Patent Application
20230144833
Additive manufacturing systems produce three-dimensional (3D) objects by building up layers of material. Some additive manufacturing systems use inkjet or other printing technology to apply some of the manufacturing materials. Additive manufacturing systems make it possible to convert a computer-aided design (CAD) model or other digital representation of an object directly into the physical object.
The accompanying drawings illustrate various examples of the principles described herein and are part of the specification. The illustrated examples are given merely for illustration, and do not limit the scope of the claims.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements. The figures are not necessarily to scale, and the size of some parts may be exaggerated to more clearly illustrate the example shown. Moreover, the drawings provide examples and/or implementations consistent with the description; however, the description is not limited to the examples and/or implementations provided in the drawings.
Additive manufacturing systems form a three-dimensional (3D) object through the solidification of layers of build material. Additive manufacturing systems make objects based on data in a 3D model of the object generated, for example, with a computer-aided design (CAD) computer program product. The model data is processed into slices, each slice defining portions of a layer of build material that are to be solidified.
In one particular example, a powder build material is deposited and a binding agent is selectively applied to the layer of powder build material. The binding agent is deposited in a pattern of a slice of a 3D object to be printed. This process is repeated per layer until the 3D object is formed. Such a binding-agent-based system may be used to generate metallic or ceramic 3D objects.
With a 3D object formed, the binding agent is cured to form a “green” 3D object. Cured binding agent holds the build material of the green object together. The binding agent is activated or cured by heating the object. Heating to form the cured green object may take place at a temperature that is capable of activating (or curing) the binder of the binding agent, but that does not thermally decompose the binder. When activated or cured, the binding agent glues the powder build material particles into the cured green object shape. The cured green object has enough mechanical strength that it is able to withstand extraction from the build material platform without being deleteriously affected (e.g., the shape is not lost).
The green 3D object may then be placed in an oven to further heat the green 3D object to sinter the build material to form the finished 3D object. Specifically, the binding agent is removed and the temperature is further raised such that sintering of the powder metal particles occurs to form a 3D object.
While in the oven, further heat is applied to sinter the 3D object thereby increasing its densification to at least about 95 percent densification, in some examples. In some examples, such sintering temperatures may range between about 1000 degrees Celsius to about 1500 degrees Celsius. It is to be understood that the term “green” does not connote color, but rather indicates that the part is not yet fully processed.
In yet another example, a laser, or other power source is selectively aimed at a powder build material, or a layer of a powder build material, to form a slice of a 3D printed object. Such a process may be referred to as selective laser melting where portions of the powder material, which may be metallic, are selectively melted together to form a slice of a 3D printed part.
In one particular example of additive manufacturing referred to as laser fusion, an array of lasers scans each layer of powdered build material to form a slice of a 3D printed object. In this example, each laser beam is turned on and off dynamically during the scanning process according to the image slice. Similar to a fusing agent-based system, this laser fusion process is also layer-by-layer.
While such additive manufacturing operations have greatly expanded manufacturing and development possibilities, further development may make additive manufacturing a part of even more industries. For example, some additive manufacturing operations result in 3D objects with pores of empty space within a material matrix. These pores affect the mechanical strength and integrity of the 3D object. 3D steel objects formed with a binding-agent based system may have a high-volume fraction of pores which may be difficult to eliminate and which may lead to grain growth. Porosity at high levels may result in a reduction in the fatigue resistance of the 3D object which may lead to unpredictable failure of the 3D objects during use.
As a specific example, SS17-4PH steel objects formed with a binder jet printer (BJP) had an observed porosity of 4.1±0.22%, which resulted in a fatigue strength of between 275-325 megapascals (MPa), which is less than half of the fatigue strength of SS17-4PH steel parts manufactured using other methods, i.e., not additively manufactured. That is, the tensile and fatigue properties of BJP SS17-4PH steel parts have properties similar to powder metallurgy processed SS17-4PH. This has inhibited the widespread adoption of BJP parts in engineering applications.
Accordingly, the present specification describes a method and system for reducing the porosity in additively manufactured objects such that the tensile and fatigue strength are increased, thus making additive manufacturing of steel objects feasible in a variety of applications. Specifically, to further reduce the porosity and enhance the mechanical properties, a BJP 3D steel object is subjected to a hot isostatic pressing treatment prior to an ageing treatment. That is, the 3D steel object is subjected to isostatic pressure and increased temperatures to reduce the density and size of pores in the 3D steel object. By doing so, the negative characteristics of a BJP 3D steel object may be alleviated or removed.
