Fabrication of Al–Si coating on Ti–6Al–4V substrate by mechanical alloying

Al–Si coatings were synthesized on Ti–6Al–4V alloy substrate by mechanical alloying with Al–Si powder mixture. The as-prepared coatings had composite structures. The effects of Al–Si ratio, milling duration and rotational speed on the microstructure and oxidation behavior of coating were investigated. The results showed that the continuity and the anti-oxidation properties of the coating were enhanced with the increase of Al–Si weight ratio. The thickness of the coating largely increased in the initial 5-hour milling process and decreased with further milling. A rather long-time ball milling could result in the generation of microdefects in coating, which had an adverse effect on the oxidation resistance of coating. Both the thickness and the roughness of the coating increased with the raise of rotational speed. The low rotational speed would lead to the formation of discontinuous coating. The rotational speed had a limited effect on the coating oxidation behavior. Dense, continuous and high-temperature protective Al–Si coatings could be obtained by mechanical alloying with Al–33.3?wt.%Si powder at the rotational speed ranging from 250 to 350?rpm for 5?h.     

Effect of microstructure on the fatigue properties of Ti–6Al–4V titanium alloys

Through an analysis on microstructure and high cycle fatigue (HCF) properties of Ti–6Al–4V alloys which were selected from literature, the effects of microstructure types and microstructure parameters on HCF properties were investigated systematically. The results show that the HCF properties are strongly determined by microstructure types for Ti–6Al–4V. Generally the HCF strengths of different microstructures decrease in the order of bimodal, lamellar and equiaxed microstructure. Additionally, microstructure parameters such as the primary α (α_p) content and the α_p grain size in bimodal microstructures, the α lamellar width in lamellar microstructure and the α grain size in equiaxed microstructures, can influence the HCF properties.     

Effect of surface roughness on fatigue performance of additive manufactured Ti–6Al–4V

Additive manufacturing is increasingly considered for production of high quality, metallic, aerospace parts. Despite the high potential of this manufacturing process to reduce weight and lead time, the fundamental understanding of additive manufactured Ti–6Al–4V material is still at an early stage, especially in the area of fatigue and damage tolerance. This paper covers the effects of inherent surface roughness on the fatigue life. In the as built condition, metallic parts have a poor surface texture, which is generally removed in fatigue critical areas. It is shown that the fatigue properties of Ti–6Al–4V samples, produced by direct metal laser sintering and electron beam melting, are dominated by surface roughness effects. A simple model based on an equivalent initial flaw size is formulated.     

Torsional fatigue resistance of plasma sprayed HA coating on Ti–6Al–4V

The torsional strength of plasma sprayed hydroxyapatite (HA) coatings was studied under static and cyclic loading. The torsional shear tests were conducted in a frustum test device developed in this laboratory, which adapted to various coating thicknesses. The interfacial fatigue resistance was measured in terms of interfacial fatigue strength defined as the average maximum stress (_fmax). A staircase fatigue method was employed to determine the interfacial fatigue strength; this method resolved the uncertainty in detecting coating failure during torsion fatigue. The values for coating shear strength and shear fatigue strength obtained from the torsional tests did not differ from those obtained by previous tensional shear tests in this laboratory. The fatigue strength of one million cycles was about 35% lower than static shear strength. This finding might be used for estimating fatigue life span without cyclic loading tests.     

Effect of primary alpha phase variation on mechanical behaviour of Ti–6Al–4V alloy

Small punch tests (SPTs) have been carried out at room temperature to correlate the microstructural variation of Ti–6Al–4V alloy with that of SPT parameters. Microstructural variation in terms of different volume fractions of primary alpha phase of Ti–6Al–4V alloy has been introduced as a result of solution annealing at different temperatures followed by thermal aging. Small punch test parameters, i.e. total area under the load vs displacement curve, area under the zone of elastic bending, plastic bending and plastic instability have been found to increase from the content of 10% primary alpha phase to 20% primary alpha phase and then these are decreasing from the content of 20% primary alpha phase to 30% primary alpha phase.     

A mechanism study on characteristic curve of residual stress field in Ti–6Al–4V induced by wet peening treatment

The microstructure evolution of Ti–6Al–4V alloy induced by wet peening treatment was studied in this work. The results show that the dislocation interaction dominates the grain refinement process. The modified layer could be divided into reconfiguration regime, unstable regime and metastable regime with depth. Corresponding to the characteristic curve of residual stress field, the maximum of compressive residual stress locates in the unstable regime and the dislocation annihilation and rearrangement lead to the decrease of the compressive residual stress in the near surface layer.     

