Investment casting process is an important way to get complex parts of titanium alloy. However there are few research on the interfacial heat transfer coefficient (h) between casting and shell thus the temperature simulation of investment casting process of titanium alloy is often inaccurate. In order to get a relatively accurate h, a one-dimensional mathematical model for the reverse calculation of h between casting and shell in investment casting process of Ti6Al4V alloy was built and the analytic relationship between temperature and time/heat flux was established. Considering the calculated h is significantly affected by the error of parameters such as the specific heat capacity and thermal diffusivity of shell and position of thermocouples, research on the error of these parameters is essential. The relationship between the error of these parameters and the temperatures in the casting and shell was studied and it was found that the effect of different kind of error on the temperature field was obviously different. An experiment based on the one-dimensional mathematical model was done and temperatures of different positions were measured. Based on the effect of different kind of error and the difference between the calculate temperature field and the measured temperatures, the proportion of effect of each kind of error was assessed. These errors were revised on the basis of the assessment, thus a relatively accurate h between the casting and shell was obtained. The relationships between h and thickness of the solidified layer on the casting/temperature at the surface of casting can be divided into 4 stages: (1) Metal was liquid and h kept about 440 W/(m2K); (2) Solid layer appeared on the surface, and h declined nearly 60%; (3) Solid layer grew up before metal became completely solid and h declined nearly 20% of its maximum; (4) After metal solidified, h declined slowly as temperature on the surface of casting dropped. These relationships were applied in a three-dimensional model for numerical simulation of the temperature field. Temperatures of different positions in casting and shell were calculated and calculated temperatures agreed with measured ones well. Thus the accuracy of h was identified and it can help solve problems in the production in investment casting process of Ti6Al4V alloy.
Rigid restraint thermal self-compressing bonding is a new solid-state bonding process. During the process, localized non-melted heating method is employed to heat the butted interface of the rigid restrained plates to be bonded. Under the localized heating, materials close to the butted interface are expanded. However, due to the existence of surrounding cool metals and rigid restraints, the expansion of the high temperature materials is restrained and thus, a compressive pressure is developed which compresses the high temperature metals near the bond interface and facilitates the atom diffusion between butt-weld specimens to produce a permanent solid-state joint. Utilizing the localized stress-strain field to accomplish atomic bonding, this process can avoid the use of external forces on which diffusion bonding and other solid-state bonding methods rely. Previous study has proven the feasibility of this process to join titanium alloys. In present work, the effect of beam power on bond interface, microstructure and mechanical properties of the TC4 joints bonded at different beam powers were analyzed through the OM observation, EBSD analysis, mechanical property test and fracture morphology analysis. Meanwhile, in order to reveal the mechanism about the effect of beam power on bond interface, the experiment study on microstructure and mechanical property and finite element analysis on present bonding were conducted to investigate the effect of beam power on the thermal stress-strain process during bonding. The results show that with the increase of beam power, the heating temperature, dwell time over high temperature, volume of materials with high temperature and the compressive plastic strain increase which promote the atom diffusion and thus bond quality of the interface is improved. At low beam power, the microstructure of the joints is homogeneous, while coarse grain with acicular a phase forms in the joint when the beam power is high. Mechanical properties of the joint are dependent on bond rate and microstructure. When the beam power is lower or higher, the compressive mechanical properties of the joints are poor because of the poor bonding quality of the interface or the coarse microstructure developed in the joint. Good comprehensive mechanical properties are obtained at the beam power of 330 W.
