System and Method for Forming Nano-Particles in Additively-Manufactured Metal Alloys

Abstract

In some embodiments, a method of producing a metallic article includes providing a metallic powder, selecting a predetermined concentration for a reactive component, providing a controlled atmosphere including the reactive component at the predetermined concentration, and additively manufacturing the metallic article from the metallic powder under the controlled atmosphere. The metallic powder includes a metallic element or metallic alloy. The reactive component reacts with the metallic powder in a weld pool formed during the additive manufacturing to form a dispersion of nano-particles in the weld pool. The nano-particles are dispersed throughout the metallic article in a substantially uniform manner. In some embodiments, the metallic powder includes the reactive component. Metallic articles formed by the disclosed methods are also disclosed.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is an application under 35 USC 111(a) and claims priority under 35 USC 119 from Provisional Application Ser. No. 62/491,792, filed Apr. 28, 2017 under 35 USC 111(b). The disclosure of that provisional application is incorporated herein by reference.

FIELD OF THE INVENTION

The described invention relates in general to additive manufacturing processes, and more specifically to a system and method for forming nano-particles in certain types of metal alloys, particularly those created by beam melting processes such as selective laser melting and electron beam melting.

BACKGROUND OF THE INVENTION

When a cast pure metal or alloy is permanently deformed in any manner, it is considered a wrought metal. Wrought metals are mostly base metal alloys, such as stainless steel, cobalt-chromium-nickel, nickel-titanium, and beta-titanium. Because of plastic deformation, the microstructure of an alloy is altered and the alloy exhibits mechanical properties that are different from those it had in the as-cast state. The most significant changes are its proportional limit and ductility. Ductility refers to a solid material stretching under tensile stress. If ductile, a material may be stretched into a wire or similar structure. Malleability, a similar mechanical property, is a material's ability to deform under pressure (compressive stress). If malleable, a material may be flattened by hammering or rolling.

Precipitation hardening, also called age hardening, is a heat treatment technique used to increase the yield strength of malleable materials, including most structural alloys of aluminum, magnesium, nickel, titanium, and some steels and stainless steels. The process of precipitation hardening produces uniformly dispersed particles (i.e., precipitates) within the grain structure of a metal that hinder dislocation motion, thereby strengthening the metal and enhancing its mechanical properties. These precipitates are generally conventionally formed using a two-step process: (i) an initial solution heat treatment (i.e. solutionization) at a high temperature; and (ii) a precipitation hardening treatment at a lower temperature, which results in the formation of precipitates. The solution heat treatment results in a single-phase solution, and the precipitation hardening treatment results in the formation of precipitates of appropriate size and morphology to enhance certain mechanical properties of the material. Precipitation hardening is typically performed in a vacuum having an inert atmosphere at temperatures ranging from 900° F. to 1150° F. (480° C. to 620° C.) for steel alloys to below 400° F. (200° C.) for some aluminum alloys. The process ranges in time from one to twenty-four hours depending on the specific material and specified characteristics. Precipitation-hardened alloys may be used for a variety of applications, including those where prolonged exposure to elevated temperatures or other harsh environments may occur.

As an alternative to using wrought alloys for creating components or parts, selective laser melting (SLM) and electron beam melting (EBM) are additive manufacturing techniques that utilize a directed energy to selectively fuse together a predetermined portion of the surface of a bed of powdered metal material. SLM uses a laser beam as the directed energy, whereas EBM uses an electron beam as the directed energy. The directed energy source is automatically aimed at points in space that are defined by a series of horizontal layers within a three-dimensional (3D) model, thereby melting/welding the powdered metal into a solid structure. SLM and EBM machines are commercially available and utilize a 3D computer-aided design (CAD) model, where a data file is created and then sent to the machine's processing unit. A technician typically manipulates this 3D model to best orient the geometry for part building and adds one or more support structures as appropriate. Once this “build file” has been completed, it is “sliced” into the horizontal layers that the machine builds in and is downloaded to the machine, allowing the build to commence.

