Ti 6Al 2Sn 4Zr 2Mo Research Articles

A novel method for investigating drilling machinability of titanium alloys using velocity force maps

During the manufacture of critical aerospace components from metastable β titanium alloys, excessive tool wear and machining cost can be significant obstacles. Although tools are highly optimised for drilling the α+β alloy Ti–6Al–4V (Ti-64), insufficient information is available to efficiently improve tools for use on higher strength alloys due to the cost intensive processes required to design and optimise new drills. To reduce this cost, a novel method is presented which enables rapid comparisons of machinability between different alloys, (Ti-64; Ti–6Al–2Sn–4Zr–6Mo (Ti-6246); and Ti–5Al–5Mo–5V–3Cr (Ti-5553)), to quickly assess the performance of specific tool characteristics using industrially relevant drilling parameters. Rapid assessment and comparison of microstructural damage at selected forces was made possible through the development of 3D tool velocity force maps (VFM) for coated TiAlN and uncoated WC/Co tools at different cutting feeds and speeds. Microstructural damage depth has been shown to decrease with increasing alloy strength even for cases where the thrust force between the work piece and the tool was the same ~1600 ​N. In addition, a lower damage depth was measured when using uncoated tools. The machinability evaluation of tool coating and its interaction with force, torque, and material properties showed that TiAlN coated tools responded differently in the case of each alloy, displaying significantly different torque profiles, thus indicating different optimum cutting speeds and feeds. Abrasive and adhesive wear mechanisms were identified in the coated tools, while the majority of wear in uncoated tools was abrasive.

Titanium alloy microstructure fingerprint plots from in-process machining

Titanium alloy components require several machining stages of forged billets which are supplied in a range of annealing conditions. Generally, the machining performance is influenced by the heat treatment and changes in billet microstructures are often overlooked by tool manufacturers and machinists. Due to the non-linear strain path during primary forging, titanium alloy billets are anisotropic in nature and require ex-situ non-destructive evaluation (NDE) during the manufacturing stages to ensure excellent service performance, particularly in safety-critical aerospace components. In this study, the local analysis of the fluctuations presented in the force response during face-turning operations is directly linked to the billet heat treatment condition and presented as microstructure fingerprint plots. The evolution of cutting forces in four different billet conditions of the alpha + beta titanium alloy Ti–6Al–2Sn–4Zr–6Mo (Ti-6246) was measured. The magnitude and fluctuations in force were directly correlated to microstructural features derived from the heat treatments. In addition, local spatial high-resolution synchronization of the cutting forces was used to determine the effects of microstructure from the heterogeneous upstream forging process and subsequent heat treatment. These rapidly produced microstructure fingerprint plots are an important development step for providing manufacturers with an in-process machining NDE method: this will help to qualify material upstream prior to expensive secondary forging or finish machining stages.

Effect of niobium addition on tensile properties and oxidation resistance of a titanium-based alloy

Previous research has inadequately elucidated the influence of Nb addition on the tensile properties and oxidation resistance of Ti-based alloys. Herein, the tensile properties and oxidation resistance of Ti–6Al–2Sn–4Zr–2Mo–0.1Si–1.0B–xNb (x = 0, 0.2, and 1.0) alloys were investigated. The oxidation resistance was significantly improved by Nb addition without significantly influencing the tensile properties. The substitution of Ti4+ by Nb5+ in TiO2 and promoted formation of a Sn-rich layer between the oxidation layer and substrate caused an increase in the oxidation resistance, thus highlighting the effectiveness of Nb addition in enhancing the oxidation resistance.

