Subsurface microstructure alteration has been a major concern to implement micromachining of titanium alloy in the high-tech industry. To quantitatively promulgate the underlying mechanisms of this alteration, a discrete dislocation dynamics-based model is proposed and used to simulate the subsurface defects and their evolution under different cutting conditions. The model considers the subsurface dislocation configuration and inner stress distribution during the orthogonal cutting of titanium alloy. The results show that subsurface defect structure consists of plenty of dislocation dipoles, twining dislocation bands, and refined grains after cutting. In the primary shear zone, two different characteristics of subsurface damage layers can be found, the near-surface damage layer and deep-surface damage layer, which have different structural natures and distribution features. Moreover, it is found that high cutting speed and small depth of the cut can suppress the formation and propagation of subsurface defects. A powerful inner stress state would promote the distortion of the lattice and result in a microcrack within the subsurface matrix. The simulation results have been compared with experimental findings on the machined surface and subsurface of similar materials, and strong similarities were revealed and discussed.

In this study, a mesoscale dislocation simulation method was developed to study the orthogonal cutting of titanium alloy. The evolution of surface grain structure and its effects on the surface mechanical properties were studied by using two-dimensional climb assisted dislocation dynamics technology. The motions of edge dislocations such as dislocation nucleation, junction, interaction with obstacles, and grain boundaries, and annihilation were tracked. The results indicated that the machined surface has a microstructure composed of refined grains. The fine-grains bring appreciable scale effect and a mass of dislocations are piled up in the grain boundaries and persistent slip bands. In particular, dislocation climb can induce a perfect softening effect, but this effect is significantly weakened when grain size is less than 1.65 μm. In addition, a Hall–Petch type relation was predicted according to the arrangement of grain, the range of grain sizes and the distribution of dislocations.

The effect of high-energy electropulsing treatment (EPT) on the microstructure evolution, mechanical properties, and fracture behavior of as-treated Ti–6Al–4V alloy strips was investigated. EPT was found to accelerate phase transition and microstructure evolution of quasi-single-phase titanium alloy strips at a relatively low temperature, and obtain characteristic duplex microstructure and Widmanstatten microstructure. The EPT-induced microstructural changes increased elongation-to-failure remarkably with a slight decrease in tensile strength. Fracture surface observation and three-dimensional analysis showed that transition from small-shallow dimple colony to big-deep colony fracture took place with an increase in frequency of EPT. The rapid phase change of the Ti–6Al–4V alloy strip under EPT was attributed to the enhancement of nucleation rate and atomic diffusion resulting from the coupling of the thermal and athermal effects. It is supposed that EPT can provide a highly efficient method for the intermediate-softening annealing of titanium alloy sheet/strips.

The feasibility of an accelerated test method on revealing the high cycle fatigue (HCF) limit stresses of a near alpha titanium alloy was successfully verified firstly during the rotary bending tests. The stress-controlled low cycle fatigue (LCF) tests at room temperature were then carried out over a range of maximum stresses and stress ratios to reveal the basic LCF strength. Finally, the core emphasis was focused on the influences of the predamage from LCF loadings on the subsequent HCF limit stresses corresponding to the life of 106 cycles. A total of eight levels of predamage from LCF with different stress levels, stress ratios, and proportions of LCF life were introduced, which resulted in obvious deterioration of the HCF stress limits (even under only 2% of expected LCF life). The fracture analysis exhibited that there were typical LCF failure characteristics in crack initiation regions of specimens under prior LCF loadings.

The microstructure of Ti-35Nb-7.2Zr-5.7Ta (TNZT) and Ti-35Nb-7.2Zr-5.7Ta-0.5B (TNZTB) alloys under different heat treatment conditions has been analyzed. The solution-treated and water-quenched TNZT sample consists mainly of β phase with a very small amount of fine athermal ω precipitate. Precipitation of α can be observed when solution-treated samples are directly aged at 580°C for 8 h. The microstructure of the samples subjected to single-stage aging at 300°C or 400°C consists of ω precipitates in equiaxed β grains. Second stage aging at 580°C for 8 h after first stage of aging at 300°C or 400°C results in the replacement of ω precipitates by secondary α. In all of these samples, the amount of ω or α phase was very small, and therefore they could not be detected by X-ray diffraction studies. However, analysis of selected area diffraction patterns obtained from transmission electron microscopy studies confirms their presence. The addition of boron leads to the formation of dispersed precipitates of TiB in the β matrix of the TNZT alloy and also refines the β grains in the microstructure. However, other microstructural features of the TNZTB alloy are similar to those of the TNZT alloy.

Optimum machining parameters are of great concern in manufacturing environments, where economy of machining operation plays a key role in competitiveness in the market. Many researchers have dealt with the optimization of machining parameters for milling operations. In this paper, optimization procedures based on particle swarm optimization algorithm are developed for find machining parameters in milling operation. It describes development and utilization of the methodology that determines optimum Pareto’s front analyzing feed, speed and depth for milling operation. The relationships between machining parameters and the performance measures of interest are obtained by using experimental data and a swarm intelligent neural network system. Results show that particle swarm optimization is an effective method for solving multi-objective optimization problems, and also, that an integrated system of neural networks and swarm intelligence can be used to solve complex machining optimization problems.

In the present study, the formation and growth of an oxide film on a titanium alloyTi6Al4V surface immersed in 3% NaCl solution using anodisation at various imposedpotentials was investigated by two electrochemical techniques, cyclic polarisation andimpedance measurements. In the cyclic polarisation studies at different scan rates, theobtained curves show a large range of passivation, due to the formation of a protectiveoxide film, generally TiO2. In addition, the negative hysteresis that appearedduring the cyclic polarisation with the shift of the corrosion potential in the more nobledirection (approximately +260 mV) may be explained by a reduction of the corrosionproducts. It was found that the lower the scan rates the lower the passivation current,and the scan rates did not have a significant effect on the potential or current ofcorrosion. The impedance measurements (Nyquist and Bode plots) at various imposedpotentials show that Ti6Al4V has a capacitive behaviour in the passive field, due to theformation and growth of the oxide film.

Deposition of Ti/N compositionally gradient film onto Ti-6Al-4V alloy substrates was carried out by reactive DC sputtering, not only to improve the blood compatibility of the alloy but also to relax the stress concentrated at the interface between the film and the alloy substrate. The compositional gradient was realized by varying continuously the nitrogen content in Ar-N2 sputter gas during deposition. In Auger electron spectroscopy (AES) analysis, Auger spectra were acquired in the N(E) mode using the beam brightness modulation (BBM) method to overcome the problem of the peak overlap of the principal Auger nitrogen transition peak (N-KLL) with one of titanium peaks (Ti-LMM). The deposited film appeared to be uniform and adhesive. TiN formation at the surface of the film was assumed, because of its yellow gold color and the X-ray diffraction (XRD) pattern for it. Under scanning electron microscopy, it was found that the surface had fine particles dispersed on a smooth accumulated deposit and that this depositing method improved the structural property of the film at the surface. According to AES in-depth profiles, the nitrogen (N) concentration in the film gradually decreased in the depth direction from the surface toward the alloy, confirming that a Ti/N compositionally gradient film had formed on the alloy substrate.