Patent Application
20100040500
The present invention relates generally to ceramic-reinforced metal alloys, and more particularly to titanium boride-reinforced titanium alloys and methods of making such alloys.
Powder metallurgy (PM) is a popular way to produce components from a wide range of materials, many of which are difficult or impossible to produce by more conventional approaches, such as casting, forming or machining. PM is particularly well-suited to making components from both refractory materials as well as materials that in other processes that do not permit the formation of a true alloy, and is especially beneficial in high-volume production (such as automobile component manufacturing) due to its repeatability and scrap avoidance attributes.
In a typical PM process, a metal powder is mixed with alloying materials, lubricants, binders or the like, pressed into a near-net shape with appropriate tooling, then sintered in a controlled atmosphere to metallurgically bind the pressed powders together. Frequently, one or more secondary operations may be undertaken, including deburring and related surface treatment, repressing, impregnation and porosity reduction.
Titanium, with is excellent corrosion resistance, relatively high temperature capability, and high specific strength, is frequently used in weight-sensitive engineering applications. The transportation industry, especially that associated with aerospace applications, has especially benefitted from the use of titanium and its alloys to create structurally efficient platforms. Nevertheless, its limited stiffness has made it hitherto difficult to fully exploit the advantages titanium has to offer relative to its more refractory counterparts. For example, the modulus of elasticity of titanium-based alloys is roughly half of that of steel and nickel-based materials. The use of additional quantities of material to compensate for these lower stiffness values reduces the efficiency advantages that titanium enjoys over nickel and iron based alternatives.
One way to increase the stiffness of titanium-based alloys is to combine them with relatively high modulus ceramic materials. Such coupling of a bulk metal with continuous or discontinuous reinforcement is part of a relatively new class of materials known as metal matrix composites (MMCs), where structural properties can be tailored to specific engineering applications by appropriate choice of constituent materials. The discontinuously-configured variant of MMCs in general and of titanium-based MMCs in particular is amenable to the PM process, as compound-forming ceramic materials can be reacted with a titanium base during sintering to produce reinforcements that improve the properties of the composite whole. Specifically, in addition to improving the stiffness of titanium, the reinforcing material, which is typically in particulate form, provides other structural benefits as well, including increased hardness for related wear, and multiple phases for enhanced fracture toughness.
In a PM process for making a titanium-based MMC, a titanium precursor can be combined with another material that, under proper temperature and pressure conditions, produces hardened, stiff ceramic reinforcing materials, such as titanium boride (TiB), titanium carbide (TiC) or titanium nitride (TiN), just to name a few. Of these, TiB has proven to be particularly compatible as the reinforcing phase of a titanium MMC, as it exhibits high strength, hardness, heat resistance and modulus of elasticity, is thermodynamically stable over all of the PM processing conditions of the titanium alloy, is insoluble in the titanium alloy, has similar coefficient of thermal expansion as the titanium alloy, and forms a stable crystallographic boundary between it and the titanium matrix. Nevertheless, TiB is unstable by itself, so it has to be produced in situ, such as through reaction of titanium diboride (TiB2) with titanium powder during sintering.
There are many challenges associated with the production of titanium-based MMCs by powder metallurgy. The most important factor for high quality titanium-based MMCs (or improved fracture toughness and fatigue resistance) is the control of the elements such as carbon, hydrogen, oxygen, nitrogen or the like. It is also important to avoid formation of magnesium and sodium compounds. Among the elements, oxygen is the most important element to be limited. For example, residual oxygen, in the form of an oxide film, may form on the surface of the titanium precursor. The presence of such oxygen can result in lower density and mechanical properties in the final product, by limiting the production of the more desirable reinforcing phases such as the aforementioned diboride. Proper mixing of the powder is also crucial to obtain a homogeneous microstructure, and prevent necklacing of the reinforcing constituent.
There exists a need for high strength titanium-based materials that also exhibit excellent toughness, corrosion resistance and high stiffness, wear resistance and heat resistance. There further exists a need to produce these materials in a cost effective way for high-throughput component manufacturing approaches.
