The subject matter of the present disclosure generally relates to metal infiltration, and more particularly relates to infiltration of three-dimensional structures having grated macro-porosity.
Metal infiltration is a processing technique by which a preform of one metal or alloy is infiltrated with a liquid infiltrant of a different metal or alloy to fill void space in the preform to create a final densified part. There are various material systems for the preform material and the infiltrant that are known in the art. It is desirable in certain instances to fabricate a preform for infiltration using three-dimensional printing processes as these processes provide excellent ability to accommodate varying part geometries, rapid prototyping and generally avoid the need for molds, which are expensive and limit part geometry. There are however certain difficulties that may be encountered in doing.
In infiltrated structures, the infiltrant and the preform material often have fairly different mechanical properties because the infiltrant typically must have a lower melting point than the preform to avoid dissolving the preform. Thus, the infiltrant is usually of lower strength, hardness and/or stiffness. As a result, the properties of the resulting material are dictated largely by the ratio of the two phases—the ratio of the infiltrant phase to the preform phase. Unfortunately, it is often difficult to independently control the ratio of the two phases when formulating the preform using additive manufacturing processes for reasons now described.
Take for example the infiltration of a preform fabricated using a powder bed additive manufacturing process. To form the preform powders are spread layer-by-layer, with binder jetted from a print head to define the preform geometry. After the preform is formed, loose powder around the preform is removed and much of the binder is removed. In an emblematic aluminum-aluminum infiltration system, the aluminum preform is then nitrided to form an aluminum nitride skeleton. The aluminum nitride preform may then be infiltrated with liquid aluminum, which has a lower melting temperature than the aluminum nitride. This nitriding and infiltration process is described in U.S. Pat. No. 7,036,550 entitled “Infiltrated Aluminum Preforms” and filed Mar. 15, 2004, the entire contents of which are incorporated by reference herein.
In a powder bed additive manufacturing process as described above, the apparent density of the powder is the density returned by measuring the volume occupied by a known mass of powder flowed freely into a standardized measuring device. The tap density of the powder is the density returned as the result of flowing an amount of powder into a graduated cylinder which is then mechanically tapped until further volume reduction is minimalized. The volume fraction of the preform material printed must, by virtue of the layer-by-layer spreading, fall between the apparent density of the powder and the tap density of the powder.
Some powders can be spread to either above or below this range, but this generally defines the range that can be achieved. For the flowable powders typically used in powder bed additive manufacturing processes, the apparent densities are typically greater than 54 percent by volume, and the tap densities are typically less than 65 percent by volume. This range, 54-63 percent, represents a very small fraction of potential design space and limits the maximum volume fraction of infiltrant. A result of this confined volume fraction range is a confined range of properties that may be achieved by the resulting composites for a given infiltrant and preform composition. It is desirable to be able to engineer a wider range of properties in such composites, and thus techniques to expand the available volume fractions range are desirable.
Similar volume fraction constraints exist for other additive manufacturing techniques such as thermoplastic extrusion of bound metal composites due to rheological concerns and concerns regarding packing and dimensional stability of the structures during the thermal processing required by such methods.
As an example of the result of such difficulties, in the aluminum-aluminum infiltration system described above, the aluminum nitride has a large difference in mechanical properties as compared to the aluminum infiltrant. Normally, one would tune the properties of the composite by adjusting the volume fraction of each. However, the above described limited volume fraction range restricts the tuning and thus mechanical properties that can be accomplished.
In certain situations, it is further advantageous or desired to vary the volume fraction of infiltrant over the course of geometry. For example, a higher volume fraction of infiltrant may be desirable at one end of a part to increase ductility, while a lower volume fraction of infiltrant is desired at the other end of a part to increase tensile strength.
Further techniques and disclosure related to infiltration can be found in U.S. Patent Publication No. 2018/0305266-A1 titled “Additive Fabrication of Infiltrable Structures” and filed Apr. 24, 2018, the entire contents of which are incorporated herein in their entirety.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.
Disclosed is method of fabricating an infiltrated part including selective areas having a high volume fraction of infiltrant.
