Experimental and theoretical investigation of forming limit diagram for Ti-6Al-4 V alloy at warm condition

Forming limit diagram (FLD) is an important performance index to describe the maximum limit of principal strains that can be sustained by sheet metals till to the onset of localized necking. It is useful tool to access the forming severity of a drawing or stamping processes. In the present work, FLD has been determined experimentally for Ti-6Al-4 V alloy at 400 °C by conducting a hemispherical dome test with specimens of different widths. Additionally, theoretical FLDs have been determined using Marciniak Kuczynski (M-K) model. Various yield criteria namely: Von Mises, Hill 1948, Hill 1993 and Cazacu Barlat in combination with different hardening models viz., Hollomon power law (HPL), Johnson-Cook (JC), modified Johnson-Cook (m-JC), modified Arrhenius (m-Arr.), modified Zerilli–Armstrong (m-ZA) have been used in M-K analysis for theoretical FLD prediction. The material properties required for determination of yield criteria and hardening models constants have been calculated using uniaxial tensile tests. The predicted theoretical FLDs results are compared with experimental FLD. It can be observed that influence of yield criterion in M-K analysis for theoretical FLD prediction is predominant than the hardening model. Based on the results; it is observed that the theoretical FLD using Cazacu Barlat and Hill 1993 yield criteria with m-Arr. hardening model has a very good agreement with experimental FLD.     

Experimental and numerical investigation of anisotropic yield criteria for warm deep drawing of Ti–6Al–4V alloy

Accuracy of the finite element simulation of sheet metal forming is significantly dependent on the correctness of input properties and appropriate selection of material models. In this work, anisotropic yield criteria namely, Hill 1948, Barlat 1989, Barlat 1996, Barlat 2000 and Cazacu Barlat have been implemented for Ti–6Al–4V alloy at 400 °C. Material constants required for the yield criteria have been determined and deformation behavior in deep drawing process has been analyzed in finite element software. Also, deep drawing experiments on Ti–6Al–4V alloy have been performed at 400 °C to validate finite element simulation results. Further, comparison of yield criteria based on thickness distribution, earing profile, complexity in material parameter identification and computational time has shown Cazacu-Barlat to be well suited for deep drawing of Ti–6Al–4V alloy.     

解析Poly6-I屈服准则对3104-H19铝合金拉深制耳预测的研究

目的 研究解析Poly6-I屈服准则预测具有高各向异性的3104-H19铝合金本构关系的能力,并将其应用于有限元仿真分析中,以实现对3104-H19铝合金拉深制耳的精确预测。方法 分析解析Poly6-I屈服准则的表达形式,减少计算参数所需的试验个数,并与经典的Yld2004-18p屈服准则进行对比,验证它对高各向异性力学性能预测的能力,将其嵌入到有限元软件中进行杯型件拉深制耳模拟,验证模型的精确性和有效性。结果 对于高各向异性材料,解析Poly6-I屈服准则所使用的试验个数可以减少到11,它预测的3104-H19铝合金屈服轨迹的各向异性系数曲线和单向拉伸曲线与Yld2004-18p屈服准则预测的结果基本相同,杯型件拉深有限元模拟结果与试验结果基本一致。结论 与Yld2004-18p屈服准则相比,考虑高各向异性特性的解析Poly6-I屈服准则所使用的试验数据更少,且无须使用优化软件求取参数,更为方便。解析Poly6-I屈服准则能精确地预测3104-H19铝合金材料在杯型件拉深试验中的制耳个数及杯型件杯壁的成形高度。     