Experimental Investigation and Optimization of Process Parameters in Ti – (6242) Alpha–Beta Alloy Using Electrical Discharge Machining

This investigation exposes electrical discharge machine (EDM) technique in expelling the metal with a conducted copper electrode from the Ti–6Al–2Sn–4Zr–2Mo alloy. The alloy Ti–6Al–2Sn–4Zr–2Mo has the highest resistance, high-temperature detection, high tensile strength, and high yield strength. Ti–6Al–2Sn–4Zr–2Mo alloy is commonly used for applications such as aerospace, medical equipment, automotive parts, and nuclear power plants. EDM provides easy machinability and excellent surface finishes. Machine parameters of primary noteworthiness, such as Discharge Current (I), Spark on time (μs), and Tool Diameter (mm), are considered using the Taguchi L27 orthogonal array. Metal removal rate (MRR), tool wear rate (TWR), and overcut (OC) perpendicularity are investigated with the help of mean effective plots. Multi response optimization to maximize the material removal rate (MRR) and minimize the tool wear rate (TWR) and over cut (OC) on the surface of the workpiece has been analyzed by Grey relational analysis (GRA). The result expresses that spark on time (Ton) contributes (6.22%) more on affecting the machining characteristics followed by electrode diameter (76.97%) and discharge current (16.97%). From the F-test, it is assumed that all the machining parameters have a significant influence on responses. Accordingly an optimum combination of machining parameters is also determined.

Local softening behavior accompanied by dislocation multiplication accelerates the failure of Ti–6Al–2Sn–4Zr–2Mo–0.1Si alloy under dwell fatigue load

In the present study, the dislocation characteristics and stress-strain curves of Ti–6Al–2Sn–4Zr–2Mo–0.1Si (Ti6242s) alloy under low-cycle fatigue and dwell fatigue at room temperature were analyzed. Ti6242s alloy exhibited local softening behavior under cyclic fatigue load as the dislocation density increased. The dislocation morphology showed that a large number of banded dislocations formed in the primary α-phase, which were not hampered by grain boundaries, phase boundaries, and other dislocations. The dislocation mobility difference in multiphase microstructure further enhanced inhomogeneous plastic deformation in local areas, which had a negative effect on the failure behavior of Ti6242s alloy under dwell fatigue.

Effects of direct aging on near-alpha Ti–6Al–2Sn–4Zr–2Mo (Ti-6242) titanium alloy fabricated by selective laser melting (SLM)

This study reported for the first time the selective laser melting of Ti–6Al–2Sn–4Zr–2Mo (Ti-6242), a near-α titanium alloy specifically designed for high-temperature services. The microstructural stability under standard aging (595 °C/8 h) was assessed after fabrication. With increasing laser scan speeds, an increase of relative density to 99.5% was observed, followed by a dramatic decrease to around 95.7%. The arithmetic mean roughness value (Ra) evolved consistently with the density, achieving the lowest value at the laser scanning speed of 950 mm/s. The as-built sample consists mainly of martensitic α′ colonies separated by prior-β grain boundaries. Martensites partially decomposed into α laths and nano β precipitates after aging treatment, but still retained initial acicular morphologies. Despite the stress relief during aging treatment, the hardness still increased from 410 HV of the as-built sample to 450 HV. The as-built and aged Ti-6242, respectively, exhibited a maximum room-temperature tensile strength of 1437 MPa and 1510 MPa, while the ductility was reduced from 5% to 1.4%. The nanotwins within martensites contribute significantly to the relatively high ductility of the SLMed sample. The aging-induced reinforcement is mainly attributed to the precipitation hardening effects of nanosized β-phase particles.

Flash-butt welded Ti6242 joints preserved base-material strength and ductility

Microstructure and mechanical properties of flash-butt welded two-phase Ti–6Al–2Sn–4Zr–2Mo (Ti6242) alloy are investigated. High ductility that is at least comparable to that of the matrix and extra strain hardening are achieved in welded joints because of the formation of a particular semi-equiaxed structure in the double-affected zone (DAZ). Also, no strength loss is observed, and this is attributed to a uniform hardness distribution in a narrower welding zone. The soft DAZ, which is sandwiched between welding seam and stress-affected zone, enables better deformation compatibility among these zones, and weakens their stress concentration. Furthermore, such semi-equiaxed structure effectively suppresses the formation of dense slip bands and crack nucleation and provides a strong barrier against its propagation. This work provides a new understanding of this particular microstructure, which is expected to have a higher combination of strength and ductility in Ti alloys.