Both selective laser melting and electron beam melting operate inside an environmentally-controlled build chamber that also includes a material dispensing system, a build platform, and a re-coater blade that is used to move new powder over the build platform. The environment inside the chamber is conventionally either an inert gas or a vacuum. Metal powder is fused into a solid part by melting or welding the powder locally using the directed energy beam. Parts are built up additively layer by layer, typically using layers between about 20 to 100 micrometers (0.8 to 3.9 mil) thick. Such beam melting processes permit highly complex part geometries to be created directly from 3D CAD data, in a fully automated manner, in a relatively short period of time, and without any tooling.

Selective laser melting and electron beam melting are net-shape processes that produces parts with high accuracy and detail resolution, good surface quality, and excellent mechanical properties. These beam melting processes have many benefits over traditional manufacturing techniques, including the ability to rapidly produce a unique part without any special tooling being required. They also provide for more rigorous testing of prototypes due to the fact that they are useable with most alloys. Accordingly, functional prototypes can be created from the same material as production components. Such systems may also be used in full-scale production in addition to prototyping. Because components are built additively, i.e., layer by layer, it is possible to include internal features and passages that could not be cast or otherwise machined. Beam melting processes are used to manufacture direct parts for a variety of industries including aerospace, dental, medical, and other industries that required small to medium size, highly complex parts.

Components or parts made using the beam melting processes may also be precipitation hardened to increase strength and durability. However, components or parts made by these processes may possess certain advantageous characteristics compared to the wrought forms of the same or similar alloys. Accordingly, there is an ongoing need for understanding and enhancing the systems and methods used in such processes to enable the creation of materials having superior performance characteristics.

BRIEF DESCRIPTION OF THE INVENTION

The following provides a summary of certain exemplary embodiments of the present invention. This summary is not an extensive overview and is not intended to identify key or critical aspects or elements of the present invention or to delineate its scope.

In accordance with one aspect of the present invention, a method of producing a metallic article includes providing a metallic powder, selecting a predetermined concentration for a reactive component, providing a controlled atmosphere including the reactive component at the predetermined concentration, and additively manufacturing the metallic article from the metallic powder under the controlled atmosphere. The metallic powder includes a metallic element or metallic alloy. The reactive component reacts with the metallic powder in a weld pool formed during the additive manufacturing to form a dispersion of nano-particles in the weld pool. The nano-particles are dispersed throughout the metallic article in a substantially uniform manner.

In accordance with another aspect of the present invention, a method of producing a metallic article includes selecting a predetermined concentration for a reactive component, providing a metallic powder, providing a controlled atmosphere, and additively manufacturing the metallic article from the metallic powder under the controlled atmosphere. The metallic powder includes a metallic element or metallic alloy and the reactive component at the predetermined concentration. The reactive component reacts with the metallic powder in a weld pool formed during the additive manufacturing to form a dispersion of nano-particles in the weld pool. The nano-particles are dispersed throughout the metallic article in a substantially uniform manner.

Additional features and aspects of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the exemplary embodiments. As will be appreciated by the skilled artisan, further embodiments of the invention are possible without departing from the scope and spirit of the invention. Accordingly, the descriptions are to be regarded as illustrative and not restrictive in nature.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments of the present invention are described below. Although the following detailed description contains many specifics for purposes of illustration, a person of ordinary skill in the art will appreciate that many variations and alterations to the following details are within the scope of the invention. Accordingly, the following embodiments of the invention are set forth without any loss of generality to, and without imposing limitations upon, the claimed invention.

The present invention relates generally to additive manufacturing processes, and more specifically to systems and methods for forming nano-particles, such as, for example, nano-oxide particles, in certain types of metal alloys, such as, for example, copper-based alloys created by additive manufacturing.

