WO1997021515A1

WO1997021515A1 - Deposition head for production of articles from powders

Info

Publication number: WO1997021515A1
Application number: PCT/US1996/018726
Authority: WO
Inventors: Gary K. Lewis; Richard M. Less
Original assignee: The Regents Of The University Of California
Priority date: 1995-11-30
Filing date: 1996-11-21
Publication date: 1997-06-19
Also published as: US5961862A; AU1121797A

Abstract

A deposition head for the production of articles from metal powder includes an optics package (201) and a delivery nozzle (241) which contains a laser beam passage (242), at least two oppositely disposed powder passages (230), upper, middle, and lower protective windows (202, 207, 209), a coolant channel (234), and upper and lower gas channels (231, 227).

Description

DEPOSITION HEAD FOR PRODUCTION OF ARTICLES FROM POWDERS

FIELD OF THE INVENTION

This invention relates to the field of fabrication of articles and, more particularly, to production of objects using particulate matter, or powder, as a starting material. This invention was made with government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

This invention is a deposition head which is useful in the process which is termed directed light fabrication (DLF) or the DLF procesε. Directed light fabrication may be used to produce articles of any material which is obtainable in the form of a powder, but its primary use is expected to be production of metal articles. Metal objects are currently produced by thermo- mechanical processes which include casting, rolling, stamping, forging, extrusion, machining, and joining operations. Multiple steps are required to produce a finished article. These conventional operations often require the use of heavy equipment and molds, tools, and dies. For example, a typical process sequence required to form a small cylindrical pressure vessel might include casting an ingot, heat treating and working the casting to homogenize it by forging or extrusion or both, then machining a hollow cylinder and, separately, end caps from the worked ingot and, finally, welding the end caps to the cylinder. The DLF process provides a method for forming such an article in a single operation and using lesε equipment. An article formed by laser deposition is relatively free of internal stresses in comparison to an article formed by welding one or more components together. Also, joining components by means of welding requires equipment just for the single step of joining and time to set up the equipment to do the job.

Conventional production methods are subtractive in nature in that material is cut away from a starting block of material to produce a more complex shape. Subtractive machining methods are deficient in many respects. Large portions of the starting material are reduced to waste in the form of cuttings. These methods produce waste materials, such as metal cuttings, oils, and solvents, which must be further processed for purposes of reuse or disposal. The articles produced are contaminated with cutting fluids and metal chips. They require cutting tools which wear and must be periodically reconditioned and ultimately replaced. Fixtures for use in manufacturing must be designed, fabricated, and manipulated during production. When a part is unusual in shape or haε internal features, machining is more difficult. Choosing the machining operations to be used and the sequence of operations requires a high degree of experience. A number of different machines are needed to provide capability to perform the variety of operations which are often required to produce a single article. Sophisticated machine tools require a significant capital investment and occupy a good deal of space. Use of DLF in place of subtractive machining provides solutions to these problems and disadvantages. The ^"DLF process may be characterized as additive in nature. The raw material which does not become part of an article is easily collected and re-used without processing. There is no need to dispose of waste liquids and metal cuttings and the articles produced are not contaminated by these materials. Fixtures and cutting tools are not required. All work needed to produce an article is accomplished using a computer workstation and a single production station. Also, this invention can be used to rapidly provide production tooling after article design is frozen.

Another difficulty with conventional machining techniques is that many objects must be produced by machining a number of parts and then joining them together. Producing parts separately and joining them requires close- tolerance machining of matching parts, provision of fastening means, such as threaded connections, and welding together of components. These operations involve a significant portion of the cost of producing an article, as they require time for design and production as well aε apparatus for performing them.

The process of designing an article for volume production often consumes a good deal of time. Much of this time is spent in producing prototypes. Prototyping may require specialized apparatus and services which are not available in-house. Molds, tools, and dies are particularly difficult to obtain rapidly. Once a prototype is made, it is often necessary to make changes in the design and make another prototype. The present invention provides a method for rapid prototyping. As commonly used today, rapid prototyping refers simply to making a pattern or non¬ functional part. A non-functional article allows fit and form to be tested. It is considerably more difficult and time-consuming to make a prototype to test function, that is, which has the mechanical properties needed to test the article in its intended use. The DLF process is capable of producing a true rapid prototype, which can be used in a service test to investigate stresses, strains, fracture properties, and wear properties. After testing, the design of the article may be changed and another true prototype made and tested. This rapid reiteration provides an ability to optimize a design in a short period of time. An article made by the DLF process has annealed properties, that is, its microstructure and metallurgical properties are similar to articles which have been annealed. If the material of a DLF article is heat-treatable, the properties of the article, such as strength, ductility, fracture toughness, and corrosion resistance, can be modified by means of heat treatment. An article formed by a conventional cold working process of a material having a large strain-hardening coefficient (such as austenitic stainless steel) will have relatively high strength compared to a DLF-formed article. Examples of cold working processes are forging and extrusion. If, however, a cold-formed article is subjected to the heat of welding, itε strength will be reduced to that of annealed articles. Thus, DLF is an attractive alternative to fabrication by subtractive machining and welding, but cannot be subεtituted for processes in which a near net shape article is fabricated by cold-forming without welding and the high strength of cold- forming is required.

