查看更多>>摘要：Multi-interlayer composed of Ni and microcracked Cr (MC-Cr), formed by electrochemical deposition, was applied to induce the formation of wavy microstructure enhancing bonding strength of titanium to carbon steel brazed joints with BAg45CuZn filler. Effect of microcrack density in Cr layer on microstructure evolution and mechanical properties of joints has been studied. Phase identification and formation mechanism of joints were analyzed systematically. Introducing multilayer with microcracks avoided the formation of brittle Ti-C and Fe-Ti phases near carbon steel substrate. During bonding, the diffusion of Ti atoms was hindered by compact Cr while promoted by Ni at microcracks. The inhomogeneous diffusion of Ti atoms led to the generation of a wavy microstructure unit that is crater-shaped structure, accompanied by the enrichment of beta-CuZn, Fe0.2Ni4.8Ti5 and Cu2TiZn phases. The fracture results showed that these structures improved the shear strength of joints up to 242 MPa by altering the direction of crack propagation.
查看更多>>摘要：Ultrasound is a potential method to enhance the mechanical properties during directed energy deposition (DED), however, its intrinsic strengthening mechanism has not been completely uncovered. In this study, the ultrasound-induced strengthening mechanism has been explored from perspectives of microstructure evolution (i.e., grain size and phase composition), and it has been compared with that of the heat treatment after building without ultrasound. The results indicate that the columnar grain (gamma) underwent obvious transition and refinement by inducing ultrasound, along with the changes in the morphology and size of Laves phase. Yet the second phase (i. e., gamma' + gamma '', Laves phase, and delta) of heat-treated samples was different than that of the samples obtained by ul-trasound. Moreover, combined with grains changes and the quantitative analysis of Laves phase, the tensile properties and fracture mechanism were analyzed. The experimental results demonstrate that the tensile properties were improved by both ultrasound and heat treatment. Among them, the average ultimate tensile stress of samples treated with ultrasound increased by 10% compared to the as-built samples along the scanning direction. Consequently, the main strengthening mechanism of samples by ultrasound was found to be grain refinement, while a large number of strengthening phases (gamma' + gamma '') precipitated to strengthen the matrix in heat-treated samples. Thus, clarifying strengthening mechanism is crucial for understanding the mechanical properties of as-built samples.
查看更多>>摘要：In this study, a novel approach of powder surface alloying is proposed to address the issue of insufficient densification of printed copper (Cu) from high laser-absorbing Ni coated Cu powder through laser powder bed fusion. Three types of powder consisting of original Ni-coated powder (surface adhesion), partially Ni-diffused powder (partial surface alloying) and surface Ni-alloyed powder (complete surface alloying) were adopted to validate the effectiveness of the approach. All these powders significantly decreased the high laser reflectivity of Cu powder. The surface Ni-alloyed powder successfully melted the Cu powder to eliminate the lack-of-fusion pores in the printed Cu samples such that best dimensional accuracy and highest relative density of up to 99.6% was obtained. The interface condition between outer Ni coating and Cu powder played an important role in the densification process. The solid solution Ni in the surface Ni-alloyed powder maximized the contact area, reduced the thermal resistance and enhanced heat transfer efficiency, which eventually promoted the melting of Cu powder and eliminated the lack-of-fusion pores. Ni content is also revealed as an important factor in balancing electrical conductivity and mechanical properties. The optimal Ni content has been determined as 0.4 wt% for powder with surface Ni-alloyed.
查看更多>>摘要：Nickel alloy Inconel 740H, a candidate material for use in ultra-supercritical power plants, is susceptible to solidification cracking during high power deep penetration laser welding. Here we examine how cracking is affected by welding variables and determine the locations where the cracks occur experimentally and theoretically. We use a solidification cracking model to calculate the effects of welding variables on cracking and the locations where the cracks form during high power laser keyhole mode welding of IN 740H. The parameters needed for the cracking model are obtained from a well-tested numerical heat transfer and fluid flow model for keyhole-mode welding. Model predictions of cracking and their locations for different welding conditions are verified by experiments.
查看更多>>摘要：Chemical-etched surface texturing is of great potential for processing technologies involving solid/liquid wetting due to its simplicity, efficiency, and applicability. In this study, the wetting behaviors of 4043 aluminum alloys on different 301L stainless steels in the as-received and two chemical-etched textured states were investigated by the modified sessile drop method. The wetting process at 650-750 C under high vacuum was dynamically captured by a digital camera. The spreading mechanism was revealed by combining the identification of precursor films and interfacial microstructures with the analysis of spreading dynamics. During the wetting process, the movement of the triple line relied mainly on the mechanism of subcutaneous infiltrate. Chemical-etched texturing achieved changes in roughness by adjusting the ratio of ridges and pits on the surface. As the etching time extended, the spreading activation energy decreased, the interfacial reactivity enhanced, and stronger capillary forces were induced by the rough surface of the substrate and IMCs, thereby leading to droplets spreading at a faster rate. The wettability was extremely improved with a maximum increase of more than 50% in spreading radius. The elucidation of the mechanism behind the effect of chemical-etched surface texturing on Al/stainless steel wetting can provide some guidance for brazing, welding, coating, and many other fields, and serve as a reference for the wetting research on other reactive systems.
