In order to improve the tensile properties of additively manufactured Ti-6Al-4V parts, specific heat treatments have been developed. Previous work demonstrated that a sub-transus thermal treatment at ...920 °C followed by water quenching generates a dual-phase α+α′ microstructure with a high work-hardening capacity inducing a desirable increase in both strength and ductility. The present study investigates the micromechanical behavior of this α+α′ material as well as the thermal stability of the metastable α’ martensite. To that end, annealing of the α+α′ microstructure is performed and the resulting microstructural evolution is analyzed, along with its impact on the tensile properties. A deeper understanding of the micromechanics of the multiphase microstructure both before and after annealing is achieved by performing in-situ tensile testing within a SEM, together with digital image correlation for full-field local strain measurements. This approach allows the strain partitioning to be quantified at a microscale and highlights a significant mechanical contrast between the two phases. In the α+α′ microstructure, the α′ phase is softer than the α phase, which is confirmed by nanoindentation measurements. Partial decomposition of the martensite during annealing induces a substantial hardening of the α′ phase, which is attributed to fine-scale precipitation and solution strengthening. A scale transition model based on the iso-work assumption and describing the macroscopic tensile behavior of the material depending on the individual mechanical behavior of each phase is also proposed. This model enables to provide insights into the underlying deformation and work-hardening mechanisms.
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Grain structure control is challenging for metal additive manufacturing (AM). Grain structure optimization requires the control of grain morphology with grain size refinement, which can improve the ...mechanical properties of additive manufactured components. This work summarizes methods to promote fine equiaxed grains in both the additive manufacturing process and subsequent heat treatment. Influences of temperature gradient, solidification velocity and alloy composition on grain morphology are discussed. Equiaxed solidification is greatly promoted by introducing a high density of heterogeneous nucleation sites via powder rate control in the direct energy deposition (DED) technique or powder surface treatment for powder-bed techniques. Grain growth/coarsening during post-processing heat treatment can be restricted by presence of nano-scale oxide particles formed in-situ during AM. Grain refinement of martensitic steels can also be achieved by cyclic austenitizing in post-processing heat treatment. Evidently, new alloy powder design is another sustainable method enhancing the capability of AM for high-performance components with desirable microstructures.
This work investigates the solution phenomenon, aging behavior and room-temperature mechanical properties of maraging steel manufactured by selective laser melting (SLM). Different heat treatment ...experiments, including solution treatment (ST), direct aging treatment (DAT) and solution + aging treatment (SAT) are designed. Microstructure analysis indicates that ST and SAT will eliminate the cellular and lath structures, but DAT has little effect on these. The content of austenite increases with the addition of DAT temperature and holding time. While austenite is almost undetectable in ST and SAT samples. Meanwhile, both the elongation and toughness of the samples with DAT gain a slight improvement with the temperature increasing. Importantly, DAT yields similar microhardness, tensile strength and impact toughness to SAT, although the resultant microstructures are completely different. The results demonstrate that DAT can achieve the similar mechanical properties to SAT samples. Samples with high mechanical properties (microhardness of 653.93 HV and ultimate strength of 2126.30 MPa) have been obtained by DAT at 520 °C for 6 h as well as solution treatment at 900 °C for 1 h and aging treatment at 520 °C for 6 h. This investigation reveals the evolution regularity of microstructure, microhardness, tensile performance and impact toughness of maraging steel manufactured by SLM after different heat treatments.
The corrosion and cavitation corrosion resistance of a nickel-aluminum bronze was improved after quenching and quenching/aging at 450 °C, which can be attributed to the refined microstructure, ...enhanced corrosion resistance, improved hardness, and weakened synergy between corrosion and mechanical attack.
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•The microstructure of as-cast NAB was refined after quenching and ageing.•Selective phase corrosion was eliminated for the refined microstructure.•Static corrosion rate reduced about 50% through microstructure design.•Improved corrosion resistance was attributed to rapid formation of a protective film.
Microstructure evolution of the nickel-aluminum bronze alloy was studied by heat treatment, including annealing, normalizing, quenching and aging. The microstructure was refined and homogenized after quenching or quenching/aging at 450 ℃, which can eliminate selective phase corrosion effectively. Compared with the current production process, static corrosion rate reduced about 50%, due to the rapid formation of a protective film. In addition, cavitation corrosion rate reduced by a factor of 4.9 and 7.9 for the quenched and quenched/aged at 450 ℃ samples, respectively. This can be attributed to the improved hardness and weakened synergy between corrosion and mechanical attack.
