Cutting temperatures when ball nose end milling γ-TiAl intermetallic alloys

Experimental results are presented for Ti–45Al–2Mn–2Nb + 0.8 vol% TiB₂XD and Ti–45Al–8Nb–0.2C alloys. Three approaches were employed involving a constantan-workpiece thermocouple arrangement, implanted K-type thermocouples and IR thermography. New and worn (~300 μm flank wear) coated carbide tools were used under dry conditions when down milling at 50–345 m/min, with workpieces mounted horizontally and at 45°. Despite slight variation in ancillary finishing parameters there was generally good agreement between data sets for the different evaluation techniques employed and for both alloys. Higher temperatures were measured with the workpiece at 45°, with constantan-workpiece thermocouple temperatures of 375 °C and 413 °C for new and worn tools respectively at 345 m/min.     

Fatigue properties of a medium-strength γ-TiAl alloy with different surface conditions

Effects of surface condition on fatigue properties of a medium-strength γ-TiAl alloy Ti-45Al-5Nb-lW(at%) were investigated.It is found that the maximum stresses of fatigue samples are lower than the yield stresses of the medium-strength γ-TiAl alloy.Meanwhile,the local plastic deformation is unconspicuous to occur at the crack tip.In this case,the fatigue strength is mainly decided by surface conditions of maximum-stressed surface,but compressive stress and deformation especially resulted from shot peening play an important role in the improvement of the condition fatigue strength.The affecting depth of shot peening is about 250 μm.As a result,the relatively weak microstructures and phases become the preferential initiation sites and propagation routes.They are observed to be equiaxed γ grains,B2 + ω grains,and α_2-γ lamellar interface in soft orientations.The existence of V-notch can significantly reduce the fatigue properties of the samples.     

Interaction between γ-TiAl alloy and zirconia

A study on the interaction between TiAI alloy and zirconia was carried out in argon atmosphere. The rnicrographic observations of the cross-section perpendicular to the interface were obtained using a scanning electron microscope with a dedicated energy dispersive spectrometer (SEM/EDS). The results showed that firstly TiAI alloy spreads on the ceramic surface, and then infiltrates into the pores between ceramic particles accompanied by a chemical reaction. The whole ceramic mold is broken into tiny blocks. A multiple fission reaction mechanism was developed to explain the interaction between TiAI alloy and zirconia.     

Interaction between γ-TiAl alloy and zirconia

A study on the interaction between TiAl alloy and zirconia was carried out in argon atmosphere. The micrographic observations of the cross-section perpendicular to the interface were obtained using a scanning electron microscope with a dedicated energy dispersive spectrometer （SEM/EDS）. The results showed that firstly TiAl alloy spreads on the ceramic surface, and then infiltrates into the pores between ceramic particles accompanied by a chemical reaction. The whole ceramic mold is broken into tiny blocks. A multiple fission reaction mechanism was developed to explain the interaction between TiAl alloy and zirconia.     

Effect of microstructural instability on the creep resistance of an advanced intermetallic γ-TiAl based alloy

Intermetallic titanium aluminides based on the γ-TiAl phase, such as the β-solidifying TNM alloy, have found application in aerospace and automotive industries. In order to adjust balanced mechanical properties in the TNM alloy a two-step heat treatment is subjected to a hot-die forged material to provide a fine nearly fully lamellar microstructure. However, during the second heat treatment step, beyond a critical temperature, discontinuous precipitation can occur which represents a coarsening process of the fine lamellar α₂/γ-colonies. This microstructural instability is a limiting factor for the TNM alloy, also regarding the maximum operation temperature. In order to investigate the influence of microstructural instability on creep behavior, microstructures with various progress of discontinuous precipitation and average lamellar spacing were adjusted, creep tested and changes in microstructure examined. The creep tests were conducted at 800 °C and 150 MPa. It is demonstrated that the creep resistance is decreased when discontinuous precipitation exceeds a certain volume fraction, thus, revoking the advantages of a narrow lamellar spacing.     

