Patent Publication Number: US-2020299793-A1

Title: Post-fabrication forging treatment

Description:
FIELD OF THE DISCLOSURE 
     The present disclosure is generally related to metallurgy, and in particular to post-fabrication forging treatment of metal articles. 
     BACKGROUND 
     Throughout the ages, many different metalworking techniques have been developed. Traditional examples of metalworking include casting, cold forging, hot forging, and machining. More recent examples of metal working include computer-controlled additive manufacturing techniques, such as selective laser sintering, selective laser melting, or binder jetting. “Casting” refers to placing molten metal in a mold and allowing the molten metal to cool and solidify in the mold to form an article. “Machining” refers to a process that is similar to sculpting, e.g., removing pieces of metal from a metal blank until what is left has a desired shape. “Forging” refers to shaping metal using pressure. In the case of a “cold forging” technique the metal is shaped using only pressure (e.g., without adding heat during shaping), whereas “hot forging” techniques apply pressure to the metal while the metal is heated to shape the metal. 
     Each of these techniques has benefits and limitations. Thus, when selecting a fabrication technique to form a metal article, many considerations come into play. Examples of such considerations include cost (including cost of materials, cost of equipment, cost of tooling, such as molds, dies, or bits, and cost of labor), when the article is needed, how many instances of the article are needed, mechanical properties that the article should have, dimensional and mechanical tolerances, as well as other factors. For example, if a casting mold for an article is available, it is often cheaper to cast the article than to machine or forge the article. As another example, a forging technique can be used to provide certain grain boundary characteristics that casting and machining generally cannot provide. As a further example, casting, machining, and computer-controlled additive manufacturing techniques can be used to generate complex shapes more readily than forging techniques. 
     As a result of such trade-offs, it can be challenging to select a fabrication technique for a particular metal article. For example, if only a single instance of a relatively simple article is needed, forging the article may be the cheapest and fastest option. However, if the article is more complex or needs to meet very strict tolerances, a computer-controlled additive manufacturing technique may be cheaper and/or faster than forging. Further, if many instances of the article are to be fabricated, the cheapest option (over the entire lifetime of the mold) may be to generate a mold and cast the articles, assuming casting can give the articles required mechanical properties. If casting cannot provide the required mechanical properties, then dies can be generated to forge the article using extrusion, drop forging, etc. 
     SUMMARY 
     In a particular implementation, a method of modifying a grain structure of a metal article includes disposing at least a portion of the metal article in a molten salt bath in a pressure vessel, where the molten salt bath is at a bath temperature that is below a melting point temperature of the metal article. The method also includes pressurizing the molten salt bath in the pressure vessel to a forging pressure sufficient to cause pressure-driven grain changes in the metal article. 
     In another particular implementation, a system for modifying a grain structure of a metal article includes a pressure vessel sized to receive the metal article, and a molten salt tank configured to store molten salt. The system also includes a first pump coupled to the molten salt tank and to the pressure vessel. The first pump is configured to provide the molten salt to the pressure vessel to at least partially submerge the metal article in a molten salt bath at a bath temperature that is below a melting point temperature of the metal article and to pressurize the molten salt bath to a forging pressure sufficient to cause pressure-driven grain changes in the metal article. 
     In a particular implementation, a forging pressure vessel includes one or more insulated walls defining an interior region and configured to withstand pressure within the interior region of at least five thousand pounds per square inch. The forging pressure vessel also includes one or more molten salt ports through the one or more insulated walls. The one or more molten salt ports are configured to transport molten salt into the interior region, out of the interior region, or both, to subject a metal article within the interior region to a molten salt bath at a bath temperature that is below a melting point temperature of the metal article and at a forging pressure sufficient to cause pressure-driven grain changes in the metal article. The forging pressure vessel further includes one or more quench medium ports through the one or more insulated walls. The one or more quench medium ports are configured to transport a quench medium into the interior region, out of the interior region, or both. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram that illustrates an example of a system for fabrication and subsequent forging treatment of a metal article. 
         FIG. 2  is a diagram that illustrates first examples of forging treatment of the metal article of  FIG. 1 . 
         FIG. 3  is a diagram that illustrates second examples of forging treatment of the metal article of  FIG. 1 . 
         FIG. 4  is a flow chart of an example of a method of forging treatment of the metal article of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure describes a post-fabrication forging techniques and systems that can be used to process metal articles to modify grain characteristics of the metal articles. For example, a casting, machining, computer-controlled additive manufacturing, or other techniques can be used to fabricate a metal article, and the post-fabrication techniques described herein can subsequently be used to modify grain characteristics of the metal article. Casting, machining, and computer-controlled additive manufacturing techniques can all be used to fabricate complex metal articles relatively quickly and inexpensively; however, metal articles fabricated using these techniques may have unacceptable mechanical properties for some applications. Many mechanical properties of metal articles are related to the grain characteristics of the metal. Thus, in the example above, changing the grain characteristics using the disclosed post-fabrication forging techniques can change the mechanical properties of the metal article. 
     Computer-controlled additive manufacturing techniques generally use layer-by-layer construction. For example, binder jetting is a process in which a binder (e.g., a polymer) is selectively applied to a layer of metal powder to bond the metal powder together in some regions. Subsequently, another layer of metal powder is applied over the first bonded layer and selectively bonded using the binder. After several bonded layers have been formed into the shape of an article, the article is baked to fuse particles of the metal powder together (e.g. to sinter the particles of the metal powder). This process forms a metal article that can have weaknesses at boundaries between particles, weaknesses between layers, and grain characteristics that limit strength and ductility of the article. 
