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Next-Generation Metal Powders for Medical Additive Manufacturing

By Gaurav Lalwani, PhD Applications Development Engineer – Medical, Carpenter Technology

Additive manufacturing (AM) has gained widespread adoption in the medical device industry. It has inherent design advantages, including easily fabricating complex implant geometries and features not possible using traditional subtractive manufacturing, such as creating porous features permitting increased cell penetration to enhance osseointegration and implant stability. Consequently, AM is routinely used in large-scale production of orthopedic devices such as acetabular cups, tibial base plates and augments, and spinal interbody cages.

Additive manufactured nitinol bone staple with shape memory properties. (All images provided by Carpenter Technologies)

Further, AM provides the opportunity to match implant to patient anatomy and potentially reduce propensity of aseptic loosening. Being a tightly regulated industry, stringent rules exist regarding the material selection process, thus restricting material choices, especially when it comes to implants. As demand for the most common materials increases, regulatory requirements change, or new mechanical properties are needed to meet the ever-advancing designs engineered by medical device producers.

Manufacturers are beginning to refocus on innovation where the additive process begins—metal powder.

Advanced Materials for Medical AM: Titanium

Among the existing metallic implant material choices, Titanium-6 Aluminum-4 Vanadium Extra Low Interstitial (Ti6Al4V ELI) commands over a 90 percent share as the material of choice for medical applications. Increasing demand for powder has renewed attention to the price, properties, and processes for producing titanium powder.

EIGA vs. Plasma Atomized Metal Powders

Ti6Al4V ELI powders of varying size distributions are used throughout additive manufacturing, including directed energy deposition (DED), laser powder bed fusion (L-PBF), and electron beam powder bed fusion (EB-PBF). Not all powder atomization methods are suitable for titanium alloys due to the harmful effect of refractory inclusions on fatigue and toughness. This leaves crucible-free plasma atomization (PA) and electrode inert gas atomization (EIGA) as the preferred methods of producing powder for additive manufacturing. PA uses a pre-alloyed wire fed into plasma torches, creating molten droplets that rapidly solidify as highly spherical powder particles. EIGA continuously feeds a rotating, high-purity, pre-alloyed bar into an induction coil to form a molten stream that free-falls directly into a high-velocity inert gas, producing highly spherical powder particles. Although wire-fed plasma atomized powders have had the first-mover advantage, recently, EIGA produced powders have been shown to be equivalent to PA powders for various physical and chemical properties.

Ti6Al4V Grade 23+ shows mechanical property improvements over standard Grade 23 titanium.

Oxygen, nitrogen, and hydrogen are interstitial elements within the Ti6Al4V alloy that strongly influence the mechanical properties of the end part, and improper control can result in parts with insufficient properties. Oxygen content is also particularly important for additive manufacturing production strategies designed for high material re-use. Powders manufactured by EIGA gain as little as 100 ppm of oxygen on top of the existing oxygen levels from the feedstock (bar). The EIGA furnace uses no refractory materials and therefore is not at risk of introducing any high-density inclusions. A white paper published by Carpenter Additive comparing PA and EIGA atomized Ti6Al4V ELI powders concludes that EIGA powders provide users an economical supply chain option to reduce cost and oxygen content in titanium while ensuring no trace contaminants are present. Additionally, a complementary white paper exploring the mechanical properties of finished parts printed with each of the powder variations concludes parts printed from EIGA powders are a viable option to reduce costs while maintaining or improving the quality of printed parts.

A New Titanium: Ti6Al4V ELI Grade 23+

Titanium alloys exhibit high strength and outstanding corrosion resistance along with favorable biocompatibility, making them suitable for a wide variety of biomedical applications. Most laser 3D-printed implantable medical devices use Ti6Al4V ELI, the extra-low interstitial variant known by its standard designation, Grade 23 (ASTM F3001). In the printed and hot isostatically pressed (HIP) condition, this alloy demonstrates very good tensile strength of over 130 ksi (890 MPa) and elongation greater than 10 percent.

However, manufacturers struggle with two key challenges using Ti6Al4V ELI in powder bed fusion. First, Ti6Al4V powder readily oxidizes during the high-temperature AM process, causing brittleness and cracking, especially at thin-walled or fine-resolution part features. Second, this inherent oxidation process limits Ti6Al4V Grade 23 powder’s reusability since as-built components must contain less than 0.13 wt percent oxygen to conform to specification.

Carpenter Additive developed a proprietary titanium solution to allow the 3D printing of complex, high-quality medical devices with increased mechanical strength and improved ductility. Ti6Al4V Grade 23+ combines controlled powder chemistry with lower oxygen content and optimized print parameters to deliver consistent, high-quality results. Coupled with topology optimization, lattice structures, and other advanced geometries, the displayed improvement in mechanical properties provide the freedom to innovate next-generation medical devices.

Grade 23+ exceeds ASTM standards for Ti6Al4V ELI.

Advanced Materials for Medical AM: Biodur 108

As patient sensitivities to materials rise and regulatory scrutiny increases, the medical design community is searching for alloy alternatives to common stainless steels or cobalt chrome molybdenum for new medical devices. Nickel sensitivity in the United States is estimated at 12 percent by the Center for Devices and Radiological Health (CDRH), and exposure to nickel ions released from the normal wear of medical implants can lead to adverse side effects such as local inflammation, aseptic loosening, and device failure. Furthermore, the EU MDR regulatory up-classification of cobalt as a class II RMR substance increases the warning label requirements on medical devices with a cobalt content higher than 0.10 percent of alloy composition. Carpenter Additive developed Biodur 108 powder to address the up-classification of cobalt and provide a nickel-free option for patients with metal sensitivities that can be effectively used in additive manufacturing production.

An FDA-approved essentially nickel- and cobalt-free alternative for medical applications, Biodur 108 has a non-magnetic, austenitic phase structure maintained by manganese (Mn) and a relatively high nitrogen content, about 1 percent. In addition to austenitic stability, the high nitrogen content improves corrosion resistance and strength, providing significant advantages compared to traditional stainless steels. Preliminary results show the realization of cold work (CW) properties in an additively manufactured component without physical cold working. 3D printed Biodur 108 can achieve strength properties meeting 20 percent wrought Biodur 108 cold work properties, equivalent to 48 percent cold work 316L properties, far exceeding ASTM F3184 AM minimum requirements.

Advanced Materials for Medical AM: Nitinol

Nitinol (NiTi) is widely used in medical devices due to its superior superelasticity, shape memory effect, low stiffness, damping, biocompatibility, and corrosion resistance. As nitinol work hardens in conventional manufacturing processes, additive presents unique opportunities in producing orthopedic implants. However, AM nitinol components’ major challenge is the effective translation of these unique properties from wrought to additive primarily due to the influence of alloy chemistry that creates a barrier to large-scale 3D production. It is, therefore, critical to understand the interplay of the atomization process along with printing parameters on the additively manufactured nitinol component.

Finished nitinol medical device properties are extremely sensitive to chemical composition and thermal gradients in production. Subtle changes in chemistry and heat treatment can lead to large variations in austenite finish (Af) temperature of the finished component. Carpenter Additive created a systematic framework for producing nitinol powder to maintain high nickel levels during atomization tailored to an application’s targeted properties. With printing parameters optimized for laser power, velocity, and hatch spacing, 99.9 percent dense components with up to 6 percent of shape memory strain recovery have been demonstrated. 3D printed bone staples with shape memory effect have been produced, with new orthopedic applications being explored routinely.

As additive manufacturing continues to be used for its unique design and production capabilities, material innovations will enable medical device manufacturers to further improve patient outcomes with next-generation products.

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