The present invention relates to a method for densifying allotropic material.
Titanium has remained an exotic metal prized for its superior strength-to-weight ratio, resilience at high temperatures, superior fatigue life, corrosion resistance, and compatibility with fibers in polymeric composites. However, high processing costs associated with the production of titanium components have kept it beyond the reach of mass production.
The present invention provides a method for densifying an allotropically transformable material, such as any powdered material that exhibits allotropic transformation, including ceramics such as silicon nitride, and including titanium and titanium alloys, into a near-net-shape component by adjusting the temperature of the material or by using thermal or magnetic cycling to cause the material to transform between a first allotrope phase and a second allotrope phase.
The method includes arranging the material in the cavity of a mold, applying pressure to the material in the mold, and applying a magnetic field to the material in the mold to cause the material to transform between a first allotrope phase and a second allotrope phase. The temperature of the material may be maintained between a determined nominal allotropic phase transformation temperature and a determined shifted allotropic phase transformation temperature, in which implementation, the strength of the magnetic field may be adjusted in a magnetic cycle to cause the material to transform between the first allotrope phase and the second allotrope phase. The method also optionally includes determining that the density of the material satisfies a density threshold. Optionally, after determining that the density of the material satisfies the density threshold, further adjusting the temperature of the material, causing the material to undergo annealing.
The present invention also provides a method for densifying a material, including arranging the material in the cavity of a mold, applying pressure to the material in the mold, setting the temperature (such as by applying heat to a selected temperature) of the material in the mold to be between a determined nominal allotropic phase transformation temperature and a determined shifted allotropic phase transformation temperature, and while applying pressure to the material in the mold and with the temperature of the material between the nominal allotropic phase transformation temperature and the shifted allotropic phase transformation temperature, applying a magnetic field to the material to cause the material to transform between a first allotrope phase and a second allotrope phase. The method also includes determining that the density of the material satisfies a density threshold. Optionally, the magnetic field may be applied to the material via magnetic cycling to adjust the magnetic field to a first strength of magnetic field and then to a second strength of magnetic field, and repeating the cycling between strengths of the magnetic field multiple times until the material transforms and achieves a desired density threshold.
The present invention also provides a method for densifying a material including arranging the material in the cavity of a mold, applying pressure to the material in the mold, and while applying pressure to the material in the mold, adjusting the temperature of the material, causing the material to transform between a first allotrope phase and a second allotrope phase. The method further includes determining that the desired density of the material is achieved.
Optionally, the present invention includes, after determining that the desired density of the material is achieved, further adjusting the temperature of the material, causing the material to undergo annealing. Optionally, adjusting the temperature of the material includes heating the material using induction heating.
These and other objects, advantages, purposes and features of the present invention will become more apparent upon review of the following specification in conjunction with the drawings.
The present invention relates to a process for densifying a material that exhibits allotropic transformation (e.g., ceramics such as silicon nitride, but more specifically titanium and titanium alloys) far below the melt temperature of the material(s). The process densifies the material into near-net-shape components using thermal and/or magnetic cycling. The resulting near-net-shape components require little machining to create final products, resulting in less waste than traditional methods. The cycling may be performed under less demanding conditions than traditional methods, such as at temperatures far below the melting temperature of the material and at more readily achievable pressures. The process may also take less time and/or energy than traditional methods, while producing final products with similar or superior material and/or mechanical properties. Furthermore, the process may be applied to low-cost raw materials (e.g., powders created for the additive manufacturing industry with grain sizes unsuitable for use by 3D printers), or titanium sponge powder or the like, thereby reducing the cost of production.
The process of thermal and/or magnetic cycling transforms the material from one allotrope phase to another allotrope phase. Each allotrope consists of atoms bonded together in a characteristic arrangement having associated characteristic physical properties. For example, the titanium molecules in the alpha titanium (α) allotrope form a hexagonal close packed structure. In contrast, the titanium molecules in the beta titanium (β) allotrope form a less closely packed, body-centered cubic structure. For pure titanium, a reduction in volume as great as 8.1% may occur during the transformation (phase change) from the β allotrope to the α allotrope. Furthermore, when sufficient pressure is applied to the material during a phase change from the β allotrope to the α allotrope, the material undergoes compaction. For example, if the material undergoes a reduction in volume during a transition from the β allotrope to the α allotrope and subsequently transitions back to the β allotrope while under sufficient pressure, the material does not simply return to the previous volume associated with the α allotrope. Instead, the material may expand to fill voids, or otherwise become more dense or compact. Therefore, iteratively cycling the material between allotrope phases, while applying sufficient pressure to the material, results in stepwise densification. The process may be repeated until a suitable density is achieved (i.e., a density associated with desired material and/or mechanical properties). For example, the process may be repeated 10-12 times until the material is greater than 99.9999% dense. Furthermore, the material may be contained in a form or mold during the densification process (e.g., a mold approximating the shape of a final product). In this case, the densification process produces near-net-shape components requiring little machining to create final products with desired material/mechanical properties.