     Selective laser sintering and selective laser melting use a layer-by-layer building process that is similar to binder jetting, but selective laser sintering and selective laser melting omit the binder. Instead, a laser is used to selectively heat portions of the metal powder to cause particles within the heated portions to be sintered together (in the case of selective laser sintering) or melted together (in the case of selective laser melting). Thus, articles manufactured using selective laser sintering and selective laser melting can have similar concerns as articles manufactured using binder jetting. 
     Post-fabrication heat treatment (e.g., in vacuum or in an inert gas environment) can be used to improve some characteristics of articles formed using computer-controlled additive manufacturing processes. For example, post-fabrication heat treatment of an article formed using selective laser sintering can encourage some additional grain growth, leading to increased density of the article. However, even after such heat treatment, articles formed using computer-controlled additive manufacturing processes tend to have characteristics (e.g., mechanical properties and grain characteristics) that are more similar to cast metal articles than to forged metal articles. 
     The post-fabrication forging treatment disclose herein uses heat and pressure to modify grain characteristics of a metal article. For example, the metal article can be disposed in a molten salt bath, and the molten salt bath can be pressurized to a forging pressure (e.g., 5000 pounds per square inch (psi) or more). The high temperature and high pressure of the pressurized molten salt bath used for the disclosed post-fabrication forging treatment refine and grow the grain structure of the metal article and exert a forging force. For example, grain boundaries of the metal article can be realigned, reshaped, merged, etc. by the heat and pressure. As a result, after the post-fabrication forging treatment, the metal article will have grain characteristics similar to grain characteristics that would result from fabricating the metal article using a hot forging process. To illustrate, after the post-fabrication forging treatment, the metal article will have grain boundaries that tend to be aligned with a surface geometry of the metal article, sometimes referred to herein as grain boundary alignment. 
     For comparison, articles that are fabricated using casting techniques tend to have randomly arranged grain boundaries. Similarly, articles that are fabricated using computer-controlled additive manufacturing processes tend to have grain boundaries that are randomly arranged or arranged in a manner that corresponds to layers of the layer-by-layer fabrication process. Additionally, metal articles fabricated by casting, machining, or additive manufacturing tend to be less dense than articles fabricated using forging techniques. The density and orientation of grain boundaries with surface geometry of the metal article that results from forging (and from the disclosed post-fabrication forging treatment) provides many of the mechanical benefits of forging. For example, the ductility and strength of forged articles (as compared to machined, cast, or additively manufactured articles) is, at least in part, attributable to the density and orientation of the grain boundaries due to forging. 
     In some implementations, equipment used for post-fabrication forging treatment (e.g., a forging pressure vessel) can also be used for other treatment processes, such as quenching or heat treatment. For example, a metal article can be disposed in a forging pressure vessel for a post-fabrication forging treatment process. Molten salt is added to the pressure vessel to submerge (or partially submerge) the metal article, and the molten salt is pressurized to a forging pressure. After a treatment period, the molten salt can be removed from the forging pressure vessel, and a quench medium (e.g., air, water, oil, etc.) can be added to the forging pressure vessel to quench the metal article. As another example, after a quenching treatment (in the forging pressure vessel or elsewhere), the metal article can be disposed in the forging pressure vessel and subject to an unpressurized or low-pressure heat treatment (e.g., molten salt or another heat source). 
     The temperature and pressure of the molten salt bath can be precisely and accurately controlled to achieve specified grain characteristics. For example, a post-fabrication forging treatment system can include a controller that controls and monitors the temperature and pressure of the molten salt bath and a duration of a forging treatment. When the post-fabrication forging treatment system also performs other operations, such as quenching or heat treatment, the controller can also control these other operations. To illustrate, the controller can control the temperature of a quench medium, the rate at which the quench medium is added to the forging pressure vessel, the quantity of the quench medium added to the forging pressure vessel, and the duration of the quenching operation. As another illustration, the controller can control the temperature of a heat treatment medium, the rate at which the heat treatment medium is added to the forging pressure vessel, the quantity of the heat treatment medium added to the forging pressure vessel, and the duration of the heat treatment. 
     In some implementations, the controller controls how long a forging operation or another operation lasts based on a timer (e.g., elapsed time since the operation started). In other implementations, data from one or more sensors can be used (alone or in conjunction with the timer) to control how long an operation lasts. For example, a forging operation can last for a specified time (e.g., 15 minutes) after the metal article or the molten salt bath within the forging pressure vessel reach a specified temperature (or pressure). In some implementations, the controller includes a memory device to store one or more process recipes that can be programmed by or selected by an operator. The process recipe(s) specify set points (e.g., temperature and pressure targets), durations, an order of operations, and other parameters that the controller uses to govern operation of the post-fabrication forging treatment system based on target grain characteristics. 
     Accordingly, the post-fabrication forging treatment systems and methods disclosed herein enable fabrication of forged articles using non-forging processes, such as casting, machining, or additive manufacturing. To illustrate, a non-forging process can be used for shaping (or coarse shaping) of the article, and subsequently a post-fabrication forging treatment can be used to modify grain characteristics of the article to be similar to grain characteristics of a forged article. This simplifies selection of a fabrication process because a fabrication process that is faster and/or cheaper than forging (such as casting, machining, or additive manufacturing) can be used to fabricate the article without sacrificing the benefits of mechanical properties that are associated with forging. 
