An ultrasonic surface rolling process (USRP) is a novel mechanical surface treatment technique for enhancing the fatigue performance of metallic materials. In this work, USRP with different repeated ...processing numbers was employed for enhancing the fatigue performance of a Ti-6Al-4V alloy. The effect of USRP on their surface integrity (including microstructure, surface quality, microhardness, and residual stress) were investigated, which were characterized by means of scanning electron microscope, transmission electron microscope, confocal laser scanning microscope, microhardness tester, and X-ray diffraction residual stress tester. Especially, a refined microstructure (grain size: ~100–400 nm) was formed on the topmost surface of twelve-repeat USRP specimen. Subsequently, the fatigue behavior of the specimens was investigated via rotating-bending fatigue tests, and the results suggested that USRP could effectively enhance the fatigue performance of the Ti-6Al-4V alloy. The USRP-induced enhancement mechanism of the fatigue performance can be ascribed to the synergistic effect of the compressive residual stress, microstructure, work hardening, and improved surface quality. The best synergistic effect and, correspondingly, the greatest improvement in the fatigue performance were realized for the one-repeat USRP specimen.
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•Fatigue limits are markedly improved via USRP with different processing numbers.•Fatigue enhancement degree decreases gradually with increasing processing number.•Moderate work hardening is more conducive to improving the fatigue performance.•The more degree of grain refinement does not mean the better fatigue performance.
High entropy alloy nitride coatings (HEAN), showing superior mechanical strength, high oxidation resistance, and thermal stability, have often been used in protective hard coatings field. However, ...the high temperature tribological field of novel HEA nitride films has not yet been well studied. The correlations among tribological properties, compressive residual stress, mechanical properties, and self-lubricating oxide are still limited.
In this research, the novel (AlCrNbSiTiMo)N coatings were fabricated on both Inconel-718 and Si (100) substrate by radio frequency (RF) magnetron sputtering via tuning both substrate bias at 300 °C deposition temperature. The (AlCrNbSiTiMo)N coatings with a specific substrate bias exhibit an outstanding hardness of 34.5±0.8 GPa.
In the wear test at 700 °C, the films deposited at −100 V revealed the lowest wear rate around 1.2 × 10−6 mm3N−1 m−1. With Molybdenum doping, the MoO3 Magnéli phase was observed on the surface at elevated temperature wearing process, and the friction coefficient at high temperature decreased significantly due to a lubricating surface. The coating exhibited the average friction coefficient value of 0.48 in the wear test of 700 °C. The high temperature tribological performance was addressed and related to the mechanical properties, the plastic deformation resistance H3/E2, and the excess residual stress of the films.
This study provides a new design for hard coatings applied to severe wearing conditions. By tuning substrate bias, the (AlCrNbSiTiMo)N coatings exhibit outstanding mechanical and tribological characteristics, which will be a promising candidate for high temperature tribological protective film.
•The high entropy alloy nitride coating (AlCrNbSiTiMo)N revealed favorable hardness of 34.5±0.8 GPa.•The (AlCrNbSiTiMo)N coatings exhibited low friction coefficient of 0.48±0.08 at 700°C owing to the Magnéli phase MoO3.•The optimal wear rate of 1.2×10−1 mm3 N−1m−1 was demonstrated by controlling residual stress via substrate bias tuning.
•The matrix–particle interaction during cooling is studied via a 2-scale FE model.•To get thermomechanical data for the matrix an equivalent alloy is cast and tested.•The predicted residual elastic ...strain is validated against X-ray diffraction data.•The sectors forming each particle create stress concentration points in the matrix.•The residual stress magnitude is comparable to the ductile cast iron yield stress.
