Titanium alloys used biomedically
Titanium alloys has been extensively used for the manufacturing of metal orthopedic joint replacements and bone plate surgeries. They are normally produced from wrought or cast bar stock by CNC, CAD-driven machining, or powder metallurgy production. Each of these techniques comes with inherent advantages and disadvantages. Wrought products come with an extensive material loss during machining into the final shape of the product and for cast samples the acquirement of a product in its final shape somewhat limits further processing and treatment (e.g. precipitation hardening), yet casting is more material effective. Traditional powder metallurgy methods are also more material efficient, yet acquiring fully dense products can be a common issue.
With the emergence of solid freeform fabrication (3D printing) the possibility to produce custom-designed biomedical implants (e.g. hip joints) has been realized. While it is not applied currently on a larger scale, freeform fabrication methods offers the ability to recycle waste powder (from the manufacturing process) and makes for selectivity tailoring desirable properties and thus the performance of the implant. Electron Beam Melting (EBM) and Selective Laser Melting (SLM) are two methods applicable for freeform fabrication of Ti-alloys. Manufacturing parameters greatly influence the micro-structure of the product, where e.g. a fast cooling rate in combination with low degree of melting in SLM leads to the predominant formation of martensitic alpha-prime-phase, giving a very hard product.
Ti-6Al-4V
This alloy has good biocompatibility, and is neither cytotoxic nor genotoxic.Ti-6Al-4V suffers from poor shear strength and poor surface wear properties in certain loading conditions:
Bio compatibility: Excellent, especially when direct contact with tissue or bone is required. Ti-6Al-4V's poor shear strength makes it undesirable for bone screws or plates. It also has poor surface wear properties and tends to seize when in sliding contact with itself and other metals. Surface treatments such as nitriding and oxidizing can improve the surface wear properties.
Ti-6Al-7Nb
This alloy was developed as a biomedical replacement for Ti-6Al-4V, because Ti-6Al-4V contains vanadium, an element that has demonstrated cytotoxic outcomes when isolated.Ti-6Al-7Nb contains 6% aluminium and 7% niobium.
Ti6Al7Nb is a dedicated high strength titanium alloy with excellent biocompatibility for surgical implants. Used for replacement hip joints, it has been in clinical use since early 1986.