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.
Hastelloy X is the trademark for a nickel-based, high-temperature superalloy that is increasingly applied in gas turbine engines because of its exceptional combination of oxidation resistance and ...high-temperature strength. The superalloy suffers from hot cracking susceptibility, however, particularly when processed using additive manufacturing and laser powder bed fusion (LPBF). This paper systematically studies for the first time the effect of post-treatment hot isostatic processing (HIP) on the microstructure and mechanical properties of LPBF-fabricated Hastelloy X, with an emphasis on fatigue performance. The experimental results demonstrate that despite the very small number of remaining gas-filled micropores due to pressure counteraction, the high temperature and high pressure during the HIP process promote recrystallisation and closing of the internal microcracks and gas-free pores. The HIP-processed specimens are shown to be roughly 130 MPa and 60 MPa weaker than the non-processed specimens in yield strength and ultimate tensile strength, respectively. The HIP-processed Hastelloy X exhibits significant improvements in fatigue life, however: the effect of the HIP processing is apparent once the applied stress decreases. This improvement in fatigue performance is attributable to the reduction in stress concentration and residual stress release caused by the HIP process. The paper also studies the hot cracking mechanism and finds that intergranular microcracks generally occur along high angle grain boundaries; the interdendritic liquid pressure drop between dendrite tip and root is found to be a significant factor in the hot crack mechanism. The significance of this research is in developing a comprehensive understanding of HIP processing on the fatigue behaviour of the LPBF-fabricated Hastelloy X. The insights on the cracking mechanism, which presents a significant step towards using additive manufacturing to produce complex crack-free parts from this superalloy.
•Effect of bio-oils on mechanical performance of asphalt mixture is investigated.•Bio-oil can improve asphalt mixture fatigue performance significantly.•Polymer modified bio-oil performs better than ...original and dewatered bio-oils.•Statistical analysis are conducted for the laboratory results.
Bio-oils derived from waste wood resources are thought to be potential alternatives for petroleum asphalt binders used in asphalt pavement. This study aims to evaluate the effect of bio-oils on asphalt mixture performance after blending bio-oil (5% and 10% by weight, respectively) into the traditional asphalt binder. Three types of bio-oils are used – the original bio-oil (OB), dewatered bio-oil (DWB) and polymer-modified bio-oil (PMB). The asphalt pavement analyzer (APA) test, four-point beam fatigue test, dynamic modulus (|E*|) test and indirect tensile (IDT) strength test were conducted to evaluate the rutting resistance, fatigue performance, dynamic stiffness, and tensile strength, respectively. The test results showed that the addition of bio-oils significantly improves the asphalt mixture fatigue performance, has no significant effect on the rutting performance and dynamic modulus, but slightly impacted the tensile strength. In addition, polymer in the bio-oil was observed to improve the asphalt mixture performance as the PMB modified asphalt mixtures performed better than the two other mixtures. Further, statistical analysis on the laboratory test results are conducted and found to be consistent with the findings above. The study shows that the bio-oils derived from waste wood resources can be a good extender and modifier for petroleum asphalt binders in the pavement industry.
•Mechanical properties significantly higher than those of cast material are achieved.•Process costs can be controlled according to the required mechanical properties.•Base plate heating can control ...the microstructure and quasistatic properties.•Base plate heating also decreases the fatigue scatter by avoiding large-size pores.•Endurance limit was successfully estimated by short-time fatigue tests.
Selective laser melting (SLM) offers high potential for manufacturing complex geometries and custom-made parts due to its unique layer-wise production process. A series of samples of AlSi12 have been manufactured by SLM process to study the effect of process parameters and post-build heat treatment on the microstructure and the corresponding mechanical properties. Optical microscope, scanning electron microscope, quasistatic tests, continuous load increase fatigue tests and constant amplitude fatigue tests have been employed for characterization. A remarkable eutectic microstructure, with dendritic width changing with SLM process parameters, has been observed. Relationship between SLM process parameters, resulting microstructure and the consequent changes in mechanical properties has been discussed. Base plate heating has been found critical in controlling the in-process microstructure. Mechanical properties of SLM parts outperform those of conventionally manufactured alloy, and can be varied as per requirement, by altering the build rate, keeping the process costs in control. Fatigue scatter can also be controlled by heating the base plate during the process.