The manufacturing process for a three-dimensional object of a desired shape begins with the forming of a build material into a skeleton, the build material including a metal powder and a binder system. The skeleton includes graded macro-porosity having a void volume. The binder system is at least partially debinded and the skeleton infiltrated with an infiltrant. The infiltrant occupies the void volume of the macro-porosity.
In certain areas portions of the object, the volume fraction of micro-porosity may be increased relative to other portions of the object, creating an object having a varied infiltrant volume fraction gradient and thus a gradient of mechanical properties.
The foregoing summary, preferred embodiments, and other aspects of the present disclosure will be best understood with reference to a detailed description of specific embodiments, which follows, when read in conjunction with the accompanying drawings, in which:
Like reference numbers and designations in the various drawings indicate like elements.
In general, by controlling an additive manufacturing process used to form an initial structure, a skeleton formed from the initial structure may define an interconnected porous network having controlled (e.g., graded) macro-porosity. As described in greater detail below, the controlled macro-porosity may be graded along the skeleton (e.g., having a predetermined variation in macro-porosity along one or more dimensions of the skeleton). Such graded macro-porosity may be useful, for example, for facilitating appropriate structural strength of the skeleton, with a lower amount of macro-porosity along portions of the skeleton requiring more support (e.g., toward the bottom of a part, overhanging features or fine details) and higher amounts of macro-porosity along portions of the skeleton requiring less support (e.g., toward the top of a part). As also described in greater detail below, infiltration of the skeleton with an infiltration material may advantageously produce a finished part having a predetermined spatial variation in material properties.
As used herein, the term “macro-porosity” should be understood to refer to pores having a smallest dimension larger than an average size of the particles of the metal powder used to form the skeleton. Thus, for example, in instances in which the average size of the particles is 20 microns, macro-porosity refers to pores having a smallest dimension greater than 20 microns. Further, macro-porosity should be understood to be distinguished from “micro-porosity,” which refers to the porosity having a smallest dimension less than the average particle size of the particles of the metal powder and is formed in the void between contacting particles of the metal powder. As a general principle, macro-porosity may be introduced into the skeleton through controlling one or more parameters of an additive manufacturing process while micro-porosity is largely a function of the average particle size of the particles used to form the skeleton.
In general, unless otherwise specified or made clear from the context, any one or more of the various different additive manufacturing techniques described herein may be used to form the initial structure with features that result in a target distribution of macro-porosity (e.g., graded macro-porosity) in the skeleton formed from the initial structure. For example, in the case of bound metal deposition, tracks of the build material may be manipulated to form the initial structure with features that result in the target distribution of macro-porosity in the skeleton formed from the initial structure. Additionally, or alternatively, in the case of powder bed binder jetting, droplet size, saturation, or a combination thereof may be controlled to control macro-porosity in the skeleton formed from the initial structure. That is, as compared to smaller droplets, the use of larger droplets may create larger capillary forces that pull the powder particles toward one another in the initial structure—a condition that is a precursor to increased macro-porosity in the skeleton. Accordingly, in instances in which the initial structure is formed through binder jetting, graded macro-porosity may be achieved in the corresponding skeleton by controlling droplet size in the binder jetting process used to form the initial structure. More generally, macro-porosity of the skeleton may be controlled by controlling surface tension of the binder delivered to a top layer of a powder bed during a binder jetting process. While the control of such surface tension has been described as being a function of the one or more of droplet size and saturation, it should be appreciated that such surface tension may further or instead be controlled through the addition of one or more other materials (e.g., moisture) to the surface to control surface tension forces of the binder.
Densification of the initial structure to form the skeleton may be carried out according to any one or more of various techniques (e.g., thermal, chemical, or a combination thereof) useful for removing one or more components of a binder system and sintering metal particles to one another. For example, unless otherwise specified or made clear from the context, densification may be carried out using any one or more of the various densification techniques described herein. In an aluminum system where an aluminum skeleton is nitrided, the skeleton may be partially sintered to form a substantially stable structure prior to infiltration.