Following are descriptions of background patents. U.S. Patent No. 4,724,299 teaches a nozzle for cladding of an object using metal powder melted by a laser beam.

U.S. Patent No. 5,111,021 deals with adding material to a surface using a laser beam and metal powder and discloεeε a nozzle for doing so.

U.S. Patent 4,743,733 teaches repair of an article by directing a laser beam and a stream of metal powder to a region of the article which requires repair.

U.S. Patent No. 4,323,756 teaches a method for producing metallic articles from metal powders and substrates which become part of the articles. A focused energy beam is used to create a molten pool on a substrate and metal powder is supplied to a point outside of the area at which the beam impinges upon the substrate. Movement of the substrate then carries the powder into the beam and molten pool, where it melts and mixes with the melted substrate material.

SUMMARY OF THF. INVENTION This invention is a deposition head for use as a part of apparatus for forming articles from materials in particulate form in which the materials are melted by a laser beam and deposited at points along a tool path to form an article of the desired shape and dimensions. The deposition head delivers the laser beam and powder to a deposition zone, which is formed at the tip of the deposition head. A controller comprised of a digital computer directs movement of the deposition zone along the tool path and provides control signals to adjust apparatus functions, such aε the speed at which the deposition head moves along the tool path. It is an object of this invention to provide apparatus for producing articles without use of molds, patterns, forming dies, or cutting tools.

Another object is to provide apparatus for producing articles having desirable mechanical properties, such as homogenous microstructure and high density.

Another object is to provide apparatus for producing near net shape articles without uεe of substractive machining methods. Another object is to provide apparatus for rapid and low-cost production of one-of-a-kind or small number of articles.

In one embodiment, the invention is a deposition head compriεed of a delivery nozzle and an opticε package for focusing a laser beam at a location in a deposition zone, where εaid laser beam paεεeε out of said optics package through a protective window, and where εaid delivery nozzle is comprised of a first end and a beam passage extending from said first end to a second end of εaid delivery nozzle, where said firεt end is attached to the optics package such that the laser beam pasεes through said beam passage and is focused at a location in said deposition zone, where the deposition zone is located adjacent to said second end of the nozzle; at least two powder inlet ports located at the first end of the nozzle; at least two powder passages, each having a first end which communicates with one of said powder inlet ports where each powder passage extends through the nozzle toward the deposition zone, where powder passages are arranged in pairs and a first passage of a pair is oriented about 180 degrees from a second passage of the pair, where the axial centerline of each powder passage forms an angle of from about 20 to about 75 degrees with the axial centerline of the nozzle, and where said axial centerlines converge in the deposition zone; a coolant inlet port and a coolant outlet port located at the first end of the nozzle; a coolant channel located inside the nozzle and extending circumferentially around the nozzle; a coolant inlet passage and a coolant outlet passage which connect said coolant inlet port and said coolant outlet port with said coolant channel; a lower window through which the laser beam passes which is disposed across the beam paεεage; a lower gas inlet port, a lower gaε channel located within the nozzle and extending circumferentially around the nozzle, and a lower gaε paεεage connecting said lower gaε port and said lower gaε channel; a plurality of gas pathways in the nozzle, each of which connects the lower gas channel with the beam pasεage, where εaid gas pathways are disposed about the beam passage adjacent to that surface of the lower window which is proximate to the second end of the nozzle; an upper gas inlet port and an upper gas inlet passage connecting said inlet port to the beam passage at a point between the protective window and the lower window; and an upper gas outlet port and an upper gas outlet passage connecting said outlet port to the beam pasεage at a point between the protective window and the lower window; and an upper gaε outlet port and an upper gaε outlet passage connecting said outlet port to the beam passage at a point between the protective window and the lower window. BRIEF DESCRI TION OF THE DRAWI GS

FIGURE 1 is a schematic and functional representation of apparatus for producing an article of this invention.

FIGURE 2 depicts a delivery nozzle attached to an optics package in a vertical section view taken along the central axis of the nozzle.

FIGURE 3 is a section plan view taken as shown by the section arrows of Figure 2. It showε powder passageε. FIGURE 4 iε a vertical partial section view of a delivery nozzle taken as εhown by section arrows of Figure 3. It shows powder paεεages.

FIGURE 5 iε a vertical partial section view of a delivery nozzle taken aε shown by section arrows of Figure 3. It shows coolant passages. FIGURE 6 is a vertical partial section view of a delivery nozzle taken as shown by section arrows of Figure 3. It showε upper gas passages.

FIGURE 7 is a vertical partial section view of a delivery nozzle taken as shown by section arrows of Figure 3. It shows a lower gas passage.

FIGURES 8 and 9 schematically depict articles which are in the process of formation.

FIGURE 10 depicts a laser beam and stream of powder. It is not to scale. FIGURES 11 and 12 each depict a rectangular portion of an article and a portion of a tool path followed in producing the article. They are not to scale.

