Fatigue performance of Ti-6Al-4V alloy after surface treatments

BYMustafa Jaleel Aziz

Master's student at the University of Kufa

Faculty of Engineering _ Department of Materials

Supervisor :. Asst. Prof. Dr. Ali Sabea Hammood

IntroductionMetallic biomaterials have essentially three fields of use; these are the artificial hip joints, wires , screw, plates and nails for internal fixation of fractures, and dental implants. Any of these devices must support high mechanical load and resistance of material against breakage is essential. High mechanical properties are needed for structural efficiency of surgical and dental implants. But their volume is restricted by anatomic realities what require good yield and fatigue strengths of metal .

The use of titanium alloys is due to:1- their excellent corrosion resistance.2-high strength to weight ratio and low elastic modulus.** Titanium continues to be widely used in biomedical applications. Ti-6Al-4V alloy is the most frequently used these days .

In orthopedic implants design, it is unavoidable the presence of geometrical fillets such as notches which cause stress locally. It's necessary to pay attention these notches, because they affect on fatigue resistance. In addition about fatigue ratio, fatigue notch factor, notch sensitivity and effect of ultimate strength and notch size on the fatigue strength of Ti-6Al-4V will be discussed.

It can be seen that fatigue-wear interaction plays a significant role in ultimate failure of these medical devices.

Fatigue fracture and wear have been identified as some of the major problems associated with implant loosening and ultimate implant failure. Although wear is commonly reported in orthopedic applications such as knee and hip joint prostheses.

Orthopedic metal alloysThe main goal of design and fabrication of an orthopedic biomaterial is to restore the function and mobility of the native tissue that is considered to be replaced.

The ideal materials for hard tissue replacement should be biocompatible and bio adhesive, possess adequate mechanical properties to tolerate the applied physiological load, be corrosion/wear resistant and finally show good bioactivity to ensure sufficient bonding at the material/bone interface.

The materials used in orthopedic surgery can be divided into five major classes of metals, polymers, ceramics, composites, and natural materials .

Standard metallic orthopedic materials include:1- stainless steels.2-cobalt-base alloys.3-titanium base alloys. with an increasing number of devices being made of titanium and titanium alloys.

The latter alloys are generally preferred to stainless steel and Co-alloys because of their lower modulus, superior biocompatibility and corrosion resistance.

Compared to other biomaterials like ceramics and polymers, the metallic biomaterials offer a wider range of mechanical properties such as high strength, ductility, fracture toughness, hardness, formability, as well as corrosion resistance, and biocompatibility.

Titanium and its alloy as orthopedic biomaterials

The need to find more reliable materials to replace broken or deteriorating parts of the human body is increasing with the increase in number of both younger and older recipients.

Modern surgery and dentistry need metals and alloys of1- extreme chemical inertness2- adequate mechanical strength.

New titanium alloy compositions, specifically tailored for biomedical Commercially pure titanium is a material of choice as an implant because of its biocompatibility resulting in no allergic reaction with the surrounding tissue and also no thrombotic reaction with the blood of human body. The average yield strength of commercially pure titanium is approximately 480 MPa. If a higher strength of the implant is necessary, for example, in hip prosthesis, titanium alloys have to be used. The most widely used alloy, Ti-6Al-4V, reaches yield strength almost double the yield strength of commercially pure titanium.

Table 1. Some characteristics of orthopedic metallic implant materials

Alpha/Beta Titanium Alloy Titanium, Ti

Table 2 show the physical and mechanical properties of Alpha/Beta Titanium Alloy and Titanium, Ti

Table 3. show the common alloying elements of titanium alloys and their stabilizer

Figure 1 . Phase diagram of the Ti-Al-V ternary system

Titanium is a transition metal has a high melting point (1678°C), exhibiting a hexagonal close packed crystal structure (hcp) α up to the beta (882.5°C), transforming to a body centered cubic structure(bcc) β above this temperature.

In contrast, α+β alloys exhibit higher strength due to the presence of both α and β phases. Their properties depend upon composition, the relative proportions of the α/β phases, and the alloy’s prior thermal treatment and thermo-mechanical processing conditions.

Figure 2. show Ti-Al phase diagram

Ti alloys were first used in orthopedics because of their unique properties, including high specific strength, light weight, excellent corrosion resistance and biocompatibility. However, for bone replacement components, the strength of pure Ti is not sufficient and Ti alloys are preferred due to their superior mechanical properties.

In general, alloying elements would lead to an improvement in the properties of Ti for orthopedic applications. Ti-6Al-4V alloy its the most commonly used in orthopedic applications because of their good combination of mechanical properties and corrosion resistance. However, the possible release of toxic ions from aluminum (Al), vanadium (V) and during in vivo corrosion of the implant remains the matter of concern.

Al for exceeding content of 7% at low temperature would lead to possible embrittlement and it may also cause severe neurological, e.g. (Alzheimer’s disease) and metabolic bone diseases, e.g.( osteomalacia)

Similarly, V can alter the kinetics of the enzyme activity associated with the cells and results in potential cytotoxic effects and adverse tissue reactions. Moreover, the oxide layer of Al2O3 and VO2 are less thermodynamically stable than that of TiO2, as their harmful debris may take place in living organism. Thus, it is necessary to develop new Ti alloys that contain non-toxic elements .