In certain implementations, the initial structure may be additively manufactured using a first metal alloy and densified through post processing to form the skeleton defining the interconnected porous network having graded macro-porosity. The skeleton may be infiltrated with a second metal alloy, different from the first metal alloy, such that the resultant infilled composite structure has a gradient of the first metal alloy (used to form the skeleton) and the second metal alloy (used as the infiltration material). Thus, to the extent the first metal alloy and the second metal alloy have different material properties, it should be generally understood that infiltration of the graded macro-porosity of the skeleton by the second metal alloy may facilitate formation of parts having spatially-varying material properties. That is, where a higher concentration of the second metal alloy is desirable in a particular area or along a particular axis, the corresponding portion of the skeleton may define a higher volume fraction of macro-porosity.
As an example, the use of a skeleton defining graded macro-porosity may be used to form aluminum parts having spatial variations in material properties. Forming aluminum-alloy parts using powdered metal additive manufacturing processes can be challenge primarily due to poor sinterability attributable to the oxide layer that forms on the skin of the aluminum alloy surface. This skin is difficult to reduce and generally hinders the sintering process which, in turn, adversely impacts the ability to form dense aluminum parts. However, as described in greater detail below, densification of aluminum-based parts through infiltration is generally not subject to the challenges associated with sintering aluminum-based parts and, therefore, may facilitate formation of dense aluminum parts.
In certain implementations, the initial structure may include a first aluminum-based material held together with a binder. The skeleton may be formed, for example, by removing the binder and sintering the initial structure to near full density using a nitrogen atmosphere. Continuing with this example, the resulting skeleton may be a dense, complex-shaped three-dimensional structure having graded macro-porosity, and the surface of the skeleton may have some aluminum-nitrided structure. This skeleton may be infiltrated with a second aluminum-based material, different from the first aluminum-based material. Because the distribution of second aluminum-based material follows the graded macro-porosity, the resulting part should be understood to have a graded distribution of the first aluminum-based material and the second aluminum-based material.
In general, the first aluminum-based material and the second aluminum-based material may be any one or more of various aluminum-based materials compatible with one another in the formation of a structure and having at least one different physicochemical property. The first aluminum-based material may include an aluminum alloy or an aluminum alloy-based composite structure. Similarly, the second aluminum-based material may include an aluminum alloy or an aluminum alloy-based composite structure. As an example, an aluminum alloy useful as the second aluminum-based material is Al-10Si alloy.
While the infiltratable structures have been described as being formed using a first aluminum-based material and a second aluminum-based material, unless otherwise specified or made clear from the context, it should be generally understood that any of the techniques associated with infiltratable structures described herein may be used with any combination of materials that may be usable in combination as a skeleton and an infiltratable material.
While infiltration may be carried out with a part under normal atmospheric pressure, other conditions may further or instead facilitate infiltration of an infiltration material into a skeleton defining an interconnected porous network. For example, the infiltration of the skeleton may be carried out in a furnace under vacuum conditions (e.g., a partial vacuum). As compared to infiltration in air under normal atmospheric pressure, infiltration under vacuum conditions may be useful for achieving improved penetration of the infiltration material into the skeleton. Additionally, or alternatively, infiltration of the infiltration material into the skeleton may be carried out in a pressurized furnace. Infiltration under such pressurized conditions may be useful for achieving faster penetration of the infiltration material into the skeleton, particularly in instances in which the skeleton has large pores.
Debinding subsystem 104 may be configured to treat the printed object by performing a first debinding process, in which the primary binder material may be removed. In some embodiments, the first debinding process may be a chemical debinding process, as will be described in further detail with reference to
In other embodiments, the first debinding process may comprise a thermal debinding process. In such embodiments, the primary binder material may have a vaporization temperature lower than that of the secondary binder material. The debinding subsystem 104 may be configured to heat the deposited build material to a temperature at or above the vaporization temperature of the primary binder material and below the vaporization temperature of the secondary binder material such that the primary binder material is removed from the printed part. In alternative embodiments, the furnace subsystem 106 rather than a separate heating debinding subsystem 104 may be configured to perform the first debinding process. For example, the furnace subsystem 106 may be configured to heat the deposited build material to a temperature at or above the vaporization temperature of the primary binder material and below the vaporization temperature of the secondary binder material such that the primary binder material is removed from the deposited build material.