     In the following, reference is made to features depicted in the drawings. In some of the drawings, multiple instances of a particular type of feature are used. Although these features are physically and/or logically distinct, the same reference number is used for each, and the different instances are distinguished by addition of a letter to the reference number. When the features as a group or a type are referred to herein (e.g., when no particular one of the features is being referenced), the reference number is used without a distinguishing letter. However, when one particular feature of multiple features of the same type is referred to herein, the reference number is used with the distinguishing letter. For example, referring to  FIG. 2 , grain structures of a metal article are illustrated and associated with reference numbers  208 A and  208 B. When referring to a particular one of these grain structures, such as a first grain structure  208 A, the distinguishing letter “A” is used. However, when referring to any arbitrary one of these grain structure or to these grain structures as a group, the reference number  208  is used without a distinguishing letter. 
     As used herein, various terminology is used for the purpose of describing particular implementations only and is not intended to be limiting. For example, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Further, the terms “comprise,” “comprises,” and “comprising” are used interchangeably with “include,” “includes,” or “including.” Additionally, the term “wherein” is used interchangeably with the term “where.” As used herein, “exemplary” indicates an example, an implementation, and/or an aspect, and should not be construed as limiting or as indicating a preference or a preferred implementation. As used herein, an ordinal term (e.g., “first,” “second,” “third,” etc.) used to modify an element, such as a structure, a component, an operation, etc., does not by itself indicate any priority or order of the element with respect to another element, but rather merely distinguishes the element from another element having a same name (but for use of the ordinal term). As used herein, the term “set” refers to a grouping of one or more elements, and the term “plurality” refers to multiple elements. 
     As used herein, “coupled” can include “communicatively coupled,” “electrically coupled,” or “physically coupled,” and can also (or alternatively) include any combinations thereof. Two devices (or components) can be coupled (e.g., communicatively coupled, electrically coupled, or physically coupled) directly or indirectly via one or more other devices, components, wires, buses, networks (e.g., a wired network, a wireless network, or a combination thereof), etc. As used herein, “directly coupled” is used to describe two devices that are coupled (e.g., communicatively coupled, electrically coupled, or physically coupled) without intervening components. To illustrate, two components that are in direct physical contact with one another are “directly coupled” to one another. 
       FIG. 1  is a diagram that illustrates an example of a system  100  for fabrication and subsequent forging treatment of a metal article  104 . The system  100  includes one or more fabrication devices  102  and a forging treatment system  101 , including a forging pressure vessel  106 , one or more pumps and tanks for materials used during treatment of the metal article  104 , and a controller  124 . Although the system  100  of  FIG. 1  illustrates the fabrication device(s)  102 , the fabrication device(s)  102  can be remote from a location of the forging treatment system  101 . For example, the metal article  104  can be fabricated at a first location and moved to a second location for post-fabrication forging treatment. In some implementations, an owner or operator of the forging treatment system  101  can be distinct from the owner or operator of the fabrication device(s)  102 . For example, the owner or operator of the forging treatment system  101  can purchase the metal article  104  from the owner or operator of the fabrication device(s)  102  and subsequently treat the metal article  104  using the forging treatment system  101 . As another example, the owner of the metal article  104  (which can be the owner or operator of the fabrication device(s)  102  or a customer of the owner or operator of the fabrication device(s)  102 ) can contract with the owner or operator of the forging treatment system  101  for forging treatment services. 
     The fabrication device(s)  102  can fabricate the metal article  104  using any metal fabrication process or a combination of metal fabrication processes. For example, the fabrication device(s)  102  can include casting devices that cast the metal article  104 . As another example, the fabrication device(s)  102  can include machining devices (e.g., a milling machine, cutters, grinders, polishers, etc.) that shape the metal article  104  from a blank. As yet another example, the fabrication device(s)  102  can include additive manufacturing devices (e.g., selective laser sintering devices, selective laser melting devices, binder jetting devices, etc.) that build the metal article  104  by fusing layers of metal particles. In still another example, the fabrication device(s)  102  can include devices that use a combination of different fabrication processes. For example, two or more machined parts can be fused together (e.g., welded) to form the metal article  104 . Thus, the metal article  104  can be cast, machined, fused layer-by-layer (also referred to as “printed”), or assembled from two or more metal parts that are fused together. In some implementations, the metal article  104  (or portions thereof) can be forged. To illustrate, if the metal article  104  is formed of two or more parts that are welded together, the forging treatment system  101  can be used to modify grain characteristics of the weld. As another example, the metal article can be forged and subsequently machined to form a particular feature. In this example, the forging treatment system  101  can be used to modify grain characteristics of a machined portion of the metal article  104 . 
     As explained above, the forging treatment system  101  includes the forging pressure vessel  106 . The forging pressure vessel  106  includes one or more insulated walls  108 . For example, if the forging pressure vessel  106  is spherical, the forging pressure vessel  106  can include a single insulated wall  108 . In another example, if the forging pressure vessel  106  is cubical, the forging pressure vessel  106  includes six insulated walls  108 . The insulated wall(s)  108  enclose a volume of space to define an interior region  110  that is sized and shaped to receive the metal article  104 . 
     One or more ports extend through the insulated wall(s)  108  to enable introduction of various process media into the interior region  110  and extraction of the various process media from the interior region  110 . For example, in  FIG. 1 , one or more molten salt ports  112  extend through the insulated wall(s)  108  to enable adding molten salt  136  to the interior region  110  to form a molten salt bath  146  in the forging pressure vessel  106 . The molten salt port(s)  112  may also enable extraction of the molten salt  136  from the interior region  110  after a forging process is complete.  FIG. 1  also illustrates one or more quench medium ports  118  extending through the insulated wall(s)  108  to enable adding a quench medium  142  to the interior region  110  to form a quench bath  164  in the forging pressure vessel  106 . The quench medium port(s)  118  may also enable extraction of the quench medium  142  from the interior region  110  after a quenching process is complete. 