These needs are met by the present invention, wherein a method and device that incorporates the features discussed below are disclosed. In accordance with a first aspect of the present invention, a method of making a composite component is disclosed. The component is a composite made from a titanium alloy matrix and TiB2 reinforcement particles dispersed within the matrix, where the method includes mixing numerous precursor (i.e., constituent) materials together to form a mixture, compacting the mixture and sintering the compacted mixture to produce the component in its composite form. The precursor materials include substantially pure titanium (for example, elemental titanium), titanium hydride (TiH2), an alloying material and a boron source material. Heat generated during the sintering process causes the boron source material to react with the titanium to produce titanium diboride (for example, as a compound in particulate form), while the TiH2 becomes activated to react with (and thereby help remove) any oxygen present in the mixture. The TiB2 reacts with the elemental titanium during sintering to produce TiB, which is only thermodynamically stable in the titanium alloy. The TiB acts as the reinforcing particulate in the MMC. Sintering, as used in the present context, is understood as being distinct from other higher temperature operations that involve melting, in that sintering involves heating the material to a temperature slightly below (typically around, but not limited to, eighty percent of) its melting point such that the disparate particles of the precursor material to adhere to one another by solid-state diffusion. Likewise, the term “compacting” and its variants are used synonymously with pressing, where rigid mechanical tooling can be used to impart a significant pressure on the mixture to give it a preferred geometric shape. By way of non-limiting example, such pressing or compacting operations may involve between five and one hundred tons per square inch of pressure.
Optionally, the precursor materials are in powder form. In such case, a substantially pure form of titanium powder may be used. In the present context, the term “substantially” refers to an arrangement of elements or features that, while in theory would be expected to exhibit exact correspondence or behavior, may, in practice embody something less than exact. As such, the term denotes the degree by which a quantitative value, measurement or other related representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue. For example, commercially available titanium is readily available with purity levels of 99.9 percent, and as such may be considered to be substantially pure. In addition, oxidation of a substantially pure metal such as titanium does not detract from its substantially pure nature. Thus, a substantially pure titanium that has an oxide film, layer or the like form on the metal surface upon exposure to the ambient atmosphere is still considered to be a substantially pure titanium in the present context.
More specifically, the diameter of the constituent titanium powder is between nine and seventy five micrometers, with a typical range between eighteen and twenty eight micrometers. The typical range of the alloying material powder is in the range of five to seventy five micrometers. Similarly, the typical range of the produced TiB2 powder is in the range of five to seventy five micrometers. While it will be appreciated by those skilled in the art that numerous titanium matrices may be used, there are certain alloys that have demonstrated particular suitability for structural components such as those encountered in aerospace and automotive applications. These include beta titanium, alpha-2 titanium, gamma titanium and combinations thereof. Examples of beta titanium which may be used in the present invention include titanium with approximately six weight percent aluminum and approximately four weight percent vanadium (i.e., Ti 6-4), and titanium with approximately six weight percent aluminum, approximately two weight percent tin, approximately four weight percent zirconium and approximately two percent molybdenum (i.e., Ti 6-2-4-2). The present inventors have found Ti 6-4 to be especially useful in the manufacture of composite-reinforced automotive components, based on its relative abundance, chemical compatibility and ease of processing. Examples of alpha-2 and gamma titanium include intermetallics, including TiAl and Ti3Al. The alloying material discussed above may be an aluminum-vanadium powder, which may include various approximate ratios including, but not limited to sixty percent aluminum to forty percent vanadium, fifty percent aluminum to fifty percent vanadium and forty percent aluminum to sixty percent vanadium.
In a particular form, the boron source material may be made up of TiB2. In another option, the mixture may include up to approximately ten percent by weight TiH2, with a more particular range being between approximately three and seven percent by weight titanium hydride. Heating that occurs during the sintering process is preferably limited to a rate of up to five degrees Celsius per minute, with a more particular range being between two and five degrees Celsius per minute. The presently disclosed sintering operation may preferably by performed in a controlled atmosphere to avoid oxidation and related contamination. Examples of such control may include evacuated or inserted environments.