Furnace subsystem 106 may be configured to treat the printed object by performing a secondary thermal debinding process (or also a primary debinding process, as in the alternative embodiment described above), in which the secondary binder material and/or any remaining primary binder material may be vaporized and removed from the printed part. In some embodiments, the secondary debinding process may comprise a thermal debinding process, in which the furnace subsystem 106 may be configured to heat the part to a temperature at or above the vaporization temperature of the secondary binder material to remove the secondary binder material. The furnace subsystem 106 may then heat the part to a temperature just below the melting point of the metal powder to sinter the metal powder and to densify the metal powder into a solid metal part.
As shown in
Metal 3D printing subsystem 102, debinding subsystem 104, furnace subsystem 106, user interface 110, and/or control subsystem 116 may each be connected to the other components of system 100 directly or via a network 112. Network 112 may include the Internet and may provide communication through one or more computers, servers, and/or handheld mobile devices, including the various components of system 100. For example, network 112 may provide a data transfer connection between the various components, permitting transfer of data including, e.g., part geometries, printing material, one or more support and/or support interface details, printing instructions, binder materials, heating and/or sintering times and temperatures, etc., for one or more parts or one or more parts to be printed.
Moreover, network 112 may be connected to a cloud-based application 114, which may also provide a data transfer connection between the various components and cloud-based application 114 in order to provide a data transfer connection, as discussed above. Cloud-based application 114 may be accessed by a user in a web browser, and may include various instructions, applications, algorithms, methods of operation, preferences, historical data, etc., for forming the part or object to be printed based on the various user-input details. Alternatively or additionally, the various instructions, applications, algorithms, methods of operation, preferences, historical data, etc., may be stored locally on a local server (not shown) or in a storage and/or processing device within or operably coupled to one or more of metal 3D printing subsystem 102, debinding subsystem 104, sintering furnace subsystem 106, user interface 110, and/or control subsystem 116. In this aspect, metal 3D printing subsystem 102, debinding subsystem 104, furnace subsystem 106, user interface 110, and/or control subsystem 116 may be disconnected from the Internet and/or other networks, which may increase security protections for the components of system 100. In either aspect, an additional controller (not shown) may be associated with one or more of metal 3D printing subsystem 102, debinding subsystem 104, and furnace subsystem 106, etc., and may be configured to receive instructions to form the printed object and to instruct one or more components of system 100 to form the printed object.
Metal 3D printing subsystem 102 may include an extrusion assembly 126 comprising an extrusion head 132. Metal 3D printing subsystem 102 may include an actuation assembly 128 configured to propel the build material 124 into the extrusion head 132. For example, the actuation assembly 128 may be configured to propel the build material 124 in a rod form into the extrusion head 132. In some embodiments, the build material 124 may be continuously provided from the feeder assembly 122 to the actuation assembly 128, which in turn propels the build material 124 into the extrusion head 132. In some embodiments, the actuation assembly 128 may employ a linear actuation to continuously grip and/or push the build material 124 from the feeder assembly 122 towards the extrusion head 132.
In some embodiments, the metal 3D printing subsystem 102 includes a heater 134 configured to generate heat 136 such that the build material 124 propelled into the extrusion head 132 may be heated to a workable state. In some embodiments, the heated build material 124 may be extruded through a nozzle 133 to extrude workable build material 142 onto a build plate 140. It is understood that the heater 134 is an exemplary device for generating heat 136, and that heat 136 may be generated in any suitable way, e.g., via friction of the build material 124 interacting with the extrusion assembly 126, in alternative embodiments. While there is one nozzle 133 shown in
In some embodiments, the metal 3D printing subsystem 102 comprises a controller 138. The controller 138 may be configured to position the nozzle 133 along an extrusion path relative to the build plate 140 such that the workable build material is deposited on the build plate 140 to fabricate a three dimensional printed object 130. The controller 138 may be configured to manage operation of the metal 3D printing subsystem 102 to fabricate the printed object 130 according to a three-dimensional model. In some embodiments, the controller 138 may be remote or local to the metallic printing subsystem 102. The controller 138 may be a centralized or distributed system. In some embodiments, the controller 138 may be configured to control a feeder assembly 122 to dispense the build material 124. In some embodiments, the controller 138 may be configured to control the extrusion assembly 126, e.g., the actuation assembly 128, the heater 134, the extrusion head 132, and/or the nozzle 133. In some embodiments, the controller 138 may be included in the control subsystem 116.