FIGURE 13 iε a vertical εection view of an article which is presented to illuεtrate the capability of the preεent invention to produce articles having overhanging structures. DETAILED DESCRIPTION OF THE INVENTION

An article is constructed by depositing molten material along a tool path which has been designed to yield the desired article shape and dimensions, where this molten material is formed from a powder, or solid particulate matter, which is supplied to points along the tool path and melted at those points. The term "tool path" is used herein because it is familiar to those skilled in the art of machining with CNC apparatus and similar apparatus is used in practice of the invention. To start fabrication of an article, laser light is focused at a point on an article support and a stream of particulate matter, or powder, is supplied to that point. A small portion of the article support is melted by the laser beam to form a molten pool. The powder is melted by heat generated when the laser light strikes the particles and by transfer of heat from the molten pool to the powder as powder impinges on the molten pool. After a small amount of material has been deposited, the molten pool is formed, in part, by re-melting the deposited material. The space containing and surrounding the molten pool, the laser spot, the "focal point" of the powder stream, the focal point of the laser beam, and that portion of the laser beam where energy density is great enough to melt material is termed the deposition zone. The deposition zone is moved along the tool path, leaving behind deposits consisting of the powder which has been consolidated by melting and, then, solidification due to loss of heat to the surroundings. A first portion of the tool path is designed to provide a base portion of the article. The base portion consists of deposited material which is in contact with the article support. After deposition along the relatively small first portion of the tool path is complete, deposition is continued along a second portion of the tool path to complete production of the article. During this second step, the molten pool formed in the deposition zone consists of previous deposits and the powder supplied to the deposition zone. It iε desirable that the molten pool be small, in order to minimize the energy requirement, facilitate control of the molten pool, and permit production of an article with tight dimensional tolerances. After depoεition iε complete, the article may be εeparated from the article support, since the support is not normally intended to be a part of the article produced by the inventive process. However, applications can be envisioned in which an article is deεigned to utilize the article support as part of the article.

Relative motion between the depoεition zone and the article support or the portion of the article already formed iε accompliεhed with apparatus similar to that used in CNC

(computer numerically controlled) machining, where motion of a cutting tool is directed by a controller comprised of a digital computer which contains machine operating code, or an operation program, or a machine perating file. The tool path is usually designed, or const; „cted, by use of a digital computer and a number of computer programs. In one embodiment of the invention, an article can be considered to be a plurality of thin layers of deposited material. For example, in fabrication of a tube, or thin-walled hollow cylinder, the tool path can be viewed as a large number of circles. In one pasε of a deposition zone, or one movement of the zone of 360 degrees around the perimeter of the tube, a tube with an outside diameter of 0.85 inch and a wall thickness of 0.075 inch was increaεed in height, or length, by 0.004 inch. The layer of material deposited in the paεε was shaped as a ring having an outside diameter of 0.85 inch, an inside diameter of 0.775 inch, and a thickness, or height, of 0.004 inch. If it had been desired to fabricate a tube with a thicker wall, a second 360 degree deposition pass could have been done in the same horizontal plane to produce a wider ring.

The shape and dimensions of an article are determined by, in addition to the tool path, control commands which are establiεhed in conjunction with the deεign of the tool path. A controller adjuεts speed of movement of the deposition zone, energy density of the laser beam, and rate of flow of powder into the deposition zone in accordance with these commands.

FIGURE 1 depicts, in a functional manner, apparatus which may be used to practice the DLF process (some features of Figure 1 are not available in the new apparatus described below) . Referring to FIGURE 1, article 2 is in the process of formation and rests upon support 4_, which in turn reεtε upon rotary table 5_. The rotary table rests on X-Y table £_. Table £ may be viewed as movable both in a side-to-side direction and in a direction normal to the plane of the drawing. Article 3_ (and subεtrate 4.) may be rotated about a vertical axiε of the article by means of rotary table 5_. Deposition head 1 is mounted on arm 22. and is capable of rotation about a horizontal shaft depicted by reference number 24.• The deposition head is comprised of delivery nozzle 41 and optics package 4J2. Arm 22 is attached to positioner 22, which is capable of movement up and down along post 21. Post 21 is capable of rotation about its vertical axis. (This rotation feature is not incorporated in the apparatus whose use is described herein.) Powder and a laser beam exit delivery nozzle Al at its lower end at a location shown by reference number 2. Laser light is produced in laser generator 14 and provided to deposition head 1 by means of flexible fiber optic 16.. Optics package 40, which is commercially available, focuses a laser beam (not shown) at a point within a deposition zone (not shown) located at article 2- Powder is provided to the deposition head from powder containers 10. and 24. by means of powder feeders H and 22 through conduits 12 and 22- The powder feeders are capable of providing a variable flow of powder. Carrier gas for delivery of powder is supplied to powder feeders 11 and 22 through conduits 15. and 21. Cooling water is provided to the delivery nozzle through conduit 20. and removed from the nozzle by means of conduit 12.. An inert gas for use in protection of the windows through which the laser beam pasεeε iε εupplied by conduit 12. The apparatus is encloεed within enclosure and conduits 12 and 12 are provided to flow inert gas into the enclosure and to evacuate the enclosure. Powder pickup 2. collects powder which is discharged from delivery nozzle 1 but not melted. This powder is returned to powder container lϋ for reuse by means of conduit 2.

