查看更多>>摘要：In this study, a novel and highly efficient isotropic etching polishing (IEP) technique electrochemically polished flat and complex 3D printed Inconel 718 (IN718) parts. The anisotropic etching was transformed into isotropic etching based on the electrolyte diffusion characteristics, and the latter was also realized as the polishing mechanism of 3D printed IN718. Wet etching revealed inhomogeneous microstructures in 3D printed IN718, which dissolved preferentially when polished for a longer duration. The initial surface roughness of 674 nm was reduced to 31.5 nm, and a maximum of 2.42 mm(3)/min material removal rate (MRR) was achieved that varied with IEP parameters. The IEP improved the corrosion resistance of 3D printed IN718 compared to the original and grounded substrates, but it was limited by the processing time. Contrarily, the surface mechanical properties of 3D printed IN718 were slightly degraded after IEP. Finally, the 3D printed IN718 comple x parts were polished for 5 min, and the Sa roughness was reduced by 76 %, demonstrating a huge potential of IEP as a contemporary industrial polishing technique.
查看更多>>摘要：Cryogenic abrasive air-jet (CAAJ) machining technology is performed simultaneously with the occurrence of the glass transition for PDMS material. However, the mask material is also cooled with cryogenic PDMS cooling. Then, they both show significant brittle characteristics. Therefore, this leads to very poor selectivity between the mask and the PDMS. To solve this problem, CAAJ direct-write machining under unmasking conditions are proposed based on the modified velocity model of abrasive particles in the radial direction. This technique was used to direct-write machined the PDMS materials under small SOD. Then, the effect of cryogenic conditions on machining characteristics of PDMS micro-channels was studied by comparing the abrasive air-jet direct-write machining at oblique angles and CAAJ direct-write machining technologies through a series of experimental studies and CFD simulations. The results show that the material removal mode is transformed to a semi-brittle one, which greatly improves erosion removal efficiency, aspect ratio and bottom surface roughness of the micro-channel. On the other hand, the change in the viscosity of the jet medium has a crucial influence on the geometry of the micro-channel. The rebound direction of abrasive particles changes, and the sidewall of microchannels is secondarily eroded due to an increase in jet medium viscosity in CAAJ. This, in turn, leads to a microchannel with "U"-shaped profile. In addition, during CAAJ direct-write machining, sliding, scratching, and rolling of abrasive particles caused by bubble bursting can effectively reduce the bottom surface roughness of the microchannel.
查看更多>>摘要：Thin-walled parts with double-sided features are widely used in many industrial sectors but their machining is particularly time consuming and challenging. Collaborative machining is a new paradigm in the development of industrial 4.0. It can potentially revolutionize the existing thin-walled part machining methods, leading to higher productivity, flexibility and sustainability. For this end, this paper introduces a novel concept with dual parallel kinematic machines (PKMs) collaboratively performing synchronized and asynchronized cutting and support from both sides of a thin-walled part, without changeover/re-clamping of the workpiece. Compared to the conventional single-sided machining, this study shows that static and dynamic performances of the workpiece are significantly improved under the dual PKM collaborative operation. A case study of milling a thin-walled part with double-sided features was conducted by PKMs under three comparative strategies, namely, double-sided synchronized milling, alternative single-sided milling, and sequential single-sided milling. Experimental re-sults show that the novel double-sided synchronized milling strategy by dual collaborative PKMs produced the best dimensional accuracy and satisfactory surface quality due to the improved static stiffness and dynamic performance, and balanced deflections. More importantly, a two-fold greater productivity has been achieved as the novel strategy doubles the material removal rate while eliminating the cumbersome in-process steps used in conventional single-sided machining.
