Patent Publication Number: US-10773340-B2

Title: Metal additive manufacturing using gas mixture including oxygen

Description:
This application is a continuation-in-part application of U.S. application Ser. No. 14/981,321, filed Dec. 28, 2015, currently pending. 
    
    
     BACKGROUND OF THE INVENTION 
     The disclosure relates generally to additive manufacturing, and more particularly, to a metal additive manufacturing system that employs a gas mixture including oxygen. The disclosure also relates to metal additive manufacturing methods and an object formed using the methods. 
     Additive manufacturing (AM) includes a wide variety of processes of producing an object through the successive layering of material rather than the removal of material. As such, additive manufacturing can create complex geometries without the use of any sort of tools, molds or fixtures, and with little or no waste material. Instead of machining components from solid billets of material, much of which is cut away and discarded, the only material used in additive manufacturing is what is required to shape the object. 
     Additive manufacturing techniques typically include taking a three-dimensional computer aided design (CAD) file of the object to be formed, electronically slicing the object into layers, e.g., 18-102 micrometers thick, and creating a file with a two-dimensional image of each layer. The file may then be loaded into a preparation software system that interprets the file such that the object can be built by different types of additive manufacturing systems. In 3D printing, rapid prototyping (RP), and direct digital manufacturing (DDM) forms of additive manufacturing, material layers are selectively dispensed to create the object. 
     In metal powder additive manufacturing techniques, such as selective laser melting (SLM) and direct metal laser melting (DMLM), metal powder layers are sequentially melted together to form the object. More specifically, fine metal powder layers are sequentially melted after being uniformly distributed using an applicator on a metal powder bed. The metal powder bed can be moved in a vertical axis. The process takes place in a processing chamber having a precisely controlled atmosphere of inert gas, e.g., argon or nitrogen, at strictly enforced oxygen levels below 500 parts per million or less than 0.1% of the volume in the processing chamber. Once each layer is created, each two dimensional slice of the object geometry can be fused by selectively melting the metal powder. The melting may be performed by a high powered laser, such as a 100 Watt ytterbium laser. The laser moves in the X-Y direction using scanning mirrors, and has an intensity sufficient to fully weld (melt) the metal powder to form a solid metal. The metal powder bed is lowered for each subsequent two dimensional layer, and the process repeats until the object is completely formed. Obtaining the appropriate gas mixture environment in metal powder additive manufacturing has proven a challenge. Consequently, achieving desired surface finishes, part efficiency and part density can be difficult using metal powder additive manufacturing. 
     BRIEF DESCRIPTION OF THE INVENTION 
     A first aspect of the disclosure provides a metal powder additive manufacturing system, the system comprising: a processing chamber; a metal powder bed within the processing chamber; a melting element configured to sequentially melt layers of metal powder on the metal powder bed to generate an object; and a control system configured to control a flow of a gas mixture within the processing chamber from a source of inert gas and a source of an oxygen containing material, the gas mixture including the inert gas and oxygen from the oxygen containing material. 
     A second aspect of the disclosure provides a metal powder additive manufacturing method, the method comprising: providing a metal powder bed within a processing chamber; controlling a flow of a gas mixture within the processing chamber from a source of inert gas and a source of an oxygen containing material, the gas mixture including the inert gas and oxygen from the oxygen containing material; and sequentially melting layers of metal powder on the metal powder bed to generate an object. 
     A third aspect of the disclosure relates to an object formed by a metal powder additive manufacturing method, the method comprising: providing a metal powder bed within a processing chamber; controlling a flow of a gas mixture within the processing chamber from a source of inert gas and a source of an oxygen containing material, the gas mixture including the inert gas and oxygen from the oxygen containing material; and sequentially melting layers of metal powder on the metal powder bed to generate the object, wherein the object has a surface porosity of no greater than approximately 0.1%. 
     The illustrative aspects of the present disclosure are designed to solve the problems herein described and/or other problems not discussed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of this disclosure will be more readily understood from the following detailed description of the various aspects of the disclosure taken in conjunction with the accompanying drawings that depict various embodiments of the disclosure, in which: 
         FIG. 1  shows a block diagram of a metal powder additive manufacturing system according to embodiments of the disclosure. 