The hot isostatic pressing operation yielded significant benefits by increasing the strength of a BJP formed 3D steel object. For example, as described above SS17-4PH steel objects that were binder jet printed had an observed porosity of 4.1±0.22%. Using the present system, i.e., hot isostatic pressing following a binder-jet printing operation, 3D steel objects had a porosity of between 2-3%. However, this porosity was still higher than porosity measurements of other manufactured objects that had been treated with hot isostatic pressing. Despite the porosity in the hot isostatically pressed binder jet printed objects being higher than the porosity of SLM manufactured 3D steel objects, high fatigue strength (in some examples more than 500 MPa) was observed in the binder jet printed steel objects formed using the hot isostatic pressing operations described herein. That is, the combination of an additively manufacturing operation to form the 3D steel object and hot isostatic pressing achieved a resulting increase in mechanical properties beyond what would be predicted based on the resulting porosity. However, the present specification describes such an increase in mechanical strength.
Moreover, it has been shown that a hot isostatic pressing treatment on an SLM fabricated steel object reduced the fatigue strength of the 3D steel object. Accordingly, the present specification describes controlling the temperature range of the hot isostatic pressing operation to initiate the formation of carbides and restrict the excessive grain growth which was observed in the reduced-strength SLM fabricated steel objects.
Specifically, the present specification describes a system. The system includes a hot isostatic pressing system. The hot isostatic pressing system includes a pressure vessel to receive the additively manufactured 3D steel object, a pressure source to apply isostatic pressure to the 3D steel object disposed therein, and a heater to heat the 3D steel object while in the pressure vessel. The system also includes a controller. The controller determines characteristics of the 3D steel object and determines, a temperature, pressure, and duration for isostatically treating the 3D steel object. The controller also activates the pressure source and heater to apply a determined pressure and temperature to the 3D steel object based on determined characteristics of the 3D steel object.
The present specification also describes a method. According to the method, an additively manufactured 3D steel object is introduced into a pressure vessel. The pore density of the 3D steel object is reduced by 1) applying hydrostatic pressure to the 3D steel object in the pressure vessel and 2) heating the 3D steel object in the pressure vessel to a first temperature for a first duration of time. The 3D steel object is solution annealed and also aged at a second temperature for a second duration of time.
The present specification also describes a non-transitory machine-readable storage medium encoded with instructions executable by a processor of an electronic device. The machine-readable storage medium includes instructions to, when executed by the processor, cause the processor to determine characteristics of the 3D steel object and determine, a temperature, pressure, and duration for isostatically treating the 3D steel object. The machine-readable storage medium also includes instructions to, when executed by the processor, cause the processor to reduce pore density in the 3D steel object by 1) activating a pressure source to apply hydrostatic pressure to the 3D steel object in a pressure vessel and 2) activating a heater to heat the 3D steel object in the pressure vessel to a first temperature for a first duration of time. The machine-readable storage medium also includes instructions to, when executed by the processor, cause the processor to age the 3D steel object at a second temperature for a second duration of time.
As described above, the pores of additively manufactured objects, may be “open” meaning that the pores are interconnected. Open porosity occurs in powder metallurgy materials when the porosity exceeds 7%. By comparison, closed porosity occurs when the porosity is less than 5%. For porosities between 5% and 7%, whether the porosity is opened or closed is dependent upon the powder shape and size distribution.
Accordingly, such systems and methods 1) provide an additively manufactured steel object with increased tensile strength, ductility, and fatigue strength as compared to other additively manufactured steel objects; 2) have equiaxed grains free from residual stress; and 3) quickly produce such additively manufactured steel objects.
Turning now to the figures,
The 3D steel object (110) may be additively manufactured in any number of ways. For example, the 3D steel object (110) may be additively manufactured via a binding agent-based system. Some experiments have found additively manufactured 3D steel objects (110) have a reduced fatigue strength due to a porosity of the additively manufactured 3D steel objects (110) and due to the formation of grains in the 3D steel object (110). However, the present system (100), by applying isostatic pressure and heat, reduced the pore density and size within the additively manufactured 3D steel object (110) and increased strengths were observed.
The 3D steel object (110) may be formed of a variety of types of steel. In one particular example the 3D steel object (110) is formed of SS17-4PH steel. However, the 3D steel object (110) may be formed through other additive manufacturing operations and of different types of steel.
