Mechanical Alloying Nanocrystalline Ti-6Al-4V And Injection Molding Technology Research

Titanium and its alloys have received considerable attention recently in aerospace, navigation, automotive, biological engineering, sports goods and other fields. The advantages of these materials include low density, high specific strength, excellent corrosion resistance, and excellent mechanical properties. Among those, Ti-6Al-4V(TC4)has been the most highly used titanium alloy, its utility ratio being more than50%of the total titanium alloy production, accounting for95%of the total titanium alloy machined part, and it is the world leading in the applications of titanium alloys. However, its shortcomings such as the high production cost, difficulty in extraction, melting and machining, limit the application of titanium and its alloys. Since the1990s, scholars around the world have launched researches on metal injection molding of titanium and its alloys. The application of this technology to titanium and its alloy forming can greatly reduce the cost, improve the utilization of a wide range of production of the high performance and complicated parts. Mechanical alloying has increasingly brought to the attention of the international academic materials for an effective method of refined grain and material microstructure structure. In this paper, mechanical alloying is adopted to prepare nanocrystalline Ti-6A1-4V alloy powder; and then systematic study is done on powder injection molding process of nanocrystalline Ti-6A1-4V alloy, thus can take advantage of MIM and can expand the application of titanium and titanium alloy components in civilian areas. The main steps carried out are as follows:(1) Mechanical alloying is adopted to prepare nanocrystalline Ti-6A1-4V alloy powder1) The initial material used in the experiments are HDH titanium powder (particle size<74μm, purity>99.5%) and Al-V powder (particle size<74μm, purity>99%).Mixtures of90wt%Ti and10wt.%Al-V alloy powder as starting powders have been milled in a QX-2planetary ball mill. With laser scattering particle analyzer, X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and other analytical testing methods, this paper observes changes in the particle size distributions of the powders, phase composition and microstructure changes in the milling process. The results show that: mechanical alloying can prepare nanocrystalline Ti-6A1-4V alloy powder, the reaction mechanism dominated by diffusion, and the solid-state reaction is the result of defects in energy and interaction of the collision energy. Organizational structure of Ti-Al-V powder mixture changed obviously with increasing milling time, and substitution solid solution Ti(V)formed in the process. After milling for40h can obtain nanocrystalline, nanocrystalline and polycrystalline mixtures obtained for60hour. The grain sizes are less than60nm and little change in grain size after milling for60h. There is no intermediate phase generated in milling process. After milling, the atomic ratio of Ti, Al, V is near to90:6:4, which is consistent with the Ti-6A1-4V elemental composition with element surface scanning and energy spectrum analysis.2) The particle shape and size of mechanically milled powder change in close response to milling process parameters, such as angular velocity of milling, ball-to-powder ratio and milling time. With the increase of the milling time, the average particle diameter of the powder particles has a continuously decreasing trend, with the largest decline in30h to60h; after being milled for60h, milling does not change the powder particle size; and particles begin to reunite after70h. Larger ball-to-powder ratio would favor powder refinement and the formation of the solid solution. Increasing the angular velocity of milling would increase milling energy and refining speed as well as promote the process of alloying. Through the above experimental research and theoretical analysis, this study selects reasonable parameters in mechanical alloying process: ball-to-powder ratio is20:1, the milling speed is330r/min, and milling time is60h.3) This paper studies the effect of the parameters of mechanical alloying process on the ball moving situation, especially impact behavior of milling ball and the deformation of the powder. The results show that velocity of milling ball vb and impact frequency f increase in proportion to angular velocity of milling. When loading capacity mp is decided, the velocity of milling ball vb reduces, average free path S decreases, and impact frequency f increases with the ball-to-powder ratio Rbp increasing. When Rbp and mp are decided, for the same material milling balls, impact frequency using a large balls is lower than that of small balls, and increasing jar mill volume can increase S because average free path of large balls is large. The maximum true strain εmax in the oblique collision increases with milling speed Ω increasing, while decreases with the impact angle Ω increasing; the shear strain γyx increases with the increasing of the milling speed Ω and the impact angle θ. These theoretical study has important theoretical significance on further study of the microscopic mechanism of mechanical alloying process, and have important guiding significance on selecting reasonable parameters in milling process.4) Results showed that temperature rise is not high in the collision, so there is no intermediate phase generated in milling process, which is consistent with the results of test and analysis. However, nanocrystalline Ti-6A1-4V can be obtained at lower temperatures for generation of a large number of defects.