SINTERING AND LASER FUSION DEVICE, COMPRISING A MEANS
FOR HEATING POWDER BY INDUCTION
5 The field of the present invention is that of the
manufacture of metal parts and more particularly that
in which a laser is used to manufacture these parts by
selectively melting a powder bed. It also covers the
case of the repair or reconstruction of parts by
10 building up material.
A manufacturing technique in which parts are
manufactured by sintering and then melting with a laser
or electron beam is already known and widely used to
produce rapid prototypes, i.e. to produce a small
15 number of parts having complex shapes in a small amount
of time. Sintering is a process that allows mechanical
parts or other objects to be produced from relatively
fine powders. In a first step, these powders are
agglomerated by various processes in order to produce a
20 preform, which is then heated in order to give it a
certain cohesion. One heating technique that is
commonly used to produce metal parts by sintering is
what is called the laser melting technique. In this
process, material in powder form is melted under the
25 action of a high-power laser (powers from 200 W to a
few kW). Repeatedly supplying powder and melting the
latter with the laser allows the thickness of the part
to be gradually increased, and choosing an appropriate
laser scanning pattern allows the desired shapes to be
30 obtained.
When this method is used to manufacture parts made
of titanium, nickel or cobalt base alloys, as is the
case for aerospace parts, high residual stresses are
generated, which stresses are due to the thermal
35 gradients generated by the melting of the layers in
succession. The geometry, thickness and changes in the
cross section of the parts to be produced are factors
that may increase these gradients. Depending on the
material, the residual stresses resulting from these
gradients may lead to deformation of the part during
construction and cracking during use.
It is therefore important for the temperature to
5 be controlled during the melting process and for a
uniform temperature to be maintained in the powder, in
order to minimize the residual stresses generated
during solidification.
There are various ways of controlling these
10 thermal gradients, such as using hot plates, heating
the powders by convection, or even preheating the
powder using a high-energy beam. However, these methods
have a number of drawbacks. The heating provided by a
hot plate is characterized by the fact that it is
15 localized in only the plate that holds the powder, the
preheating temperature is limited and the heating is
nonuniform through the build tray thickness; heating of
powders by convection is for its part localized in the
top face of the build tray and is non-uniform through
20 the thickness of the powder; lastly, beam preheating is
also localized in the top face of the build tray and is
also non-uniform through the thickness of the build
tray.
Overall, these methods enable only very localized
25 temperature control and do not guarantee a uniform
temperature in the part during its construction.
The aim of the present invention is to remedy
these drawbacks by providing a device and a process
allowing the above drawbacks to be mitigated and
30 therefore parts to be produced or built-up by laser
melting of a powder bed, the resulting parts, after
solidification, containing residual stresses that are
as small as possible.
For this purpose, one subject of the invention is
35 a device for producing or building up a metal part by
laser sintering and melting, comprising a generator for
generating a laser beam, a means for deviating said
beam in order to sweep it over the surface of the part
to be produced, and a sintering tray containing a metal
powder intended to cover the surface of the part and to
be melted by the laser beam in order to increase the
thickness of said part, characterized in that it
furthermore comprises at least one inductive heating
means for heating the powder contained in a zone of
said sintering tray.
The inductive heating allows the temperature of
the part and that of the surrounding powder to be
controlled and therefore temperature gradients within
the part to be controlled.
Advantageously, the sintering tray has a
cylindrical shape the sidewalls of which (i-e. the
walls formed by the generatrices of the cylinder) hold
a number of inductive heating means, said walls being
made of a material that is not susceptible to inductive
heating.
Even more advantageously, the cylinder-shaped
sintering tray comprises a vertically movable base (the
base being defined as a surface cutting all of the
generatrices of the cylinder), the sidewalls being
encircled by a number of layers of heating means, said
layers being tiered over the entire length of travel of
the movable base, each layer consisting of a number of
inductive heating means positioned at the same distance
from said movable base.
These multiple heating means make it possible to
regulate the temperature to the desired value in each
zone of the powder bed contained in the sintering tray.
