Journal of Materials Processing Technology最新文献

Ceramic coatings have great potential in protecting vital metal structural components in hydrogen energy and nuclear fusion reactors. Fracture toughness and hydrogen permeation resistance are two key characteristics that determine life span and performance of these coatings. However, there is still no strategy to simultaneously enhance both of these two characteristics. Herein, we developed a one-step approach to fabricate AlPO₄ nanosheet reinforced α-Al₂O₃ ceramic coating with simultaneously enhanced fracture toughness and hydrogen permeation resistance. Different from the external nanosheet addition method of the conventional techniques for fabricating nanosheet reinforced coatings, AlPO₄ nanosheets were in-situ formed within α-Al₂O₃ ceramic coating though a one-step heat treatment process. Owing to the positive reinforcing effect of AlPO₄ nanosheets, the fracture toughness of the coating increases by 3 times, achieving an ultrahigh K_IC value of 7.1 MPa/m². Meanwhile, the hydrogen permeation resistance of the coating increases by 78 %, reaching as high as 3440 times that of the steel substrate. Additionally, the Cr–P bonds formed between the coating and substrate ensure their good bonding, with their bonding strength increasing up to 36 MPa. The combination of high hydrogen permeation resistance, high fracture toughness, and high bonding strength makes the AlPO₄ nanosheet reinforced ceramic coating a promising alternative for reducing hydrogen permeation in various hydrogen-related fields. This study also provides critical insights and practical guidelines for constructing high-performance functional coatings via nanosheet reinforcement.

Torsion extrusion (TE) is a severe plastic deformation (SPD) technique with the potential to be used for the preparation of large bulk materials with enhanced mechanical properties. Shear deformation is the key factor for SPD to enhance the materials properties. However, it is not clear by far that what shear strains are dominant in torsion extrusion process and how they distribute in the extruded materials. These issues are crucial to the working parameters design of the torsion extrusion process for maximizing the shear effect. To this end, a plasticine billet with two colors was applied as experimental material, and the material flow pattern was revealed by measuring the twisted bicolor interfaces. The torsion extrusion was implemented using a die with a wavy cross-sectional bearing segment (W-TE), which is able to prevent the circumferential sliding of the materials against the die. Flow velocities were re-built according to the bicolor interfaces at each section in a cylindrical system. Strain rates and shear strains induced by torsion deformation along the radius of the material were formulated based on the velocity field and Eulerian variable derivatives, from which the dominant shear strain can be recognized. A shear effect index (SEI) was proposed to evaluate the contribution of the shear strains induced by torsion, and the distribution of SEI was analyzed. Furthermore, the W-TE experiments on the as-cast TiB₂/2026 Al composite were carried out to demonstrate the positive correlation between the shear effect and the microstructural improvement, and finite element simulation for such experiments showed the consistency of the simulated deformation patterns with the plasticine measured ones. The evaluation method and the experimental results deepened the understanding to the shear effect of torsion extrusion process.

Yttria-stabilized zirconia (YSZ) is an outstanding ceramic material with applications in dentistry, biomedical, and mechanical device, where a smooth and durable surface is required. This study employed ultrasonic vibration-assisted burnishing (UVB) to enhance the surface hardness of YSZ while simultaneously reducing its roughness. The surface topography formation, subsurface microstructure evolution, and surface hardness change of YSZ were investigated. It was found that a smooth surface with an average roughness (Ra) of 0.12 μm was achieved using UVB, representing 67.6 % reduction in surface roughness from the original sintered surface, which is attributed to the burnishing-to-cutting phenomenon during the processing. The UVB-induced compression and material densification prevented the formation of the monoclinic phase during the processing of YSZ. Grain refinement and crystal lattice distortion occurring in the subsurface layer led to an increase in surface hardness, reaching up to 19.7 % higher than that of the original sintered surface. Moreover, the UVB-processed surface demonstrated a high resistance to mechanical impacts, effectively inhibiting the tetragonal-to-monoclinic phase transformation in indentation testing. These findings demonstrate that UVB is an effective approach for enhancing the surface and subsurface properties of the material.

