Patent Application
20200130268
Embodiments of the present disclosure generally related to additive manufacturing, and more particularly, to cooling of a deposit formed via an additive manufacturing process.
Additive manufacturing techniques, such as directed energy deposition for example, may be used to fabricate a component having a predetermined three-dimensional structure. Such a process typically includes depositing a material layer-by-layer or volume-by-volume to form the structure, rather than by removing material from an existing component. Additive manufacturing techniques may improve efficiency and reduce waste while expanding manufacturing capabilities, such as by permitting seamless construction of complex configurations which, when using conventional manufacturing techniques, would have to be assembled from two or more separate components. The opportunity for additive techniques to replace subtractive processes depends on several factors, such as the range of materials available for use in the additive processes, the size and surface finish that can be achieved using additive techniques, and the rate at which material can be added. Additive manufacturing techniques may advantageously be capable of fabricating complex precision net-shape components ready for use. In some cases, however, the additive techniques may generate “near-net shape” products that require some degree of finishing.
One type of additive manufacturing process is a direct energy deposition process. During a direct energy deposition process, powder is injected from one or more nozzles into a focused beam of a laser. The localized heating melts a small area on the substrate and powder contacting the melt pool will also melt to generate a deposit on the substrate upon solidification.
Direct energy deposition can generate high residual stresses as a result of the geometry being built, the build strategy employed, use of materials with very different thermal expansions, or other factors. In addition, it is difficult to control the microstructure, and therefore the mechanical properties, of the component being formed.
According to an embodiment, an additive manufacturing assembly includes a substrate, a nozzle for depositing additive material onto the substrate, and at least one cooling nozzle for supplying a cooling fluid to at least a portion of the substrate. The at least one cooling nozzle is movable relative to the substrate. A controller is operably coupled to the cooling nozzle. The controller is programmed to control operation of the at least one cooling nozzle to achieve a desired convection heat transfer coefficient of the additive material.
In addition to one or more of the features described above, or as an alternative, in further embodiments the controller is programmed to control an angle of the at least one cooling nozzle relative to the substrate.
In addition to one or more of the features described above, or as an alternative, in further embodiments the controller is programmed to control a flow rate of the cooling fluid output from the at least one cooling nozzle.
In addition to one or more of the features described above, or as an alternative, in further embodiments comprising an energy source for directing an energy beam onto the substrate, the energy source being coupled to the controller and movable relative to the substrate.
In addition to one or more of the features described above, or as an alternative, in further embodiments the at least one cooling nozzle is movable with the energy source.
In addition to one or more of the features described above, or as an alternative, in further embodiments the energy beam forms a melt pool in the substrate, and the controller is programmed to position the at least one nozzle such that the cooling fluid is directed toward a trailing edge of the melt pool.
In addition to one or more of the features described above, or as an alternative, in further embodiments the at least one cooling nozzle includes a plurality of cooling nozzles, the plurality of cooling nozzles being positioned circumferentially about the energy source.
In addition to one or more of the features described above, or as an alternative, in further embodiments the energy source is movable relative to the substrate in a scan direction, a portion of plurality of cooling nozzles is arranged forward of the energy source with respect to the scan direction and another portion of the plurality cooling nozzles is disposed behind the energy source with respect to the scan direction.
In addition to one or more of the features described above, or as an alternative, in further embodiments the controller controls the another portion of the plurality cooling nozzles to achieve the desired convection heat transfer coefficient.
In addition to one or more of the features described above, or as an alternative, in further embodiments the portion of plurality of cooling nozzles arranged forward of the energy source with respect to the scan direction is non-operational.
In addition to one or more of the features described above, or as an alternative, in further embodiments the desired convection heat transfer coefficient is selected to control a morphology of the additive material deposited on the substrate.
According to an embodiment, a method of forming a three-dimensional build object on a substrate comprising depositing additive material onto the substrate and controlling a convective heat transfer coefficient of the additive material deposited onto the substrate.