As a solid state technology, friction stir welding (FSW) has been used to join titanium alloys for avoiding the fusion welding defects. So far, many previous studies have attempted to elucidate the microstructure characteristics and evolution during the FSW process of titanium alloy, but few are about the mechanism of microstructure transformation along the thickness direction of joint. For solving this problem, in this work, 2 mm thick TC4 titanium alloy is successfully welded by FSW. On the basis of numerical simulation, the effects of temperature distribution on the microstructure along the weld thickness direction and the tensile strength of welding joint were investigated. The results show that the peak temperatures of material close to weld surface exceed b phase transus temperature under the rotational speed of 300 r/min and the welding speed of 50 mm/min. With the increase of distance away from the weld surface, the peak temperature decreases. The peak temperature of weld bottom near the backing board is difficult to be higher than b phase transus temperature owing to quick heat radiation. The region, where the peak temperature is higher than b phase transus temperature, consists of primary a, lath-shape a and residual b phases. The size of lath-shape a inside the weld is larger than that near the weld surface. Primary a and b phases with smaller size are attained in the weld bottom owing to the dynamic recrystallization, and the distribution of b phase on primary a matrix is more homogeneous. When the rotational speed reaches 350 r/min, the area where the peak temperature is higher than b phase transus temperature becomes wider along the thickness direction, which makes the size and quantity of lath-shape a phase increase and then the lath-shape a clump appears. Lath-shape a phase with different orientations hinder the propagation of crack and be beneficial for the tensile strength of FSW joint.
Weld deformation of the thin-wall weldment used in fighter aircraft not only hinders its subsequent procedure of fabrication and assembling, but also reduces its fatigue strength. As a result, weld deformation shortens its service life essentially. Dustpan deformation is always produced in the thin-wall weldment after multiple-pass weld. In this work, combining with the experiment, the finite element method was adopted to analysis the deformation of the thin-wall weldment by multiple-pass weld and its shape correction by post weld heat treatment. For obtaining the fundamental properties such as thermal parameters and mechanical parameters of TA15 titanium alloy, a series of experiments were conducted at room temperature and high temperatures. Additionally, creep behaviors of TA15 titanium alloy were studied at the temperatures of 500, 550, 600, 650, 700 and 750 ℃, and the parameters of creep constitutive equations of the alloy were obtained with considering the analysis of post weld heat treatment. A thermal coupled temperature-displacement analysis for welding and post weld heat treatment was performed on a three dimensional shell model of protective grille. Experiments of multiple-pass weld and post weld heat treatment were used to testify the reliability of the finite element model of welding and post weld heat treatment. With using the reliable finite element model, the parameters of heat treatment were studied. The study indicates that, the fabrication on the crossing of structure section and fillet after fillet-wallboard weld leads the compression deformation release along the fillet, after that, the shrinkage distortion produced during spot welding of fillet-structural section mainly contributes the large dustpan deformation of the thin-wall weldment; increasing temperatures, enlarging loads and prolonging the hold time can improve the shape correction of the thin-wall weldment during post weld heat treatment, hence the guide maps of the post weld heat treatment for shape correction of the thin-wall weldment under 700 and 750 ℃ are worked out.
The service environment faced by marine equipment and its key friction pair parts are much more severe than that on land surface. The life cycle and safety of the hydraulic and power transmission system, which directly get in touch with the seawater, depends largely on the tribological behavior of the components in the seawater. Titanium alloy is an ideal material used for ocean engineering, however due to its poor friction performance its life cycle may be shortened when working in the environment with friction and wear. In order to improve the tribological performance of titanium alloy in seawater, laser processing was used to build super hydrophobic with grid and dot micro-structure on Ti6Al4V alloy surface. Tribological performance was evaluated by HSR-2M high speed reciprocating friction test machine in artificial seawater, and compared with in water (distilled water). The results show that the friction coefficients and wear losses (volume) of super hydrophobic Ti6Al4V alloy surface are significantly smaller than that of the Ti6Al4V alloy substrate. The friction coefficients of surface with dot and grid reduced by 17.8% and 11.7%, and wear losses (volume) reduced by 36.8% and 57.5% respectively in artificial seawater. The friction coefficient of super hydrophobic Ti6Al4V alloy surface in artificial seawater is smaller than that in water while the wear loss has the opposite phenomena. The tribological performances of titanium alloy in artificial seawater are significantly improved by the build of super hydrophobic Ti6Al4V alloy surface.
