International Journal of Machine Tools & Manufacture最新文献

In the laser powder bed fusion (LPBF), the grains grow in preferential directions depending on the scanning strategies, which results in layer-by-layer builds of particular crystallographic textures. The unique microstructure formed by LPBF results in anisotropic properties of the built structure at both macro and micro levels. To understand the grain deformation of the textured alloy fabricated by LPBF in the high-strain-rate shear process, Alloy 718 was used as an example in this work. Bulk samples with different metallurgical textures were deliberately fabricated by LPBF via three laser rotation angles, namely 0°, 67° and 90°, and then four thin slices obtained from bulks were subjected to “quasi-in-situ” grain deformation investigation through orthogonal cutting (a simple shear loading condition). The evolution of crystal orientations and morphologies, including size and shape, were traced before and after shear deformation. A full-field crystal plasticity simulation was used to quantify the stress status for grains obtained from EBSD data. This for the first time reveals the crystallographic level deformation history for hundreds of microns during a high strain rate shear removal deformation. Due to the carefully retained deformation history (i.e., typical bulges and slip bands) on the surface, a repeated deformation pattern was observed, attributing to the non-homogeneous deformation of typical build-directional blocks. The most active slip trace of deformed grain was calculated and verified based on the dominated slip bands within individual grains. The slip trace direction and intensity were quantified for different textured Alloy 718. Since the slipping-based deformation for an orientated grain is represented by its most active slip trace, a deformation tendency map is obtained by combining the shear direction, slip system and grain morphology. It reveals that grains in high texture intensity workpieces generally follow the macro shear-based deformation, while with the decrease in texture intensity, the plastic anisotropy is significant at the grain scale. Grains with similar orientations may also result in localised deformation anisotropy due to the different morphologies.

In robotic milling with large allowance process, low-frequency chatter (LFC) is an important factor observed in high-speed and low-speed milling, affecting the processing efficiency and quality. Previous research has used the regenerative chatter theory, ignoring modulated tool-workpiece engagement conditions, or mode coupling theory under the assumption of threading operations to explain the LFC mechanism and predict the stability boundary. However, these models overlook or inaccurately characterize the modulation effect, leading to inaccurate modeling of dynamic chip thickness changes during milling, making it difficult to understand the mechanism of LFC. Here, we propose an LFC stability model that considers modulated tool-workpiece engagement conditions and the mode coupling effect of the robotic structure for robotic milling. This approach allows us to reveal the mechanism of LFC and identify the characteristic signal of low-frequency vibration, which is the sideband frequency signal. Initially, the evolution of LFC is analyzed, and its characteristics are summarized. Further, a surface renewal (SR) model is proposed to accurately calculate the dynamic cutting force caused by modulated tool-workpiece engagement conditions in LFC. Furthermore, the LFC stability model, considering the modulated tool-workpiece engagement conditions and mode coupling effect, is established based on impulse response function (IRF) method. Finally, we verify the accuracy of our model through milling experiments and compare it with that of the classical stability prediction model. Our results show that LFC is highly dependent on speed, and our stability model can effectively predict the stability boundary of LFC in robotic milling with large allowance process.

Making laser powder bed fusion (L-PBF) additive manufacturing process sustainable requires effective powder recycling. Recycling of Ti6Al4V powder in L-PBF can lead to powder oxidation, however, such impact on laser-matter interactions, process, and defect dynamics during L-PBF are not well understood. This study reveals and quantifies the effects of processing Ti6Al4V powders with low (0.12 wt%) and high (0.40 wt%) oxygen content during multilayer thin-wall L-PBF using in situ high speed synchrotron X-ray imaging. Our results reveal that high oxygen content Ti6Al4V powder can reduce melt ejections, surface roughness, and defect population in the built parts. With increasing oxygen content in the part, there is an increase in microhardness due to solid solution strengthening and no significant change in the microstructure is evident.