In addition to one or more of the features described above, or as an alternative, in further embodiments controlling the convection heat transfer coefficient includes directing a flow of cooling fluid toward the substrate.
In addition to one or more of the features described above, or as an alternative, in further embodiments directing a flow of cooling fluid toward the substrate includes operating at least one cooling nozzle positioned behind an energy source relative to a scan direction.
In addition to one or more of the features described above, or as an alternative, in further embodiments controlling the convection heat transfer coefficient includes controlling an orientation of one or more cooling nozzles relative to the substrate.
In addition to one or more of the features described above, or as an alternative, in further embodiments controlling the convection heat transfer coefficient includes controlling a flow rate of the cooling fluid.
In addition to one or more of the features described above, or as an alternative, in further embodiments depositing additive material onto the substrate further comprises directing an energy beam toward a surface of the substrate to form a melt pool and directing a mixture of additive material and propellant gas onto the melt pool.
In addition to one or more of the features described above, or as an alternative, in further embodiments directing the flow of cooling fluid toward the substrate includes directing the flow of cooling fluid toward a trailing edge of the melt pool.
In addition to one or more of the features described above, or as an alternative, in further embodiments comprising selecting the convective heat transfer coefficient based on a desired morphology of the additive material.
The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike:
A detailed description of one or more embodiments of the disclosed apparatus and method are presented herein by way of exemplification and not limitation with reference to the Figures.
With reference now to
The melt-pool 28 may include liquefied material from the substrate 26 as well as additive material. In the illustrated, non-limiting embodiment, additive material, such as a powdered material for example, may be directed onto the melt-pool 28 in an additive material and propellant gas mixture 30 exiting one or more nozzles 32. The nozzles 32 may fluidly communicate with a material reservoir 34 and a propellant gas reservoir 36. The nozzles 32 create a flow of the additive material and propellant gas mixture 30 that may substantially converge into an apex or region of smallest physical cross-section so that the material of the additive material and propellant gas mixture 30 is incorporated into the melt-pool 28. The material deposition assembly 20 is movable relative to the substrate 26 to forms a layer on the substrate 26. Additional layers may be formed adjacent to or on top of the initial layer to fabricate solid, three-dimensional objects.
While the additive material deposited onto the substrate 26 is illustrated and described as being in powdered form, it should be understood that embodiments where the additive material has another form are also within the scope of the disclosure. For example, the additive material may be provided as a wire feed material, a foil material, or any other type of material known for use in additive manufacturing processes.
In certain applications, the additive manufacturing process can generate high residual stresses in the component or build part being formed. These high residual stresses are a result of several factors including, but not limited to, the geometry of the component, the build strategy used, and the materials being used. However, by controlling the rate at which the component is cooled during formation, these residual stresses may be reduced. Accordingly, it may be advantageous to control the cooling of the deposited additive material and/or the area surrounding the deposited additive material during an additive manufacturing process.
In an embodiment, the rate at which the component is cooled during formation may be controlled via the heat transfer coefficient of the deposited additive material, also referred to herein as the “convection coefficient.” More specifically, this convection coefficient of the additive material may be manipulated to achieve a desired temperature gradient ratio at the melt pool 28. The ratio of the temperature gradient is a comparison of the temperature gradient adjacent a top surface of the melt pool 28 to a temperature gradient adjacent a bottom surface of the melt pool 28. During a typical additive manufacturing process, the temperature gradient adjacent the top surface of the melt pool 28 and the temperature gradient adjacent the bottom surface of the melt pool 28 are distinct such that the ratio is very small, such as having a magnitude of about 10−4 for example. By controlling the convection coefficient of the additive material, the temperature gradient adjacent the top surface of the melt pool 28 may be increased, thereby altering the ratio of the temperature gradient.
