Fatigue Resistance Of Titanium Alloy Research Articles

Improving the fatigue resistance of plasma electrolytic oxidation coated titanium alloy by ultrasonic surface rolling pretreatment

Plasma electrolytic oxidation (PEO) damages fatigue strength of titanium alloy, ultrasonic surface rolling process (USRP) pretreatment is used to mitigate this adverse effect. The microstructure, stress distribution and fatigue performance of Ti6Al4V alloy with different treatment are tested by FESEM, TEM, X-ray stress tester and rotating-bending fatigue tests, and anti-fatigue mechanism of co-strengthened sample is discussed. USRP treatment refines surface grains and introduces a compressive stress field of above 560 μm, which helps to improve the compactness and reduce surface roughness of PEO coating, and significantly increase fatigue resistance of PEO coated samples. Under covering the coating of 20 μm, the fatigue life of co-strengthened sample is about 14 times higher than that of simple PEO sample at 600 MPa, and fatigue crack source also changes from multi-source to single-source and transfers to the subsurface. PEO consumes part of surface nanocrystalline layer and releases some compressive stress for ultrasonic-rolled sample, but the effective compressive stress field with a thickness of 180 μm is still present, and the grains at coating/substrate interface also remain at the nanoscale (∼81 nm), which are the key reasons for USRP to improve fatigue resistance of titanium alloy treated by PEO.

Effects of ultrasonic shot peening on fatigue behavior of TA15 titanium alloy fabricated by laser melting deposition

Due to the influence of tensile residual stress and pores, the fatigue resistance of titanium alloy fabricated by laser melting deposition (LMD) still needs to be improved. In this study, ultrasonic shot peening (USP) was utilized to strengthen the TA15 titanium alloy manufactured by LMD. The surface topography, microstructure, microhardness, residual stress and fatigue behavior were analyzed. After USP treatment, the root-mean-square deviation of the three-dimensional profile (Sq) and the local stress concentration factor (Kst) increased. High-density dislocations (dislocation tangles, dislocation walls, dislocation cells) and deformation twins were generated on the specimen subsurface. The existence of high-density dislocations improved the surface microhardness. Meanwhile, changes in microstructure affected the state and distribution of residual stress. The surface tensile residual stress was completely transformed into compressive residual stress. The fatigue behavior was significantly influenced. Under 720 MPa and 760 MPa stress levels, the safe fatigue life (reliability R = 0.95, confidence level C = 0.95) of TA15 titanium alloy manufactured by LMD increased by 200 % and 43 %, respectively. In the pore-profile valley competition, the pore showed greater fatigue sensitivity. The high-density dislocations and compressive residual stress accounted for the improvement in fatigue life. The high-density dislocations hindered the crack nucleation and short crack propagation at pores near the surface. The compressive residual stress reduced the crack propagation rate in the stable crack propagation stage. The results indicated USP as an effective method to improve the fatigue performance of the TA15 titanium alloy manufactured by LMD.

Theoretical investigation of machining-induced residual stresses in longitudinal torsional ultrasonic–assisted milling

In order to realize the compressive stress and antifatigue manufacturing of titanium alloy TI-6Al-4V, a compound processing of longitudinal-torsional ultrasonic vibration and milling (LTUM) is proposed, a theoretical prediction model of machining-induced residual stress (RS) is established, and it is validated by experiments. The trajectory model of cutting edge in LTUM is constructed; furthermore, the undeformed chip thickness (UCT) model of LTUM is structured; The mechanical stress model of LTUM is established from the shear stress and plow stress; the thermal stress model of LTUM is established from shear thermal stress and plow thermal stress; Considering mechanical and thermal stress, the residual stress of longitudinal-torsional ultrasonic milling is established by loading and releasing stress. From numerical simulation of mechanical stress and thermal stress model, it shows that in LTUM, stress fluctuates with ultrasonic vibration, and mechanical stress absolute value is larger than that of traditional machining (TM); thermal stress absolute value is less than that of TM. A series of experiments are carried out to verify the RS model of LTUM. From present work, through theoretical prediction and experimental verification of machining-induced residual stress, it is concluded that established theoretical model predicts properties, and distribution of residual stress with high accuracy and the LTUM significantly increases surface compressive stress and compressive stress layer depth. It lays a foundation for the compressive stress and fatigue resistance of titanium alloys.

