Electrochemical Hydrogen Charging on Corrosion Behavior of Ti-6Al-4V Alloy in Artificial Seawater

This study employs advanced electrochemical and surface characterization techniques to investigate the impact of electrochemical hydrogen charging on the corrosion behavior and surface film of the Ti-6Al-4V alloy.The findings revealed the formation ofγ-TiH andδ-TiH_(2) hydrides in the alloy after hydrogen charging.Prolonging hydrogen charging resulted in more significant degradation of the alloy microstructure,leading to deteriorated protectiveness of the surface film.This trend was further confirmed by the electrochemical measurements,which showed that the corrosion resistance of the alloy progressively worsened as the hydrogen charging time was increased.Consequently,this work provides valuable insights into the mechanisms underlying the corrosion of Ti-6Al-4V alloy under hydrogen charging conditions.

Ti-6Al-4V ELI钛合金的宽温域(-253~350℃)力学性能研究

钛及钛合金是一种重要的太空金属材料,在航空航天领域发挥重要的作用。研究了Ti-6Al-4V ELI钛合金在室温(20℃)、高温(350℃)以及超低温(-253℃)环境下的拉伸力学性能,比较了Ti-6Al-4V ELI钛合金在不同温度下的力学性能指标,分析了其断口形貌特征。结果表明,Ti-6Al-4V ELI钛合金在超低温下表现出较高的力学性能,抗拉强度可达1900 MPa以上,其断口形貌呈现脆性断裂特点。在室温和高温下,Ti-6Al-4V ELI钛合金表现出良好的塑性和韧性,具有较为明显的韧性断裂特点。因其良好的宽温域力学性能,Ti-6Al-4V ELI钛合金是未来发展航天航空领域必不可少的重要金属材料。

大厚度Ti-6Al-4V ELI钛合金电子束焊接头的组织及力学性能

采用不同的焦点偏移量和扫描幅值对55 mm厚Ti-6Al-4V ELI钛合金板进行电子束焊接试验,通过金相分析、室温拉伸及显微硬度测试,研究了焊缝成型及接头的组织、力学性能。结果表明:当采用较小的焦点偏移量焊接时,扫描幅值的增加会显著降低焊缝深宽比,当焦点偏移量过大时,焊缝易出现咬边及明显的钉尖缺陷。焊缝组织为粗大的柱状晶,其内部由十字交叉状排列的针状马氏体α’相构成。当焦点偏移量为-15 mm、扫描幅值为0.2 mm时,接头抗拉强度比母材的高,沿熔深方向抗拉强度无明显差异,力学性能稳定。焊缝处平均硬度为323 HV,比母材高40 HV。

Stitch welding of Ti-6Al-4V titanium alloy by fiber laser

Stitch welding of plate covered skeleton structure of Ti-6Al-4V titanium alloys has a variety of applications in aerospace vehicle manufacture. The laser stitch welding of Ti-6Al-4V titanium alloys was carried out by a 4 kW ROFIN fiber laser. Influences of laser welding parameters on the macroscopic geometry, porosity, microstructure and mechanical properties of the stitch welded seams were investigated by digital microscope, optical microscope, scanning electron microscope and universal tensile testing machine. The results showed that the three-pipe nozzle with gas flow rate larger than 5 L/min could avoid oxidization, presenting better shielding effect in comparison with the single-pipe nozzle. Porosity formation could be suppressed with the gap between plate and skeleton less than 0.1 mm, while the existing porosity can be reduced with remelting. The maximum shear strength of stitch welding joint with minimal porosity was obtained by employing laser power of 1700 W, welding speed of 1.5 m/min and defocusing distance of ＋8 ram.

Development of processing windows for diffusion bonding of Ti-6Al-4V titanium alloy and 304 stainless steel with silver as intermediate layer

As titanium alloy is chemically reactive,it is very difficult to join by conventional welding techniques.Titanium alloys can easily pick up nitrogen and oxygen from the atmosphere.In the fusion welding method,brittle intermetallic compounds are formed when joining titanium alloy and stainless steel,which decrease the mechanical behavior of the couples.Hence,for joining of titanium alloy,diffusion bonding is recommended.This work dealt with the measurement of feasible process parameters for diffusion bonding of Ti-6Al-4V and AISI 304 stainless steel with silver as an intermediate layer.The quality of the bonds was confirmed by the lap shear test and microstructural analysis.With the experimental results obtained,diffusion bonding windows were constructed and this will act as reference maps to identify the process parameters for obtaining defect free bond.Bonding was successful in the temperature range of 750-800 °C.Maximum lap shear strength was achieved under a bonding pressure of 5 MPa and holding time of 90 min.

