Showing papers in "International Journal of Machine Tools & Manufacture in 2023"

Microstructure and residual stress modulation of 7075 aluminum alloy for improving fatigue performance by laser shock peening

Abstract: Laser shock peening (LSP) is an advanced surface-strengthening technology that improves the anti-fatigue performance of metallic components. However, there is a significant barrier to the application of thin-walled components because the high-energy laser causes deformation and nonuniformity of compressive residual stress, thereby reducing fatigue performance. In this study, an LSP technology based on a low-pulse-energy laser was developed. We applied it to a thin-walled AA7075 aluminium alloy specimen (∼4 mm thickness) and achieved an improvement in the high-cycle fatigue limit of 20.4 and 37.0% for the smooth and pre-cracked fatigue specimens, respectively, in the absence of deformation. It was discovered that the enhanced dynamic nanoscale precipitation and dislocation multiplication effects of the high-pressure shock wave contribute to microstructure stability under cyclic loading, resulting in high compressive residual stress stability. Moreover, the unique heterogeneous grain structure on the surface layer subjected to LSP at low pulse energy effectively restrains crack initiation and propagation. Because these findings apply to a wide range of alloys, the current results create new avenues for improving the fatigue performance of thin-walled components.

Review on field assisted metal additive manufacturing

Abstract: 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.

Grain refinement and columnar-to-equiaxed transition of Ti6Al4V during additive manufacturing via different laser oscillations

Abstract: 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.

Gradient microstructure and prominent performance of wire-arc directed energy deposited magnesium alloy via laser shock peening

Abstract: 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.

A review of static and dynamic analysis of ball screw feed drives, recirculating linear guideway, and ball screw

Abstract: Static and dynamic analysis of key structural components of the machine tool is the crucial stage in transferring the physical to the virtual domain for digital manufacturing trends. The modeling technique of rolling kinematic joints with high nonlinearity can directly influence the accuracy and efficiency of prediction. Existing literature replicates the nonlinear static and dynamic characteristics considering the rolling element contact interface and proposes the theoretical modeling approach for the feed drives. However, there is a lack of systematic literature surveys. This paper reviews the current progress placed at the nonlinear analytical model of rolling kinematic joints, including ball screw feed drives, recirculating linear guideways, and ball screws. Advanced investigations on nonlinear dynamic stiffness and vibration response associated with ball screw feed drives are covered. Specifically, for linear guideways and ball screws, the stiffness and load distribution models can be divided into two categories: with and without consideration of the component structural deformations. Moreover, the corresponding detailed modeling process is introduced. The time-dependent modeling principle highlighting the recirculation motion of rolling elements is summarized, and friction and wear behavior is briefly discussed. The paper ends with the current research advancement and scarcity and recommends promising modeling tendencies. Particularly, the modeling tendencies require integrated model research on ball screw feed drives considering the more detailed nonlinear joint. Moreover, a fusion of multi-physics parameters is expected to achieve the high-fidelity mechanical model of key structural components for intelligent manufacturing demand.

Recent progress on the additive manufacturing of aluminum alloys and aluminum matrix composites: Microstructure, properties, and applications

Abstract: 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 additive manufacturing of aluminum alloys.

The curved uncut chip thickness model: A general geometric model for mechanistic cutting force predictions

Abstract: The curved uncut chip thickness model is introduced to predict the cutting forces for general uncut chip geometries using the mechanistic approach. Classical geometric models assume that the cutting force is distributed along straight elementary sections of the uncut chip area, which has limited physical validity, but makes mathematical treatments easier for simple cases. The new model assumes that the flow of the material on the contact area of the tool is given by a continuous vector field, according to which the curved uncut chip thickness is measured. The cutting force is distributed along these paths, which leads to a mathematically unique and consistent solution for regular and complex cutting edge geometries. These curved paths can be generated by basic mechanical models, which mimic the more realistic motion of the chip segments along the rake face, without the need of explicit time-consuming cutting simulations. The presented computational procedure generalizes cutting force prediction based on geometric parameters, orthogonal cutting data and the orthogonal to oblique transformations only. The effectiveness of the model for various cutting edge geometries (e.g., thread turning inserts) under extreme cutting conditions is presented in case studies, laboratory and industrial experiments.

