Review article
Microstructure and mechanical behavior of Ti–6Al–4V produced by rapid-layer manufacturing, for biomedical applications

https://doi.org/10.1016/j.jmbbm.2008.05.004Get rights and content

Abstract

The microstructure and mechanical behavior of simple product geometries produced by layered manufacturing using the electron beam melting (EBM) process and the selective laser melting (SLM) process are compared with those characteristic of conventional wrought and cast products of Ti–6Al–4V. Microstructures are characterized utilizing optical metallography (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and included α (hcp), β (bcc) and α (hcp) martensite phase regimes which give rise to hardness variations ranging from HRC 37 to 57 and tensile strengths ranging from 0.9 to 1.45 GPa. The advantages and disadvantages of layered manufacturing utilizing initial powders in custom building of biomedical components by EBM and SLM in contrast to conventional manufacturing from Ti–6Al–4V wrought bar stock are discussed.

Introduction

Metal orthopedic joint replacements (hip or knee joints) and bone plate surgeries now number in the millions worldwide annually, with knee joint replacement surgeries in the US alone numbering in excess of 300,000 annually. Most hip and knee implants are fabricated from wrought or cast bar stock by CNC, CAD-driven machining, or powder metallurgy (PM) production methodologies; including HIP and powder injection molding of near-net-shape components (Long and Rack, 1998, Gibson, 2005). Most of these millions of joint replacements, bone plates, etc. are generic, mass-produced components which do not work well with patients having an abnormal or unusual anatomy. In these situations, custom-designed implant components are preferred or required. This is also particularly true for cranioplasty especially where the missing piece of bone is large, requiring an implant component to be fabricated to follow the overall skull curvature (Eufinger and Saylor, 2001). A further challenge in implant component fabrication is the necessity to manufacture complex shapes, including thin-walled sections, where cutting operations can take a long time owing to significant material removal; up to 80% of bar stock from which knee implants are fabricated is converted to metal chips or scrap material.

In recent years solid freeform fabrication (SFE) — also variously called rapid prototyping (RP), layered manufacturing (LM) or rapid manufacturing (RM) — or direct digital manufacturing (DDM) has provided a “renaissance in manufacturing” (Chuna et al., 2003). SFF technologies are able to fabricate complex shapes of a custom-designed component using precursor powders to build these shapes through computer-controlled, self-assembly by sintering or melting powder layers using either a laser or an electron beam. Until recently, SFF technologies were not able to build custom-designed components from biocompatible metal or alloy powders such as stainless steel, Ti, Ti–6Al–4V and other Ti-alloys, and cobalt–chromium (Co–Cr) alloys (29% Cr, 6% Mo, balance Co by weight). Furthermore, desirable monodispersed metal or alloy powders with a uniform rapid solidification microstructure were also not generally available until early in this century.

Cormier et al. (2004) recently described the characterization of H13 steel (components using electron beam melting (EBM), while Harrysson and Cormier (2005) described the EBM fabrication of custom orthopedic implants from commercially available powders of H13 steel and Ti–6Al–4V. However neither the powders nor the resulting EBM fabricated components were characterized in the context of their microstructures or associated mechanical behavior. More recently, Hiemenz (2007) discussed the EBM process generally while Murr et al. (in press) have compared the microstructure and mechanical behavior of both the precursor Ti–6Al–4V powder and simple EBM fabricated components along with the microstructure and mechanical properties of several commercial, wrought products of Ti–6Al–4V. In addition to these papers describing EMB-DDM utilizing metal powders to fabricate biomedical and related components, Vandenbroucke and Kruth (2007) have also recently described selective laser melting of biocompatible metals and alloys for RM.

Niinomi (2008) has intimated in a recent review article that, “Among metallic biomaterials such as stainless steels and Co–Cr alloys, Ti and its alloys exhibit the most suitable characteristics for biomedical applications because of their high biocompatibility, specific strength, and corrosion resistance (Niinomi, 2001)”. Indeed, the majority of hip and knee implants over the past decade have been variously processed Ti–6Al–4V (Long and Rack, 1998, Chuna et al., 2003, Niinomi, 2007, Yaszemski, 2004) and recent EBM-RM produced implants and other experimental products fabricated from commercial Ti–6Al–4V powders have shown exceptional promise for custom-designed implant components (Harrysson and Cormier, 2005, Christensen et al., 2007, Christensen, 2007, Murr et al., in press).