热处理对Ti-6Al-4V ELI合金厚板组织与性能的影响

研究了热处理对Ti-6Al-4V超低间隙(ELI)合金厚板组织与性能的影响.结果表明,Ti-6Al-4V ELI合金经α+β区热处理后得到双态组织,强度、塑性都较高;经β区热处理后得到片层组织,细片层组织强度较高.片层粗化后,强度降低.当片层尺寸小于某一临界值(～5μm)时,延伸率随着片层的粗化升高,当片层继续粗化时,延伸率反而下降.

Constitutive Modeling for Ti-6Al-4V Alloy Machining Based on the SHPB Tests and Simulation

A constitutive model is critical for the prediction accuracy of a metal cutting simulation. The highest strain rate involved in the cutting process can be in the range of 104-106 s 1. Flow stresses at high strain rates are close to that of cutting are difficult to test via experiments. Split Hopkinson compression bar （SHPB） technology is used to study the deformation behavior of Ti-6Al-4V alloy at strain rates of 10 -4-10 4s- 1. The Johnson Cook （JC） model was applied to characterize the flow stresses of the SHPB tests at various conditions. The parameters of the JC model are optimized by using a genetic algorithm technology. The JC plastic model and the energy density-based ductile failure criteria are adopted in the proposed SHPB finite element simulation model. The simulated flow stresses and the failure characteristics, such as the cracks along the adiabatic shear bands agree well with the experimental results. Afterwards, the SHPB simulation is used to simulate higher strain rate（approximately 3 × 10 4 s -1） conditions by minimizing the size of the specimen. The JC model parameters covering higher strain rate conditions which are close to the deformation condition in cutting were calculated based on the flow stresses obtained by using the SHPB tests （10 -4 - 10 4 s- 1） and simulation （up to 3 × 10 4 s - 1）. The cutting simulation using the constitutive parameters is validated by the measured forces and chip morphology. The constitutive model and parameters for high strain rate conditions that are identical to those of cutting were obtained based on the SHPB tests and simulation.

Effect of wet shot peening on Ti-6Al-4V alloy treated by ceramic beads

Ti-6Al-4V alloy was processed by wet shot peening with ceramic beads. The effects of the shot peened intensity on the microstructure, surface morphology, and residual stress were investigated. A tensile-tensile fatigue test was performed and the fracture mechanism was proposed. The results demonstrate that the surface roughness after wet shot peening is obviously lower than that after dry shot peening. With the increase of the shot peened intensity, the depth of the residual stress layer increases to 250 ktrn, and the maximum stress in this layer increases to -895 MPa. The fatigue strength also increases by 12.4% because of the wet shot peening treatment. The dislocation density of the surface layer is significantly enhanced after the wet shot peening with ceramic beads. The microstructure of the surface layer is obviously refined into ultra-fine grains.

Mechanical properties of strengthened surface layer in Ti-6Al-4V alloy induced by wet peening treatment

A modified surface layer was formed on Ti-6Al-4V alloy by wet peening treatment. The variations of the residual stress,nano-hardness and microstructure of the modified layer with depth from surface were studied using X-ray diffraction analysis,nano-indentation analysis, scanning electron microscopy and transmission electron microscopy observations. The results show thatboth the compressive residual stress and hardness decrease with increasing depth, and the termination depths are 160 and 80 μm,respectively. The microstructure observation indicates that within 80 μm, the compressive residual stress and the hardness areenhanced by the co-action of the grain refinement strengthening and dislocation strengthening. Within 80–160 μm, the compressiveresidual stress mainly derives from the dislocation strengthening. The strengthened layer in Ti-6Al-4V alloy after wet peeningtreatment was quantitatively analyzed by a revised equation with respect to a relation between hardness and yield strength.

显微组织对Ti-6Al-4VELI合金疲劳裂纹扩展速率的影响

研究了Ti-6Al-4VELI合金板材的显微组织对疲劳裂纹扩展速率的影响。用金相显微镜对不同热处理制度下该合金α和β转变组织的变化进行了观察分析。采用MTS-810低周疲劳试验机测试合金的裂纹扩展速率。通过Origin6.0软件对数据进行处理并绘制裂纹扩展速率(△a/△N)与应力强度因子幅△K的关系曲线。结果表明:在Pairs区范围内,疲劳裂纹扩展速率对双态组织中初生α相含量的多少不敏感,而在近门槛区和快速扩展区,裂纹扩展速率对组织比较敏感;在本实验研究的条件下,细针编织状魏氏组织的da/dN<平直状片层组织的da/dN<双态组织的da/dN。