This study presents the effects of solution annealing and subsequent aging on shape memory response of Ni-rich Ni50.8Ti49.2 alloys fabricated by Selective Laser Melting. After solutionizing, samples ...were heat treated at selected times at 350 °C and 450 °C and their shape memory effect, superelasticity, and transformation temperatures were determined. It was found that transformation temperatures, transformation behavior, strength, and recoverable strain are highly heat treatment dependent. Samples aged at 350 °C showed better recovery in superelasticity tests where 350 °C-18 h aged samples exhibited almost perfect superelasticity with 95% recovery ratio with 5.5% strain in the first cycle and stabilized superelasticity with a recoverable strain of 4.2% after 10th cycle. Meanwhile, 450 °C-10 h aged sample exhibited 68% recovery with a recoverable strain of 4.2% in the first cycle and stabilized recoverable strain of 3.8% after 10th cycle.
•Transformation behavior of SLM fabricated Ni50.8Ti49.2 was systematically studied.•Shape memory behavior of SLM NiTi samples was found to be highly aging dependent.•Transformation temperatures and strength of SLM NiTi were tailored with aging.•Almost perfect superelasticity with 95% strain recovery was observed under 1 GPa.•Superelastic stability of the SLM NiTi was investigated under cycling experiments.
•Superelastic shape memory alloy (SMA) cables are comprehensively studied.•Thermal-mechanical characterisations of the SMA material are carried out.•A series of SMA cable specimens are tested under ...various cyclic loading protocols.•A simple yet effective numerical modelling method is proposed for SMA cables.•A prototype bridge employing SMA cables as restrainers is designed and analysed.
This paper reports a comprehensive study on the mechanical behavior, annealing (heat treatment) scheme, hysteretic modelling strategy, and potential seismic application of superelastic shape memory alloy (SMA) cables. The study commenced with the thermal-mechanical characterization of monofilament SMA wires, and in particular, the influence of annealing scheme on the mechanical and phase transformation characteristics of the material was revealed. A series of 7 × 7 SMA cable specimens were subsequently tested at room temperature under various cyclic loading protocols. It is observed, among other findings, that the SMA cables are able to reasonably “scale up” the satisfactory properties of the SMA wires, and the mechanical behavior of the SMA cables may be improved by annealing. Moderate annealing temperature and duration (i.e., 350–400 °C for 15 min) can generally increase the stiffness, energy dissipation, and form setting ability of the SMA cables considered in this study, whereas an overly high annealing temperature tends to compromise these characteristics. Following the experimental study, an effective numerical modelling approach is proposed which reliably captures the basic mechanical behavior of the SMA cables. A model bridge, where SMA cables are adopted as restrainers, is finally designed and analyzed to demonstrate the efficiency of the SMA components for seismic damage mitigation. The analysis result shows that the SMA-cable restrainers can effectively control the peak and residual displacements of the bridge girder, and make the bridge more resilient.
Color changes were tested and compared for heat-treated Paulownia tomentosa and Pinus koraiensis wood treated with hot oil or hot air for further utilization of these species. Hot oil and hot air ...treatments were conducted at 180, 200, and 220 °C for 1, 2, and 3 h. Heat-treated wood color changes were determined using the CIE-Lab color system. Weight changes of the wood before and after heat treatment were also determined. The weight of the oil heat-treated wood increased considerably but it decreased in air heat-treated wood. The oil heat-treated samples showed a greater decrease in lightness (L*) than air heat-treated samples. A significant change in L* was observed in Paulownia tomentosa. The red/green chromaticity (a*) of both wood samples increased at 180 and 200 °C and slightly decreased at 220 °C. The yellow/blue chromaticity (b*) in both wood samples increased at 180 °C, but it rapidly decreased with increasing treatment durations at 200 and 220 °C. The overall color change (ΔE*) in both heat treatments increased with increasing temperature, being higher in Paulownia tomentosa than in Pinus koraiensis. In conclusion, oil heat treatment reduced treatment duration and was a more effective method than air heat treatment in improving wood color.
