Critical assessment of the fatigue performance of additively manufactured Ti–6Al–4V and perspective for future research

To realize the potential benefits of additive manufacturing technology in airframe and ground vehicle applications, the fatigue performance of load bearing additively manufactured materials must be understood. Due to the novelty of this rapidly developing technology, a very limited, yet swiftly evolving literature exists on the topic. Motivated by these two points, we have attempted to catalog and analyze the published fatigue performance data of an additively manufactured alloy of significant technological interest, Ti–6Al–4V. Focusing on uniaxial fatigue performance, we compare to traditionally manufactured Ti–6Al–4V, discussing failure mechanisms, defects, microstructure, and processing parameters. We then attempt to identify key knowledge gaps that must be addressed before AM technology can safely and effectively be employed in critical load bearing applications.     

Microstructure and inclusion of Ti–6Al–4V fabricated by selective laser melting

Selective laser melting (SLM) was used in fabricating the dense part from pre-alloyed Ti-6Al-4V powder. The microstructural evolution and inclusion formation of as-fabricated part were characterized in depth. The microstructure was characterized by features of columnar prior β grains and acicular martensite α'. High density defects such as dislocations and twins can be produced in SLM process. Investigations on the inclusions find out that hard alpha inclusion, amorphous CaO and microcrystalline Al₂O₃ are three main inclusions formed in SLM. The inclusions formed at some specific sites on melt pool surface. The microstructural evolution and inclusion formation of as-fabricated material are closely related to the SLM process.     

Effect of surface roughness on fatigue performance of additive manufactured Ti–6Al–4V

Additive manufacturing is increasingly considered for production of high quality, metallic, aerospace parts. Despite the high potential of this manufacturing process to reduce weight and lead time, the fundamental understanding of additive manufactured Ti–6Al–4V material is still at an early stage, especially in the area of fatigue and damage tolerance. This paper covers the effects of inherent surface roughness on the fatigue life. In the as built condition, metallic parts have a poor surface texture, which is generally removed in fatigue critical areas. It is shown that the fatigue properties of Ti–6Al–4V samples, produced by direct metal laser sintering and electron beam melting, are dominated by surface roughness effects. A simple model based on an equivalent initial flaw size is formulated.     

Synthesis,corrosion and wear of anodic oxide coatings on Ti–6Al–4V

Electrodeposited anodic oxide coatings were produced on Ti–6Al–4V substrates using aqueous electrolytes containing dissolved calcium and phosphorus. Different coatings were produced by varying the time periods. The coatings were characterised by XRD technique and TEM. The coatings were exposed to Simulated Body Fluid (SBF). Electrochemical polarisation and ac impedance studies too were performed on the coatings in SBF. Pins were coated and run against wooden disc in pin-on-disc type of wear tests. Coatings produced from long time electrolysis showed very good resistance to the attack of SBF and less wear compared to those produced from short time exposure.     

Microstructure and mechanical properties of as-built and heat-treated electron beam melted Ti–6Al–4V

This paper investigates the effect of post-deposition heat treatment on porosity, microstructure, and mechanical properties of Ti–6Al–4V produced via an Electron Beam Melting process. Samples were studied in the conditions of as-built and heat treated at 920°C and 1030°C. The as-built samples were characterised by columnar β grains consists of α+β microstructure with Widmanstätten and colony morphologies were found. Heat treatment resulted in increased α lath width. The yield strength and ultimate tensile strength was greater in the as-built condition than wrought material. Porosity re-growth occurred after heat treatment but it did not affect the tensile properties. Greater ductility after heat treatment was attributed to the larger α lath width which increases effective slip length.     

Microstructure study and constitutive modeling of Ti–6Al–4V alloy at elevated temperatures

A reliable and accurate prediction of flow behavior of metals in industrial forming process considering the coupled effects of strain, strain rate and temperature is crucial in understanding the workability of the metal and optimizing parameters for hot forming process. In this study, the tensile fracture behavior of the Ti–6Al–4V alloy is examined with scanning electron microscope (SEM) over the range of magnifications. SEM study revealed that microvoids and shallow dimples are observed at the fracture surface which indicates the fracture is predominately ductile in nature. Also, an investigation on flow behavior of Ti–6Al–4V alloy is done using constitutive models. Four constitutive models; modified Johnson-Cook (m-JC), modified Arrhenius type equations (m-Arr), modified Zerilli–Armstrong (m-ZA) and Rusinek–Klepaczko (RK) models are developed to predict the flow stress. The predictions of these constitutive models are compared with each other using statistical measures like correlation coefficient, average absolute error and its standard deviation. Comparing the statistical measures, m-Arr model is a better model for predicting the flow stress, but considering the fact that m-ZA model is a physical based model, m-ZA model is preferred over the m-Arr model.     