     When the quench medium  142  is a liquid (e.g., oil, water, a water/glycol solution, or brine), quenching the metal article  104  can liberate gases  168  due to combustion, evaporation, or both. To facilitate use of a liquid quench medium  142 , the forging pressure vessel  106  can include a vapor control system  114  that includes one or more vapor ports  116 . The vapor port(s)  116  are configured to vent from the forging pressure vessel  106  the gases  168  released by the quench medium  142 . 
     The insulated wall(s)  108  are configured to retain heat (i.e., are insulated) to facilitate control of the temperature within the interior region  110 . Further, the insulated wall(s)  108  are configured to withstand forging pressures that are applied to the metal article  104 . Generally, the forging pressure is at least 5000 psi; however, even higher pressures can be used depending on the particular metal or alloy from which the metal article  104  is fabricated and depending on the goals of the forging treatment process (e.g., target grain characteristics  162  desired in the metal article  104  after the forging treatment). In some implementations, the forging pressure can be greater than or equal to 8000 psi; greater than or equal to 10,000 psi; greater than or equal to 12,000 psi; greater than or equal to 15,000 psi; greater than or equal to 17,000 psi; greater than or equal to 20,000 psi; or even greater than or equal to 22,000 psi. In various implementations, the insulated wall(s)  108  are configured to withstand any of these forging pressures. 
     In some implementations, one or more sensors are disposed within the forging pressure vessel  106 . The sensor(s) can be coupled to connectors external to the insulated wall(s)  108  to provide sensor data signals to the controller  124  or other components (not shown), such as a fabrication data tracking system. For example, in  FIG. 1 , the sensor(s) include one or more temperature sensors  150  coupled to one or more first connectors  120  and one or more pressure sensors  152  coupled to one or more second connectors  122 . In other implementations, the forging pressure vessel  106  includes fewer sensors (e.g., only the pressure sensor(s)  152 ), more sensors (e.g., a liquid level sensor or a chemical composition sensor in addition to the temperature and pressure sensors  150 ,  152 ), or different sensors (e.g., a liquid level sensor or a chemical composition sensor without the temperature or pressure sensors  150 ,  152 ). 
     The temperature sensor(s)  150  are configured to provide one or more temperature feedback signals  154  via the first connector(s)  120  to the controller  124 . The temperatures sensor(s)  150  can include, for example, thermocouples, optical temperature sensors, temperature-sensitive diodes, thermistors, or other electrical or electronic devices that generate a signal having characteristics indicative of temperature or changes in temperature. The temperature sensor(s)  150  are configured to (e.g., positioned to or positionable to) monitor a temperature of the molten salt bath  146 , a temperature of the quench bath  164 , a temperature of the metal article  104 , a temperature of a mold or die  148 , or another temperature associated with the interior region  110 . In some implementations, the temperature sensor(s)  150  include sensors to monitor two or more of the temperature of the molten salt bath  146 , the temperature of the quench bath  164 , the temperature of the metal article  104 , the temperature of the mold or die  148 , or another temperature associated with the interior region  110 . In some implementations, the temperature sensor(s)  150  include one or more sensors to monitor a temperature difference within the interior region  110 , such as difference between the temperature of the metal article  104  and the temperature of the molten salt bath  146 . 
     The pressure sensor(s)  152  are configured to provide one or more pressure feedback signals  156  via the second connector(s)  122  to the controller  124 . The pressure sensor(s)  152  can include, for example, an optical pressure sensor, a piezoelectric sensor, a capacitive sensor, or other electrical or electronic devices that generate a signal having characteristics indicative of pressure within the interior region  110 , a pressure change within the interior region  110 , or a difference in pressure between interior region  110  and outside the forging pressure vessel  106 . 
     The vapor control system  114  can be active or passive. For example, if the vapor control system  114  is passive, the vapor port(s)  116  can be configured to (e.g., calibrated to) open responsive to a specific pressure within the forging pressure vessel  106 . To illustrate, the force on the vapor port(s)  116  due to the pressure within the forging pressure vessel  106  can push the vapor port(s)  116  open. In this example, the vapor port(s)  116  can be manually reset after opening or can automatically reset responsive to the pressure within the forging pressure vessel  106  falling below a specific pressure. If the vapor control system  114  is active, the vapor port(s)  116  can be opened responsive to a control signal from the controller  124  or another control device, such as a controller of the vapor control system  114  or a safety system. The control signal can be sent responsive to value of the pressure feedback signal(s)  156  or based on an operation that is being performed. To illustrate, the vapor port(s)  116  can be opened at the beginning of a quench operation. 