Referring now to the drawings and the illustrative embodiments depicted therein,
The tooling may include a base mold having a near-net-shape cavity (e.g., suggesting or approximating the shape of a final product) and an upper die or plate capable of applying pressure to the material contained in the base mold. Alternatively, the tooling may include a substantially planar base plate supporting a perimeter mold containing the material and an upper plate capable of applying pressure to the material contained in the perimeter mold. Other tooling configurations are possible. For instance, the mold and/or tooling may consist of several pieces that can be disassembled after the component is processed (e.g., to facilitate removal of components with complex geometries).
The system is operable to apply pressure to the material contained in the tooling (e.g., via a hydraulic or pneumatic ram assembly 106). Here, the system includes a cylinder ram assembly disposed above the tooling and capable of applying downward pressure (e.g., uniaxial pressure) to the material via the tooling (e.g., applying pressure to the upper plate of the tooling, the upper plate, in turn, transmitting the pressure to the material). For material consisting of titanium or titanium alloy, the applied pressure may be in the range from 1 MPa to 1,000 MPa, and preferably about 50 MPa. The ram assembly includes layers or portions that perform additional functions while also transmitting and applying pressure to the material. These additional functions include providing thermal isolation between layers of the ram assembly or between layers of the ram and the tooling/material, conforming the ram assembly to a surface of the tooling, or the like. The ram assembly and the tooling may include portions consisting of ferrous and non-ferrous materials including, but not limited to, steel, stainless steel, molybdenum, tungsten, cobalt, or any combination thereof, and/or ceramics including, but not limited to, silicon nitride, silicon carbide, alumina, boron nitride, Zirconia, or any combination thereof.
Optionally, the ram assembly include one or more extensometers, pressure sensors, and/or position sensors. The position sensors may include an encoder, linear variable differential transformer (LVDT), or other sensor or sensor combination capable of accurate measurement in the environment. For example, a sensor may monitor the position of the ram progressively moving to a lower position, indicating a change in one or more dimensions of the material as the density of the material increases (accounting for other relevant parameters, such as temperature and allotropic composition of the material). The sensors may determine that the ram has moved to a position indicating that a desired material density has been achieved (e.g., taking into account the weight of the material and the volume of the mold).
Alternatively, the position sensors may determine that the ram has stopped moving to a lower position during the densification process (or is moving by an amount less than a threshold distance), indicating that the desired material density has been achieved. An extensometer (e.g., an optical distance sensor capable of resolving distances of 10−6 meters) may be used to measure the position of the ram, as described above, or to measure deformation of the material (e.g., as a function of pressure applied by the ram). In this way, the sensors indicate when the material has achieved a threshold density (e.g., at least 99.999%) so that the final product will have suitable structural and/or mechanical properties.
The system is also operable to adjust the temperature of the material. Here, the system includes an induction-heating system 108. The induction-heating system generates time-varying electromagnetic fields that induce eddy currents in the electrically conductive tooling and/or material. The induced eddy currents flow through the resistive tooling or material, generating heat. Heat generated in the tooling is thermally conducted to the material in contact with and contained within the tooling. Optionally, the tooling consists of a semiconductor ceramic, such as silicon nitride doped with titanium nitride. Above a particular temperature, the semiconductor ceramic ceases to be electrically conductive. Therefore, eddy currents are not induced in the semiconductor ceramic tooling above that temperature. In this way, the tooling may be configured to self-limit the range of temperature adjustment of the material.
Alternatively, the temperature of the material may be adjusted by applying electrical current directly to the resistive tooling, or by applying electrical current to resistive cartridges in contact with the tooling or the material, or by wrapping the tooling in heating “blankets,” or “pads.” The temperature of the material may be adjusted by applying infrared radiation, heat from a flame, or other suitable heat source. Optionally, the system provides for monitoring the temperature of the material using one or more thermocouples (TC), resistive temperature detectors (RTD), infrared temperature detectors (IR), or any other contact or non-contact style temperature sensor capable of accurate measurement in the environment. An operator may use the temperature measurements from the sensor(s) to manually adjust the temperature of the material.