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     

Microstructure and mechanical properties of in situ formed TiC-reinforced Ti–6Al–4V matrix composites

Carbon nanotubes were blended into a Ti–6Al–4V matrix to synthesize titanium carbide (TiC) in situ, via spark plasma sintering. The microstructure and mechanical properties of both the monolithic Ti–6Al–4V alloys and the TiC/Ti–6Al–4V composites were studied to evaluate the strengthening effects of TiC on the Ti–6Al–4V matrix. The morphologies obtained by scanning electronic microscopy and optical microscopy indicated that the grain size of both the Ti–6Al–4V alloy and the TiC/Ti–6Al–4V composite decreased with increasing planetary ball-milling (PBM) speed, leading to an increase in the hardness of the investigated materials. The compressive yield strength of the monolithic Ti–6Al–4V alloys and the TiC/Ti–6Al–4V composites initially increased and then decreased with increasing PBM speed. The strengthening and fracture mechanisms were studied.     

Thermal analysis and phase transformation behaviour during additive manufacturing of Ti–6Al–4V alloy

Despite the fact that the additive manufacturing (AM) technique has been established for almost two decades, its optimisation is still performed by trial and error experimentation. In the present work, a finite element modelling approach was used to study both the temperature distribution and heat flux vector characteristics during multi-layer deposition of a Ti–6Al–4V alloy that take place in the AM process. The results revealed the difference between different powder deposition time intervals on thermal cycles, heat flux vectors and the resulting microstructures. Good agreement between the numerical and experimental results was found. The results obtained can be used for process optimisation.     

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.     

Microstructure and notched tensile fracture of Ti–6Al–4V to Ti–4.5Al–3V–2Fe–2Mo dissimilar welds

Dissimilar welding of Ti–6Al–4V (Ti-6-4) to Ti–4.5A1–3V–2Fe–2Mo (SP-700) alloys was performed using a CO₂ laser. The microstructure and notched tensile strength (NTS) of the dissimilar welds were investigated in the as-welded and post-weld heat treatment (PWHT) conditions. Moreover, the results were compared with homogeneous laser welds with the same PWHT. The dilution of SP-700 with the Ti-6-4 alloy caused the formation of fine needle-like α + β structures, resulting in the exhibition of a moderately high fusion zone (FZ) hardness of HV 398. The high FZ hardness (HV 438) for the weld with the PWHT at 482 °C was associated with low NTS or high notch brittleness. The fracture appearance of the notched tensile specimen was related to its inherent microstructure. With increasing the PWHT temperature, the thickness of grain boundary α increased, which promoted an intergranular dimple fracture. By contrast, fine shallow dimples were present in the peak-aged weld, which was induced by the refined α + β microstructures in the basket-weave form.     

Effect of Vacuum Heat Treatment on the Microstructure and Tribological Property of Ti–6Al–4V Alloy with Cu Coating

The effect of vacuum heat treatment on the interface microstructure and tribological property of Cu-coated Ti – 6Al – 4V alloy is investigated herein. After the vacuum heat treatment process, a diffusion layer is formed at the interface between the Cu coating and the Ti – 6Al – 4V substrate. The formed intermetallic compounds at the interface between the Ti – 6Al – 4V substrate and Cu coating are CuTi₂, CuTi, Cu₄Ti₃, and β-Cu₄Ti. The activation energy of intermetallic compound growth in the diffusion zone of Cu-coated Ti – 6Al – 4V is 126.0 kJ mol⁻¹, and the pre-exponential factor is 0.1 m² s⁻¹. The tribological properties of the Cu-coated Ti – 6Al – 4V alloy are best when subjected to diffusion treatment at 700 °C for 300 min, with weight loss reduced by 58.2% compared to the Ti – 6Al – 4V alloy. The wear resistance of the Ti – 6Al – 4V alloy can be enhanced by Cu coating and vacuum diffusion heat treatment, and the formation of the Cu – Ti intermetallic compound contributes to this improvement. These findings offer new insights for further advancements in the tribological properties of titanium alloys.     