Most titanium alloys have been designed for aeronautical applications, where their excellent specific properties are fully employed and weldability is a classic problem with Ti and its alloys. Microstructure and mechanical properties of the electron beam weldments of TC17 alloy were investigated in this work. The results showed that there exhibited three zones across the TC17 electron beam weldment: the fusion zone (FZ), heat affected zone (HAZ) and base metal (BM). It was also observed that the as-welded FZ consisted of metastable β columnar grains, while the HAZ consisted of acicular α/α′ phase, equiaxed α phase and metastable β phase. Furthermore, it was indicated that the transformation from metastable β phase to α+β phase happened when the FZ and HAZ were post-weld heat treated at 630~800 ℃, the coarsening of α laths and the grain boundary α were also observed when the heat treatment temperature increased. The increasing of 450 ℃ ultimate tensile strength of FZ was ascribed to the precipitation of secondary acicular α platelets during tensile testing in the as-welded and 800 ℃ heat treated conditions, which led to the low yield ratio of FZ. The tensile failure location of the weldments was found to occur in preference in the low tensile yield strength area, or in the low hardness area when the difference between yield strength across the weldments is very small. It was concluded that the optimal post-weld heat treatment for the TC17 alloy weldment was 630 ℃, 2 h, A.C., at which the weldments showed good combination of tensile strength and elongation.
TC4 titanium alloy is usually used to manufacture engine blades, blings or blisks and fatigue is the main failure of these components due to its high strength, good corrosion resistance and light weight. In engineering applications, three typical surface modification processes such as shot peening (SP), laser shock peening (LSP) and low plasticity burnishing (LPB) were employed to improve fatigue performance. In this work, SP, LSP and LSB were taken to enhance surface layer of TC4 titanium alloy. The surface integrity of specimens including surface roughness, microhardness, residual stresses and microstructure was investigated to obtain the effects of modification on surface layer by different methods. The rotating-bending fatigue performance was tested at room temperature and fatigue fracture surfaces were analyzed by SEM. Fatigue life was compared at the same stress 760 MPa with the reference machinced specimen. Fatigue strength was determined by stair method for 1×107 cyc. The results show that both the rotating-bending fatigue life and fatigue strength of TC4 titanium alloy are increased by these surface modification processes. The fatigue life prolonging factor (FLPF) for SPed specimens is 20.4, and FLPF for LSPed specimens and LPBed specimens is 89.6 and 99, respectively. Meanwhile, fatigue strength improvement percentage (FSIP) for SPed, LSPed and LPBed specimens is 36.3%, 37.8% and 38.8%, respectively. Moreover, the fatigue cracks initiate beneath surface enhanced layer for surface-modified specimens, while they are located at surfaces for un-surface-enhanced ones. Based on dislocation theory, the subsurface cracks initiation resistance and fatigue strength for surface-enhanced specimens were analysied. Finally, surface modification mechanisms were discussed and some quantitative analysis methods on surface modification effects were proposed. For surface-enhanced smooth specimens, the FSIP limit is 40% based on proposed analysis model and it is verified in this work by different surface layer enhancement processes (36.3% for SPed specimens, 37.8% for LSPed and 38.8% for LPBed specimens are near to 40%). Fatigue total life including initiation and propagation is a complex problem, and therefore it is difficult to give accurate life prediction and analysis, especially for small crack growth, although some invesitigations on total fatigue life can be roughly estimated based on Basquin relation for stress fatigue life or Coffin and Marson eqution for strain fatigue life which have not any physical meaning or any mechanism.
The prospect of joining titanium and aluminum components into structures is desirable for a wide range of aerospace and automobile industry applications. One of the problems related with the joining processes for dissimilar metals such as Ti and Al is the formation of residual stress in the bonded joint, which has significant effect on the joint mechanical properties. In this work, joining of a titanium alloy to an aluminum alloy by ultrasonic assisted brazing using a Zn-Al filler metal was investigated. The microstructures of the titanium/aluminum brazed joints were determined by OM, SEM and TEM. The local tensile deformation characteristics of the brazed joints were also examined using the digital image correlation (DIC) methodology by mapping the local strain distribution during in situ tensile tests. The results showed that the Ti7Al5Si12 phase and the TiAl3 phase were formed at the titanium/brazing seam interface. The brazing seam was primarily composed of a Zn-rich phase and a Zn-24.14%Al (mass fraction) eutectoid structure. At the aluminum/brazing seam interface, no interfacial reaction layer was observed and the primary phase Zn-Al dendrites nucleated at the aluminum base metal and grew into the inside of the bonding region. A diffusion layer was formed in the aluminum base metal. It was found that the tensile deformation of the brazed joints was highly heterogeneous, which led to the deflection of the crack during propagating in the joint. The fracture initiated at the Zn-rich phases, where contained the highest stress concentration due to their low elastic modulus, and propagated in the Zn-rich phases or through the interface between Zn-rich phase and Zn-Al eutectoid structure.
