Electric heating – Metal heating – By arc
Reexamination Certificate
2002-11-12
2004-06-29
Elve, M. Alexandra (Department: 1725)
Electric heating
Metal heating
By arc
C219S121630
Reexamination Certificate
active
06756561
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH/DEVELOPMENT
Not Applicable
BACKGROUND OF THE INVENTION
1. 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.
2. 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 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, Apr. 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 side nozzle powder delivery builds unevenly in various directions in the xy-plane, the additional required step of machining after each deposition 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 machining 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 directions. In addition it is apparent that the incident laser beam is always normal 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 (Buongiorno, Dec. 19, 1995) and U.S. Pat. No. 5,111,021 (Jolys, May 5, 1992).
U.S. Pat. No. 5,731,046 to Mistry (Mar. 24, 1998) discloses a technique for fabricating 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 constituent 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. Ministry 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 melt pool required to make precise 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 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 Mar. 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 off, 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 controlled 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, Sand
Islam Mahmud-Ul
McGregor Gavin
Elve M. Alexandra
Jones Tullar & Cooper PC
National Research Council of Canada
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