查看更多>>摘要：Laser peen forming (LPF) is a metal forming process that utilizes laser-induced mechanical shock waves to form desired shapes or modify bent structures. The present work focuses on the applicability of LPF to Ti-6Al-4V sheets, to identify an optimal LPF process parameter window and achieve desired bending without compromising the surface quality within the peened region. The effect of LPF process parameters, i.e. laser power density, overlap, type of sacrificial overlay, and the number of peening sequences was investigated for specimens with different thicknesses. The laser power density and number of peening sequences were the most influential parameters that affect the bending of the specimens. Using sacrificial overlay has a significant effect on the bending and surface quality of the specimens. Surface quality after LPF was assessed by measuring the roughness in the peened region. In experiments without a sacrificial overlay, a black titanium oxide residue on the peened region was observed and additionally, small micro-cracks were found in the near surface region. Further characterization of the peened region revealed that the average crack length increased with increase in laser power density. Two possible LPF process parameter combinations were identified to obtain bending in the peened region, where LPF with sacrificial overlay resulted in no surface damage. Furthermore, residual stresses were determined at various LPF process parameters by incremental hole-drilling method in the peened region.
查看更多>>摘要：Mechanical surface modification such as shot peening offer powerful enhancement of fatigue properties of metals and other materials. Cavitation usually causes surface damage in hydraulic machineries. However, careful selection of process parameters allowed developing an approach known as "cavitation peening." Its advantage is surface roughness increase is lower than in conventional shot peening as there are no solid collisions involved. As cavitation is a hydrodynamic phenomenon, an understanding of both fluid dynamics and materials science is required. Cavitation peening is distinguished from "water jet peening," in which water column impulse is used. Another flavor is "Submerged laser peening" that involves the use of a pulsed laser and can be considered a type of cavitation peening where cavitating bubbles are generated due to laser ablation. Ultrasound vibration, a popular method for generating cavitation for cleaning, has also been adapted for cavitation peening. The present comparative review presents key insights and achievements and addresses future directions that are required for advancing cavitation peening technology by considering the mechanisms of cavitation peening based on the reported data for water jet, pulsed laser, and ultrasonic cavitation peening. The data and methods are critically considered and summarized in comparison with shot peening. Strategic view of future challenges is presented.
查看更多>>摘要：Al2O3-based ceramic tool has been widely utilized in hard cutting. It is more prone to failure in interrupted hard cutting because of the fierce cutting condition. In addition, green cutting has aroused great interest due to the increasing attention to environmental issues. Taking the advantages of surface microstructure into account, it is crucial to propose ceramic tool surface microstructure suitable for green interrupted hard cutting. A novel bionic multifunctional surface microstructure was designed for efficient performance improvement of Al2O3/TiC ceramic tool in green interrupted hard cutting. Minimum quantity lubrication (MQL) and vegetable oil were employed to achieve green cutting. Manis pentadactyla and honeycomb were used for reference in the design of the novel bionic microstructure with multiple functions such as impact resistance, lubricant storage and spontaneous lubricant transport. The first two functions served as the basis of spontaneous lubricant transport. Surface modification and nanosecond laser processing were utilized in microstructure fabrication. Different combinations of nanosecond laser parameters such as laser angle theta(l) and scan number ns were employed in the fabrication. This study analyzed the geometry features and various behaviors for different microstructures. Then, the external cutting load, the fracture toughness and the internal micromechanical features were integrated in a theoretical performance index. This work examined the influences of the microstructures on the performance index and the experimental performance of the ceramic tool. It was found that multifunction was achieved for the bionic microstructure. The proposed bionic microstructure provided a new way for efficient improvement of tool performance.
查看更多>>摘要：This paper addresses the problem of material removal in electrorheological (ER) polishing with a mini annular integrated electrode both theoretically and experimentally. The model of the material removal profile in ER polishing is proposed based on the evaluation of the amount of material removed from the surface along the direction orthogonal to the tool path. Firstly, the electric field distribution around the tool head is deduced theoretically. On this basis, the contact pressure model in the ER polishing area is established. Considering the feed rate and tool spindle speed of ER polishing, the distribution of the relative sliding velocity in the polishing area is modeled. On the basis above, the innovative approach to calculate the material removal profile is developed for the ER tool polishing along a straight path. The model is validated by a series of designed polishing experiments, which demonstrates that the material removal model established in this paper has high prediction accuracy of material removal profile in ER polishing.
NSTL
Elsevier