Note: Descriptions are shown in the official language in which they were submitted.
CA 02448519 2003-11-06
TITLE OF THE INVENTION
"Apparatus For Laser Consolidation And Manufacturing"
BACKGROUND OF THE INVENTION
Field of the Invention:
This invention relates to an apparatus for building precise 3D components
and structures by a material addition process called laser consolidation, more
particularly an arrangement for the vertical delivery of metallic powder, or
wire into
a precisely formed melt pool created in a substrate by laser beams having a
specific angular orientation relative to the substrate.
Background Information:
Rapid Prototyping (RP) is a related technique based on layered
manufacturing where a part is built as a series of horizontal layers, each one
being formed individually and bonded to the preceding layer. Various processes
have been used differing in the way each layer is formed and the raw materials
used but the underlying methodology is essentially the same in each case.
Stereolithography (SLA) and Selected Laser Sintering (SLS) are the two
most common rapid prototyping processes. In both cases, a three dimensional
CAD model of a part is generated and sliced into horizontal layers. The sliced
files
are used for tool path generation to make a solid part layer by layer. The
thickness of each slice is controlled and is determined by the degree of
accuracy
required and the capability of the system, viz-a-viz the maximum thickness
that
can be cured or sintered by the specific process.
The SLA process uses a photosensitive monomer, which is cured layer by
layer using an ultraviolet laser resulting in a cured polymer part. In the SLS
process a carbon dioxide laser of appropriate power is used to scan across the
surface of a bed of a powdered thermoplastic material, sintering the powder
into
the shape of the required cross-section. A major limitation of the SLS process
is
its inflexibility in the selection of metals that can be used. To generate
metallic
parts, thermoplastic coated metal powders are used to create a "green shape"
of
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CA 02448519 2003-11-06
the component. The thermoplastic plastic is removed in a"burn-off'step and
replaced by infiltrating a lower melting point metal.
In order to produce dense three dimensional metal/alloy parts, Los Alamos
National Laboratory in the U.S. developed a process called "Directed Light
Fabrication of Complex Metal Parts" (1994 ICALEO conference). In this process
a
coaxial powder delivery nozzle is used with a normal laser incident angle. The
focussed laser beam enters a chamber along the vertical axis of the nozzle
that
also delivers metal powder to the focal zone. The deposition is done on a base
plate, which is removed after the part is built. The powders used for part
build-up
are 316 stainless steel, pure tungsten, nickel aluminide and molybdenum
disilicide.
In a paper presented at a "Rapid Prototyping and Manufacturing "96"
conference (SME, Michigan, April 23-25, 1996) Dave Keicher of Sandia National
Laboratories dealt with "Laser Engineered Net Shaping (LENS) for Additive
Component Processing". This process uses a Nd:YAG laser and a special nozzle
arrangement for powder delivery. Four streams of powder are fed into a melt
pool
which is created and sustained by a central laser beam. It is pointed out that
this
arrangement avoids the situation in off-axis single side feed powder delivery
system where there is a strong directional dependence. The symmetrical (quasi
coaxial) arrangement permits uniform cladding independent of direction.
A rapid prototyping technique has also been used by Prof. W. Steen (1994
ICALEO conference paper). A machining pass is added after each build-up pass,
and a high power carbon dioxide laser (> 2 kw) is used. Optics for the beam
delivery system are incorporated on an automatic tool changing system. The
process requires that after each laser build-up pass, the metal layer is
machined
back to required dimensions, necessary because of a lack of control on the
laser
build-up. It was also found that a change in cladding direction has a
significant
influence on the shape and quality of the build-up. Good quality clad with a
regular shaped bead was obtained parallel to the flow direction but as the
angle to
the flow direction increased the quality deteriorated until clad perpendicular
to the
flow was of poor quality. Machining is used to remove the imperfections in
shape
and size of each built up layer arising from the change in the clad direction.
As
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side noale powder delivery builds unevenly in various directions in the xy-
plane, the
additional required step of machining after each deposiition pass makes the
process
cumbersome and expensive. As the control on the build-up process is poor, most
of
the material is removed to maintain the geometry creating unnecessary waste of
expensive material.
It is evident from the above that in building up metal parts using a carbon
dioxide or Nd:YAG laser and metallic powder, single nozzle side delivery
always
involves a directional dependence, and is either abandoned in favour of
coaxial
powder delivery or machlning is employed after every pass to maintain
dimensions.
The trend is to use a coaxial powder delivery to obtain equal layer build-up
in all
direCfions. In addition it is apparent that the incident laser beam is always
normafl to
the surface of the base plate.