Controller 22 provides control signals for operating the apparatus to produce an article. Control leads 22. and 39 depict the signalε required to cause movement of the X-Y table and the rotary table. Control lead 22 provideε a signal to actuator base 22 for control of rotation of post 21- The control signal for movement of delivery nozzle 1 in a vertical direction is provided to positioner 22 by control lead 22- Control lead 22 provides a signal to actuator 22, which causes the delivery nozzle to rotate about shaft 24. Commands to vary the power level of the laser beam and to turn the beam on and off are provided to laser generator 14 by control lead 2£- Flow rate of powder is adjusted by controller 22 by means of control leads 22 and 22- Sensors, depicted by reference number 42, provide information on deposition head and article configuration to controller 22 by means of control lead 42. These sensors will be further described below.

FIGURE 2 depicts a deposition head of thiε invention. The head iε comprised of optics package 2Q_1 and delivery nozzle 241- The optics package is commercially available apparatus for receiving and focusing laser light and is mated to the delivery nozzle. Laser beam 203 pasεeε through protective window 202 and beam passage 242 and is focused at the point denoted by reference number 220. Protective window 202 is supplied as a part of optics package 2111. Optics package 221 is fastened to flange 224 by means of bolts and nuts (such as shown by reference numbers 222 and 226) disposed around the circumferences of flange 224 and flange 222, which is a part of the optics package. Element 222 connects the deposition head to the motion apparatus. The laser light passes through element 222 and into and through the optics package. Outer element 212 of the delivery nozzle is attached to bulkhead ring 212 by means of upper clamp ring 214. which is threaded to the bulkhead ring. Extension 217 is attached to outer element 212 by means of lower clamp ring 216 , which is threaded to the lower portion of outer element 215. Inner nozzle tip 221 and outer nozzle tip 222 are attached to extension 217 by means of screw threads. Inner element 204 iε attached to outer element 212 by meanε of screw threads. Coolant channel 234 is defined by an outer surface of the inner element and an inner surface of the outer element and extends circumferentially around the nozzle. Water or another coolant is circulated through channel 234 in order to cool the deposition head. Coolant inlet and outlet passages are shown in Figure 5. Lower gas channel 227 is in the shape of a ring or portion of a ring and is provided with gas as shown in Figure 7. Also shown in Figure 7 are passageε for flow of gaε into the beam passage. Figure 6 showε passages for flow of gas into the beam pasεage. Gaε is flowed into the deposition head to purge the beam pasεage of powder and other material, thereby protecting the depoεition head. Lower window 209 iε disposed across beam passage 242. The window is of quartz and serves to protect the optics package from heat, particulate matter, molten metal, etc. Lower window 209 rests in lower window housing 210 and is held in place by lower window retainer 222-

Lower window housing 210 is retained in extension 212 by means of screw threads (not shown) located at the surface indicated by reference no. 243 in Figure 4. Lower window retainer 208 has threads on its external circumferential surface (not shown) which engage threads in lower window housing 210 at the surface indicated by reference no. 244 of Figure 4. Upper window 207 may be provided for the purpose of protecting the optics package in the event that the lower window becomes damaged. The upper window restε in inner element 204 and iε held in place by upper window retainer 205. which has threads on its outer circumference which engage threads on the inner element. The terms upper window and lower window are used because these terms describe their positions when the deposition head is oriented as shown in Figure 2, even though the head may be tilted during operation.

Powder is supplied to the deposition head by meanε of conduits 211, which are connected to bulkhead ring 213 by means of tubing fittings 212. The powder iε transported by means of a flowing gas stream. The powder and itε carrier gaε flow through powder passages 222 and exit the delivery nozzle at its lower end, as depicted by arrows 219. Each powder passage begins in bulkhead ring 212 and passeε through outer element 215. extenεion 212, and the tip of the delivery nozzle, which iε formed by inner nozzle tip 221 and outer nozzle tip 222. Reference no. 231 denoteε a flow channel in bulkhead ring 212- The channel εo denoted is a part of powder passage 230. There are other flow channels in the bulkhead ring for powder, coolant, and gas, as described below. Reference no. 239 denoteε an exemplary O- ring channel for retaining an O-ring (not εhown) for purposeε of εealing. Other O-ring channels are shown, but not numbered. Those skilled in the art are capable of providing such sealing means and other features, such aε gaεkets at the outer surfaceε of the window . Reference no. 206 denoteε a portion of the opticε package which is called out only because it appears in Figures 4-7.

FIGURES 4 through 7 depict that portion of the delivery nozzle below the section line for Figure 3 which is shown in Figure 2. The manner in which each section is taken is shown in Figure 3, which is a plan view in section. Reference numbers are consiεtent with those of Figures 2 and 3 to permit correlations of elements from drawing to drawing. FIGURE 3 depicts inlet ports 222 through 222 in bulkhead ring 212- Clamp ring 214. is εhown. Eight powder inlet ports denoted by reference no. 250 are shown. A powder conduit will be connected at each powder inlet port. A conduit carrying coolant will be connected to coolant inlet port 251. Coolant will leave the deposition head by means of a conduit connected to coolant outlet port 222. Gaε iε provided to the depoεition head by meanε of conduitε which will be connected to gas inlet ports 253 and 255. Gaε iε removed by meanε of gas outlet port 254. Flow channels pass through bulkhead ring 212, starting at each of the ports, as represented by reference no. 221.