Effect of α precipitation on β texture evolution during β-processed forging in a near-β titanium alloy

The effects of α precipitation on β texture development during β-processed forging in Ti–6Al–2Sn–4Zr–6Mo alloy were investigated. Typical {001} and {111} β textures of β phase were developed, and the {001} texture intensity monotonically increased with increasing deformation ratio during forging in the β single-phase region with its promotion because of dynamic recrystallization (DRX), where DRX grains inherited the crystal orientation of the parent β phase. However, the {001} texture intensity decreased, accompanied by α precipitation during forging in the (α + β) dual-phase region, where the forging was performed after cooling from the β single-phase region. DRX hardly occurred during forging in the (α + β) dual-phase region. The precipitated α phase showed strong {11–20} transformation texture according to the Burgers orientation relationship under deformation. The {10–11}<11–20> pyramidal slip system was dominant in the α phase, which is nearly parallel to several {110}<111> β slip systems. It enhanced slip transmission between α and β phases increasing the {110}<111> slip activity in the β phase and it weakened the {001} β texture intensity after α precipitation.

Interfacial characteristics and cohesion mechanisms of linear friction welded dissimilar titanium alloys: Ti–5Al–2Sn–2Zr–4Mo–4Cr (Ti17) and Ti–6Al–2Sn–4Zr–2Mo (Ti6242)

A detailed microstructural examination endeavoring to understand the interfacial phenomena yielding to cohesion in solid-state assembling processes was performed. This study focuses on the transition zone of a dissimilar titanium alloy joint obtained by Linear Friction Welding (LFW) the β-metastable Ti17 to the near-α Ti6242. The transition zone delimitating both alloys is characterized by a sharp microstructure change from acicular HCP (Hexagonal Close-Packed) α′ martensitic laths in the Ti6242 to equiaxed BCC β (Body-Centered Cubic) subgrains in the Ti17; these α′ plates were shown to precipitate within prior-β subgrains remarkably more rotated than the ones formed in the Ti17. Both α′ and β microstructures were found to be intermingled within transitional subgrains demarcating a limited gradient from one chemical composition to the other. These peculiar interfacial grains revealed that the cohesive mechanisms between the rubbing surfaces occurred in the single-phase β domain under severe strain and high-temperature conditions. During the hot deformation process, the mutual migration of the crystalline interfaces from one material to another assisted by a continuous dynamic recrystallization process was identified as the main adhesive mechanism at the junction zone. The latter led to successful cohesion between the rubbing surfaces. Once the reciprocating motion stopped, fast cooling caused both materials to experience either a βlean→α′ or βlean→βmetastable transformation in the interfacial zone depending on their local chemical composition. The limited process time and the subsequent hindered chemical homogenization at the transition zone led to retaining the so-called intermingled α’/βm subgrains constituting the border between both Ti-alloys.

Contributions of an innovative post-weld heat treatment to the micro-tensile behavior of two mono-material linear friction welded joints using: The β-metastable Ti–5Al–2Sn–2Zr–4Mo–4Cr (Ti17) and the near-α Ti–6Al–2Sn–4Zr–2Mo (Ti6242) Ti-alloys