Titanium alloys are widely used as structural material in aerospace, automobile, biomedical and other fields because of its low density, high specific strength, good corrosion resistance and good biocompatibility. But high coefficient of friction, poor wear resistance and high-temperature oxidation resistance are the main reasons for limiting the use of titanium alloy in complex working conditions. In order to improve the high-temperature oxidation resistance and optimize the microstructure of titanium alloy, high Nb content Ti-Al intermetallic composite coating was fabricated by laser in situ synthesis technique on BT3-1 titanium alloy surface. Phase structure of the composite coating was analyzed according to XRD spectra. Unit area oxidation weight gain of the titanium alloy substrate and coating before and after heat treatment were tested by GSL-1600X tube furnace under 950 ℃. The oxidation kinetics curves were drawn and the high temperature oxidation resistance was compared. The microstructures of coating before and after oxidation were observed by OM and SEM, and the high-temperature oxidation resistance mechanism was analyzed. The results show that the coating mainly consists of Nb, intermetallic γ-TiAl, α2-Ti3Al and Ti3Al2 phases before heat treatment. But after heat treatment, Nb is dissolved in γ-TiAl and α2-Ti3Al, and a new phase Ti3AlNb0.3 is generated in the coating. The coating is approximately γ-TiAl+α2-Ti3Al duplex structure. The oxidation kinetics curves of coating is between linear and parabolic rule before heat treatment, its high temperature oxidation resistance increased by 2 times of titanium alloy substrate. The oxidation kinetics curves of coating is approximately parabolic law after heat treatment, and the rate of oxidation is small, its high temperature oxidation resistance increased more than 20 times of titanium alloy substrate. Under 950 ℃ cyclic oxidation conditions, the oxide layer surface of coating forms a continuous dense capsule oxide, and oxide layer closely connect the unoxidized coating portion, the oxide layer plays a good protective role of the composite coating. But for titanium alloy substrate, the oxide layer surface is loose and porous oxide, the oxide layer is fractured and removed from the substrate surface. Nb alloying significantly improves the high temperature oxidation resistance of Ti-Al intermetallic coating.
Grain refinement is a challenging topic to improve mechanical properties of metallic materials, especially for titanium alloys which show great potential in aerospace and medical implants areas due to the low density and good corrosion resistance. However, severe plastic deformation (SPD) technologies which have been commonly used in laboratory in smaller scale are difficult to be realized in industrial. Considerable researches are therefore paying attention to the development of new technologies for improvement of grain refinement at relatively lower strains. In this work, the dual phase TC16 titanium alloy showing excellent room temperature ductility was investigated with emphasis on the feasibility of producing ultrafine grains by roller die drawing at room temperature. The techniques of XRD, SEM, TEM, Vickers hardness test and tensile test were employed to analyze the phase constitutes, microstructure evolutions and preliminary mechanical properties of the alloy deformed at different conditions. Results reveal that TC16 titanium alloy mainly consists of α and β phases after roller die drawing at room temperature, and a small quantity of stress-induced α" martensite can be additionally identified inside β grains. The grain sizes of α phase and β phase decrease with strain increasing, which result to enhanced tensile strength and Vickers hardness. Indeed, the fibrous morphology of both α phase and β phase with 0.3 μm in thickness and a high value of 365 HV in Vickers hardness were revealed at the applied true strain of 2.14. Ultra-fine grains evidenced by a near-ring SAED spots were therefore achieved in the present case.
