Journal of Materials Processing Technology最新文献

Continuous fiber reinforced metal matrix composites (CFMMCs) offer higher specific strength, specific modulus, and operating temperature than matrix metals due to the unique enforcement mechanism of the one-to-one scale arrangement of matrix and reinforced phases. Due to the heterogeneous characteristics between the plastic matrix and brittle fibers, the removal mechanism of CFMMCs during processing is exceptionally complex. Ultrasonic vibration-assisted grinding (UVAG) shows great advantages in machining difficult-to-cut materials (i.e., ceramics and composites) by changing the motion trajectory between grains and workpieces, effectively reducing grinding force and improving machining quality. However, little is known about the removal mechanism of UVAG for CFMMCs composed of the ductile (i.e., metal matrix) and brittle (i.e., SiC fiber) phases with highly anisotropic structure characteristics. This raises the question of how CFMMCs with heterogeneous components perform under abrasive processing and how to predict their processing forces. Hence, UVAG and conventional grinding (CG) experiments with single CBN grain were carried out on SiC fiber reinforced TC17 matrix composites (SiC_f/TC17) in this work. A grinding force model considering both phases and materials structure from energy aspect was proposed. A theoretical model for suppressing SiC fiber damage has been proposed, which is expected to guide low-damage processing of brittle materials. According to the results, the removal models of CFMMCs are revealed including: i) macro fracture of SiC fiber, ii) neat fracture of SiC fiber, and iii) TC17 matrix massive adhesion on the SiC fiber. Besides, no cracks crossing fibers are observed on the subsurface of SiC fiber under both UVAG and CG due to the good support of the TC17 matrix on SiC fibers. The grinding force predicted model error decreases as a_p increases. When a_p is 50 μm, the errors between predicted and experimental values are 7.8 % and 9.1 % for normal forces (F_n) and tangential forces (F_t), respectively. Ultrasound suppresses the severe wear behavior of grains, thereby improving the tool life. This paper aims to comprehensively reveal the characteristics of abrasive processing of CFMMCs from various aspects (surface morphology, subsurface features, grinding force prediction, and tool wear), which will promote the industrial application of CFMMCs.

To precisely control the melting of two base metals, laser offset welding is an effective method for managing the interfacial microstructure in dissimilar metal welds. Macrosegregation exhibits a markedly different morphology near the steel-nickel fusion line, which plays an important role in premature creep failure. In this study, it was found that an offset of the laser beam towards the nickel correlated with an increase in macrosegregation. When the laser beam was biased towards the steel, the partially mixed zone (PMZ) and transition zone (TZ) of 9Cr steel were identifiable within the macrosegregation, displaying a distinct solidification mode. Lath martensite and fine austenite formed in the PMZ, with alternating distributions of martensite and austenite observed in the TZ. Conversely, when the laser beam was biased towards the nickel, the unmixed zone (UMZ) of 9Cr steel was observed within the macrosegregation. Nanoindentation tests revealed that the maximum nanohardness in macrosegregation reached 5.90 GPa in the PMZ due to the formation of lath martensite, while the minimum nanohardness was 1.30 GPa in the TZ due to reduced martensite formation. Additionally, the TZ exhibited a significant reduction in resistance to plastic deformation. The various microstructures of macrosegregation were attributed to the solidification process and the dilution rate of 9Cr steel. When the liquidus temperature of WM exceeded 1455℃, a wide TZ formed in the macrosegregation due to adequate mixing. In contrast, when the WM had a significantly lower liquidus temperature than the 9Cr steel, only UMZ and PMZ formed in the macrosegregation due to preferential solidification and insufficient mixing. To minimize macrosegregation and eliminate the softening TZ, a laser beam offset towards the steel between 0.1 mm and 0.2 mm is recommended.

The formation of wavy interface is one of the distinctive features of high-velocity impact welding. It is known that the geometric parameters of waves (amplitude $a$ and length $\lambda$) depend not only on the impact conditions, but also on the mechanical properties of the materials being welded. However, until now, the impact of mechanical properties on the formation of waves has only been explored in a limited number of studies. To address this issue, in this paper, we use extensively a numerical simulation. First, we demonstrate that the numerical model, in conjunction with the smooth particle hydrodynamics (SPH) solver, effectively replicates the outcomes of a carefully controlled high-velocity impact welding experiment. Secondly, based on the validated model, we conducted a systematic study of the influence of strength on the wave formation process. Using numerical simulations with Johnson-Cook and ideal elastic-plastic strength models, we show that various characteristics of strength have a profound influence on the wave formation process. Furthermore, it is crucial to consider not only the yield strength of a material, but also factors such as strain and strain-rate hardening, along with thermal softening, to fully understand the wave formation during high-velocity impact welding.

