Experimental and theoretical investigation of forming limit diagram for Ti-6Al-4 V alloy at warm condition

Forming limit diagram (FLD) is an important performance index to describe the maximum limit of principal strains that can be sustained by sheet metals till to the onset of localized necking. It is useful tool to access the forming severity of a drawing or stamping processes. In the present work, FLD has been determined experimentally for Ti-6Al-4 V alloy at 400 °C by conducting a hemispherical dome test with specimens of different widths. Additionally, theoretical FLDs have been determined using Marciniak Kuczynski (M-K) model. Various yield criteria namely: Von Mises, Hill 1948, Hill 1993 and Cazacu Barlat in combination with different hardening models viz., Hollomon power law (HPL), Johnson-Cook (JC), modified Johnson-Cook (m-JC), modified Arrhenius (m-Arr.), modified Zerilli–Armstrong (m-ZA) have been used in M-K analysis for theoretical FLD prediction. The material properties required for determination of yield criteria and hardening models constants have been calculated using uniaxial tensile tests. The predicted theoretical FLDs results are compared with experimental FLD. It can be observed that influence of yield criterion in M-K analysis for theoretical FLD prediction is predominant than the hardening model. Based on the results; it is observed that the theoretical FLD using Cazacu Barlat and Hill 1993 yield criteria with m-Arr. hardening model has a very good agreement with experimental FLD.     

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.     

Joint properties of dissimilar Al6061-T6 aluminum alloy/Ti–6%Al–4%V titanium alloy by gas tungsten arc welding assisted hybrid friction stir welding

Hybrid friction stir butt welding of Al6061-T6 aluminum alloy plate to Ti–6%Al–4%V titanium alloy plate with satisfactory acceptable joint strength was successfully achieved using preceding gas tungsten arc welding (GTAW) preheating heat source of the Ti alloy plate surface. Hybrid friction stir welding (HFSW) joints were welded completely without any unwelded zone resulting from smooth material flow by equally distributed temperature both in Al alloy side and Ti alloy side using GTAW assistance for preheating the Ti alloy plate unlike friction stir welding (FSW) joints. The ultimate tensile strength was approximately 91% in HFSW welds by that of the Al alloy base metal, which was 24% higher than that of FSW welds without GTAW under same welding condition. Notably, it was found that elongation in HFSW welds increased significantly compared with that of FSW welds, which resulted in improved joint strength. The ductile fracture was the main fracture mode in tensile test of HFSW welds.     

Cryorolling and warm forming of AA6061 aluminum alloy sheets

Cryorolling is a severe plastic deformation (SPD) process used to obtain ultrafine-grained aluminum alloy sheets along with higher strength and hardness than in conventional cold rolling, but it results in poor formability. An alternative method to improve both strength and formability of cryorolled sheets by warm forming after cryorolling without any post-heat treatment is proposed in this work. The formability of cryorolled AA6061 Al alloy sheets in the warm working temperature range is characterized in terms of forming limit diagrams (FLDs) and limiting dome height (LDH). Strain distributions and thinning in biaxially stretched samples are studied. Hardness of the formed samples is correlated with ultimate tensile strength to estimate post-forming mechanical properties. The limit strains and LDH have been found to be higher than in the case of the conventional processing route (cold rolled, annealed and formed at room temperature), making this hybrid route capable of producing sheet metal parts of aluminum alloys with high strength and formability. In order to combine the advantages of enhanced formability and better post-forming strength than the conventional cold rolled and annealed sheets, warm forming at 250°C has been found to be suitable for this alloy in the temperature range that has been studied.     

