Stitch welding of Ti–6Al–4V titanium alloy by fiber laser

Stitch welding of plate covered skeleton structure of Ti–6Al–4V titanium alloys has a variety of applications in aerospace vehicle manufacture. The laser stitch welding of Ti–6Al–4V titanium alloys was carried out by a 4 kW ROFIN fiber laser. Influences of laser welding parameters on the macroscopic geometry, porosity, microstructure and mechanical properties of the stitch welded seams were investigated by digital microscope, optical microscope, scanning electron microscope and universal tensile testing machine. The results showed that the three-pipe nozzle with gas flow rate larger than 5 L/min could avoid oxidization, presenting better shielding effect in comparison with the single-pipe nozzle. Porosity formation could be suppressed with the gap between plate and skeleton less than 0.1 mm, while the existing porosity can be reduced with remelting. The maximum shear strength of stitch welding joint with minimal porosity was obtained by employing laser power of 1700 W, welding speed of 1.5 m/min and defocusing distance of +8 mm.     

Fatigue in Ti–6Al–4V–B alloys

The addition of small amounts of B to Ti–6Al–4V alloy reduces the as-cast grain size by an order of magnitude and introduces TiB phase into the microstructure. The effects of these microstructural modifications on both the high cycle fatigue and cyclic stress–strain response were investigated. Experimental results show that B addition markedly enhances the fatigue strength of the alloy; however, the influence of prior-β grain size was found to be only marginal. The presence of TiB particles in the matrix appears to be beneficial with the addition of 0.55 wt.% B to Ti–6Al–4V enhancing the fatigue strength by more than 50%. Strain-controlled fatigue experiments reveal softening in the cyclic stress–strain response, which increases with the B content in the alloy. Transmission electron microscopy of the fatigued specimens indicates that generation of dislocations during cyclic loading and creation of twins due to strain incompatibility between the matrix and the TiB phase are possible reasons for the observed softening.     

Continuous and directional solidification technology of titanium alloys with cold crucible

The experiments of continuous and directional solidification of titanium alloys with cold crucible were carried out in a multifunctional electromagnetic cold crucible apparatus. Parameters and factors influencing the surface crack and macrostructure of titanium alloy ingots were studied. The mechanism of the parameters and factors influencing the surface crack and macrostructure of the ingots were interpreted. The results show that the surface cracks of the prepared ingots decrease with the increase of the input power from 50 to 60 kW or with the increase of the coil turns from 3 to 5 circles. The surface cracks increase with the increase of withdrawal velocity from 3 to 5 mm/min or the height of the primer from 2 to 3 cm, then decrease with the increase of withdrawal velocity from 5 to 8.7 rnm/min or the height of the primer from 3 to 4 cm. Coil turns is the most important one in all parameters effect on the surface crack, the input power is more important, then the withdrawal velocity is important and the height of the primer is the least important. Withdrawal velocity is the most important factor affecting the macrostructure, and effects of other factors on maerostructure is slight. With the decrease of velocity from 8.7 to 0.5 mm/min, the quantity of grains reduces, the grain orientation degree becomes small, and the solidification fronts change from concave to plane to convex. The ingot can be directional solidified at velocity of 1 mm/min. The ingot with free surface crack and directional macrostructure is prepared under definite conditions.     

Large gap joining of Ti–6Al–4V alloy with mixed powder interlayers

Abstract

Titanium based brazing alloys containing chromium, iron, copper, and nickel as β stabilisers have been studied for joining the titanium alloy Ti–6Al–4V. Two of these alloys were selected for use in producing large gap joints. The first brazing alloy, Ti–12Zr–14Cr–12Cu–12Ni (type 1), can be used to braze Ti–6Al–4V below its β transus temperature. Joints of thickness up to 150 μm can be made in a normal brazing cycle without prolonged holding. The interlayer consists of a β titanium alloy with no precipitation of intermetallic compounds. The second brazing alloy, Ti–12Zr–14Cr–6Fe–5Cu–5Ni (type 2), has to be brazed above the β transus temperature of Ti–6Al–4V. Its powders were mixed with pure titanium and Ti–6Al–4V powders and the mixture was used as the joining interlayer. Interlayers 5 mm in thickness were used to produce joints for microstructural examination and mechanical testing. It was found that residual pores in the interlayers were related to the amount of the brazing alloy in the interlayer. A fully dense interlayer could be obtained with 60 wt-% brazing alloy in the interlayer. The as bonded joints revealed tensile strength equal to 50% of that of the base metal. Diffusional treatment of the joints improved the joint efficiency to about 70%, compared with the base metal.     