Preferably, the sintering tray comprises a base
intended to receive the part to be produced, said base
being equipped with a heating means and a means for
regulating its temperature. Thus possible thermal
pumping effects in the vicinity of the base are
prevented.
In a particular embodiment the device furthermore
comprises at least one means for measuring the
temperature of the powder at a point located within the
sintering tray.
5 Preferably, the sintering tray holds a measuring
rod equipped with at least one thermocouple, said rod
extending in order to pass at least partway through the
powder bed contained in said tray.
Advantageously, the device furthermore comprises a
10 means of regulating the temperature of at least one
point in the powder bed by way of at least one heating
means, said heating means being controlled depending on
the value delivered by said means for measuring the
temperature of the powder.
15 Another subject of the invention is a process for
producing or building up a metal part by laser
sintering and melting, said part being placed in a
sintering tray containing a metal powder intended to be
melted by a laser beam in order to increase the
20 thickness of said part, the process comprising a step
of covering that surface of the part the thickness of
which is to be increased with a thickness of powder, a
step of melting the powder by sweeping said laser beam
over it, and a step in which the molten material is
25 solidified by cooling, characterized in that it
furthermore comprises a step of inductive heating of
the powder contained in said sintering tray.
In one particular embodiment, the inductive
heating is carried out before the laser melting.
30 In another particular embodiment, the inductive
heating is carried out after the laser melting in order
to regulate the temperature of the powder contained in
the sintering tray'during the phase in which the liquid
portion of the part solidifies.
35 The invention will be better understood and other
aims, details, features and advantages thereof will
become more clearly apparent from the following
detailed explanatory description of an embodiment of
the invention, given by way of purely illustrative and
nonlimiting example, and with reference to the appended
schematic drawings.
5 In these figures:
- figure 1 is a schematic view of a laser
sintering/melting machine;
- figure 2 is a schematic vertical cross-sectional
view of a laser sintering/melting machine according to
10 one embodiment of the invention; and
- figure 3 is a top view of the machine in figure
Figure 1 shows a machine for producing a metal
part by laser sintering and melting.
15 A laser-beam generator 1 emits a laser beam 2 that
is directed toward a set of reflective mirrors 3, the
last mirror 4 of which can pivot in order to allow the
beam to be swept over the surface of the part 5 to be
produced. It will be noted that the laser beam is not
20 necessarily routed by mirrors; an optical fiber could
be used, depending on the wavelength of the laser
. employed, and the laser beam could be swept over the
surface using other means, such as F-theta lenses.
The part 5 is placed on a build plate 10 located
25 facing the laser beam 2; it is moreover submerged in a
tray 6 so that it can be regularly covered with a layer
of metal powder 7 suitable for sintering. A second tray
8 for supplying powder is positioned beside the
sintering tray 6 and is filled with the sintering
30 powder 7. A piston-type device 9 allows an amount of
powder 7 to be moved from the supply tray 8 to the
sintering tray 6 in order to cover the part 5 with a
powder layer of given thickness. The thickness of this
layer corresponds to that by which the thickness of the
35 part will be increased during one melting sweep of the
laser beam 2, after allowing for compaction and
solidification shrinkage. Devices for lowering the
sintering tray 6 and raising the supply tray 8 allow,
on the one hand, the part 5 to be sintered to be kept
flush with the walls of the tray 6, and on the other
hand, a layer of metal powder 7 of the correct
thickness to be spread by the piston 9 from the supply
tray 8.
Sintering of the part 5 and melting of the powder
by the laser is achieved by successive elementary
operations that are carried out in the following way:
the part 5 is positioned flush with the top of the
walls of the sintering tray 6, the piston 9 is moved in
the direction of this tray 6 so that it deposits the
desired thickness of powder 7 on the part 5, it then
returning to its standby position at the end of the
supply tray 8. The laser beam 2 is swept over the
surface of the part using the oscillating mirror 4,
thereby causing the layer of metal powder to melt and
aggregate on the part 5, therefore causing the
thickness of the latter to increase. The part 5 is then
drawn downward in order to compensate for the increase
in its thickness and so that its surface once more lies
flush with the sintering tray 6, whereas the supply
tray 8 is raised in order to again place a suitable
amount of metal powder 7 facing the piston 9. This
process is repeated the number of times required to
obtain the desired geometry and dimensions of the part
5.
