Hydroxyapatite coating on Ti6Al4V alloy by a sol–gel method

Using trimethyl phosphate and calcium nitrate tetrahydrate as the calcium and phosphorus precursors, respectively, HA films were prepared layer by layer by a sol–gel method. The phase constitution, microstructure and calcium/phosphorus (Ca/P) molar ratio of the sol–gel films were studied by X-ray diffraction (XRD) and electronic probe microanalysis (EPMA). The results show that the sol–gel films have high crystallinity and are composed of HA and CaO phases, and the Ca/P ratio is slightly higher than the theoretical value in HA because of the loss of phosphorous element. Two typical cauliflower-like and lamellar structures were observed in the films. Cauliflower-like structure, which increases the biological reactivity of the implant surface towards natural bone, formed mainly at low drying temperature and high calcining temperature, while the lamellar structure formed when the drying temperature is high (500 °C or above).     

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.     

Amorphous Ti–Cu–Ni–Al alloys prepared by mechanical alloying

Ti _x (CuNi)_90–x Al₁₀ (x = 50, 55, 60) amorphous powder alloys were synthesized by mechanical alloying technique. The evolution of amorphization during milling and subsequent heat treatment was investigated by scanning electron microscopy, X-ray diffraction, differential scanning calorimetry and transmission electron microscopy. The fully amorphous powders were obtained in the Ti₅₀Cu₂₀Ni₂₀Al₁₀, Ti₅₅Cu_17.5Ni_17.5Al₁₀ and Ti₆₀Cu₁₅Ni₁₅Al₁₀ alloys after milling for 30, 20 and 15 h, respectively. Differential scanning calorimetry revealed that thermal stability increased with the increasing (CuNi) content: Ti₆₀Cu₁₅Ni₁₅Al₁₀, Ti₅₅Cu_17.5Ni_17.5Al₁₀ and Ti₅₀Cu₂₀Ni₂₀Al₁₀. Heating of the three amorphous alloys at 800 K for 10 min results in the formation of the NiTi, NiTi₂ and CuTi₂ intermetallic phases.     

Hardness and wear resistance improvement of surface composite layer on Ti–6Al–4V substrate fabricated by powder sintering

A wear resistant surface composite layer on Ti–6Al–4V substrate was fabricated using powder sintering method. The surface composite layer consisted of Ti–6Al–4V matrix and different fractions of TiN particles as reinforcement phase. The surface layer and the substrate were directly bonded together while the powders were cold formed and then sintered at an elevated temperature. The two layers showed good metallurgical bond. In this study, 5%, 10% and 15% TiN weight fractions were adopted to fabricate the surface composite layer. Effects of TiN addition on the microstructure, hardness and wear resistance were investigated. It was found that the wear resistance of the surface composite layer was improved due to the addition of TiN compared to that of pure Ti–6Al–4V.     

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.     

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.     

Effects of milling intensity on amorphisation of Al–Fe mixed powders by mechanical alloying

Abstract

The effects of milling intensity on the amorphisation of Al–Fe powder mixtures containing 10, 15, 20, and 25 at.-% Fe, during mechanical alloying by a high energy planetary ball mill, have been investigated. Two levels of milling intensity, 80g and 150g (where g is acceleration due to gravity), were adopted. Two different pathways of amorphisation reaction were found; Al–Fe solid solution → amorphous phase; and Al–Fe solid solution → formation of Al₅Fe₂ phase → amorphous phase. The former reaction was observed in Al–10, –15, –20, and –25 at.-%Fe compositions at the lower milling intensity and in Al–10 at.-%Fe at the higher milling intensity. The latter reaction was observed in Al–15, –20, and –25 at.-%Fe compositions at the higher milling intensity. The two pathways of amorphisation reaction are explained mainly by a kinetics approach.     