FIGURE 4 shows powder passages 230. each of which extends from a powder inlet port 250 toward a deposition zone which surrounds laser beam focal point 222, as shown in Figure 2. Inner nozzle tip 221 and outer nozzle tip 222 each have a hollow fruεtoconical configuration and are diεpoεed such that a frustoconical annular space is formed between the outer surface of inner tip 221 and the inner surface of outer tip 222. The central axes of the laser beam passage, the inner tip, and the outer tip are 17

coincident. The powder passages extend through the annular space, but are not defined within the annular space, that is, powder from the eight separate powder pasεages through extension 217 may mix in the annular space. The powder passages are disposed around the circumference of the nozzle and are provided in pairs, with one of each pair about 180 degrees from the other. It is not necesεary that the paεsages be equally spaced, but it is highly desirable that they be provided in pairs to avoid biasing the growth of an article. At least two powder streams should be provided. Reference no. 240 depicts certain locations having screw threads. In some applicationε, it may be desirable to provide ribs on either the inner nozzle tip or outer nozzle tip to separate the powder passages from one another. This embodiment will permit variations in powder type and flow. For example, if 8 powder streams are used, 3 streams could provide one metal to the depiction zone while 5 streams provide another metal, thereby forming an alloy. Powder carrier gas pressure might be varied, with one or more streams having a higher pressure when the head is tilted in order to compensate for the gravitational effects of the differing orientation of the powder paths. A one-piece nozzle tip may be used instead of the two piece nozzle tip which is shown in Figure 4; it could be attached by means of the threads used to attach inner nozzle tip 221. A one- piece nozzle tip might contain 2, 4, or more tubular powder passages. The powder pattern may be varied by providing a nozzle tip having powder passages which are not uniformly oriented in a conical manner, as depicted in Figure 4. Powder passage angles may be varied. FIGURE 5 depicts coolant inlet paεεage 222, which provides coolant from a conduit attached to bulkhead ring 213 at coolant inlet port 221 to coolant channel 224. Coolant outlet pasεage 222 carrieε coolant away from the coolant channel to coolant outlet port 252.

FIGURE 6 depictε upper gaε channelε. Gas flows into bulkhead ring 212 at gas inlet port 222 and passes through one of the flow channels 221, which iε aligned with upper gaε inlet channel 222 in outer element 215. Gaε flows from upper gas inlet channel 222 through upper gas flow channel

222, which is a part of inner element 204. into beam passage 242. Gas flows out of the beam passage through upper gaε flow channel 237 and upper gas outlet channel 238, exiting the nozzle at upper gas outlet port 224.. The flow of gas through the space between protective window 202 and upper window 222, or the space between the protective window and lower window 209 if the upper window is not used, serves to protect the optics package.

FIGURE 7 depicts lower gaε passage 229, which extends from lower gaε inlet port 255 through bulkhead ring 213 and outer element 212 into extension 217. Gaε flowε from gaε passage 229 into lower gas channel 227. which distributeε the gaε to flow paεsages in lower window housing 210. Lower gas channel 222 extends entirely around or subεtantially around the nozzle in a circumferential manner in order to communicate with the gas flow passages. Reference no. 222 denotes one such passage. There are 5 additional passages (not shown) spaced around lower window housing 212, all of which connect with lower gas channel 227. The gas flow pasεageε in the lower window housing are oriented such that gas flowing out of them impinges on the surface of the lower window which is proximate to the lower end of the nozzle. The gas exits the nozzle through the lower end of the beam passage. The purpose of the gas flow is to remove powder and other material from the lower window and aid in protection of the deposition head from heat, molten metal splatters, etc.

The delivery nozzle may be fabricated of any material suitable to withstand the heat produced by the laser beam. Stainless steel is one choice. The inner nozzle tip and outer nozzle tip, which are those parts of the nozzle cloεest to the molten metal of the depoεition zone, may be fabricated of a metal or a machinable ceramic which has a high service temperature rating. One εuch ceramic iε Macor (tradename) , which iε available from Corning Glass and is comprised of silicon dioxide, alumina, magnesium oxide, potassium oxide and several lesser compoundε.

The powder, gas, and water pasεages of a prototype delivery nozzle have their central axes at an angle of about 40 degrees to the central axis of the beam pasεage, so that the total angle between powder streams leaving the annular space at the tip of the nozzle is about 80 degrees. Values of from about 20 to about 75 degrees may be used. The outer radius of the bulkhead ring of the prototype nozzle is about 2.45 in. (62.2 mm) and its thicknesε is about 1.2 in. (30.5 mm) . The vertical height of the outer element is about 1.2 in. (30.9 mm) and that of extenεion 12 is about 0.7 in. (17.6 mm). The outer radius of the upper end of the inner element is about 1.58 in. (40.1 mm) and that of the lower end is 0.81 in. (20.5 mm) . The inside radius of the inner nozzle tip at itε upper end iε about 0.42 in. (10.7 mm) and that of its lower end is 0.18 in. (4.6 mm). The outer surface of the inner nozzle tip is at an angle of about 50 degrees to the central axis of the nozzle, in comparison to the 40 degrees used in the upper portion of the nozzle. The inner surface of the outer nozzle tip iε at an angle of 40 degrees to the axial centerline of the nozzle. The inner radius of the upper end of the outer nozzle tip is about 0.56 in. (14.3 mm) and that of the lower end is about 0.24 in. (6.1 mm) .