Several nozzle designs for coaxial powder feeding during laser cladding have
been disclosed, for example: U.S. Pat. No. 4,724,299 (Hammeke, Feb. 9, 1988);
U.S.
Pat, No. 5,418,350 (Freneaux, May 23, 1995); U.S. Pat. No. 5,477,026
(Buongiomo,
Dec. 19, 1995) and U.S. Pat. No. 5,111,021 (Jolys, May 5, 1992).
U.S. Pat. No. 5,731,048 to Mistry (Mar. 24, 1998) discloses a technique for
fabr9cating diamond and diamond-like coatings on a substrate. Mistry also
discloses
that complex shapes can be fabricated as coating structures on the surface of
the
substrate. Mistry discloses using a plurality of lasers each having different
and
specific temporal and spectral characteristics to perform the following
functions: one
laser to ablate the constitauent element, a second to initiate chemical
reaction, and a
third to provide overall thermal balance. Mistry discloses that shaped
coatings can be
made on the surface of the substrate by the relative movement of the laser
system
and the substrate. Mistry does not teach the importance of the critical angle
of the
lasers relative to the powder feed nozzle, the symmetrical arrangement of the
laser
beams relative to the material feed system nor the control over and the shape
of the
me{t pool required to make pnecise structures and components with smooth
walls.
The inventors' U.S. Pat. No. 5,855,149 (Canadian application 2,242,082
published Dec. 30, 1999) teaches a method of producing a sharpened edge on a
cutting die by having a laser beam or beams impinge on a
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base surface at an angle to the normal of between 5 and 45 to fuse
sucCessive thin
layers forming a metal ridge to the cutting edge. The inventors' Canadian
application
No. 2,215,940 published March 23, 1998 discloses an apparatus and method for
material disposition on a surface using a laser beam or beams impinging on the
surface at an angle to the normal of between 5 and 45 .
Generally laser based material addition processes rely on focussing a laser
beam to create a small molten zone in a suitable starting material
(substrate). New
material, usually in powder form, is added and melted to increase the volume
of the
molten zone. When the laser is shut oft=, or moved to a new location, the
molten
material rapidly cools and solidifies. When the process is sustained by moving
the
laser and material addition system across the substrate, at a controlled
speed, it is
possible to make a uniform ridge. The ridge can take on geometric forms when
the
laser and powder feed systems are moved across the substrate by following a
predetermined path as desCribed by a computer numerically controAed system. By
repeating the operation using the original ridge as a new substrate,
eventually after
subsequent layers are added, a walled structure is formed.
All of the processes reported, can be described as near net shape, For
example, Sandia National Laboratories, using their Laser Engineered Net Shape
(LENS'M) process, can produce parts with complex shapes having surface
finishes
that resembles a fine sand casting and having dimensional tolerances at best
of +!-
100 microns. To obtain better dimensional control and surface finishes
requires
secondary operations.
The arrangements commonly used in the prior art as illustrated in Figures
1 a-7 b of the drawings have a centrai laser source 20 with powder 21 entering
symmetncally from the sides around the circumference. In this way the
relationship to
the pool remains the same regardless of the wall path. The arrangement is
symmetrical but powder entering from the sides causes thermal and viscosity
gradients 22, leading to incomplete melting where the wall surfaces are
forming. Line
23 indicates the boundary of the molten zone. Unmelted or partially melted
particles
of powder 24 tend to stick on the surface as the wall is cooling. Attempting
to oorrect
the situation by adding more energy is not
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successful because the surface where the energy enters starts to evaporate
causing a plasma to form which absorbs the incoming laser energy. The mass of
"soupy" unmelted material in the vicinity of sides of the wall tends to slump
outside the dimensions of the pool. Subsequent passes, or layers, applied in
this
slumped condition result in a wall where each layer has a convex curved
surface
25. These curves at the surfaces of the layers produce variations in the wall
thickness 26. The resultant wall has the appearance similar to 27 shown in
figure
1a.
The practice of making precise structures in the prior art is to form a rough
shape then use a material removal operation such as machining to create the
final shape and surface finish. The present application describes a
methodology
and apparatus for making precise structures, for example, in the form of
shells, in
one operation.
When a focussed laser is used to rapidly melt a zone in a substrate, and
the zone is cooled quickly, the surface of the solidified zone is smooth. When
the
melting takes place in a non-oxidizing, dust and vibration free environment,
and
the molten zone is maintained close to the flow temperature of the substrate
material, sub-micron finishes can be obtained on the solidified surface. If
the
melting process is controlled it is possible to get high quality surface
finishes.