Deformation characteristics of Ti–6Al–4V

Abstract

The deformation characteristics of Ti–6Al–4V have been established by torsion testing in the temperature range 800–1150°C. Constitutive equations are proposed for both the β-region and the α+β-region which, it is suggested, may have some practical applications. Extensive optical and electron microscopy have established that dynamic recovery is the operative deformation mode in the β-region, while dynamic recrystallisation predominates in the α+β-region.

MST/806     

Formation of hydroxyapatite layer on bioactive Ti and Ti–6Al–4V by simple chemical technique

Bioactive coatings on cp-Ti and Ti–6Al–4V were prepared by a simple chemical technique. Specimens of cp-Ti and Ti–6Al–4V were initially immersed in a 5 M NaOH solution at 60 °C for 24 h which resulted in the formation of a porous network structure composed of Na₂Ti₅O₁₁ and TiO₂. The specimens were then immersed in a Ca-rich solution either at 60 °C or at 36.5 °C for 24 h. During this treatment Na⁺ was released and Ti–OH groups were formed. Subsequently, TiO₂ dissociated from the Ti–OH group and combined with calcium ions to form calcium titanate (CaTiO₃), which was embedded in a titania gel layer during the immersion period. The specimens were then immersed in r-SBF at 36.5 °C for 1–30 days. After immersion in r-SBF for 3 days, HAp (hydroxyapatite) spheroids began to deposit on the substrates, and within a week the surfaces were covered. The HAp spheroids were 5 μm in size with a Ca/P ratio of 1.68 which was close to bone-like apatite (1.67). The average thicknesses of HAp layer after immersion in r-SBF for 3 days, 1 week, and 2 weeks were 3.8, 5.6, and 6.4 μm, respectively. A scratch test, used to evaluate the adhesive strength of the HAp layer, showed that the HAp layer was not scraped off until the applied load reached 26 N.     

Low cycle fatigue life of Ti–6Al–4V alloy under non-proportional loading

Multiaxial low cycle fatigue life of Ti–6Al–4V under non-proportional loading was studied. Strain-controlled multiaxial fatigue tests at room temperature were carried out using tubular specimens. The strain paths employed were push–pull loading, reversed torsion loading, and two kinds of 90° out-of-phase loadings. The former two loadings are proportional loading tests where the principal directions of stress and strain are fixed in the cycle. The latter two are non-proportional loading tests where there is a 90° phase difference between axial and shear loadings, and the principal directions are cyclically rotated continuously. Failure lives are reduced obviously by non-proportional loadings in comparison with those in proportional loading tests. This paper focuses on determining a suitable fatigue model for evaluating the failure lives of Ti–6Al–4V under multiaxial loading.     

Low cycle fatigue properties of AA2519–Ti6Al4V laminate bonded by explosion welding

The paper presents the effect of heat treatment on low cycle fatigue properties and the course of fatigue cracking in AA2519/Ti6Al4V sandwich laminates. The laminate is made by explosion welding, using an intermediate layer in the form of an alloy AA1050. The strength properties of bonds were determined during fatigue tests conducted in conditions of constant total strain amplitude ε_ac. This paper describes the impact on the test conditions on the shape of the hysteresis loop, the course of the cyclical strengthening/weakening of the material and the fatigue strength. Test results were extended to the analysis of the surface of the resulting fatigue fractures. The characteristic traits of fatigue cracking were investigated by examining the microstructure of the fracture surface using the scanning electron microscope.     

Investigation of joining Al–C–Ti cermets and Ti6Al4V by combustion synthesis

The paper has addressed a route for the welding of titanium alloy (Ti6Al4V) and Al–C–Ti powders by the combustion synthesis (CS) method. Al–C–Ti powders were compressed in the titanium alloy pipes with relative densities of 65%, and then the powder compact was sintered by two reaction mode at the same time as the annulus of titanium alloy and the synthesized product were joined. The paper has studied the effects of reaction mode and Al content in starting powders on the structure and property of the welded joints. And it has also discussed the microstructure of welded joints by laser-induced combustion synthesis (LCS). The mechanical properties of the welding seam have been also tested. The results show that LCS welding has realized fusion welding and the welding seam has good mechanical properties. Furthermore, SEM analysis has indicated that nano-size grains of TiC were formed in the joint layer.     

Internal fatigue crack initiation in drawn Ti–6Al–4V wires

Fatigue cracks in titanium alloys are often found to initiate at faceted alpha grains. In the very high cycle fatigue regime, crack initiation tends to shift from the surface towards the interior of the material, and more initiation facets can be found on the fracture surface. In this study, fatigue tests were performed on drawn and heat-treated Ti–6Al–4V wires. Only a few samples fractured due to interior initiation. The facets at the initiation sites of these samples were not flat, but had markings on the nano-scale, and were highly inclined. A possible explanation for these aspects is the crystallographic texture of the wire, and a reflection is made on the suggested mechanisms of facet formation.

