Alloy Powders for Medical Applications

By: K. Murray, M. Kearns, N. Mottu

Powder metallurgy offers a number of advantages in the fabrication of high-quality, high-value medical products. It allows refined microstructures with homogeneous properties to be produced and enables near net shape manufacturing, which results in high material utilisation. Near net shape manufacture means less waste and avoids costly machining operations, which is increasingly critical because raw materials costs continue to be volatile. This article reviews the development of gas-atomised (GA) alloy powders for the fabrication of medical products by a variety of established and emerging processes. It includes examples of cobalt alloy powders used in metal injection moulding (MIM) and rapid manufacturing of orthopaedic implants, and MIM of stainless steel powders for medical instruments and orthodontic brackets.

Gas-atomised powders

Specialised powder metallurgy processes are required to meet the rigorous demands of medical applications with respect to mechanical properties, corrosion resistance and biocompatibility. The technologies described here, all of which provide the rapid manufacture of near net shaped parts, are hot isostatic pressing (HIPing) to produce fully dense parts via powder encapsulation, MIM, and emerging technologies using laser or electron beam methods for rapid manufacturing. Hip and knee implants are produced using HIPing and orthopaedic implants such as joint prostheses and post-tumour bone reconstructions are made using laser and electron beam technologies. Dental, laparoscopic and biopsy instruments and dental brackets can also be produced using laser or electron beam methods.

The powders that are essential to the success of these manufacturing processes have common attributes of cleanness, sphericality and good flowability. They are produced by inert gas atomisation and have different particle sizes, which are controlled by the atomising process. HIPing powders are typically coarse and less than 500 µm in size; MIM powders are fine and typically less than 32 µm; and powders for rapid manufacturing are intermediate in size depending on the fabrication process. Each process has its own well defined area on the component size versus component number chart shown in Figure 1.

The processes employed for the different demands of each near net shape technology have been optimised and a range of alloy powders is available including all stainless steels and popular cobalt alloys as well as nickel-free grades and proprietary alloys. Depending on the level of densification achieved, GA powder parts can offer superior performance compared with cast or wrought materials. Refined, homogenous microstructures with minimal inclusion levels can be achieved by limiting the powder size to less than 50 µm. This immediately excludes potentially damaging inclusions that may exceed that size in a conventional casting.

Development of the alloys

The convergence of relatively new powder metallurgy technologies such as MIM and rapid manufacturing and the growing availability of suitable medical alloy powders with the right size and chemistry have provided opportunities to manufacture a widening range of medical products. Examples include permanent hip and knee implants, medical instruments and orthodontic brackets. The selection of alloy powders for medical applications includes various stainless steels, cobalt nickel chromium (Co35Ni20Cr10Mo) and cobalt chromium (CoCrMo ASTM F75) alloys.

The composition of alloy powder can be tailored. For example, 316L stainless steel powder is available at a high nickel level, which reduces the incidence of residual magnetism, and at a low nickel level, which maximises the sintered density. A medical implant grade is also offered, which complies with ISO 5832-1, Implants for Surgery, Metallic Materials, Part I, Wrought Stainless Steel. Similarly, ASTM F75 CoCrMo is produced with low and high carbon levels, which helps to control final mechanical properties and wear resistance.

GA powders are sieved or air classified to produce standard as well as tailored size distributions. MIM grades range from sieved fractions of less than 32 µm down to finest grades at 80% less than 5 µm.
As size decreases, sinterability increases together with precision, surface finish and tolerance control.

Metal injection moulding

MIM successfully combines shape complexity and dimensional control in high-performance materials, while achieving low production and ultimately low product costs. Mass production of medical devices is becoming a practical option for manufacturers thanks to the availability of biocompatible alloy powders and the adoption of MIM as a mainstream fabrication process.

There is a three stage process to produce parts by MIM. The first stage is feedstock formulation, which is achieved by mixing and blending alloy powders (60% by volume or 94% by weight) with surfactants and polymers that lubricate and bond powder particles during sintering. Binder and feedstock formulation need to be adjusted according to the size of powders to achieve adequate melt flow index and injection performance.

The second stage involves injection moulding the feedstock using equipment that is similar to plastic injection moulding machines. Mould tooling used to create three dimensional products is designed to take into account the shrinkage of up to 15% that occurs during sintering.

In contrast to plastic injection moulding, the final and most critical stage in the MIM process is the thermal debinding and sintering cycle. Batch or continuous sintering furnaces produce sintered components with high densities (more than 97%) that require few finishing operations. These operations may include polishing or limited machining to guarantee a specific product’s dimensions and tolerances.

Orthopaedic and orthodontic applications

Traditional investment cast ASTM F75 CoCrMo orthodontic brackets have largely been replaced by those made by MIM, which provides a significantly more cost-effective production route because it reduces the number of process steps and radically reduces the scrap rate. Moulded parts can be reground and remoulded into a new design. In a recent review, Williams highlights some of the successes of MIM in the dental sector including a number of past award winning designs.1

ASTM F75 CoCrMo and stainless steel powders are exploited in other medical applications. There is the potential to progress from an established base in small orthodontic brackets and medical instruments towards larger implants such as knee implants. In spite of a natural conservatism related to lengthy qualification processes, medical device manufacturers, including those who have traditionally relied on investment casting for larger implants, are giving attention to MIM options. Rising raw materials prices and the cost of machining hard alloys weigh in MIM’s favour. Although it is uncertain whether MIM is capable of delivering
satisfactory large scale prosthetics, smaller joints and partial implants could be produced this way.