When material is added and melted into the melt pool, to increase its
volume, it is more difficult to maintain a smooth finish. The problems with
existing
state of the art near net shape processes that feed powder into the pool from
the
sides stem from the thermal, and hence viscosity gradients, created in the
pool
and from powder particles sticking to the side walls as the pool solidifies.
SUMMARY OF THE INVENTION
In the invention:
Laser energy enters the molten pool at an angle of about 30 degrees to the
vertical and symmetrically around the pool in the form of an annulus.
Powder is injected vertically at the top dead centre of the melt pool through
a fine nozzle.
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The advantages are:
In forming the pool of molten material, energy enters symmetrically around
the pool allowing the temperature to rise uniformly and rapidly avoiding
local evaporation or the creation of serious thermal gradients within the
pool.
Surface tension is maintained uniformly around the pool and hence results
in a pool with a surface that is close to hemispherical in shape.
The temperature of the pool in the regions where the walls will form is
uniform from side to side and is controlled above the melt temperature so
that all the powder is completely melted. Thus the walls formed on cooling
have a precise width and the surfaces are smooth. There are no visible or
metallurgical discontinuities to show that the structure has been formed in
a series of passes.
Directing the powder into the pool at the top ensures a high capture rate of
powder and any stray particles are directed through the incoming beam
and away from the solidifying wall surfaces.
The symmetry of the total system permits the substrate to be moved in any
direction relative to the laser powder feed arrangement without changing
the thermal balance within the melt pool.
The apparatus of this invention meets the criteria for making precise walls.
However, in practice there may be a need to make minor adjustments in wall
thickness. A fixed focus rigid 360 degree focussing mirror precludes any
adjustment.
Variations in the apparatus are disclosed which permit fine adjustments.
Specifically, the invention relates to apparatus for deposition and machining
of an
article comprising: a laser, focussing means producing a convergent beam from
the laser, and reflecting means defining a path resulting in the focal point
of the
beam being positioned close to a work surface. The focussing means is adapted
for movement whereby the focal point can be moved above and below the work
surface for improved precision.
In another aspect the invention relates to apparatus for deposition and
machining of an article comprising: a laser, focussing means producing a
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convergent beam from the laser and means adapted for movement of the focussing
means to vary the position of the focal point of the beam. A beam splitter and
reflective means define paths for the split beams to impact on a work surface.
The
reflective means includes a plane reflecting surface adapted for movement in a
direction to alter the length of the paths. When the beams overfap on the work
surface they produce an area of enhanced energy useful for cutting, welding or
machining.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing, and additional objects, features, and advantages of the
present invention will become apparent to those of skill in the art from the
following
detailed description of preferred embodiments thereof, taken with the
accompanyirig
drawings, in which:
FIGS. 1a and 1b are schematic diagrams showing sources of dimensional
inaccuracy
and surface roughness in prior art equipment,
FIGS. 2a-2d contain schematic diagrams illustrating surface tension forces.
FIG. 3 shows a cross-sectional view of 360 focussed laser beam consofidator,
FIGS. 4a-4b show the direction and arrangement of laser beams and powder feed,
FIG. 5 shows a cross-sectional view of the apparatus using a pyramidal mirror,
FIG. 6 shows a cross=sectionai view of apparatus using lenses,
FIG. 7 shows a variation of the apparatus shown in Figure 5 using plane rather
than
spherical reflectors,
FIG. 8 shows the beam paths in the apparatus of Figure 7 when the focussing
lens is
moved, and
F1G. 9 shows the beam paths in the apparatus of Figure 7 when the plane
reflecting
mirrors are moved.
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DESCRIPTION OF THE INVENTION
In building components using the laser consolidation process, the shape and
position of the liquid to vapour and the liquid to solid surface tension
interfaces are
critical.
Using the consolidation process to create precise structures with smooth
sides, requires the volume of molten material to be as close to spherical as
possible
where the diameter of the sphere is equal to the thickness of the wall under
construction. It follows that the process depends on being able to control the
diameter and location of the sphere.
It is well known that when a liquid body is free to do so, it minimizes its
energy
content by assuming a spherical shape. This is the shape that has the least
surface
area to volume. In practice this is difficult to attain. One example, is when
a liquid
freezes during a free fall, such as in forming lead shot. It is also possible
using the
controlled conditions of the consolidation process to create essentially a
spherically
shaped volume of molten material.