According to Hall-Petch relationship, high strength of nano-grain and ultrafine-grain meta-llic materials are always accompanied by the cost of ductility because of the lack of work hardening induced by rare or absent dislocation or slip band. And various strategies including semi-solid processing accompanied by rapid solidification, recrystallization induced by plastic deformation and heat treatment, consolidation of blended powders with different grain sizes, and so on, have been developed to fabricate so-called bimodal/multimodal microstructures in the pursuit of high strength and no sacrificing ductility. As one of the most significant types of phase transformation in metallography, eutectic reaction was frequently utilized to tailor phase constitution and its microstructure due to high strength resulted from resultant lamellar eutectic structure. Generally, eutectic structure is more common in solidification and even traditional semi-solid processing for low melting point alloys (such as aluminum and magnesium alloys). In this work, a fundamentally novel approach of semi-solid sintering stemmed from the formation of liquid phase induced by eutectic transformation is introduced. Through regulation of the phase composition of eutectic transformation (or eutectic liquid content), novel bimodal Ti52.1Fe21.7Co8.2Nb12.2Al5.8 alloy with high-strength and large-ductility was successfully fabricated by semi-solid sintering of amorphous alloy powder with multi-phase eutectic system. The fabricated bimodal microstructure consists of fine nearly equiaxed fcc Ti2(Co, Fe) embedded into ultrafine lamellar eutectic matrix containing bcc β-Ti and bcc Ti(Fe, Co) lamellae, which is different from bimodal microstructures reported so far. The fabricated bimodal alloy exhibits ultra-high yield strength of 2050 MPa and large plastic strain of 19.7%, superior to those of bimodal titanium alloys reported so far. The method is conducive to process high-performance new structural metallic alloys in high melting point alloy systems.
In order to improve the inoxidizability of TC4 alloy at high temperatures, hot dip aluminizing process is an efficient and economical way for industrial application. In this process, the wetting of TC4 alloy by molten Al alloy is the main factor which determined the coating quality. In this work, wetting of TC4 alloys by two industrial grade Al alloys (i.e., 6061 Al and 4043 Al alloys) were studied by using the modified sessile drop method at 600~700 ℃ under high vacuum. The results show that Al/Ti system is a typical reactive wetting, and the spreading dynamics can be described by reaction product control model, further the whole wetting behavior can be divided into two stages: the first stage for the nonlinear spreading and the second stage for the linear spreading. The small amount of alloying element Si in the Al alloys can cause significantly segregation at liquid/solid interface and formation of the Si-rich phase (Ti7Al5Si12). Ti7Al5Si12 decomposition is responsible for the nonlinear spreading, and Ti7Al5Si12 decomposition and Al3Ti formation are together responsible for the linear spreading. The formation of precursor film accompanies with the good final wettability.
TC4 titanium alloy is highly promising for aerospace and medical implant applications due to its low density, high strength, corrosion resistance and biocompatibility, and the ultra-fine grains of TC4 alloy by accumulative roll bonding (ARB) can efficiently improve the low temperature super-plasticity and biocompatibility for its widespread applications. However, the ARB process for TC4 alloy has been limited due to the high deformation resistance and low anti-oxidant ability. In this work, ARB was conducted for the ultra-fine grains of TC4 titanium alloy, and the effects of ARB temperatures and layer numbers on the bonding interface and microstructure were investigated as well as the deformation mechanism of the mixed α /β phase structure, and the influences of ARB processing on the mechanical properties were studied. The good interface bonding could be fabricated by the proper ARB temperature (near 720 ℃), the anti-oxidation treatment and the multilayer with the high deformation, which always takes on the hardened interface with the high oxidation contents, and the interface bonding strength increases with the increase of the ARB layers and temperature through the process of the diffusion and the necking fracture. The deformation process is composed by the cooperation deformation of α /β structure and the shear deformation during ARB processed TC4 alloy, during which the β phase at the grain boundary changes from the long strips to the short bands to deform with hcp α phase, while the shear bands with severe local-deformation is used to adapt the severe plastic deformation. The deformed microstructure is composed of the equiaxed structure (about 300 nm spacing) and the elongated deformation structure (about 400 nm spacing), in which the equiaxed structure comes from the function of the deformation temperature, local shear deformation and the local overheat. Additionally, the inhomogeneous microstructure and properties along the thickness direction can be observed, and the high hardness can be obtained at the bonding interface, which gradually distributes homogeneous with the increase of ARB layers. The strength of ARB processed TC4 sheets increases with the increase of ARB layers, which can get to 1325 MPa after 16 ARB layers, and simultaneously the plasticity decreases to 5.4%. The ductile fracture can be observed with the low ARB layers, while the mixed structure of the quasi-cleavage and ductile fracture is obtained with the increase of ARB layers.