FIGURES 8 and 9 each depict an article which restε on εupport 4. and is in the process of formation by use of laser beam 22. Particulate matter, or powder, iε represented aε shown by reference number 22- FIGURE 8 εhows rod 22, which is made by moving the deposition zone only in a vertical direction. Rod 23. conεiεtε of solid zone 22, mushy zone 91, and liquid zone 22- The sizes of these zones, the laser beam, and the powder are for drawing convenience and are not intended to show size relationshipε. Mushy zone 21 iε the tranεition space, where solid dendrites are growing, or εolidifying, between the already εolidified portion of the rod and the material which is liquid form. In FIGURE 9, the first pass of the delivery nozzle produced solid portion 22 of article 22. and the second pasε iε in progress. The second pasε haε cauεed to be formed the portion of article 99 denoted by reference number 24._/ which portion has solidified. The direction of movement of the laser beam is to the right, as shown by arrow 100. Mushy zone 22 separates solid zone 24 from liquid zone 96. FIGURE 10 depicts the end portion of a laser beam and powder stream when the deposition head is vertically oriented, that is, when its axial centerline is vertical. The exterior envelope of the generally conical beam is εhown by reference number 22. Laser spot 100 is the pattern formed by laser light on a plane which is normal to the central axis of the beam and located at the article being formed. When the deposition head is operating in a position other than vertical, powder particle velocity should be increased, in order to overcome gravitational effects which may cause bending of the powder stream. The powder delivery spot 102 may assume an oval configuration and the center of the spot may not be coincident with the central axis of the delivery nozzle when the delivery nozzle of FIGURE 2 is tilted. While it is convenient to visualize the laser beam as focused at a single point, available optical apparatus iε not capable of accomplishing thiε, εo the diameter of the laser spot has a minimum value fixed by the optics. The focal point is also termed the theoretical diffraction- limited spot. The εize of the focal zone will normally be about 0.060 in. (1.5 mm) above and below the focal point. The depoεition head may be poεitioned εuch that the laεer εpot may be located above, below or at the focal point. Maximum energy density of the beam is at the focal point and energy density is less at points on either side of the focal spot. Reference number 102 εhows a powder delivery spot and dashed lines 122 indicate a conical delivery envelope for streams of powder converging in the depoεition zone. However, it is not necessary to have εuch a uniform curtain of powder, but only that at least two streams of powder enter the deposition zone. These streams must converge within the deposition zone, that is, they must meet in the depoεition zone. It iε not necessary that all powder leaving the delivery nozzle enter the deposition zone. If only two streams of powder are used, they should enter the deposition zone at about 180° from one another to avoid biasing "growth" of the article being deposited toward the powder exit port of the delivery nozzle. It is preferable to use four or more powder paths entering the deposition zone at equally spaced points. In the case of four εtreamε, it iε preferable that they be spaced at about 90° apart. If the powder spot is larger than the laser spot, causing powder to accumulate around the article, thiε powder may be recovered and re-used. It is desirable to achieve a powder delivery spot which iε equal to or smaller than the laser spot. This will minimize the quantity of powder for recycle and contribute to dimensional precision of the process.

FIGURES 11 and 12 are top views of a portion of article 114. which has a rectangular cross-section. Portions of tool paths by which the article may be formed are shown by reference numbers 115 and 116. A layer of the article may be formed by a roughing pass following tool path 115 or 112 and then by a finishing pasε which forms the outer portion of the layer. Alternatively, a finishing pasε may create a εhell and then the shell may be filled in with material using a roughing pass. In a roughing pasε, operating parameters are established which maximize rate of material deposition, while parameters for a finishing pass are optimized for dimensional precision and surface finish. The DLF molten pool is generally about 10% of the size of a molten pool obtained when doing TIG welding. Dimensional accuracy is improved by decreasing the size of the molten pool. Size of the molten pool is dependent upon such factors as laser beam power, abεorptivity of the material being heated, and heat flow away from the pool.