Machining of long ceramic fibre reinforced metal matrix composites – How could temperature influence the cutting mechanisms?

Abstract: Metal matrix composites (MMCs) offer a unique set of properties due to the ductile-brittle combination produced by the matrix and the reinforcements. Conventional MMCs are usually particle-reinforced, and their cutting mechanisms have been thoroughly studied, showing that they tend to follow traditional cutting theory as the particles roll within the surface/chip or are pushed in/pulled out of the machined surfaces. However, while the enforcement mechanism is quite unique in fibre reinforced MMCs, very little is known about the cutting mechanisms of this kind of materials. These materials are distinguished for having a, roughly, one-to-one scale alternation of the ductile (i.e., matrix) and hard/brittle (i.e., ceramic fibres) phases; key characteristic that is likely to heavily influence the material removal mechanism. Further, there is an open question on how the (temperature-dependent) stiffness of the matrix would affect the cutting mechanism when considering the hybrid machining process (e.g., heat assisted/cryogenic machining) to improve their machinability. To elucidate these aspects, here, by means of cutting a SiCf/Ti-6Al-4V MMC, the following particularities/peculiarities of the cutting mechanism of these structures are reported: (1) the chip formation includes, up to now unobserved, extrusion of the ductile component of the MMC (Ti-6Al-4V matrix) between the fractured hard phase (SiC); (2) the properties and deformation mechanisms of the matrix (adjusted by temperature control: −180 °C; 24 °C; 400 °C) will affect the crack initiation of the SiC hard/brittle fibre which is manifested underneath the machined surface. Thus, this work is unique in its approach as it opens the understanding of how these complex and heterogeneous structures could be “activated” (e.g., by thermal means to change the stiffness of a particular phase) for improved cutting conditions.

Investigation of the grain deformation to orthogonal cutting process of the textured Alloy 718 fabricated by laser powder bed fusion

Abstract: 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.

Introduction of rolling motion at the tool-tip in metal cutting

Abstract: In metal cutting, severe sliding contact at the tool-chip interface is unavoidable by a conventional cutting tool, which results in sloth motion of chip, higher chip thickness, formation of stagnation zone, higher power consumption, and poor surface finish. To overcome this limitation, rolling motion is introduced at the tool-chip interface by mounting a roller at the tip of a cutting tool – termed as Rotating Tip Cutting Tool (RTC-tool). The roller in RTC-tool rotates during cutting and establishes rolling-sliding contact. A model in-situ experimental configuration is used to study the plane strain flow characteristic of pure copper, notoriously known for higher chip thickness and power consumption, during cutting at low speed. The performance of RTC-tool is compared with sharp cutting-edge and blunt cutting-edge tools (fixed curvature and stationary-roller-tip tool). It is found that rolling motion at the tool-tip decreases the chip thickness and average plastic strain within the chip by about two times than sharp tool and more than two times than the blunt tools. Additionally, there is a significant improvement in surface roughness than the sharp tool. The digital image correlation techniques reveal that flow characteristics within the chip and near the tool-tip interface (retardation region) are influenced by the non-laminar plastic flow of materials. As opposed to the unstable retardation region and periodic cracking at the tool-tip of sharp tool, additional rolling motion at the tool-tip cuts the chip at the incipient stage, forms a stable retardation region, and increases the average velocity of the material in this region. Large retardation region and sloth motion of materials within the retardation region produce large chip thickness during cutting by blunt edge tool. As the chip thickness rather power consumption of the sharp tool is less than the blunt tool, force measurement in nearly orthogonal cutting configuration of RTC-tool compared with the sharp cutting edge. These tests are performed at moderate speed range in a lathe machine. The force measurement data and post-cutting characterization are aligned with the in-situ observations. As the frictional resistance of the roller controls the chip thickness further improvement in the performance of the RTC-tool is possible by reducing the frictional resistance of the roller.

Femtosecond laser printing patterned nanoparticles on flexible substrate by tuning plasmon resonances via polarization modulation

Abstract: 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.