查看更多>>摘要：In-situ preparation of particle reinforced Al matrix composites (PRAMCs) by laser powder bed fusion (LPBF) is a promising strategy to strengthen Al-based alloys. The laser-driven thermite reaction can be a practical mechanism to in-situ synthesise PRAMCs. However, the introduction of elements oxygen by adding Fe2O3 makes the powder mixture highly sensitive to form porosity and Al2O3 film during LPBF, bringing challenges to prepare dense materials. This work develops an LPBF processing strategy combined with consecutive high-energy laser melting scanning and low-energy laser remelting scanning to prepare dense PRAMCs from Fe2O3/AlSi12 powder mixture. A high relative density (98.2 +/- 0.55 %) was successfully obtained by optimising laser melting (E-melting) and remelting energy density (E-remelting) to E-melting = 35 J/mm(2) and E-remelting = 5 J/mm(2). Results reveal the necessity to increase Emelting to improve metal liquid's spreading/wetting by breaking up Al2O3 films surrounding molten pools; however, the high-energy laser melting produced much porosity. Low-energy laser remelting could close the resulting internal pores, backfill open gaps and smoothen solidified surfaces. Although with two-times laser scanning, the microstructure still shows fine cellular Si networks with Al grains inside (grain size 370 nm) and in-situ nano-precipitates (Al2O3, Si and Al-Fe(-Si) intermetallics). Finally, the fine microstructure, nano structured dispersion strengthening and high-level densification strengthen the prepared in-situ PRAMCs, reaching yield strength of 426 +/- 4 MPa and tensile strength of 473 +/- 6 MPa. Furthermore, the results can provide valuable information to process other powder mixtures with severe porosity/oxide-film formation potential considering the evidenced contribution of laser melting/remelting strategy to densify material and obtain good mechanical properties during LPBF.
查看更多>>摘要：Current research activities involving the laser directed energy deposition (DED) of Ti-6Al-4V alloy focus on its microstructure and performance. This paper details an approach for fine-tuning the content of Al and V during DED Ti-6Al-4V and explores a heat treatment strategy to enhance mechanical properties. A Ti-6Al-4V feedstock powder was adapted to provide the basis of Ti-5.6Al-3.8V, Ti-5.3Al-3.6V, and Ti-5.0Al-3.4V alloys. The effect of alloy composition on tensile properties of DED these alloys were determined. It was confirmed that the tensile properties of DED Ti-5.6Al-3.8V and Ti-5.3Al-3.6V alloys exceeded those of forging standard of Ti-6Al-4V, while the tensile strength of DED Ti-5.0Al-3.4V was lower than the forging standard of Ti-6Al-4V due to coarsened alpha laths in the prior beta grains. The changes to microstructure and tensile properties of DED Ti-5.6Al-3.8V and Ti5.3Al-3.6V alloys after heat treatment were analyzed, and the results showed that the elongation of the DED Ti-5.3Al-3.6V after heat treatment was significantly improved, from 11.3%-17.2%, while its tensile strength slightly decreased 1.7 %. The results are expected to guide the fabrication of DED near Ti-6Al-4V alloy to produce products with high ductility at a minimal compromise in ultimate tensile strength.
查看更多>>摘要：Conventional residual stress mitigation techniques involve long processing times at high temperatures and/or mechanical loading to build plastic compressive stress below the surface. In this study, we present a new residual stress mitigation methodology at near ambient temperature in less than a minute. This is demonstrated on a welded joint of 316 L stainless steel, where low-frequency DC current pulses are shown to recrystallize the specimen and reduce residual stress. We present experimental evidence of -30 % reduction in electrical resistance, which corresponded to -40 % decrease in both microhardness and residual stress, measured by the X-ray diffraction tests. Similar improvement was qualitatively observed through significant decrease in the low-angle grain boundary density, which also reflects the decrease of the residual stress. The technique can be applied to relieve residual stress in conditions difficult for the conventional processing, such as locations with extreme space constraints or objects that cannot be heat treated.
查看更多>>摘要：Compound forging is a promising approach for the manufacturing of bimetal parts. However, reliable material interlock is difficult to be obtained in compound forged bimetal parts due to the asynchronous material flow. To overcome this problem, in this work, the previous proposed thixotropic-core compound forging (TCF1) method was applied in the precision forming of a bimetal cylinder spur gear. This investigation focuses on the feasibility of this new forming method. An aluminum alloy core and a steel shell were used as raw materials. The effects of the aluminum alloy core billet status on the material interlock and the shell thickness distribution were studied. Moreover, the microstructures and mechanical properties of the aluminum alloy core in the bimetal gear were investigated. The results suggested that the synchronous material flow could be achieved in the TCF. On this basis, reliable material interlock was obtained. Moreover, the thixotropic core is helpful to improve the uniformity of the shell thickness distribution by diminishing the friction. The liquid segregation in the thixotropic core can be remarkably diminished by the three-dimensional compressive pressures provided by the outer shell during the synchronous material flow. Therefore, uniform microstructure and property uniformities can be obtained in the core of bimetal parts manufactured by TCF. Furthermore, thixotropic core billets with a liquid fraction (f(l)) lower than the requirements of conventional thixoforming were found to be viable in TCF.
