Daniel Eylon

Effect of various surface conditions on fretting fatigue behavior of Ti–6Al–4V

Abstract An experimental investigation was conducted to explore the fretting fatigue behavior of Ti–6Al–4V specimens in contact with varying pad surface conditions. Four conditions were selected: bare Ti–6Al–4V with a highly polished finish, bare Ti–6Al–4V that was low-stress ground and polished to RMS #8 (designated as ‘as-received’), bare Ti–6Al–4V that was grit blasted to RMS #64 (designated as ‘roughened’) and stress relieved, and Cu–Ni plasma spray coated Ti–6Al–4V. Behavior against the Cu–Ni coated and as-received pads were characterized through determination of a fretting fatigue limit stress for a 107 cycle fatigue life. In addition, the behavior against all four-pad conditions was evaluated with S-N fatigue testing, and the integrity of the Cu–Ni coating over repeated testing was assessed and compared with behavior of specimens tested against the as-received and roughened pads. The coefficient of friction, μ, was evaluated to help identify possible crack nucleation mechanisms and the contact pad surfaces were characterized through hardness and surface profile measurements. An increase in fretting fatigue strength of 20–25% was observed for specimens tested against Cu–Ni coated pads as compared to those tested against as-received pads. The experimental results from the S-N tests indicate that surface roughness of the coated pad was primarily responsible for the increased fretting fatigue capability. Another factor was determined to be the coefficient of friction, μ, which was identified as ~0.3 for the Cu–Ni coated pad against an as-received specimen and ~0.7 for the bare as-received Ti–6Al–4V. Specimens tested against the polished Ti–6Al–4V pads also performed better than the specimens tested against as-received pads. Fretting wear was minimal for all cases, and the Cu–Ni coating remained intact throughout repeated tests. The rougher surfaces got smoother during cycling, while the smoother surfaces got rougher.

Diffusion during Powder Metallurgy Synthesis of Titanium Alloys

In the present study titanium alloys were synthesized by the blended elemental press-andsinter powder metallurgy approach using hydrogenated titanium powder. Experimental investigation and modeling of the homogenization processes during synthesis were used to analyze peculiarities of mass transfer and factors affecting diffusion. Processes of alloying elements redistribution during chemical homogenization of powder blends are shown to be strongly dependent on the chemical composition of the initial powders. Optimization of the processing parameters allows to synthesize uniform, nearly-dense material with reduced grain size, at relatively low temperatures and short time. This will provide improvement of mechanical properties simultaneously with better cost-effectiveness of the process.

Effects of ballistic impact damage on fatigue crack initiation in Ti–6Al–4V simulated engine blades

Abstract The ingestion of debris into jet engines creates nicks and dents on the leading edges of blades and vanes. This is commonly known as foreign object damage (FOD). Such damage, which can often result in premature failure, was simulated in the laboratory using diamond cross-section axial fatigue samples that were impacted with 1 mm diameter glass beads at 305 m s −1 at either 0 or 30° angle of incidence. The samples had either a thin leading edge (LE) with a radius of 0.127 mm or a thick LE with a radius of 0.381 mm. Fatigue strength of impacted specimens showed degradation of 10–50% due to LE damage, regardless of the depth of the damage zone. FOD related impact notch depth, loss of material (LOM), shear, folds, embedded shattered glass, and microstructural damage were characterized by SEM. Fatigue strength degradation was found to be higher for the 30° impacts than for the 0° impacts. No clear correlation between notch depth or LE thickness and fatigue strength was found.

Effects of conventional machining on high cycle fatigue behavior of the intermetallic alloy Ti47Al2Nb2Cr (at.