TA4纯钛带材各向异性屈服行为表征与研究

目的 基于复杂加载状态试验和先进屈服准则,实现考虑塑性演化的TA4纯钛在复杂加载状态下塑性各向异性行为的精确表征。方法 通过0°、45°、90°方向的单拉试验和复杂加载比例的十字形试件双向拉伸试验,获得TA4纯钛的基本力学性能参数和拉伸屈服轨迹,采用不同的屈服准则对试验屈服轨迹进行预测,并通过变r值的屈服准则预测其屈服轨迹的塑性演变规律。结果 在小变形范围内,Yld2000?2d屈服准则对TA4屈服轨迹的预测精度最高;塑性变形过程中,呈线性增大趋势的r值与TA4纯钛的屈服轨迹演变现象直接相关。结论 试验与理论屈服轨迹的对比表明,Yld2000?2d屈服准则可以实现TA4纯钛初始屈服行为的精确表征。TA4纯钛带材的r值随塑性变形呈线性增大趋势,考虑塑性演化的Barlat89屈服准则预测的TA4屈服轨迹外凸性更显著。在TA4纯钛带材冲压成形过程的有限元分析、模具设计和工艺优化中,仅考虑初始屈服轨迹时,可采用Yld2000?2d屈服准则;当各向异性特征存在较强的塑性演化相关性时,可采用形式相对简单的Barlat89屈服准则。     

Interfacial reaction kinetics of SiC fiber-reinforced Ti3Al matrix composites

• 1.(1) SiC fibers reacted with Ti₃Al + Nb to form multilayer reaction products during consolidation and extended isothermal exposure. Complex carbides and silicides genertated from the reactions between Ti, Nb, Al and SiC appears to be the major components in the reaction zone. The C-rich layer on the surface of a SiC fiber affects the development of the reaction zone and the distribution of the reaction products.
• 2.(2) The fiber/matrix interfacial reaction is diffusion-controlled with an activation energy of 271.49 KJ/mole and 218.11 KJ/mole for the SCS-6/Ti-24A1-11Nb and Sigma/Ti-24Al-11Nb composites, respectively, at the early stage of reaction.
• 3.(3) The results of this study indicate that in the SCS-6/Ti-24Al-11Nb composite, the activation energy is higher, the growth rate of reaction zone is slower, and the consumption of C-rich layer is much slower than that in the SCS-6/Ti-6Al-4V composite.

     

3D shear-mode fatigue crack growth in maraging steel and Ti-6Al-4V

Fatigue crack growth tests in mixed-mode II + III were performed on maraging steel and Ti-6Al-4V. The 3D evolutions of the crack fronts -measured by SEM after interrupted tests- were analyzed, taking into account the reduction in effective crack driving force by the interlocking and friction of the asperities of the crack surface. Under small-scale yielding conditions, the mixed-mode crack growth rates were found to correlate best with \({\sqrt{{\Delta {\rm K}}_{\rm II}^{{\rm eff}^{2}}+1.2\Delta {\rm K}_{\rm III}^{{\rm eff}^{2}}}}\) in maraging steel, while for Ti-6Al-4V, \({\sqrt{\Delta {\rm K}_{\rm II}^{{\rm eff}^{2}}+0.9\Delta {\rm K}_{\rm III}^{{\rm eff}^{2}}}}\) appeared suitable. For extended plasticity, a crack growth prediction method is proposed and validated for Ti-6Al-4V. This method is based on elastic-plastic F.E. computations and application, ahead of each node of the crack front, of a shear-dominated fatigue criterion.     

Ion nitriding of explosively-clad titanium/steel tandems

Ti-6Al-4V, that was explosively welded to a 304 stainless steel plate, was ion nitrided in a d.c. plasma. An effective nitriding was achieved due to the high dislocation density and vacancy concentration that emaneted from the shock wave exposure of explosive welding which, in turn, accelerated diffusion of nitrogen into the titanium matrix. Processed Ti-6AI-4V developed a surface layer of TiN () followed by a Ti₂N () and interstitial nitrogen containing diffusion layer of -titanium. The growth rate of compound layer ( + ) and case depth were found to be controlled by the diffusion of nitrogen. Depending on the temperature of ion nitriding, high Knoop hardness values, between 800 and 2520, were obtained and these values show two to sixfold increase in the hardness when compared with unprocessed samples. XRD results showed VN formation also, together with TiN and Ti₂N nitrides and a preferred TiN growth in (002) orientation. Interfaces between Ti-6Al-4V and 304 stainless steel showed higher hardnesses as was seen following explosive welding than associated cores and a small amount of recrystallization was observed in the Ti-6Al-4V layer at the higher temperature processing. Nitriding of clad-Ti-6Al-4V thus provides an excellent opportunity of cladding surfaces with titanium alloys, in advanced structural applications without the expense of their monolithic counterparts.     