Additive manufacturing, as used herein, refers to any three-dimensional (3D) printing process by which a metallic article is formed using a directed energy source to additively melt and fuse a metallic powder layer-by-layer to build up the article. Additive manufacturing processes include, but are not limited to, selective laser melting (SLM), selective laser sintering (SLS), direct metal laser sintering (DMLS), direct metal laser melting (DMLM), and electron beam melting (EBM).

Selective laser melting, as used herein, refers to any additive manufacturing process using a laser beam as the directed energy source. Selective laser melting processes include, but are not limited to, SLS, DMLS, and DMLM.

A nano-particle, as used herein, refers to any dispersoid particle formed in a melt pool during an additive manufacturing process by reaction between a reactive component and a metal. In some embodiments, the nano-particle has a size in the range of about 1 nm to about 100 nm, alternatively in the range of about 1 nm to about 200 nm, alternatively in the range of about 1 nm to about 1000 nm, or any value, range, or sub-range therebetween, depending on the conditions under which the nano-particle is formed.

A non-reactive gas, as used herein, refers to a gas that does not react with the melt pool formed by the metallic powder of a metallic element or alloy during an additive manufacturing process. As certain gases may react with one metallic element or alloy but not another, whether a particular gas is non-reactive may be dependent on the specific metallic element or alloy being used to additively manufacture an article.

A reactive component, as used herein, refers to a compound or element that reacts with the melt pool formed by the metallic powder of a metallic element or alloy during an additive manufacturing process. The reactive component may be provided in the atmosphere of the additive manufacturing device or in the powder from which the article is additively manufactured.

One aspect of this invention involves modifying or otherwise manipulating the composition of the metallic powder and/or the composition of the gas or gases contained in the atmosphere inside the printing chamber used in an additive manufacturing system (referred to as “headspace modulation”) to form metal compounds that confer desired characteristics such as nano-particles. In some embodiments, the atmosphere includes primarily a non-reactive gas but also a predetermined level of a reactive component. In some embodiments, the atmosphere is a reduced pressure atmosphere or vacuum with a predetermined concentration of a reactive component. In some embodiments, the powder includes a predetermined concentration of a reactive component. In some embodiments, the predetermined concentration of the reactive component is selected to produce an article having specific mechanical properties or a specific concentration of nano-particles.

The metallic powder may have any composition that reacts with a reactive component. In some embodiments, the metallic powder is a pure metallic element. In some embodiments, the metallic powder is a metallic alloy. In some embodiments, the metallic powder is pure copper, a copper-based alloy, a copper-tin alloy, or a copper-nickel-silicon alloy. In some embodiments, the metallic powder is an iron-based alloy or a steel alloy. In some embodiments, the metallic powder is not a steel alloy. In some embodiments, the metallic powder is not an iron-based alloy.

In some embodiments, the reactive component is oxygen. The unique effect from the additive manufacturing process, however, may not be limited to the use of oxygen as the reactive component to form of nano-oxide particles, but may be extended to other metal compounds that may be formed in the presence of suitable gases within the chamber gases, such as nitrides (through the presence of small quantities of nitrogen in a vacuum or an argon atmosphere) or silicides/carbides (through the presence of volatile silicon or carbon compounds in a vacuum or in a nitrogen or an argon atmosphere). In some embodiments, the reactive component is not oxygen but instead is a reactive component other than oxygen. Changing the relative concentrations and/or the composition of the purge gases surrounding the printing parts would presumably control the type and quantity of other metal compounds within the printed parts, thereby yielding desired improvements in mechanical properties.

The melt pool formed during the additive manufacturing process is understood to be very dynamic, with significant flow occurring within the melt pool leading to significant mixing, stirring, and entraining. This movement permits an increased level of incorporation of a reactive component from the atmosphere into the melt pool and also an increased rate of reaction between the reactive component and the metallic element or alloy to form the nano-particles. In some embodiments, the amount of formed dispersoids from the additive manufacturing and precipitates from the precipitation hardening is greater than what is possible by solutionizing and precipitation hardening for the same metallic element/alloy. The rapid cooling which occurs upon removal of the directed heat source essentially then leads to freezing of the nano-particles in place and a relatively homogeneous distribution of the nano-particles in the additively-manufactured article.