Abstract The TiAl based alloy, Ti47Al2Nb2Cr (at.%), was used as a model system to explore effects of machining-induced surface deformation on fatigue behavior of an intermetallic alloy with limited ductility. Conventional machining processes, such as grinding and turning, harden TiAl to depths ranging from 40 to 180 μm. Turning doubled the hardness to approximately 500 VHN in the outer 20 μm, and hardness increased at least 50 VHN to a depth of 180 μm. The deformed layer formed during machining recrystallized after 1 h at 760°C. Axial fatigue tests were performed by step loading every 10 6 cycles through a fixed set of stress levels until failure. At room temperature, mean fatigue strength was not affected by surface condition. The outer 20 μm of the electropolished surface hardened to the same level as the turned samples during the fatigue test. After hardening, these two surfaces would have similar crack initiation resistance and, therefore, similar fatigue strengths. At 760°C, turning improved the average fatigue strength by 5%, and the average life at the final stress level by about 1.5 orders of magnitude. Fatigue resistance of the turned samples was improved by formation of a continuous, crack initiation resistant, recrystallized layer in the outer 30–50 μm during the test.

Hot strength and hot ductility of titanium alloys—a challenge for continuous casting process

Abstract Hot workability of Ti-6Al-2Sn-4Zr-2Mo, Ti-10V-2Fe-3Al, Ti-15V-3Cr-3Sn-3Al and NiTi have been examined up to the melting point, and compared with those of Ti-6Al-4V and carbon steels. Titanium alloys do not show any embrittlement in zone II, which is the best region of hot rolling. The embrittlement occurs just below the β transus temperature. Based on these results, it is suggested that titanium alloys can be successfully produced by the continuous casting and hot direct rolling processes if cast material passes the unbending point at temperature above β transus in order to avoid the embrittlement in zone III. Since the strength level of titanium alloys in the β region is very low, it is also easier to unbend the strand. Hot direct rolling immediately after the casting is also an effective method for obtaining good product shapes free from surface oxidation and also has substantial cost benefits.

Fracture characteristics and microstructural factors in single and duplex annealed Ti–4.5Al–3V–2Mo–2Fe

Abstract The influence of microstructure on fracture toughness of single and duplex annealed Ti–4.5Al–3V–2Mo–2Fe alloys was investigated. For the single-annealing treatment between 1103 and 1173 K for 3.6 ks, fracture toughness, J IC , decreases first with the increase of annealing temperature from 1103 to 1123 K and then increases with increasing annealing temperature upto 1173 K. As a result, there is a minimum of fracture toughness at 1123 K. This anomalous relation between fracture toughness and annealing temperature also exists in the duplex-annealed specimens at 993 K for 3.6 ks followed by the above single-annealing treatment. Examining the change of J IC and microstructure with annealing temperature, it can be found that J IC is related to volume fraction, aspect ratio and grain size of primary α phase, width of acicular α phase and prior β grain size. The decrease of fracture toughness in the temperature range from 1103 to 1123 K is mainly due to the decrease of volume fraction of primary α phase, while the increase of fracture toughness above 1123 K is mainly caused by the increase of prior β grain size.

The influence of texture and phase distortion on ultrasonic attenuation in Ti-6Al-4V

A high resolution experimental capability has been developed to map the phase and magnitude of ultrasonic waves transmitted in a solid. The advancement presented in this paper is provided by laser detection of the ultrasonic energy over a microscopic aperture of approximately 50 μm. The system is built around a computer controlled scanner and a confocal Fabry-Perot interferometer, which uses a diode pumped, frequency-doubled Nd:YAG laser as a light source. Wave propagation in the axial and radial directions of a 2.5″ diameter bar of textured Ti-6Al-4V was investigated in this study. Measurements were also taken on samples cut with angles between the surface normals and the axis of the bar of 0, 30, 45, 60, and 90 degrees. The work was motivated by the observation of unusually high apparent attenuation in the axial direction of the as-received bar, thought to be associated with phase distortion rather than actual energy loss. The current phase mapping results, using a focused laser spot, show relatively high wavefront distortion and more nonuniform distribution of the transmitted energy in the axial direction. The contribution to attenuation associated with phase cancellation loss was also investigated. These measurements show the laser detected attenuation to be substantially lower than the piezoelectrically measured attenuation. However, even the relative phase insensitivity of focused laser detection approach clearly indicates the attenuation to be strongest in the axial direction. This paper demonstrates the orientation dependence of attenuation stems from scattering effects associated with texturing and the elongated macroscopic grain structure in the mill annealed Ti-6Al-4V bar generated during processing, which may also affect diffraction and beam divergence.

