Ti6Al4V Alloy Specimens Research Articles

Influence of the Heat Treatment on the Microstructure, Mechanical Properties and Fatigue Behavior of Additively Manufactured Ti6Al4V Alloy

This contribution deals with the selective laser melting (SLM), which is one of the additive manufacturing (AM) technologies enabling the production of complex parts from metal powder, layer-by-layer wise. This technology uses laser as source of energy to melt a powder to compact state. Properties of final products can be significantly influenced by the process parameters and post-fabricated heat treatments. The purpose of this study is to determine the effect of a heat treatment on properties of the Ti6Al4V alloy specimens manufactured by Eosint M280 machine by the SLM. Three sets of specimens, treated at different temperatures (730 ˚C, 900 ˚C, 1200 ˚C), resulting in a different structure, associated mechanical and fatigue properties, were investigated.

Influence of surface topography on fatigue behavior of Ti6Al4V alloy by laser powder bed fusion

This article deals with the understanding of the influence of surface topography on fatigue behavior of Ti6Al4V alloy specimens produced by laser powder bed fusion (LPBF). The same laser parameters and scan strategy were used for all specimens, giving a sample density higher than 99.5 %. Two different surface topographies were obtained by using the top and side surfaces of the specimens.The surface topography and morphology were investigated by optical surface profilometry and focus variation microscopy. Four-point bending fatigue test was performed on specimens with top and side surfaces as the highest stressed surface respectively. Machined specimens were used as reference. The features of the fracture surface, such as crack initiation and propagation, were analyzed by focus variation and scanning electron microscopy (SEM). Both, fatigue results and fracture surface investigations, were correlated and discussed in relation to surface topography and microstructure, as well as manufacturing parameters.

Mechanical and tribological properties of ice-bonded abrasive polishing tools

This article covers the efforts on characterising ice-bonded abrasive polishing tool in terms of the mechanical and tribological properties such as hardness, coefficient of friction, and wear rate. These studies were attempted on the tools prepared at different temperatures ranging from −10 °C to 0 °C with a view to identify the condition suitable to prepare ice-bonded abrasive polishing tool for effective polishing of Ti–6Al–4V alloy specimen. It also presents the methods adopted to determine various properties of ice-bonded abrasive polishing tool. Hardness was estimated from the measured penetration depth of cone shape indenter into the tool, coefficient of friction was determined from the change in power drawn by the motor rotating the tool mould, and wear behaviour of tool was assessed from the melting rate of the tool determined from the change in height of ice-bonded abrasive polishing tool at different stages of polishing. From the results of this study, it is clear that ice-bonded abrasive polishing tool prepared at −4 °C has possessed sufficient hardness, coefficient of friction, and reasonable wear rate suitable for polishing of Ti–6Al–4V specimens. This article also covers the details of low-temperature coolant supply unit developed to prepare the ice-bonded abrasive polishing tool at any desired temperature between 0 °C and −40 °C and thus to maintain it for a long time. Polishing studies with such ice-bonded abrasive polishing tool showed 72% improvement in finish after 90 min of polishing of Ti–6Al–4V specimen with tool, prepared at −4 °C.

High Cycle Fatigue Life of Ti6Al4V Alloy Produced by Direct Metal Laser Sintering

High cycle fatigue life of Ti6Al4V alloy specimens manufactured by Direct Metal Laser Sintering (DMLS) was experimentally determined. The DMLS fabrication process was characterized by a 400 W laser power and 50 μm layer melted thickness. Post-fabrication heat treatment consisted in stress relieving for 3 h at 720 °C in vacuum with subsequent cooling in argon atmosphere. Fatigue testing of specimens oriented in three different directions with respect to the material build direction was performed with the aim to examine the influence of the layered microstructure on the fatigue behavior. Results of measurement of surface roughness, metallographic examinations of the layered material and fractographic investigation of the fatigue fracture surfaces were employed in the discussion of fatigue crack initiation in DMLS fabricated Ti6Al4V alloy.

Residual Stress in SLM Ti6Al4V Alloy Specimens

Selective Laser Melting (SLM) presents a modern manufacturing process with an innovative technology which allows the production of full-density objects or fine-structured parts with complex geometry and inner structures. Stability and certification of the properties of SLM parts are important tasks for all producers and end-users. One of the drawbacks of this technology is high residual stress in as-made SLM objects. In this study X-ray diffraction technique was used for investigating the residual stress induced into SLM Ti6Al4V alloy samples. Principal stresses were estimated for the cut rectangular specimen. Two types of the cantilevers were produced and numerical simulation of the stress was performed. The bending of cut cantilevers was measured before and after heat treatment. Next series of the samples had rectangular shapes and different thicknesses from 1 to 46 layers. All as-manufactured specimens attached to the substrate showed the presence of tensile residual stresses near the top surface. Residual stress along the laser scanning direction had magnitudes twice that of the stress in the perpendicular direction. Conclusions regarding directions and values of stresses in SLM objects from Ti6Al4V powder are given.