Titanium alloys have been widely used in bearing force components in aeronautical structures, such as blades and beams to withstand the high frequency dynamic loads, which requires an outstanding fatigue resistance performance in very high cycle regime during their service life. In this work, very high cycle fatigue failure property of TC17 alloy used as aircraft engine blade material was studied by ultrasonic fatigue test and electromagnetic resonance fatigue test under 110 Hz and 20 kHz sinusoidal load, and crack initiation mechanism of different failure mode was analyzed. The results showed that, fatigue failure modes of TC17 alloy could be divided into surface induced failure and interior induced failure. Surface induced failure was caused by the machine defect and surface slide trace that triggered by the asymmetric loading. Interior induced failure was caused by slid fracture of primary α phase under asymmetric loading. Fatigue resistance of TC17 alloy was influenced by the fatigue crack initiation mechanism but concerned little about the loading frequency. The variation of the fatigue failure mechanism resulted in the S-N curves presenting bilinear. A fatigue strength predicted model is established based on the parameter of the weak crystal orientation area, which is in good agreement with the fatigue test result.
The titanium alloy parts, which have been formed by traditional laser additive manufacturing (LAM) method, usually have obviously different microstructure from wrought microstructure of titanium alloy and show room temperature mechanical anisotropy. In order to make the LAMed titanium alloy parts get the same microstructure and mechanical properties as wrought titanium alloy, a new technology of LAM called consecutive point-mode forging and laser rapid forming (CPF-LRF) has been proposed. During CPF-LRF process, deposited TC11 titanium alloy by laser rapid forming (LRF) was deformed by consecutive point-mode forging (CPF), and then on the surface of the deformed TC11 titanium alloy, new LRF process started over again. Both LRF and CPF were performed alternatively throughout the process of the fabrication of a TC11 titanium alloy part. Microstructures and mechanical properties of the CPF-LRFed TC11 alloy sample have been investigated. The average grain size of equiaxed grains of the CPF-LRFed TC11 alloy sample is 48.7 μm. The equiaxed grains have continuous grain boundary α phase. The microstructure of the equiaxed grain is bimodal microstructure consisting of primary α phase lath and transformed β. During CPF-LRF process, being plastically deformed by CPF, the surface deformation zone of the thick-wall TC11 titanium alloy part is 1.5 mm depth and its deformation degree is 20%. During a new layer deposited on the surface of the CPF cold deformed TC11 titanium alloy part, when laser beam scans through, about 1 mm thick (four layers) cold deformed titanium alloy in the heat affected zone of laser melting pool is heated up above β-transus temperature of TC11 titanium alloy in which static recrystallization complete within time interval of 0.86 s. The mechanical properties indicate that compared with the tensile properties at room temperature of TC11 alloy forged piece, the CPF-LRFed TC11 alloy has higher strength and less ductility. Fracture analysis indicates that intergranular fracture is mainly responsible for the poor ductility of CPF-LRFed TC11 alloy.
Investment casting is widely used in producting complex thin-wall titanium alloy components. In this process, the β→α phase transformation decides the final microstructures of these components. However most of present studies on phase transformation of titanium alloys focus on the microstructure evolution in heat treatment process or after deformation rather than in casting process now. It is a main reason only this work aims at the solid phase transformation of Ti-6Al-4V alloy in investment casting. In this work, the growth model of edge of α phase plates based on multi component Zener-Hiller model, and the growth model of broad face of α phase plates based on diffusion and conservation of multi components were established. The growth competition of different colonies, which consist of α phase plates in same orientation, was simulated and the microstructures and their evolution with temperature were obtained. The comparison between simulated microstructures and their evolution with temperature and experimental data indicated that the proportion of undercooling degree caused by impurities in the alloy is about 45% of the total undercooling degree in broad face of α phase plates and a much smaller portion in edge of α phase plates. The comparison also showed that the enthalpy change of solid phase transformation of titanium alloy is about 70 kJ/kg. The simulated and experimental morphologies look like similar and the simulated growth rate is also in good accordance with experiment inferred growth rate.