     
    
    
     It is noted that the drawings of the disclosure are not to scale. The drawings are intended to depict only typical aspects of the disclosure, and therefore should not be considered as limiting the scope of the disclosure. In the drawings, like numbering represents like elements between the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     As indicated above, the disclosure provides a metal powder additive manufacturing system and method that uses a gas mixture including oxygen at higher levels than conventionally employed to better control, for example, object surface porosity and effective density. It has also been discovered that the teachings of the disclosure provide dramatic increases in effective filter life on metal powder additive manufacturing systems. 
     Referring to the drawings,  FIG. 1  shows a schematic/block view of an illustrative metal powder additive manufacturing system  100  for generating an object  102 , of which only an upper surface is shown. In this example, system  100  is arranged for direct metal laser melting (DMLM). It is understood that the general teachings of the disclosure are equally applicable to other forms of metal powder additive manufacturing such as selective laser melting (SLM). Object  102  is illustrated as a double walled turbine element; however, it is understood that the additive manufacturing process can be readily adapted to manufacture many other parts. 
     System  100  generally includes a metal powder additive manufacturing control system  104  (“control system”) and an AM printer  106 . As will be described, control system  104  executes code  108  to generate object  102 . Control system  104  is shown implemented on computer  110  as computer program code. To this extent, computer  110  is shown including a memory  112 , a processor  114 , an input/output (I/O) interface  116 , and a bus  118 . Further, computer  110  is shown in communication with an external I/O device/resource  120  and a storage system  122 . In general, processor  114  executes computer program code  108  that is stored in memory  112  and/or storage system  112 . While executing computer program code  108 , processor  114  can read and/or write data to/from memory  112 , storage system  122 , I/O device  120  and/or AM printer  106 . Bus  118  provides a communication link between each of the components in computer  110 , and I/O device  120  can comprise any device that enables a user to interact with computer  110  (e.g., keyboard, pointing device, display, etc.). Computer  110  is only representative of various possible combinations of hardware and software. For example, processor  114  may comprise a single processing unit, or be distributed across one or more processing units in one or more locations, e.g., on a client and server. Similarly, memory  112  and/or storage system  122  may reside at one or more physical locations. Memory  112  and/or storage system  122  can comprise any combination of various types of non-transitory computer readable storage medium including magnetic media, optical media, random access memory (RAM), read only memory (ROM), etc. Computer  110  can comprise any type of computing device such as an industrial controller, a network server, a desktop computer, a laptop, a handheld device, etc. 
     As noted, system  100  and in particular control system  104  executes code  108  to generate object  102 . Code  108  can include, inter alia, a set of computer-executable instructions  108 S for operating AM printer  106  and a set of computer-executable instructions  108 O defining object  102  to be physically generated by AM printer  106 . As described herein, additive manufacturing processes begin with a non-transitory computer readable storage medium (e.g., memory  112 , storage system  122 , etc.) storing code  108 . Set of computer-executable instructions  108 S for operating AM printer  106  may include any now known or later developed software code capable of operating AM printer  106 . Set of computer-executable instructions  108 O defining object  102  may include a precisely defined 3D model of an object and can be generated from any of a large variety of well-known computer aided design (CAD) software systems such as AutoCAD®, TurboCAD®, DesignCAD 3D Max, etc. In this regard, code  108 O can include any now known or later developed file format. Furthermore, code  108 O representative of object  102  may be translated between different formats. For example, code  108 O may include Standard Tessellation Language (STL) files which was created for stereolithography CAD programs of 3D Systems, or an additive manufacturing file (AMF), which is an American Society of Mechanical Engineers (ASME) standard that is an extensible markup-language (XML) based format designed to allow any CAD software to describe the shape and composition of any three-dimensional object to be fabricated on any AM printer. Code  108 O representative of object  102  may also be converted into a set of data signals and transmitted, received as a set of data signals and converted to code, stored, etc., as necessary. In any event, code  108 O may be an input to system  100  and may come from a part designer, an intellectual property (IP) provider, a design company, the operator or owner of system  100 , or from other sources. In any event, control system  104  executes code  108 S and  108 O, dividing object  102  into a series of thin slices that it assembles using AM printer  106  in successive layers of material. 