The equivalent plastic strain-dependent Yld2000-2d yield function and the experimental verification

An equivalent plastic strain-dependent anisotropic material model was developed for 5754O aluminum alloy sheet. In the developed model, the anisotropy coefficients for Barlat’s Yld2000-2d anisotropic yield function were established as a function of the equivalent plastic strain. The developed anisotropic material model was implemented into the commercial FEM code ABAQUS as a user material subroutine (UMAT) for simulations. In order to evaluate the accuracy of the developed material model, biaxial tensile tests were carried out using cruciform specimens and a biaxial loading testing machine. The results show that the developed material model predicts the experimental results better than the other three material models (Yld2000-2d, Mises and Hill48 yield functions). It is also found that the developed material model describes the uniaxial tensile test curves better than Yld2000-2d yield function. The deep drawing test for 5754O aluminum alloy sheet was carried out and was simulated with different material models. The comparison between the experimental and simulation results indicates that the developed material model predicts the earing profile better than other material models. It is concluded that the equivalent plastic strain-dependence of the material coefficients should be considered for the accurate prediction of the anisotropic deformation behavior of materials.     

Surface characteristics of a porous-surfaced Ti-6Al-4V implant fabricated by electro-discharge-compaction

Electro-discharge-compaction (EDC) is a unique method for producing porous-surfaced metallic implants. The objective of the present studies was to examine the surface characteristics of the Ti-6Al-4V implants formed by EDC. Porous-surfaced Ti-6Al-4V implants were produced by employing EDC using 480 F capacitance and 1.5 kJ input energy. X-ray photoelectron spectroscopy was used to study the surface characteristics of the implant materials. C, O, and Ti were the main constituents, with smaller amounts of Al and V. EDC Ti-6Al-4V also contained N. Titanium was present mainly in the forms of mixed oxides and small amounts of nitride and carbide were observed. Al was present in the form of aluminum oxide, while V in the implant surface did not contribute to the formation of the surface oxide film. The surface of conventionally prepared Ti-6Al-4V primarily consists of TiO₂, whereas, the surface of the EDC-fabricated Ti-6Al-4V consists of complex Ti and Al oxides as well as small amounts of titanium carbide and nitride components. However, preliminary studies indicated that the implant was biocompatible and supports rapid osseointegration.     

Mechanism of surface modification of a porous-coated Ti-6Al-4V implant fabricated by electrical resistance sintering

A porous-coated Ti-6Al-4V implant was fabricated by electrical resistance sintering, using 480 F capacitance and 1.5 kJ input energy. X-ray photoelectron spectroscopy (XPS) was used to study the surface characteristics of the implant material before and after sintering. There were substantial differences in the content of O and N between as-received atomized Ti-6Al-4V powders and the sintered prototype implant, which indicates that electrical resistance sintering alters the surface composition of Ti-6Al-4V. Whereas the surface of atomized Ti-6Al-4V powders was primarily TiO₂, the surface of the implant consisted of a complex of titanium oxides as well as small amounts of titanium carbide and nitride. It is proposed that the electrical resistance sintering process consists of five stages: stage I – electronic breakdown of oxide film and heat accumulation at the metal-oxide interface; stage II – physical breakdown of oxide film; stage III – neck formation and neck growth; stage IV – oxidation, nitriding, and carburizing; and stage V – heat dissipation. The fourth stage, during which the alloy repassivates, is responsible for the altered surface composition of the implant.     

Antibiotic release from F-doped nanotubular oxide layer on TI6AL4V alloy to decrease bacterial viability

We aimed to evaluate the release of two antibiotics: gentamicin and vancomycin loaded into F-doped nanotubular anodic oxide layers, as well as their bactericide effect. F-doped nanotubular oxide layers fabricated on Ti-6Al-4V loaded with gentamicin (Gm), vancomycin (Vm) and their mixture (Gm?+?Vm) by a previously described loading method. Antibiotic release was studied by RP-HPLC and by a biological method. Bactericidal activity was evaluated by a bacterial adherence protocol described previously using on three clinically important bacterial species. The antibiotic release steady up to 120 and 180?min for Gm and Vm, respectively, and despite the antibiotic concentration decreased, their biological activity was maintained over time. The number of living bacteria of three species tested on NT-Gm specimens was significantly lower than on NT specimens without antibiotics (P?<?0.01). There are significant differences among NT-Gm and NT-Gm?+?Vm specimens (P?<?0.05) for S. aureus 15981, S. epidermidis ATCC 35984, and P. aeruginosa ATCC 27853 and no differences between NT-Vm and NT-Gm?+?Vm for staphylococci (P?>?0.05). In conclusion, this Gm?+?Vm loading method added to the properties of F-doped nanotubular oxide layers fabricated on Ti-6Al-4V, and therefore surfaces with antibacterial, biocompatible, tissue integration stimulating and spread-spectrum bactericidal properties can be obtained.