This paper is part of a Themed Issue on Euromech 570: Interface-dominated materials.     

Strain-controlled fatigue properties of dissimilar welded joints between Ti–6Al–4V and Ti17 alloys

The aim of this study was to evaluate the microstructure, hardness and cyclic deformation behavior of electron beam welded dissimilar joints of Ti–6Al–4V and Ti17 (Ti–5Al–4Mo–4Cr–2Sn–2Zr) titanium alloys. The welding resulted in a significant microstructural change across the joint, with hexagonal close-packed (hcp) martensite α′ and orthorhombic martensite α″ in the fusion zone (FZ), α′ in the heat-affected zone (HAZ) of Ti–6Al–4V side, and coarse β in the HAZ of Ti17 side. A characteristic asymmetrical hardness profile across the dissimilar joint was observed with the highest hardness in the FZ and a lower hardness on the Ti–6Al–4V side than on the Ti17 side, where a soft zone was observed. The dissimilar joint exhibited a lower Young′s modulus and higher cyclic strain hardening exponent than both Ti–6Al–4V and Ti17 base metals (BMs), and had the monotonic and cyclic yield strengths lying in-between those of two BMs with higher values for Ti17 alloy. Both BMs and joint showed essentially symmetrical hysteresis loops and equivalent fatigue life, and exhibited cyclic stabilization at lower strain amplitudes up to 0.6%, while cyclic softening occurred after initial cyclic stabilization at higher strain amplitudes. The initial cyclic stabilization was shortened with increasing strain amplitude. In the Ti–6Al–4V BM fatigued at a high strain amplitude of 1.2%, a short initial cyclic hardening emerged, corresponding to the presence of twinning and its resistance to the dislocation movement. Fatigue failure of the dissimilar joint occurred in the HAZ of Ti17 side where the soft zone was present, with crack initiation from the specimen surface or near-surface defect and crack propagation characterized by typical fatigue striations.     

Microindentation study of Ti–6Al–4V alloy

In order to study the micromechanical behavior of Ti–6Al–4V alloy, microindentation experiments were performed with five different maximum loads of 100, 150, 200, 250 and 300 mN, and with three loading speeds of 6.4560, 7.7473 and 9.6841 mN/s respectively. The experimental results revealed that loading speed has little influence on microhardness and Young’s modulus. Microindentation hardness experiments showed strong indentation size effects, i.e. increase of indentation hardness with the decrease of indentation load or depth. Then microindentation constitutive equation that described the stress as a function of the strain was proposed through dimensional analysis. And the finite element simulation results showed that the predicted computational indentation data from developed constitutive equation can track the microindentation experimental data of Ti–6Al–4V alloy.     

Microplasticity in HCF of Ti–6Al–4V

Previous research has shown that Ti–6Al–4V exhibits pronounced stress ratio effects under high cycle fatigue (HCF) loading. At high stress ratios (R>0.7), a transition of failure mode occurs from traditional surface fatigue crack initiation and growth to bulk-dominated damage initiation and coalescence of multiple microcracks consistent with a ductile tensile test. At these high stress ratios, ratchetting was shown to occur (Int. J. Fatigue 21 (1999) 679; Mech. Time-Dependent Mater. 2 (1999) 195), leading to progressive strain accumulation until final failure. This study explores the microstructural origins of this stress ratio transition in HCF using computational micromechanics. The material being studied is a two-phase Ti–6Al–4V plate forging, consisting of a duplex microstructure with a hexagonal close-packed (hcp) α-phase and lamellar grains with layers of body-centered cubic (bcc) β-phase and secondary hcp α-phase. Crystallographic slip is the dominant mode of plastic deformation in this material. A 2-D crystal plasticity model that incorporates nonlinear kinematic and isotropic hardening at the slip system level is implemented into the finite element method to simulate the cyclic plasticity behavior. The finite element model is used to qualitatively understand the distribution of microplasticity in this alloy under various loading conditions. For typical HCF stress amplitudes, it is shown that microstructure scale ratchetting becomes dominant at R=0.8, but is insignificant at R=0.1 and 0.5. Reversed cyclic microplasticity is insignificant at all three stress ratios. The effects of phase morphology and orientation distribution are shown to affect the microscale plastic strain distribution in terms of the location and magnitudes of the plastic shear bands that form within clusters or chains of primary α grains. The results of the finite element modeling are also considered in light of previous experimental results.     