The rapid cooling of the melt pool in the additive manufacturing process provides homogeneity in the formed article similar to a first solutionizing step of a conventional precipitation hardening process of a wrought material. With regard to certain copper/nickel/silicon (CuNiSi) alloys, the micro-precipitation of a Ni—Si compound improves the mechanical properties, and the additional formation of the nano-oxide particles further enhances the mechanical properties. These enhanced mechanical properties are maintained over an extended exposure of the alloy to elevated temperatures that would normally have resulted in a decrease in such mechanical properties for a wrought form of the same alloy. Accordingly, precipitation hardening from nano-particles formed during an additive manufacturing process may be utilized to create Cu-based structures that maintain their mechanical properties for longer time periods at elevated temperatures. Such materials are beneficial for electrical connector applications in harsh environments, where extreme temperatures typically degrade the performance of conventional copper-based contacts, or for other applications that require copper-based parts that maintain their structural integrity and electrical characteristics for prolonged periods of time.

As previously discussed, the precipitation of fine inclusions (precipitation hardening) is a mechanism for modification of the mechanical properties of metal/metal alloys. In the processing of metals, the approach typically used to create a particular material is to form a melt with a desired composition, solidify that material, and then apply a variety of post-solidification heat treatments to the material. However, the compositions attainable (and consequently the types of precipitated inclusions that are possible) are generally limited to those that form melts.

The precipitation hardening process confers greater hardness to the article subjected to the process. Heat aging processes increase the mechanical properties of metals up to a maximum before experiencing a loss of the enhanced mechanical properties as the exposure of the materials to elevated temperatures continues. The systems and methods of the present invention provide additively-manufactured metals and alloys that may be precipitation hardened in a simpler and more efficient manner than the wrought versions of these alloys and that may retain the beneficial aspects of precipitation hardening for a longer period of time or at higher temperatures due to the presence of nano-particles in the materials, either as additively-manufactured or after precipitation hardening.

Copper-based alloys made by an SLM process have been observed to include dispersoids in the form of nano-particles, more specifically nano-oxide particles, and such alloys possess certain advantageous characteristics compared to the wrought forms of the same or similar alloys. In addition, the very rapid cooling of materials formed by additive manufacturing simplifies the process of precipitation hardening of such materials by making the solution treatment step (i.e., the first step in the process) unnecessary. Accordingly, the present invention includes methods for modifying or manipulating the additive manufacturing process to affect the formation of advantageous dispersoids.

In some embodiments, the enhanced mechanical properties of the article include a tensile strength, more specifically an ultimate tensile strength, that is better maintained at an elevated temperature over an extended period of time. This enhanced mechanical property provides at least two potential advantages. First, it makes the article easier to process than a wrought article, because the length of time for the precipitation hardening is less critical for achieving a predetermine tensile strength or ultimate tensile strength. Second, it permits use of the article for a longer period of time without risk of failure in a high-temperature application compare to a wrought article. In some embodiments, the enhanced mechanical properties include an enhanced tensile strength, more specifically an enhanced ultimate tensile strength. In some embodiments, the enhanced mechanical properties include an enhanced tensile strength, more specifically an enhanced ultimate tensile strength, that is better maintained at an elevated temperature over an extended period of time.

EXAMPLES

An article was formed by an additive manufacturing process, more specifically an SLM process of DMLS/DMLM, from a powder of a copper-tin (Cu—Sn) Cu-4% Sn alloy (95.5-96.5 wt % copper and 3.5-4.5 wt % tin; bronze). In analyzing the structural characteristics of the article, the nano-oxide particles of the additively-manufactured alloy were observed to be very stable at an annealing temperature of 600° C. (1100° F.). Dispersed nano-oxide particles were identified within the bulk of the SLM-formed Cu-4% Sn alloy.