Mechanical properties of titanium-based Ti–6Al–4V alloys manufactured by powder bed additive manufacture

Additive manufacturing is currently a topic of considerable interest at both academic and industrial levels. While a significant amount of data exists on the mechanical properties and structure–property relationships of traditional wrought alloys, less information is available on alloys manufactured by additive manufacture. This review examines current state-of-the-art manufacture of titanium-based Ti–6Al–4V alloys by powder bed additive manufacture. Published mechanical properties to date are collected which include tensile strength, yield strength, hardness, wear, fracture toughness and fatigue. Differences in microstructure and properties compared to conventional wrought alloys of the same composition are described.     

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.     

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.     

Influence of Processing Conditions on Microstructure and Mechanical Properties of Large Thin-Wall Centrifugal Ti–6Al–4V Casting

In this work,the effects of mould pre-heating temperatures and hot isostatic pressing(HIPping)process on the microstructural characteristics and mechanical properties,including static tensile and damagetolerance properties of large thin-wall cylindrical Ti–6Al–4V casting,have been studied.The experimental results show that with the increasing mould pre-heating temperature from 673 to 873 K,the casting microstructures change from a mixture of Widmanst?tten and colony microstructure to a primary colony.The centre of the thick wall section has relatively coarse microstructure than the edge and thin section.Lower mould pre-heating temperature brings about more porosities.HIPping process,which not only reduces the casting pores effectively but also increases the priorβgrain boundary cohesion and coarsens the microstructure,is essential to improving the ductility of the casting.Due to the oxygen contamination and finer microstructure on the surface,micro-hardness profiles on the cross section present a decreasing tendency from the surface to inner.The thickness of the reaction layers for the different mould pre-heating temperatures is nearly the same(~450μm).On the whole,the tensile strength and microhardness decrease with increasing mould pre-heating temperature from 673 to 873 K.However,the fracture toughness and fatigue crack growth resistance of the castings increase with increasing mould preheating temperature.     

Effect of severe plastic deformation on creep behaviour of a Ti–6Al–4V alloy

This paper examines the effect of severe plastic deformation on creep behaviour of a Ti–6Al–4V alloy. The processed material with an ultrafine-grained (UFG) structure (d ≈ 150 nm) was prepared by multiaxial forging. Uniaxial constant stress compression and constant load tensile creep tests were performed at 648–698 K and at stresses ranging between 300 and 600 MPa on the UFG processed alloy and, for comparison purposes, on its coarse-grained (CG) state. The values of the stress exponents of the minimum creep rate n and creep activation energy Q _c were determined. Creep behaviour was also investigated by nanoindentation method at room temperature under constant load. The microstructure was examined by transmission electron microscopy and scanning electron microscope equipped with an electron back scatter diffraction unit. The results of the uniaxial creep tests showed that the minimum creep rates of the UFG specimens are significantly higher in comparison with those of the CG state. However, the differences in the minimum creep rates of both states of alloy strongly decrease with increasing values of applied stress. The CG alloy exhibits better creep resistance than the UFG one over the stress range used; the minimum creep rate for the UFG alloy is about one to two orders of magnitude higher than that of the CG alloy. The indentation creep tests showed that annealing had little effect on the creep behaviour in UFG Ti alloy at room temperature.     

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.     

Static coarsening behaviour of lamellar microstructure in selective laser melted Ti–6Al–4V

Static coarsening mechanism of selective laser melted (SLMed) Ti–6Al–4V with a lamellar microstructure was established at temperatures from 700 °C to 950 °C. Microstructure evolution revealed that high heat treatment temperature facilitated martensite decomposition and promoted lamellae growth. At each temperature, the growth rate decreased with increasing holding time. The static coarsening behaviour of SLMed Ti–6Al–4V can be interpreted by Lifshitz, Slyozov, and Wagner (LSW) theory. The coarsening coefficient were 0.33, 0.33–0.4, 0.4–0.5 for 700–800 °C, 900 °C and 950 °C, respectively. This indicated the coarsening mechanism was bulk diffusion at 700–800 °C, and a combination of bulk diffusion and interface reaction at 900 °C and 950 °C conditions.