Friction stir welding (FSW) is widely used to join large-size Al alloy components in advanced manufacturing industries with high requirements for mechanical properties. However, due to the huge differences in microstructure and local mechanical properties between different regions of FSW Al alloy joint, early yielding and strain localization tend to occur in low-strength regions, limiting the further improvement of the tensile properties. In this study, laser shock peening (LSP) was applied as a post-weld surface treatment to FSW Al-Cu alloy joints, resulting in increases of 52 %, 10 %, and 21 % in yield strength, tensile strength, and elongation, respectively. The effects of LSP on local mechanical properties, microstructure and overall mechanical properties were systematically studied. Compared to the stirring zone LSP induced more significant grain refinement and higher dislocation density in the thermo-mechanically affected zones (TMAZs) due to the lower initial dislocation density and fewer precipitates presented in the TMAZs. LSP generated more significant grain boundary and dislocation strengthening on TMAZs to obtain high strength improvement and achieve local mechanical property homogenization. The homogenization of local microstructure and mechanical properties hindered strain localization, facilitating the full utilization of the dislocation storage capacity of each region to accommodate plastic deformation, thereby enhancing the overall elongation of the joint. The research findings provide new insights based on LSP for improving the tensile properties of heterogeneous welded joints.

Efficient and cost-effective manufacturing of turbine slots has always been a challenge for aeroengine production. Wire electrochemical machining (WECM) is a feasible alternative because of its inherent characteristics. However, it readily leads to profile errors because of the uneven material removal that results from an inconsistent electric quantity when trimming complex profiles in an equidistant offset. To improve the machining accuracy of WECM for trimming turbine slots, this study proposes an electric-quantity manipulation strategy that regulates the trimming velocity of wire electrodes to facilitate equal material removal for complex profiles. The evolution of complex profiles was revealed through simulation, which led to the derivation of profile shape-dependent, regulated trimming velocities that enabled an equal electric quantity and amount of material removal. Subsequently, the electric quantity distribution, material-removal depth, and profile deviation of the turbine slot structure with and without the manipulation strategy were compared through simulations and experiments. The surface integrity of the machined turbine slots was verified. The experimental results indicate that the WECM trimming technique with the manipulating strategy prevents overcutting at convex arcs and undercutting at concave arcs, reduces the overall profile deviation of the turbine slot from ±50 to ±18 μm, and reduces the deviation of the mortise–tenon contact surface from ±30.7 to ±3.6 μm compared to that without the manipulating strategy. The fir-tree turbine slots were prepared efficiently and precisely via one-pass WECM trimming. The equalizing electric quantity strategy effectively improves the machining accuracy of WECM trimming for a complex profile.

The inter-electrode gap (IEG) is a key factor in electrochemical machining (ECM), which directly governs the electric resistance of machining and affects the flow field. In conventional electrochemical milling, the actual IEG expands with the material removal of the workpiece, which increases the electric resistance and renders the electrolyte flow ineffective in transporting the electrolytic products. In this paper, a sinking push mode for electrochemical milling is proposed to minimise the IEG, thus improving material removal rate (MRR). Under a small IEG, electrochemical discharges are observed and damages the workpiece. Arising from this observation, electrochemical discharges are intentionally introduced to further improve MRR. And the material removal process is transformed from mere ECM to electrochemical discharge machining (ECDM). Furthermore, a novel ECDM-ECM mode is developed to eliminate the recast layer produced by discharge action. In this mode, the machining behaviour from ECM to ECDM can be altered by simply manipulating IEG distribution. Multiphysics simulations coupling electric field and flow field are conducted to better understand the mechanisms of the proposed modes. The IEG distribution, transient current behaviour, MRR, energy efficiency, surface integrity and tool wear are discussed by experiments. The ECDM-ECM mode successfully eliminates the recast layer with high MRR in a single controllable process, demonstrating its potential for producing high quality surfaces with high throughput.

Edge crack formed during the hot rolling process significantly affected the quality of high-grade non-oriented electrical steel (NOES), posing substantial challenges to production. This study investigated the formation mechanism of serrated edge cracks in hot rolled strip, characterized by unusually coarse strip-like grains with typical thicknesses ranging from 0.5 to 0.8 mm and lengths ranging from 10 to 15 mm. These coarse strip-like grains evolved from abnormal columnar grains at the edges of reheating slab. During the continuous casting process, the bulge phenomenon occurred easily in high-grade NOES slabs, due to its lower yield strength at high temperatures. This bulge induced high internal stress at the slab edge, leading to abnormal growth of the columnar grains during the slab reheating process at high temperature. The orientation of coarse grains gradually changed to a rotated cube texture ({100}<011>) throughout the hot rolling process. These grains with rotated cube texture exhibited fewer slip systems and lower deformation storage energy, which is not conducive to the plastic deformation in thickness direction. Consequently, coarse grains at the slab edge deformed along TD direction and overflowed towards the lower surface of hot rolled strip, resulting in the original edge boundary enveloped by the overflowed metal to generate crack source. Finally, the serrated edge crack was formed from crack source under tensile stress during finish hot rolling.