(2) Research on MIM of nanocrystalline Ti-6Al-4V1) Starting from the basic theory of rheology, the paper analyzes the rheological behaviors of Ti-6A1-4V feedstocks, and deeply discusses the influence of the shear rate, temperature, mechanical alloying time,powder loading on shear viscosity. The experimental results show that:①The viscosity of feedstocks reduces with the increasing of the shear rate, temperature, and milling time; the viscosity of feedstocks increases with the improving of powder loading; The n value of the all feedstocks in this experiment are smaller than1, and the value of n decreases with the increasing of shear rate. Under a certain shear rate, n value increases with temperature raised; under the same temperature and shear rate, the value of n decreases with powder loading increasing, the value of n increases with the milling time prolonged.②By the density components experiment and mathematical calculations of empirical models, the critical powder loading of Ti-6A1-4V feedstocks is69vol%.③The flow activation energyEa and A value change with the increasing of shear rate, Ea value of the feedstocks fabricated from different milling time meet the index relationship between shear rate; A value of the feedstock fabricated from milling for60h satisfies index relationship between shear rate, but A value of the feedstocks fabricated from milling for20h or40h has a linear relation with the shear rate. From the Arrhenius equation, a set of empirical constitutive equations are established, two important factors, namely, temperature and shear rate are contained in these equations. The equation as a guidance to the injection molding production.2) The fitting empirical parameters by using the least square method are extracted. For Ti-6A1-4V feedstocks fabricated by the powder milling for60h, zero shear viscosity η0(T)value is13464.97Pa·s,8080.82Pa·s and6030.5Pa·s at140℃,150℃and160℃, respectively. The master curve (η/η0～η0γ) developed from time-temperature superposition is used to obtain a master curve for Ti-6A1-4V feedstocks. With the master curve, it be beneficial to further studies of MIM feedstocks as well as production practice.3) This paper proposes to optimize of injection process parameters by the application of DOE (Design of Experiment) method. On this basis, a large number of injection experiments are carried out, the results of which are analyzed by MiniTab software. A new design of experiment-fractional factorial design is put forward to study the interaction effect between process parameters. The important interaction and single factors are thus screened. Fractional factorial experiment results show that the injection pressure, holding pressure, injection temperature, injection speed, holding pressure and injection temperature interaction, injection temperature and injection pressure interaction significantly affect the density of injection part. The Taguchi orthogonal experimental design methods are then employed to design L27(313) test matrix and to analyze the influence of four significant factors and two interactions on the density of injection part. The optimized process parameters are:injection temperature150℃, the injection pressure100bar, the holding pressure90bar, and injection speed62%.4) Studied the debinding behavior and mechanism of nanocrystalline Ti-6A1-4V injection parts, and then a two-step debinding process is selected:solvent debinding and thermal debinding to remove the binder. Parameters of solvent debinding are:debinding for4hours in normalheptane at60℃; parameters of thermal debinding are:temperature500℃,heating ratel℃/min,and the gas flow rate200ml/min.①Effects of debinding temperature, debinding time, part geometry, solvent on binder removal are investigated, it is found that debinding rate increases with extension of the debinding time; and the higher the debinding temperature is, the higher the debinding rate is. But after10hours, the effects of temperature on the debinding rate is less clear than before, and the debinding rate at different temperature gradually converge. By study of the solvent debinding kinetic study, solvent debingding process of this experiment sample is found to be predominantly diffusion-controlled. From the mathematical model, the diffusion active energy of normalheptane is calculated as40.39KJ·mol-1·K-1, which is higher than methylene chloride (38.41kJ·mol-1·K-1), The diffusion coefficient of normalheptane is higher than methylene chloride at the same temperature.②Relationships between removed binder quantities, debinding time and temperature are described by Ricardo V. B. Olivira equation. Through calculation, with the surface area/volume ratio As/V increasing, debinding rate at the same time increases and solubilization energy Q decreases.For sample with constant As/V, time has no significant effect on the solubilization energy.③A thermal debinding curve is established by thermal gravimetric analysis(TGA) and Ti-6A1-4V alloy features. The discussion is mainly done on the influence of thermal debinding temperature and the gas flow rate of protective atmosphere on debinding rate and residual carbon content and oxygen in debinding stage, which results in the determination of a reasonable thermal debinding temperature being500℃and the gas flow rate being200ml/min.5) By research on sintering process, the nanocrystalline Ti-6A1-4V was found to have a equiaxed structure, revealing a grains with intergranular β phase. The percentage of a phase in the alloys increased with increasing sintering temperature and times. Theoretical densities and hardness of samples increased, porosity decreased with increasing sintering temperature and times. Theoretical densities, maximum ultimate tensile strength and elongation were obtained 97.92%,783MPa and6.04%at1200℃for3h.