Tribocorrosion behaviors of Ti-6Al-4V and Monel K500 alloys sliding against 316 stainless steel in artificial seawater

The tribocorrosion behaviors of Ti-6Al-4V and Monel K500 alloys sliding against 316 stainless steel were investigated using a ring-on-block test rig in both artificial seawater and distilled water. It is found that friction coefficients are in general larger in distilled water compared with seawater. The wear losses of Ti-6Al-4V and Monel K500 alloys are larger in seawater compared with distilled water. The mechanical action can destroy the passive film and increase the corrosion rate. The synergism effect between corrosion and wear occurs. The synergism action between corrosion and wear is related to the corrosion rate and with the increase of corrosion rate, the synergism becomes more important. 316 stainless steel suffers severe wear sliding against Monel K500 alloy compared with sliding against Ti-6Al-4V alloy in both distilled water and seawater.

Process parameters optimization of Ti-6Al-4V alloy sheet during hot stretch-creep forming

Hot stretch-creep forming （SCF） is a novel technique to produce hard-to-form thin-walled metal components. Comprehensively considering the analysis results of the springback angle, yield strength and microstructure, four hot SCF process parameters including temperature, stretch velocity, post stretch percentage and dwelling time of a Ti-6Al-4V alloy sheet were optimized using an orthogonal experiment. The results reveal that temperature is the most important factor on springback angle. The yield strength of the deformed material in 0° direction increases, while those in directions of 45° and 90° fluctuate around the original value. After hot SCF, the shape of some a phases changes from short thin grains to long slender ones, and the microhardness changes very little. The optimized parameters with temperature of 700 ℃, stretch velocity of 5 mm/min, post stretch percentage of 2% and dwelling time of 8 min are achieved finally.

Influence of hydrogen content on room temperature compressive properties of Ti-6Al-4V alloy at high strain rate

Electromagnetic forming tests were done at room temperature to reveal the influence of hydrogen content on the compressive properties of Ti-6Al-4V alloy at high strain rate. Microstructure was observed to reveal the mechanism of hydrogen-enhanced compressive properties. The experimental results indicate that hydrogen has favorable effects on the compressive properties of Ti-6Al-4V alloy at high strain rate. Compression of Ti-6Al-4V alloy first increases up to a maximum and then decreases with the increase of hydrogen content at the same discharge energy under EMF tests. The compression increases by 47.0% when 0.2% (mass fraction) hydrogen is introduced into Ti-6Al-4V alloy. The optimal hydrogen content for cold formation of Ti–6Al–4V alloy under EMF was determined. The reasons for the hydrogen-induced compressive properties were discussed.

Effect of grain size of primary α phase on bonding interface characteristic and mechanical property of press bonded Ti-6Al-4V alloy

The effect of grain size of primary α phase on the bonding interface characteristic and shear strength of bond was investigated in the press bonding of Ti-6Al-4V alloy. The quantitative results show that the average size of voids increases from 0.8 to 2.6 μm and the bonding ratio decreases from 90.9% to 77.8% with an increase in grain size of primary α phase from 8.2 to 16.4 μm. The shape of voids changes from the tiny round to the irregular strip. The highest shear strength of bond can be obtained in the Ti-6Al-4V alloy with a grain size of 8.2 μm. This is contributed to the higher ability of plastic flow and more short-paths for diffusion in the alloy with smaller grain size of primary α phase, which promote the void closure process and the formation of α/β grains across bonding interface.

Dynamic fracture of Ti-6Al-4V alloy in Taylor impact test

The dynamic fracture behaviors of Ti-6Al-4V alloy at high strain rate loading were investigated systemically through Taylor impact test, over the range of impact velocities from 145 m/s to 306 m/s. The critical impact velocity of fracture ranges from 217 m/s to 236 m/s. Smooth surfaces and ductile dimple areas were observed on the fracture surfaces. As the impact velocity reached 260 m/s, the serious melting regions were also observed on the fracture surfaces. Self-organization of cracks emerges when the impact velocity reaches 260 m/s, while some special cracks whose ＂tips＂ are not sharp but arc and smooth, and without any evidence of deformation or adiabatic shear band were also observed on the impact end surfaces. Examination of the sections of these special cracks reveals that the cracks expand along the two maximum shear stress directions respectively, and finally intersect as a tridimensional ＂stagger ridge＂ structure.

