Journal of Materials Processing Technology最新文献

The use of SiC wafer is widespread in many fields, especially in aerospace, energy, 5 G communications, and microelectronics. Chemical-mechanical polishing (CMP) is the primary method used for achieving an ultra-smooth surface on SiC wafers. However, CMP suffers from low efficiency, leading to increased processing time and costs. To address this issue, we developed a novel diamond gel polishing disc that incorporates SiO₂/Fe₃O₄ (S/F) powder. The disc enhances polishing efficiency through a solid-phase Fenton reaction between the disc and SiC. The research investigates the reaction mechanism and the material removal model of the polishing process using SEM, TEM, and XPS analysis. Experimental studies are conducted to assess the polishing performance and validate the effectiveness of the theoretical model. The findings indicate that SiC undergo a solid-phase Fenton reaction with polishing disc mixed S/F powder (SG-S/F disc) during polishing. The Fenton reaction generates hydroxyl radicals (·OH), which break the Si-C and Si-Si bonds in the crystal structure, leading to the formation of a softer nanoscale amorphous oxide layer on the SiC surface. The cyclic generation and removal of this oxide layer enable highly efficient polishing of SiC wafers. Compared to a gel disc without S/F (SG disc), SiC polished with the SG-S/F disc exhibits superior surface quality. Additionally, the material removal rate (MRR) of the SG-S/F disc reaches 1.42 μm/h, representing a 51.1 % improvement over that of the SG disc. These results clearly demonstrate that the solid-phase Fenton reaction significantly enhances the polishing performance of the gel polishing disc.

This is the first report introducing the idea of processing Nb-silicide-based in-situ composite through electron beam powder bed fusion (PBF-EB). A high energy input together with a line order strategy resulted in cracking, whilst a lower energy input without the line order led to microstructural inhomogeneity characterised by alternating dark- and bright-contrast bands. In samples printed using the line order strategy, long cracks initiated close to the melt pool boundary and propagated into the previous layer. These cracks were due to the weak interface between the primary Nb₃Si columnar grains and the equiaxed (Nb,Ti) solid solution phase or Nb₃Si-(Nb,Ti) solid solution eutectic structure. The porosity level increased from 0.122 % to 0.547 % with the increase of area average energy input from 2.22 J/mm² to 6.67 J/mm². The density decreased to 6.80 g/cm³ in the line order strategy samples due to the presence of cracks, compared to 6.84 g/cm³ in samples without employing the line order strategy. The dark-contrast band consisted of (Nb,Ti) solid solution, Nb₃Si and Nb₅Si₃, while the bright-contrast band was composed of (Nb,Ti) solid solution and Nb₃Si. Within the bright-contrast band, a recurring layered microstructural inhomogeneity appeared, with columnar grains at the bottom, whilst fine- and coarse-grained eutectic networks in the middle and upper regions. The Nb₃Si phase exhibited a pronounced {001} texture, whereas the (Nb,Ti) solid solution phase displayed a weaker texture. The Nb-silicide-based in-situ composite fabricated through PBF-EB had a microhardness of ∼700 HV_0.5. This value is higher than that achieved through conventional means but falls within the mid-range for laser-based additive manufacturing counterparts.

The effect of superimposed low-frequency vibration (LFV) during deformation is somewhat similar to that of ultrasonic-vibration, however its mechanism is still unclear. In order to clarify the mechanism of LFV, the LFV-assisted compression experiments of Al-Zn-Mg-Cu alloy with different amplitudes were conducted. The length of the elastic deformation zone (L_E) of the material was considered when selecting the relative amplitude parameters. The macro-mechanical response, distribution of secondary phases, dislocation evolution and microhardness were determined. The results show that LFV with different amplitudes exerted different effects on the material. LFV at vibration amplitude < L_E exerted a superimposed effect on the elastic stress. The proportion of micron-diameter secondary phases with an equivalent diameter < 3 μm increased by 16.67 % and dislocation proliferation was promoted. LFV at amplitude > L_E exerted the coupling effects of elastic stress superposition and plastic vibration softening. The proportion of micron-diameter secondary phases with an equivalent diameter < 3 μm increased by 42.77 %, the proportion of nanoscale precipitates with an equivalent diameter < 2.5 nm decreased by 63.21 %, and dislocation annihilation was accelerated. This study reveals the amplitude correlation to LFV mechanism and provides a new idea for the application of vibration-assisted forming.