As described above, the system (100) reduces the pore density of the 3D steel object (110) by exposing the 3D steel object (110) to elevated pressures and temperatures. Accordingly, the system (100) includes an isostatic pressing system (102) that includes a pressure vessel (104) to receive an additively manufactured 3D steel object (110).
Within the pressure vessel (104), the 3D steel object (110) is subjected to isostatic gas pressure. Isostatic pressure is pressure that is uniformly applied across all surfaces of the 3D steel object (110). Hydrostatic pressure provides such uniform application via a fluid or gas introduced into the pressure vessel (104). In such an example, the hot isostatic pressing system (102) includes a pressure source (106) that pumps and pressurizes a fluid or gas to generate the isostatic pressure. As described above, the pressure within the pressure vessel (104) may be between 100 MPa and 200 MPa. The pressure within the pressure vessel (104) may be selected based on any number of criteria including the characteristics of the 3D steel object (110) itself or of the additive manufacturing process. For example, different binding agents, build materials, and additive manufacturing parameters may impact the quantity and size of pores within the 3D steel object (110) matrix. The characteristics of the pores may trigger particular pressure and heater settings that would result in reduced pore density. Accordingly, different pressure and heat settings may be implemented to reduce the openness of the pores by a determined amount or to reduce the pore size and density to a target value.
As another example, the pressure may be set based on target mechanical properties. For example, an object may be exposed to a high number of fatigue cycles during use such that a robust fatigue strength is desired. In other examples, the object may not be exposed to high fatigue during its life such that a reduced fatigue strength is acceptable. As such, a reduced pressure may be generated within the pressure vessel (104).
In an example, the pressure source (106) introduces argon gas into the pressure vessel (104) to provide the isostatic pressure. An inert gas such as argon is used such that the 3D steel object (110) does not chemically react. In some examples, the pressure vessel (104) may include a lid or other opening through which the additively manufactured 3D steel object (110) may be introduced into the pressure vessel (104).
The hot isostatic pressing system (102) also includes a heater (108) to heat the 3D steel object (110) while in the pressure vessel (104). That is, the volume within the pressure vessel (104) is heated, causing the pressure inside the pressure vessel (104) to increase. The heater (108) may heat the 3D steel object (110) to between 1100 and 1400 degrees Celsius. The heater may include conductive plates coupled to an electrical source. As depicted in
That is, the metal material is compressed by the applied pressure in all directions (x, y and z) at high temperature. At temperatures close to solidus temperature (0.8 Tsolidus), the metal is soft and easy to compress. This facilitates reduction in size of large pores or complete elimination by pore closure.
The system (100) also includes a controller (112) to control the hot isostatic pressing system (102). Specifically, the controller (112) determines the characteristics of the 3D steel object (110) and determines the temperature and pressure settings for the isostatic pressing operation as well as a duration for isostatically treating the 3D steel object (110).
That is, as described above, various properties of the 3D steel object (110) define the pore density and size within the object material matrix. Examples of such characteristics include a binding agent that is used in additively manufacturing the 3D steel object (110), a powder material used in additively manufacturing the 3D steel object (110), and a powder size of the powder material. That is, each of these characteristics may impact the formation and characteristics of pores within the 3D steel object (110). The characteristics of the pores may impact the temperature and pressure settings to be used. For example, larger pores may trigger higher pressure and temperature treatment for a longer period of time as compared to smaller pores. The powder material, and more particularly the particle size of the powder material, may also impact the pore size. Accordingly, the controller (112) receives this information, for example via user input or via metadata associated with a file that describes the 3D steel object (110) and determines what pressure, temperature, and time settings to implement in the isostatic pressing operation. As such, the controller (112) activates the pressure source (106) and the heater (108) to apply the determined pressure and temperature to the 3D steel object (110) based on the determined characteristics of the 3D steel object (110).
As described above, target properties may also be used to select the particular temperature and pressure settings to implement. As such, a combination of target properties and raw materials may be used to select particular temperature and pressure settings for the hot isostatic pressing system (102). While particular reference is made to particular criteria by which temperature and pressure settings are selected, other criteria may be used in determining these settings.
Moreover, while temperature and pressure settings are particularly described, the controller (112) may determine other settings as well. For example, the controller (112) may determine the duration of isostatic pressing. For example, the duration of isostatic pressing may be between 1 and 4 hours. As with the temperature and pressure settings, the duration setting may be based on any of the afore-mentioned criteria.
