Process and properties of hydride formed on gamma titanium aluminide during cathodic charging

A cathodic charging procedure was used to study the process of the formation of the hydride layer on gamma titanium aluminides. This electrolytic process was carried out at constant current densities of 1 and 2 A/m² for 24 h of charging in a 1N sulfuric acid solution. The hydride layer formed as a result of the charging process was observed using scanning electron microscopy. Nanohardness and microhardness of this hydride layer were also measured. Results show that the hydride forms initially as isolated islands and becomes continuous with increased charging time. The hydride layer is quite brittle and its degree of porosity increases from the metal surface outward. The thickness of the hydride layer also increases with charging current density. This is confirmed by the hardness measurements. EDS signals show the presence of the constitutive elements of gamma titanium aluminide in the hydride. ICPS analysis of the electrolyte indicates increasing metal content with increasing time of exposure probably as a result of the brittle hydride flaking off and falling into the electrolyte during the charging process.     

Incremental deformation and the forgeability of γ titanium aluminide

Hot die forging has been undertaken on cube shaped workpieces of cast two-phase γ titanium aluminide. The inhomogeneous as-cast microstructure causes non-uniform deformation and low ductility. Incremental deformation using hot dies aims to break down large α₂+γ lamellae in the structure by using an accumulation of small amounts of deformation with interspersed with periods for material recovery. The effect of various forms of incremental deformation, with or without subsequent isothermal forging, on microstructure is discussed. It is shown that the use of hot-die forging to refine microstructure is limited by chilling of the workpiece and loss in ductility.     

Prediction of recrystallization behaviors in a steel billet during hot forging

Multi-recrystallizations occurring in a steel billet during hot forging were predicted. The prediction was obtained by coupling rigid thermoviscoplastic finite element analysis and recrystallization modeling equations. The finite element analysis was preformed for nonisothermal deformation as well as heat transfer for the entire forging process. As a result, it was predicted that three or four times of recrystallizations occurred in the billet and that resulting grains varied in size for different locations. The final grain sizes at the locations in the billet were measured and compared with predicted ones. It was found that they were in good agreement.     

An axisymmetric forging approach to preform design in ring rolling using a rigid–viscoplastic finite element method

In this paper, a new method for approximately predicting the deformation of material in ring rolling is presented. The plastic flow of material in ring rolling is assumed to be axisymmetric and thus the ring rolling process is considered as a sequence of consecutive forging processes. The problem, having tool velocity as well as material velocity field as unknown variables, is formulated by an axisymmetric rigid–viscoplastic finite element method. The unknown tool velocity is determined by making the circumferential stress on the ring cross-section exist in the range of user-specified values. The approach is applied to preform shape design in ring rolling of bearing races. The predicted results are compared with the experimental ones. It has been shown that the approximate approach presented is useful for engineering design of preform in ring rolling of bearing race-like rings.     

Effects of cutting angle,edge preparation,and nano-structured coating on milling performance of a gamma titanium aluminide

Gamma titanium aluminides are intermetallic alloys. Recently, they have been evaluated as important contenders for structural applications in the automotive and aerospace sectors. This is due to their excellent high-temperature performances and their significantly lower density compared to nickel-based superalloys. In this paper, an analysis of machinability of a gamma TiAl obtained via an electron beam melting (EBM) process is presented. The effects of tool geometry modifications, in terms of cutting tool angles and cutting edge preparation, were investigated. The reduction of radial rake angle and the drag finishing process for cutting edge preparation resulted in an increase of the tool life of the carbide end mills. Nanogradient tool coatings were also observed to affect tool wear during milling tests, and the results highlight that AlSiTiN coating performs better compared to CrAlSiN coating. A post-coating polishing treatment was also taken into account, and it allowed a further reduction of tool wear. The overall results indicate that the machinability of this difficult-to-cut material can be significantly improved by an adjustment of the cutting edge geometry, and by using an AlSiTiN coating system.     

Grain size and texture changes of magnesium alloy AZ31 during multi-directional forging

Grain size and texture changes of magnesium alloy AZ31 were studied in multidirectional forging(MDF) under decreasing temperature conditions.MDF was carried out up to large cumulative strains of 4.8 with changing the loading direction during decrease in temperature from pass to pass.MDF can accelerate the uniform development of fine-grained structures and increase the plastic workability at low temperatures.As a result,the MDFed alloy shows excellent higher strength as well as moderate ductility at room ...     