The uneven metal flow and inhomogeneous microstructure on the cross-section of the extruded bar are mainly induced by the uncoordinated deformation during the traditional extrusion process, which seriously restricts its production and application. These defects are more prominent for the dual-phase Mg-Li alloy due to the phase transformation and the difference in flow between soft and hard phases. In order to solve the uncoordinated deformation in traditional extrusion, the reciprocating rotary extrusion (R-RE) process based on harmonic oscillation of die is proposed. The experiment and numerical simulations of the reciprocating rotary extrusion process were carried out at rotating frequency of 2.5 and 5 Hz, extrusion velocity of 1 mm/s, forming temperature of 290℃, die extrusion ratio of 12 and die rotating angle of ±6°. The coordinated deformation mechanism from macroscopical flow and microstructure in reciprocating rotary extrusion was investigated deeply. Meanwhile, a novel theoretical method was proposed to describe coordinate deformation characteristics quantitatively. The results indicated that the reciprocating rotary extrusion significantly reduces the forming load and accumulates more strain. The more uniform metal flow contributes to coordinated deformation. The extrusion deformation factors are proposed to reveal the coordinated deformation mechanism. In addition, the deformation body characteristic zone is novelly divided into six zones by combination of flow pattern and microstructure evolution.

RS-SiC faces limitations in surface accuracy with conventional finishing processes due to its high mechanical hardness and multiphase nature. In this study, vibration-assisted photocatalytic oxidation is introduced to enhance the efficiency and precision of RS-SiC diamond polishing. By utilizing the photocatalytic reaction, the hard heterogeneous SiC phase and Si phase on the RS-SiC surface are transformed into a "softer" homogeneous oxide layer that can be removed easily. The introduction of vibration increases the relative velocity between the workpiece and the abrasive and prevents the aggregation of the photocatalyst, thus improving polishing efficiency. By reducing the abrasive size to the nanometer scale, the boundary between the Si phase and SiC phase can be smoothed out to further improve the workpiece surface finish. Using TiO₂ as the photocatalyst, H₂O₂ as the oxidant, 25 nm diamond as the abrasive, and introducing vibration, an ultra-smooth surface with a surface roughness of 0.195 nm in Ra is obtained by the finishing method proposed in this paper. The proposed photocatalysis/vibration-assisted finishing approach offers a novel method for ultra-smooth polishing reaction-sintered silicon carbide.

Collapse has consistently posed a critical bottleneck in the laser penetration welding of thick plates. This study proposed a novel method involving the application of bottom airflow to suppress collapse. The suppression mechanism was elucidated through high-speed imaging and numerical simulations, and effects of airflow on the microstructure and properties of welded joints were clarified. Firstly, during the welding process, the keyhole underwent periodic changes in a "penetration-closure-penetration" cycle. With the application of bottom airflow, the cycle duration was reduced from 5 ms to 0.4 ms. The more frequent opening of the keyhole facilitated the timely release of pressure and heat from within the molten pool, thereby avoiding the downward movement even the loss of melts. Secondly, the application of bottom airflow provided an upward force of approximately 7.7 mN at the backside of the molten pool, compensating for insufficient surface tension and helping to further suppress the downward collapse of the molten pool. Finally, the bottom airflow introduced beneficial disturbances in flow and cooling, refining the bainite and enhancing the strain degree in ferrite. The changes contributed to strengthen the tensile properties of the welds. This study validated the effectiveness of bottom airflow in suppressing collapse during laser penetration welding, offering a significant solution to this technical bottleneck and presenting a new approach for large-scale equipment manufacturing.

