Chapter 1c Metallic Biomaterials: Titanium and Titanium Alloys

Abstract

Comparison of international standards for titanium and titanium alloys (Refs. 1, 2)

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1c.1 Composition

Table 1c.1 Comparison of international standards for titanium and titanium alloys (Refs. 1, 2a)

Table 1c.2 Chemical composition of commercially pure (cp)-titanium (wt%) (Refs. 1–3)

Table 1c.3 Chemical composition of (α + β)-titanium alloys (wt%) (Ref. 1)

Table 1c.4 Chemical composition (wt%) of β and near-β titanium alloys (Refs. 4–7)

1c.2 Physical Properties

Table 1c.5 Physical properties of cp-Ti grade 1 (Refs. 8–11)

Table 1c.6 Physical properties of (α + β)-titanium alloys (Refs. 4, 8–9, 12–14)

Table 1c.7 Influence of alloying elements and heat treatment on Young’s modulus (Refs. 5, 15–18)

1c.3 Processing of cp-Ti and Ti Alloys

Provided that the following characteristics of titanium are taken into consideration, almost all processing procedures are possible:

• 1. High affinity to oxygen, nitrogen, and hydrogen gases

• 2. High reactivity to all metals to produce intermetallic compounds

• 3. Relatively low Young’s modulus and therefore backspringing

• 4. Relatively low thermal conductivity

• 5. Tendency to stick to tools

1c.3.1 Hot Working and Heat Treatment

Titanium and titanium alloys are fabricated into semifinished products by conventional methods such as forging, rolling, pressing, and drawing. When Ti materials are heated, care must be taken to avoid an excessive adsorption of oxygen, nitrogen, and hydrogen. Therefore, heating and annealing should take place in a neutral or slightly oxidizing atmosphere. During heating in a gas-fired furnace, direct contact with the flame must be avoided because of the risk of hydrogen pickup and local overheating. In a short heating period, oxygen pickup is restricted to the surface area. This surface zone must be removed by chemical or mechanical methods. Hydrogen is able to penetrate the matrix rapidly; therefore, a reducing atmosphere must be avoided. The hot working temperature depends on the alloy composition and should be selected to obtain the best mechanical properties and grain structure (A1–A9 according to ETTC2, Ref. 17). Table 1c.8 summarizes the deformation temperatures for the various Ti materials. Table 1c.9 lists the temperature ranges and recommended annealing times for stress relieving, soft annealing, and solution treating with age hardening. When the cross section is very small, annealing is favorably carried out in a high-vacuum furnace. Prior to this annealing treatment, the oxide film must be removed from the surface to avoid diffusion of oxygen into the material.

Table 1c.8 Deformation temperatures for various titanium materials (Refs. 19, 20)

Table 1c.9 Recommendation for the heat treatment of cp-Ti and Ti alloys (Refs. 2, 8)

1c.3.2 Working of Sheet

At room temperature cp-Ti grades 1 and 2 can be worked very well, and grades 3 and 4 only moderately well. Titanium alloys, because of their high yield/tensile strength ratio, can be worked only under certain conditions.

For deep drawing, special coatings in the form of polymer foils have proved to be effective. At high temperatures colloidal graphite and common hot press greases with graphite or molybdenum disulfide additives have been successful. The Fe-, Ni-, and Cr-contents should be limited to 0.05, 0.1, and 0.33 wt%, respectively, to allow during a short annealing treatment (1–5 min at 750–850 °C) a grain growth producing an average grain size of 50–70 μm. Due to this grain growth a deformation by twinning, with a resulting increased deep drawability, occurs (Ref. 19).

Superplastic forming is a material-saving and cost-reducing process for manufacturing parts of a complicated shape because it can be carried out together with diffusion welding in a single operation. Fine-grained alloys like Ti6Al4V and Ti5Al2.5Fe can be used for superplastic deformation. Other special deformation processes such as stretch forming, spinning, or explosion forming are also possible.

1c.3.3 Descaling

The average thickness of the oxide layer on the surface of cp-Ti as a function of temperature can be found in Table 1c.10 and the composition of the oxide layer on cp-Ti and Ti alloys can be found in Table 1c.11. The oxide layer on the surface of thick-walled pieces generated during deformation and/or heat treatment is removed by sandblasting and/or pickling. The workpiece is treated in an aqueous solution of 20 wt% HNO₃ and 2 wt% HF.

Table 1c.10 Thickness of oxide formed on the surface of cp-Ti as a function of temperature (Ref. 2)

Table 1c.11 Nature of oxides formed on cp-Ti and titanium alloys (Ref. 22)

Thin-walled pieces are merely pickled in an electrolytic solution or salt bath. It is important that not only the surface layer of oxide but also the underlying oxygen enriched diffusion zone is removed. Otherwise, the machinability and the service life of turning and milling tools would be negatively affected.

