Method for manufacturing a part by melting powder, the powder particles reaching the bath in a cold state

Abstract

A method of fabricating a part includes: a) supplying powder particles; b) melting a first quantity of power with a beam and forming, on a support, a first pool including the melted powder and a portion of the support; c) forming a second pool by melting a second quantity of powder on the support; d) repeating c) to form a first layer; e) heating an [n].sup.th quantity of the powder, and forming an [n].sup.th pool above the first layer; f) heating an [n+1].sup.th quantity of the powder, and forming an [n+1].sup.th pool downstream from the [n].sup.th pool above the first layer; g) repeating f) to form a second layer above the first layer; and h) repeating e) to g) until the part is constructed. The powder particles reaching each pool are at a temperature well below the pool temperature.

Claims

1. A method of fabricating a part having a shape, the method comprising: a) supplying a material in the form of powder particles forming a powder beam; b) heating a first quantity of said powder to a temperature higher than a melting temperature of the powder with the help of a high energy beam, and forming, at a surface of a support, a first pool comprising the melted powder and a portion of the support; c) heating a second quantity of said powder to a temperature higher than the melting temperature with the help of said high energy beam, and forming, at the surface of the support, a second pool comprising the melted powder and a portion of the support downstream from the first pool; d) repeating step c) until a first layer of said part is formed on said support; e) heating an [n].sup.th quantity of said powder to a temperature higher than the melting temperature with the help of a high energy beam, and forming an [n].sup.th pool comprising in part the melted powder above a portion of said first layer; f) heating an [n+1].sup.th quantity of said powder to a temperature higher than the melting temperature with the help of said high energy beam, and forming an [n+1].sup.th pool comprising in part the melted powder downstream from said [n].sup.th pool above a portion of said first layer; g) repeating step f) so as to form a second layer of said part above said first layer; and h) repeating steps e) to g) for each layer situated above an already-formed layer until the shape for said part has been reconstituted; wherein the powder beam and the high energy beam are substantially coaxial during steps b) through h) to form each layer so that an angle between an axis of the powder beam and an axis of the high energy beam is equal to or less than 20 and wherein the powder particles reach each of the pools during steps b) through h) at a temperature that is cold relative to the temperature of said pool, and wherein each of said pools has a shape defined by <90, H.sub.app/e.sub.app<1, and H.sub.ZR/H.sub.app0.6, where designates the angle between a top surface of said pool and a working plane, H.sub.app designates an apparent height of a fillet that corresponds to the portion of the pool above the working plane, e.sub.app designates a width of the fillet, and H.sub.ZR designates a height of a remelted zone, the working plane being defined as the plane containing the surface on which said layers are being formed.

2. The method of fabricating a part according to claim 1, wherein the powder particles have a size lying in a range from 25 m to 75 m.

3. The method of fabricating a part according to claim 2, wherein the powder particles have a size lying in a range from 25 m to 45 m.

4. The method of fabricating a part according to claim 1, wherein a focal point of the high energy beam is situated above a working plane or in said working plane, and a focal point of the powder beam is situated beneath the working plane, such that powder particles do not at any time cross the high energy beam between an outlet of a nozzle and the working plane, the working plane being defined as the plane containing the surface on which said layers are being formed.

5. The method of fabricating a part according to claim 4, wherein in order to obtain the focal point of the high energy beam and the focal point of the powder beam, a defocus distance of the powder beam is increased, or a divergence half-angle of the high energy beam relative to a perpendicular to said working plane is decreased, or both the defocus distance of the powder beam is increased and the divergence half-angle of the high energy beam relative to a perpendicular to said working plane is decreased, or a defocus distance of the high energy beam is decreased.

6. The method of fabricating a part according to claim 1, wherein the three quantities , H.sub.app/e.sub.app, and H.sub.ZR/H.sub.app satisfy the following relationships:
1560, 0.04H.sub.app/e.sub.app0.75, and 1H.sub.zR/H.sub.app6.