Whilst the adoption of additive manufacturing (AM) of aluminum alloys is relatively slower compared with that of steels and titanium alloys, it has undergone a flourishing trend in the past 15 years. Significant progress, such as the development of novel processes, novel alloys, novel heat treatment profiles, and applications, has been made through the combined efforts from academic and industry fields. This state-of-the-art review presents a detailed overview of the process technology, microstructure, and properties of different aluminum alloys and aluminum matrix composites fabricated using various additive manufacturing technologies, including laser powder bed fusion, electron beam powder bed fusion, laser powder direct energy deposition, wire arc additive manufacturing, binder jetting, and additive friction stir deposition. The pros and cons of each technology in fabricating aluminum alloys are evaluated. As the dominant additive manufacturing technology for aluminum alloys, an emphasis is put on the laser powder bed fusion technology by reviewing the effect of various factors, such as post-heat treatment, powder feedstock, oxidation, and element evaporation, on the microstructure and properties. We close the review with the outlook listing the remaining challenges associated with the additive manufacturing of aluminum alloys.

Conventional additive manufacturing produces coarse columnar grains, which affect the mechanical properties of additively manufactured titanium alloys. This study developed a novel integrated additive manufacturing technology termed oscillation laser melting deposition, including linear, circular, 8-shape, and infinite, was developed to modify the microstructure and improve the mechanical properties of Ti6Al4V. The results showed that significant grain refinement and columnar-to-equiaxed transition (CET) can be induced by laser oscillation. The prior β grain size of the sample with infinite laser oscillation decreased by 54.24% in the single-track zone and by 42.55% in the overlap remelting zone. The ultimate tensile strength of the sample with infinite laser oscillation increased by 16.95% and 32.37% in the parallel and vertical directions, and the elongation also increased by 83.60% and 13.77%, respectively. The anisotropy of (10-10) and (11-22) was also significantly eliminated. The temperature variation and thermal field evolution were also investigated, and the complex oscillation changed the fluid flow velocity orientation, reduced the temperature gradient, and promoted the nucleation of equiaxed grains. In addition, the strengthening mechanisms of the different laser oscillations were revealed. Therefore, the oscillation laser melting deposition technology can become a new approach for overcoming the key bottlenecks of additive manufacturing.

The formation of burrs is among the most significant factors affecting quality and productivity in machining. Burrs are a negative byproduct of machining processes that are difficult to avoid because of a limited understanding of the complex burr formation mechanisms in relation to cutting conditions, including both process parameters and tool condition. Thus, the objective of this work was to characterize burr formation under finish machining conditions via a high-speed, high-resolution in-situ experimental method. Various parameters pertaining to burr geometry such as height, thickness, and initial negative shear angle were measured both during and after cutting. Results showed that varying the conditions of uncut chip thickness, tool-wear, and cutting speed all have a significant effect on burr formation, although certain burr metrics were found to be insensitive with respect to different process conditions because the difference was statistically insignificant. This study provides new insights into the relationships between the workpiece material's microstructure, machining parameters, and tool condition on both crack formation and propagation/plasticity during burr formation. Using digital image correlation (DIC) and a physics-based process model not previously utilized for burr formation analysis, the displacement and corresponding flow stress were calculated at the exit burr root location. This novel semi-analytical approach revealed that the normalized stress at the exit burr root was approximately equal to the flow stress for a variety of different conditions, indicating the potential for model-based prediction of burr formation mechanics. Finally, this study investigates factors that influence fracture evolution during exit burr formation. It was found that negative exit burrs are a direct result of high strain rate and high uncut chip thickness, which was expected, but also a microstructural size effect and a tool-wear effect, neither of which have been previously reported. By harnessing ultra-high-speed imaging and advanced optical microscopy techniques, this manuscript deals with the fundamentals of burr formation, including new insights into material response at the grain-scale to the loads imposed with both sharp and worn tools.