Rapidly solidified MC carbide morphologies of a pulsed laser surface alloyed γ-TiAl intermetallic with carbon

Rapidly solidified TiC type MC carbide morphologies were studied as a function of cooling rate using a Nd–YAG pulsed laser surface alloying with carbon. At a low cooling rate, the carbide morphology was well-developed dendrite. As the solidification cooling rate increased, the growth morphology of MC carbide changed to cross-petal-like with symmetrical arms and irregular block or undeveloped dendrite.     

High speed ball nose end milling of γ-TiAl alloys

Following a brief review on the development and intended application of γ-TiAl for aeroengine components, the paper details statistically designed machinability experiments involving high speed ball end milling in order to benchmark 4th generation alloy Ti–45Al–8Nb–0.2C against Ti–45Al–2Mn–2Nb +0.8%vol. TiB_2, which has been the prime focus of machinability evaluation and associated publications. Testing utilised 8 mm diameter, solid WC, AlTiN coated ball nose end cutters in a factorial design and involved measurement of cutting temperatures and workpiece surface roughness in addition to tool life and cutting forces. The trials established alloy Ti–45Al–8Nb–0.2C as providing a ∼25% increase in tool life based on a maximum flank wear criteria of 300 μm. On average, alloy Ti–45Al–2Mn–2Nb +0.8%vol. TiB₂ had a surface roughness Ra of ∼0.1 μm higher than Ti–45Al–8Nb–0.2C with values <0.6 μm Ra possible. Maximum cutting temperatures for Ti–45A–8Nb–0.2C measured using infra red pyrometry were ∼250 °C at a cutting speed of 160 m/min for a new tool, rising to ∼430 °C at a cutting speed of 340 m/min with a tool that had 300 μm of flank wear. For both γ-TiAl alloys, fracture/pullout was observed on every milled surface that was assessed and varied from a few microns to a few millimeters depending on test operating levels, within general, higher cutting speeds and tool flank wear levels causing an increased level of damage.     

A positron investigation of electron-irradiation induced defects in γ-TiAl intermetallic compounds

The isochronal recovery of irradiation induced defects was investigated in γ-TiAl intermetallic compounds (with 50 and 54 at% Al) by positron lifetime measurements after 2.5 MeV electron irradiation at 21 K. In the as-irradiated condition, the analysis of the results and the comparison with published data led to a value τ_d=230±5 ps for the lifetime of vacancy-trapped positrons. The lifetime variations observed during isochronal anneals at increasing temperatures are consistent with vacancy migration around 450 K. The observation of a progressive decrease in the lifetime of trapped positron, during the migration and elimination of vacancies, suggests that they do not form unrelaxed three-dimensional clusters, and that another type of positron traps is simultaneously present.     

Effect of fully lamellar morphology on creep of a near γ-TiAl intermetallic

Creep properties of a polycrystalline binary near γ-TiAl intermetallic in two fully lamellar microstructural conditions are presented. Creep tests (760°C/240 MPa) indicate that a lamellar structure with fine interface spacing and planar grain boundaries improves creep resistance. A lamellar structure with wide lamellar interface spacing and interlocked grain boundaries has less than half the creep life, five times higher minimum creep strain rate and a greater tertiary creep strain. The deformation substructures are presented in terms of the lamellar orientation to the stress axis and indicate that creep strain is accommodated by dislocation motion in soft oriented grains, but the creep strain rate is controlled by hard oriented grains. The extent of tertiary creep is controlled by the grain boundary morphology, with planar grain boundaries susceptible to intergranular cracking. The results suggest that to maximize the creep resistance of near γ-TiAl intermetallics with lamellar microstructures requires narrow lamellar interface spacing and interlocked lamellae along grain boundaries.     