7. The method of fabricating a part according to claim 1, wherein the powder particles are at ambient temperature when reaching the pool.

8. The method of fabricating a part according to claim 7, wherein the ambient temperature is 20 C.

9. The method of fabricating a part according to claim 1, wherein the powder particles all have the same size.

10. The method of fabricating a part according to claim 1, wherein the angle between the axis of the powder beam and the axis of the high energy beam is equal to or less than 10.

11. The method of fabricating a part according to claim 10, wherein the angle between the axis of the powder beam and the axis of the high energy beam is equal to or less than 5.

12. The method of fabricating a part according to claim 1, wherein the powder particles have a size distribution that is in a range from 25 m to 75 m.

Description

(1) 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:

(2) FIG. 1 is a diagram showing one possibility for positioning the high energy beam and the powder beam in the method of the invention;

(3) FIG. 2, described above, is a diagram for explaining the prior art method and shows the device for the DMD method;

(4) FIG. 3, described above, shows the effect of the diameter D.sub.P of the particles of Ti-6Al-4V powder on their temperature at the outlet from the nozzle when they reach the liquid pool;

(5) FIG. 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

(6) FIG. 5, described above, is a diagrammatic cross-section of the liquid pool formed in the support.

(7) In the invention, the powder particles are cold when they reach the pool formed at the surface of the preceding 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.

(8) In comparison, the temperature of the liquid pool T.sub.BL is higher than the melting temperature T.sub.F of the material constituting the powder, but lower than the boiling temperature T.sub.evap of that material. This melting temperature is higher than 550 C. for aluminum alloys, higher than 1300 C. for nickel-based alloys, higher than 1450 C. for steels, and higher than 1550 C. for titanium alloys.

(9) FIG. 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 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.

(10) FIG. 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 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 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 following are measured: the laser defocus distance Defoc.sub.L, the powder defocus distance Defoc.sub.P, the working distance WD, the diameter .sub.L of the laser beam, and the diameter .sub.P of the powder beam.

(11) 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 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.

(12) In the invention, the nozzle 190 is configured and positioned in such a manner that the focal point F.sub.L of the high energy beam 95 is situated above the working plane P or in that plane, and the focal point F.sub.P 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 beam between the outlet from the nozzle and the working plane P.

(13) In an implementation other than that shown in FIG. 1, the focal point F.sub.P of the powder beam may lie within the support. Under such circumstances, the powder defocus distance Defoc.sub.P is smaller than that shown in FIG. 1. As a result, the diameter .sub.L of the laser beam in the plane P is closer to the diameter .sub.P of the powder beam in the plane P, for given parameter settings (F.sub.L, V, D.sub.m).

(14) By way of example, the diameter .sub.L of the laser beam in the plane P is slightly less than the diameter .sub.P of the powder beam in the plane P.

(15) As shown in FIG. 1, such a configuration is obtained by moving the nozzle 190 closer to the working plane P relative to the prior art configuration (FIG. 4), i.e. by reducing the working distance WD.

(16) Such a working configuration is particularly adapted to making wide fillets 105, i.e. fillets 105 of width that is greater than the diameter .sub.L0 of the high energy beam 95 at the laser focal point.

(17) The diameter of the liquid pool .sub.BL is then greater and more cold powder particles reach the liquid pool 102, which is beneficial as explained above.

(18) The focal point F.sub.L 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 F.sub.P of the powder beam 94 may be situated in the working plane P. The focal point F.sub.P of the powder beam 94 may also be situated below the working plane P.

(19) In order to optimize the method of the invention, it is possible to adapt certain parameter settings accordingly, in particular the laser power P.sub.L, the scanning speed V, and/or the powder mass flow rate D.sub.m.

(20) Nevertheless, in the implementation shown in FIG. 1, it may be necessary to provide (additional) cooling of the nozzle 190 since the nozzle 190 is heated by radiation due to its proximity to the liquid pool 102. Such cooling requires a device that is expensive.

