Abstract:Micro-arc oxidation is a new surface treatment method, but the structure of the micro-arc oxidation coating is limited to the electrolyte composition. In this paper the micro-arc oxidation coating on Ti6Al4V alloy was modified by addition of micro- Al₂O₃ particles to a sodium phosphate solution. The coating structure and phase was characterized by scanning electron microscopy and X-ray diffraction, and the oxidation resistance and thermal shock properties of the coating were investigated. Results showed that a coating denser than the original coating was produced. This new coating was composed of Al₂TiO₅ and TiO₂. The oxidation resistance and thermal shock property of the coating improved with addition of Al₂O₃ particles to the electrolyte relative to the sample prepared without the particles in the electrolyte. Moving Al₂O₃ particles were adsorbed on the coating surface and penetrated through it. As a result, the phase structure and properties of the original coating were modified.

Abstract:In this paper, the microstructure and mechanical properties of a high strength titanium alloy (Ti–6Al–6Mo–4V) were firstly investigated. The relationship between microstructures and properties of the alloys were investigated after solution treatment at α/β and β regions and then aging at five different temperatures ranging from 460℃ to 620℃ for 6h. The results illustrate the alloys after α/β region solution treatment and aging show a more attractive combination of strength and elongation than β solution treatment and aging. After solution treatment at 850℃ (α/β region) and aging at 460℃, the alloy obtains the highest strength (1572MPa) with elongation (2.63%). When aging at 620℃, the alloy obtains the highest elongation (11.46%) but lower strength (1201MPa). After solution treatment at 825℃ plus aging at 540℃, this alloy reaches a great combination of strength (1328MPa) and elongation (7.58%).Meanwhile, due to the large β grains after β region solution treatment and fine secondary α forms during aging, it does not obtain an attractive strength after β region solution treatment plus aging.

Abstract:Electron beam cold hearth melting (EBCHM) is a promising technique for achieving premium titanium alloys for critical rotating parts of the aero-engine and to recycle the titanium scraps. This article studied the heat transfer, the fluid flow and solidification in the cold hearth during EBCHM process of Ti-6Al-4V alloy. The results show that the melt is constrained to a very shallow depth which is 15mm in our experiment and the melt velocity is about a few centimeters per second. The melt temperature increases with the increase of melting power and decreases with the increase of melting rate. The melt depth increases with the increase of melting power, while the effect of melting rate on the shape of melt pool isn’t obvious. The trajectories of inclusions with different densities and sizes were simulated. The density has great influence on the trajectories of particles. The evaporation model was established to study the influence of melting parameters on the ingot composition. The results indicate that melting temperature, melting rate and composition of row materials have great influence on the ingot composition. Round ingots were produced using the commercial EBCHM furnace in the factory. The morphology of the shell was investigated and the ingot composition was tested. The calculating results are in good agreement with experiment.

Abstract:Macrostructural characterisation was performed for Ti-6Al-4V components produced by wire and arc additive manufacturing (WAAM) to investigate the effects of different deposition direction and interlayer dwell time on grain morphology of prior-β. By extending the interlayer dwell time between each deposited layer, the full equiaxed β grain were obtained. The results indicated that the equiaxed β grains at the bottom were caused by recrystallization. The sizes of the equiaxed β grains at the bottom were both affected by the thermal cycle and the degree of recrystallization.

Abstract:Due to the defects of high aluminum residue and high oxygen residue, high titanium ferro alloy prepared by thermite reaction outside the furnace cannot be directly used in liquid steel refining process. In this paper, thermodynamics of the high titanium ferro with high aluminum used in liquid steel refining process was researched, and then effects of the amount and the aluminum content in high titanium ferro onthe liquid steel refining process were also studied. The results indicate that: using Ti and Al as composite deoxidizers, the deoxidizing product at 1873K is Ti2O3 when the aTi/aAl in liquid steel is above 8; but in fact, only when the aTi/aAl value in liquid steel is above 10, Ti2O3 precipitates as deoxidizing product. Using high titanium ferro with high aluminum content as deoxidizer, the content of aluminum and titanium in liquid steel can meet the requirements of composition for the related steel. With the increasing amount of high titanium ferro, inclusions in cast steel after refining transformed from silicate into Ti-Al-Mn composite inclusions；and at the same time, obvious or part of radial acicular ferrite forms around those inclusions, which refines the microstructure of steel. The aTi/aAl value in the actual system is 17.78(>8), which is consistent with the theoretical results. It is vital to control the values of aTi/aAl in the liquid steel when the oxygen content in liquid steel is high.