大锻件锻造过程中温度场测定及其结果分析

大型锻件锻造过程中工件温度场的变化规律对于保证锻件质量具有重要作用,由于锻件受环境温度、保温情况等诸多因素影响,生产中很难准确预测锻件锻造过程中温度变化规律。通过实验模拟实际生产中锻件加热曲线,在不同保温措施下,取锻件芯部至表层不同测控点进行锻造过程中的温度测量,通过数据分析建立了Mn18Cr18N护环钢锻造过程中的温度场,得出锻件的温度差与时间变化曲线及温度变化速率图,描述了工件在一定保温措施下的温度场变化情况,其结论对准确了解Mn18Cr18N护环钢锻造过程中的温度变化规律具有实际指导意义。     

Hot forging process design and parameters determination of magnesium alloy AZ31B spur bevel gear

Magnesium alloys have relatively low workability at room temperatures due to hexagonal crystal structure. In general, the forging process of magnesium alloys is considered to be very difficult because of the poor flowability and the sensitivity to the temperature and strain rate. Taken spur bevel gear as an example, the hot forging process of the complicated shape parts of magnesium alloy AZ31B was investigated by means of finite element (FE) simulations combined with experiments. After the two-stage hot forming process (preforming operation without gear shape and finish forging operation) was determined, the influence of various shapes of preform dies on the hot forging process was discussed by the commercial finite element analysis software Marc, and the optimum preform die shape was obtained. According to the numerical simulation results, the hot forging experiments of magnesium alloy AZ31B spur bevel gear were successfully conducted. By comparison between experimental load–stroke curves and the calculated ones, it shows that the calculated results are consistent with the experimental ones.     

爪极热锻模具失效机理的数值模拟与实验研究

针对爪极热锻模具寿命低、耗费大的问题,对热锻成形过程进行金属变形、温度以及模具应力和磨损等有限元分析。为验证有限元分析结果,对实际失效热锻模具进行微观组织和力学性能实验分析。有限元预测的模具受力、磨损和温度与实验结果吻合较好,共同揭示了上模因应力集中开裂失效、下模因磨损和高温软化导致塑性变形失效的机理。     

GH864合金涡轮盘锻造过程中微观组织的数值模拟

为了预测GH864合金φ1250 mm涡轮盘锻造过程中微观组织的演变规律,将微观组织模型加入到模拟软件MSC Super-form中,实现了高温合金塑性变形-传热-微观组织演变的耦合.研究了不同变形工艺参数对涡轮盘组织分布的影响,包括初始变形温度、变形速率、模具预热温度、摩擦系数及保温措施等的影响.结果表明:实际φ12...     

Ductility of a magnesium alloy in warm forging with controlled forming speed using a CNC servo press

The influence of press ram motion on forging performance for magnesium alloys was examined at elevated temperatures using an upsettability test on a CNC servo press. The forging limit of a wrought AZ31B (Mg–3 mass%Al–1 mass%Zn) magnesium specimen, forged using applied deceleration of the press ram motion, was found to be 30% higher than the forging limit achieved without press ram motion control. A finite element analysis was conducted to model the experimental results. The calculated temperature distribution was relatively uniform during upsetting with decelerated ram motion, and the maximum equivalent strain was maintained at a low level. Experimental and simulation results suggested that the influence of the press ram motion on the ductility of the Mg specimen could be described by a strain localization model. Furthermore, the ductility of Mg specimens during decelerated ram motion was improved in backward extrusion.     

镍基合金大锻件成形过程流变行为与晶粒尺寸预测

基于IN718合金热态成形过程中流动应力软化特征,采用动态再结晶软化本构模型来描述锻造过程中变形-传热-微观组织演变的相互作用。借助DEFORM-3D二次开发,将本构模型和微观演变模型进行耦合,模拟预测了IN718合金在多道次锻造过程中流变行为与晶粒尺寸演变,为大锻件锻造热力参数的合理制定与控制提供了依据。     

铝合金锻件成形工艺及三维有限元分析

为了揭示叶轮件模锻成形过程中金属的变形流动规律,采用有限元模拟对不同形状坯料成形过程的塑性变形流动行为进行对比分析.结果表明:叶片处是较难以充填成形的部位,且随着截面位置的下移,金属向该处型腔充填流动的趋势更为显著;与圆柱形坯料相比,阶梯形坯料成形过程中叶片部位更易于被充填成形,金属流动的均匀性也较好;通过工艺实验验证了利用阶梯形坯料可一次性成形出尺寸精度和性能均符合设计要求的叶轮锻件.     