     In  FIG. 1 , the forging treatment system  101  includes a molten salt tank  134  coupled to the forging pressure vessel  106  via a first pump  138 . The molten salt tank  134  is configured to store the molten salt  136  (or a portion of the molten salt  136  that is not being used in the molten salt bath  146  at any particular time). In a particular implementation, the molten salt tank  134  includes or is coupled to a heater to control a temperature of the molten salt  136 . In some implementations, the heater is also configured to melt a solid salt material to form the molten salt  136 . In other implementations, the solid salt material can be melted elsewhere and disposed in the molten salt tank  134  in a liquid (e.g., molten) state. Although only one molten salt tank  134  is shown in  FIG. 1 , in other implementations, the forging treatment system  101  can include more than one molten salt tank  134 . For example, a first molten salt tank can be used to store fresh or unused molten salt  136 , and a second molten salt tank can be used to store used molten salt  136  extracted from the forging pressure vessel  106 . In this example, the used molten salt  136  can be processed (e.g., filtered, reheated, etc.) before being returned to the fresh molten salt tank, or the used molten salt  136  can be disposed of. As another example, a first molten salt tank can be used to store a first type of molten salt  136  (e.g., molten sodium chloride), and a second molten salt tank can be used to store a second type of molten salt  136  (e.g., molten sodium nitrate). In this example, different types of molten salt  136  can be used depending on chemical compatibility with a metal or alloy of the metal article and the target grain characteristics  162 . 
     The first pump  138  is coupled to the controller  124  to receive first control signals  158 , and the controller  124  is configured to control operation of the first pump  138  using the first control signals. The first pump  138  is configured to (responsive to the first control signals  158 ) pump the molten salt  136  into the forging pressure vessel  106  from the molten salt tank  134  to form the molten salt bath  146  (e.g., to at least partially submerge the metal article  104 ). In some implementations, the first pump  138  is also configured to pressurize the molten salt bath  146  to the forging pressure after filling the interior region  110  with molten salt  136 . In other implementations, such as when the molten salt bath  146  does not completely fill the interior region  110 , the molten salt bath  146  can be pressurized using a pressurized gas (e.g., an inert gas, such as argon). In some implementations, the first pump  138  is also configured to extract the molten salt  136  from the forging pressure vessel  106  and return the molten salt  136  to the molten salt tank  134 . In other implementations, the molten salt bath  146  can be dumped (e.g., driven by pressure and gravity) to extract the molten salt  136  from the forging pressure vessel  106 . 
     In  FIG. 1 , the forging treatment system  101  also includes a quench medium tank  140  coupled to the forging pressure vessel  106  via a second pump  144 . The quench medium tank  140  is configured to store the quench medium  142  (or a portion of the quench medium  142  that is not being used in the quench bath  164  at any particular time). Although only one quench medium tank  140  is shown in  FIG. 1 , in other implementations, the forging treatment system  101  can include more than one quench medium tank  140 . For example, a first quench medium tank can be used to store fresh or unused quench medium  142 , and a second quench medium tank can be used to store used quench medium  142  extracted from the forging pressure vessel  106 . In this example, the used quench medium  142  can be processed (e.g., filtered) before being returned to the fresh quench medium tank, or the used quench medium  142  can be disposed of. As another example, a first quench medium tank can be used to store a first type of quench medium  142  (e.g., brine), and a second quench medium tank can be used to store a second type of quench medium  142  (e.g., oil). In this example, different types of quench media  142  can be used depending on chemical compatibility with a metal or alloy of the metal article and the target grain characteristics  162 . 
     The second pump  144  is coupled to the controller  124  to receive second control signals  166 . The second pump  144  is configured to (responsive to the second control signals  166 ) pump the quench medium  142  into the forging pressure vessel  106  from the quench medium tank  140  to form the quench bath  164  (e.g., to at least partially submerge the metal article  104 ). In some implementations, the second pump  144  is also configured to extract the quench medium  142  from the forging pressure vessel  106  and return the quench medium  142  to the quench medium tank  140 . In other implementations, the quench bath  164  can be dumped (e.g., driving by pressure and gravity) to extract the quench medium  142  from the forging pressure vessel  106 . 
     As illustrated in  FIG. 1 , the controller  124  includes one or more processors  126  and memory  128  (e.g., one or more memory devices). The memory  128  is accessible to the processor  126  and stores instructions  130  that are executable by the processor  126  to initiate or control one or more operations of the forging treatment system  101 . For example, the processor  126  can execute the instructions  130  to process the temperature feedback signal(s)  154  and the pressure feedback signal(s)  156  to determine whether process conditions within the forging pressure vessel  106  correspond to respective set points as specified in one or more process recipes  132 . The processor  126  can also execute the instructions  130  to generate the first control signals  158 , the second control signals  166 , other control signals (not shown, such as a control signal to the vapor control system  114 ), etc. 
     The process recipe(s)  132  specify process conditions for particular operations performed by the forging treatment system  101 . For example, for a particular forging operation the process recipe(s)  132  can specify a bath temperature of the molten salt bath  146 , a forging pressure, a type of molten salt  136  to be used, a forging time  160 , a fill level of the molten salt bath  146 , etc. As another example, for a particular quenching operation the process recipe(s)  132  can specify a type of quench medium  142  to be used, a quenching time (or a quenching endpoint, such as an endpoint temperature of the metal article  104 ), a fill level of the quench bath  164 , etc. In some implementations, the process recipe(s)  132  can also specify other process parameters, such as a duration or temperature of a pre-heating operation performed before a forging operation, a delay between a forging operation and a quenching operation, or process conditions for other processes (besides forging and quenching) performed in the forging pressure vessel  106  (such as a duration and temperature of a heat treatment). 
     In an illustrative example of operation described below, processing the metal article  104  using the forging treatment system  101  includes a forging operation, a quenching operation, and a heat treatment operation. In other examples, the metal article  104  can undergo other processes using the forging treatment system  101 , or one or more of the described operations can be repeated, omitted, or performed in a different order. The specific order of the operations, process conditions used in each operation, and the types of operations used will depend on the metal or alloy of the metal article  104  and the target grain characteristics  162 . 