Microstructures and mechanical properties are closely related to thermal cycles during additive manufacturing. For maraging steel, the research on the effect of thermal cycles during additive manufacturing is limited. Based on the above issues, this work investigated the effect of thermal cycles in the process of wire arc additive manufacturing Co-free maraging steel on microstructure evolution and mechanical properties, and attempted to establish the relationship between thermal cycles and microstructure as well as mechanical properties of maraging steel on the basis of quantitative thermal cycle data. The results show that in the additive manufacturing process, the thermal cycles affect the cooling rate, so that the primary dendrite arm spacing and grain size gradually increase along the height direction. For maraging steel, in additive manufacturing, welding or other hot processing processes, the thermal cycles make the martensite reverse change, resulting in an increase in austenite content, resulting in grain refinement. Thermal cycles in additive manufacturing result in differences in the grain size, grain boundary ratio, dislocation density and primary dendrite arm spacing, resulting in inhomogeneity of the mechanical properties in the height direction. The difference in microstructure in different directions of additive manufacturing samples leads to anisotropy of tensile properties. The results of this work can elucidate and refine the action mechanism of thermal cycles on maraging steel. In addition, this work can be used to control thermal cycles by changing the process and cooling conditions, etc., to obtain maraging steel samples with homogeneous or gradient properties, which is highly important.

In this work, a high frequency induction heating (HFIH) assisted plasma arc welding (IPAW) technique is proposed to weld 6 mm thick S690QL high strength low alloy steel plates in square butt joint configuration and without using any filler material. A 3D finite element based coupled electromagnetic-thermal analysis is carried out to ascertain the weld's thermal characteristics. Microstructure evolution and mechanical performance are investigated using scanning electron microscopy, electron back scattered diffraction, transmission electron microscopy, tensile test, Charpy impact test and micro hardness test. The results demonstrate that 15 s initial static heating using a co-directional current coil with magnetic flux concentrator predominantly improves the weld penetration by 48 % and joint efficiency reaches 90 % to the base plate strength. Microstructural study shows that the addition of HFIH promotes low angle grain boundaries (LAGB) with bainite ferrite and granular bainite micro-constituents in the fusion zone, which causes localised yielding and failure in this zone during tensile test. Further investigation reveals that the HFIH encourages the formation of BI-type bainite ferrite laths of 1.3–2.2 μm width, combined with slender martensite-austenite (M-A) island at the bainite ferrite lath boundaries. The results also reveal that the induction heating promotes the formation of M₂₃C₆ precipitates and B2 structured Cu-enrich nano precipitates in the fusion zone (FZ). The microhardness distribution indicates that the FZ and coarse grain heat affected zone are significantly reduced due to the assistance of the HFIH. The impact test result shows a lower energy absorption by the IPAW weldments due to dominance of the LAGB with unfavourable strain distribution.

Perforated sheets exhibit strong anisotropy related to the arrangement of holes during plastic deformation, which poses significant challenges for accurate prediction of the macroscopic plastic behavior in stamping. Addressing this, unit cell simulations were conducted to determine the homogenized yielding and hardening behaviors of perforated sheets with hexagonal or square arrays of circular holes under in-plane loading conditions. The local deformation mode, which determines the macroscopic anistropy, is unveiled. A mechanism-motivated homogenized yield criterion was proposed based on the local deformation modes, which provides a concise yet unified approach to modeling complex material anisotropic behavior with high accuracy. Additionally, a mixed hardening strategy was developed to capture the evolution of yield loci in term of size and shape. The proposed constitutive model demonstrates precise predictions of flow stress variations and the apparent r-value with loading angles during uniaxial tension. Furthermore, it successfully forecasts two distinct types of earing profiles in the deep drawing of perforated sheets with square arrays of circular holes at different hole fractions. This modeling approach provides a feasible way for predicting the deformation behavior of perforated sheets during stamping with high computational efficiency.