Low cyclic fatigue behavior of electron-beam-welded Ti–6Al–4V titanium joint

The low cycle fatigue(LCF) tests were carried out using symmetrical cyclic loading under total strain amplitude control conditions.The present paper is devoted to investigating the cyclic deformation response of Ti–6Al–4V titanium and the electron-beam-welded(EBW) joint in the following aspects,i.e.,cyclic deformation behavior,fatigue life and fatigue fracture behavior.The results show that the softening of the joint is significant at larger strain ranges,while not obvious at smaller strain ranges.The joint shows shorter fatigue life at larger strain ranges and equivalent fatigue life at smaller strain ranges compared with Ti–6Al–4V base metal.A fatigue crack of the joint not only originates at the surface or subsurface,but also at defects in the fusion zone(FZ).The crack propagation zone of Ti–6Al–4V base metal shows ductile fracture mechanism,while the joint shows brittle fracture mechanism.In all the fatigue fracture zones many dimples appear,showing the typical ductile fracture.     

Properties of passive film formed on CP titanium,Ti–6Al–4V and Ti–13.4Al–29Nb alloys in simulated human body conditions

The passive film behavior of CP Titanium, Ti–6Al–4V and Ti-based intermetallic Ti–13.4Al–29Nb was investigated as a function of immersion time in simulated body condition (Hank's solution), utilizing potentiodynamic polarization and electrochemical impedance spectroscopy techniques. All the alloys were spontaneously passivated on immersion in the electrolyte. Potentiodynamic polarization experiments conducted after 1 and 168 h of immersion in Hank's solution indicated similar passive current densities. The susceptibility of Ti–13.4Al–29Nb alloy for passive film breakdown was ascribed to the greater Al content in the alloy. Electrochemical impedance spectroscopy studies indicated that the resistance of the passive film increased with duration of immersion for CP Titanium and Ti–13.4Al–29Nb. Therefore, the order of corrosion resistance under simulated human body conditions is Ti–Nb>Ti>Ti–V.     

Enhanced densification of Ti–6Al–4V powders by transformation-mismatch plasticity

The densification kinetics of Ti–6Al–4V powders with spherical or angular shapes are compared in uniaxial die pressing experiments between isothermal conditions (at 1020 °C, in the β-field, where deformation occurs by creep) and thermal cycling (between 860 and 1020 °C, within the range of the α–β phase transformation of the alloy, where transformation-mismatch plasticity is activated). Densification kinetics are only moderately affected by powder shape, but are markedly faster under thermal cycling than under isothermal conditions, as expected from the higher deformation rate achieved under transformation-mismatch plasticity conditions as compared to creep conditions. The densification curves for both creep and mismatch plasticity deformation mechanisms are successfully modeled for various applied stresses and for partial cycling, when transformation is incomplete. Tensile properties of specimens fully densified under thermal cycling conditions are similar to literature values from Ti–6Al–4V densified by isothermal hot isostatic pressing.     

Optimization of Ti6Al4V titanium alloy laser–arc hybrid weld parameters

This article reports the results of a study aimed at using statistical methods to optimize the parameters for laser–arc hybrid butt welding of Ti6Al4V titanium alloy sheets with a thickness of 3.0 mm. The study has examined the effects of the hybrid welding process parameters, such as laser beam power, arc pulse frequency, arc length, arc current, wire speed, laser and arc relative positions, and weld speed. Microstructure has been studied using light microscopy and morphological analysis of weld bead cross sections. This article reports the results of energy and morphological tests.     

Machinability improvement of titanium alloy (Ti–6Al–4V) via LAM and hybrid machining

Titanium alloy (Ti–6Al–4V) is one of the materials extensively used in the aerospace industry due to its excellent properties of high specific strength and corrosion resistance, but it also presents problems wherein it is an extremely difficult material to machine. The cost associated with titanium machining is also high due to lower cutting speeds (<60 m/min) and shorter tool life. Laser-assisted machining (LAM) and consequently hybrid machining is utilized to improve the tool life and the material removal rate. The effectiveness of the two processes is studied by varying the tool material and material removal temperature while measuring the cutting forces, specific cutting energy, surface roughness, microstructure and tool wear. Laser-assisted machining improved the machinability of titanium from low (60 m/min) to medium-high (107 m/min) cutting speeds; while hybrid machining improved the machinability from low to high (150–200 m/min) cutting speeds. The optimum material removal temperature was established as 250 °C. Two to three fold tool life improvement over conventional machining is achieved for hybrid machining up to cutting speeds of 200 m/min with a TiAlN coated carbide cutting tool. Tool wear predictions based on 3-D FEM simulation show good agreement with experimental tool wear measurements. Post-machining microstructure and microhardness profiles showed no change from pre-machining conditions. An economic analysis, based on estimated tooling and labor costs, shows that LAM and the hybrid machining process with a TiAlN coated tool can yield an overall cost savings of ～30% and ～40%, respectively.     