Morphological Analyses and Crystalline Structures of Anodic TiO<sub>2</sub> Thin Film on Ti6Al4V Alloy Using Phosphate and Calcium Containing Electrolyte under Different Voltage and Calcium Molarity

This study was aimed at evaluating morphological and crystalline structures of anodic and hydrothermal titanium oxide thin films formed on Ti6Al4V alloy in a mixture of glycerol phosphate disodium salt (GP) and calcium acetate (CA) solution at various forming environments such as CA molarity and applied voltage. Anodic oxide films were hydrothermally treated and their morphological surfaces, crystal structures and chemical compositions were characterized. It was found that TiO2film was formed in all surfaces of Ti6Al4V alloy specimen with porous and rough morphology and the crystal structures were mainly anatase. The anodized film morphology and the crystals structure were dependent on CA molarity and anodizing voltage. Increasing CA molarity from 0.15 to 0.30M increased the number and size of micro porous and the intensity of anatase crystals, but further increasing to 0.45M, the number and size did not change and the intensity of anatase peaks became weak. Except in 0.45M-CA, the number and size of micro porous and the intensity of anatase crystals increased gradually with increasing anodizing voltage. Ca-P crystals were precipitated on the surface of hydrothermally treated films. CA molarity associated to the crystal shape while anodizing voltage related to the amount and size precipitated crystals. CA molarity was also associated to the atomic compositions of Ca and P that precipitated on the hydrothermally treated surface film.

Relationship between constant-load creep, decreasing-load creep and stress relaxation of titanium alloy

The mechanism of sheet metal parts hot sizing is considered as constant-strain stress relaxation due to creep. This creep deformation in stress relaxation is limited within the elastic strain range, unlike multi-step decreasing-load creep test in which the creep deformation is unlimited. In this paper, the short-term constant-load, decreasing-load creep and stress relaxation tests were performed on Ti6Al4V alloy specimens at 700°C. The initial stress in the range of 5–126MPa was applied in constant-load and decreasing-load creep, and initial strain of 0.002, 0.004, 0.0065 and 0.02 was loaded during stress relaxation. The two kinds of creep tests were performed for 3600s while stress relaxation tests for 1800s. The creep rate–stress, creep rate–time and creep strain–time relationships were studied respectively based on the test data. Constitutive creep models were developed according to constant-load creep and stress relaxation data, respectively. Application of the established models on simulation of constant-strain, constant-load and decreasing-load creep was introduced. Results show that the creep model from the SRT test is able to predict the stress relaxation behavior well while creep model from the constant-load creep tests is reliable in the simulation of constant-load and decreasing-load creep deformation.

The effect of different methods to add nitrogen to titanium alloys on the properties of titanium nitride clad layers

In this investigation, titanium nitride (TiN) reinforcements are synthesized in situ on the surface of Ti–6Al–4V substrates with gas tungsten arc welding (GTAW) process by different methods to add nitrogen, nitrogen gas or TiN powder, to titanium alloys. The results showed that if nitrogen gas was added to titanium alloys, the TiN phase would be formed. But if TiN powder was added to titanium alloys, TiN+TiNx dual phases would be presented. The results of the dry sliding wear test revealed that the wear performance of the Ti–6Al–4V alloy specimen coated with TiN or TiN+TiNx clad layers were much better than that of the pure Ti–6Al–4V alloy specimen. Furthermore, the evolution of the microstructure during cooling was elucidated and the relationship among the wear behavior of the clad layer, microstructures, and microhardness was determined.

A study on the tribological characteristics of duplex-treated Ti–6Al–4V alloy under oil-lubricated sliding conditions

The tribological characteristics of the polished, dimpled and over-coated dimpled specimens were investigated. Dimples were produced on a Ti–6Al–4V alloy specimen using a laser surface texturing (LST). A Cr-doped diamond-like carbon (DLC) film was deposited on a dimpled specimen using an unbalanced magnetron sputtering (UBMS). The effects of dimples and over-coated Cr-doped DLC film on the tribological characteristics were investigated by performing the friction tests against a Cr-plated steel pin. The test results showed that the over-coated dimpled specimen exhibited a lower friction coefficient and wear compared to those of the polished and dimpled specimens, which may be attributed to the storage of wear debris and high hardness. A model of the wear reduction mechanism of the specimens was discussed.