The controller (112) may include various hardware components, which may include a processor and memory. The processor may include the hardware architecture to retrieve executable code from the memory and execute the executable code. As specific examples, the controller (112) as described herein may include computer readable storage medium, computer readable storage medium and a processor, an application specific integrated circuit (ASIC), a semiconductor-based microprocessor, a central processing unit (CPU), and a field-programmable gate array (FPGA), and/or other hardware device.
The memory may include a computer-readable storage medium, which computer-readable storage medium may contain, or store computer usable program code for use by or in connection with an instruction execution system, apparatus, or device. The memory may take many types of memory including volatile and non-volatile memory. For example, the memory may include Random Access Memory (RAM), Read Only Memory (ROM), optical memory disks, and magnetic disks, among others. The executable code may, when executed by the controller (112) cause the controller (112) to implement at least the functionality of isostatically pressing a 3D steel object (110).
A test was performed to confirm the results that isostatically treating a 3D steel object (110) did in fact increase the strength of a 3D steel object (110). Specifically, the test indicated that the system (100) as described herein led to reduced porosity in binding-agent printed SS17-4PH steel bars. Specifically, porosity of binding-agent printed steel bars initially were observed to have a porosity of 4.1±0.22%. Using the system (100) and methods described herein, the porosity of SS17-4PH steel bars was reduced to 2.37±0.46% and 2.76±0.57% for H1150 and H900 ageing treatments, respectively. The porosity was further reduced to 1.3±0.06% and 0.84±0.09%, respectively, after treatment in the hot isostatic pressing system (102) as described herein.
Additionally, while binding-agent printing may result in 3D steel objects with pores having an average diameter of less than 30 microns, treating the 3D steel objects (110) with the system (100) described herein may result in pores having an average diameter of less than 10 microns. This reduction in overall porosity as well as size of the pores resulted in fatigue strength measurements of 500-540 MPa, which is comparable to fatigue strength achieved in non-additively manufactured objects. Furthermore, the grain growth during isostatic pressing was impeded, possibly due to pinning of the grain boundary by the pores present along the grain boundaries. As such, despite previous expectations where it was believed isostatic pressing would not increase the fatigue strength given the characteristics of additively manufactured object pores, the 3D steel objects (110) produced by a BJP and treated with the system (100) described herein had similar yield strength and ultimate tensile strength as compared to non-additively manufactured counterparts.
That is, high cycle fatigue of alloys depends on the crack initiation life i.e., time spent in crack initiation rather than propagation. The fatigue crack initiation is relatively easier in materials with pores because of higher stress intensity factor (Kt) around them. Stress at pore corner increases multiplicatively by a factor of Kt with applied stress (σ∞). The value of Kt is determined by the size and shape of the pores, which in turn is a factor controlling the fatigue crack initiation life. That is, a higher Kt leads to easy crack initiation and poor fatigue properties. Accordingly, given the characteristics of the pores and the grain growth that propagated therefrom in other additively manufactured steel objects (110), HIP was shown to be ineffective to increase the strength of the 3D steel object (110). However, as indicated in the graphs depicted below, increased strength was observed. Moreover, the isostatic pressing changed the shape of the pores, such that they were made more spherical, and therefore less likely to be an origin of a fatigue crack. That is, the reduced pore size and reduced percentage of pores provides better fatigue strength.
According to the method (200) a pore density within the 3D steel object (
While under pressure, the 3D steel object (
In an example, the 3D steel object (
In an example, the 3D steel object (
As described herein, the method (200) reduces the pore density of an additively manufactured 3D steel object (
As depicted in Table 1, the average yield and ultimate tensile strength for 3D steel objects (
Table 1 also lists the fatigue strength (for 107 cycles) obtained from different batches of BJP and CM specimens. The BJP 3D steel objects (
As described above, the test demonstrated the utility of hot isostatic pressing in additively manufactured, and specifically binding-agent printed, 3D steel objects (
Once the 3D steel object (
The 3D steel object (
In an example, the 3D steel object (
Referring to
Accordingly, such systems and methods 1) provide an additively manufactured steel object with increased tensile strength, ductility, and fatigue strength as compared to other additively manufactured steel objects; 2) have equiaxed grains free from residual stress; and 3) quickly produce such additively manufactured steel objects.
