Journal of Materials Processing Technology最新文献

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.

During laser welding of pure copper, instabilities such as violent molten pool oscillation, large spatters, and melt ejection severely damage weld quality, which are caused by copper’s high reflectivity on commonly used infrared laser. The seriously unstable molten pool and keyhole and complicated laser-material interactions result in complex process signal emission waveforms, adding to the difficulties in process stability monitoring tasks. In this work, to break down different contents in the complex signals and deeply analyzing signal-process relation, combinative spatial optical sensor system was designed, and time-frequency signal analysis in multi-scale windows was performed. It was found that the infrared radiation at the front and end side of molten pool indicates the oscillation behavior of liquid metal surface, and the signal fluctuation patterns of visible radiation from different height of metal vapor varied when meeting severe instability like melt ejections. Signal features were extracted based on the understanding of process mechanism and signal behaviors. A cascade model combining Artificial Neural Network (ANN) and Support Vector Machine (SVM) was introduced to predict weld seam quality, where the ANN model focused on short-time stability status perception and the SVM model was used to decide macroscopic seam formation defects based on combining outputs of the ANN model in a long-term sampling window. Application results showed that the recognition accuracy of pit was 100 % and the accuracy of uneven toe reached 86.3 %. The multi-source signals of unstable molten pool recognized by the cascade model were summarized. The evolution process of copper molten pool ejection was revealed.

As the scale of additive manufacturing process (e.g., laser spot size, powder particle size, powder layer thickness) decreases, the application of micro laser powder bed fusion (μ-LPBF) involves novel mechanisms for process, microstructure and performance coordination. This study provides a systematic view of the processing window and performance enhancement of high-strength Al-Mg-Sc-Zr alloy fabricated by μ-LPBF. The effects of μ-LPBF parameters on defect control and densification activity of the printed parts were analyzed, so as to obtain the suitable processing window. The influence of building orientation and heat treatment on microstructural characteristics and mechanical properties of μ-LPBF processed parts was studied. The μ-LPBF Al-Mg-Sc-Zr exhibited sound surface quality (R_a of 6.088 μm) and considerably refined grains with an average size of 1.102 μm, which was related to the high cooling rate (8.6 × 10⁷ K/s) induced by a small-sized laser beam (25 μm) and a tiny powder particle size distribution (2–20 μm) applied in μ-LPBF. After aging treatment (325 °C/4 h), the superior ultimate tensile strength of 590.24 ± 4.75 MPa combined with the sufficiently high elongation of 11.99 ± 1.17 % was achieved. Due to the significantly decreased scale of μ-LPBF production, the anisotropy caused by the variation of building directions was negligible. These enhanced mechanical properties were attributed to the combined effect of the grain size refinement, the higher number density (1.2 × 10²⁴/mm³) of interior precipitates within grains, and the small-sized molten pool size of μ-LPBF. Computational fluid dynamics (CFD) simulation was applied to reveal the molten pool thermodynamics, indicating that a higher thermal temperature gradient (up to 9.8×10⁷ K/m), a smaller molten pool size (with the width of 38.7–69.8 μm and depth of 8.7–30.0 μm) were generated in μ-LPBF. This work presents great potential in processing high-precision metallic components with fine structural feature size and satisfactory manufacturing quality.

The characteristics of a new alternating current (AC) heterogeneous twin-wire indirect arc welding (TWIAW) process were comprehensively studied. A mathematical model of non-uniform melting was designed to evaluate the control stability of a twin-wire indirect arc and also in guiding the wire feeding speeds. The effect of AC frequency, current amperage, and current waveform on the stabilities of indirect arc and droplet transfer characteristics were investigated. The results show, the arc was relatively stable within an AC frequency range of 2.5–10 Hz. Droplet transition diagram and force analyses have been established to study the arc characteristics and droplet transfer mechanisms, respectively. Current waveforms with peak and base currents were investigated to study the stability of droplet transfers. Current waveform with two peak values and one base value improved the stability of the arc. When the peak current was 130 A/10 ms and the base current was 110 A/40 ms, the arc shape and droplet transfer were the most stable, along with a continuous and smooth weld seam. Through theoretical analysis and experiments, the rules of heterogeneous double-wire AC indirect arc current voltage and droplet transfer were clarified for the first time, and a waveform adjustment method that can control droplet transfer was designed. The heat input and the amount of cladding metal can be controlled by adjusting the current waveform. It has stable droplet transfer and extremely high cladding efficiency, which is of great significance to the indirect arc process in the heterogeneous multi-wire arc additive manufacturing of high-strength aluminum alloys.

In order to overcome the defects during the process of additive manufacturing for AA6061, a novel ultrasonic rolling assisted direct energy deposition method with semi-solid thixo-forming characteristics was proposed in this study, which used cold deformed AA6061 wire as raw material, and a corresponding process platform was established. The results showed that under the conditions of laser heating temperature of 750 ℃ and heating plate temperature of 200 ℃, the sample could achieve continuous multi-pass deposition while retaining some of the cold-drawn plastic deformation energy. When the vertical deformation rate was 0.153 and the ultrasonic amplitude was 20 μm, the microstructure of deposition sample after semi-solid heat-treat exhibited typical equiaxed grain characteristics, corresponding to average grain diameter of 95.6 μm and shape coefficient of 1.263. Thin-walled samples with different shapes were formed to verify the capability of process platform. The inference of the formation mechanism of semi-solid microstructure was revealed at last: the introduction of ultrasonic rolling supplemented the plastic deformation energy, and semi-solid heat-treat provided temperature and time for the incubation of equiaxed grains, both of which were crucial for the formation of the semi-solid microstructure.

Plasma anisotropic etching polishing (plasma-AEP), a non-contact polishing method, is proposed to achieve highly efficient planarization of polycrystalline diamond (PCD) films. Inductively coupled plasma, with a high concentration of reactive radicals, serves as the source of plasma-AEP. In-situ observation confirms that the planarization effect of plasma-AEP is realized through the preferential removal of the top areas of the pyramid-shaped protrusions, despite the entire surface being uniformly irradiated by the plasma. The material removal rate in plasma-AEP for PCD achieves 127 μm/min. Plasma-AEP is proven effective for PCD films with thicknesses of 0.5, 1, and 2 mm, demonstrating a generic semi-finishing approach for PCD regardless of thickness. Atomic-scale nudged elastic band calculations revealed that the energy barriers for CO and CO₂ desorption from 1- and 2-coordinated C atoms are significantly lower than those for 3- and 4-coordinated ones. ReaxFF molecular dynamics simulations showed that at the top areas of the pyramid-shaped protrusions, 1- and 2-coordinated C atoms with a higher etching priority remained dominant during plasma-AEP, leading to the preferential removal of C atoms forming these protrusions. Furthermore, contact polishing was added to complete the finishing of the PCD film, followed by plasma-AEP, resulting in a nanoscale smooth surface with a roughness of 3.4 nm. Transmission electron microscopy confirmed that the crystal structures on the surface and subsurface of the PCD film were well ordered. Overall, this paper displays that plasma-AEP is a promising approach for highly efficient semi-finishing of PCD films.