Low-temperature copper (Cu) sinter-joining technology has attracted increasing attention in high-power electronic device packaging because of its low material cost, good electrical and thermal conductivity, low-temperature joining processes, and high-temperature service characteristics. However, due to the high oxidation risk and agglomeration tendency of Cu particles, the stability of Cu pastes is poor, the sinter-joining conditions become harsh, and the performances of sintered parts could be better. These defects heavily hinder the application of low-temperature Cu sinter-joining technology in power electronic packaging. In this review, the research of the low-temperature Cu sinter-joining technology is reviewed, including its background, development, challenges, and perspectives. Based on the sinter-joining mechanism, the advanced progress of Cu paste formulas and sintering processes are described in detail, and the mainstream strategies for improving Cu paste's stability and oxidation resistance are discussed, highlighting scientific issues and advocating for best practices in conducting low-temperature Cu sinter-joining technology. In addition, the reliability and failure mechanisms of sintered Cu joints are summarized, which provides theoretical support for further improving the performance and service life of the joint. Finally, the development trend and future research directions of low-temperature Cu sinter-joining technology are described.

Graded composite fillers exhibit unique advantages in alleviating residual stress in ceramic/metal brazed joints. However, traditional methods for preparing graded composite fillers are often complex and may deplete their strengthening phases. In this study, cold spray additive manufacturing was employed to fabricate yttria-stabilized zirconia (YSZ)-Ti₂₅Zr₂₅Ni₂₅Cu₂₅-Ag graded composite coatings on Cu substrates to serve as fillers. The graded composite coatings and the resultant Ti₃SiC₂/Cu brazed joints were systematically characterized to elucidate the deposition mechanism of the coatings and its residual stress-relaxed mechanism. The bondings between particles in the coatings, based on their plastic deformation ability, were classified into noncrystalline weak bonds and homogeneous strong bonds. Both bondings facilitated the elemental interdiffusion between filler and Cu substrate during the heating process, which contributed to the formation of Ag-Cu eutectic. This significantly reduced the required brazing temperature and its induced residual stress. Furthermore, the solid phase transformation of graded-distributed YSZ within the brazing seam from tetragonal to monoclinic achieved a smooth thermal transition and releasing residual stress at the same time. Consequently, the shear strength of the brazed joint reached to the highest value of 110.23 ± 3.45 MPa, which was Much higher than the previously reported values. This study introduces a novel method for residual stress relaxation assisted with cold spray technology and broadens its application in the brazing field.

The machining of honeycomb cores presents challenges due to exerted unbalanced cutting forces from discontinuous hexagonal structures, where severe tool wear and cracking on the machined surface occurs. In this study, the established cutting force models consider alterations of the wall thickness and immersion angles when mono-oblique and duplex-oblique cutting tools are engaged. The maximum cutting forces and the produced damages compared to the tool positions are analytically simulated and validated. The increased cutting temperatures and the shear stress gradients with respect to cutting speeds and feed rates were analysed after fitting them with the experimental results. Moreover, the vibration-induced cutting-edge-rounding and fracturing on the machined surface were identified. In the validation experiments, a wide range of cutting speeds (100–300 m/min) and feed rates (0.2–1.0 mm/rev) were operated for practical utilities. It was proven that the highest cutting forces (∼26.59 N) took place at the entry node of the hexagonal cell; whilst the cutting forces’ fluctuations (±20.35 N) under high frequency (∼2.5 kHz) and high intensity (∼ 8290 dB) could result in cutting-edge rounding from tool wear. The damage locations on the machined surfaces, as well as the corresponding fracturing mechanisms of fibres incubating the in-plane and out-of-plane cracks were verified and discussed.

Additive manufacturing (AM) is an automated process for fabricating complex structures, especially for continuous fiber-reinforced polymer (CFRP) composites with exceptional mechanical performance and reduced weight. The extrusion-based CFRP-AM process has been widely adopted but faces challenges with insufficient impregnation/adhesion and lack of geometrical accuracy. Existing research efforts exploit various mechatronics approaches and printing parameter adjustment techniques to enhance CFRP quality. Nevertheless, most of them are static or slow-varying; thus, they cannot dynamically adjust to local geometrical features and faults, which are particularly representative of the CFRP-AM process in high-curvature regions. To enable dynamic and rapid adjustment, this study exploits contact forces as major real-time feedback signals to dynamically control the feed velocity, layer height, and fiber extrusion rate. To create a continuous feedback control environment for dynamic adjustment, the study also proposes a novel helical CFRP-AM printing trajectory generation method for closed shell structures, such that the helical angles indirectly adjust the layer height of the parts. A material extrusion CFRP-AM platform combining a co-extrusion nozzle, an industrial robot, a six-degree-of-freedom load cell, and a real-time closed-loop control system is built to verify the proposed control framework. Three representative case studies are introduced to show the enhancements of the closed-loop CFRP-AM process, particularly in layer height adjustment, high-speed printing, and large curvature regions.