Quantitative characterization of β-solidifying γ-TiAl alloy with duplex structure

For precise plastic deformation, microstructure control is essential especially for β-solidifying γ-TiAl alloy with duplex structure. Based on stereology, the microstructure of isothermally compressed γ-TiAl alloy was divided into β₀ grains, remnant α₂/γ lamellar colonies, α₂ and γ grains. The results show that the volume fraction of β₀ grains slightly increases in the isothermally compressed γ-TiAl alloy with the increase of height reduction. Meanwhile, the volume fractions of remnant α₂/γ lamellar colonies and α₂ grains decrease. However, the volume fraction of γ grains increases from 64.39% to 78.47%. According to the quantitative results, the α→γ phase transformation was investigated in-depth, and it is found that isothermal compression accelerates the α→γ phase transformation. The first α→γ phase transformation is similar to ledge-controlled transformation, through which remnant α₂/γ lamellar colonies finally convert into γ grains in isothermal compression. The second is achieved by α/γ phase interface immigration.     

Surface integrity of biodegradable orthopedic magnesium�Ccalcium alloy by high-speed dry face milling

Biodegradable magnesium?Ccalcium (MgCa) implants have the ability to gradually dissolve and be absorbed in human body after implantation. Since the implant surface is in direct interaction with body fluids, surface integrity of MgCa implants are key factors influencing degradation rate of the biodegradable implants. Machining is often very necessary to make geometric features of implants. The effects of process parameters in dry high-speed face milling of MgCa0.8 (wt%) alloy on surface integrity characteristics, i.e. surface topography, surface roughness, microstructure, microhardness, and residual stresses, are investigated in this study. Polycrystalline diamond inserts are used to avoid material adhesion in milling MgCa alloy. High cutting speeds of up to 2,800 m/min and a broad range of feed and depth-of-cut values are selected to cover the regimes of finish and rough cutting. Average roughness value of 0.4 ??m and shallow strain hardened depths are achieved. Little change of grain size can be observed in the near surface even for very slow feed value of 0.05 mm/rev. The surface residual stresses are measured to be highly compressive.     

Microstructure and technological plasticity of cast intermetallic alloys on the basis of γ-TiAl

This work is devoted to the estimation of the technological plasticity of binary and alloyed γ titanium aluminides by conducting compression tests at T = 1000°C. The technological plasticity was shown to grow with decreasing size of grains and grain colonies and with increasing amount of β-stabilizing elements in the alloys. The best technological properties are characteristic of the alloys that solidify completely through the β phase, containing β-stabilizing additions of niobium and molybdenum and microadditions of boron. These alloys are characterized by a small size of crystallites in the cast state; the use of special heat treatments makes it possible to substantially decrease the fraction of the lamellar component and to increase the content of the β(B2) phase in them. For the most technological alloy, tensile tests in the cast state have been carried out. In the temperature range of T = 900–1100°C, superplastic elongations have been achieved.     

Effect of carbon addition on solidification behavior,phase evolution and creep properties of an intermetallic β-stabilized γ-TiAl based alloy

Improving mechanical properties of advanced intermetallic multi-phase γ-TiAl based alloys, such as the Ti-43.5Al-4Nb-1Mo-0.1B alloy (in at.%), termed TNM alloy, is limited by compositional and microstructural adaptations. A common possibility to further improve strength and creep behavior of such β-solidifying TiAl alloys is e.g. alloying with β-stabilizing substitutional solid solution hardening elements Nb, Mo, Ta, W as well as the addition of interstitial hardening elements C and N which are also carbide and nitride forming elements. Carbon is known to be a strong α-stabilizer and, therefore, alloying with C is accompanied by a change of phase evolution. The preservation of the solidification pathway via the β-phase, which is needed to obtain grain refinement, minimum segregation and an almost texture-free solidification microstructure, in combination with an enhanced content of C, requires a certain amount of β-stabilizing elements, e.g. Mo. In the present study, the solidification pathway, C-solubility and phase evolution of C-containing TNM variants are investigated. Finally, the creep behavior of a refined TNM alloy with 1.5 at.% Mo and 0.5 at.% C is compared with that exhibiting a nominal Ti-43.5Al-4Nb-1Mo-0.1B alloy composition.     