Surface modification of Ti–6Al–4V alloys using triode plasma oxidation treatments

In this study, triode plasma oxidation (TPO) has been used to improve the tribological characteristics of Ti–6Al–4V. The effect of TPO on ball-on-plate reciprocating-sliding, impact, and micro-abrasion wear resistance of this alloy is investigated. Surface micro-profilometry, nano-/micro-indentation hardness testing, scratch-adhesion testing, scanning electron microscopy (SEM), atomic force microscopy (AFM), glancing-angle X-ray diffraction (GAXRD), and glow-discharge optical emission spectroscopy (GDOES) data is presented to corroborate the effects of the oxidation process. ‘Traditional’ thermal oxidation processes were used to benchmark this novel treatment. Following TPO treatment at 700 °C for only 4 h, a hard (exceeding 11 GPa) and well-adhered oxide layer, composed of mixtures of the anatase and rutile polymorphs of TiO₂, was formed at the surface of the Ti-alloy. This layer is accompanied by a much larger oxygen-solution strengthened zone which creates a gradual chemical and mechanical gradient from the hard oxide ‘compound layer’ into the ductile substrate core. The various wear testing methods employed revealed excellent wear resistance of the TPO-treated alloy—compared both to the untreated alloy and to conventional, thermally oxidised samples.     

Dynamic stress–strain properties of Ti–Al–V titanium alloys with various element contents

A series of Ti–Al–V titanium alloy bars with nominal composition Ti–7Al–5V ELI,Ti–5Al–3V ELI,commercial Ti–6Al–4V ELI and commercial Ti–6Al–4V were prepared.These alloys were then heat treated to obtain bimodal or equiaxed microstructures with various contents of primary a phase.Dynamic compression properties of the alloys above were studied by split Hopkinson pressure bar system at strain rates from 2,000 to 4,000 s-1.The results show that Ti–6Al–4V alloy with equiaxed primary a(ap)volume fraction of 45 vol%or 67 vol%exhibits good dynamic properties with high dynamic strength and absorbed energy,as well as an acceptable dynamic plasticity.However,all the Ti53ELI specimens and Ti64ELI specimens with ap of 65 vol%were not fractured at a strain rate of4,000 s-1.It appears that the undamaged specimens still have load-bearing capability.Dynamic strength of Ti–Al–V alloy can be improved as the contents of elements Al,V,Fe,and O increase,while dynamic strain is not sensitive to the composition in the appropriate range.The effects of primary alpha volume fraction on the dynamic properties are dependent on the compositions of Ti–Al–V alloys.     

High-Temperature Oxidation Kinetics and Behavior of Ti–6Al–4V Alloy

Owing to the high-temperature reactivity of titanium, the oxidation and alloying of titanium during hot working processes is an important variable. The oxidation behavior of Ti–6Al–4V alloy in air was investigated at various temperatures between 850 and 1100 °C for different times. The oxidation kinetics were determined by isothermal oxidation weight gain experiments. The results showed that the oxidation kinetics approximately obeyed a parabolic law. The activation energy of oxidation was estimated to be 199 and 281 kJ mol^?1 when temperature was above and below the beta transformation temperature (T _β), respectively. A model to predict oxidation extent was established based on experimental observations. The oxide scales mainly consisted of TiO₂ with a small amount of Al₂O₃ and TiVO₄. The alpha case was defined as solid solution formed because of oxygen diffusion into the substrate. The difference in the morphology and the formation mechanism of the alpha case at different temperature ranges was mainly owing to the participation of the grain boundary and grain orientation of the nucleation site.     

Diffusion bonding of Ti—6Al—4V titanium alloy powder and solid by hot isostatic pressing

The Ti—6Al—4V (TC4) alloy powder and forged solid were diffusion bonded by hot isostatic pressing (HIP) to fabricate a powder—solid part. The microstructure of the powder—solid part was observed by scanning electron microscope (SEM). The microhardness and tensile tests were conducted to investigate the mechanical properties. The results showed that the powder compact was near-fully dense, and the powder/solid interface was tight and complete. The microhardness of the interface was higher than that of the powder compact and solid. The fractures of all powder—solid tensile specimens were on the solid side rather than at the interface, which indicated that a good interfacial strength was obtained. The tensile strength and elongation of the powder compact were higher than those of the solid. It is concluded that the HIP process can successfully fabricate high-quality Ti—6Al—4V powder—solid parts, which provides a novel near net shape technology for titanium alloys.     

