For the last several decades, orthopedic medical implants have been manufactured mainly from austenitic stainless steels, titanium and titanium alloys, and cobalt-based alloys. Alloy selection for a specific application has depended upon a variety of design criteria, including biocompatibility, corrosion resistance, tensile strength, fatigue strength, modulus, wear resistance, processing, and cost.

The vast majority of cobalt-based orthopedic implants worldwide have been manufactured using castings of ASTM F75, a cobalt-based alloy. Casting in this alloy provides desirable processing flexibility and lower initial costs than forging. However, distinct limitations have been associated with castings. Limitations include coarse grain size, nonuniform microstructural segregation, lower tensile strength, and lower fatigue strength than a forging. These drawbacks can be overcome by manufacturing cobalt-based implants from cobalt-chromium-molybdenum wrought bar stock. Wrought bar is essentially an ingot (a large casting) that has been reheated and forged down to a smaller size. Working the material is the process of reducing the dimension of a cast ingot to a smaller forged size, in order to refine macrostructure and impart strength.

Three cobalt-chromium-molybdenum (Co-28Cr-6Mo) wrought alloys are covered under ASTM F1537 and used for orthopedic medical implants. Of the three, the lowest- carbon (0.14% max) alloy 1 (UNS R31537) has been used most often. This alloy has been traditionally manufactured using conventional cast-wrought processing, but it can also be manufactured using powder-metallurgy processing.

In a comparison of the two manufacturing methods, the powder-metallurgy process provides higher strength, improved fatigue resistance, and enhanced microstructural characteristics at both room and elevated temperatures (see Tables I and II). Data collected have confirmed that both methods of manufacturing wrought feedstock are superior to conventional casting.

The Powder-Metallurgy Process

When compared with the conventionally produced cast-wrought alloy, bar stock made using the powder-metallurgy process exhibited higher tensile and fatigue strength, increased hardness, finer grain size, and a more-uniform structure that was less prone to segregation (see Table II). These attributes were seen in bars in the typical as-supplied, unannealed, warm-worked conditions.

The powder-metallurgy processed alloy exhibited these same relative attributes after exposure to elevated temperatures that are typically associated with annealing or with the forging of orthopedic implants. Such attributes would likely improve the performance and life of joint-replacement implants and fracture-fixation devices. These devices, which are typically machined or forged, include total hip, knee, and shoulder replacements.

Because of the characteristics typically produced by the powder-metallurgy process, F1537 Alloy 1 can be produced in the warm-worked or hot-worked unannealed conditions in smaller-diameter bar and wire products than the conventionally cast-wrought alloy. Also, powder-metallurgy processed stock can be made without the need for cold drawing and annealing, two processes that can be detrimental to fatigue strength.

Because of its fatigue strength, bar stock manufactured using the powder-metallurgy process could be used to manufacture small-diameter parts such as pins, rods, and wire used in spinal applications. The powder-metallurgy process allows for the production of fully wrought near-net shapes. A near-net shape is one that is close to that being machined. Rather than making a round bar and machining the part, the powder-metallurgy process can be used to make a shape closer to the machined size. These fully wrought near-net shapes are suitable for applications in which higher tensile and fatigue strength is required than is possible with castings.

Processes

A typical conventional cast-wrought process uses the following steps:

• Melt the alloy using vacuum induction.
• Electroslag remelt the ingots.
• Hot forge the alloy to billets.
• Hot roll the alloy into wrought bar stock.
• Turn and grind the alloy to the finished condition.

The powder-metallurgy process (see Figure 1) is as follows:

• Melt a heat of high-purity gas-atomized powder using vacuum induction. A heat is the industry term for a lot or batch.
• Screen the powder to a predetermined mesh size.
• Blend several heats to make one master blend.
• Fill stainless canisters and hot isostatically press (HIP) to full denseness. The HIP process is much like a big pressure cooker. The powder is poured into a stainless-steel container and then heated under pressure. Full denseness means that during the course of being placed under heat and pressure, the powder solidifies within the container to 0% porosity (100% solid).
• Hot roll the alloy into wrought bar.
• Turn and grind the alloy to the finished condition.

Properties

Conventional cast-wrought alloys are available in the annealed condition or, more typically, in the hot-worked or warm-worked conditions. Powder-metallurgy-processed alloys are typically available in either the annealed or, more commonly, the warm-worked condition. When manufactured to the same metallurgical condition (such as warm worked), a powder-metallurgy alloy typically exhibits higher yield and higher ultimate tensile strength than a conventionally cast alloy. Typical mechanical properties for both processes are shown in Table II.

Both the powder-metallurgy alloy and the conventional alloy were tested in the warm-worked condition using an R. R. Moore rotating-beam fatigue-testing machine. The test was conducted at a frequency of 6000 rpm on bar stock samples. The powder-metallurgy alloy showed elevated estimated endurance limits (see Table I).