With regard to the Cu-4% Sn alloy, the observed nano-oxide particles are assumed to have been continuously created during the SLM process and were observed to be dispersed throughout the bulk interior of the copper alloy. The nano-oxide particles may be formed either as a result of the molten metal pool scavenging residual oxygen present in the otherwise inert (nitrogen) atmosphere within the deposition chamber or from traces of oxides in the metallic powders used in the SLM process.

An article was formed by an additive manufacturing process, more specifically an optimized SLM process of DMLS/DMLM, from a powder of a commercial Corson alloy based on CuNiSi, referred to as 70250, having a composition of 2.2-4.2 wt % Ni, 0.25-1.2 wt % Si, 0.05-0.30 wt % magnesium (Mg), up to 0.20 wt % iron (Fe), up to 1.0 wt % zinc (Zn), up to 0.1 wt % manganese (Mn), up to 0.05 wt % lead (Pb), and a balance of Cu. The article was then subjected to a precipitation hardening process to maximize its physical and mechanical properties. The nano-particles in the article after the precipitation hardening were observed to be substantially spherical with an average diameter of about 33 nm and to be present at a concentration of about 0.25 vol % of the article. Similar to other copper-based alloys, Corson alloys (i.e., alloys that derive their enhanced mechanical properties from a precipitation process carried out at elevated temperatures) demonstrate decreased mechanical properties under longer aging times at elevated temperatures as a result of a strengthening precipitate coarsening process. In a separable electrical contact that depends on a spring force to maintain good conductivity, the loss of mechanical properties may result in a decreased normal force on the separable interface and a degradation of the electrical performance across the separable interface.

The precipitation hardening process followed the additive manufacturing of the CuNiSi-based article without a solutionizing step between the additive manufacturing and the precipitation hardening. The precipitation hardening process was thus simplified because an initial solutionizing step was not required.

In this embodiment, the Corson alloy based on CuNiSi (70250) alloy powder was created using an optimized SLM process for printing high density parts and components. This process, which optimized laser power, laser travel speed, beam focus, spacing between laser lines, beam offsets, and width of laser raster scan, was developed to: (i) obtain a relatively smooth and defect free finish on external surfaces; (ii) attain interior sections with a density of 98%-100% relative to the reported density of the wrought form of the alloy; and (iii) create strong and continuous support structures that sufficiently bond to steel build plates as well as to the initial layers or printed parts. The laser parameters for a commercially available bronze alloy were found to be inadequate to process the CuNiSi alloy. Accordingly, the laser energy (power/laser scan speed) was increased and a more focused beam setting was applied.

Another aspect of this invention involves the formation of nano-particles, specifically nano-oxide particles, in additively-manufactured CuNiSi alloys. Precipitation-hardened CuNiSi materials normally experience a decrease in mechanical properties when exposed to elevated temperatures for extended times. As discussed above, nano-oxide particles were identified within the bulk of both a Cu-4% Sn alloy (bronze) and a CuNiSi alloy, and are believed to be responsible for an observed reduced rate of microstructural grain coarsening at 600° C. (1100° F.). These nano-oxide particles are presumably created during the additive manufacturing process, which disperses the particles throughout the bulk of the interior of the material. The nano-oxide particles may originate from trace oxide on the surface of the metallic powder used in the DMLS printing process or may be created during the additive manufacturing process by the molten metal scavenging trace oxygen from the otherwise inert atmosphere within the internal environment of an additive manufacturing machine/system. In either case, the oxide particles are dispersed throughout the bulk interior of the printed material. Laboratory observations suggest that a similar phenomenon occurs in a CuNiSi alloy and may be responsible for enhancing the mechanical properties during an over-aging condition when fabricated using DMLS. The unique effect from the additive manufacturing process may not be limited to Cu/Sn alloys, but are expected to extend to other copper-based alloys, and even other metal/metal alloy systems. The unique effect(s) of the observed nano-particles are not expected to be limited to oxide containing particles, but to extend to other reactive complexes with metals.