Investigation on WC–Al composite coatings of AZ91 alloy by mechanical alloying

In this paper, WC–Al composite coatings of AZ91 alloy prepared by mechanical alloying have been investigated in detail. It was found that the premixing process of composite powders has no significant effect in promoting the formation of uniform coating of WC–Al powders. Under the optimised conditions, i.e. the composition of composite powders of (10 g WC–6·5 g Al–3 g AZ91–0·5 g Mg), ball-to-powder weight ratio of 14∶1 and milling duration of 12 h, the average thickness of composite coating can be remarkably increased to 38·01 μm. Compared with the bare substrate, the Brinell hardness of the specimen with WC–Al composite coating can be significantly increased by about 90·01%.     

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.     

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.     

Fabrication of ultrafine W – Cu powders by mechanical alloying

Abstract

Ultrafine composite powders of W – 15 wt-%Cu, W – 25 wt-%Cu, and W – 35 wt-%Cu have been fabricated by mechanical alloying. The effects of type of mill, process control agent, temperature of milling, and ball/powder ratio on the final products have been evaluated. The results show that the planetary ball mill possesses a higher impact energy intensity than that of the vibratory ball mill. The optimum milling time is confirmed by the formation of a nanocrystalline microstructure in the planetary ball mill after optimisation of the milling parameters. A steady state between cold welding and fracture is attained with a milling time of up to 25 h in the planetary ball mill under optimised conditions. Crystallites with sizes of 7 – 8 nm for W – Cu composite powders have been obtained after 25 h of ball milling. The powders obtained after mechanical alloying have been characterised in terms of their size, shape, phase constitution, and microstructural features using X-ray diffraction and scanning electron microscopy.     

Survey about effects of shot peening conditions on fatigue performances of Ti–6Al–4V mechanical specimens featured by different cross-section geometries

Abstract

The present article provides a technical survey of the effects of shot peening conditions on the fatigue performance of Ti–6Al–4V specimens representative of the material (and the surface treatment) used in helicopter rotor hubs. As the effects of shot peening on non-plain surfaces have been fairly neglected in the scientific literature, the present work attempts to define the effects of shot peening on different specimens, featuring specific cross-section geometries, namely smooth and sharp edged specimens. Experimental tests also include measurements of the residual stress field caused by shot peening and the definition of the fatigue limit (by means of the ‘staircase method’) for all the tested specimen configurations. The present study proceeds with an optical and scanning electron microscopic investigation of the dynamics and causes of the different fatigue limits associated with the geometrical features. The present study conveys a strong correlation between specimen geometry and shot peening microstructural effects, resulting in different fatigue performances. The present work concludes that, whenever surface treatment is involved in the manufacturing process, the component’s design must be included, in addition to the required geometrical features for the operative conditions, an evaluation of how these provided features might affect the surface treatment outcome.     

Enhanced wear and fatigue properties of Ti–6Al–4V alloy modified by plasma carburizing/CrN coating

In this study, a newly developed duplex coating method incorporating plasma carburization and CrN coating was applied to Ti–6Al–4V and its effects on the wear resistance and fatigue life were investigated. The carburized layer with approximately150 μm in depth and CrN coating film with 7.5 μm in thickness were formed after duplex coating. Hard carbide particles such as TiC And V₄C₃ were formed in the carburized layer. XRD diffraction pattern analysis revealed that CrN film had predominant [111] and [200] textures. The hardness (Hv) was significantly improved up to about 1,960 after duplex coating while the hardness value of original Ti–6Al–4V was 402. The threshold load for the modification and/or failure of CrN coating was measured to be 32 N using the acoustic emission technique. The wear resistance and fatigue life of duplex-coated Ti–6Al–4V improved significantly compared to those of un-treated specimen. The enhanced wear resistance can be attributed to the excellent adhesion and improved hardness of CrN coating film for the duplex-coated Ti–6Al–4V. The initiation of fatigue cracks is likely to be retarded by the presence of hard and strong layers on the surface, resulting in the enhanced fatigue life.     