The present work focuses on heat treatment effects on cyclic plasticity behavior, multiscale strengthening mechanisms and low cycle fatigue performance of laser melting nickel-based superalloys. ...Microscopic indentation analysis, mesoscopic DIC analysis and macroscopic material testing were conducted to identify multiscale mechanical properties. A DIC-aided indentation method was developed to identify the constitutive parameters of the laser melting material by introducing the reference material. The dendritic–cellular microstructures in the remelting zone (RZ) significantly decrease the effects of the grain size and morphology. Heat treatments can improve the strength but decrease the ductility of RZ and do not affect the macroscopic stress–strain responses of the multilayered material. Moreover, the strengthening mechanisms in the laser melting material were verified to reveal size- and plasticity-dependent effects. The strengthening contributed by grain boundaries and twin boundaries yields little effect on the strength at micro scales and the influences increase with plastic deformations. Furthermore, the aging treatment increases the fatigue life of RZ by factor 2 and, however, decreases the low cycle fatigue performance of the multilayered material, meaning that the under-matched material can be stronger than the base material in certain configurations. The stabilized fatigue load of the laser melting material has strong correlations with the mismatching and loading levels. The present study provides a deep insight into the strengthening mechanisms and cyclic plasticity behaviors of laser-manufactured materials.
•Developed a DIC-aided indentation reverse method for multilayered materials.•Identified multiscale strengthening mechanisms in multilayered materials.•Quantified the heat treatment effects on mechanical properties and plasticity modeling.•Unraveled non-proportional variations of fatigue performance in laser melting materials.
•Full-scale steel-UHPC composite deck specimen is tested under cyclic loadings.•Transverse fatigue behavior of steel-UHPC composite deck is evaluated.•The fatigue failure mode of steel-UHPC composite ...deck is determined.•Effects of damage in UHPC layer and studs are quantified using a damage index.
With increasing traffic volumes, fatigue of orthotropic steel decks (OSDs) has become a critical issue in recent years. In this study, an innovative composite deck composed of large-size U-ribs, a thin ultra-high-performance concrete (UHPC) layer and headed studs is proposed to enhance the fatigue properties of OSDs. Experimental and numerical approaches were combined to investigate the transverse fatigue behaviour of the proposed composite deck. A large-scale composite deck specimen was fabricated and tested under cyclic loading. The experimental results revealed the progressive failure process of the composite deck. The effects of damage accumulation in UHPC layer and headed studs on the mechanical behaviour of steel-UHPC composite deck were evaluated. The results indicated that the mechanical degradation in the composite deck system accelerated the damage accumulation in the fatigue-prone details of OSDs. It is necessary to consider the effects of mechanical degradation in fatigue resistance evaluation.
•A fatigue simulation algorithm based on WRsum failure criterion was validated for bio-binders.•Bio-oil modification reduced binder stiffness but improved resistance to fatigue cracking.•Aging ...enhanced binder deformation resistance but compromised fatigue performance.•Consistent observations on fatigue were made from rheological parameters and LAS approach.
This study was aimed to evaluate the fatigue characteristics of asphalt binders modified with different dosages of bio-oil under a variety of aging conditions. The frequency sweep test was performed to determine four rheological parameters indicative of cracking resistance. The linear amplitude sweep (LAS) test was conducted to yield damage characteristic relationship, failure criterion, and failure envelope based on the viscoelastic continuum damage theory. In general, consistent observations were obtained between the rheological parameters and the LAS-based fatigue analysis, in that higher percentages of bio-oil in modification provided improved resistance to cracking, while higher aging intensities produced lower fatigue performance.