In an embodiment, the natural convection heat transfer coefficient of an additive material is usually of the order of about 1-5 W/m2K. In an embodiment, additional cooling, such as forced convection for example, may be provided to the deposited additive material during the additive manufacturing process to increase the convection heat transfer coefficient approximately two orders of magnitude greater than the natural convection heat transfer coefficient. For example, with the application of forced cooling, the convection heat transfer coefficient may be 100 W/m2K. The additional cooling provided by this increased convection coefficient may be used to remove heat from the hot sections of the melt pool 28 and/or from the material being deposited to form the build part.
In an embodiment, the convection coefficient, and the resulting temperature gradient at the top surface of the melt pool 28 is controlled to achieve desired mechanical properties or a morphology of the deposited material. For example, by controlling the convention coefficient, the deposited additive material may have a refined microstructure. In an embodiment, the structure of the deposited material may include columnar grains having a fine dendritic structure. With the additional cooling resulting in an increased convection coefficient, the dendrite spacing may be reduced from about 1 micron (in embodiments absent forced cooling) to about 250 nm as a result of the fast cooling. Similarly, the convection coefficient may be controlled to achieve a reduction in grain size. In an embodiment, the grain size achieved during an additive manufacturing process including forced cooling is less than half the grain size achieved during an additive manufacturing process with a natural convection coefficient (absent forced cooling). In addition, the yield strength of the structure being formed may increase by up to, or even more than, 30%.
As previously suggested, the convection coefficient may be controlled during an additive manufacturing process by applying forced cooling to the process, such as forced convection for example. With reference now to
The cooling nozzles 40 positioned about the energy source 22 may be controlled to facilitate further or forced cooling via convection to the substrate 26 based on the scan direction. As shown, the cooling nozzles 40 may be divided into various zones or regions based on their position about the energy source 22 and the scan directions available to the assembly 20. In an embodiment, the cooling nozzles 40 are operated to cool the non-solidified region or mushy portion of the build object, indicated at 42 in
In the illustrated, non-limiting embodiment, the cooling nozzles 40 are generally positioned within a first zone 46, disposed adjacent a first side of the energy source 22, or a second zone 48, disposed adjacent a second, opposite side of the energy source 22. As the energy source 22 and cooling nozzles 40 move in a first scan direction, indicated by arrow A, the cooling nozzles 40 in the first zone 46 are operated to deliver a fluid flow, thereby providing forced convection, that supplements the natural convection of the substrate 26 and deposited material. During movement in this first scan direction, the cooling nozzles 40 within the second zone 48 may be non-operational. When the energy source 22 and cooling nozzles 40 move in a second scan direction, indicated by arrow B, the cooling nozzles 40 within the second zone 48 are operated to provide forced convection, and the cooling nozzles 40 within the first zone 46 are non-operational. In an embodiment, the cooling nozzles 40 are movably mounted. Accordingly, during operation of one or more of the cooling nozzles 40, an orientation of the nozzle relative to the surface of the deposited additive material and/or substrate 26 is adjustable, as indicated by the angle θ2. In addition, the velocity or flow rate at which a fluid is output from the cooling nozzles 40 may be controlled.
The cooling system may additionally include a controller, illustrated schematically at 50, operatively coupled to the one or more cooling nozzles 40. The controller 50 may be the same controller, or alternatively, may be a different controller than the controller associated with operation of the energy source 22 and/or the deposition nozzle 32 (
Controlling the convective coefficient of the additive manufacturing process through the use of one or more cooling nozzles 40 enables a reduction in distortion for a component having a complex geometry. In addition, the cooling allows for a refinement of the microstructure of the component being formed, such as a reduction in grain size and dendrite spacing, and increased mechanical properties.
The term “about” is intended to include the degree of error associated with measurement of the particular quantity based upon the equipment available at the time of filing the application.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present 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, element components, and/or groups thereof.
While the present disclosure has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the essential scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this present disclosure, but that the present disclosure will include all embodiments falling within the scope of the claims.