In another option, the process of mixing can serve two purposes. In addition to the primary benefit of evenly distributing the powder or other constituents, the inventors have determined that a more aggressive mixing approach helps to strip away oxide layers that may have built up on the surface of the titanium during the exposure of the metal to the atmosphere or related oxygen-containing environment. In this way, the mixing further comprises removing at least a portion of such oxygen-based material. More particularly, the removing comprises placing the precursor materials in an inert environment (for example, argon after oxygen evacuation) and subjecting them to rotational mixing until such time as the mixed materials have predetermined characteristics, such as a maximum powder size, surface smoothness, evidence of pre-sintering alloying, as well as tap density increases, where the latter corresponds to the bulk density of the mixed material after having been shaken or compacted to promote settling. The rotational mixing may more specifically include using rotational speeds of agitators, a mixing drum or other mixing member at high rotational speeds for extended lengths of time. The inventors have found that rotational speeds of approximately 3600 revolutions per minute for between approximately four and twelve hours produces the degree of mixing necessary to effect one of the aforementioned predetermined characteristics, oxygen film removal, or both.
The present invention is well-suited to producing numerous titanium-based structural components, although the present inventors have found them to be particularly appropriate for automotive and related transportation components. In the present context, the term “automotive” is intended to refer to not only cars, but trucks, motorcycles, buses and related vehicular modes of transportation. Within the group of automotive applications, the inventors have found that components made from the materials and methods disclosed herein are especially useful in engine-related applications, where high mechanical loading and high temperatures are both in existence. Examples of automotive component uses include valves, retainers, valve springs, connecting rods, bolts, fasteners, coil suspension springs and exhaust systems.
In addition to the mixing, compacting and sintering operations discussed above, other optional steps may be undertaken. For example, one or more surface-modifying operations, such as deburring, surface compressive peening, porosity reduction or impregnation (the last to introduce lubricants into the component such as may be used in bearings, journals or related friction-reducing parts), may be undertaken to improve the functionality of the finished component.
According to another aspect of the invention, a method of preparing a material for powder metallurgy processing is disclosed. The method includes placing numerous precursor powder materials comprising substantially pure titanium, titanium hydride, an alloying material and a boron source material into a titanium-based mixing container, substantially replacing an ambient atmosphere in the mixing container with an inert fluid, rotating an agitator at a minimum predetermined speed for a minimum predetermined time until a mixture evidences at least a twenty percent reduction in powder size, at least a thirty percent increase in tap density of the mixture, or a substantial removal of an oxide film from the titanium powder. After completion of the mixing, the mixture is sintered.
Optionally, the agitator can be configured in various forms. In one form, the agitator is made up of numerous titanium-based spheres or balls that can be made to rotate within the mixing container, such as by container movement or the like. In another form, the agitators can be paddles, bars or related members that radially extend from an elongate rotating shaft such that upon shaft rotation, chums powder. In a preferred option, the minimum predetermined rotational speed of the spheres or members is approximately 3600 rotations per minute (RPM), and the minimum predetermined time is approximately four hours.
According to another aspect of the invention, a method of making a titanium boride reinforced titanium-based metal matrix composite component is disclosed. The method includes mixing at least a substantially pure titanium powder with an alloying material and a boron source material. The degree of mixing is similar to that previously discussed, where the mixing is more vigorous than that required for the mere substantially even distribution of the constituent materials in that by the frictional rubbing action between colliding materials in powder form, most or all of any oxide layers that may have formed on the titanium powder is removed. In addition to mixing, the mixture must be compacted into a shape of the component, after which the compacted mixture is sintered to a degree sufficient to have the boron source material react with the titanium to produce a reinforcing phase made up of the titanium boride.
In one optional form, the method further includes adding titanium hydride to the titanium powder, alloying material and boron source material constituents such that it can be mixed along with them. Additional steps may include conducting one or more of forging and annealing operations once the component has been sintered.
The following detailed description of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which:
Referring initially to
Referring next to
The inventors have discovered that a modified mechanical pulverization treatment (MPT) is beneficial in that it promotes tap and final density. One example of a simplified set of process conditions associated with MPT that the inventors have used includes (1) evacuating and then providing argon protection (with pressure higher than atmosphere) to the powder in the chamber, (2) cooling the chamber to keep the powder temperature no higher than 35 C, (3) providing a weight ratio of 1:12 for the balls and powder materials respectively, (4) providing a ratio of ball sizes of 3:3:1 for ball diameters of 20 mm, 10 mm and 8 mm respectively, and (5) subjecting the powder to a grinding time of between 4 and 12 hours at a speed of 3600 RPM. These steps should (1) produce powder size decreases by 20 to 60 percent, (2) result in a more smooth powder surface, (3) promote pre-alloying and (4) increasing tap density between 30 and 40 percent with sintering density increases commensurate with MPT treatment time.