The mechanical behavior of two mono-material Ti-alloy joints obtained by Linear Friction Welding (LFW) were investigated through micro-tensile testing combined with full-field measurements; the resulting tensile behaviors were examined in dependence of the local microstructures and crystallographic features. The near-α Ti–6Al–2Sn–4Zr–2Mo (Ti6242) and the β-metastable Ti–5Al–2Sn–2Zr–4Mo–4Cr (Ti17) welds were tested up to failure in both as-welded (AW) and heat treated (PWHT) configurations. In the AW Ti17 joint, the dissolution of the hardening α precipitates in favor of a βmetastable phase resulted in a remarkable loss in stiffness and strength compared to the as-received Widmanstätten α+β base material (BM). The gradients of microstructure led to significant heterogeneities in the local mechanical behaviors and subsequent strain incompatibilities between the refined weld center line (WL) and the neighboring coarse-grained thermo-mechanically affected zone (TMAZ). These features caused an early and intense strain localization in the joint ultimately causing failure by quasi-cleavage at the WL/TMAZ border. Transforming the AW microstructures through β annealing followed by α+β ageing permitted to form back a Widmanstätten α+β microstructure in the whole assembly resulting in homogenized elastic and plastic behaviors. In the joint core of the AW Ti6242, the precipitation of α’/α” martensitic laths or αsecondary fragments upon cooling the high temperature βtransformed microstructure caused a noteworthy refinement and a consistent weld overmatch; a slight loss of stiffness was also noted. The microstructures neighboring the refined joint did not exhibit singular microstructural changes but the dissolution of αsecondary platelets initially hardening the β matrix and a probable resorption of hardening particles. These phenomena appeared to have weakened these heat-affected zones (HAZ) leading to an intense slip occurrence in the latter and a quasi-cleavage fracture. The heat treatment consisting of an α+β annealing followed by ageing permitted to homogenize the stiffness values through the reprecipitation of equilibrium α+β microstructures across the joint. The formerly softened HAZ was hardened back and exhibited a mechanical behavior similar to the neighboring BM. A gradual microstructure refinement was preserved across the joint ensuring a remaining slight weld overmatch.

Investigation of Friction and Wear Properties of Duplex-Annealed Ti–6Al–2Sn–4Zr–2Mo Against Hardened AISI E52100 at Linear Reciprocating Motion

Near-α titanium alloys possess enhanced properties such as high specific strength, low density, corrosion resistance and an excellent creep behaviour at high operating temperatures which makes them suitable for a wide range of aerospace application areas. However, titanium and its alloys are known for their poor tribological properties which limit their applicability to structural materials. Due to increasing demands on efficiency and environmental restrictions, the application of Ti–6Al–2Sn–4Zr–2Mo as bearing material should also be considered. Experiments were performed on a linear reciprocating tribometer for a constant sliding distance of 300 m, a fixed stroke of 10 mm, at normal loads between 5 and 20 N varying the frequency between 2 and 4 Hz. For the lubrication tests a biodegradable hydraulic oil based on synthetic esters was applied between the two solid bodies. Mechanical properties of the tribotechnical system were determined by tensile tests, hardness and roughness tests as well as microstructural observations. Under dry condition, the Coefficient of Friction (CoF) decreases with increasing load and increasing frequency, whereas the wear volume increases with increasing load and decreasing frequency. Generally, the CoF significantly decreases by applying a lubricant and tends to increase with increasing load and increasing frequency whereas the wear volume increases at higher loads and frequencies. The protective effect of the lubricant against wear is absent. Either the wear volume shows the same values or possesses higher values compared to dry condition. The occurring wear mechanism was determined using a scanning electron microscope to observe the wear track and conduct an elemental analysis using energy-dispersive X-ray spectroscopy.

A Comparison of Porosity Analysis Using 2D Stereology Estimates and 3D Serial Sectioning for Additively Manufactured Ti 6Al 2Sn 4Zr 2Mo Alloy

Abstract Porosity is a typical defect in additively manufactured (AM) parts. Such defects limit the properties and performance of AM parts, and therefore need to be characterized accurately. Current methods for characterization of defects and microstructure rely on classical stereological methods that extrapolate information from two dimensional images. The automation of serial sectioning provides an opportunity to precisely and accurately quantify porosity in three dimensions in materials. In this work, we analyzed the porosity of an additively manufactured Ti 6Al 2Sn 4Zr 2Mo sample using Robo-Met.3D®, an automated serial sectioning system. Image processing for three dimensional reconstruction of the serial-sectioned two dimensional images was performed using open source image analysis software (Fiji/ImageJ, Dream.3D, Paraview). The results from this 3D serial sectioning analysis were then compared to classical 2D stereological methods (Saltykov stereological theory). We found that for this dataset, the classical 2D methods underestimated the porosity size and distributions of the larger pores; a critical attribute to fatigue behavior of the AM part. The results suggest that acquiring experimental data with equipment such as Robo-Met.3D® to measure the number and size of particles such as pores in a volume irrespective of knowing their shape is a better choice.

