ORIGIN OF THE INVENTION

The present disclosure is based on work performed by employees of the United States Government and may be manufactured and used by or for the U.S. Government for Government purposes without the payment of any royalties thereon or therefore.

FIELD

The present invention generally relates to the powder metallurgy production of metallic components and more specifically to the method of making small Nickel-Titanium metal alloy components. More specifically, the invention relates to overcoming the previous impediment in making small Nickel-Titanium metal alloy components that exhibit the formation of soft phases in the metal alloy wherein such soft phases prevent hardening due to the inability to rapidly quench the hardened component, such as bearing balls.

BACKGROUND

Nickel-Titanium metal alloy, commonly referred to as Nitinol, is an intermetallic compound of nickel and titanium, discovered in 1959. Nitinol, which is presented in roughly equal atomic percentages, has unique properties that cannot be found in other materials. Common compositions of Nitinol are Nitinol 55 (55 weight % Ni-45 weight % Ti) and Nitinol 60 (60 wt % Ni-40 wt % Ti) which are widely used due to their unique properties. However, other compositional variations of Nickel and Titanium could be used and still utilized the unique properties of the resultant metal alloy.

Nitinol compositions can be heat treated to a hardness of Rockwell C 60 or higher and are wear resistant and non-galling, despite a high titanium content. In addition, regardless of the high nickel content, these Nitinol compositions are non-magnetic and are highly corrosion resistant in a variety of media. The density of these Nitinol compositions is typically only 86 percent of the density of steel, which is advantageous in applications where weight is a consideration, and also displays superelastic and shape memory properties.

Even though these Nitinol compositions have a number of attractive properties, reasonable commercialization efforts did not take place until a decade after its discovery. Further significant usage of Nitinol has been slowly adopted or non-existent because it is a difficult composition to process by the common metallurgical practice of ingot melting followed by hot and cold working. Nitinol compositions in cast form can be brittle and can crack unexpectedly under otherwise normal processing conditions. Several attempts have been made to broadly manufacture Nitinol components. However, due to the difficulties in conventional ingot metallurgy processing, Nitinol compositions have not been widely used.

NASA Glenn researchers have been using and investigating Nitinol or NiTi intermetallic materials for numerous terrestrial and space applications. Most notably, NASA Glenn researchers are developing corrosion immune, shockproof ball bearings for terrestrial and aerospace applications like aircraft control surface joints and actuator gearboxes, utilizing emerging superelastic intermetallic materials based upon NiTi intermetallic materials. Gears, ball bearing races, and large bearing balls (⅜ inch diameter and larger) have been produced from the baseline 60NiTi alloy (60 wt % Ni and 40 wt % Ti) using the methods described in the NASA co-owned patents identified as U.S. Pat. Nos. 8,182,741; 8,377,373; and 9,393,619 and each hereby incorporated by reference herein. However, these patented methods and other known NiTi production methods have been incapable and previously inadequate for producing small components, particularly small diameter balls, having the required characteristics and hardness of the larger NiTi components.

The patented NiTi production methods developed in collaboration with NASA, identified above, for make bearing balls involves pouring NiTi alloy powder into spherical cavities machined into graphite molds. This approach works well for relatively large balls (˜0.375 inch diameter or larger). However, this approach is inconsistent and not efficient or effective for the production of smaller ball diameter sizes down to 0.25 inch diameter and not at all below 0.25 inch diameter. One significant problem is that it is difficult to feed powdered metal through the necessarily small fill tunnels that lead to the mold cavities. Large ball molds use relatively large and easy to fill tunnels but small balls must use very small fill tunnels and powder doesn't flow well through these fill tunnels.

In addition, it has been found that small parts made of the baseline 60NiTi alloy (60 wt % Ni and 40 wt % Ti) are difficult to heat treat and attain high hardness levels. The baseline 60NiTi alloy must be rapidly cooled after heating to attain high hardness. Small parts, like balls less than ˜0.25 inches in diameter, cool excessively while being removed from the oven before they can be quenched. This slow cooling or relative slow cooling in relation to the size of the balls leads to the formation of undesired soft phases in the alloy. These undesired soft phases prevent the alloy from attaining the required hardness as comparable to the hardness levels achieved from larger baseline 60NiTi parts having sufficient size so as to reduce or eliminate the formation of undesired soft phases in the alloy.

Therefore, there is a significant need in the art to overcome the production obstacles in the art so as to enable the production of small NiTi components exhibiting the beneficial features of NiTi alloy such as high hardness and corrosion resistance.

SUMMARY

The following presents a simplified summary of the innovation in order to provide a basic understanding of several aspects of the innovation. This summary is not an extensive overview of the innovation. It is not intended to identify key/critical elements of the innovation or to delineate the scope of the innovation. Its sole purpose is to present some concepts of the innovation in a simplified form as a prelude to the more detailed description presented later.

A method for making small NiTi metal alloy components, including balls, comprises providing a metal powder comprising nickel, titanium, and a transition metal, consolidating the metal powder into cylindrical rods, and cutting the cylindrical rods into segments. The segments are then machined into spheres slightly larger than the finished ball size diameter. The spheres are heat treated to solutionize and dissolve all phases and subsequently cooled without the need for rapid quenching due to the influence of the transition metal to suppresses the formation of soft phases in the spheres, wherein such soft phases prevent hardening, to achieve a Rockwell hardness of HRC 58-62. Finally, the hardened spheres are polished until the desired finished ball size diameter and surface finish is achieved.

Through innovative research, development, and testing, for the first time, small NiTi alloy parts can be made where none could be made in the past. Included with the development of these small NiTi alloy parts, small diameter NiTi alloy bearing balls can now be made. The ready availability of hard, high grade bearing balls made from NiTi alloy is a major advancement to the ball bearing field.