In this paper we review the thermomechanical processing of wrought and cast Ti–6Al–4V products along with their associated microstructure and mechanical behavior. Microstructures are characterized by optical and electron microscopy (including scanning electron microscopy (SEM) and transmission electron microcopy (TEM) and include α(hcp:a=0.295nm;c=0.466nm) and β(bcc:a=0.33nm) phase morphologies, dislocation substructures, twinning, martensitic transformations, and other phases (Destefani, 1990, Lampman, 1990, Eylon and Froes, 1990, Niinomi, 1998, Ding et al., 2002, Froes, 2004, Leutjering and Williams, 2003, Williams and Leutjering, 1980, Williams et al., 1987). These phase morphologies as well as dislocation, twin, and martensitic substructures affect mechanical behavior — in particular indentation hardness and tensile properties. Additionally and more importantly, we discuss and compare Ti–6Al–4V products fabricated from precursor powders utilizing EBM and laser melting (specifically selective laser melting (SLM)); especially comparing their microstructure and mechanical behavior. Finally, we discuss the advantages and disadvantages of EBM and laser melting-rapid prototype manufacturing especially in the context of prospects for Ti–6Al–4V custom implant production and the tailoring of biomedical component properties and performance through systematic microstructure-mechanical behavior control during powder layer manufacturing.

Section snippets

Wrought and cast Ti–6Al–4V: Thermomechanical processing, microstructure, and mechanical behavior

Manufacturing processes associated with Ti products include (1) casting (2) wrought (forging/milling from ingots), (3) powder metallurgy (P/M), and relatively new processing methods by direct digital manufacturing such as electron beam melting (EBM), direct metal laser sintering (DMLS), laser-engineered net-shaping (LENS) and ultrasonic consolidation (UC). Wrought products account for 70% of the Ti and Ti alloy market (Lampman, 1990). Wrought Ti materials undergo several melt cycles in order to

Direct digital manufacturing by electron beam melting (EBM): Microstructures and mechanical behavior of Ti–6Al–4V components

Fig. 6 illustrates a schematic view of the ARCAM EBM S400 system. The system builds layers approximately 100 μm thick from atomized powder as represented in Fig. 7 for Grade 5 Ti–6Al–4V powder. The EBM system in Fig. 6 builds parts from the bottom up by scanning the focused electron beam ((2) and (3) numbered circles) at nominally 103 mm/s to selectively melt specific areas of the powder bed using a 3D-CAD system while powder is continuously added from the powder cassettes (4) to the top of the

Direct digital manufacturing using selective laser melting (SLM): Microstructures and mechanical behavior of Ti–6Al–4V components

Fig. 14 illustrates a schematic view of the EOSINT M270 selective laser melting (SLM) or direct metal laser sintering (DMSL) system. This system builds layers approximately 30 μm thick from atomized powder represented in Fig. 15. Fig. 15(b) illustrates that this powder, in contrast to nominal EBM Ti–6Al–4V precursor powder shown in Fig. 7(b), has a skewed, smaller size distribution with an average particle size (diameter) of 21 μm. The smaller size powder for SLM facilitates laser beam melting

Comparative advantages and disadvantages of EBM and SLM in contrast to wrought Ti–6Al–4V manufactured products

The standard production methods for generic mass-produced medical implants involves 5-axis or 6-axis CNC machining of bar stock. A typical knee implant component machined from bar stock produces as much as 80% metal waste chips. Lengthy machining times, material and equipment costs not only exacerbate generic component production, but in many instances generic components will not work well with patients having abnormal anatomy, requiring reconstruction of the patient joint to fit the implant.

Summary

Solid freeform fabrication (SFF) methodologies or rapid (prototype) manufacturing have been demonstrated to be extremely successful in fabricating complex shapes of custom-designed components from CT or other 3D CAD data utilizing polymer or ceramic powders to build these components layer-by-layer. In this review we have demonstrated the EBM and SLMS processes to be viable layer manufacturing technologies utilizing metal or alloy powders; specifically Ti–6Al–4V atomized powders which can be

Acknowledgements

This research was supported in part by Mr. and Mrs. MacIntosh Murchison Chair Endowments at the University of Texas at El Paso. We thank Terry Hoppe and Wayne Meyers of Stratasys, Inc. for providing test materials built on the ARCAM system, along with Ulf Lindhe of ARCAM-Sweden. We also thank Shane Collins of EOS of North American, Inc. for providing the SLM built specimens examined in this paper.

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