Microstructure and mechanical properties of Al–Si cast alloy with additions of Zr–V–Ti

The microstructure and tensile properties at temperatures up to 300 °C of an experimental Al–7Si–1Cu–0.5Mg (wt.%) cast alloy with additions of Ti, V and Zr were assessed and compared with those of the commercial A380 grade. The microstructure of both alloys consisted of Al dendrites surrounded by Al–Si eutectic containing, within its structure, the ternary Al–Al₂Cu–Si phase. Whereas the Al₁₅(FeCrMn)₃Si₂ phases were present in the A380 alloy, Ti/Zr/V together with Al and Si phases, Al(ZrTiV)Si, were identified in the experimental alloy. As a result of chemistry modification the experimental alloy achieved from 20% to 40% higher strength and from 1.5 to 5 times higher ductility than the A380 reference grade. The role of chemistry in improving the alloy thermal stability is discussed.     

Interfacial characterization of plasma-spray coated calcium phosphate on Ti–6Al–4V

Microstructural characterization was performed on plasma-spray coated calcium phosphate and its interface with Ti–6Al–4V using scanning and transmission electron microscopy. The amorphous calcium phosphate phase was found near the coating/substrate interface, usually in between the crystalline hydroxyapatite grains and the Ti–6Al–4V substrate. The presence of the amorphous calcium phosphate and the channels extending from the interface to the coating surface was attributed to the low interfacial bond strength in a physiological environment. There was an indication of titanium diffusing into the amorphous regions. However, no reaction phases were found between the coating and the Ti–6Al–4V substrate. In the coating area remote from the interface, no intergranular amorphous calcium phosphate phase was observed. The microstructure of the Ti–6Al–4V substrate near the interface was observed to be affected by heat generated during plasma spraying. © 1998 Kluwer Academic Publishers     

Microstructure and mechanical properties of TiC/Ti–6Al–4V composites processed by in situ casting route

Abstract

TiC/Ti–6Al–4V composites containing various volume fractions of TiC were produced by induction skull melting and common casting utilising in situ reaction between titanium and carbon powder. The microstructure and room tensile properties of as cast and heat treated TiC/Ti–6Al–4V composites were investigated. Bar-like or small globular eutectic TiC were found in 5 vol.-%TiC/Ti–6Al–4V composite, whereas the equiaxed or dendritic primary TiC particles were found to be the main reinforcements in 10 and 15 vol.-%TiC/Ti–6Al–4V composites. The as cast TiC/Ti–6Al–4V composites have shown higher strength but lower ductility than those of monolithic Ti–6Al–4V alloy. The shape and fracture of TiC particles can strongly influence the fracture and failure of the composites, and so the ultimate tensile strengths and elongations of as cast composites reduce with the increase in volume fraction of TiC. TiC particles appear to be spheroidised, and titanium precipitation can be found within large TiC particles after heat treatment at 1050°C for 8 h, which can promote the resistance to fracture of composites. Therefore, the elongations of the composites increase significantly, and the ultimate tensile strengths also have marginal increase especially for the 10 and 15 vol.-%TiC/Ti–6Al–4V composites after heat treatment.     

Influence of defects on mechanical properties of Ti–6Al–4 V components produced by selective laser melting and electron beam melting

This study evaluates the mechanical properties of Ti–6Al–4 V samples produced by selective laser melting (SLM) and electron beam melting (EBM). Different combinations of process parameters with varying energy density levels were utilized to produce samples, which were analyzed for defects and subjected to hardness, tensile, and fatigue tests. In SLM samples, small pores in amounts up to 1 vol.% resulting from an increase in energy density beyond the optimum level were found to have no major detrimental effect on the mechanical properties. However, further increase in the energy density increased the amount of porosity to 5 vol.%, leading to considerable drop in tensile properties. Samples produced using lower-than-optimum energy density exhibited unmelted powder defects, which, even at 1 vol.% level, strongly affected both tensile and fatigue properties. In EBM, insufficient energy input was found to result in large, macroscopic voids, causing serious degradation in all mechanical properties. These findings are helpful in process optimization and standardization of SLM and EBM processes.