From a theoretical point of view, to attain verdcal and smooth side wall
construction, requires the shape of the molten surface exposed to the
atmosphere to
be essentially hemispherical. It is equally important that the portion of the
molten
zone cradled within the substrate is also essentially hemispherical, so the
total
molten volume is spherical. This spherical condition results in a smooth
vertical
transition as each pass of material is added. In the spherical condition the
components of surface tension, liquid to solid (yLs), liquid to vapour (yLv)
and solid to
vapour (ySv) are balanced. Various models of melt pool configurations are
shown in
Figures 2a-2c, in which this balance is not maintained and where material
slumping
is experienced. Slumping is caused by the mass of added material overcoming
the
surface tension force's effort to fomt a sphere.
To aid in achieving the total spherical shape it is necessary to create within
the wall, or substrate, a cradle which is essentially hemispherical to support
the
liquid. It has been found experimentally that the molten material has to be
cradled in
such a way that the liquid to solid surface tension interface (Ls) blends
smoothly and
vertically into the liquid to vapour surface tension interface (Lv).
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Since there are no angular forces in this condition the surface tension force
system
can be expressed as yLv = yLs + y5v. Because the consolidation process forms a
cradle with a sharp edge at the wall surface, ySv is negligible, thus yLs =
yLv which
is what one would expect in a perfect sphere. ff the smooth vertkal blend is
not
achieved some form of aben'ation will occur in the forming side wall.
The applicants have found that to create a suitable support cradle the entry
angle of the laser beam has to be between 25-30 degrees to the vertical. It
may be
possible to use smaller angles but practiCal limitations imposed by the powder
feed
system prevented exploration of this logical possibility, Increasing the angle
produces
shallower cradles and an imbalance in the surface tension force system
resufting in
discontinuities in the wall surfaces.
Figures 2a-2d illustrate surface tensions components for various systems. In
Figures 2a, 2b, 2c, 2d the liquid-to-vapour interface (Lv) is represented by
arrow 50,
the liquid-to-solid interface (Ls) is represented by arrow 51 and the solid-to-
vapour
interface (Sv) is represented by arrow 52. In all of the conditions shown in
Figures
2a, 2b, 2c (representing prior art) the surface tension components have
angular
relationships to each other and can be expressed as yLv + yLs cose1 + ybtir
cosA2 _
9, where 91, eZ represent the angular relationships between the surface
tension
components.
In Figure 2a (prior art) the shallow cradle 53 limits the build up capacity
because of the onset of slumping, or overflowing. In Figure 2b (prior art) the
laser
energy distribution has created shallow areas 54 near the sidewalls and is
unable to
support the build up of material. In Figure 2c (prior art) the deeper cradle
56
improves the build capacity, but the surface tension forces in attempting to
attain a
spherical form cause the material to bulge 57 resulting in poor dimensional
control
and undulating surfaces. In Figure 2d (representative of the present
invention), the
hemispherical cradle allows material to build up such that the surface tension
components 50, 51, 52 at the point of maximum build up are vertically aligned
and
thus surface tension is able to form a spherical pool.
Figure 3 shows a 360 degree focussed laser beam consolidator 30. A low
energy density expanded laser beam 31 enters at the top of the consolidator
30.
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The laser beam is reflected by a conical mirror 32 forming a divergent hollow
conical beam 33 which impinges on a 360 degree spherical mirror 34. The
spherical mirror reflects the hollow conical beam in the form of a convergent
hollow cone 35 and focuses the energy in an annular pattern 36 on the
substrate
surface 37. The angle d that the converging beam makes with the vertical axis
of
the system is 30 degrees. The diameter of the melt pool made by the annular
ring of energy determines the wall thickness of the structure to be built. A
powder
feed tube 38 passes through the centre of the low energy density incoming
laser
beam and through the centre of the conical mirror and terminates in a
precision
powder feed nozzle 39 positioned directly above the melt pool. Powder is
propelled through the powder feed tube and injected from the nozzle into the
centre of the melt pool. Additionally the consolidation system features a gas
purge system 40 to keep the mirrors clean and as the gas exists from the
nozzle
of the protective cone 41 provides a cover gas 42 over the melt pool that
inhibits
oxidation.