Abstract:Macrosegregation is a typical defect occurred during the vacuum arc remelting (VAR) of titanium alloy ingot. In the subsequent heat treatment serious segregation of special elements would lead to the so called beta-fleck, which will significantly reduce the property of product. Reducing the incidence of macrosegregtion requires a thorough understanding of the formation mechanism of such defect. In this paper a continuum model for alloy solidification has been used to simulate the temperature evolution, solute distribution and liquid flow. The segregation patterns obtained in simulations are consistent with the observations in experiments. The development of segregation in the conditions with different thermal and solute buoyancy forces were analyzed. It was shown that the inhibition of flow can minimize the macrosegregation to the greatest extent. It was also found that the solute partition coefficient determines the segregation pattern and severity in the same fluid flow conditions.

Abstract:In Wire Arc Additive Manufacturing (WAAM), residual stress is easily generated during solidification and subsequent cooling, which affects the formation and use of the components. In this paper, finite element simulations are carried out to investitgate the influence of roller reductions on the temperature, stress, strain and residual stress distribution in components during the hybrid forming of Ti-6Al-4V alloy by WAAM and interlayer rolling. The results show that interlayer rolling reduce the residual macroscopic stress significantly in the deposited metal, and at the same time reduce the overall stress on the substrate. Increasing reduction can reduce significantly the macroscopic residual stresses and distorsion of the components. The microstructure of the material can be changed dramatically by the plastic deformation after rolling, which lead to a new route for the optimization of the microstructure during additive manufcturing.

Abstract:With the large-scale development of marine equipment and the improvement of its performance, it is necessary to research and develop ultra-large sized titanium alloy ingots with high uniform composition. Based on the simulation results by MeltFlow, the 1st 12 tons ingot of Ti80 alloy is melted in Chinese and then was verified by two times. The chemical testing results show that the composition difference of the upper, middle and bottom of the ingot is less than 1000ppm, and the composition difference of the 17 positions of the riser is less than 3000ppm. Both of them are equivalent to that of the 5 tons ingot. Further, the macrosegregation at the riser was investigated and it is found that Al, Nb, Mo are enriched in the edge of the ingot and poor in the center of the ingot, whereas Zr has the opposite phenomenon. EDS was used to study the microsegregation between the grain boundary and interior. The results show that Nb and Mo are enriched within grain and poor at grain boundary. On the contrary, Al and Zr are enriched in grain boundary and poor within grain. The macro and micro segregation are consistent for Nb, Mo and Zr and opposite for Al. This is largely because the distribution of Nb, Mo and Zr is more largely depended on the solute redistribution at the solidification interface. By contrast, due to the highSsaturated vapor pressure at high temperature, Al is volatilized severe during the long time and high vacuum melting process and the solute redistribution at the solidification interface.

Abstract:As oxidizable metals, titanium alloys are very active on melting point. It is easier for large-size titanium castings to react with the casting materials which will affect the surface quality of the castings during casting . In this experiment, large-scale titanium alloy castings were casted by Yttrium oxide mould in vacuum consumable shell furnace, and the surface layer of samples with different thickness was tested and analyzed by SEM, XRD and EDS. The results show that there is a certain thickness of oxygen diffusion layer on the surface of the samples. And there is a certain thickness of tissue transition zone on the surface. Finally, the experiment was carried out by acid. Finally, the acid pickling method was used to remove the reaction layer and the transition zone of the sample surface, which effectively improved the surface quality of the castings.

Abstract:Lamellar Ti–55511 near-beta Ti alloy was conducted on 750°C hot rolling and subsequent 600°C annealing to investigate the microstructure evolution and mechanical properties. The Burgers orientation relationship between α and β phase is broken down owing to dynamic recrystallization (DRX) of lamellar α phase during hot deformation, forming fine and equiaxed α plates, which leads to the increasing tensile strength and elongation. During annealing, secondary α grains are precipitated from β matrix, resulting in higher strength but lower elongation. Meanwhile, further spheroidization and growth of α phase via static recrystallization (SRX) happen during annealing. The recrystallization of β phase only happens in subsequent annealing rather than hot rolling. At the beginning of the deformation, two twin variants can be found in lamellar α phase. With an increase in deformation, three twin variants are formed in α phase. The nano-scaled twins and stacking fault was formed resulting from twins decomposing during annealing.

Abstract:Micro TiB and nano La₂O₃ particles hybrid reinforced difficult-to-deformation Ti-6Al-4V TMCs were processed by severe plastic deformation of equal-channel angular pressing (ECAP). The effect of ECAP temperature on microstructure, formation mechanism of ultrafine grains and mechanical properties was studies by SEM, TEM and room temperature tensile in detail. The results show that ultrafine-grained (UFG) structure has formed in ECAPed matrix, and ECAP temperature have significant impact on the formation mechanism. Plenty of dislocation pile-ups and tangling contribute to cell structures of hundreds of nanometers in matrix at lower ECAP temperature, while dynamic recrystallization occurs at higher ECAP temperature, which promotes the formation