The debinding fluid contained in the storage chamber 156 may be directed to the process chamber 150 containing the inserted printed object 130. In some embodiments, the build material that the printed object 130 is formed of may include a primary binder material and a secondary binder material. In some embodiments, the printed object 130 in the process chamber 150 may be submerged in the debinding fluid for a predetermined period of time. In such embodiments, the primary binder material may dissolve in the debinding fluid while the secondary binder material stays intact.
In some embodiments, the debinding fluid containing the dissolved primary binder material (hereinafter referred to as “used debinding fluid”) may be directed to a distill chamber 152. For example, after the first debinding process, the process chamber 150 may be drained of the used debinding fluid, and the used debinding fluid may be directed to the distill chamber 152. In some embodiments, the distill chamber 152 may be configured to distill the used debinding fluid. In some embodiments, the debinding subsystem 104 may further include a waste chamber 154 fluidly coupled to the distill chamber 152. In such embodiments, the waste chamber may collect waste accumulated in the distill chamber 152 as a result of the distillation. In some embodiments, the waste chamber 154 may be removably attached to the debinding subsystem 104 such that the waste chamber 154 may be removed and replaced after a number of distillation cycles. In some embodiments, the debinding subsystem 104 may include a condenser 158 configured to condense vaporized used debinding fluid from the distill chamber 152 and return the debinding fluid back to the storage chamber 156.
The build material may be a bulk metallic powder delivered and spread in successive layers. The binder material may be, for example, a polymeric liquid that may be deposited onto and may be absorbed into layers of the build material. One or more of binder jet fabrication subsystem 202, de-powdering subsystem 204, and furnace subsystem 106 may include a shaping station to shape the printed object and a debinding station to treat the printed object to remove a binder material from the build material. Furnace subsystem 106 may heat and/or sinter the build material of the printed object. System 200 may also include a user interface 210, which may be operatively coupled to one or more components, for example, to binder jet fabrication subsystem 202, de-powdering subsystem 204, and furnace subsystem 106, etc. In some embodiments, user interface 210 may be a remote device (e.g., a computer, a tablet, a smartphone, a laptop, etc.). User interface 210 may be wired or wirelessly connected to one or more of binder jet fabrication subsystem 202, de-powdering subsystem 204, and furnace subsystem 106. System 200 may also include a control subsystem 216, which may be included in user interface 210, or may be a separate element.
Binder jet fabrication subsystem 202, de-powdering subsystem 204, furnace subsystem 106, user interface 210, and/or control subsystem 216 may each be connected to the other components of system 200 directly or via a network 212. Network 212 may include the Internet and may provide communication through one or more computers, servers, and/or handheld mobile devices, including the various components of system 200. For example, network 212 may provide a data transfer connection between the various components, permitting transfer of data including, e.g., geometries, the printing material, one or more support and/or support interface details, binder materials, heating and/or sintering times and temperatures, etc., for one or more parts or one or more parts to be printed.
Moreover, network 212 may be connected to a cloud-based application 214, which may also provide a data transfer connection between the various components and cloud-based application 214 in order to provide a data transfer connection, as discussed above. Cloud-based application 214 may be accessed by a user in a web browser, and may include various instructions, applications, algorithms, methods of operation, preferences, historical data, etc., for forming the part or object to be printed based on the various user-input details. Alternatively or additionally, the various instructions, applications, algorithms, methods of operation, preferences, historical data, etc., may be stored locally on a local server (not shown) or in a storage and/or processing device within or operably coupled to one or more of binder jet fabrication subsystem 202, de-powdering subsystem 204, furnace subsystem 106, user interface 210, and/or control subsystem 216. In this aspect, binder jet fabrication subsystem 202, de-powdering subsystem 204, furnace subsystem 106, user interface 210, and/or control subsystem 216 may be disconnected from the Internet and/or other networks, which may increase security protections for the components of system 200. In either aspect, an additional controller (not shown) may be associated with one or more of binder jet fabrication subsystem 202, de-powdering subsystem 204, and furnace subsystem 206, etc., and may be configured to receive instructions to form the printed object and to instruct one or more components of system 200 to form the printed object.
