International Journal of Machine Tools & Manufacture最新文献

An in-depth knowledge and understanding of residual stress in dissimilar ultrasonic vibration-assisted friction stir welding (UVaFSW) are crucial for the performance evaluation of multimaterial structure designs; however, extensive research is still lacking. The present study evaluated the residual stress of dissimilar aluminium (Al)/magnesium (Mg) alloy joints produced by traditional FSW and UVaFSW to elucidate the ultrasonic effect mechanism with the aid of process simulation and microstructural evaluation. The weld surficial residual stress measured by X-ray diffraction (XRD) using the cos α method indicated the generation of predominantly compressive stress in UVaFSW welds. In agreement with the XRD measurements, the stress maps evaluated using the contour method (CM) exhibited an expanded compressive stress region and a mitigated tensile stress region in the UVaFSW welds. The Al/Mg interfacial mismatch of thermal expansion led to a tensile stress state on the Mg side and a compressive stress state on the Al side near the Al/Mg interface. The maximum compressive stress in the UVaFSW weld was ∼100 MPa higher than that in the FSW weld. The ultrasonic effect proficiently reduced the layer thicknesses of the intermetallic compounds (IMCs), promoting grain recrystallisation behaviour due to improved material transfer and mixing. Consequently, more homogeneous hardness distributions and improved tensile properties were formed in UVaFSW welds. However, ultrasonic vibration had an insignificant effect on the density of geometrically necessary dislocations and stored strain energy, indicating limited effects on microscopic residual stress in the studied condition. The ultrasonic vibration was found to positively mitigate residual tensile stresses and macroscopic distortion by increasing the temperature and encouraging material mixing within the stirred zone, as well as enhancing the stress interaction of the Al/Mg interface related to thinner IMCs. The UVaFSW has considerable potential to in-process co-optimise residual stress and microstructure for dissimilar Al/Mg welds.

Mechanical micromachining has become a leading approach to fabricating complex three-dimensional microscale features and miniature devices on a broad range of materials. To satisfy the accuracy and productivity demands of various micromachining applications, the tool-tip dynamics, i.e., the dynamic behavior of the tool-ultra high-speed spindle assembly as reflected at the cutting edges of a microtool, should be well-understood. However, existing techniques for predicting tool-tip dynamics pose strict limitations in frequency bandwidth and do not capture the effect of the spindle speed on tool-tip dynamics. In addition, those techniques cannot be applied broadly to predict tool tip dynamics for a myriad of microtool geometries. This paper presents a systematic approach to predicting the tool-tip dynamics accurately in micromachining when using ultra-high-speed (UHS) spindles and for arbitrary microtool geometries. The speed-dependent dynamics of the UHS spindle are obtained using an experimental approach. The dynamics of microtools are obtained analytically using the spectral Tchebychev technique, such that any microtool geometry can be modeled accurately and does not require new testing. The tool-tip dynamics are then predicted by combining (coupling) the spindle and microtool dynamics using a novel modal-Tchebychev domain coupling technique. This technique enabled accurate coupling/decoupling of substructure dynamics within a broad frequency bandwidth (up to 15 kHz) and at different spindle speeds (up to 120,000 rpm). Furthermore, an empirical model for the mode-splitting effect is derived to capture the effect of spindle speeds on tool-tip dynamics. The overall approach is demonstrated and experimentally validated on a UHS spindle with microtool blanks and micro endmills at operational speeds. We conclude that the presented methodology can be used to determine the tool-tip dynamics accurately.