The accurate characterization of the anisotropic thermal-mechanical constitutive properties of structural sheet metals at elevated temperatures and under nonuniform stress/strain states is crucial for the precise hot plastic forming and structural behavior evaluation of an engineering sheet part. Traditional thermal-mechanical testing methods rely on the assumption of states homogeneity, leading to a large number of tests required for the characterization of material anisotropy and nonlinearity at various high temperatures. In this work, a highly efficient identification method is proposed that allows the simultaneous characterization of the anisotropic yielding, strain hardening and elasto-plasticity thermal softening material properties using the minimum number of tests, releasing the limitations of traditional identification methods. This is implemented by performing a digital image correlation and infrared thermography assisted heterogeneous high temperature test and processing the full-field measurement data based on the principle of virtual work. A double-notched tensile specimen configuration combined with a center-to-periphery temperature gradient is designed first to enable the high heterogeneity of stress/temperature states. Simulations of the heterogeneous tests are then performed to supply idealized reference data that can be used to evaluate the identification sensitivity of the proposed identification algorithm. After the numerical validation of the methodology, it is then applied to the TC4 titanium alloy sheet specimen using the self-developed experimental setup. The experimental identification results verify that the multiple anisotropic thermal-mechanical elasto-plasticity constitutive parameters can be accurately identified from the heterogeneous test with high computation efficiency, significantly simplifying the testing process in comparison to applying multiple traditional homogeneous tests. The current work provides an effective and convenient alternative to high temperature identification strategies used by the material processing community.

Although nanocrystalline (NC) metals and alloys have been studied for nearly 40 years, their preparation is limited to the laboratory as large-scale; low-cost commercial production remains a challenge. In this study, high-strength bulk NC Cu–Al alloys were prepared from coarse-grained Cu–Al alloy rods via rotary swaging. Rotary swaging is characterized by the low cost and infinite length of the processed samples; therefore, it can advance the industrial application of bulk NC alloys. Core–shell-structured Cu–Al alloy rods with a hard NC core (diameter of 2.2 mm) wrapped in a soft ultrafine-grained (UFG) shell with a thickness of 1.75 mm were prepared using a rotary swage. Tensile tests revealed that the hard NC Cu–Al alloy core exhibited an ultimate tensile strength of 1034 MPa, which surpassed current strength records. Microstructural characterization showed that the hard NC core was composed of NC fiber grains with widths of 45 nm and lengths of 190 nm. The edge of the rod contained numerous low-angle grain boundaries and shear bands, which provided it with a lower strength and higher elongation than those of the center. During swaging, strong (200) and (111) fiber textures perpendicular to the cross-section were produced during the early stages of deformation. In the latter deformation stages, the polar densities of the (200) and (111) textures weakened, and some complex textures were formed along with high-angle grain boundaries. The grain refinement mechanisms were dominated by multiple deformation twinning, stacking faults, and dislocation slips. Finite elemental analysis showed that triaxial compressive stress and a high strain rate were applied to grain refinement. In addition, the softer shell protects the harder core during deformation, preventing fracture. This study verified an effective preparation technique for bulk NC materials via rotary swaging because it is a simple process with low cost and broad industrial prospects.

Numerical simulations have investigated the effects of electromagnetic drive waveform amplitude and pulse width on the state of metal droplet generation during drop-on-demand printing of molten metal aluminum by electromagnetic drive. It indicates that positive and negative pulses of magnetohydrodynamic(MHD) actuation applied to the molten metal in the crucible can form a so-called “push-pull” effect, which is the main mechanism affecting droplet generation and breakup. The positive pressure pulse causes the liquid metal to be extruded outward, and the negative pressure pulse promotes the fluid retraction at the nozzle to promoting the thinning of the liquid bridge and the generation of liquid droplets. With the increase in the amplitude of the applied current, the ejected molten aluminum from the crucible experiences three states: no droplet, single droplet, and droplet with satellite droplets. In the single drop state, the droplet formation time is shortened as the current peak increases and the droplet volume increases with the increase of pulse width. The energy conversion and force changes during droplet formation and falling are analyzed. When the peak kinetic energy of the ejected molten metal exceeds its surface energy, the jet breaks up and generates a single droplet. For the actuation with constant amplitude and duration for positive pulses, extending the duration of negative pulses weakens the "pull" effect, leading to a transition from the generation of a single droplet to multiple droplets. The numerical simulation results provide information on the current amplitude, pulse width, and Weber number range necessary for stable single droplet generation.