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Characterization and tribological evaluation of duplex treatment by depositing carbon nitride films on plasma nitrided Ti-6Al-4V

Carbon nitride (CN_X) films (with N/C ratio of 0.5) were deposited on both untreated and plasma nitrided Ti-6Al-4V substrates by D.C. magnetron sputtering using a graphite target in nitrogen plasma. TEM and XPS analysis revealed the formation of both amorphous CN_X structure and crystalline -C₃N₄ phases in the deposited coatings. Nano-indentation tests showed that the film hardness was about 18.36 GPa. Both the scratch tests and indentation tests showed that compared with CN_X film deposited directly on Ti-6Al-4V, the load bearing capacity of CN_X film deposited on plasma nitrided Ti-6Al-4V was improved dramatically. Ball-on-disk wear tests under both dry sliding and lubricated conditions (with simulated body fluids) were performed to evaluate the friction and wear characteristics of the deposited coatings. Results showed that under both dry and lubricated conditions, the duplex treated system (i.e., with CN_X film deposited on plasma nitrided Ti-6Al-4V substrate) was more effective in maintaining a favorable low and stable coefficient of friction and improving wear resistance than both individual plasma nitriding and CN_X films on Ti-6Al-4V substrate. Under dry sliding conditions, the generated wear debris of spalled films were accumulated on the wear track, mechanically alloyed and graphitized, thus significantly reducing the coefficient of friction and preventing wear of the substrate. However, under lubricated conditions, due to the flowing of the fluids, the lubricating wear debris was taken away by the fluids, and therefore, the direct contact of two original surfaces resulted in high coefficient of friction and extensive abrasive wear of the substrate for CN_X films deposited on Ti-6Al-4V substrate. Also when there was some small-area spallation of CN_X films, the fluids could seep into the interface between the film and substrate, thus degrading the interfacial adhesion and resulting in a large area spallation.     

Interface contribution to the SiC-titanium and SiC-aluminium tensile strength prediction

Fragmentation tests of single SiC filaments embedded in an aluminium (1050 and 5083 alloys) or a titanium (Ti-6Al-4V) matrix have been analysed in an effort to obtain the interface contribution in terms that could be incorporated into a tensile fracture model for unidirectional composites. Depending on the matrix, two regimes of interfacial stress transfer can be distinguished within the whole range of tested temperatures. For the SCS2/5083 system, plastic deformation of the alloy limits the stress transfer, and the interface contribution thus finds its expression in the shear stress of the matrix. for the SCS6/Ti-6Al-4V system, friction is the leading process and the interface contribution strongly depends on the stress state around the fibre. Assuming a temperature dependent compressive radial stress up to 925C, an effective transfer shear stress may be easily calculated for unidirectional SCS6/Ti-6Al-4V composites.     

Experimental and theoretical formability analysis using strain and stress based forming limit diagram for advanced high strength steels

In this study, experimental and numerical analyses of Forming Limit Diagram (FLD) and Forming Limit Stress Diagram (FLSD) for two Advanced High Strength Steel (AHSS) sheets grade DP780 and TRIP780 were performed. Initially, the forming limit curves were experimentally determined by means of the Nakazima forming test. Subsequently, analytical calculations of both FLD and FLSD were carried out based on the Marciniak–Kuczinsky (M–K) model. Additionally, the FLSDs were calculated using the experimental FLD data for both investigated steels. Different yield criteria, namely, von Mises, Hill’s 48, and Barlat2000 (Yld2000-2d) were applied for describing plastic flow behavior of the AHS steels. Both Swift and modified Voce strain hardening laws were taken into account. Hereby, influences of the constitutive yield models on the numerically determined FLDs and FLSDs were studied regarding to those resulted from the experimental data. The obtained stress based forming limits were significantly affected by the yield criterion and hardening model. It was found that the forming limit curves calculated by the combination of the Yld2000-2d yield criterion and Swift hardening law were in better agreement with the experimental curves. Finally, hole expansion tests were conducted in order to verify the different failure criteria. It was shown that the stress based forming limit curves could more precisely describe the formability behavior of both high strength steel sheets than the strain based forming limit curves.     