Recent X-ray diffraction (XRD) measurements have revealed that plastic deformation and a residual elastic strain field can be present around the graphite particles in ductile cast iron after manufacturing, probably due to some local mismatch in thermal contraction. However, as only one component of the elastic strain tensor could be obtained from the XRD data, the shape and magnitude of the associated residual stress field have remained unknown. To compensate for this and to provide theoretical insight into this unexplored topic, a combined experimental-numerical approach is presented in this paper. First, a material equivalent to the ductile cast iron matrix is manufactured and subjected to dilatometric and high-temperature tensile tests. Subsequently, a two-scale hierarchical top-down model is devised, calibrated on the basis of the collected data and used to simulate the interaction between the graphite particles and the matrix during manufacturing of the industrial part considered in the XRD study. The model indicates that, besides the viscoplastic deformation of the matrix, the effect of the inelastic deformation of the graphite has to be considered to explain the magnitude of the XRD strain. Moreover, the model shows that the large elastic strain perturbations recorded with XRD close to the graphite–matrix interface are not artifacts due to e.g. sharp gradients in chemical composition, but correspond to residual stress concentrations induced by the conical sectors forming the internal structure of the graphite particles. In contrast to common belief, these results thus suggest that ductile cast iron parts cannot be considered, in general, as stress-free at the microstructural scale.
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300M ultra-high strength steels (300M steels) are frequently used in the manufacture of aircraft landing gear due to their high strength and ductility. However, their high sensitivity to surface ...defects accelerates fatigue failure and hinders their wider application. In this work, ultrasonic surface rolling processing (USRP) was used to process 300M steel. The surface roughness, hardness, microstructure, and residual stresses before and after USRP treatment were compared. The surface roughness for USRP-treated samples (0.062 μm) was found to be lower than that for untreated samples (0.32 μm). In addition, a plastically deformed layer was generated on the surface of USRP-treated samples that resulted in higher hardness. Beneficial compressive residual stresses were introduced as a result of USRP treatment. The better surface finish, higher surface hardness and compressive residual stresses lead to significant improvement in the resistance of the 300M steels to fretting fatigue and corrosion fatigue. The fretting fatigue life increased from 11.9 K cycles to 56.3 K cycles, while the corrosion fatigue life increased from 29.9 K cycles to 702.1 K cycles.
The corrosion resistance of 7075 aluminum (Al) alloy treated by ultrasonic surface rolling process (USRP) in a chloride environment was studied in this work. The effects of different USRP passes on ...the surface state (surface roughness and residual stress), surface microstructure, and corrosion resistance of this alloy were investigated through microstructural characterization, stress relaxation, immersion testing and electrochemical measurement. The results revealed that all USRP-treated samples demonstrated significantly improved corrosion resistance. However, the main factors that led to this performance enhancement were not the same. After 3 USRP treatment passes, the reduction in surface roughness and the increase in surface compressive residual stress improved the corrosion resistance of the 7075 Al alloy. After 7 USRP passes, the larger number of grain boundaries caused by surface grain nanocrystallization led to the rapid enrichment of passive elements, which formed a dense passive film on the alloy surface. At the same time, the dissolution of precipitates and the disappearance of precipitation-free zones (PFZ) reduced the corrosion caused by anodic dissolution. Meanwhile, the primary mechanism that controlled the corrosion rate was transformed from the surface state to the surface microstructure, which further improved the corrosion resistance of the 7075 Al alloy.
•In our work, the surface state and surface microstructure of 7075 aluminum alloy are tailored by changing the ultrasonic rolling passes.•The corrosion resistance of 7075 Al alloy is significantly improved by ultrasonic surface rolling process (USRP) in this study.•The effects of surface roughness, residual stress, surface nanocrystalline and precipitates on the corrosion resistance of 7075 Al alloy after USRP are studied by stress relaxation and surface roughness analysis.
Additive manufacturing (AM) has been widely used to fabricate functional metal parts in automobile, aerospace, energy, and medical device industries due to its flexible process capacity including ...complex geometry, functional graded materials, and free usage of tool. For the two major categories of metal additive manufacturing processes include powder bed fusion (PBF) and directed energy deposition (DED), parts are fabricated through melting of feed stock materials in the form of either powders or wires directly from a CAD model. The unique thermal cycle of metal additive manufacturing is characterized by: (1) rapid heating rate due to high energy intensity with steep temperature gradients; (2) rapid solidification with high cooling rates due to the small volume of melt pool; and (3) melt-back involving simultaneous melting of the top powder layer and re-melting of underlying previously solidified layers. Residual stress caused by the unique thermal cycle in AM is the critical issue for the fabricated metal parts since the steep residual stress gradients generate part distortion which dramatically deteriorate functionality of the end-use parts. This paper comprehensively assessed the current research status on residual stress sources, characteristics, and mitigation. First, the relationship between residual stress and microstructure is highlighted in AM metal parts. Then, the measurement methods and characteristics of residual stress in both as-build metal parts and post-processed ones were summarized. Third, residual stress mitigation and control methods including in-situ and post-process control methods were thoroughly discussed. Furthermore, future work directions are provided in this work.