A new laεer depoεition εyεtem which will utilize thiε invention is presently being constructed. Laser welding apparatus which is designated Lumonics MW2000 by the manufacturer is being incorporated into the new DLF syεtem. This apparatuε includes a 5 axis positioning system (designated Model 550) supplied by the Laserdyne Division of Lumonics Corp. of Livonia, MI. A Laserdyne System 94 Controller will be used. The opticε package component of the depoεition head, which iε capable of providing a laεer spot having a diameter of 0.5 mm, is provided by Laserdyne. It is expected that the maximum deposition rate for the new system will be in excess of 2 g/sec. The new apparatuε includes a Z-axis positioner to move the deposition head in a vertical direction and a table on which an article support is placed so that the support can be moved along the X-axis, along the Y-axis, and rotated about a vertical axis. The table will be cooled to prevent damage due to heat transferred to it during fabrication of an article. Cooling water will be circulated εuch that it contacts the lower surface of the top plate upon which rests a support for fabrication of an article. A circular trough will surround the table for collection of excesε powder. During and after fabrication of an article, flow of powder may be diεcontinued while the powder carrier gas is left on and the deposition head moved so that excess powder is blown into the trough by the carrier gaε. The table will be deεigned εo that an article support or fixture or partially finished article can be placed on it or clamped to it. A fixture may be used to hold a support or partially finished article in a particular position chosen by a designer as optimum for fabrication of the article. An article may be manually repositioned at a particular point in its fabrication sequence. The deposition head of the new syεtem will be mounted on apparatuε which has the capability of rotating it 135 degrees from the vertical in each direction in a single plane, in the manner described in connection with FIGURE 2. Thus, the new εystem will provide the capability of depositing material in a sidewayε direction and an upward direction, that is, on the bottom side of a feature of an article. This is illustrated by reference to FIGURE 13, which is a sectional view of flange 112- First, the large vertical portion of the flange, denoted by reference number ill is deposited with the delivery nozzle in a vertical or near vertical position εo that the laser beam and powder stream are directed in a downward direction. Then, the delivery nozzle is rotated to produce the horizontal portion of the flange denoted by reference number 112. Lastly, the nozzle is rotated further, so that the laser beam and powder stream are directed upward at an angle of up to 45 degrees to the horizontal, in order to produce the overhanging vertical portion of the flange denoted by reference number 113. The normal deposition head orientation is such that the axial centerline of the head will pasε through the center of the deposition zone. However, the new system has a higher power laser than the three-axis apparatus and may be operated with the laser head slightly tilted from this position, perhaps at a tilt angle of about 20°, to avoid reflection of light from the deposition zone into the laser delivery system. Such reflections may cause damage to optical components. It may be necessary to tilt the head when fabricating an article of aluminum, which is quite reflective, whereas a normal head orientation may be used for a stainless steel article, since SS is lesε reflective than Al.

The new system will utilize a 2 kW maximum continuous wave Nd-YAG laser having one lasing cavity with four lasing rods. Peak power in pulsed mode will be 5.4 kW. The generator will be capable of operation in a pulsed mode or with any degree of modulation. The laser light will be delivered to the depoεition head by use of a fiber optic cable and mirrors.

Movement of the deposition zone and controlling functions such as powder feed rate could be done manually, but it is very difficult to achieve specific article dimensions when doing so. Attempts to manually grow articles were considered to be unεucceεsful. Practically, it is necessary to utilize CNC machining motion apparatus to accomplish precise movement of the deposition zone. Such apparatus is normally sold with a controller which provides control signals to the motion apparatus which accomplishes relative motion between a workpiece and a cutting tool. A controller also provides other machine control commands. For example, to carry out a conventional machining operation of grooving an article, a CNC milling machine controller may be programmed so that it causeε movement of the εpindle of the machine, and thus the cutting tool contained in the εpindle, downward from a starting location (this is termed a plunge) to a point near the edge of a flat plate, causes rotation of the spindle to start, and then causeε movement of the tool along the X axiε of the machine for a specified distance, thereby cutting a straight groove in the plate. After cutting the groove, the spindle may be moved upward (this is termed a retract) to its starting position or to another position in preparation for another plunge. Just before starting the cutting movement, the controller may provide a signal which causeε a cutting fluid, or coolant, to start flowing to the location at which cutting is taking place. The milling machine controller may also have spare, or user-defined, control commands available so that the user may program the controller to send a voltage to a set of terminals at a particular point in the machining εequence. For example, if the machine doeε not have the capability for automatic changing of cutting toolε, a εpare command may be uεed to εound a buzzer to alert an operator to change the tool. The controller of a CNC milling machine iε normally provided to a cuεtomer with εoftware which a technician may uεe to program the controller. For example, in creating a machine operating program, the technician may add the lines of code "P102" and "M108". Thiε, in accordance with the protocol established for the machine by its manufacturer, "tells" the machine to execute a plunge and then to turn on coolant flow at the point in the operating sequence at which the lines of code are located. The movements of the spindle are called "tool paths" and commands such as "coolant on" are referred to as being embedded in the tool paths. A εimple machining sequence, such as the grooving operation described above, is easily created by a technician by use of the programming software. However, controllers of sophisticated CNC milling machines are usually provided by the user with machine operating code which is created by use of commercially available software, thus avoiding the laborious task of programming a sequence of motions and commands for the controller line-by-line.

The deposition head and article support of this invention are mounted on commercially available motion apparatuε. Movements of the depoεition head, article support, and control commandε can be programmed into a controller in the same manner as if a cutting tool were mounted on the motion apparatus. In practice of this invention, the singular term "tool path" iε uεed to refer to the total sum of the movement of the deposition zone required to produce an article. Also, terminology specific to CNC machining is used for purposes of convenience and clarity. For example, the laser deposition apparatus may be referred to as a machine. A preferred embodiment of this invention utilizes commercial εoftware, though significant modifications to the software are made, to create electronic files which reεide in a digital computer. It iε more efficient to adapt CNC εoftware for use in DLF than to create thouεands of lines of machine code from scratch. The adaptation process generally will be carried out just once. Of course, a DLF sequence may be programmed line-by-line, using the machine commands provided by the machine supplier.