Quasi-static and dynamic tensile behaviors in electron beam welded Ti-6Al-4V alloy

The quasi-static and dynamic tensile behaviors in electron beam welded（EBW） Ti-6Al-4V alloy were investigated at strain rates of 10-3 and 103 s-1,respectively,by materials test system（MTS） and reconstructive Hopkinson bars apparatus.The microstructures of the base metal（BM） and the welded metal（WM） were observed with optical microscope.The fracture characteristics of the BM and WM were characterized with scanning electronic microscope.In Ti-6Al-4V alloy joint,the flow stress of WM is higher than that of BM,while the fracture strain of WM is less than that of BM at strain rates of 103 and 10-3 s-1,respectively.The fracture strain of WM has apparent improvement when the strain rate rises from 10-3 to 103 s-1,while the fracture strain of BM almost has no change.At the same time,the fracture mode of WM alters from brittle to ductile fracture,which causes improvement of the fracture strain of WM.

MODELING OF 'BANDING' MICROSTRUCTURE FORMATION IN CENTRIFUGALLY SOLIDIFIED Ti-6Al-4V ALLOY

Numerical investigations of the ＇banding＇ microstructure formation during solidification of Ti-6Al-4 V alloy in the centrifugal casting are conducted using a multi-scale model, which combines the finite difference method （FDM） at the macroscale with a cellular automaton （CA） model at the microscale. The macro model is used to simulate the fluid flow and heat transfer throughout the casting. The micro model is used to predict the nucleation and growth of microstructures. With the proposed model, numerical simulations are performed to study the influences of the nucleation density, mould rotation speed, and casting size upon the ＇banding＇ microstructure formation. It is noted that changing the nucleation density has a minor effect on the microstructure formation. The rotation speed promotes the formation of ＇banding＇ microstructure, which is more noticeable for larger size castings. The ＇major mechanism responsible for this ＇banding＇ phenomenon is the spatial variation in cooling rates created by centrifugal force.

Modeling of Tool-Chip Contact Length for OrthogonalCutting of Ti-6Al-4V Alloy Considering Segmented Chip Formation

In this work, the orthogonal cutting experiments on Ti-6Al-4V alloy were conducted at different cutting speeds(10—160 m/min)and feed rates(20—160 μm/rev). The tool-chip contact length was measured by the track of tool rake face; meanwhile, the chip morphology caused by the localized and overall chip deformation was characterized by the degree of segmentation and the chip compression ratio, respectively. These parameters were analyzed and calculated according to the segmented chip morphology. In addition, three modified models considering the overall chip deformation and the localized deformation of adiabatic shear band were proposed, and the constants of the models were calculated by the genetic algorithm optimization. Considering the overall and localized chip deformation, the value and variation trend of the tool-contact length predicted by these three models agreed well with the experimental results.

Ti-6AL-4V ELI宽幅板坯锻造工艺研究

为了研制出Ti-6AL-4V ELI钛合金4500 m深潜器用宽幅厚板,宝鸡钛业股份有限公司(简称宝钛股份)采用φ920 mm/7200 kg铸锭,利用80/100 MN快锻机锻造设备的优势,并通过合理的锻造工艺和变形参数,在国内首次成功研制出230 mm×1750 mm×4000 mm的Ti-6AL-4V ELI钛合金宽幅板坯。利用光学显微镜、拉伸试验机和探伤设备分析了板坯的显微组织、力学性能、探伤水平。结果表明,经过充分变形后组织为典型的两相区变形组织,由等轴或长条的α相和β相转变组织组成;板坯抗拉强度大于900 MPa,延伸率大于14%;超声波探伤杂波水平到平底孔Φ2.0-6 d B。板坯各项指标均满足设计要求。

Effect of electropulsing on deformation behavior of Ti-6Al-4V alloy during cold drawing

Electron backscattered diffraction （EBSD） and transmission electron microscopy （TEM） were used to investigate effect of electropulsing on microstructure and texture evolution of Ti-6Al-4V during cold drawing. Research results demonstrate that the electropulsing treatment （EPT） can enhance the deformability of the grains with unfavorable orientations, which makes the compatibility of deformation among grains much better. A comparison in texture evolution between conventional cold drawing and EPT cold drawing indicates that the EPT promotes prismatic 〈a〉 slip moving, restricts pyramidal 〈c＋a〉 slip occurring and accommodates the deformation with c-component by grain boundary sliding. The fraction decrease of low-angle grain boundaries for samples deformed with EPT reveals that the application of electropulsing restricts the formation of the incidental dislocation boundaries and the geometrically necessary boundaries.