Oxidation behaviour of Ti6Al4V titanium alloy in oxygen

Abstract

The isothermal oxidation behaviour of two phase (α + β) titanium base alloy Ti6Al4V (coupons) has been studied at 1050, 1150, 1250, and 1340 K in O₂ gas at atmospheric pressure for 2, 4, 6, 8, and 12 h. Investigations on kinetic behaviour followed by the metallographic examination of oxidised scale morphology was carried out. Thermogravimetric data (weight gain v time) exhibited parabolic behaviour. Below 1250 K, the rate of oxidation substantially decreased after 8 h exposure, however, at 1340 K the oxidation rate was markedly high over the whole 12 h period. Parabolic rate constants were 0.234×10^-7, 3.67×10^-7, 10.72×10 ^-7, and 31.17×10^-7 kg² m^-4 s^-1 at 1050, 1150, 1250, and 1340 K respectively. The effective activation energy of oxidation was 88 kJ mol^-1. The instantaneous rate constant k _i exhibited a marked deviation from parabolic behaviour at high temperatures e.g. 1150, 1250, and 1340 K, however, k _i at lower temperature (1050 K) remained broadly unchanged with time exhibiting no deviation from parabolic behaviour. Metallographic observation of the sample coupons treated at 1340 K revealed an identical oxide scale morphology with increased thickness over the time.     

On the correlation of mechanical and physical properties of 6061-T6 and 7249-T76 aluminum alloys

Al 6061-T6 was exposed to 75 heat treatments and Al 7249-76 was exposed to 90 heat treatments. The solution treatments, cooling rates and age hardening treatments were varied to simulate pitfalls that heat treaters may encounter. The physical and mechanical properties of thermally treated alloys were correlated and discussed.     

Experimental evaluation and prediction of forming limit of FSW blanks made of AA 6061 T6 sheets at different weld orientations and weld locations

The main objective of the present work is to predict the forming limit of friction stir welded (FSW) sheets made of AA 6061T6, having different weld orientations, weld locations, and made at two different welding speeds. The predicted forming limit curves (FLCs) are validated with experimental FLCs. The thickness gradient based necking criterion (TGNC) and major strain‐rate ratio based necking criterion (MSRC) are used to predict the forming limit. The significance of single zone model and double zone model in FLC prediction is discussed. A decrease in hardness is witnessed in the weld zone as compared to base material. With increase in shoulder diameter and decrease in rotational speed, hardness has improved in the weld zone. The forming limit predictions of un‐welded sheets and FSW sheets coincide well with experimental results. The predicted FLCs of FSW sheets from TGNC and MSRC are equally accurate as compared to experimental FLCs in all the weld locations. Both TGNC and MSRC predict almost the same forming limit in 90° weld orientation, while TGNC showed better prediction in 45° weld orientation. FSW sheets with double zone models show better prediction accuracy than single zone models in most of the cases, except in the case of weld at centre location and at longitudinal orientation. There is only slight deviation between single zone and double zone model predictions. The failure location and failure pattern predictions are also agreeing well with the experimental FLCs.     

Foaming behavior of Ti6Al4V particle-added aluminum powder compacts

The foaming behavior of 5 wt.% Ti6Al4V (Ti64) particle (30–200 μm)-added Al powder compacts was investigated in order to assess the particle-addition effects on the foaming behavior. Al compacts without particle addition were also prepared with the same method and foamed. The expansions of Ti64 particle-added compacts were measured to be relatively low at small particle sizes and increased with increasing particle size. At highest particle size range (160–200 μm), particle-added compacts showed expansion behavior similar to that of Al compacts without particle addition, but with lower expansion values. Expansions studies on 30–45 μm size Ti64-added compacts with varying weight percentages showed that the expansion behavior of the compacts became very similar to that of Al compact when the particle content was lower than 2 wt.%. However, Ti64 addition reduced the extent of drainage. Ti64 particles and TiAl₃ particles formed during foaming increased the apparent viscosity of the liquid foam and hence reduced the flow of liquid metal from cell walls to plateau borders. The reduced foamability in the compacts with the smaller size Ti64 addition was attributed to the relatively high viscosities, due to the higher cumulative surface area of the particles and higher rate of TiAl₃ formation between liquid Al and Ti64 particles.     