TC6合金叶片预锻过程中微观组织的数值模拟

根据实验数据建立了TC6钛合金的动态再结晶模型,并对大型模拟软件Deform进行了二次开发,使其具有微观组织预测功能。通过对圆柱体的镦粗模拟验证了Deform软件二次开发后的的可靠性。采用二次开发后的软件,对叶片的预锻成形过程进行了数值模拟,分析了不同变形温度和变形速度对叶片预锻成形时晶粒尺寸及分布的影响。模拟结果表明,随着变形温度的升高,锻件主要变形区的平均晶粒尺寸略有增大,晶粒间晶粒尺寸的差别缩小,晶粒分布比较均匀;随着变形速度的增大,平均晶粒尺寸的分布越来越不均匀。因此,在合适的变形温度及变形速度下能够使锻件获得良好的综合机械性能。     

Temperature dependent flow softening of titanium alloy Ti6Al4V: An investigation using finite element simulation of machining

Titanium alloy Ti6Al4V is the most commonly used titanium alloy in the aerospace and medical device industries due to its superior properties. There has been a considerable amount of research to better understand the serrated chip formation mechanism of titanium alloy Ti6Al4V by using finite element simulation of machining. An accurate representation of the behavior of the material is important in order to obtain reliable results from the finite element simulation. Flow softening behavior has been integrated into the material constitutive models to simulate adiabatic shear bands and serrated chips. Flow softening is usually related to the dynamic recrystallization phenomenon which initiates after a critical temperature. The aim of this study is to investigate the influence of various flow softening conditions on the finite element simulation outputs for machining titanium alloy Ti6Al4V. For this purpose, a new flow softening expression, which allows defining temperature-dependent flow softening behavior, is proposed and integrated into the material constitutive model. The influence of flow softening below the critical temperature, as adopted in recent studies, is also investigated. Various temperature-dependent flow softening scenarios are tested using finite element simulations, and the results are compared with experimental data from the literature. The results showed that the flow softening initiating around 350-500 °C combined with appropriate softening parameters yields simulation outputs that agree well with the experimental measurements.     

Microstructure prediction of two-phase titanium alloy during hot forging using artificial neural networks and FE simulation

The microstructural evolution of titanium alloy under isothermal and non-isothermal hot forging conditions was predicted using artificial neural networks (ANN) and finite element (FE) simulation. In the present work, the change in phase volume fraction, grain size, and the volume fraction of dynamic globularization were modelled considering hot working conditions. Initially, an ANN model was developed for steady-state phase volume fraction. The input parameters were the alloy chemical composition (Al, V, Fe, O, and N) and the holding temperature, and the output parameter was the alpha/beta phase volume fraction at steady state. The non-steady state phase volume fraction under non-isothermal conditions was subsequently modelled on the basis of 4 input parameters such as initial specimen temperature, die (or environment) temperature, steady-state phase volume fraction at die (or environment) temperature, and elapsed time during forging. Resulting ANN models were coupled with the FE simulation (DEFORM-3D) in order to predict the variation of phase volume fraction during isothermal and non-isothermal forging. In addition, a grain size variation and a globularization model were developed for hot forging. To validate the predicted results from the models, Ti-6Al-4V alloy was hot-worked at various conditions and then the resulting microstructures were compared with simulated data. Comparisons between model predictions and experimental data indicated that the ANN model holds promise for microstructure evolution in two phase Ti-6Al-4V alloy.     

Oxidation and alkali sulfate‐induced corrosion of aluminide diffusion coating with and without platinum

A platinum‐free (MDC210) and a platinum‐rich (MDC150L) aluminide diffusion coating applied to a CMSX4 single crystal Ni‐based superalloy were investigated after exposure to sulfate‐induced corrosion and oxidation at 900 °C for a total of 100 h. Weight changes, microstructural, and microchemical analyses of the products were performed by the means of gravimetry, scanning electron microscopy (SEM), energy dispersive X‐ray (EDX), and X‐ray diffraction (XRD). Platinum was found to have a beneficial effect on both the oxidation and corrosion resistance of the coating. In both cases the scale was thicker on the platinum‐free coating. However, the difference in the extent of oxidation of the two coatings was small.     

Growing ledge structures of AlN crystals in a Fe–Mn–Al–C alloy

The Fe–30.4Mn–8.7Al–1.0C (wt.%) alloy was nitridized at 1000 °C. The AlN formed into a Widmanstätten side-plate shape. The side-plate of the AlN is parallel to the basal plane of the HCP crystal. There are three possible growing riser planes; namely ()_AlN, ()_AlN, and ()_AlN planes. The angle for growing riser planes met on the (0 0 0 1) terraces is 120°.