The figure depicts the operation of a mixing device as used in the present invention is shown. Mixing device includes a mixing drum or similar container 150, precursor feed line 160 with pump 170, rotating shaft 180 and agitators 190 rigidly affixed to shaft 180. In the present invention, the inventors have discovered that a modified mechanical pulverization treatment approach works especially well with the precursor materials in producing a preferred mixture. In one particular form, the mixing process, referred to as process uses a modified MPT, utilizes substantially pure titanium spheres or balls 195 of different sizes, where the ball diameters can be 20 millimeters, while others are 10 millimeters and 8 millimeters. Once the precursor materials are placed inside the mixing drum 150, the container can be evacuated to remove residual oxygen. Afterwards, argon gas (over 99.999%) or a related inerting fluid is pumped into the container to a slightly elevated pressure (for example, up to about 1.2 atmospheres) to prevent powder oxidation. To avoid contamination, the entire interior of the container is made of substantially pure titanium, while a coolant, such as cooling water pumped through coolant circuit 155, can be used to keep the chamber temperature no higher than 350 C. The weight ratio of titanium balls to the process powders is 1:12. The mixing is conducted at 3600 rotations per minute for 4 to 12 hours, with each batch of precursor materials being between five and ten kilograms.
The presence of the titanium hydride 120 in the mixture will further reduce the presence of oxygen in the sintered component. Moreover, the decomposition of the titanium hydride produces a fine powder of titanium that is beneficial in increasing component density as it fills up the interstitial spaces between the other mixed powders. The inventors have discovered that there is a preferred range of TiH2 addition, as too little may not provide enough additional oxygen removal, while too much may cause non-uniform cracking during sintering.
As mentioned above, there are numerous ceramic-based titanium compounds that may be used as reinforcement of the titanium matrix. Nevertheless, the present inventors have determined that some are better-suited to the manufacture of Ti MMC components than others. For example, TiN is a weak reinforcing phase relative to TiC and TiB, and of these remaining two, the latter worked better. The concentration of thermodynamically stable ceramic particles (such as TiB and TiB2) is chosen based on the applications. For many automotive applications, the inventors have discovered that an upper limit may be approximately eighteen percent by weight, with a lower limit of as little as one percent. From phase diagram information, the present inventors expect that the reaction between Ti and TiB2 during sintering process would form the thermodynamically stable phase of TiB in Ti alloys.
The second step employed in making the material includes compacting, pressing or otherwise forming the mixture. This is shown as step 200. As with the mixing step discussed above, there are various ways in which the component can be formed into its green (i.e., pre-sintered) state. Such ways include isostatic forming, die forming or the like. The pressure imparted to the mixture during compaction 200 is sufficient to maintain the part substantially in a near-net shape while awaiting the sintering step 300 (discussed below).
The third step is the sintering step 300. During sintering, the compacted green component is heated such that the titanium (for example, elemental titanium) and the alloying material are alloyed, thereby producing the titanium-based matrix. As stated above, controlled environments may be used to reduce the likelihood of contamination. Temperatures at which the sintering step 300 may be conducted are preferably between 1200° C. and 1450° C. The sintering step 300 may include a ramped heating schedule, such as between 2 and 5 degrees Celsius per minute. One example of the effects of sintering temperature, specifically, on the amount of hydrogen present in a Ti-MMC, is shown in
Afterwards, a cooling schedule may be used, where the sintered component is cooled over the course of 7 hours. In such circumstance, cooling rate may be approximately 200° C. per hour. Also during the sintering step 300, reactions are taking place between the boron source material 140 and the titanium 110 to form TiB, as well as the titanium alloy (for example, Ti6Al4V). Likewise, the step may also include closed die forging or phase-transformation densification 400, where small voids left over from the sintering process can be removed by using a hot press forging. Such a step is preferably conducted at a high temperature. Coatings applied to the part at room temperature help to prevent part oxidation at high temperature. In one form, the coating contains Al2O3, SiO2 and B2O3, as well as organic binder. It can be applied with a brush for a few coats when the parts are heated to 70 C, after which the part is dried. Typical forging temperatures range from 900 to 1400 C, and more particularly between 1200 and 1350 C, depending upon the TiB2 content, with higher levels requiring that a higher forging temperature be used. The typical reduction ratio (which is the ratio of the cross sectional area before and after forging, sometimes referred to percent reduction in thickness, and related to the size of the processed MMC material) should be broadly between 300 and 800 percent, with a more particular range of 500 to 700 percent. In such a range, the sintering temperature would be 1350 C. The typical anneal temperature should be within a broad range of 550 to 950 C, and more particularly between 650-740 C. The time should be between one half and two hours.