► Responses of SLM-produced and wrought Ti6Al4V to heat treatment are compared. ► Temperature is found to be the controlling parameter for treatments in the α+β range. ► Ductility could be improved ...by a factor of 85%, from 7.27% to 13.59%. ► An optimal heat treatment for SLM produced Ti6Al4V is proposed.
The present work shows that optimization of mechanical properties via heat treatment of parts produced by Selective Laser Melting (SLM) is profoundly different compared to conventionally processed Ti6Al4V. In order to obtain optimal mechanical properties, specific treatments are necessary due to the specific microstructure resulting from the SLM process. SLM is an additive manufacturing technique through which components are built by selectively melting powder layers with a focused laser beam. The process is characterized by short laser-powder interaction times and localized high heat input, which leads to steep thermal gradients, rapid solidification and fast cooling. In this research, the effect of several heat treatments on the microstructure and mechanical properties of Ti6Al4V processed by SLM is studied. A comparison is made with the effect of these treatments on hot forged and subsequently mill annealed Ti6Al4V with an original equiaxed microstructure. For SLM produced parts, the original martensite α′ phase is converted to a lamellar mixture of α and β for heat treating temperatures below the β-transus (995°C), but features of the original microstructure are maintained. Treated above the β-transus, extensive grain growth occurs and large β grains are formed which transform to lamellar α+β upon cooling. Post treating at 850°C for 2h, followed by furnace cooling increased the ductility of SLM parts to 12.84±1.36%, compared to 7.36±1.32% for as-built parts.
In this work, a novel and simple strategy, i.e., solvent activation before heat-treatment was proposed to improve reverse osmosis (RO) membrane performance. This activation strategy is different from ...that adopted in previous reports. With this strategy, solvent activation was conducted before heat-treatment. While in previous reports, solvent activation was conducted after heat-treatment. By systematic characterization, solvent activation before and after heat-treatment were compared for the first time. The results showed that exchanging the order of solvent activation and heat-treatment created completely different effects on the RO membrane structures and performance. Specifically, solvent activation before heat-treatment affected the RO membrane structures more obviously and was superior in terms of improving RO membrane performance. Besides, the solvent activation was discussed deeply. The results showed that it had better choose an activation solvent, which is non-polar and low viscous as well as molecular size is small. By using an appropriate activation solvent, such as hexane, solvent activation before heat-treatment improved the RO membrane performance markedly: the flux was improved from 46 ± 1 L m−2· h−1 to 75 ± 5 L m−2 h−1 (by 63%) and meanwhile a high rejection of 98.97 ± 0.08% was retained (the testing condition: 15.5 bar pressure, 2000 ppm NaCl feed and 25 °C). Additionally, an unexpected phenomenon was found: increasing the activation time led to the decrease of flux and increase of rejection, which is because more activation solvent (hexane) molecules remained in the polyamide layer and then impeded the transport of water and salt.
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•Solvent activation before heat-treatment was proposed to improve RO membrane performance.•Solvent activation before heat-treatment was superior to solvent activation after heat-treatment.•The activation could improve the flux by 63% meanwhile retaining a high rejection (98.97%).•More activation solvent molecules could remain in the polyamide layer when increasing the activation time.
Inconel 718 (IN718) samples have been produced by selective laser melting (SLM). The effects of solution temperature, time and cooling rate as well as aging hardening on the microstructure and ...mechanical properties of SLMed IN718 have been studied. It is found that the as-fabricated IN718 is characterized with fine cellular dendrites with Laves phase precipitating in the subgrain boundaries, which is profoundly different from cast and wrought materials and needs different heat treatment schedules. The relationship between the minimum solution time and solution temperature is established and it provides a basis for the selection of solution treatment parameters. In addition, decreasing the cooling rate of solution treatment will contribute to the precipitation of strengthening phases. The precipitation temperatures of γ′ and γ″ are about the same for SLMed and wrought IN718, but the former has a faster aging response. The tensile properties of SLMed IN718 can be tuned in a large range by properly varying the microstructure. The highest elongation of 39.1% can be obtained after solution treatment (water quenching) without aging treatment and the highest yield and tensile strength (1374/1545 MPa) can be obtained after the direct aging treatment. The match of strength and ductility is able to be tailored by controlling the amount of strengthening phases, which can be realized by adjusting the cooling rate of solution treatment and aging time.
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