Claims

WHAT IS CLAIMED IS:

1. A deposition head comprised of a delivery nozzle and an optics package for focusing a laser beam at a location in a deposition zone, where said laser beam passes out of said optics package through a protective window, and where said delivery nozzle is comprised of: a. a first end and a beam passage extending from said first end to a second end of said delivery nozzle, where said first end is attached to the optics package such that the laser beam passeε through said beam paεεage and is focuεed at a location in εaid deposition zone, where the deposition zone is located adjacent to said second end of the nozzle; b. at least two powder inlet portε located at the firεt end of the nozzle; c. at least two powder passageε, each having a firεt end which communicates with one of said powder inlet portε, where each powder passage extends through the nozzle toward the deposition zone, where powder pasεages are arranged in pairs and a first passage of a pair is oriented about 180 degrees from a second pasεage of the pair, where the axial centerline of each powder paεεage formε an angle of from about 20 to about 75 degreeε with the axial centerline of the nozzle, and where εaid axial centerlines converge in the deposition zone; and d. a lower window through which the laser beam passes which is disposed across the beam pasεage.

2. The apparatus of Claim 1 further including means for cooling the delivery nozzle.

3. The apparatus of Claim 1 further including means for cooling the delivery nozzle comprised of: a. a coolant inlet port and a coolant outlet port located at the first end of the nozzle; b. a coolant channel located inside the nozzle and extending circumferentially around the nozzle; and c. a coolant inlet pasεage and a coolant outlet passage which connect said coolant inlet port and said coolant outlet port with εaid coolant channel.

4. The apparatus of Claim 1 further including means for flowing gas through the delivery nozzle.

5. The apparatus of Claim 1 further including means for flowing gas through the delivery nozzle comprised of: a. a lower gas inlet port; b. a lower gas channel located within the nozzle and extending circumferentially around the nozzle; c. a lower gaε paεsage connecting said lower gas port and said lower gas channel; and d. a plurality of gas pathways in the nozzle, each of which connects the lower gas channel with the beam passage, where said gas pathways are dispoεed about the beam pasεage adjacent to that εurface of the lower window which is proximate to the second end of the nozzle.

6. The apparatus of Claim 1 further including means for flowing gas through the delivery nozzle comprised of : a. an upper gas inlet port; b. an upper gas inlet passage connecting said inlet port to the beam pasεage at a point between the protective window and the lower window; and c. an upper gaε outlet port and an upper gas outlet pasεage connecting said outlet port to the beam passage at a point between the protective window and the lower window.

7. The apparatus of Claim 1 further including an upper window through which the laser beam pasεeε which iε disposed across the beam passage between the lower window and said protective window.

8. The apparatus of Claim 1 where a detachable tip is located at said second end of the delivery nozzle.

9. The apparatus of Claim 1 where a detachable tip is located at said second end of the delivery nozzle and εaid tip is comprised of a first frusto-conical shape and a second frusto-conical shape disposed on a common central axis to form an annular space between the exterior surface of said first shape and the exterior surface of said second shape, where said annular space comprises said powder passages.

10. The apparatuε of Claim 1 where the central axiε of each powder passage and the central axis of the laser beam form an angle of from about 20 to about 75 degrees.

11. A deposition head comprised of a delivery nozzle and an optics package for focuεing a laser beam at a location in a deposition zone, where said laser beam pasεes out of said optics package through a protective window, and where said delivery nozzle is comprised of: a. a first end and a beam passage extending from said first end to a second end of said delivery nozzle, where said first end is attached to the optics package such that the laser beam passes through said beam pasεage and is focused at a location in said deposition zone, where the deposition zone is located adjacent to said second end of the nozzle; b. at least two powder inlet ports located at the first end of the nozzle; c. at leaεt two powder passages, each having a first end which communicates with one of said powder inlet ports, where each powder passage extends through the nozzle toward the deposition zone, where powder pasεageε are arranged in pairs and a firεt paεsage of a pair is oriented about 180 degrees from a second passage of the pair, where the axial centerline of each powder passage forms an angle of from about 20 to about 75 degrees with the axial centerline of the nozzle, and where said axial centerlines converge in the depoεition zone; and d. A lower window through which the laεer beam passes which is disposed across the beam pasεage; e. a coolant inlet paεsage and a coolant outlet pasεage which connect said coolant inlet port and said coolant outlet port with said coolant channel; f. a lower window through which the laser beam paεses which iε disposed across the beam passage; g. a lower gas inlet port, a lower gas channel located within the nozzle and extending circumferentially around the nozzle, and a lower gas pasεage connecting said lower gaε port and εaid lower gas channel; h. a plurality of gas pathways in the nozzle, each of which connects the lower gaε channel with the beam paεεage, where εaid gaε pathwayε are diεposed about the beam pasεage adjacent to that surface of the lower window which is proximate to the second end of the nozzle; i. an upper gaε inlet port and an upper gas inlet pasεage connecting εaid inlet port to the beam paεεage at a point between the protective window and the lower window; and j . an upper gaε outlet port and an upper gaε outlet passage connecting said outlet port to the beam pasεage at a point between the protective window and the lower window; and k. an upper gas outlet port and an upper gas outlet passage connecting said outlet port to the beam passage at a point between the protective window and the lower window.