Alternatively, the system may provide automatic temperature control based on the temperature measurements, using an automatic control algorithm such as a proportional-integral-derivative (PID) loop or the like to achieve a target or desired material temperature. The temperature of the material may be adjusted up and down repeatedly, or “cycled” over time between values such that the material transforms back and forth between different allotrope phases. For example, a titanium alloy may transform to the alpha titanium (α) allotrope at a temperature less than 882 degrees C. (1620 degrees F.) and to the beta titanium (13) allotrope at a temperature greater than 882 degrees C. (the α allotrope having a hexagonal close-packed structure (see 202 in
The densification process described above is effective when the material completely transforms from one allotrope to another during temperature cycling. Furthermore, the densification process is also effective when a significant or merely a non-trivial amount of the material transforms to another allotrope during temperature cycling. The number of cycles or iterations required to achieve the desired densification, however, may depend on the proportion of the material undergoing allotropic transformation (as well as the relative spacing of atoms in each allotrope, and other factors). That is, if only a small percentage of the material transforms to another allotrope during temperature cycling, more cycles may be required than if a larger percentage of the material transforms to the other allotrope. For example, and with reference to
Cycling the temperature above and below the allotropic transformation temperature causes the titanium to transform between α and β allotropes. As the Oxygen content of the titanium increases, the transformation between primarily alpha titanium (α) and primarily beta titanium (β) occurs over an increasingly large temperature range, referred to as the transus phase. That is, at certain temperatures, the titanium consists of both alpha titanium (α) and beta titanium (β). The transus phase is indicated in
Referring to
Referring now to
Referring now to
Referring now to
The temperature of the material, therefore, may be maintained at a target temperature between the nominal allotropic transformation temperature and the decreased allotropic transformation temperature. The magnetic field may then be cycled on and off, causing the material to transform between allotropic states at the constant (or substantially constant) temperature. Here, when the magnet field is applied to the material at the target temperature, the material transforms to the β allotrope. When the magnet field is cycled off, the material transforms to the α allotrope.
Therefore, cycling the magnetic field on and off at while the material is at the target temperature effects a transformation between titanium allotrope phases. Thus, titanium may be effectively densified by the present invention using magnetic cycling, with or without temperature cycling. Transformations between α and β-phases tend to be exothermic or endothermic. With titanium, this transformation tends to produce approximately 90 kilojoules per kilogram of material. That is, the amount of heat released or absorbed by the material is proportional to the mass of the material. The time between successive magnetic cycles is sufficiently long for the system to maintain the material at the target temperature. The time between magnetic cycles may still be shorter than the time required for temperature cycling, reducing the overall time to achieve the desired density, particularly for less massive parts and less demanding densities (e.g., greater than 99.9%).
Referring now to
Referring now to
Referring now to
At step 1118, the temperature of the tooling and material is cycled between temperatures causing the material to undergo annealing. The temperatures associated with the annealing process are independent of the temperatures associated with the densification process. However, the annealing step may be performed using the same tooling and heat source as the densification process. At step 1118, after the densification and annealing is complete, the mold is removed from the system. At step 1120, the mold is removed from the fixture. Optionally, the mold is disassembled to facilitate removing the part (e.g., to accommodate parts having complex geometries). At step 1122, the part is removed from the mold. At step 1124, the controlled atmosphere is released.
Referring now to
Referring now to
Referring now to
Thus, the present invention provides a cost-effective process for densifying any material that exhibits allotropic transformation (e.g. metal alloys, such as Ti-6Al-4V), into a near-net-shape components using thermal and/or magnetic cycling. The thermal and/or magnetic cycling may repeat until the desired density is achieved. Optionally, the invention also provides for annealing the component (e.g., to remove internal stresses created by the densification process). The densification process produces near-net-shape components requiring little machining to create final products with desired material/mechanical properties.
Changes and modifications to the specifically described embodiments may be carried out without departing from the principles of the present invention, which is intended to be limited only by the scope of the appended claims as interpreted according to the principles of patent law including the doctrine of equivalents.
The present application claims the filing benefits of U.S. provisional application Ser. No. 62/971,305, filed Feb. 7, 2020, which is hereby incorporated herein by reference in its entirety.
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/US2021/070128 | 2/5/2021 | WO |
Number | Date | Country | |
---|---|---|---|
62971305 | Feb 2020 | US |