Microstructure and tribological performance of Ti–6Al–4V cladding with SiC powder

In this investigation, titanium carbide (TiC) and Ti 5Si 3 reinforcements were synthesized in situ using a gas tungsten arc welding (GTAW) cladding process in which silicon carbide (SiC) powder was used to clad Ti–6Al–4V substrate. The evolution of the microstructure during cooling was elucidated and the relationship among the wear behavior of the clad layer, microstructures, and microhardness was determined. The experimental results reveal that the SiC clad layer specimen under a dry sliding wear test condition exhibited better wear performance than that of the un-clad Ti–6Al–4V alloy specimen.

Tribological characterization of a biocompatible thin film of UHMWPE on Ti6Al4V and the effects of PFPE as top lubricating layer

Ultra-high molecular weight polyethylene (UHMWPE) thin film was coated onto Ti6Al4V alloy specimens using dip coating method. Tribological performance of this coating (thickness of 19.6±2.0 μm) was evaluated using 4 mm diameter Si 3N 4 ball counterface in a ball-on-disk tribometer. Tests were carried out for different normal loads (0.5, 1.0, 2.0 and 4.0 N) and rotational speeds of the disk (200 and 400 rpm). UHMWPE coating formed in this study exhibits high hydrophobicity with water contact angle of 135.5±3.3° and meets the requirements of cytotoxicity test using the ISO 10993-5 elution method. This coating shows low coefficient of friction (0.15) and high wear durability (>96,000 cycles) for the tested conditions. PFPE overcoat on UHMWPE has further increased the wear durability of UHMWPE coating as evaluated at even higher rotational speed of 1000 rpm.

Effect of micro ploughing during fine particle peening process on the microstructure of metallic materials

In this study, the mechanism of microstructural changes induced by fine particle peening (FPP) treatment was analyzed. Ti–6Al–4V alloy specimens treated with FPP, using tool steel particles with a peening time ranging from 0.1 to 30 s were prepared and examined. Observation of the specimen with short-term FPP treatment revealed that two types of collision dents were formed by the micro abrasive behavior between collided particles and specimen surface. One was the “indented dent,” formed by particles that collided with the surface vertically, and the other was the “ploughed dent,” accompanied by a heap of piled-up material that was ploughed aside during the formation of this dent. Analyses using energy dispersive X-ray spectroscopy (EDS) revealed the presence of particle fragments transferred to the ploughed dents, although no fragments were observed on the indented dents. With an increase in peening time, transferred fragments were embedded into treated materials by deformed material due to a micro ploughing effect. The Fe-rich region became deeper as the particle fragments were embedded. Finally, a lamellar microstructure with a high concentration of Fe was formed by FPP treatment. The effects of micro ploughing during the FPP process on the lamellar modified microstructure formation were clarified. One effect was an enhancement of metal transfer from particles to substrate because virgin metallic surface was exposed; and the other was the mechanical mixing of transferred elements into the substrate.

Effects of combined plasma-carburizing and shot-peening on fatigue and wear properties of Ti–6Al–4V alloy

The effects of combined plasma-carburizing and shot-peening on fatigue and wear properties of Ti–6Al–4V alloy specimen were investigated. Surface morphology and roughness, microstructure, compressive residual stress, work hardening state, and micro-hardness on the surface modified layer in the Ti–6Al–4V alloy specimen were measured and analyzed. The shot-peening effectively induces highly compressive residual stress and work hardening states on the surface layer of the plasma-carburized Ti–6Al–4V alloy specimen. Consequently, the fatigue life of plasma-carburized Ti–6Al–4V alloy specimen has been significantly improved by subsequent shot-peening. The cracks of both shot-peened and shot-peened carburized specimens initiated on the surface at higher applied stress levels. On the contrary, interior-originating fractures occurred at lower applied stress levels. Corresponding to this behavior, the S– N curves show the shape of two-step stages. The wear resistance of Ti–6Al–4V alloy specimen was also significantly improved by the hardness increase on the surface layer by combination of plasma-carburizing and shot-peening.

Dynamic Recrystallization as a Potential Cause for Adiabatic Shear Failure

Dynamic recrystallization (DRX) is almost universally observed in the microstructure of adiabatic shear bands. It is usually admitted that DRX results from the large temperatures that develop in the band along with very high local strains. This paper reports the observation of dynamically recrystallized nanograins in Ti6Al4V alloy specimens that were impact loaded to only half the failure strain at which the adiabatic shear band develops. This observation shows that DRX not only precedes adiabatic shear failure but it is also likely to be a dominant micromechanical factor in the very generation of the band. This result means that adiabatic shear failure is not only a mechanical instability but also the outcome of strong microstructural evolutions leading to localized material softening prior to any thermal softening.