Hot-compression behavior and microstructure evolution of pre-alloyed powder compacts of a near-γ titanium aluminide alloy

Abstract The hot-compression behavior and microstructure evolution of pre-alloyed powder compacts of the near-γ titanium aluminide alloy Ti48at.%Al2at.%Cr2at.%Nb were determined and compared with results for the same alloy processed via ingot metallurgy methods. Gas-atomized powder was by consolidated hot isostatic pressing at a low temperature (1010°C) to retain a fine microstructure. Samples of this material were upset isothermally at temperatures between 1000 and 1260 °C and strain rates between 10 −3 and 10 −1 s −1 . The stress-strain curves revealed moderate amounts of flow softening which were attributed primarily to dynamic recrystallization and secondarily to deformation heating (at the highest strain rate studied). The absence of large lamellar colonies in the powder metallurgy (PM) material was deduced to be the reason for peak flow stresses which were much lower than those previously noted for cast plus hot isostatically pressed Ti48at.%Al2at.%Cr2at.%Nb. As for ingot metallurgy near-γ titanium aluminides, the flow stress and grain size showed a strong dependence on temperature and strain rate. The steady state flow stresses for the PM alloy were almost identical with those for cast plus hot isostatically pressed and for the cast plus hot isostatically pressed plus isothermally forged Ti48Al2Cr2Nb. The PM materials also revealed a noticeable degree of microstructure non-uniformity which persisted even after hot-compression testing at various sub-α transus temperatures.

Effect of particle morphology on flow characteristics of a composite plasma spray powder

The effects of BaF2-CaF2 particle morphology on National Aeronautics and Space Administration (NASA) PS304 feedstock powder flowability were investigated, BaF2-CaF2 eutectic powders were fabricated by comminution (producing an angular morphology) and by gas atomization (producing a spherical morphology). The fluoride powders were added incrementally to the other powder constituents of the NASA PS304 feedstock, (Ni-Cr, Cr2O3, and Ag powders). A linear relationship between flow time and concentration of the BaF2-CaF2 powder was found. The flow of the powder blend with spherical BaF2-CaF2 was better than that with angular BaF2-CaF2. The flowability of the powder blend with angular fluorides decreased linearly with increasing fluoride concentration. However, the flow of the powder blend with spherical fluorides was independent of fluoride concentration. The results suggest that for this material blend, particle morphology plays a significant role in flow behavior, offering potential methods to improve powder flowability and enhance the commercial potential. These findings may be applicable to other difficult-to-flow powders such as cohesive ceramics.

Creep Deformation and Damage in a Continuous Fiber-Reinforced Ti-6Al-4V Composite

Mechanisms of longitudinal creep deformation and damage were studied in an eight-ply unidirectional-reinforced SCS-6/Ti-6Al-4V composite. The composite was creep tested in air under constant tensile load at temperatures from 427 °C to 650 °C and stresses from 621 to 1380 MPa.In situ acoustic emission (AE) monitoring and post-test metallographic evaluation were used to study fiber fracture and damage during creep. At low creep stresses, creep rates continuously decreased to near-zero values. This was attributed to a mechanism of matrix relaxation and the time-dependent redistribution of load from the ductile matrix to the elastic fibers. At higher stresses, progressive fiber overload occurred during creep loading. In this case, the composite exhibited a stage of decreasing creep rate (due primarily to matrix relaxation), followed by a secondary stage of nearly constant creep rate due to fiber fracture. The results indicate that interfacial oxidation damage and inefficient load transfer at elevated temperatures significantly decreased the capability of broken fibers to carry load. As a result, additional time-dependent stress redistribution occurred in the composite, which was responsible for the secondary creep stage.