New titanium alloys are being introduced to change the chemical composition and the mechanical properties. Some titanium alloys that are in use today or are being considered for use as implant materials are listed in table (3)along with their mechanical properties. The properties in table (3) result from specific heat treatments and will vary depending on their processing parameters. Information in this table permits a comparison of mechanical properties of pure titanium, some alpha/beta titanium alloys and some beta titanium alloys .

Table 3 . Mechanical properties of selected titanium alloys

Some implants are used for short time duration in the human body whereas others remain in place providing a continuous and trouble free function for decades. To avoid a reoperation caused by the implant material, the material must meet certain chemical and mechanical requirements:

1-chemical requirement include high biocompatibility without altering the environment of the surrounding tissue even under deformation.

2- Mechanical property requirement relate to specific strength, modulus, fatigue, creep and fracture toughness which, in turn, relate to microstructures.

In general, most of the Ti alloys offer appropriate mechanical properties for orthopedic applications. The modulus of Ti alloys is closer to those of bone and theoretically provides less stress shielding than those of stainless steel and Co-Cr alloys.

Figure 3. Comparison of Young’s modulus of cortical bone, β type Ti–13Nb–13Zr, α + β type Ti–6Al–4V, 316L stainless steel and Co–Cr–Mo alloy for biomedical applications.

The Young’s modulus of α + β type titanium alloy, Ti–6Al–4V that is the most widely used titanium alloy for biomedical applications, is much lower than those of stainless steel and Co based alloy. However, its Young’s modulus is still much greater than that of cortical bone.

Fatigue of biomaterials

Safety is of concern for any biomaterial and its mechanical aspect, breakdown of a metallic implant, is a major complication. High mechanical properties are needed for structural efficiency of surgical and dental implants. But their volume is restricted by anatomic realities what require good yield and fatigue strengths of the metal.

Cyclic loading is applied to orthopedic implants during body motion, resulting in alternating plastic deformation of microscopically small zone of stress concentration produced by notches or microstructural inhomogeneity. The interdependency between factors such as implant shape, material, processing and type of cyclic load in determination of the fatigue resistance of a component an intricate, but critical, task.

Standard fatigue tests include tension/compression, bending, torsion, and rotating bending fatigue testing. Unfortunately, no standard for fatigue evaluation of biomaterials testing has yet been established, a variety of testing conditions being encountered in reported fatigue studies of orthopedic materials .

One of the main reasons for concern about fatigue of biomaterials arises from the adverse host-tissue response to wear debris generated by the fatigue process. This appears to be a natural defence mechanism of the body. The wear debris often invokes an inflammatory and immunological response. This in turn causes blood clotting processes, leukocytes, macrophages and, for severe cases, giant cells to move in on the foreign wear particles resulting in interfacial problems between the implant and the host tissue.

Metal fatigue has been extensively studied . The fatigue strengths of commonmetallic implant alloys used in hip replacements such as stainless steel, cobalt chrome and titanium, and their relationship to their microstructures, surface and corrosion properties have been reported .

Fig. 4 shows the fatigue strength (in air) of some common implant alloys using the S/N approach. The use of hot isostatic pressing (HIP) which introduces fine microstructures also has a pronounced improvement.

Figure 4 . Fatigue strength of some common implant alloys

The combined mechanical and chemical processes play a vital role in crack initiation. Fig. 5 shows schematically how the formation of slip planes can break through the protective oxide film during fatigue .

Figure 5 . Schematic illustration of the formation of fresh slip planes in a body fluid environment during fatigue, exposing unprotected regions to electrochemical and biological activities

Figure 6. Fretting fatigue in PBS environment of Ti-6Al-4V

Fretting fatigue of implant alloys based on the S/N approach has been studied. In titanium implants, wear debris has given rise to blackening of surrounding tissue. Wear particles also cause implant loosening. Fretting fatigue is essentially a micromotion phenomenon and often occurs at interfaces such as between the metal and the cement in the case of a hip prosthesis. This can result in a drastic reduction in fatigue strength.

Table 4. Effect of surface preparation on the fatigue of α and α/β titanium alloys

Table 4. illustrates the effect of surface finishing techniques on the fatigue strength of titanium alloys:

Influence of Surface Treatments on Fatigue Strength of Ti6Al4V Alloy

Titanium alloys do not exhibit good seizure toughness. In general, a surface treatment must be applied if the component is subjected to sliding contact. For surface hardening, it is well known that oxygen and nitrogen treatments can be used because they are easily available, inexpensive elements. However, these treatments cause the fatigue strength to decrease which is a problem.

The reason for the fatigue strength decrease due to oxygen diffusion treatment is residual tensile stress on the surface. Shot peening is an effective technique to improve the fatigue strength of oxygen diffusion treated titanium.