Spreader 222 may be movable across powder bed 224 to spread a layer of powder, from powder supply 220, across powder bed 224. Print head 226 may comprise a discharge orifice 230 and, in certain implementations, may be actuated to dispense a binder material 232 (e.g., through delivery of an electric current to a piezoelectric element in mechanical communication with binder material 232) through discharge orifice 230 to the layer of powder spread across powder bed 224. In some embodiments, the binder material 232 may be one or more fluids configured to bind together powder particles.
In operation, controller 228 may actuate print head 226 to deliver binder material 232 from print head 226 to each layer of the powder in a pre-determined two-dimensional pattern, as print head 226 moves across powder bed 224. In embodiments, the movement of print head 226, and the actuation of print head 226 to deliver binder material 232, may be coordinated with movement of spreader 222 across powder bed 224. For example, spreader 222 may spread a layer of the powder across powder bed 224, and print head 226 may deliver the binder in a pre-determined, two-dimensional pattern, to the layer of the powder spread across powder bed 224, to form a layer of one or more three-dimensional objects 234. These steps may be repeated (e.g., with the pre-determined two-dimensional pattern for each respective layer) in sequence to form subsequent layers until, ultimately, the one or more three-dimensional objects 234 are formed in powder bed 224.
Although the example embodiment depicted in
An example binder jet fabrication subsystem 202 may comprise a powder supply actuator mechanism 236 that elevates powder supply 220 as spreader 222 layers the powder across powder bed 224. Similarly, build box subsystem 208 may comprise a build box actuator mechanism 238 that lowers powder bed 224 incrementally as each layer of powder is distributed across powder bed 224.
In another example embodiment, layers of powder may be applied to powder bed 224 by a hopper followed by a compaction roller. The hopper may move across powder bed 224, depositing powder along the way. The compaction roller may be configured to follow the hopper, spreading the deposited powder to form a layer of powder.
For example,
Hopper 221 may be any suitable metering apparatus configured to meter and/or deliver powder from powder supply 220′ onto a top surface 223 of powder bed 224′. Hopper 221 may be movable across powder bed 224′ to deliver powder from powder supply 220′ onto top surface 223. The delivered powder may form a pile 225 of powder on top surface 223.
The one or more spreaders 222′ may be movable across powder bed 224′ downstream of hopper 221 to spread powder, e.g., from pile 225, across powder bed 224. The one or more spreaders 222′ may also compact the powder on top surface 223. In either aspect, the one or more spreaders 222′ may form a layer 227 of powder. The aforementioned powder delivery and spreading steps may be successively performed in order to form a plurality of layers 229 of powder. Additionally, although two spreaders 222′ are shown in
Print head 226′ may comprise one or more discharge orifices 230′ and, in certain implementations, may be actuated to dispense a binder material 232′ (e.g., through delivery of an electric current to a piezoelectric element in mechanical communication with binder material 232′) through discharge orifice 230′ to the layer of powder spread across powder bed 224′. In some embodiments, the binder material 232′ may be one or more fluids configured to bind together powder particles.
In operation, controller 228′ may actuate print head 226′ to deliver binder material 232′ from print head 226′ to each layer 227 of the powder in a pre-determined two-dimensional pattern, as print head 226′ moves across powder bed 224′. As shown in
Although the example embodiment depicted in
As in
Although not shown, binder jet fabrication subsystems 202, 202′ may include a coupling interface that may facilitate the coupling and/or uncoupling of the build box subsystems 208, 208′ with the binder jet fabrication subsystems 202, 202′, respectively. The coupling interface may comprise one or more of (i) a mechanical aspect that provides for physical engagement, and/or (ii) an electrical aspect that supports electrical communication between the build box subsystem 208, 208′ to the binder jet fabrication subsystem 202, 202′.
