Papers by Keyword: Ti₆Al₄V Alloy

Abstract: Orthogonal metal cutting process involves large plastic deformation accompanied by excessive heat generation. This work addresses the thermal-mechanical responses of the workpiece material at the tool-workpiece contact. In this respect, the orthogonal cutting process of Ti-6Al-4V using CVD diamond tool is simulated using finite element method. The cutting condition consists of cutting speed, V=180 m/min, feed rate, t=0.125 mm/rev and width of cut of 1.25 mm. Eulerian formulation with coupled thermal-mechanical analysis is employed in the model. The Johnson- Cook constitutive equation is employed for Ti-6Al-4V workpiece material to accurately simulate the formation of shear bands. The stick-slip friction condition is modeled at the tool-chip interface. The sliding coefficient of friction of 0.8 and the limiting shear stress of 700 MPa for stick-slip condition are determined experimentally. Results show that high temperature and temperature gradient concentrate in the primary shear zone and the contact area between the tool rake face and the chip. A primary shear band is predicted in the workpiece ahead of the tool-workpiece contact face while the secondary shear band is formed in the chip. This highly-deformed shear band is revealed in the microstructure of etched chips. The predicted high strain rate results in build-up edge at tool cutting edge-chip contact. Low cutting condition of V=150 m/min, t=0.125 mm/rev promotes stagnant zone formation that helps preserve the cutting edge of the tool. The maximum predicted temperature at the cutting edge is in excess of 700 °C. Such high temperature level facilitates diffusion of carbon elements into the chips and conversely, elements of titanium into the CVD diamond tool.

Abstract: In the present work two different hydroxyapatite nanofilms (50 and 100 nm thick) have been successfully deposited on titanium implants that were previously laser macrostructured in order to assess the influence of the thickness of nanometric calcium phosphate coatings on the osseointegration. Cylindrical implants were tested in a sheep tibia model together with titanium alloy controls achieving very good osseointegration results. Laser macrostructured titanium alloy implants have shown improved bone regeneration when coated with nanometric films of carbonated HA. The pulsed laser deposited nanofilm has promoted bone in-growth deep into the laser ablated craters. There were no significant differences between the two coating thicknesses, neither when assessed with electron microscopy or classical optical methods. This result suggests that the 50 nm coating is as effective as the 100 nm one, therefore implying that the thickness limit for such a bioactive layer to stimulate bone growth may be even further below.

Abstract: Apatite-mullite glass-ceramics have been developed as an alternative to hydroxyapatite for use in vivo as a bioactive, osseoconductive biomedical alloy coating. In the cerammed state, they present a number of advantages including control over dissolution rates and mechanical properties by altering the composition of the parent glass or heat treatment regime. In the present study, a simple sedimentation route was used to coat a biomedical titanium alloy, commonly used for orthopaedic applications. The material was deposited as a glass and cerammed in situ to create a well adheared coating that resisted delamination or cracking. To investigate the nature of the coating-substrate reaction, a number of characterization techniques were used to examine the crystallization behaviour of the glass, the glass-ceramic microstructure, and the interfacial reaction region composition. The presence of products such as titanium silicides and unexpected pores are explained by proposed reaction routes between the titanium and glass coating.

Abstract: The effect of initial lamellar structure of β heat treated Ti-6Al-4V alloy on the globularization behavior during the multi-step forging was investigated. Specimens of different lamellar thicknesses were upsetted and stretched by side pressing repeatedly, i.e. multi-step forging, at the sub-transus temperature to break down the lamellar structure. The microstructural changes after multi-step forging were analyzed in the light of globularization behavior. The results showed that the thick lamellar structure was more difficult to be transformed to homogeneous equiaxed structure than thin lamellar structure.

Abstract: The dynamic globularization behavior during hot working of Ti-6Al-4V alloy was investigated by high temperature torsion tests. The torsion tests were carried out to investigate microstructure evolution occurring during dynamic globularization in Ti-6Al-4V alloy. The torsion tests were performed under a wide range of temperatures and strain rates with true strain up to 2. The flow curves revealed that the amount of flow softening for the fine alpha-lamellae structure was higher than that for the coarse alpha-lamellae structure under the temperature of 900oC. The effects of hot deformation parameters and initial microstructures on the dynamic globularization were analyzed.

Abstract: Ti6Al4V alloys are considered difficult materials to machine, especially at high cutting speeds due to their low thermal conductivity and specific heat which causes high localized cutting temperatures at the tool-workpiece interface. For these reasons machining titanium alloys usually results in excessive tool wear and its low modulus promotes chatter. This regenerative vibration or chatter is a significant problem. In the investigation reported here, vibration monitoring has been used to optimise machining processes by correlating machining process parameters with vibration severity. Machining experiments were carried out under wet and dry machining conditions while vibrations were measured and analysed. The results have demonstrated that the application of a vibration monitoring system can be an important tool to increase machining speed.

Abstract: The surface of Ti-29Nb-13Ta-4.6Zr (TNTZ) subjected to gas nitriding at 1023–1223 K was investigated in comparison with the conventional biomedical titanium alloy, Ti-6Al-4V ELI (Ti64). After gas nitriding, the microstructures near the surface of these alloys were observed by optical microscopy, X-ray diffraction, Auger electron spectroscopy, and X-ray photoelectron spectroscopy. In both alloys, two titanium nitrides (TiN and Ti2N) are formed and the α phase precipitated by gas nitriding. Furthermore, oxygen impurity in the gas nitriding atmosphere reacts with the titanium nitrides; thus, TiO2 is formed at the outermost titanium nitride layer. The surface hardening was also evaluated by Vickers hardness measurement. The Vickers hardness near the surface of TNTZ and Ti64 increases significantly by gas nitriding.

Abstract: In this paper, based on analyzing the properties of medical Ti-6Al-4V Titanium alloy, the author takes the purpose of studying the machinability of the medical Ti-6Al-4V Titanium alloy and aims at improving the tool durability. The study starts from the tool material, geometrical parameters of the tools, usage for the cutting and other aspects in order to achieve the suitability of selecting cutting tool as well as the optimization of choosing cutting usage which lays the foundation for further investigation of the machinability of the medical Ti-6Al-4V Titanium alloy and carry out the online optimization of cutting parameters.

Abstract: A niobium modified layer on Ti-6Al-4V surface was obtained by means of the plasma surface alloying technique. The oxidation behavior of the modified layer was investigated and compared with Ti-6Al-4V at 700°C~900°C for 100h. Composition and microstructure of Ti-6Al-4V and the modified layer after oxidation at 900°C for 100h were analyzed using XRD and SEM respectively. The experimental results showed the oxidation behavior of Ti-6Al-4V at 900°C for 100h was obviously improved after the niobium alloying process.