Deformation of titanium alloy Ti–6Al–4V under dynamic compression

Deformation localisation is the main reason for material failure in cold forging of titanium alloys and is thus closely related to the production yield of cold forging. Recent research has revealed that the width of shear band of titanium alloys after dynamic compression is related with their static and dynamic mechanical properties and processing parameters. To explore the influences of these factors on titanium alloys in dynamic compression, the distributions of stress, strain, strain rate and temperature of the specimens over the macro and microscales have been systematically studied. This work can be beneficial to process parameter optimisation and material designing for cold forging. In the study of the influence of process parameters on dynamic compression, considering material constitutive behaviour, physical parameters and process parameters, a numerical dynamic compression model for titanium alloys has been constructed. The entire dynamic compression process is simulated and a good agreement with experiments is observed. By extracting and comparing the stress, strain and temperature distribution under prescribed conditions, the effects of friction and compression velocity on the macrostate and distribution of strain and stress of compression samples are studied. Friction and compression rate are important factors influencing the spread and the stress state of deformation localisation zone. When friction is reduced to a certain level, deformation localisation can be effectively alleviated. The increase of friction and compression rate can lead to early appearance of tension stress in the deformation localisation zone, which may explain the experimental finding that crack tendency increases with higher compression rate and poorer lubrication. By adjusting the process parameters, the severity of strain localisation and stress state in the localised zone can be controlled thus enhancing the compression performance of titanium alloys.     

Experimental and numerical investigation of anisotropic yield criteria for warm deep drawing of Ti–6Al–4V alloy

Accuracy of the finite element simulation of sheet metal forming is significantly dependent on the correctness of input properties and appropriate selection of material models. In this work, anisotropic yield criteria namely, Hill 1948, Barlat 1989, Barlat 1996, Barlat 2000 and Cazacu Barlat have been implemented for Ti–6Al–4V alloy at 400 °C. Material constants required for the yield criteria have been determined and deformation behavior in deep drawing process has been analyzed in finite element software. Also, deep drawing experiments on Ti–6Al–4V alloy have been performed at 400 °C to validate finite element simulation results. Further, comparison of yield criteria based on thickness distribution, earing profile, complexity in material parameter identification and computational time has shown Cazacu-Barlat to be well suited for deep drawing of Ti–6Al–4V alloy.     

Cladding of titanium/hydroxyapatite composites onto Ti6Al4V for load-bearing implant applications

To improve the bioactivity of Ti6Al4V alloy for use as a load-bearing hard tissue replacement, titanium/hydroxyapatite (Ti/HA) composites were bonded to a Ti6Al4V substrate by a novel cladding method. With the aid of a silver foil as the interlayer and an external pressure during sintering, the interfaces between the composites and the substrate were free of defects. The bioactivity of the fabricated materials was evaluated in the simulated body fluid (SBF) and the results demonstrated that the materials could induce nucleation and growth of bone-like apatite in the SBF. Factors that contributed to the bioactivity of the materials were discussed. The release of Ag⁺ ions from the materials was also detected, which is expected to impart antibacterial effect after implantation, and further enhance the functionalities of the materials.     

Failure modes and fatigue life estimations of spot friction welds in cross-tension specimens of aluminum 6061-T6 sheets

Failure modes of spot friction welds (SFWs) in cross-tension specimens of aluminum 6061-T6 sheets are investigated. Micrographs of the SFWs before and after failure under quasi-static and cyclic loading conditions are examined. Two different nugget pullout failure modes can be seen. A fatigue crack growth model based on the paths of the dominant kinked fatigue cracks is adopted to estimate the fatigue lives of SFWs. The computational stress intensity factors for finite kinked cracks and the Paris law for fatigue crack propagation are considered. The fatigue life estimations based on this model agree well with the experimental results.     