1c.3.4 Machining

The machining of titanium materials presents no difficulties provided that the following properties are taken into account:

• 1. Relatively low thermal conductivity which may cause high thermal stresses at the cutting edge of the tool

• 2. Low Young’s modulus which applies pressure to the tool

• 3. Tendency to stick to the tool

Titanium materials must be machined at low cutting speeds, at a relatively high feed rate, and with an ample supply of coolant (sulfur-containing oil; mixture of tetrachloride carbon, molybdenum sulfide, and graphite; 5 % aqueous solution of sodium nitrite, 5–10 % aqueous solution of water-soluble oil or sulfurized chlorinated oil). The cutting tools should be sharp and mounted as rigidly as possible. Recommended parameters for turning and milling are given in Table 1c.12. Since titanium dust and chips can easily catch fire, safety precautions must be taken. Threads should be cut on a lathe, as thread-cutting discs are subject to seizure.

Table 1c.12 Recommendations for the cutting and milling of cp-Ti and Ti alloys (Ref. 8)

Sawing causes no difficulties if a high blade pressure is used and the coolant supply is sufficient. Coarse-toothed blades (4 teeth per inch) are recommended.

For grinding, aluminum oxide (5–10 m/s) and silicon carbide (20–30 m/s) can be used.

1c.3.5 Soldering and Brazing

Immediately before soldering and brazing, the oxide layer present on the surface of titanium material must be removed. For direct applications using a torch, aluminum–zinc and tin–zinc solders are suitable. The higher temperatures required for brazing present the difficulty of avoiding the formation of intermetallic phases. As with almost all metals, titanium forms brittle intermetallic phases in the fusion zone. The only exception is silver, so that this metal forms one of the main constituents of brazers. The sources of heat used for brazing and soldering are the acetylene torch, high-frequency induction coils, infrared radiation, an inert-gas-shielded arc with graphite or tungsten electrodes, furnaces with an argon atmosphere (min. 99.99 % and/or a dew point below −50 °C), and high-vacuum furnaces. If brazing is not performed under vacuum or in a controlled atmosphere, fluxes are necessary to dissolve the oxide layers and prevent a pickup of gases.

1c.3.6 Welding

The inert-gas-shielded arc processes (TIG and MIG) are mainly used for fusion welding. In special cases resistance, ultrasonic, electron beam, diffusion, and laser welding are applied. With the cp-Ti grades the weld attains mechanical properties approximating those of the base metal. A slight decrease in ductility may occur with high tensile grades. Under passivating conditions, titanium welds have the same corrosion resistance as the base metal. On the contrary, in reducing media the weld may be subjected to a more severe corrosive attack than the base metal. During the welding operation, the weld, the heat-affected zone, and the underside of the weld are shielded from the atmosphere. The filler rod used is an uncoated wire of the same grade or of a grade with a lower hardness than the base metal. Careful preparation of the joint is necessary; that is, surface impurities must be removed by grinding or pickling in order to avoid porosity. Even fingermarks can produce a hardening of the weld. A single layer can weld sheets up to 2.5 mm thick. In order to avoid local oxygen concentrations oxidation products, such as those found at the tip of the electrode, must be cut off. The effectiveness of the inert gas is responsible for the welding rate. The optimum argon flow rate has proved to be about 6–8 l/mm. After welding, the appearance of a dark blue or gray oxide layer indicates an insufficient inert gas shielding and an embrittlement of the weld due to oxygen and/or nitrogen pickup. The hardness of a good weld may exceed that of the fully recrystallized base metal by a maximum of 50 VHN. If, after a slight grinding of the surface, a hardness test should give a higher value, the weld must be completely removed because of embrittlement.

Electron beam welding is particularly suitable for titanium materials. It offers many advantages such as very narrow seams and small heat-affected zones, weldability of thick diameters, high welding speed, and reproducibility of even complex welds.

Titanium materials can be spot welded without any particular preparation under similar conditions to those of stainless steel. Using flat-tipped electrodes, spot welding can be performed without inert gas. A hardening of the zone by up to 50 VHN compared with the base metal is regarded as normal and does not diminish the strength of the joint. Seam and flashbutt welding are also possible if an argon atmosphere is used.

Diffusion welding is of particular importance for titanium materials because these materials are more amenable to a homogeneous band in the solid state than other metals. After welding, the joint zone shows a higher temperature under high vacuum or, in an inert atmosphere, a microstructure very similar to that of the base metal.