     AM printer  106  may include a processing chamber  130  that is sealed to provide a controlled atmosphere for object  102  printing. A metal powder bed or platform  132 , upon which object  102  is built, is positioned within processing chamber  130 . A melting element  134  is configured to sequentially melt layers of metal powder on metal powder bed  132  to generate object  102 . In this regard, melting element  134  may generate one or more laser or electron beams  138  that fuse particles for each slice, as defined by code  108 . An applicator  140  may create a thin layer of raw material  142  spread out as the blank canvas from which each successive slice of the final object will be created. Various parts of AM printer  106  may move to accommodate the addition of each new layer, e.g., a metal powder bed  132  may lower and/or chamber  130  and/or applicator  140  may rise after each layer. The process may use different raw materials in the form of fine-grain metal powder, a stock of which may be held in a chamber  144  accessible by applicator  140 . In the instant case, object  102  may be made of a “metal” which may include a pure metal or an alloy. In one example, the metal may include practically any non-reactive metal powder, i.e., non-explosive or non-conductive powder, such as but not limited to: a cobalt chromium molybdenum (CoCrMo) alloy, stainless steel, an austenite nickel-chromium based alloy such as a nickel-chromium-molybdenum-niobium alloy (NiCrMoNb) (e.g., Inconel 625 or Inconel 718), a nickel-chromium-iron-molybdenum alloy (NiCrFeMo) (e.g., Hastelloy® X available from Haynes International, Inc.), or a nickel-chromium-cobalt-molybdenum alloy (NiCrCoMo) (e.g., Haynes 282 available from Haynes International, Inc.), etc. 
     With further regard to processing chamber  130  and the atmosphere therein, as noted herein, in conventional systems, the chamber is filled with an inert gas such as argon or nitrogen and controlled to minimize or eliminate oxygen. In accordance with embodiments of the disclosure, increased trace amounts of oxygen have been found to advantageously change the melt pool characteristics of object  102  resulting in a higher density part and substantial improvements in the overall surface finish of the as-built part. In contrast to conventional systems, embodiments of the disclosure have control system  104  configured to control a flow of a gas mixture  160  within processing chamber  130  from a source of inert gas  154  and a source of an oxygen containing material  162 . In this case, control system  104  may control a pump  150 , and/or a flow valve system  152  for inert gas and a flow valve system  164  for oxygen containing material, to control the content of gas mixture  160 . That is, control system  104  controls at least one of: flow valve system  152  coupled to source of inert gas  154 , flow valve system  164  coupled to source of oxygen containing material  162 , and pump  150  coupled to source of inert gas  154  and source of oxygen containing material  162 . Each flow valve system  152 ,  164  may include one or more computer controllable valves, flow sensors, temperature sensors, pressure sensors, etc., capable of precisely controlling flow of the particular gas or material. Pump  150  may be provided with our without valve systems  152 ,  164 . Where pump  150  is omitted, inert gas and oxygen containing material may simply mix together in a conduit or manifold prior to introduction to processing chamber  130 . Source of inert gas  154  or source of oxygen containing material  162  may take the form of any conventional source for the material contained therein, e.g. a tank, reservoir or other source. Any sensors (not shown) required to measure the particular materials may be provided. In any event, gas mixture  160  includes inert gas and oxygen from the oxygen containing material. In one embodiment, a volume percentage of oxygen in gas mixture  160  may be between approximately 0.25% to approximately 1%, i.e., significantly higher than in conventional systems. As used herein, “approximately” indicates +/−10% of the value or, if a range, values stated. Gas mixture  160  may be filtered using a filter  170  in a conventional manner. 
     The inert gas can include any of the aforementioned gases, e.g., argon or nitrogen, perhaps with some helium. The oxygen contain material can take a variety of forms. In one embodiment, the oxygen containing material may include an oxygen containing gas, such as air or pure oxygen (the latter having &gt;99% O 2 ), each of which can be provided in a compressed form in a conventional fashion. Where the oxygen containing material includes inert gases, such as the case with air carrying inert gases such as nitrogen, the amounts of inert gas (and/or oxygen containing material) may be altered to accommodate the increased volumes of inert gas. In another embodiment, the oxygen containing material may include water, which may be applied, for example, by introducing water to paper gas filters in a filtering system, allowing the gas mixture to absorb moisture as it passes through the paper filters. In this latter example, provisions (not shown) may be necessary to remove condensate within processing chamber  130 . According to embodiments of the disclosure, introduction of water provides additional oxygen to gas mixture  160 , as described herein, and also improves filter  170  operation. Accordingly, as an optional embodiment, filter  170  may be moistened with water, e.g., by spraying water or dipping filter  170  into water prior to operation, to improve its filtering effectiveness and to introduce some oxygen to gas mixture  160 . It is also envisioned that a variety of other oxygen containing materials could be employed within the scope of the disclosure, perhaps with other structure to separate the oxygen out of any carrier such as, but not limited to: carbon dioxide (CO 2 ), iron oxide (FeO), silicon dioxide (SiO 2 ), chromium oxide (Cr 2 O 3 ) and manganese monoxide (MnO). 