Chemical milling effect on the low cycle fatigue properties of cast Ti–6Al–2Sn–4Zr–2Mo alloy

The current research work presents the chemical milling effect on the low cycle fatigue properties of cast Ti–6Al–2Sn–4Zr–2Mo alloy. Chemical milling treatment is one of the final steps in manufacturing titanium alloy components that removes the brittle alpha-case layer formed during various thermal processes. The treatment includes immersion of the components in solutions containing hydrofluoric (HF) and nitric (HNO3) acids in relevant molar ratios. Although this treatment demonstrates advantages in handling components with complex net geometries, it may have detrimental effects on the surface, by introducing pitting and/or intergranular corrosion and thereby adversely affecting in particular the fatigue strength. The first series of specimens were tested in as-machined condition. Two more series were, prior to fatigue testing, subjected to 5 and 60min chemical milling treatment. It was found that the fatigue lives were substantially decreased for the chemically treated specimens. The fractographic investigation of all mechanically tested samples revealed multiple fatigue crack initiation sites in the chemically milled samples. These cracks were located either at the prior beta grain boundary or the prior beta grain boundary triple joints. The prior beta grain boundaries were found to have deep ditch-like appearance which depth increased with increasing milling time. These ditch-like grain boundaries acts as stress raisers and thereby promote early fatigue crack initiation and thus lower fatigue life.

Mechanical properties and microstructure of laser welded Ti–6Al–2Sn–4Zr–2Mo (Ti6242) titanium alloy

Room temperature tensile properties and microhardness of a laser welded Ti–6Al–2Sn–4Zr–2Mo (Ti6242) titanium alloy sheet were examined and correlated to the microstructure evolution across the weld. Tensile testing integrated with the optical image correlation Instron® system indicated that the average yield strength (YS), ultimate tensile strength (UTS), and total elongation of the weldment were respectively 88%, 87%, and 69% of the corresponding base material (BM) values. Electron probe microanalysis (EPMA) demonstrated a uniform distribution of the main alloying elements across the weld. The hardness raised increasingly from the BM toward the heat affected zone (HAZ) and the fusion zone (FZ) due to mainly a higher α volume fraction in HAZ and acicular α′ martensite formation in the FZ. Because of the higher hardness of the HAZ and FZ, a higher YS for the weldment relative to the BM would be expected. However, the lower YS as well as the lower UTS of the weldment can be explained by presence of some porosity and underfill in the FZ. The lower total elongation of the weldment compared to the BM can be related to the higher hardness of the HAZ and FZ.

Local strain rate sensitivity of single α phase within a dual-phase Ti alloy

We have performed in-situ micropillar compression to investigate the local strain rate sensitivity of single α phase in dual-phase Ti alloy, Ti–6Al–2Sn–4Zr–2Mo (wt%). Electron backscatter diffraction (EBSD) was used to identify two grains, anticipated to primarily activate a slip on the basal and prismatic plane respectively. Comparative micropillars were fabricated within single α laths and load-hold tests were conducted with variable strain rates (on the order of 10−2 to 10−4 s−1). Local strain rate sensitivity exponent (i.e. m value) is determined using two types of methods, constant strain rate method (CSRM) and conventional stress relaxation method (SRM), showing similar rate sensitivity trends but one order higher magnitude in SRM. We thus propose a new approach to analyse the SRM data, resulting in satisfactory agreement with the CSRM. Significant slip system dependent rate sensitivity is observed such that the prism slip has a strikingly higher m value than the basal. Fundamental mechanisms differing the rate sensitivity are discussed with regards to dislocation plasticity, where more resistance to move dislocations and hence higher hardening gradients are found in the basal slip. The impact of this finding for dwell fatigue deformation modes and the effectiveness of the present methodology for screening new alloy designs are discussed.