Although only certain specific reactive components and metallic elements and alloys are described herein, the methods described herein may be applied to any pair of a reactive component and a metallic element or alloy to produce nano-particles in an additively manufactured article. As such, the metallic element or alloy may be any composition capable of reacting with a reactive component when in a melted state to form a nano-particle dispersoid in a melt pool of the metallic element or alloy. In some embodiments, the additively-manufactured article has a composition that is not capable of being precipitation hardened after being formed. In some embodiments, the additively-manufactured article is subsequently subjected to a precipitation hardening process, as described herein.

While the present invention has been illustrated by the description of exemplary embodiments thereof, and while the embodiments have been described in certain detail, it is not the intention to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to any of the specific details, representative devices and methods, and/or illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the general inventive concept.

Claims

1. A method of producing a metallic article, the method comprising:

providing a metallic powder, wherein the metallic powder comprises a metallic element or metallic alloy;

selecting a predetermined concentration for a reactive component;

providing a controlled atmosphere comprising the reactive component at the predetermined concentration; and

additively manufacturing the metallic article from the metallic powder under the controlled atmosphere such that the reactive component reacts with the metallic powder in a weld pool formed during the additive manufacturing to form a dispersion of nano-particles in the weld pool;

wherein the nano-particles are dispersed throughout the metallic article in a substantially uniform manner.

2. The method of claim 1, wherein the metallic powder is copper or a copper-based alloy.

3. The method of claim 2, wherein the copper-based alloy is a copper-nickel-silicon alloy or a copper-tin alloy.

4. The method of claim 1, wherein the controlled atmosphere further comprises an inert gas.

5. The method of claim 4, wherein the inert gas is selected from the group consisting of argon, nitrogen, and a combination thereof.

6. The method of claim 1, wherein the controlled atmosphere is a vacuum.

7. The method of claim 1, wherein the reactive component comprises an element selected from the group consisting of oxygen, nitrogen, silicon, carbon, and a combination thereof.

8. The method of claim 1, wherein the nano-particles are nano-oxide particles.

9. The method of claim 1 further comprising subjecting the metallic article to a single-step precipitation hardening process, without solutionizing the metallic article between the additive manufacturing and the precipitation hardening, to enhance at least one mechanical property of the metallic article.

10. The method of claim 1, wherein the additive manufacturing comprises selective laser melting or electron beam melting.

11. A metallic article formed by the method of claim 1.

12. A method of producing a metallic article, the method comprising:

selecting a predetermined concentration for a reactive component;

providing a metallic powder, wherein the metallic powder comprises a metallic element or metallic alloy and the reactive component at the predetermined concentration;

providing a controlled atmosphere; and

additively manufacturing the metallic article from the metallic powder under the controlled atmosphere such that the reactive component reacts with the metallic powder in a weld pool formed during the additive manufacturing to form a dispersion of nano-particles in the weld pool;

wherein the nano-particles are dispersed throughout the metallic article in a substantially uniform manner.

13. The method of claim 12, wherein the metallic element or metallic alloy is copper, a copper-based alloy, a copper-nickel-silicon alloy or a copper-tin alloy.

14. The method of claim 12, wherein the controlled atmosphere is an inert gas atmosphere.

15. The method of claim 14, wherein the inert gas is selected from the group consisting of argon, nitrogen, and a combination thereof.

16. The method of claim 12, wherein the controlled atmosphere is a vacuum.

17. The method of claim 12, wherein the reactive component comprises an element selected from the group consisting of oxygen, nitrogen, silicon, carbon, and a combination thereof.

18. The method of claim 12, wherein the nano-particles are nano-oxide particles.

19. The method of claim 12 further comprising subjecting the metallic article to a single-step precipitation hardening process, without solutionizing the metallic article between the additive manufacturing and the precipitation hardening, to enhance at least one mechanical property of the metallic article.

20. The method of claim 12, wherein the additive manufacturing comprises selective laser melting or electron beam melting.