     In the illustrative example of operation, the metal article  104  is disposed in the forging pressure vessel  106 . In some circumstances, as described further with reference to  FIG. 3 , the metal article  104  can be coupled to, placed on, or enclosed within a mold or die  148  that defines a target feature shape for the metal article  104 . After the metal article  104  is disposed within the forging pressure vessel  106 , an operator can start the forging process by selecting or programming a process recipe  132 . The controller  124  subsequently initiates or controls operations performed by the forging treatment system  101 . 
     The forging process includes pumping molten salt  136  into the forging pressure vessel  106  to at least partially submerge the metal article  104 . After the metal article  104  is at least partially submerged in the molten salt bath  146  and before the molten salt bath  146  is pressurized, a pre-heating operation can be performed to gradually heat the metal article  104 . For example, the bath temperature of the molten salt bath  146  is generally much hotter than room temperature. To illustrate, the bath temperature can be set to a forging temperature of the metal article  104 . The forging temperature is below the melting point temperature of the metal of the metal article  104  but is hot enough that the metal begins to soften somewhat (e.g. greater than a recrystallization temperature of the metal article  104 ). As an example, the forging temperature can be greater than 500 deg. F. Suddenly exposing the metal article  104  to such a dramatic temperature change (e.g., from room temperature to the forging temperature) can cause shock. The pre-heating operation can be used to mitigate or avoid such shock. One way to perform the pre-heating operation is to heat the molten salt  136  in the molten salt tank  134  to a bath temperature that is lower than the forging temperature. The molten salt  136  is then pumped into the forging pressure vessel  106  to form the molten salt bath  146 . The molten salt  136  can be circulated (e.g., by the first pump  138 ) between the forging pressure vessel  106  and the molten salt tank  134  to avoid cold spots (e.g., to achieve or maintain a relatively uniform temperature distribution within the molten salt bath  146 ). In this example, the temperature of the molten salt  136  in the molten salt tank  134  and the molten salt bath  146  can be gradually increased to the forging temperature. 
     After the molten salt bath  146  is at the forging temperature, the metal article  104  can be allowed to soak in the molten salt bath  146  for a period (e.g., a soak time). Soaking the metal article  104  in the molten salt bath  146  helps to achieve a relatively uniform temperature distribution within the metal article  104  (or within the portion of the metal article  104  that is submerged in the molten salt bath  146 ). 
     When a forging operation begins, the molten salt bath  146  is pressurized to a forging pressure. For example, the first pump  138  can pump molten salt  136  into the interior region  110  to fill and pressurize the molten salt bath  146 . Alternatively, if the molten salt bath  146  does not completely fill the interior region  110 , then a pressurized gas source (not shown) can be used to pressurize the interior region  110  and the molten salt bath  146 . The metal article  104  remains in the molten salt bath  146  at the forging pressure for a duration indicated by the forging time  160 . The duration of the forging time  160 , the value of the forging pressure, and the bath temperature are selected based on the properties of the metal of the metal article  104  and the target grain characteristics. 
     While the metal article  104  is in the pressurized molten salt bath  146 , the pressure and heat applied to the metal article  104  drive changes to the grain structure of the metal article  104 . For example, a density of the metal article  104  can be increased due to grain growth. Additionally, grain boundaries can shift. For example, dislocations at the grain boundaries can consolidate and, to some extent, be reoriented (e.g., to align more closely with surface geometry of the metal article  104 ). The amount and type of pressure-driven grain changes that occur depend on the forging pressure, the bath temperature, and the forging time  160 . 
     When the duration of the forging time  160  is complete, the molten salt bath  146  is extracted from the forging pressure vessel  106  (e.g., by the first pump  138 ). The metal article  104  can be removed from the forging pressure vessel  106 , or can remain in the forging pressure vessel  106  for further treatment. For example, the metal article  104  can remain in the forging pressure vessel  106  for a quench treatment. 
     The quench treatment is performed by introducing the quench medium  142  into the forging pressure vessel  106  to form the quench bath  164  at least partially submerging the metal article  104 . The goal of quenching is to control the rate of cooling of the metal article  104  after the forging operation to achieve particular grain characteristics and corresponding material properties of the metal article  104 . The rate of cooling can be controlled based on the type of the quench medium  142  that is used. For example, quenching with air provides a slower cooling rate than quenching with water. When a liquid quench medium  142  is used, it is common for gases  168  to be liberated (e.g., due to evaporation and/or combustion). The vapor control system  114  vents the gases  168  from the forging pressure vessel  106  during the quench operation if needed. 
     The quench operation can end when a quench time is complete or based on temperature of the quench bath  164  or of the metal article  104 . At the end of the quench operation, the quench bath  164  can be extracted from the forging pressure vessel  106  (e.g., by the second pump  144 ). The metal article  104  can also be removed from the forging pressure vessel  106 , or can remain in the forging pressure vessel  106  for further treatment. For example, the metal article  104  can remain in the forging pressure vessel  106  for a heat treatment. 
     Heat treatment is performed to relieve some of the internal stresses introduced into the metal article  104  due to the quenching operation. The heat treatment is performed by introducing molten salt  136  into the forging pressure vessel  106  to at least partially submerge the metal article  104  in a molten salt bath  146 . The heat treatment can use the same type of molten salt  136  that was used for the forging treatment or a different type of molten salt  136  can be used. Additionally, the bath temperature of the molten salt bath  146  used for the heat treatment can be the same as the bath temperature used for the forging treatment (e.g., the forging temperature) or the bath temperature can be different (e.g., lower) during the heat treatment than during the forging treatment. For example, the forging temperature can be greater than a recrystallization temperature of the metal article  104  and the heat treatment can be performed at a temperature that is less than the recrystallization temperature of the metal article  104 . 