•There are two stages for the surface residual stress relaxation in carburized case.•Relaxation mainly occurs near the surface and it depends on the amplitude of load.•Residual stress relaxation ...mechanism under alternating loading was revealed.•A model of surface residual stress relaxation under cyclic loading was established.•Rotating bending fatigue behaviors before and after LTGC treatment were analyzed.
Low-temperature gaseous carburization is an effective thermochemical surface modification process for austenitic stainless steels. The treatment results in the development of a carbon-enriched case with high hardness, improved localized corrosion performance and high compressive residual stress, that in principle improves the fatigue performance. Residual stress relaxation occurs during cyclic loading and the residual stress relaxation in carburized case on 316L austenitic stainless steel under cyclic loading was investigated with rotating bending fatigue testing. Two stages of residual stress relaxation in the surface region were identified: an instantaneous and large relaxation in the first cycle followed by gradual relaxation with the number of fatigue cycles. Depth-resolved analysis of residual stress showed that relaxation mainly occurs in the surface-near region of the carburized case. The rate of stress relaxations depends on the amplitude of the alternating load. The occurrence of plastic accommodation provoked by lattice expansion and the additional plastic deformation imposed during loading is decisive for stress relaxation. A model of surface residual stress evolution for carburized case under cyclic loading was established, which could accurately describe the relaxation.
Residual stresses and distortion in Additive Manufactured (AM) parts are two key obstacles which seriously hinder the wide application of this technology. Nowadays, understanding the thermomechanical ...behavior induced by the AM process is still a complex task which must take into account the effects of both the process and the material parameters, the microstructure evolution as well as the pre-heating strategy. One of the challenges of this work is to increase the complexity of the geometries used to study the thermomechanical behavior induced by the AM process. The reference geometries are a rectangular and a S-shaped structures made of 44-layers each. The samples have been fabricated by Directed Energy Deposition (DED). In-situ thermal and distortion histories of the substrate are measured in order to calibrate the 3D coupled thermo-mechanical model. Once the numerical results showed a good agreement with the temperature measurements, the validated model has been used to predict the residual stresses and distortions. Different process parameters have been analyzed to study their sensitivity to the process assessment. Different preheating strategies have been also analyzed to check their effectiveness on the mitigation of both distortions and residual stresses. Finally, some simplifications of the actual scanning sequence are proposed to reduce the computational cost without loss of the accuracy of the simulation framework.
When rock salt is subjected to discontinuous fatigue, time intervals (periods during which the stress remains constant) can accelerate plastic deformation and reduce the fatigue life due to the ...internal residual stress generated between various defects and the host material. However, the time interval effect on rock salt in a three-dimensional stress state has rarely been investigated despite its great significance in the use of underground salt caverns as storages. In this research, a series of triaxial discontinuous cyclic compression tests were conducted on salt and the residual stress was reproduced through numerical simulations. Experimental results show that the interval effect decreases with the increase of the confining pressure. The change in plasticity caused by time intervals is small and constant when the stress level (ratio of the maximum stress to strength) is below “60%” but then increases as a linear function of the stress level. During simulations, it was observed that the greater the difference in the elasticity modulus between the host material and impurities, the larger the residual stress. The moderate effect of time interval under confinement is the result of the fact that a higher confining pressure strengthens the host material, reduces the volume of impurities and thus leads to a smaller residual stress. In addition, when the stress level is below a certain threshold (which is shown to be at 0.8–0.85 of the critical yield stress), the residual stress is relatively low in magnitude and area of influence. Above that threshold, the residual stress grows rapidly. This is the reason why the interval effect displays a distinct behavior before and after dilatancy point.
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