Effect of the presence of macrozones on short crack propagation in forged two-phase titanium alloys

Macrozones, which are regions containing grains with similar orientations, are commonly reported to be present in two-phase titanium alloys and to be detrimental to their fatigue resistance. The effect of macrozones on fatigue crack initiation is established, but their effect on short crack propagation remains unknown. In this study, the effect of the presence of macrozones on short crack propagation is investigated under four-point bend fatigue testing on both as-forged and powder hot isostatic pressed Ti-64. Micronotches were machined into the samples as crack initiation sites and the short crack propagation behaviour was recorded by replica. It was found that the crack propagation in macrozones can be faster than in random texture because most of the crack planes in the grains of the same macrozone are aligned. Furthermore, macrozones oriented favourably for basal 〈a〉 slip systems are less resistant to short crack propagation than macrozones oriented favourably for prismatic 〈a〉 slip systems. This is because the basal planes of the grains within the same macrozone are aligned, while the prismatic planes of the grains within the same macrozone may not be aligned. This study provides an important fundamental understanding of the effect of macrozones on short crack propagation in forged two-phase titanium alloys and thus on fatigue resistance of titanium alloys and could be useful for the titanium industry in predicting the effects of the presence of macrozones in their products.

Raising the Fatigue Resistance of Titanium Alloys by High-Speed Deformation in the Range of Polymorphic Transformations

Methods for raising the fatigue limit of critical parts by forming the required properties and structure in preforms from α + β titanium alloys by high-speed pressing at a temperature above the point of polymorphic transformations and subsequent heat treatment are described. Amethod for experimental determination of expedient thermomechanical regimes for raising the fatigue limit of the materials of articles is considered for billets from alloy VT9.

Beam Assisted Deposition Film for Improving Fretting Fatigue Resistance of Ti-8Al-1Mo-1V Alloy at Elevated Temperature

Ion-beam-assisted deposition (IBAD) was investigated as a potential way to increase the fretting fatigue resistance of Ti-8Al-1Mo-1V alloy at elevated temperature. Three coating systems, hard TiN film with good toughness and soft Al film of low friction and Cu/Ni multilayer films with modulation period thickness of 20~600nm have been applied on the base material. Coefficients of friction and fretting fatigue lives of the specimens with and without film were compared. The film damage was characterized through scanning electron microscopy. The results indicate that the IBAD technique can prepare all films with high bonding strength and excellent lubricating properties. The fretting fatigue life of the Ti-8Al-1Mo-1V alloy with the TiN film was improved by a factor of 2.4 as compared to the uncoated substrate at elevated temperature because of the excellent wear and fatigue resistance and good toughness of the film. Excellent wear resistance and good anti-fatigue properties could be simultaneously obtained by a single hard film to control the fretting fatigue damage. The IBAD Al film significantly improved the fretting fatigue resistance of the Ti-8Al-1Mo-1V alloy at elevated temperature for good lubricating property. The fretting fatigue resistance of the Ti-8Al-1Mo-1V alloy was improved by all the Cu/Ni multilayer films. However, the fretting fatigue resistance did not increase monotonically with the modulation period of the films. Films with a modulation period of 200 nm had the highest fretting fatigue resistance among the multilayer films prepared owing to their high toughness and strength and good lubricating and anti-fatigue action. The fretting fatigue resistance of films with a modulation period of 20 nm was low because of the poor fracture toughness and crack propagation resistance, even though these films had the highest hardness and good fretting wear resistance. Thus, comprehensive properties, including high toughness and strength, must be considered for multilayer films used to improve the fretting fatigue resistance of titanium alloys.

Effect of mechanical surface treatments on Ti–6Al–4V direct metal deposition parts

The effect of surface treatments, including aggressive milling, rotational burnishing, and nonrotational burnishing, on Ti–6Al–4V DMD (direct metal deposition) parts was investigated. Particular emphasis is on the question of whether these surface treatments could induce the plastically deformed and work-hardened layer that was proven to enhance the fatigue resistance of titanium alloys and was a key step for recrystallization. Through the microhardness examination and microstructure analysis, it was found that the rotational burnishing process was able to work harden the material deeper than 1000 μm, while the work-hardened layer generated by the nonrotational process was ∼600 μm and that for aggressive milling was less than 10 μm. Because surface finish is another critical factor for the resistance of fatigue crack initiation, it was also evaluated for these treatments.