     The heat treatment can end when a heat treatment time is complete or based on temperature of the molten salt bath  146  or of the metal article  104 . At the end of the heat treatment, the molten salt bath  146  can be extracted from the forging pressure vessel  106  (e.g., by the first pump  138 ). The metal article  104  can also be removed from the forging pressure vessel  106 , or can remain in the forging pressure vessel  106  for further treatment, such as cleaning to remove salt residue from the metal article  104 . After a combination of the forging treatment, quench treatment, heat treatment, cleaning, and/or other processes, the metal article  104  has the target grain characteristics  162 . 
       FIG. 2  is a diagram that illustrates first examples of forging treatment of the metal article  104  of  FIG. 1 .  FIG. 2  illustrates a cross-sectional view  202  of an example of the metal article  104  at the beginning of the forging treatment (e.g., while the metal article  104  is submerged in the molten salt bath  146  and the molten salt bath  146  is pressurized to the forging pressure, as indicated by arrows  210 ).  FIG. 2  also illustrates a cross-sectional view  204  of an example of the metal article  104  after the forging treatment. 
     As illustrated in the cross-sectional view  202 , at the beginning of the forging treatment, the metal article  104  has a first grain structure  208 A. In the illustrated example, the first grain structure  208 A includes a plurality of relatively well-defined layers or grain boundaries. Relatively well-defined layers may be present, for example, in articles fabricated via additive manufacturing processes. In another example, relatively well-defined grain boundaries may be present in articles that are machined from forged blanks. 
     During or before the forging treatment illustrated in the cross-sectional view  202 , the metal article  104  is heated to a forging temperature by the molten salt  136 . The forging temperature is below the melting point temperature of the metal article  104 , but heats the metal article  104  enough that the metal article  104  is slightly softened. During the forging treatment, the pressure of the molten salt bath  146  exerts a force on every surface of the metal article  104  that is exposed to the molten salt bath  146 . In the cross-sectional view  202 , the metal article  104  has a shape defined by the external surfaces  206  of the metal article  104 , and each of the external surfaces  206  is exposed to the molten salt bath  146 . Thus, the forging pressure exerts a forging force against each of the external surfaces  206 . The forging force at each point on the external surface  206  has a component that is normal to the external surface  206  at that point. 
     The forging force (resulting from application of the forging pressure to the external surfaces  206 ) tends to press grains of the metal article  104  together, which can cause several changes to the grain characteristics. For example, some grains can merge, which tends to increase the density of the metal article. As another example, grains tend to flatten in a direction parallel to the forging force at each location and elongate in other directions. The flattening and elongation of the grains tends to realign grain boundaries to be roughly parallel with the external surfaces  206 . Thus, in the cross-sectional view  204 , the metal article  104  has a second grain structure  208 B. In the second grain structure  208 B, grain boundaries are more aligned with the external surfaces  206  of the metal article  104  than was the case for the first grain structure  208 A. The second grain structure  208 B can also be denser than the first grain structure  208 A. As a result of the changes in the grain structure  208 , the metal article  104  will tend to be stronger and more ductile after the forging treatment than before the forging treatment. 
       FIG. 3  is a diagram that illustrates second examples of forging treatment of the metal article  104  of  FIG. 1 .  FIG. 3  illustrates a cross-sectional view  302  of an example of the metal article  104  at the beginning of the forging treatment (e.g., while the metal article  104  is submerged in the molten salt bath  146  and the molten salt bath  146  is pressurized to the forging pressure, as indicated by arrows  210 ).  FIG. 3  illustrates a cross-sectional view  304  of an example of the metal article  104  after the forging treatment. In  FIG. 3 , the forging treatment illustrated uses a mold  148 . For example, as illustrated in the cross-sectional view  302 , the mold  148  has a circular cross-sectional shape that is sized to define a desired inner diameter of the metal article  104 . The metal article  104  is coupled to or positioned with respect to the mold  148  before the forging treatment starts. 
     As illustrated in the cross-sectional view  302 , at the beginning of the forging treatment, the metal article  104  has a first grain structure  308 A. In the illustrated example, the first grain structure  308 A is fairly random or disorganized. A random or disorganized grain structure may be present, for example, in articles fabricated via casting. 
     During or before the forging treatment illustrated in the cross-sectional view  302 , the metal article  104  is heated to the forging temperature by the molten salt  136 , as described with reference to  FIG. 2 . During the forging treatment, the pressure of the molten salt bath  146  is exerts a force on every surface of the metal article  104  that is exposed to the molten salt bath  146 . In the cross-sectional view  302 , the metal article  104  has a shape defined by an outer surface  306  and an inner surface  312 . In the example illustrated, the inner surface  312  is adjacent to or in contact with the mold  148  and is not directly exposed to the molten salt bath  146 ; however, the outer surface  306  is exposed to the molten salt bath  146 . Thus, the forging pressure exerts a forging force against the outer surface  306 . At each point on the outer surface  306 , the forging force has a component that is normal to the outer surface  306 . 