(21) 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 the high energy beam, the inventors have devised an implementation that consists advantageously either in reducing the distance Defoc.sub.L, or in reducing the divergence half-angle of the laser beam 95 relative to the axis Z, which amounts either way to reducing .sub.L so as to ensure that it is smaller than .sub.P.

(22) Alternatively, the distance Defoc.sub.P of the powder beam 94 is increased in order to compensate for the reduction in .sub.P when increasing WD, thereby keeping .sub.P greater than .sub.L.

(23) This reduction in the distance Defoc.sub.L and in the angle , and this increase in the distance Defoc.sub.P may be performed jointly.

(24) 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 particles 60 reach the working plane P immediately outside the zone of the working plane P that is covered by the laser beam 95.

(25) Thus, given that the liquid pool 102 extends by conduction a little beyond that zone, the majority of the 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 any change of shape, to avoid agglomerates forming, and to avoid harmful oxidation of the powder particles 60.

(26) This explains why tests undertaken by the inventors show that the melting mass efficiency R.sub.m in the method of the invention is higher than the melting mass efficiency when the powders reach the pool while hot, or indeed while partially or completely melted.

(27) 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 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).

(28) 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+recycling) method of the invention is thus indeed greater than the total mass efficiency of the prior art method.

(29) 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 <90, H.sub.app/e.sub.app<1, and H.sub.ZR/H.sub.app0.6, where is the angle made by the top surface of the pool 102 with the working plane P, H.sub.app is the apparent height of the fillet (portion of the bath 102 above the working plane P), e.sub.app is its width, and H.sub.ZR is the height of the remelted zone or diluted zone (portion of the pool below the working plane P) (see FIG. 5).

(30) Preferably, the three quantities , H.sub.app/e.sub.app, and H.sub.zR/H.sub.app satisfy the following relationships:
1560, 0.04H.sub.app/e.sub.app0.75, and 1H.sub.zR/H.sub.app6.

(31) When material is being built up on a part for repair purposes, these quantities preferably satisfy the following relationships:
3060, 0.151H.sub.app/e.sub.app0.25, and 0.01H.sub.ZR/H.sub.app0.025.

(32) Advantageously, the size distribution of powder particles 60 is narrow (which corresponds to particles all having substantially the same size, which size is appropriate for the temperature and the volume of the liquid pool so as to be molten at all times throughout the duration of laser/pool interaction). Under such circumstances, the probability is high that all of the powder particles 60 have sufficient time to melt in the pool 102 before the laser beam 95 has moved on (and thus ceased to heat the pool 102). The method consisting in feeding the pool with powder particles that are cold and that have a size distribution that is narrow is then more effective in terms of stability and build speed since the temperature of the pool decreases more quickly and the apparent height of the fillet becomes greater. This apparent height increases with finer particles since the temperature of the pool decreases progressively and remains constant (solidification threshold reached) as the particles penetrate into the pool 102.

(33) For example, the powder particles 60 present sizes lying in the range 25 micrometers (m) to 75 m. Preferably, these sizes lie in the range 25 m to 45 m.

(34) In the prior art method, a wider distribution of powder particles 60 is more harmful. In the presence of interaction between the laser and the powder, powder particles 60 of different sizes reaching the pool at different temperatures leads to the temperature of the pool fluctuating, and runs the risk of making the pool unstable.

(35) Advantageously, the positioning of the nozzle 190, i.e. the working distance WD, is servo-controlled to spatial variations of the working plane P (variations in the consolidated material height H.sub.app of a layer of the part to be built, while the raising increment Z up the Z axis of the nozzle 190 is kept constant by preprogramming) such that, for each layer, the focal point F.sub.L of the laser beam 95 is situated at the same height above the working plane P, and the focal point F.sub.P of the powder beam 94 is situated at the same height below the working plane P.

(36) Alternatively, the increment Z may be servo-controlled to the variations in the consolidated material height H.sub.app of a layer.

(37) Such servo-control is performed by using a process control program of known type, that does not need to be described herein.