Influence of surface treatment and its condition on fatigue strength1- Influence of notch on fatigue strength:Figure 7. shows the fatigue strength of plane and notched specimens for fatigue test. The fatigue strength of the plane specimens is 600 MPa, which decreased to 315MPa with a U notch, for fatigue strength reduction of about 50%. For the V notch specimen, the fatigue strength deteriorated to 230 MPa. Fatigue strength is thus clearly affected by the presence of a notch.

Figure 7 . Results of fatigue tests (without the OD treatment).

2- Influence of the OD treatment on fatigue strength:Figure 8. shows the fatigue strength of the OD treated specimens in fatigue tests. Fatigue strength decreased after the OD treatment was applied. In the OD treated fatigue test specimens, fatigue strength decreased by 460 Mpa compared to that of the non-OD treated test specimens. We think this is caused by the surface oxygen diffusion layer. In addition, notch sensitivity increased due to the surface oxidation diffusion layer, and fatigue strength decreased. Furthermore, the fatigue strength of the OD treated U notch and V notch test specimens decreased to 160 MPa and 120 MPa, respectively.

Figure 8 .Result of fatigue tests (with the OD treatment).

Improvement method of fatigue strength after the OD treatmentChange in fatigue strength by shot peening:Figure 9 . shows the fatigue strength after shot peening, for both OD treated and non-OD treated specimens. The date indicate an improvement in fatigue strength for the OD treated after shot peening test specimen .The surface residual stress is changed to compressive stress of compression from tensile stress by shot peening.

Figure 9 . Results of fatigue test of shot peenig specimens.

Effect of Laser Surface Treatment on Ti-6Al-4V

The present study aims at enhancing the wear resistance of Ti-6Al-4V by laser surface melting and nitriding and subsequently, studying the influence of laser surface treatment on the corrosion resistance in a simulated body fluid and also the bio-compatibility.

** Laser surface melting leads to an increased volume fraction of acicular martensite and a decreased volume fraction of the β phase in the microstructure. Laser surface nitriding leads to the formation of titanium nitride dendrites. The micro-hardness could be improved up to a maximum of 450 Hv in laser surface melting and 900–950 Hv in the case of laser surface nitriding as compared to 260 Hv of the as-received substrate.

Detailed characterisation of the microstructure and phases in the laser surface melted and laser surface nitrided Ti-6Al-4V was ascertain the characteristics of the modified layer. In addition, mechanical measurements and the corrosion properties of the surface melted and surface nitrided layers were evaluated and compared with the as-received Ti-6Al-4V.

Characteristics of the melted/nitrided zoneFigure 10. shows the scanning electron micrograph of the top surface of the as received Ti-6Al-4V substrate used in the present study. The microstructure consists of elongated α-Ti and the presence β-Ti lamellae in the inter-granular region. The as-received microstructure showed that it had been cold rolled. The average grain size varied between 15 µm and 25 µm.

Figure 10 . Scanning electron micrograph (SEI) of as received Ti-6Al-4V.

Figures 11 .(a, b) show the microstructures of (a) laser surface melted and (b) laser surface nitrided Ti-6Al-4V at a power of 600 W. Laser surface melting led to a significant refinement of the microstructure and due to the very high cooling rate caused acicular α’ martensite to form.

Figure 11. Scanning electron micrograph (SEI) of the top surface of (a) laser surface melted and (b) laser surface nitrided Ti-6Al-4V lased with a laser power of 600 W.

Properties of the melted/nitrided layer

Micro-hardness of the top surface and cross section of the laser surface melted and laser surface nitrided Ti-6Al-4V was carefully measured t observe the influence of laser surface modification on micro-hardness and its distribution. Figure 12. shows the variation of micro-hardness with depth from the surface of laser surface melted (plot 1 and plot 2) and laser surface nitrided (plot 3 and plot 4) Ti-6Al-4V samples lased under different processing conditions.

Laser surface melting was found to improve the micro-hardness of the melted region from 260 Hv of as received surface to a maximum of 490 Hv. The micro-hardness in the melted region was however, almost uniform along depth and decreased to a value of substrate micro-hardness at the solid-liquid interface. The improvement in micro-hardness of the melt zone is attributed to refinement of microstructure and formation of acicular martensite during rapid quenching. On the other hand, micro-hardness of the nitrided layer was improved to 2–3 times (to a maximum of 950 Hv) as compared to that of as-received Ti-6Al-4V.

Figure 12 .Variation of micro-hardness with depth from the surface of laser melted Ti-6Al-4V with a power of 800 W (plot 1), 600 W (plot 2) and laser nitrided Ti-6Al-4V with a power of 800 W (plot 3), 600 W (plot 4) respectively.

Q / An alloy of titanium, Ti-6Al-4V, has a fracture toughness of 50 MP m1/2. A plate of this material has a penny-shaped crack of diameter 10 mm. Take the yield stress of this alloy to be 980 MPa. What is the stress that can be applied in service to this plate without causing failure? Given: K = 2σ(a/π)1/2 .

Sol// D = 10 mm 2a = 10 mm a = 0.005 m

K = 2σ(a/π)1/2 50 = 2σ * (0.005 / π)1/2

σ = 626.4 Mpa

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