A new way to produce Al + Cr coating on Ti alloy by vacuum fusing method and its oxidation resistance

An Al + Cr coating successfully produced by the vacuum fusing method was proposed for improving the oxidation resistance of Ti–6Al–4V alloy specimens. Unlike powder pack cementation, this technique was much more effective and cheap without the need for long time diffusion, and the Al concentration and the thickness of vacuum fusing coating layer could be controlled by adjusting powder mixing ratio and adding the immersed times, respectively. Compared with the specimen with an Cr-modified aluminide coating, the oxidation resistance of the specimens with vacuum fused Al + Cr coating was about two times than that of the specimens with Cr-modified aluminide coating. During oxidation, Cr additions suppressed metastable θ-alumina formation. It is only one phase α-alumina scale that developed on the vacuum fused Al + Cr coating surface, while a single metastable θ-alumina scale formed on the Cr-modified aluminide coating surface.

Surface deformation behavior of beta solution treated and overaged Ti–6Al–4V during laser shock processing

The surface of a beta solution treated and overaged Ti–6Al–4V alloy specimen deformed by laser shock processing was studied using electron backscatter diffraction, scanning electron microscopy, and atomic force microscopy. Slip steps were observed within grains oriented with their c axis nearly parallel to the specimen surface normal. Based on the slip step traces and orientation information, the slip planes were determined to be {112¯2} for grains with their c axis within 15° of the specimen surface normal and {112¯1} for grains with their c axis between 15° and 40° away from the specimen surface normal. Although both these planes are known to belong to twinning systems, {112¯2}⟨112¯3¯⟩ and {112¯1}⟨112¯6¯⟩, respectively, the latter has not been observed to operate as a slip system. Examination of the Taylor factors associated with these slip systems shows that the grains with slip steps have the lowest Taylor factors. Determination of localized lattice rotations showed a unique behavior in grains with slip steps, such that all the lattice rotations were concentrated about the steps, with almost no orientation variations in between slip steps. This distribution indicates that stress concentrations exist at the slip steps, which could potentially affect the performance of the material.

Formation of titanium hydride in Ti–6Al–4V alloy

Use of hydrogen as a temporary element in titanium alloys is an attractive approach for enhancing processability, and also for modification of microstructures and thereby improving final mechanical properties. In this article, microstructures, phases and phase transformations in Ti–6Al–4V alloy specimens containing 0.1, 0.3 and 0.5 wt.% hydrogen has been investigated using optical microscopy, XRD and TEM. It was shown that there are fcc δ titanium hydride plates precipitated in the specimens containing hydrogen 0.302 and 0.490 wt.%. Simultaneously, mass orthorhombic martensite α″ has been found in the specimens as well as δ hydride. One eutectoid transformation mechanism from β H to δ hydride and α based on diffusion transformation theory was presented.

Microstructural and surface characterization of Ti–6Al–4V alloys after fretting fatigue

Abstract Ti–6Al–4V alloy specimens were tested under conditions of fretting fatigue, with the contact geometry, the normal stress, as well as the cyclic stress selected such that the mixed, slip-stick regime prevails during the experiments. Following testing, the specimens were characterized using white light interference profilometry, scanning electron microscopy, microhardness, and electron dispersive spectroscopy (EDS). The results revealed that the surface roughness of the slip region increases compared to the roughness of the stick, and non-contact ones. In addition, at the higher spatial frequencies, the power spectral density (PSD) of the slip region increases compared to the PSD of the stick and non-contact regions, thus revealing that an increase of the population of the smaller size asperities occurs. The microstructure of the material below the slip zone was found to be transformed to a finer one; and the percentage of the transformed β phase has been decreased substantially. This area of the transformed microstructure, has also a higher hardness compared to the hardness of the bulk structure. EDS analysis revealed a high concentration of oxygen on the specimen’s surface at the slip region of the two contacting bodies. This finding indicates that elevated temperatures are developed during fretting fatigue and enable the diffusion of oxygen from the atmosphere to the alloy.

X-ray photoelectron study of surface layers on orthopaedic alloys. I. Ti–6Al–4V (ASTM F-136) alloy

It is now widely acknowledged that the stability, performance, and life of prostheses fabricated from metallic materials are directly related to the compositional details of the layers formed on the surfaces of these components at the time of implantation. The thrust of the present investigation is thus the use of angle-resolved x-ray photoelectron spectroscopy for the identification of the differences in composition and thickness between the layers formed on the surface of a Ti–6Al–4V alloy specimen following exposure for 1 h to either laboratory air or wet steam.