The occurrence of wrinkling and splitting in forming integral aluminum alloy thin shells using traditional forming processes is extremely difficult to preclude. Accordingly, a novel forming process at ultra-low temperature gradient is proposed in this paper. The process leverages the abnormal ‘dual enhancement effect’ of hardening and ductility at ultra-low temperatures. In this proposed approach, the risk unsupported region is fundamentally cooled to ultra-low temperatures to avoid splitting, and the tension-compression stress state is then adjusted by ultra-low-temperature gradient cooling and blank-holder force to control wrinkling. Hyper-hardening and high-ductility properties at ultra-low temperatures are simultaneously utilised to adjust the deformation considering these properties. Mechanical and numerical analyses were conducted to reveal the deformation mechanism, and the effects of ultra-low-temperature gradient, blank-holder force and thickness-to-diameter ratio were studied. The forming defects, thickness, and stress and strain distributions were determined to reflect the deformation behavior. The blank needs to withstand larger deformation to form the thinner components without wrinkling. The maximum radial strain increases by 50% when the thickness-diameter ratio decreases from 13.3% to 3.3‰. A smaller temperature gradient and larger blank-holder force can be used to reduce hoop compressive stress and prevent wrinkling defects. A bigger temperature gradient may be used to increase the stress difference between flange and unsupported regions to further improve forming limit or deformation uniformity, accompanying with easier engineering implementation for large-sized components. An ultra-low temperature forming device was developed to verify the feasibility of this new forming process. The forming limit was significantly improved by cooling the unsupported region, and a more uniform thickness was obtained at a larger ultra-low temperature gradient. The depth of the hemispherical specimen improved by 54.5%, and the average thickness deviation was only 6.9%. Through fundamental research, an integral dome with a diameter of 2.25 m was formed at an ultra-low temperature gradient, surpassing the wrinkling limit and overcoming splitting. The new forming process has considerable potential to fabricate large thin-shell components made of aluminum alloy.

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 SiC_f/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.

Gradient heterostructures typically exhibit excellent mechanical properties. The traditional laser shock method can produce only 1D or 2D gradient structures along the thickness of a material. In this study, we propose a technique called 3D gradient laser shock peening without coating (3LSPwoC) for manufacturing 3D gradient metal structures. An excellent application of this method is the manufacture of multi-scale hydrophobic surfaces with integrated enhanced armor (IE-armor) in a flexible, large-scale and low-cost manner. Hydrophobic surfaces of metals are of great importance, but are typically mechanically fragile and degrade quickly, as the surface nanostructures tend to break under mechanical forces. Current approaches either expose the functional large-aspect-ratio nanostructures directly to external forces or have unbalanced strength-ductility synergy for dynamic loads, resulting in degradation of the properties. A self-armored hydrophobic surface structure was obtained by a combination of laser shock and low surface energy treatment. An IE-armor structure with a well-designed strength-ductility synergy was considered to protect the rich nano-hydrophobic structures. The arrayed micro-pits and abundant micro-nano structures in the pits realized a stable Cassie-Baxter state, resulting in a superhydrophobic surface. The alternating regular distribution of hard and sub-hard domains on the metal surface, together with the soft domain in the core, formed a 3D gradient structure, which achieved excellent synergistic plastic deformation and provided superior mechanical robustness. The 3D gradient metal structure manufactured using the 3LSPwoC process is expected to play a crucial role in highly reliable functional surfaces in aerospace, locomotive manufacturing, and ocean engineering.

This study investigates the deformation and fracture mechanisms of two testing methods, tension under cyclic bending (TCB) and tension under cyclic bending plus compression (TCBC) and their relationship to single point (SPIF) and double-sided (DSIF) incremental sheet forming processes. Experimental tests were carried out by using a bespoke TCBC test rig and a DSIF machine with grade 1 pure Ti samples. The results show the elongation-to-fracture has a high relevance to the bending depth and compression, which leads to detailed investigation to the stress and strain evolutions in the local bending region using finite element (FE) method. A new Gurson-Tvergaard-Needleman (GTN) model is proposed with a modified shear damage mechanism utilising experimental fracture strain loci to calibrate the Lode angle effect under low stress triaxiality. It is found the bending and reverse-bending stages correspond to different stress states and significantly affect the fracture occurrence in TCB, TCBC and SPIF, DSIF processes. For the first time, the stress paths in the plane of stress triaxiality and Lode parameter are used to reveal the transition of deformation modes from equi-biaxial to plane strain tension in SPIF and DSIF, as compared to the plane stress tension in TCB and TCBC. Using the new GTN model, the simulation gives accurate predictions to the elongation-to-fracture in TCB and TCBC, and the fracture depth in SPIF and DSIF with an error of less than 8% in comparison to the experimental results. Although there is a distinction between the equi-biaxial and uniaxial tension deformations, the study concludes that the TCB and TCBC tests provide an insight into the formability improvement and represent intrinsic deformation mechanisms of SPIF and DSIF processes, an ongoing research question, which has drawn considerable attention in recent years.