Ultraviolet photofunctionalization of nanostructured titanium surfaces enhances thrombogenicity and platelet response

The purpose of this study was to evaluate blood and platelet response to nanostructured TiO₂ coatings and to investigate the effect of Ultraviolet (UV) light treatment on blood clotting ability, platelet activation and protein adhesion. Ti-6Al-4V titanium alloy plates (n?=?138) were divided into three groups; a sol–gel derived MetAlive^TM coating (MA); hydrothermal coating (HT); and a non-coated group (NC). Sixty nine titanium substrates were further treated with UV light for 1?h. The thrombogenicity of the titanium substrates was assessed using fresh human blood with a whole blood kinetic clotting time method. The platelet adhesion test was conducted to evaluate the morphology and adhesion behavior of the platelets on the titanium substrates. Human diluted plasma and bovine fibronectin were used to evaluate protein adsorption. Total clotting time for the UV treated HT, MA and NC titanium substrates was almost 40?min compared to 60?min for non-UV substrates, the total clotting time for the UV treated groups were significantly lower than that of the non UV NC group (p?<?0.05). UV light treatment had significantly enhanced coagulation rates. The HT and MA substrates presented more platelet aggregation, spreading and pseudopod formation in comparison with the NC substrates. UV treatment did not affect the platelet activation and protein adsorption. This in vitro study concluded that nanostructured titanium dioxide implant surfaces obtained by sol–gel and hydrothermal coating methods increased coagulation rates and enhanced platelet response when compared with non-coated surfaces. UV light treatment clearly improved thrombogenicity of all examined Ti-6Al-4V surfaces.

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Toughening of titanium alloys by twinning and martensite transformation

Two kinds of titanium alloys, titanium alloy (Ti-10V-2Fe-3Al) and titanium alloy (Ti-5Al-2.5Sn) were used to investigate the toughening mechanisms with new approaches. The results show that Ti-5Al-2.5Sn alloy possesses good combination of strength and ductility as well as satisfied low-cycle fatigue life both at 293 K and 77 K. As for Ti-10V-2Fe-3Al alloy, the microstructure with metastable phase shows lower strength and ductility but higher threshold stress intensity factor (K _th) than solution treated and aged microstructure composed of and phases. The microstructures also show that twinning occurs in deformation of Ti-5Al-2.5Sn alloy at 77 K. Twinning seems to be helpful for improving the low-cycle fatigue life to a great extent at cryogenic temperature. It's also found that owing to stress-assisted martensite transformation in metastable Ti-10V-2Fe-3Al alloy, the fatigue crack propagation path shows a very tortuous way, which decrease the effective stress intensity factor (K _eff) at crack tip, and increase threshold stress intensity factor (K _th).     

Sources of acoustic emission during fatigue of Ti-6Al-4V: effect of microstructure

The fundamentals of acoustic emission (AE) analysis of fatigue cracking were applied to Ti-6Al-4V. The effect of microstructure on the characteristics of the AE events generated and the failure mechanisms which produced AE in Ti-6Al-4V were established. Lamellar microstructures generated one to two orders of magnitude more emission than equiaxed microstructures. The combination of larger grain size, more continuous / interfaces, more tortuous crack-front geometry, cleavage and intergranular fracture in lamellar microstructures accounts for the greater amount of emission. For lamellar microstructures, most AE events were generated in the upper 20% of the stress range, whereas in equiaxed microstructures, most events were generated at lower stresses. Most AE events were generated during crack opening and also at low stresses. AE events having high level intensities were also generated at stresses other than the peak stress. This is because in titanium alloys, which have both high strength and toughness, AE events are generated from both plastic zone extension and crack extension.