A device allowing parts to be produced by laser
sintering and melting will now be described with
reference to figures 2 and 3.
The part 5 to'be produced is placed on the build
plate 10, which is able to move vertically under the
action of a lowering piston 11, and covered with a
sintering powder 7 spread by a supply piston 9 from a
supply tray (not shown). The sintering tray 6 shown
here is cube-shaped, although this is not necessarily
the case. A series of inductors 12 are embedded inside
the walls of this sintering tray; these inductors,
which are connected to a power supply (not shown), are
intended to give the powder bed the desired
temperature. Horizontally, these inductors are
5 regularly spaced around the periphery of the sintering
tray 6 in order to make the powder bed temperature as
uniform as possible; vertically, a number of series of
inductors are stacked one above the other so as to
allow the powder to be heated whatever the size
10 obtained by the part 5, i.e. whatever the position
occupied by the build plate 10 in the vertical plane.
The series of inductors thus extends as far as the
bottom portion of the vertical walls of the sintering
tray 6.
15 A control rod 13 is located in the middle of the
sintering tray, at a distance away from the walls of
the sintering tray and from the part to be manufactured
that is compatible with the operations to be carried
out, said control rod 13 passing through the build
20 plate 10 and extending vertically across the entire
height of the sintering tray. The build plate 10 is
thus drilled with a hole that allows it to move
vertically as the thickness of the part 5 increases
without interfering with the control rod 13. This rod
25 holds a number of means for measuring the temperature
of the powder, such as thermocouples for example, which
are regularly spaced over its height. They are used to
measure the temperature of the powder 7 when the
position of the plate 10 is such that these
30 thermocouples 14 lie above said plate and thus make
contact with the powder 7.
How the device according to the invention produces
a part by laser sintering and melting will now be
described. A part is repaired by material deposition in
35 an analogous way.
The part is produced in substantially the same way
as in a conventional device, i.e. powder, taken from
the supply tray 8, is spread by the supply piston 9
over the part 5 the thickness of which it is desired to
increase. A laser beam 2 is swept over this powder with
a scanning pattern that describes the area to be
5 thickened and that locally melts the powder, so as to
agglomerate it with the existing part.
However, the invention differs therefrom in that
the device also comprises a series of inductors 12 the
function of which is to regulate the temperature of the
10 powder 7 during the phase in which the molten metal
solidifies and agglomerates with the existing part.
These inductors, which are fitted around the
perimeter of the build plate and envelop the article
during manufacture, form a system for heating the
15 powder because of the metallic nature of the latter.
They are separated from the powder 7 by the walls of
the sintering tray 6, which walls are made of a
material allowing the powder to be heated by induction
but that themselves undergo almost no heating under the
20 effect of induced currents.
This heating system is controlled by a system for
regulating the temperature of the powder in its various
zones based on temperature information measured by the
control rod 13 and its thermocouples 14. These
25 temperature measurements allow the heating provided by
the inductors 12 to be controlled in order to regulate
the temperature of the part 5 in construction and the
surrounding powder 7. On account of the many inductors
present around the sintering tray 6, the temperature of
30 the powder may be regulated on a zone-by-zone basis,
thereby providing better control of the cooling and
solidification by allowing certain particular
parameters to be taken into account, such as the
thickness of the previously agglomerated material at
35 each point of the part and therefore its local
properties in terms of conduction and convection.
A program for controlling the inductive heating,
that it is within the abilities of a person skilled in
the art to develop, defines the electrical current that
must pass through each inductor in order to obtain the
5 desired temperature at each point in the tray of
powder. If required, one phase of the development of
this program may involve carrying out a calibration,
using a reference part, of each of the types of alloy
from which it is envisioned to produce a part.