Calibration rod selection strategy in RCSA-based method for reliable calculation of milling tool-tip FRFs in rotating conditions

Abstract: The measurement of milling tool-tip frequency response functions (FRFs) in rotating conditions is challenging in practice. Methods based on the receptance coupling substructure analysis (RCSA) can obtain rotating tool-tip FRFs using normal modal test devices; thus, they have received extensive attention in the research community. The typical RCSA framework first adopts a calibration rod for measuring rotating FRFs. Then, it analytically calculates the desired tool-tip FRFs through the RCSA theory. As the calculation process involves matrix inversion, high-quality FRF data is required. However, experimentally measured FRFs in rotating structures contain severe noise, leading to an unreliable calculation. This paper presents a novel error analysis model to investigate the propagation mechanism of measurement errors in the typical RCSA framework. Results show that measurement errors would cause errors in the length of the coupled substructure while introducing scaling effects. The calibration rod is found to be vital for RCSA calculation reliability. The patterns of the calculation error are opposed when adopting a short or long calibration rod. Then, a calibration rod selection strategy is proposed. The strategy makes full use of the high measurement quality near the resonance in rotating FRFs and achieves the dominant mode frequency matching between the clamped rod and the clamped tool by adjusting the rod length. Simulations validate the error analysis model and the calibration rod selection strategy. Experimental results also show that the optimal selection of the calibration rod could improve the calculation reliability of rotating tool-tip FRFs in the typical RCSA framework.

Investigation of the low-frequency chatter in robotic milling

Abstract: 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.

Analytical modelling of transient thermal characteristics of precision machine tools and real-time active thermal control method

Abstract: Thermal error is one of the primary factors affecting the machining accuracy of precision machining tools. Therefore, it is important to study the transient thermal characteristics of machine tools and the thermal-error control strategies. Thus far, a transient analytical modelling method for characterising the thermal characteristics of machine tools was proposed and an active error control strategy was provided. First, temperature-field modelling was conducted using an analytical method based on the Fourier series method and partial differential equations of heat conduction. Second, using the derived temperature field, the thermal deformation field was calculated based on finite element theory. Subsequently, the continuous real-time effect of the thermal power per unit heat source on the temperature and deformation fields of precision machine tools was studied. The proposed analytical modelling method not only predicts the machine tool heat deformation based on the working conditions of the heat source, but also matches the thermal control source power with the demand of the machine tool heat deformation. The optimal real-time power of the thermal control source is dynamically iterated and matched, such that the thermal deformation caused by the heat and thermal control sources can be balanced in real time at the displacement control point. Finally, the volumetric thermal error was actively controlled by adjusting the temperature field of the machine tool. The simulated and experimental results indicate that the transient analytical model can accurately predict the real-time thermal characteristics of the machine tool and that the real-time active thermal control method can effectively reduce volumetric thermal errors. Using active thermal control, the squareness error in the YZ-plane was reduced by approximately 45%, the spindle thermal elongation was reduced from 23 μm to 7 μm, and the volumetric thermal error in the X, Y, and Z directions were reduced by approximately 16, 14, and 17 μm, respectively.

Analysis of burr formation in finish machining of nickel-based superalloy with worn tools using micro-scale in-situ techniques

Abstract: 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.

Effects of laser oscillation on metal mixing, microstructure, and mechanical property of Aluminum–Copper welds

Abstract: Laser welding of dissimilar metals is important in many industrial applications. However, as dissimilar metals get mixed during the melting process, intermetallic compounds are often formed in the welds which can significantly undermine the electrical and mechanical properties of the welds. This poses a critical challenge to the widespread utilization of this welding technique. Compared with conventional line-scan laser welding, oscillating laser welding offers additional processing parameters to control the welding process. Although some work has been reported on oscillating laser welding of dissimilar metals, a mechanistic understanding of this process was still missing. The research objective of this work was to reveal the physical mechanisms and evaluate their relative significance to the fluid flow, metal mixing, and microstructure evolution in the molten pool in oscillating laser welding of dissimilar metals. A combination of experiments and simulations was leveraged to achieve the objective. Four fluid flows have been found to determine the metal mixing in the molten pool, and their dependences on the laser oscillating parameters were discussed. In addition, the thermo-solutal conditions of the molten pool solidification were quantified as functions of the laser oscillating parameters, and the effects of the thermo-solutal conditions on the final weld microstructures were analyzed.

In situ monitoring the effects of Ti6Al4V powder oxidation during laser powder bed fusion additive manufacturing

Abstract: 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.