Relationship between the volume of the unit cell of hexagonal-close-packed Ti, hardness and oxygen content after α-case formation in Ti–6Al–2Sn–4Zr–2Mo–0.1Si alloy

In this study, the influence of oxygen diffusion on the physical properties of Ti–6Al–2Sn–4Zr–2Mo–0.1Si was examined. Measurements were carried out directly on sample cross sections which were preoxidized at high temperature. The lattice parameter evolution was measured using synchrotron X-ray diffraction and was coupled with microhardness and electron probe microanalyzer results with the aim of highlighting their relationships. The results show that the hardness and oxygen gradients along the oxygen diffusion zone in the alloy are similar to the evolution of the α-phase unit-cell volume quantified by X-ray diffraction. Linear relationships were found between these three parameters.

Local deformation mechanisms of two-phase Ti alloy

This paper describes a study of local deformation mechanisms in two-phase Ti alloy, Ti–6Al–2Sn–4Zr–2Mo, by performing in-situ micropillar compression tests. A colony microstructure was examined and select grains identified for examination were chosen with EBSD measurements. These grains were chosen to isolate individual slip systems within each test. Micropillars of tri-crystal (α–β–α) structure were fabricated from four determined regions, and compression tests were performed using a displacement-controlled nanoindenter set inside a SEM, with a constant displacement rate. The results show that the α/β morphology significantly affects the local deformation behaviour. For these colony structures, Schmid's law in general enables anticipation of local slip activity, but the presence and morphology of the β phase can significantly alter the apparent yielding point and work hardening response. The role of interfaces within these tri-crystal pillars is discussed.

Environmentally assisted fatigue crack nucleation in Ti–6Al–2Sn–4Zr–6Mo

An unexplained feature was observed at the fatigue crack origin of a number of α/β titanium specimens tested at 450°C in the low cycle fatigue regime. The origin was discoloured blue but this was not a result of temper colouration; this feature sometimes resulted in large reductions in fatigue lives. A number of specimens were examined to determine the cause and formation mechanism of these “blue spots.” This feature was associated with elevated oxygen and chloride levels and the presence of sodium. A mechanism based on hot-salt stress-corrosion cracking is proposed and the implications for service components are discussed.

Tool wear behaviour and workpiece surface integrity when turning Ti–6Al–2Sn–4Zr–6Mo with polycrystalline diamond tooling

The paper details the performance of a range of polycrystalline diamond (PCD) tools (∼1.3–39μm average diamond grain size) when turning Ti–6Al–2Sn–4Zr–6Mo with 150bar cutting fluid. The alloy is used for aeroengine components such as turbine discs due to its superior mechanical and elevated temperature properties. Tool life improved from ∼30 to 80min with increasing grain size except with the ultra-coarse (∼39μm) PCD grade, which failed via chipping/edge fracture after ∼8min. In general, the principal wear modes were crater formation and workpiece adhesion. Workpiece integrity assessment showed no major subsurface damage with surface compressive residual stresses of ∼600MPa.

Oxidation and alpha-case formation in Ti–6Al–2Sn–4Zr–2Mo alloy

Isothermal heat treatments in ambient air were performed on wrought Ti–6Al–2Sn–4Zr–2Mo (Ti-6242) material at 500, 593 and 700°C for times up to 500h. In the presence of oxygen at elevated temperatures simultaneous reactions occurred in Ti-6242 alloy, which resulted in the formation of an oxide scale and a layer with higher oxygen concentration (termed as alpha-case). Total weight gain analysis showed that there was a transition in the oxidation kinetics. At 500°C, the oxidation kinetics obeyed a cubic relationship up to 200h and thereafter changed to parabolic at prolonged exposure times. At 593°C, it followed a parabolic relationship. After heat treatment at 700°C, the oxidation obeyed a parabolic relationship up to 200h and thereafter changed to linear at prolonged exposure times. The observed transition is believed to be due to the differences observed in the oxide scale. The activation energy for parabolic oxidation was estimated to be 157kJ/mol. In addition, alpha-case layer was evaluated using optical microscope, electron probe micro-analyser and microhardness tester. The thickness of the alpha-case layer was found to be a function of temperature and time, increasing proportionally, and following a parabolic relationship. The activation energy for the formation of alpha-case layer was estimated to be 153kJ/mol.