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.

Laser powder bed fusion (LPBF) technology has the potential to revolutionize the fabrication of complex metal components in the aerospace, medical, and automotive industries. However, keyhole pores may be induced during the rapid laser-metal interaction (∼10⁻⁵ s) of the LPBF. These inner porosities can potentially affect the mechanical properties of the fabricated parts. Here, based on the experimentally observed keyhole-penetration pore (KP-pore) led by the keyhole splitting of the molten pool in LPBF, a multi-physics finite volume model was established to reveal this mechanism, where keyhole pores were formed under the direct contact of keyhole and solid metal substrate, which is different from the previously reported gas–liquid interaction. The formation mechanisms of the KP-pore, rear-front pore (RF-pore), and rear pore (R-pore) could be attributed to different keyhole fluctuation modes. The effects of the powder on the characteristics of the keyhole, molten pool, and pore formation were explored. The increased pore counts and decreased size were owing to the powder-promoting keyhole and molten pool oscillation. In addition, a relationship map between the input energy density and pore number was built via a high-throughput simulation, providing a strategy to reduce or remove the pores in laser powder bed fusion.

Laser machining is promising in shaping the brittle carbon/carbon composites (C/Cs) with deep holes, sharp edges, or thin walls. However, there are still many unknowns relating to the laser ablation of carbon materials, and the existing theory and practice is insufficient to guide the industrial machining of C/Cs. Herein the laser drilling of C/Cs was experimentally conducted and numerically modeled to probe into the mechanisms responsible for the material removal, surface formation, and damage evaluation. Firstly, the intrinsic correlations among the anisotropic hole feature, the fiber yarn alignment and the steady-state thermal conduction are revealed. The detailed characterizations of the ablated surface and the recast layer clearly prove that sublimation of the graphitic carbon dominates the material removal process under laser ablation. Furthermore, it is proposed that the greater portion of crystalized graphene layers enables the lower ablation rate of the pyrocarbon matrix than the carbon fibers. Secondly, the combination of the experimental and simulated results unravels that the continuously evolved surface slope and the redeposited recast layer are the decisive factors in the laser-carbon interaction, which affect the efficient absorption coefficient of the laser and result in the nonlinear drilling rate and the self-limiting of the drilling. Finally, the roles of the laser heating and the subsequent rapid cooling in damage initiation and propagation are identified: nanoscale splitting of the pyrocarbon occurs due to the growth and realignment of the graphene layers upon laser heating, and the tensile thermal stress induced by the cooling drives the further growth of high-density but discrete microcracks from these splitting sites. The load bearing capability of the carbon fibers, however, is retained in this severe thermal shock. As a result, the laser drilling induces only a slight degradation of the mechanical strength of the C/Cs.

Polishing-based post-processing is essential for removing the undesired surface diffraction on diamond-turned microstructured surfaces that is enhanced by periodic tool marks. To overcome challenges in existing micropolishing methods, a fast-tool-servo-controlled shear-thickening micropolishing method was proposed for the non-contact and controllable polishing of microstructured surfaces. The operating kinematics and material removal mechanism are modeled analytically and investigated experimentally. The comprehensive principal stress in front of the rake face of the tool is found to mainly contribute to the material removal. The fast tool servo can tune the principal stress and the viscosity of the slurry by flexibly adjusting the gap width between the surface and the tool edge. Thus, the material removal can be controlled at any operating position. Meanwhile, although the material removal rate is nonlinearly related to the rotation radius and gap width, the constraint between these two factors is linear for achieving a fixed material removal rate. Finally, the feasibility of the proposed micropolishing method is demonstrated by successfully polishing rotationally symmetric and asymmetric microstructured surfaces to achieve improved surface smoothness and conformal surface shapes.