Demonstrated Embodiment
Now described is experimentation conducted to demonstrate the disclosed subject matter, particularly the efficacy of infiltrating three-dimensional printed parts. The experimentation concerned an aluminum-based infiltration compatible with the Studio Printing System produced by DESKTOP METAL, INC. of Burlington, Mass. However, one of ordinary skill in the art to which the present disclosure pertains will readily understand that the results of the below described experimentation is applicable to parts manufactured by other systems or methods and in the use of alternative material systems. Further, the conditions under which the experiment was performed should be understood as exemplary and subject to alteration depending on various criteria, including the desired mechanical characteristics of the final part.
The experimental process involved the production of a resin bonded aluminum powder green part, thermal debinding, partial transformation of the aluminum of the part into an interconnected aluminum nitride network, and finally infiltration with a second aluminum alloy.
The primary conclusions of the experimentation were:
Materials and Experiment Processes
Powders and Feedstocks
There were three different aluminum powders used in the experiment. A metal injection molding (MIM) grade 6061 aluminum from AMPAL, INC. of Palmerton, Penn., a Selective Laser Melting grade 6061 aluminum alloy of LPW TECHNOLOGY INC of Pittsburgh, Penn., and feedstock used to produce 3D printed parts from aluminum feedstock from DESKTOP METAL, INC. of Burlington, Mass. Additions of magnesium and tin were also made to some alloys. The tested three-dimensional printed parts did not contain any magnesium or tin, however this was due only to a lack of immediately available feedstock containing these elements at the time the experiment was performed. Such feedstocks will in the future be readily available, and expected to be mechanically the same or similar to the other tested materials.
The majority of the experimentation was undertaken using cast bars, which were produced from resin bonded powder, as these were easily produced and the results thereof will be applicable to preforms manufactured by a variety of additive manufacturing methods. In this case the resin was a polypropylene powder (SPP-10) from SHAMROCK TECHNOLOGIES, INC. of Newark, N.J. The Magnesium powder used was significantly coarser than desired, and while this this did not seem to have an effect on the process, it would be expected to be advantageous to use finer powder in three-dimensional printing.
The particle size distribution is shown in
Scanning electron images of the powder are shown in
Part Production
The resin-metal powders mixtures were blended for 30 minutes in a Tubular mixer and then poured into steel molds. These were then heated for 30 minutes at 175° C., which melted the resin to hold the metal powder particles together. On cooling, the cast parts had shrunk slightly and could therefore be removed from the mold, as depicted in
Furnace and Furnace Cycle
The furnace used in this work was a 3 zone tube furnace, which was fitted with a 160 mm diameter stainless steel tube and gas/vacuum tight end caps, as depicted in
The stainless steel crucibles had a loose fitting lid that contained two 3 mm diameter holes. This allowed removal of the decomposed resin vapor, but contained any Magnesium vapor and limited the exchange of the atmosphere with the part. This is important as an extremely low oxygen content environment is needed to initiate the nitridation reaction. Samples were placed into the crucible, which also contained a small alumina crucible that was filled with Magnesium turnings, as depicted in
The furnace cycle is summarized in
Results
Weight Gains Under Different Nitridation Conditions
One of the important factors in the success of the infiltration process for the given material is to first create an interconnected aluminum nitride skeleton. The amount of nitride was determined by measuring the weight increase of the preforms, after taking into account the resin loss. Assuming complete resin removal, a weight gain of ˜5% is usually sufficient for structural integrity during infiltration, while full conversion to AlN results in >30% weight gain. In terms of the effect of powder type, alloy composition, flow rate and gas purity on the weight gain of 6061 preforms, it was found that:
The Effect of Time
Generally, the amount of nitridation increases with time, as shown in
The Effect of Gas Purity and Flow Rate
The effect of flow rate and gas purity on the weight gain of the 6061 aluminum preforms is shown in
For the pure 6061 alloys (LPW, Ampal and 3D Printed), the weight gain was strongly dependent on the gas flow rate. This was true for both gas purities. For the LPW and 3D printed parts, there appears to be a critical flow rate above which nitridation occurs. For the finer Ampal powder, there is a more gradual, but still significant, increase in the amount of nitridation with flow rate. In contrast, the LPW magnesium-tin alloy is almost insensitive to the flow rate. The gas purity does not change the overall effect of the flow rate and does not have as big an effect on the nitridation as the flow rate. This indicates that the addition of the magnesium turnings to the crucible is successful in acting as a getter and controlling the local oxygen content.