     In operation, metal powder bed  132  is provided within processing chamber  130 , and control system  104  controls flow of gas mixture  160  within processing chamber  130  from source of inert gas  154  and source of an oxygen containing material  162  such that gas mixture includes the inert gas and oxygen from the oxygen containing material. Control system  104  also controls AM printer  106 , and in particular, applicator  140  and melting element  134 , to sequentially melt layers of metal powder on metal powder bed  132  to generate object  102 . Object  102  created according to embodiments of the disclosure exhibit a surface porosity of no greater than approximately 0.1%, which is an improvement over conventional processes that do not employ as much oxygen. “Surface porosity” as used herein indicates accessible void space over total external space of an object. Further, object  102  may have an effective density of greater than approximately 99.9%, which is also an improvement over conventional processes that do not employ as much oxygen. As used herein, “effective density” indicates an average density (e.g., in kilograms per cubic meter) of the object formed versus the density of the material of which it is made (in same units). 
     In most cases, control system  104  acts to maintain oxygen content in gas mixture  160  in the afore-mentioned range. Alternatively, control system  104  may also change a percentage of oxygen in gas mixture  160  during operation of AM printer  106 , e.g., metal powder bed  132  and melting element  134 . In this fashion, object  102  manufacture could be ensured to have consistent physical attributes, e.g., surface porosity, effective density, etc., where other system parameters change, or object  102  can have its physical properties altered in portions thereof, as dictated by code  108 O. In any event, once complete, object  102  may be exposed to any variety of finishing processes, e.g., minor machining, sealing, polishing, etc. 
     In one embodiment, system  100  can be created by modifying a number of conventional metal powder additive manufacturing systems such as a model 280HL selective laser melting system available from SLM Solutions Group AG, Lubeck Germany, or similar models available from EOS GmbH Electro Optical Systems, Munich Germany; Concept Laser GmbH, Lichtenfels Germany; and 3D Systems, Rock Hill Inc., South Carolina, USA. 
     System  100  provides a better as-built surface finish for object  102 , e.g., surface porosity, and denser as-built objects  102 . Consequently, system  100  provides a wider process window for acceptable part quality. In addition, system  100  has been found to increase the life of gas mixture filter  170 , resulting in decreased consumable operating costs. 
     As will be appreciated by one skilled in the art, parts of system  100  of the present disclosure may be embodied as a system, method or computer program product. Accordingly, control system  104  may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” Furthermore, the present disclosure may take the form of a computer program product embodied in any tangible medium of expression having computer-usable program code embodied in the medium. 
     Any combination of one or more computer usable or computer readable medium(s) may be utilized. The computer-usable or computer-readable medium may be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. More specific examples (a non-exhaustive list) of the computer-readable medium would include the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a transmission media such as those supporting the Internet or an intranet, or a magnetic storage device. Note that the computer-usable or computer-readable medium could even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured, via, for instance, optical scanning of the paper or other medium, then compiled, interpreted, or otherwise processed in a suitable manner, if necessary, and then stored in a computer memory. In the context of this document, a computer-usable or computer-readable medium may be any medium that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The computer-usable medium may include a propagated data signal with the computer-usable program code embodied therewith, either in baseband or as part of a carrier wave. The computer usable program code may be transmitted using any appropriate medium, including but not limited to wireless, wireline, optical fiber cable, RF, etc. 
     Computer program code for carrying out operations of the present disclosure may be written in any combination of one or more programming languages, including an object oriented programming language such as Java, Smalltalk, C++ or the like and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The program code may execute entirely on the user&#39;s computer, partly on the user&#39;s computer, as a stand-alone software package, partly on the user&#39;s computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user&#39;s computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). 
     These computer program instructions may also be stored in a computer-readable medium that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable medium produce an article of manufacture including instruction means which implement the function/act specified in the flowchart and/or block diagram block or blocks. 
     The computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present disclosure has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the disclosure in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the disclosure. The embodiment was chosen and described in order to best explain the principles of the disclosure and the practical application, and to enable others of ordinary skill in the art to understand the disclosure for various embodiments with various modifications as are suited to the particular use contemplated.