Also as shown in
Referring with particularity to
Pre-sintering compaction and related tap density can be achieved by numerous vibration, shaking or related agitation means. One approach to increasing the pre-sintering compaction is through cold die compaction. In one form, this can be conducted at room temperature at 190-360 MPa (i.e., approximately 28,000 to 52,000 psi) for 3 minutes, with a typical range of 1 to 6 minutes, and a typical pressure range of between 230 and 270 MPa. The inventors have discovered that to achieve a best green density and green strength, a preferred titanium particle size should be 22 and 34 micrometer within a broader range of 5 to 75 micrometers. The sintering process includes heating these green parts at a rate of 2 to 5 degrees Celsius per minute until they reach the desired sintering temperature of approximately 1300 degrees Celsius, with a typical range (as mentioned above) of 1200 to 1450 degrees Celsius, for 3 hours, with a typical range 2 to 8 hours. During sintering, it is advantageous to maintain a vacuum of 103 Pa for a between 2 and 8 hours, with a more specific range of 3 to 6 hours in order to achieve 99% theoretical density. Longer sintering times can further improve the sintered density.
Precursor material sizes may vary, although the typical sizes of the titanium powders used for making Ti6Al4V MMC range broadly between 9 and 75 micrometers, with a narrower range between 18 and 28 micrometers. There are several possible methods for producing titanium powder. One of them is done by a hydride-dehydride titanium powder making process with a varied rotation speed during jet milling to get different powder sizes. Referring with particularity to
The typical TiB2 particle size is in the range of 5 to 75 micrometers, and can be prepared by a self-propagating high-temperature synthesis-process, such as that shown in the following reaction:
Ti+2B→TiB2+Q (324 KJ/mol)
The physical properties of TiB2 powder are shown in the following table, while
TiB2 and Ti6Al4V have similar densities. It is advantageous to mix them. However, it has been found that TiB2 significantly decreases the sintered density of titanium MMC. When the content of TiB2 is higher than 7% by weight, the forged density also starts to decrease significantly. One possible explanation may be the significant difference in the relative densities of boron (approx. 2.34 g/cc) and titanium (4.5 g/cc).
The typical particle size of Al—V alloying material is in the range of 5 to 75 micrometers. In powder form, these alloying materials are also prepared by a commercially available self-propagating high-temperature synthesis-process shown in simplified form according to the following reaction:
Al+V205→AIVx+Al203+Q
Three different Al—V alloy powders were prepared with aluminum to vanadium ratios of 60/40, 50/50 and 40/60 respectively, for making Ti6Al4V MMC. The chemical compositions were primarily aluminum and vanadium with traces amounts of oxygen, carbon, iron and silicon.
Referring next to
Referring again to
Compared to unreinforced Ti6Al4V alloy, the Ti6Al4V MMC discussed herein have higher strength and elastic modulus. As such, TiB2 is an excellent reinforcement for Ti6Al4V titanium alloy. For example, the elastic modulus of the reinforced Ti6Al4V is over 140 GPa, with an average of 155 GPa, in comparison with 100 GPa average for unreinforced Ti6Al4V. The ultimate tensile strength of over 1350 MPa (average 1450 MPa) is significantly greater than the 1140 MPa average for the unreinforced Ti6Al4V, with a 0.2% yield strength of over 1250 MPa (average 1300 MPa), in comparison with average of 980 MPa for unreinforced Ti6Al4V. The Rockwell hardness is above 43. One example of a structural component made according to one of the aspects of the present invention is a connecting rod for use in an automotive engine, although it will be appreciated by those skilled in the art that numerous other components may also be manufactured.
While certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes may be made without departing from the scope of the invention, which is defined in the appended claims.
