beam.
5 Simultaneously with projecting powder particles 60, the
nozzle 190 emits a laser beam 95 coming from a generator 90.
The first orifice 191 of the nozzle 190 through which the
powder is projected onto the support 80 is coaxial with the
second orifice 192 through which the laser beam 95 is
10 emitted, such that the powder is projected in the laser beam
95. The powder forms a cone of particles, this hollow cone
presenting a certain thickness (powder beam 94), and the
laser beam is conical.
In the invention, the nozzle 190 is configured and
15 positioned in such a manner that the focal point FL of the
high energy beam 95 is situated above the working plane P or
in that plane, and the focal point Fp of the powder beam 94
is situated beneath the working plane P, such that the
powder particles 60 do not at any time cross the high energy
20 beam between the outlet from the nozzle and the working
plane P.
In an implementation other than that shown in Figure 1,
the focal point Fp of the powder beam may lie within the
support. Under such circumstances, the powder defocus
25 distance Defocp is smaller than that shown in Figure 1. As a
result, the diameter 0L of the laser beam in the plane P is
closer to the diameter 0P of the powder beam in the plane P,
for given parameter settings (PL, V, Dm) .
By way of example, the diameter 0L of the laser beam in
30 the plane P is slightly less than the diameter 0P of the
powder beam in the plane P.
As shown in Figure 1, such a configuration is obtained
by moving the nozzle 190 closer to the working plane P
relative to the prior art configuration (Figure 4), i.e. by
35 reducing the working distance WD.
Such a working configuration is particularly adapted to
making wide fillets 105, i.e. fillets 105 of width that is
13
greater than the diameter 0LO of the high energy beam 95 at
the laser focal point.
The diameter of the liquid pool 0BL is then greater and
more cold powder particles reach the liquid pool 102, which
5 is beneficial as explained above.
The focal point FL of the high energy beam (95) may
alternatively be situated in the working plane P, which is
preferable when making fine fillets of smaller width. Under
such circumstances, the focal point Fp of the powder beam 94
10 may be situated in the working plane P. The focal point Fp
of the powder beam 94 may also be situated below the working
plane P.
In order to optimize the method of the invention, it is
possible to adapt certain parameter settings accordingly, in
15 particular the laser power PL, the scanning speed V, and/or
the powder mass flow rate Dm.
Nevertheless, in the implementation shown in Figure 1,
it may be necessary to provide (additional) cooling of the
nozzle 190 since the nozzle 190 is heated by radiation due
20 to its proximity to the liquid pool 102. Such cooling
requires a device that is expensive.
In order to mitigate this problem and thus conserve a
working distance WD (distance of the nozzle from the pool)
that is sufficient, while avoiding the powder beam crossing
25 the high energy beam, the inventors have devised an
implementation that consists advantageously either in
reducing the distance DefocL, or in reducing the divergence
half-angle p of the laser beam 95 relative to the axis Z,
which amounts either way to reducing 0L so as to ensure that
30 it is smaller than 0P.
Alternatively, the distance Defocp of the powder beam 94
is increased in order to compensate for the reduction in 0P
when increasing WD, thereby keeping 0P greater than 0L.
This reduction in the distance DefocL and in the angle
35 P, and this increase in the distance Defocp may be performed
jointly.
14
These variations in these three variables may be
performed independently or in addition to increasing the
working distance WD. In practice, the nozzle 190 is thus
configured and positioned in such a manner that the powder
5 particles 60 reach the working plane P immediately outside
the zone of the working plane P that is covered by the laser
beam. 95.
Thus, given that the liquid pool 102 extends by
conduction a little beyond that zone, the majority of the
10 powder particles 60 drop into the pool 102 without
interacting with the laser beam 95. The powder particles 60
are thus still cold before they penetrate into the pool 102.
An advantage of this absence of interaction between the
laser and the powder upstream from the pool 102 is to avoid
15 any change of shape, to avoid agglomerates forming, and to
avoid harmful oxidation of the powder particles 60.
This explains why tests undertaken by the inventors
show that the melting mass efficiency R^ in the method of the
invention is higher than the melting mass efficiency when
20 the powders reach the pool while hot, or indeed while
partially or completely melted.
Furthermore, the pool 102 is thermally more stable
since the powder particles 60 cool the pool 102 quickly
(thereby increasing the surface tension between the liquid
25 and the vapor of the pool, and very certainly leading to
changes in convection movements within the pool as a result
of variation in the density of the liquid by adding "cold"
powders and by changing the temperature gradient within the
pool) .
30 An additional advantage of the method of the invention
is that the powder particles 60 that have not participated
in forming the liquid pool (since they drop outside the pool
102) remain cold and are thus almost all suitable for
recycling. The total mass efficiency of the (melting +
35 recycling) method of the invention is thus indeed greater
than the total mass efficiency of the prior art method.
15
Advantageously, for greater stability of the pool 102
and for better material soundness once a steady temperature
regime has been established locally around the pool in the
part being built, the pool has an oblong shape defined by
5 9<90°, Happ/eapp0.6, where 8 designates the angle between the
top surface of said pool and said working plane P, H
19
designates the apparent height of the fillet, eapp
designates its width, and HZR designates the height of
the remelted zone.
6. A method of fabricating a part according to claim 5,
characterized in that the three quantities 6, Happ/eapp,
and HZR/Happ satisfy the following relationships:
15°<8<60°, 0.04