Nanoparticles patterned on stretchable films for broad applications lack efficient fabrication methods. In this study, femtosecond laser-induced transfer was employed to assemble nanoparticles into a well-defined array on a flexible substrate while mitigating the inevitable plasmon resonances. The metal islands patterned on the substrate are regularly transferred as spherical nanoparticles onto the polymer, with a small deposition deviation and large embedded depth after laser irradiation. However, inhomogeneous laser absorption in the patterned array severely amplifies the printing deviation and narrows the process window, particularly for smaller patterns and complex arrangements. Plasmon resonance excited by an incident laser causes a localized optical field distribution, which accounts for absorption enhancement or suppression. The field distribution from the numerical simulation exhibited periodicity related to the laser parameters and array geometry. A theoretical model was established to clarify the propagation of plasmon resonance waves. The field distribution was modulated by adjusting the polarization direction, guided by theoretical and simulation analyses. Finally, regular and complex nanoparticle arrays were successfully fabricated after tuning the plasmon resonances. This study provides an effective method for fabricating programmable nanoparticle arrays on flexible films.

Additive manufacturing (AM) offers unprecedented design freedom and manufacturing flexibility for processing complex components. Despite the numerous advantages of AM over conventional manufacturing methods, there are still some issues and bottlenecks that hinder the wide-scale industrial adaptation of AM techniques. The emerging field-assisted additive manufacturing (FAAM) is a designation that combines different auxiliary energy fields (e.g., ultrasound, magnetism, etc.) to overcome limitations in AM by benefiting from the intrinsic advantages of auxiliary fields. This work provides an up-to-date and dedicated review of FAAM in metallic materials, assisted by mainstream auxiliary magnetic, acoustic, mechanical, and thermal fields, as well as some emerging fields. The work principle and interaction mechanism between the field and the deposited metallic materials are elucidated. FAAM processes simulation and modelling are also reviewed. The auxiliary fields can affect the melt pool convection and dynamics, alter the temperature profile and thermal history during material solidification and induce stress or plastic deformation to the deposited materials. Hence, the effects of the auxiliary fields on the melt pool dynamics, solidification kinetics, densification behaviour, microstructure and texture, mechanical properties and fatigue performance are reviewed and discussed in detail. The perspectives on the research gap and further development trends of FAAM are also discussed.

Wire-arc directed energy deposition (DED) has attracted significant interest for the fabrication of large-sized, lightweight Mg-alloy components. However, these components generally exhibit poor mechanical properties and limited corrosion resistance owing to their inherent residual stress and non-equilibrium microstructures. Herein, laser shock peening (LSP) was adopted to successfully modify the stress state and microstructure of AZ31 Mg-alloy fabricated using wire-arc DED. The influence of LSP on the residual stress, mechanical properties, electrochemical behaviour, and microstructural evolution was systematically investigated. The experimental results indicate that, compared with the as-built specimen, the performance of the LSP-treated specimen was notable, with a ≈63.8% decrease in the corrosion current density and ≈30% and ≈13% decreases in the yield strength (YS) and ultimate tensile strength, respectively. The enhanced corrosion resistance can be attributed to the LSP-induced compressive residual stress, nanograins, and nanoparticles. Nanocrystallisation, particle refinement, dense mechanical twins (MTs), and planar dislocation arrays (PDAs) jointly contributed to the enhancement of the YS. The LSP-induced nanocrystallisation was rationalized by the accumulation of PDAs, the intersection of multiple nano-MTs, and the transformation of nano-MTs blocks into sub-grains and then into nanograins owing to continuous dynamic recrystallisation. The particle refinement mechanism involved dislocation proliferation and the development of dislocation slip bands, which eventually led to fragmentation and separation. Therefore, this study introduces a LSP post-treatment technology for the residual stress regulation, microstructural modification, and performance enhancement of Mg alloys fabricated using wire-arc DED. Based on the ability of LSP to tailor the microstructure and performance of Mg alloys, a novel method of wire-arc DED with online LSP treatment is proposed. This method can achieve in-situ surface strengthening and the integrated formation of large-sized components with complex geometries.