Improving uniform elongation in metals typically involves enhancing the work hardening rate, as elevated work hardening can delay necking and fracture. However, our investigation into commercial pure titanium reveals a counterintuitive relationship between these properties. We find that high uniform elongation correlates with low work hardening capability, while a high work hardening rate results in reduced ductility. Two types of ultrafine-grained pure titanium, prepared by rotary swaging, subsequent rolling, and annealing, exhibit different mechanical properties. Microstructural and deformation mechanism analyses reveal that the difference arise from variations in texture. Specifically, extensive activation of <c+a> dislocations in the former sample leads to premature, intense work hardening that is quickly exhausted, while the latter sample shows a steady, uniform work hardening progression that delays necking. Our findings challenge the conventional understanding that high work hardening rates ensure high uniform elongation. Instead, we propose that optimizing ductility requires a strategic allocation of work hardening throughout the tensile deformation to delay necking. This study reveals the intrinsic relationship between work hardening and ductility, offering new strategies for designing stronger and tougher materials.

Additive manufacturing technologies, such as laser powder bed fusion (LPBF), have attracted a significant amount of attention for their capability in fabricating components of complex geometries with improved efficiency and design flexibility. However, the presence of pores in additively manufactured high pressure die casting dies has emerged as a critical issue affecting the performance and reliability of the dies. A numerical model for the formation of pores due to the instability of the keyhole and molten pool in a 18Ni300 steel is created. Computational fluid dynamic (CFD) simulation revealed that the keyhole in the molten pool undergoes inward ebbing and is wrapped by the liquid interface behind the laser beam, forming gas bubbles along the laser tracks. Some of the bubbles will either coalesce or breakup in remelting by the laser beam above in building the next layer forming a string of pores along the LPBF build direction, while others remain unchanged. The line distributed pores are the primary sites for cracking, and the cracks grow along the LPBF build direction. As a result, these longitudinal cracks cause the premature failure of dies with conformal cooling channels (CCC).

The fabrication of superhard micro milling cutters with extreme sharpness is highly demanded to meet the strict requirements of micro structures in terms of machining accuracy, surface roughness and burr height. In this study, nanosecond laser induced graphitization assisted grinding was proposed by considering machining efficiency and machining quality simultaneously for chemical vapor deposited (CVD) diamond micro flat-end milling cutters. An optimal cut-section was obtained by adopting moderate laser cutting energy and avoiding unstable thermodynamic coupling effect. The following laser induced graphitization mechanism on the cut-section was studied by analysing the different graphitization behaviours. The extensive graphitization caused sintered and banded graphite while the weak graphitization could not eliminate the defects caused by laser cutting. The uniform and dense graphitization resulted from appropriate energy distribution contributed to the smooth transition layer which was the significant factor in subsequent grinding. Moreover, the subsequent grinding behaviours of different transition layers were investigated, the grooved defects on transition layer caused the unstable grinding and fracture removal model, leading to the cracked ground surface and blunt cutting edge. Nevertheless, the grinding model of smooth transition layer was scratching without transcrystalline cracks, resulting in the smooth ground surface and sharp cutting edge. Thus, the laser parameters were finally optimized based on the feedback. Furthermore, the even graphitization, undamaged transition layer, scratching, as well as less material stress resulted in the extreme sharpness. In this way, the CVD diamond micro flat-end milling cutter (CVDM) with a diameter of 200 μm, surface roughness Sa of 30 nm, aspect ratio of 3, and edge radius of around 0.12 μm was prepared, the extreme sharpness was indicated by comparing with the edge radius of 1–5 μm reported heretofore. Finally, the micro grooves with surface roughness Sa of 17.8 nm and tiny burrs were machined on Ti-6Al-4 V by the homemade CVDM, which thereby indicated the outstanding cutting performance of the CVDM in micro-milling.

Ultrasonic vibration was applied when soldering carbon fiber-reinforced aluminum composites (C_f/Al) in this work. The effects of ultrasonic action time on joint formation, carbon fiber distribution, and mechanical properties of the joints were studied. Results show that, with the assistance of ultrasonic vibration, inactive solder can wet the carbon fiber within dozens of seconds at low temperature (250 °C). Short ultrasonic action time leads to minor erosion of aluminum substrate, thereby restricting the movement of carbon fibers into joint. Extensive erosion of the aluminum occurs with prolonged ultrasonic action time. A uniform distribution of carbon fibers in joint is achieved at 60 s. Wetting of carbon fiber is characterized by amorphous-nanocrystalline oxides on its surface, which can be attributed to the elevated temperature and pressure induced by the cavitation of the solder. Extending ultrasonic time enhances the hardness and the shear strength of the joint. The failure cracks propagate through the substrate when ultrasonic time exceeds 15 s. This work provides referential value when soldering materials with low surface energies.