查看更多>>摘要：The effects of adhesive-backed overlays, specifically vinyl tape and aluminum tape, on the peak pressures generated by laser shock peening (LSP) of 7085 aluminum alloy and Ti-6Al-4V were investigated and the results are reported. Peak pressures were determined from photon doppler velocimetry measurements of rear surface velocity. Incorporating adhesive-backed opaque overlays in LSP provided up to 50 % pressure enhancement compared to processing without the overlays. Contrary to processing without the overlays, the temporal characteristics of LSP impulses with adhesive-backed overlays are shown to be independent of the laser pulse width in the range studied. This finding is an important impulse consideration in LSP parameter selection and has not been investigated or reported in the literature. A novel physics-based analytical model to predict shockwave pressures is proposed and validated with experimental data agreeing nominally within 15 % of the predicted values across the parameter space. The results provide insight into the improved residual stresses and enhanced material properties of published opaque overlay studies. Moreover, the results provide vital information for selecting LSP parameters for optimizing component performance and potential extension into previously unreachable pressure regimes.
查看更多>>摘要：In near-isothermal local loading forming (NLLF) of large-size Ti-alloy parts, one of the challenging issues is to achieve accurately prediction of complex microstructural development. In this work, a thermo-mechanical-microstructural FE model considering complicated phase transformation and morphology transformation of the whole forming process is developed and experimentally verified. The simulated distribution of phase amount for primary alpha (alpha(p)) and secondary alpha (alpha(s)) phases agrees well with the experimental results. Based on the model, the evolution characteristics of alpha(p) phase, lamellar alpha(s) (alpha(ls)) phase, and globularized alpha s (alpha(gs)) phase for large-size Ti-alloy parts at different stages were investigated. It is found that alpha(s) phase amount in the first loading region is significantly higher than that in the second loading region, while alpha(gs) phase amount shows a reverse trend. Moreover, processing parameters including initial heating temperature, die loading speed, and loading pass greatly affect the relative amount and distribution uniformity of constituent phases among different loading regions. Finally, the formation mechanism of quasi-trimodal microstructures under NLLF of complex Ti-alloy parts is clarified by analyzing the coupling effects of applied deformation and phase transformation on the microstructure morphology. To achieve the regulation and control of the tri-modal microstructure, an optimized processing scheme is proposed.
查看更多>>摘要：In this paper, a novel method of electromagnetic partitioning forming with elastic cushion is proposed. Its main principle is based on that the parts vibrate at high speed under the action of the electromagnetic force and the rebound force of the elastic pad. The state of the elastic parts inside the workpiece is changed into plastic state, and then, springback is eliminated through vibration. A common spiral coil is used to generate the magnetic field. A finite element model has been developed to analyze the effects of coil structure and coil discharge position on the stress, strain, and surface quality of the parts. It is found that a 1.5-mm bump is generated on the sheet surface, which corresponds to the middle of the coil. This is because this sheet region corresponds to the coil edges moving towards each other in opposite directions. Subsequently, the electromagnetic shielding method is used to isolate half of the spiral coil. Both the simulation and experiment demonstrated that no bulge was produced on the sheet surface. Moreover, the internal stress and springback of the parts are significantly reduced due to that the sheet vibrates at high speed. Large-scale skin was obtained after the coil was discharged multiple times in six different positions. Compared to traditional stretching, the springback of the parts is significantly reduced and the plate surface is smooth after electromagnetic forming. Consequently, the highspeed vibration induced by electromagnetic forming can reduce springback and enables the manufacturing of large size parts with smooth surface.
NSTL
Elsevier