For CoCrMo alloys, carbon content is critical in determining mechanical properties. At low levels, CoCrMo has a small grain size and high fatigue strength, but poor wear resistance. If carbon levels are raised, wear resistance improves at the expense of fatigue resistance. Nitrogenation of low carbon alloys is an alternative approach to achieving high performance and Johnson et al. have reported the use of different sintering atmospheres to control strength and ductility levels.2 It was shown that GA and water-atomised (WA) powders can give excellent mechanical properties comparable with wrought ASTM F75 CoCrMo (0.2% yield strength ~680 MPa, ultimate tensile strength (UTS) ~1000 MPa, hardness ~26 HRC and elongation to failure ~18%). However, although the high initial oxygen level in WA powders prompted concerns that inclusions will be detrimental to fatigue performance, the GA powder product had low oxygen levels and satisfactory levels of cleanliness.

Rapid manufacturing

One type of rapid manufacturing process, selective laser melting (SLM) is an additive production process that is driven by computer aided design (CAD) files, computed tomography (CT) scan or magnetic resonance imaging (MRI) data, or laser scanned moulds of, for example, dentures. The CAD data is sectioned and redrawn by a laser in a linear pattern directly onto the surface for the powder bed to create a structure in two dimensions. The powder bed then drops as an additional alloy powder layer that is deposited by rolling or wiping across the surface of the bed. This provides a new surface upon which the laser can operate to create the next solidified two dimensional section. The process is repeated and the file is translated into three dimensions with each successive layer of powder (Figure 2). The resolution of the process is impressive with additive layers of 20 µm in height creating fine detail, as well as building hierarchical unit cell type structures (Figure 3). Porous surfaces of this type are ideal for bone and tissue ingrowth3 and can form the basis of lightweight structural members such as orthopaedic implants (joint prostheses) and post-tumour bone reconstruction.

Rapid manufacturing of medical devices using SLM has been employed successfully in various applications, including osteotomy and drilling guides (Figure 4). Commercialisation of SLM technology is in progress with orthodontic applications, including the manufacture of customised prostheses such as crown and bridge frameworks and removable partial denture framework.4 SLM technology utilises high power lasers to melt and bond powder particles in contrast to electron beam melting (EBM), which is a highly controllable process performed in a partial vacuum. The electron beam operates on a coarser powder bed and consequently achieves lower resolution and rougher surface finish. The latter may be advantageous for bone ingrowth.

Cranio-maxillo facial implants made by EBM have been successfully fitted to an increasing number of patients with severe head injuries often associated with road traffic accidents.3 It is estimated that there are approximately 5000 custom made implants provided in Europe per year at a cost of US$6000–7000 each, which represents a market value of approximately US$30–35 million. CT scan, cone beam or MRI data are reverse engineered using the relative symmetry of the skull to produce suitable CAD data, which is manipulated using specialist software to create a stereolithography file suitable for computer aided manufacturing by the electron beam melting process. Fully dense ASTM F75 CoCrMo products are produced with impressive properties that exceed the ASTM minimum requirements (0.2% yield strength ~600 MPa, UTS ~900 MPa, hardness ~34 HRC and elongation to failure ~10%).5

An efficient future option

GA powders are essential precursors for high-performance near net shape manufacturing processes such as HIPing, rapid manufacturing and MIM. Each fabrication route demands distinct powder size ranges for optimum performance and these can be achieved by tuning atomising conditions appropriately. The range of powder compositions available is increasing and includes ISO5832-1 grade stainless steels and CoCrMo alloys. Net shape technologies such as those described are efficient in reducing manufacturing time and increasing materials utilisation. Growth in these areas is therefore set to continue.

1. B. Williams, “Powder Injection Moulding in the Medical and Dental Sectors,” Powder Inj. Moulding Int., 1, 12–19 (2007).

2. J.L. Johnson and L.K.Tan, “Processing of MIM Co-28Cr-6Mo,” Advances in Powder Metallurgy and Particulate Materials, MPIF, Princeton, New Jersey, USA, Part 4, 13–21 (2005).

3. J. Poulkens, “Rapid Manufacturing of Cranio-Maxillo Facial Implants,” 2nd International Conference on Rapid Manufacturing, Loughborough, UK, 11–12 July 2007.

4. R. Bibb, “Rapid Manufacturing of Medical Devices Using Selective Laser Melting,” 2nd International Conference on Rapid Manufacturing, Loughborough, UK, 11–12 July 2007.

Keith Murray, Sales and Marketing Manager; Martin Kearns, Director; Natalie Mottu, Global Tecnhincal Marketing Manager for Sandvik LTD, Long Acre Way, Holbrook, Sheffield, S20 3FS, UK, tel. +44 1142 633 100, e-mail: medicalsolutions@sandvik.com [8], www.sandvik.com/medical. [9]