The effect of a current pulse on the fatigue of titanium alloy

The fatigue resistance of titanium alloy increases under the influence of a strong current pulse. Surface residual stresses, as well as the electric resistance of specimens, decrease. An ‘aiming healing’ crystal lattice defects is supposed to be the cause of the properties improvement.

The effect of current pulse on the fatigue of titanium alloy

The fatigue resistance of titanium alloy increases under the influence of a strong current pulse. Surface residual stresses, as well as the electric resistance of specimens, decrease. An ‘aiming healing’ crystal lattice defects is supposed to be the cause of the properties improvement.

Effect of shot peening on the fretting wear of Ti–6Al–4V

In this paper, we report on the fretting wear behaviour of polished and shot peened Ti–6Al–4V specimens. For fretting experiments, due to micro-displacements at the interface between two contacting surfaces, two types of damage can be observed: crack initiation and debris formation. Shot peening, which is already well known for improving fatigue resistance of titanium alloys, is shown to have a beneficial effect on the crack initiation and propagation under fretting wear loading, as cracks observed on specimens after cylinder-on-flat fretting tests are shorter in shot peened specimens than in polished ones. It is also demonstrated that shot peening decreases the friction coefficient only at the beginning of the test, as long as the asperities induced by shot peening are not worn-off. The effects of displacement amplitude, normal force and test duration on the wear volume have been investigated: in all cases, shot peening has no significant impact on the wear process. The same amount of debris are formed and ejected for both polished and shot peened specimens. Moreover, it is found that, for both types of specimens, the linear relation, developed for steels and hard coatings, between wear volume and cumulated dissipated energy is not valid in the present case as different wear volumes are measured for the same cumulated dissipated energy, depending on the experimental conditions (normal force, displacement amplitude). Using the test duration as the variable parameter, energy wear coefficients are calculated for different experimental conditions.

Improvement in fretting wear and fatigue resistance of Ti–6Al–4V by application of several surface treatments and coatings

Application of surface modification methods is expected to be an ideal solution to mitigate fretting damage. In this study, our aim was to improve the fretting wear and fretting fatigue resistance of titanium alloys by using several types of surface treatments and thin films, including shot-peening, ion-beam-enhanced deposition (IBED) CrN films, shot-peening+IBED CrN films as well as IBED CuNiIn films. Results showed that with the application of all the above surface coatings and treatments, the fretting wear and fretting fatigue resistance of Ti–6Al–4V were improved. However, the mechanisms and effects of several surface modification methods to mitigate the fretting damage were quite different. IBED CrN film exhibited the best fretting fatigue performance while the duplex treatment by shot-peening/IBED CrN film exhibited the highest fretting wear resistance. There are four mechanisms which can be used to explain the different fretting performance of these surface treatments and coatings: (1) to induce a compressive residual stress; (2) to decrease the coefficient of friction; (3) to increase the hardness; (4) to increase the surface roughness.

Effect of the mechanical state of the specimen surface on the fatigue resistance of titanium alloy

Effect of the mechanical state of the specimen surface on the fatigue resistance of titanium alloy

Fatigue resistance of titanium alloys with detonation coatings : V.V. Sergeev et al.Poroshkovaya Metallurgiya, no 1/2, 1995, 47–52. (In Russian.)

Fatigue resistance of titanium alloys with detonation coatings : V.V. Sergeev et al.Poroshkovaya Metallurgiya, no 1/2, 1995, 47–52. (In Russian.)

Effect of chemical heat treatment on the fatigue resistance of titanium alloys

In order to improve the wear resistance of titanium alloy articles, chemical heat treatment, in particular, oxidation and nitriding diffusion impregnation of the article is performed. As a result of this treatment there is a marked increase in surface layer hardness. The change in structure and properties of surface layers have a marked effect on material resistance in fatigue failure as well as on sensitivity to stress concentration. This is the subject of the research discussed here.