Figures 4a and 4b illustrate the arrangement for the delivery of energy
and powder. Laser energy 70 enters the substrate material 71 symmetrically in
an annular pattern from several directions up to a full 360 degrees at an
angle of
30 degrees to the normal of the substrate surface, or vertical axis. The
cumulative effect of the energy in the beams creates within the substrate a
hemispherical pool of molten material 72, the viscosity of the molten material
is
relatively constant throughout the pool. A controlled stream of powdered
material
73 is injected into the top of the molten pool 74. The volumetric rate of
material
entering the pool is critical to ensure that the mass of the increased height
is
balanced against the ability of the surface tension force to maintain the
spherical
shape 75 of the pool. Subsequent passes are required to build-up a wall of
material. The height of each pass 76 is small and consistent with the
balancing of
surface tension against gravity. Because the region close to the edge 77 is
kept
in a molten state, the surface tension in the material is able to maintain the
verticality of wall between passes. Because of the small mass of molten
material,
relative to that of the substrate, the material solidifies quickly leaving a
smooth
surface 78 and a fine grained metallurgical structure in the material.
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By replacing the conical mirror shown in Figure 3 with a multifaceted
pyramidal mirror, a number of beamlets equal to the number of facets will be
reflected on to the concave spherical mirror. The beamlets are then focussed
by
the spherical mirror to form an annulus of energy in the melt pool that is
similar to
the 360 degree arrangement shown in Figure 3. The multi-beamlet arrangement
has the advantage of providing space for the powder feeder or sensors to enter
between the beamlets, and thereby simplify the construction of the
consolidation
system.
In the multi-beamlet configuration individual mirrors for each beamlet can
replace the single spherical mirror Figure 5. The incoming beam 80 is split
into
four beamlets by the four sided pyramidal mirror 82. The four beamiets with D-
shaped cross-sections are reflected towards the individual mirrors 81 that
have a
concave spherical surface. The beamlets in turn are reflected and focussed
onto
the substrate 83. The individual mirrors can be moved horizontally so that the
axis of the focussed beamlet is shifted laterally in, or out. This lateral
adjustment
is used to change the melt pool diameter and hence the thickness of the part
being produced. The amount of movement is small, in the order of 13 microns
(0.0005") and the focussing angle is kept at 30 degrees. The arrangement also
shows the powder feed tube 84 entering the system between the beamlets and
connecting to the injection nozzle below the pyramidal mirror.
It is further possible to reduce the number of facets on the splitter mirror
to
two, that is, a wedge rather than a cone. This two-beamlet arrangement
produces
less precise parts, as the energy distribution in the D-shaped beamlets is not
completely uniform during directional changes.
The two-beamlet arrangement is capable of producing a nominal wall
thickness controlled to +/-25 microns and surface finishes better than 2
microns
Ra.
Standard optics can be used to perform in a similar manner to the multi-
beamlet approach, such as that shown in Figure 6. This apparatus employs
lenses 91 positioned to focus supplied laser beams at the desired focussing
angle
d of 30 degrees. The powder feed system is directly vertical 92. The optics
and
the powder feed system are accurately position in a rigid body 93 that can
also
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support a protective feed cone 94 and an inert gas cover purge and system 95
and
96. The number of beams that can be physically acoommodated is limited in this
design. Energy 97 may be supplied to the focussing lenses through fibre optic
delivery systems commonly used with Nd;YAG lasers. A structure 98 is shown
being
built on an original substrate 99.
Figure 7 illustrates a modification of the apparatus shown in Figure 5 in
which the spherically mirrored sections 81 are repl2Ced by movable plane
mirrors
101. A movable focussing lens 102 is provided in the path of the laser beam
prior to
the beam impacting on the flat faceted surfaces of the reflector 82, which as
shown is
pyramidal-shaped.
Movement of the focussing lens 102 results in movement of the focal point of
the beam 103 above and below the work surFace, as is shown in the ray diagram
of
Figure 8. A dx displacement of the lens 102 leads to a similar displacement of
the
focai point. Movement of the reflecting plane mirror 101 results in sideways
displacement of the beam with respect to the work surface as shown at 104 in
the ray
diagram of Figure 9. A displacement d in the mirror 101 leads to a slightly
larger
lateral displacement d' of the focal point, where d' = 2d cosO. This
arrangement
provides improved control with a structure less complex than that shown in
Figure 5.
Although the movement of mirrors 101 is shown as being perpendicular to their
planar surface, which facilitates control thereof, it will be appreciated that
the
movement could be lateral or radial with respect to the central axis of the
apparatus.
A further advantage of the apparatus of Figure 7 is that it is possible to
have
the separate beams overlap originating at the beam splitter on the work
surface
thereby producing increased intensity which is useful for cutting, welding and
machining.
It will be clear to persons skilled in the art, that other numbers of beams
could
be used and would fall within the scope of this invention. It will also be
clear to those
skiiled in the art that materials other than powdered metai could be used and
that
other forms of material such as wire could be used in the material feed
without
deviating from the scope of the present invention.
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