The fatigue results from this study are significantly higher than those found in previous in-house tests that were conducted to evaluate the fatigue properties of annealed-plus-cold-drawn alloy bar stock. Both cast-wrought unannealed bar stock and the powder-metallurgy unannealed bar stock had significantly higher fatigue properties than the annealed-plus-cold-drawn alloy. Both the high tensile strength and the fatigue strength of the powder-metallurgy alloy are attributable to the fine grain size and uniform microstructure produced by the powder-metallurgy process.

In the warm-worked condition, the powder-metallurgy alloy has a slightly finer grain size than the conventionally produced alloy. The standard cast-wrought alloy has an ASTM grain size of 12.5 with an average grain dimension of 7 and an average cross-sectioned grain area of 50, as shown in Figure 2. (ASTM grain size is determined via a comparison method, so there is no unit of measure. The grain is rated as the number it most closely matches on a grain size chart. To find the grain area, multiply the length of the grain by the width, in microns.) The powder-metallurgy alloy has an ASTM grain size of 13.6 with an average grain dimension of 4.6 and an average grain area of 22, as indicated in Figure 3. Grain size represents the size of the internal structure for a metal. Typically the smaller the grain dimension measured, the stronger the material might be. The conventionally produced hot-worked alloy has an ASTM grain size of 11.5 with an average grain dimension of 8.7.

Thermal Treatment

Tests were conducted to evaluate the effects of various thermal treatments on the microstructure and hardness of the conventional alloy and the powder-metallurgy alloy. Samples of the conventional alloy were tested in both the unannealed hot-worked and unannealed warm-worked conditions. Samples of the unannealed warm-worked powder-metallurgy alloy also were tested. The samples received 30-minute air-cooled heat treatments using a temperature range from 1500ÞF (815ÞC) to 2100ÞF (1149ÞC).

Grain structure and hardness were evaluated on the as-received samples and after each heat treatment cycle. Microstructure comparison showed that the powder-metallurgy alloy exhibited a finer ASTM grain size in the as-received unannealed condition than the conventionally cast alloy. In addition, the powder-metallurgy alloy maintained its fine grain structure after each heat treatment (see Figure 4).

The data clearly indicate that the powder-metallurgy processed alloy maintains a consistently finer grain size than the cast-wrought alloy throughout the heat treatment cycle, especially after exposure to temperatures above 1900ÞF (1038ÞC).

Of particular interest is the grain size difference shown in Figures 5a and 5b after a 2100ÞF, 30-minute cycle. This is a relatively common forging temperature used during the processing of orthopedic implants. After exposure to a temperature of 2100ÞF (1149ÞC), cast-wrought Alloy 1 developed a grain size of ASTM 4.5 µm (see Figure 5a) with an average grain area of approximately 11,000 µm2. In contrast, the powder-metallurgy alloy developed an ASTM grain size of 7 µm2 with an average grain area of approximately 2000 µm2 (see Figure 5b).

Other Characteristics

At 1900ÞF (1038ÞC), which is within the carbide precipitate range for the F1537 alloy tested, a significant difference was observed in the nature of the carbide precipitate between the cast-wrought alloy and the powder-metallurgy alloy.

As shown in Figures 6a and 6b, the cast-wrought alloy developed a banded carbide precipitate, while the powder-metallurgy alloy developed more-uniformly dispersed carbide precipitate. The powder-metallurgy process greatly decreases the likelihood for localized segregation and possible banding, which can occur in the cast-wrought alloy. Co-Cr-Mo alloys are prone to segregation, which can occur in the form of carbide banding. Carbides are chromium- and molybdenum-rich carbon-containing particles. When chromium and molybdenum are captured by cobalt, they are no longer available to fight corrosion or provide strength in the alloy. It is best not to have any carbides; however, if they are present, they should be randomly dispersed so that they do not form a weak spot in the material.

The precipitate in both the cast-wrought alloy and the powder-metallurgy material was completely dissolved at 1950ÞF (1066ÞC). Once dissolved, the carbide does not tend to reprecipitate if exposed again to temperatures from the 1600ÞF (871ÞC) to 1900ÞF (1038ÞC) range. When the alloy is heated to 1950ÞF, all of the phases (in this case, all of the carbide) become a solution (just like sugar in water).

In addition to microstructure evaluations, surface-to-center hardness profiles were also completed on each sample in the as-received, unannealed condition and after each heat-treatment cycle. As a result, when compared with both the hot-worked and warm-worked cast-wrought alloy, the powder-metallurgy alloy was found to have consistently higher hardness on the surface (see Table II), at midradius (see Figure 7), and at the center in the as-received, unannealed condition, as well as after each heat-treatment cycle.

Conclusion

The powder-metallurgy process results in an F1537 bar material that exhibits higher strength, enhanced fatigue resistance, increased hardness, improved microstructural uniformity, and finer grain size in the unannealed condition, as well as after exposure to elevated temperatures. Such attributes allow the material to be manufactured to smaller diameters without the need for cold working and annealing, which can be detrimental to grain size and, subsequently, fatigue strength. In addition to smaller diameter, a powder-metallurgy-processed alloy also lends itself to the manufacturing of near-net special shapes, which could replace castings in certain applications.

Michael Walter is medical alloy specialist for Carpenter Technology Corp. (Reading, PA). He can be contacted at [email protected].

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