Tensile properties of gas tungsten arc weldments in commercially pure titanium,Ti–6Al–4V and Ti–15V–3Al–3Sn–3Cr alloys at different strain rates

Abstract

The influence of microstructure and strain rate on the mechanical behaviour of three titanium alloys having applications in aerospace, namely, commercially pure titanium (α phase), Ti–6Al–4V (α + β phases) and Ti– 15V–3Cr–3Sn–3Al (β phase) is investigated for both the parent metals and their gas tungsten arc weldments. The results indicate that the tensile strengths of the three as received titanium alloys and their weldments increase with increasing strain rate. However, their elongations decrease with increasing strain rate. The as received Ti–6Al–4V alloy and its weldment, with a mixed α and β phase microstructure, have the maximum strength and microhardness. Commercial purity titanium metal and its weldment exhibit the minimum strength and microhardness. The tough Ti–15V–3Cr–3Sn–3Al alloy and its weldment, having a fully β phase microstructure, appear to have optimum strength and microhardness. The tensile properties of all three titanium alloy weldments are inferior to those of the as received metals.     

Wetting of porous titanium carbonitride by Al–Mg–Si alloys

Wetting of porous TiC_0.17N_0.83 by six alloys from the Al–Mg–Si system (pure Al, pure Mg, Al–15 at.% Mg, Al–10 at.% Si, Mg–5 at.% Si, and Al–10 at.% Mg–10 at.% Si) in an argon atmosphere was studied using the sessile drop experiment. The contact angle of the liquid drops on TiC_0.17N_0.83 substrates was measured as a function of temperature. Aluminium, Al–10 at.% Si, and Al–10 at.% Mg–10 at.% Si did not wet TiC_0.17N_0.83 in the studied temperature range. Magnesium always wetted TiC_0.17N_0.83 with a minimum contact angle of ≈44° at 900°C, and alloying with Mg significantly lowered the contact angle of Al on TiCN. Alloying with Si deteriorated the wetting of TiCN by Mg. A comparative study between the systems was conducted, based on the results and on data available in the literature. The improvement of the wetting of TiCN by Al due to alloying with Mg can be explained by the segregation of Mg to the interface with TiCN, where it lowers the interface energy. The addition of Si to pure Mg or to Al–Mg results in an increase in the contact angle on TiCN.     

Tensile properties and failure analysis of Ti–6Al–4V joints by electron beam welding

The microstructural and mechanical characterization of electron beam welded joints of forged Ti–6Al–4V were investigated. Microhardness tests indicate that the hardness of the fusion zone(FZ) is higher than that of the heat-affected zone(HAZ) and base metal. The tensile results show that the mechanical properties of the welded joints are comparable with those of the base metal in terms of static strength and are in accordance with the relationship between microstructure and mechanical properties of welded joints. The ultimate tensile strength of the weld is equal to that of the hourglass joint, which indicates that the mechanical properties of the longitudinal FZ and those of the transverse FZ are the same. Macromechanical behavior and macrofracture and microfracture of the base material,joint, and weld specimens are observed. A comparison among the three types of specimen fracture phenomena reveals the following distinctive differences:(1) the fracture mode,(2) the micrograph of the dimple pattern at the central region, and(3) the size of the dimple at the central region and the transition region.     

Characterization of Ti6Al4V–Ti6Al4V/30Ta Bilayer Components Processed by Powder Metallurgy for Biomedical Applications

Metals and Materials International - The design and fabrication of a bilayer Ti6Al4V–Ti6Al4V/30Ta component were performed by using the powder metallurgy process and solid-state sintering as...     

A coupled thermal and metallurgical model for welding simulation of Ti–6Al–4V alloy

The accurate prediction of the mechanical behavior of welded components made of Ti–6Al–4V requires a particular material model considering the significant effects of the material behavior during welding. Especially, phase transformations are assumed to have an influence on the temperature distribution. The flow curves of the material are changed during welding by complex mechanisms which might be describable using time-temperature history dependent flow curves. The following paper shows (as a step forward), how the influence of phase transformations on the transient heat conduction in components made of Ti–6Al–4V during tungsten inert gas (TIG) arc welding is investigated using a coupled thermal and metallurgical model. Kinetics of α+β→β phase transformation during heating and β→α phase transformation during cooling are studied using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation. A numerical heat transfer model is used to calculate the transient temperature field during welding. The thermal properties are calculated by a linear mixing rule based on the phase fractions and the thermal properties of each pure phase. Using these obtained thermal properties, the welding process of Ti–6Al–4V alloy is modeled using finite-elements for the spatial discretization and finite-differences to predict the transient temperature field. Additional calculations neglecting the phase changes are carried out to compare the temperatures and visualize the effects of phase transformations on the cooling behavior. The comparison of these models with measurements shows that the model considering the influence of solid phase transformations describes the temperature profile during cooling accurately.