The optical microstructure of the powders after nitriding for 12 h at 540° C. at 1 slpm in UHP N2 are shown in
Infiltration
Infiltration of the preforms was performed at 700° C. At this temperature, the nitride skeleton maintains structural integrity of the preform as both the base powder and infiltrant are both fully molten. Using this approach, it is possible to change the composition (and therefore properties) of the parts by using different infiltrants.
In order to arrest the nitridation reaction, it is critical that the nitrogen be removed from the furnace at the end of the hold at 540° C. and before heating to 700° C. and holding for 1 h. In this work, three different atmospheres were used for infiltration. In all cases the amount of infiltrant was 120% of the weight of the preform. The first was to evacuate the furnace and immediately back fill with argon prior to heating to 700° C. Infiltration was then completed under argon. The second approach used vacuum through the entire infiltration step, while the third evacuated the furnace and heated to 700° C. and held under vacuum for 50 minutes, after which argon was introduced for the last 10 minutes of the 700° C. hold. As can be seen from
One of the important aspects of the infiltration is to ensure the correct volume of infiltrant is added to the part. Insufficient infiltrant results in large areas of porosity and therefore low properties, while excess infiltrant can bleed out of the part and onto the surface. For cast bars, the optimum amount of infiltrant for the experiment was ˜120% of the weight of the preform, as depicted in
Infiltration of 3D printed parts was more challenging because these parts contained no magnesium or tin, complete nitridation occurred. Thus infiltration only occurred in the spaces between the print lines and also in the voids created by the in-fill pattern, as shown in
The elemental distribution of the final part was determined using EDS mapping and is shown in
Properties
Critical to the success of this alloy system will be the ability to produce acceptable mechanical properties. It has been previous established that the ductility is determined by the amount of nitridation, with low ductility associated with high nitride content. Hence, there is a balance that needs to be made between structural stability (which requires a high amount of nitride) and ductility. The pure 6061 aluminum parts (either cast or 3D printed) completely nitrided during processing to form a porous aluminum nitride structure (see
To gain some insight into the properties of the 3D printed bars, a single bar was tested using 3-point bending. The flexural strength was well under half (and in some cases ¼) the strength on LPW magnesium tin case bars. These properties and that of all the tensile tests are summarized in
The work below was all performed using cast LPW magnesium tin preforms.
Hardness Results
Tensile Properties
The tensile properties of LPW-2Mg-1Sn which had been nitrided for various times at 540° C. are shown in
Increasing the Ductility
There is a limit by which the ductility can be increased via reducing the nitride thickness. At some point, a percolating skeleton does not form and the geometric stability is lost. An alternate way to increase the ductility is to deliberately produce voids in the green part, which would then be filled by the infiltrant. Since these volumes would not contain any aluminum nitride, they should enhance the ductility. Macro-porosity in the form of channels were produced in parts via the addition of thin plates to the mold. These plates were then removed and a second full bar was placed on top. After infiltrating, the bars were cut in two (the lower section contained the channels, while the top section did not) and then the parts were machined into tensile bars.
Schematically, this is shown in
Electrical Conductivity
Electrical conductivity was measured using a handheld eddy current probe. The results, along with that for wrought 6061 are presented in
3.4 Demonstration Part
A 3D printed impeller (inner diameter of 30 mm) was infiltrated using a 12 h hold at 540° C. and then 1 h at 700° C., as depicted in
Summary
In summary, it was confirmed there is an ability to produce dimensional stable infiltrated aluminum preforms. Specifically, it was shown:
Although the disclosed subject matter has been described and illustrated with respect to embodiments thereof, it should be understood by those skilled in the art that features of the disclosed embodiments can be combined, rearranged, etc., to produce additional embodiments within the scope of the invention, and that various other changes, omissions, and additions may be made therein and thereto, without parting from the spirit and scope of the present invention.
Filing Document | Filing Date | Country | Kind |
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PCT/US2019/041697 | 7/15/2019 | WO | 00 |
Number | Date | Country | |
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62697886 | Jul 2018 | US |