Diffusion bonding of γ-TiAl alloy to Ti-6Al-4V alloy under hot pressure

1 Introduction Alloys based on the ?-TiAl intermetallic compounds have been of much interest in recent years as light-mass structural materials for elevated temperature aerospace applications[1- 5]. Successful joining and cost effective fabrication method…     

Effect of electrolyte temperature on the nano-carbonitride layer fabricated by surface nanocrystallization and plasma treatment on a γ-TiAl alloy

The distribution of nano-carbonitrides produced by the treatments of surface nanocrystallization and plasma electrolytic carbonitriding on a γ-TiAl was investigated by means of figure analysis. The skewness and kurtosis of Gaussian shape distribution curves were studied and the effect of electrolyte temperature was determined. The usage of lower temperatures of the electrolyte is more suitable for achieving lower sizes of complex nano-carbonitrides. The surface roughness of treated samples was measured and it was observed that there is an optimum level of electrolyte temperature for surface roughness increase (difference between two measured data).     

Enhancement of creep properties and microstructural stability of intermetallic β-solidifying γ-TiAl based alloys

Carbon, Mo and Si represent promising alloying elements with respect to increase the operating temperature of intermetallic titanium aluminides. The influence of these elements on microstructural stability and creep properties is investigated on a β-solidifying γ-TiAl based alloy, named TNM, with a nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in at.%) in two different microstructural conditions. The first condition after casting and hot isostatic pressing has a coarse “nearly lamellar γ” microstructure. The second condition is adjusted by a subsequent heat treatment and shows a “nearly lamellar β” microstructure with finely spaced α₂ and γ lamellae and areas of discontinuous precipitation (cellular reaction) in the α₂/γ-colonies. Creep tests were carried out at 815 °C and 150 MPa to examine the influence of microstructure and its change on and during creep. Alloying of C and Si or Mo to the TNM based material led to improved creep properties and microstructural stability by inhibiting the progress of discontinuous precipitation.     

Synthesis of γ-TiAl based alloy by mechanical alloying and reactive hot isostatic pressing

Elemental Ti-48Al-2Cr-2Nb powders were mechanically alloyed (MA) for 8 h and 45 h. The MA powders were then consolidated by reactive hot isostatic pressing (HIP). The microstructure of the HIPed materials consisted of equiaxed γ-TiAl and α₂-Ti₃Al phases. During the high-temperature annealing of the HIPed materials, the α₂-Ti₃-Al phase transformed into a lamellar structure consisting of alternating laths of α₂-Ti₃Al and γ-TiAl. It is suggested that a high content of interstitial elements together with the microalloying elements of niobium and/or chromium in MA powders raises α/(α + γ) transus to a higher temperature.     

The application of transformation matrices on the determination of γ/γ interface types in a γ-TiAl alloy

The domain orientations and lamellar interfaces play a key role in the mechanical properties of TiAl-based alloys. All six domain orientations in the lamellar structure of a modern (α₂+γ) two-phase alloy were therefore measured by means of convergent-beam electron diffraction. The transformation matrices necessary for indexing crystallographically differently oriented γ lamellae were systematically calculated. Based on these calculated matrices, the interface types between neighboring γ laths could be determined unambiguously. The frequency of different interface types was measured systematically and compared with the frequency distribution in binary TiAl alloys.     

Analysis of local microstructure after shear creep deformation of a fine-grained duplex γ-TiAl alloy

The present work characterizes the microstructure of a hot-extruded Ti–45Al–5Nb–0.2B–0.2C (at.%) alloy with a fine-grained duplex microstructure after shear creep deformation (temperature 1023 K; shear stress 175 MPa; shear deformation 20%). Diffraction contrast transmission electron microscopy (TEM) was performed to identify ordinary dislocations, superdislocations and twins. The microstructure observed in TEM is interpreted taking into account the contribution of the applied stress and coherency stresses to the overall local stress state. Two specific locations in the lamellar part of the microstructure were analyzed, where either twins or superdislocations provided c-component deformation in the L1₀ lattice of the γ phase. Lamellar γ grains can be in soft and hard orientations with respect to the resolved shear stress provided by the external load. The presence of twins can be rationalized by the superposition of the applied stress and local coherency stresses. The presence of superdislocations in hard γ grains represents indirect evidence for additional contributions to the local stress state associated with stress redistribution during creep.