High and low cycle fatigue behavior of linear friction welded Ti–6Al–4V

Linear friction welded Ti–6Al–4V was investigated in fatigue at various stress amplitudes ranging from the high cycle fatigue (HCF) to the low cycle fatigue (LCF) regime. The base material was composed of hot-rolled Ti–6Al–4V plate that presented a strong crystallographic texture. The welds were characterized in terms of microstructure using electron backscatter diffraction and hardness measurements. The microstructural gradients across the weld zone and thermomechanically affected zone of the linear friction welds are discussed in terms of the crystallographic texture, grain shape and hardness levels, relative to the parent material. The location of crack nucleation under fatigue loading was analyzed relative to the local microstructural features and hardness gradients. Though crack nucleation was not observed within the weld or thermomechanically affected zones, its occurrence within the base material in LCF appears to be affected by the welding process. In particular, by performing high resolution digital image correlation during LCF, the crack nucleation site was related to the local accumulation of plastic deformation in the vicinity of the linear friction weld.     

Effect of rotation speed on microstructure and mechanical properties of Ti–6Al–4V friction stir welded joints

The effect of tool rotation speed on microstructure and mechanical properties of friction stir welded joints was investigated for Ti–6Al–4V titanium alloy. Joints were produced by employing rotation speeds ranging from 400 to 600 rpm at a constant welding speed of 75 mm/min. It was found that rotation speed had a significant impact on microstructure and mechanical properties of the joints. A bimodal microstructure or a full lamellar microstructure could be developed in the weld zone depending on the rotation speeds used, while the microstructure in the heat affected zone was almost not influenced by rotation speed. The hardness in the weld zone was lower than that in the base material, and decreased with increasing rotation speed. Results of transverse tensile test indicated that all the joints exhibited lower tensile strength than the base material and the tensile strength of the joints decreased with increasing rotation speed.     

Antimicrobial effects of oxygen plasma modified medical grade Ti–6Al–4V alloy

Biomedical titanium alloy (Ti–6Al–4V) has good mechanical properties and cytocompatibility but post-operative implant-related bacterial infection is a big concern. Therefore, it is very important to suppress adhesion of bacteria on the implants or even kill the bacteria by proper surface modification. In this study, oxygen plasma immersion ion implantation (O-PIII) is employed to produce antibacterial effects on medical grade titanium alloy. The correlation between the surface chemistry and topography on antibacterial behavior is systematically investigated. Colony forming unit (CFU) counting is carried out to evaluate the adhesion of bacteria on the surface and enhanced green fluorescent protein (EGFP) mouse osteoblastic cells are used to study the cytocompatibility after the plasma treatment. Our results suggest that the nanostructured TiO₂ layer produced by O-PIII can significantly suppress bacterial adhesion while the original cytocompatibility can be retained. The nanoscale TiO₂ layer is promising in the prevention of implant-related bacterial infection on orthopedic implants.     

Comparative experimental study on effects of conventional and ultrasonic deep cold rolling processes on Ti–6Al–4V

Abstract

The present paper presents the experimental studies on the treatment of Ti–6Al–4V using conventional and ultrasonic deep cold rolling (CDCR and UDCR) processes. The UDCR process is a novel technique in which the ultrasonic technology was incorporated into the concept of the CDCR process, which enables use of superimposed dynamic forces onto considerably reduced static forces applied on a part surface during treatments by UDCR. Considerable improvements in surface topography and near surface characteristics of treated material were achieved in the present study. Both processes exhibited comparable results in surface finish and induced compressive residual stresses, while the UDCR process produced superior results than the CDCR process in work hardening. The investigations suggest that the UDCR process could provide technological benefits for the treatment of thin walled aerospace components to be carried out without geometry distortion, which would lead to improved fatigue life and resistance to failure mechanisms.     

Life assessment methodologies incoroporating shot peening process effects: mechanistic consideration of residual stresses and strain hardening Part 1 – effect of shot peening on fatigue resistance

Abstract

Shot peening is a well known process applied to components in order to improve their fatigue resistance. In recent years, there has been an increasing interest in including the effects of the shot peening process in life assessment models since this would allow a reduction in conservatism compared to those in current application. The present paper seeks to review firstly the effects of the shot peening process (surface roughening, strain hardening and compressive residual stresses) and how the magnitude of these effects can be determined both experimentally and numerically. The reasons for the beneficial effect of shot peening on fatigue resistance are reviewed; this includes consideration of how different operating conditions can affect the magnitude of the benefit. The second part of the review details the life assessment approaches which have been developed to date incorporating these effects and seeks to identify the areas in which further development is still required before the models can be applied in structural integrity assessments.