1c.4 Mechanical Properties

Table 1c.13 Mechanical properties of commercially pure titanium (Ref. 3, 21)

Table 1c.14 Influence of a cold deformation on the mechanical properties of commercially pure titanium (Ref. 23)

Table 1c.15 Mechanical properties of β and near-β-titanium alloys (experimental alloys) (Refs. 6, 15, 24–26)

Table 1c.16 Influence of heat treatment on the mechanical properties of β- and near-β-titanium alloys (Refs. 6, 15)

Table 1c.17 Mechanical properties of (α + β)-titanium alloys (Refs. 2, 27–29)

Table 1c.18 Influence of a solution treating and ageing on the mechanical properties of Ti6Al4V

Table 1c.19 Influence of a plasma nitriding (PVD) on the mechanical properties of Ti6Al4V (Ref. 30)

Table 1c.20 Fracture toughness of Ti-alloys (Refs. 6, 31)

1c.5 Fatigue

Table 1c.21 High cycle fatigue strength σ _B and rotating fatigue strength σ _R of pure titanium and titanium alloys (Wöhler curves) (Refs. 15, 32–37)

Table 1c.22 Rotating bending fatigue tests of unnotched and notched titanium alloys (Ref. 31)

Table 1c.23 High cycle fatigue strength of hip endoprostheses of titanium alloys, measured in 0.9 % NaCl solution at 37 °C. Testing conditions similar to DIN 58840 (simulated loosened shaft, 50 mm) (Refs. 34, 36, 38)

Table 1c.24 Influence of the mean stress S _m on the fatigue strength of Ti6Al4V (Ref. 39)

Table 1c.25 Influence of the notch factor on the fatigue strength of Ti6Al4V (Refs. 40–43)

Table 1c.26 Influence of interstitial elements on the rotating bending strength of Ti6Al4V (Ref. 44)

Table 1c.27 Influence of texture and test directions on the rotating bending fatigue strength of Ti6Al4V (fine equiaxed microstructure in rolled plates) (Ref. 45)

Table 1c.28 Influence of heat treatment (annealing and precipitation hardening, respectively) on the fatigue strength of Ti6Al4V (Ref. 43, 46)

Table 1c.29 Influence of the beta field heat treatment on the fatigue strength of Ti6Al4V (Ref. 47)

Table 1c.30 Influence of the surface treatment on the rotating bending fatigue (fine lamellar microstructure, produced by annealing in 15 min/1050 °C/H₂O + 1 h/800 °C/H₂O) (Ref. 48)

Table 1c.31 Influence of the surface treatment on the rotating bending fatigue of Ti6Al4V (fine equiaxed microstructure produced by rolling at 800 °C/H₂O + 1 h/800 °C (HP)) (Ref. 48)

Table 1c.32 Influence of surface working on the rotating bending of Ti6Al4V (Ref. 40, 49)

Table 1c.33 Influence of plasma nitriding (PVD) on the rotating bending fatigue of Ti6Al4V (Ref. 30)

1c.6 Corrosion and Wear

Table 1c.34 Electrochemical data for titanium and titanium alloys in 0.1 M NaCl under different conditions (Refs. 24, 50–54)

Table 1c.35 Polarization current (i) and polarization resistance (R _c) of titanium and titanium alloys in pure saline at 37 °C (Ref. 50) and in 0.9 % NaCl with a stable redox system [Fe(CN)₆ ⁴VFe(CN)₆ ³] (Ref. 55)

Table 1c.36 Repassivation time in 0.9 % NaCl and breakdown potential in Hanks’ solution of cp-Ti and Ti alloys (Refs. 24, 32, 56, 57)

Table 1c.37 Electrochemical data for anodic titanium and Ti6Al4V at 37 °C in different solutions (de-aired) versus standard calomel electrode (SCE) (Ref. 58)

Table 1c.38 Electrochemical data for cp-Ti and Ti alloys after 7 days in artificial saliva (Ref. 59)

Table 1c.39 Repassivation time of titanium and titanium alloys in contact with different metallic materials (Ref. 60)

Table 1c.40 Influence of the surface treatment on the fretting corrosion behavior of Ti6Al4V (Ref. 61)

Table 1c.41 Influence of the surface treatment on the wear behavior of Ti6Al7Nb as a result of a pin-on-disk test (Ref. 62)

Table 1c.42 Volumetric wear rate of Ti6Al4V and Ti6Al7Nb under different sliding speeds and normal load (Ref. 63)

Table 1c.43 Influence of ion implantation of nitrogen on the wear properties of commercial cp-Ti and Ti6A14V (Ref. 64)

Table 1c.44 Rate of formation of corrosion products for cp-Ti in Hanks’ solution during current-time-tests (Ref. 65)

1c.7 Biological Properties

Table 1c.45 Biocompatibility of cp-Ti and Ti alloys, survival rate of L132 cells incubated with powders (Ref. 66)

Table 1c.46 Influence of the implantation time (in vivo) on the surface roughness and peak-to-valley (P–V) height of Ti6A14V femoral heads (Ref. 67)

1c.8 Nitinol: Shape Memory

Table 1c.47 Properties of Nitinol (shape memory, Ni45Ti) alloy (Refs. 50, 68–71)

Table 1c.48 Resulting oxide layer thickness on Nitinol stents using different preparation techniques (Ref. 72)

Table 1c.49 Ion release from Nitinol incubated with L132 cell culture (Ref. 73)

Table 1c.50 Survival rate of L132 cells incubated with Nitinol powder (Ref. 74)