     The forging force tends to press the metal article  104  against the mold  148  such that the shape of the metal article  104  (e.g., the shape of the inner surface  312 ), is conformed to a target feature shape defined by the mold  148 . For example, after the forging treatment, the inner surface  312  has an inner diameter corresponding to an outer diameter of the mold  148 . Further, as explained with reference to  FIG. 2 , the forging force tends to press grains of the metal article  104  together, which can increase the density of the metal article  104  and realign grain boundaries to be roughly parallel with the outer and inner surfaces  306 ,  312 . Thus, in the cross-sectional view  304 , the metal article  104  has a second grain structure  308 B. In the second grain structure  308 B, grain boundaries are more aligned with the outer surface  306  of the metal article  104  than was the case for the first grain structure  308 A. The second grain structure  308 B can also be denser than the first grain structure  308 A. As a result of the changes in the grain structure, the metal article  104  will tend to be stronger and more ductile after the forging treatment than before the forging treatment. 
       FIG. 4  is a flow chart of an example of a method of forging treatment of the metal article of  FIG. 1 . For example, the method  400  or one or more operations of the method  400  can be performed by the system  100 . In  FIG. 4 , the method  400  includes, at  402 , fabricating a metal article using one or more of an additive manufacturing process, a casting process, or a machining process. For example, the fabrication device(s)  102  of  FIG. 1  can fabricate the metal article  104  as explained with reference to  FIG. 1 . In some implementations, the forging treatment system  101  can be performed separately from the fabrication process, in which case, the method  400  can omit the fabrication of the metal article. For example, the metal article can be purchased or otherwise obtained rather than fabricated. 
     In  FIG. 4 , the method  400  also includes, at  404 , disposing at least a portion of the metal article in a molten salt bath in a pressure vessel. The molten salt bath is at a bath temperature that is below a melting point temperature of the metal article. For example, the metal article  104  of  FIG. 1  can be placed in the forging pressure vessel  106  and the first pump  138  can pump molten salt  136  from the molten salt tank  134  into the forging pressure vessel  106  to form the molten salt bath  146 . The molten salt bath  146  at least partially submerges the metal article  104  in the forging pressure vessel  106 . 
     The method  400  also includes, at  406 , pressurizing the molten salt bath in the pressure vessel to a forging pressure sufficient to cause pressure-driven grain changes in the metal article. For example, the first pump  138  can pressurize the molten salt bath  146  to the forging pressure. The metal article  104  can remain in the pressurized molten salt bath  146  for a forging period (e.g., corresponding to the forging time  160 ) that is specified in a process recipe  132  and is selected to allow the metal article  104  to undergo pressure-drive grain changes (e.g., forging) sufficient to achieve the target grain characteristics  162 . 
     In some implementations, the method  400  can also include post-forging processes. For example, in  FIG. 4 , the method  400  includes a quenching operation that includes, at  408 , after maintaining the molten salt bath at the forging pressure for the forging period, disposing at least the portion of the metal article in a quench medium. Performing the quenching operation can include extracting the molten salt from the pressure vessel, at  410 ; introducing the quench medium into the pressure vessel, at  412 ; and venting from the pressure vessel gases released by the quench medium, at  414 . For example, the first pump  138  can extract the molten salt  136  of the molten salt bath  146  from the forging pressure vessel  106 . Subsequently, the second pump  144  can introduce the quench medium  142  into the forging pressure vessel  106  to form the quench bath  164 . The gases  168  liberated when the quench medium  142  is introduced into the forging pressure vessel  106  can be vented by the vapor control system  114 . 
     In the example illustrated in  FIG. 4 , the method  400  also includes a heat treatment process, which includes, at  416 , after disposing at least the portion of the metal article in the quench medium, heat treating the metal article in the molten salt bath at a pressure less than the forging pressure. For example, the heat treatment process includes extracting the quench medium from the pressure vessel, at  418 , and introducing molten salt into the pressure vessel, at  420 . 
     The method  400  enables modifying a grain structure of a metal article after the metal article is fabricated. For example, the metal article can be fabricated using a fabrication technique that is selected based on cost, availability of equipment or tools, availability of materials, speed, efficiency (e.g., the amount of waste produced), availability of skilled labor, or any other relevant factor. Subsequently, the metal article can be treated via the method  400  to modify the grain structure of the metal article to have a target set of characteristics. 
     The illustrations of the examples described herein are intended to provide a general understanding of the structure of the various implementations. The illustrations are not intended to serve as a complete description of all of the elements and features of apparatus and systems that utilize the structures or methods described herein. Many other implementations may be apparent to those of skill in the art upon reviewing the disclosure. Other implementations may be utilized and derived from the disclosure, such that structural and logical substitutions and changes may be made without departing from the scope of the disclosure. For example, method operations may be performed in a different order than shown in the figures or one or more method operations may be omitted. Accordingly, the disclosure and the figures are to be regarded as illustrative rather than restrictive. 
     Moreover, although specific examples have been illustrated and described herein, it should be appreciated that any subsequent arrangement designed to achieve the same or similar results may be substituted for the specific implementations shown. This disclosure is intended to cover any and all subsequent adaptations or variations of various implementations. Combinations of the above implementations, and other implementations not specifically described herein, will be apparent to those of skill in the art upon reviewing the description. 
     The Abstract of the Disclosure is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, various features may be grouped together or described in a single implementation for the purpose of streamlining the disclosure. Examples described above illustrate but do not limit the disclosure. It should also be understood that numerous modifications and variations are possible in accordance with the principles of the present disclosure. As the following claims reflect, the claimed subject matter may be directed to less than all of the features of any of the disclosed examples. Accordingly, the scope of the disclosure is defined by the following claims and their equivalents.