Thermo-mechanical aspects of adiabatic shear failure of AM50 and Ti6Al4V alloys

The thermo-mechanical aspects of adiabatic shear band (ASB) formation are studied for two commercial alloys: Mg AM50 and Ti6Al4V. Tests are carried out on shear compression specimens (SCS). The evolution of the temperature in the deforming gauge section is monitored in real-time, using an array of high-speed infrared detectors synchronized with a Kolsky apparatus (split Hopkinson pressure bar). The evolution of the gage temperature is found to comprise three basic stages, in agreement with Marchand and Duffy’s simultaneous observations of mechanical data and gauge deformation patterns (1988). The onset and full formation stages of ASB are identified by combining the collected thermal and mechanical data. Full development of the ASB is identified as the point at which the measured and calculated temperature curves intersect and diverge thereon. At that stage, the homogeneous strain assumption used in calculating the maximum temperature rise is no longer valid.     

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.     

Surface properties of CrN-coated Ti–6Al–4V alloys by arc-ion plating process

Ti–6Al–4V alloys which are widely used in various industries were successfully coated by a hard CrN layer using an arc-ion plating process (AIP). The CrN layer coated on the Ti–6Al–4V substrate was found to improve the surface properties such as hardness, smoothness and wear resistance compared with the substrate materials. Relatively good bonding force between the both materials was obtained.     

Biocompatible and mechanical properties of low temperature deposited quaternary (Ti, Al, V) N coatings on Ti6Al4V titanium alloy substrates

A series of quaternary (Ti, Al, V) N coating layers were obtained by low temperature reactive plasma sputtering in differing deposition conditions to improve the wear resistance and the biocompatibility of a titanium surgical alloy, specifically Ti-6Al-4V. Characterization of the mechanical properties, structure and the chemical composition of the coating layer was explored by microhardness test, ball against flat wear test, scanning electron microscopy and X-ray diffraction. The biocompatibility of the optimum coating layer (as determined by mechanical performance) was examined by a modified MTT toxicity test and by monitoring cell growth assessed by quantitative stereological analysis. The experimental results are encouraging, indicating that this low temperature deposited, dense, quaternary (Ti, Al, V) N coating layer exhibits improved mechanical properties such as high hardness and excellent adhesion to a Ti alloy substrate and is highly biocompatible.     

Time dependence of microstructure and hardness in plasma carbonized Ti–6Al–4V alloys

Ti–6Al–4V alloy was treated by plasma carburizing process at 950 °C for different durations of 1 h, 2 h, 3 h, and 4 h. Plasma carburizing was performed in pure Ar gas under 32 ± 2 Pa. Graphite rod was employed as carbon supplier. Optical microscope (OM) observations showed a carburizing layer formed after carburizing. Further FESEM examinations and XRD analysis confirmed that the carburizing layer consists of a TiC/α-Ti mixed layer and a thin compound (TiC) layer on the mixed layer. With increasing the carbonizing time, the thickness of the carburizing layer increased and the specimen treated at 950 °C for 3 h obtains maximum values of the hardness.     

A comparative study of the impurity segregation from commercially pure Ti, Ti6Al4V and Ti3Al8V6Cr4Zr4Mo

The surface composition of commercially pure Ti, Ti6Al4V and Ti3Al8V6Cr4Zr4Mo during annealing at different constant temperatures was experimentally investigated. Auger electron spectroscopy was used to monitor the APPHs of the specified elements present on the surfaces. The surfaces of Ti and its alloys were contaminated by oxygen and carbon, and the contamination is attributed to the continual uptake of the background gases, even in the UHV chamber. It was found that mainly C and S segregated at 400 °C, and Cl at higher temperatures (500–630 °C) for commercially pure Ti. However, S was the main segregating species for all three samples. The segregation of Al was measured for the Ti6Al4V and Ti3Al8V6Cr4Zr4Mo samples at higher temperatures. The linear least-square fit method was employed to determine the contribution of pure Ti and TiC from the measured APPH's. The AES fitting confirmed the formation of TiC on the surface at temperatures 400–500 °C.