10 Thus, the invention allows, because measurements
are carried out at various heights using the
thermocouples 14 of the rod 13, the correct powder
temperature to be obtained at every point in the build
tray, and therefore correct cooling of the part 5
15 during its production to be guaranteed.
In addition, the device may also comprise, to
improve the regulation provided by the system, a system
for heating the plate, preventing thermal pumping
effects that could otherwise appear due to the presence
20 of a cold build plate at the base of the tray 6. This
plate may be heated by any conventional means, such as
for example a set of heating rods that pass through its
thickness.
In a particular embodiment, the position of the
25 control rod 13 might not be fixed but might instead be
tailored to the part to be produced and the shape of
the latter. For this purpose, several alternative
locations are provided for the orifice in the build
plate 10 through which the rod 13 passes. It is thus
30 possible to refine the temperature measurements and
optimize.the heating at each point of the powder 7.

CLAIMS
1. A device for producing or building up a metal
5 part by laser sintering and melting, comprising a
generator (1) for generating a laser beam (2), a means
(4) for deviating said beam in order to sweep it over
the surface of the part (5) to be produced, and a
sintering tray (6) containing a metal powder (7)
10 intended to cover the surface of the part (5) and to be
melted by the laser beam (2) in order to increase the
thickness of said part,
characterized in that it furthermore comprises at least
one inductive heating means (12) for heating the powder
15 contained in a zone of said sintering tray.
2. The device as claimed in claim 1, in which the
sintering tray (6) has a cylindrical shape the
sidewalls of which hold a number of inductive heating
20 means (12), said walls being made of a material that is
not susceptible to inductive heating.
3. The device as claimed in claim 2, in which the
cylinder-shaped sintering tray (6) comprises a
25 vertically movable base (10). the sidewalls being
encircled by a number of layers of heating means, said
layers being tiered over the entire length of travel of
the movable base, each layer consisting of a number of
inductive heating means (12) positioned at the same
30 distance from said movable base.
4. The device as claimed in one of claims 1 to 3,
in which the sintering tray (6) comprises a base (10)
intended to receive the part to be produced, said base
35 being equipped with a heating means and a means for
regulating its temperature.
5. The device as claimed in one of claims 1 to 4,
furthermore comprising at least one means (14) for
measuring the temperature of the powder (7) at a point
located within the sintering tray (6).
6. The device as claimed in claim 5, in which the
sintering tray (6) holds a measuring rod (13) equipped
with at least one thermocouple (14), said rod extending
in order to pass at least partway through the powder
10 bed (7) contained in said tray (6).
7. The device as claimed in either. of claims 5 and
6, furthermore comprising a means of regulating the
temperature of at least one point in the powder bed (7)
15 by way of at least one heating means (12), said heating
means being controlled depending on the value delivered
by said means for measuring the temperature of the
powder (14).
8. A process for producing or building up a metal
part by.laser sintering and melting, said part being
placed in a sintering tray (6) containing a metal
powder (7) intended to be melted by a laser beam (2) in
order to increase the thickness of said part (5), the
25 process comprising a step of covering that surface of
the part the thickness of which is to be increased with
a thickness of powder, a step of melting the powder by
sweeping said laser beam over it, and a step in which
the molten material is solidified by cooling,
30 characterized in that it furthermore comprises a step
of inductive heating of the powder contained in said
sintering tray.
9. The process as claimed in claim 8, in which the
35 inductive heating is carried out .before the laser
melting.
10. The process as claimed in either of claims 8
and 9, in which the inductive heating is carried out
after the laser melting in order to regulate the
temperature of the powder (7) contained in the
5 sintering tray during the phase in which the liquid
portion of the part (5) solidifies.
Dated this 19/08/2013 w RAN A MEHTA-DUTT
^OF REMFRY & SAGAR
ATTORNEY FOR THE APPLICANT[S]