Mg alloys offer potential advantages over conventional biomedical implant materials because of their biodegradability and biocompatibility, but could be limited by their poor mechanical properties. ...In this study, laser shock peening (LSP), a surface processing technique, was applied to improve the mechanical properties of the AZ31B magnesium (Mg) alloy. It was demonstrated that LSP increased the hardness and yield strength of the Mg alloy. Due to the hardening, LSP significantly improved the wear resistance and fatigue performance of the Mg alloy. In addition, immersion tests carried out in cell culture medium revealed that LSP did not significantly increase Mg2+ release and weight loss. Furthermore, an in vitro cell culture study showed that the LSP-treated samples have cell-compatibility comparable to untreated samples. Thus, the LSP technique could, with further study, advance the clinical utility of Mg alloys in the orthopedic field.
•LSP significantly decreased the coefficient of friction and improved the wear resistance of AZ31B alloy.•LSP significantly improved the tensile strength and fatigue resistance of AZ31B alloy.•LSP-treated samples have cell-compatibility comparable to untreated samples.
•Fatigue performance of repaired Al alloy is recovered to original state.•The bonding mode was transformed from mechanical bonding to metallurgical bonding.•Massive Al2O3 reinforcements of nano/micro ...size were formed in the coating.•Laser shock peening is used to optimise the residual stress distribution.
The cold spray (CS) process has been demonstrated to be an attractive additive manufacturing technology for the development of freeforms and coatings from temperature-sensitive materials. However, owing to the lower bonding strength and inhomogeneous residual stress field distribution, the fatigue performance of CS deposition cannot satisfy the requirements of numerous engineering conditions. This study proposes a novel and promising approach to recover the fatigue performance of CS-repaired Al alloy components using two laser beam modulations. In the treatment, a millisecond laser is used during the CS process to heat the impact location in situ, by which the bonding mode is transformed from mechanical bonding to metallurgical bonding, and massive Al2O3 reinforcements of nano/micro size are formed in the coating. Thus, the bonding strength is significantly improved. Next, another nanosecond laser is used as a post-treatment to act on the area except the repaired region of the sample, which optimised the residual stress distribution around the repaired zone by the mechanical effects of the laser-induced ultra-high shock waves. Through the combination of these two lasers, the fatigue performance of CS-repaired Al alloy samples is successfully restored to its original condition. This study demonstrates that this compound process of CS technology and laser processing presents a wide range of possibilities for further development of advanced repair technology.
•Surface finishing improves the fatigue performances of SLMed Ti-6Al-4V.•HIP and heat treatments coarsen the α laths and improve the ductilities.•HIP and heat treatments increase the fatigue limits ...of SLMed Ti-6Al-4V.•Pores accelerate the fatigue crack initiation and lower the fatigue limit.
In this study, a detailed evaluation on the effects of different surface finishing processes, hot isostatic pressing (HIP) and heat treatments on the fatigue performance of SLMed Ti-6Al-4V is conducted. It is found that surface finishing processes of turning, grinding, grinding followed by sandblasting and polishing can effectively reduce the surface roughness of as-SLMed surfaces (Rz = 68.66 μm) and significantly improve the fatigue performances. The HIP, HT-920 and HT-850-550 treatments can significantly improve the ductilities of SLMed Ti-6Al-4V due to the coarsening of α laths and formation of β grains. The ultimate tensile strengths (UTSs) and yield strengths (YSs) of HIPed and HT-920 specimens reduce more than 140 MPa, while those of the HT-850-550 specimens show no obvious reduction. The SLMed Ti-6Al-4V specimen has a fatigue limit lower than 300 MPa, while the HT-920 and HT-850-550 treatments enhance the threshold of crack growth and increase the fatigue limit to 350–400 MPa. The reduction or elimination of pores and the modification of microstructure in HIPed specimens both significantly extend the period of fatigue crack initiation, leading to an increased fatigue limit of 450–500 MPa.
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