查看更多>>摘要：Material mixing and intermetallic compounds (IMCs) are the key factors that affect the mechanical properties of dissimilar metal joints. In this paper, a feasible method of stationary-dynamic shoulder friction stir welding has been developed, which reduces the mixing of materials and inhibit the growth of IMCs during the Al-Cu welding process. The stationary-dynamic shoulder was composed of a stationary shoulder with a larger diameter and a rotational shoulder with a smaller diameter. While the stationary shoulder provided the upsetting force to guarantee the weld formation, the rotational shoulder served for material stirring and heat input. Compared with conventional friction stir welding, the mechanical properties of joints obtained by the new method were significantly improved. The maximum tensile strength was 243 MPa, which was equivalent to 90 % of Cu base metal and 54 % of Al base metal. Furthermore, the evolution of IMCs across the transition layer was analyzed in depth. The results indicated that, with an increase in the rotation speed, the IMCs layer gradually became thicker. However, when the thickness reached a certain level, it promoted the generation of micro-cracks on IMCs and reduced the strength of the joint. According to thermodynamic and kinetic analysis, IMCs were formed with respect to Al2Cu-Al4Cu9 sequence, which was consistent with transmission electron microscopy results.
查看更多>>摘要：Blank wrinkling is the most frequently encountered material processing failure in spinning. Design of toolpath profiles and selection of process parameters are important in preventing wrinkling. Existing studies employ simplified toolpath profiles and rely on the calculation of maximum and critical stresses of the blank to determine the onset of wrinkling, often requiring time-consuming Finite Element simulations. To overcome these limitations, this paper develops an analytical wrinkling prediction model integrated with toolpath design in developing multi-pass conventional spinning. The toolpath design is parameterised for concave, convex and linear profiles and a wrinkling-wave function using a concave profile is developed to capture geometrical characteristics of a wrinkled blank. The forming depth is introduced as a critical variable in designing a toolpath profile to control wrinkling. Material plastic deformation is analysed by employing the Donnell-Mushtari-Vlasov theory and wrinkling initiation is predicted by the occurrence of the instability of the blank as a doubly-curved thin shell. Experimental tests of first-pass conventional spinning using concave, convex and linear toolpath profiles are performed to validate the developed wrinkling prediction model. The experimental validation confirms that the proposed critical forming depth of toolpath profile is an accurate and effective measure in predicting the onset of wrinkling. The results show the significant effect of the design of toolpath profile, blank thickness, spin ratio and feed ratio on the onset of the wrinkling. The wrinkling prediction model is capable of producing processing maps of these key parameters to prevent wrinkling for the spinning process development in practical applications.
查看更多>>摘要：High-quality joints of ultra-high strength C-Si-Mn quenched and tempered martensitic steel were successfully fabricated by double-sided friction stir spot welding (FSSW) with adjustable probes. We found that the mechanical properties of the welded joints increased with the rotation speed, due to the increase of hard phase and strengthened welding interface. In the surroundings of the shoulder/probe interface, a large amount of martensite was formed resulting from the higher austenitizing temperature and the higher cooling rate. In addition, severe material flow vertical to the welding interface and strong relative slip along the interface conspicuously favored the fragmentation and dispersion of the oxides, and the drive force for grain boundary migration introduced by severe material flow further promoted the dispersion of the oxides. Consequently, a high-quality welding interface was fabricated, and a high-strength welded joint with a stable plug failure was obtained. Furthermore, we pointed out that soundly welded high strength interface of ultra-high strength steel can be obtained by higher welding temperature and introducing strong material flow vertical to the interface.
查看更多>>摘要：The welding of thick W onto Cu, with good bonding, has been a big challenge due to the large differences in physical properties between W and Cu. Among various novel methods, explosive welding is the promising one to produce bimetals with large size and great thickness. However, the cracking of brittle W under high strain rate limits its application. In this work, hot-explosive welding technique was explored to overcome this problem. A 2 mm thick W plate was preheated to 500 celcius and was successfully welded with pure Cu plate, without any cracks formed in W layer. The result suggests that preheating W to over its dynamic ductile-to-brittle transition temperature and decreasing the imported kinetic energy are two most important factors for the successful welding of thick W plate. The weldability window calculated using the parameters at 500 degrees C predicted the formation of a good wavy interface. The microstructures at W-Cu interface were characterized by optical microscope, SEM, EBSD and TEM. The mechanically mixed W-Cu phase and the 2 similar to 6 nm thick amorphous layer along the interface created strong bonding between the immiscible W and Cu. The measured interfacial compressive shear strength reached 188 MPa, indicating a good bonding strength of the interface.
查看更多>>摘要：Although lack-of-fusion porosity due to incomplete melting of powder can limit the mechanical properties of additively manufactured metals, quantification and prediction of these defects remains challenging. We compare three common strategies to measure porosity: the Archimedes, micrograph-based, and micro-computed tomography approaches. We find that while these methods work equally well at low void fraction, their predictions diverge at higher void fractions (> 5 %). We find that the disparity comes since the Archimedes method measures the total amount of solid in the sample, while micrograph-based approach neglects loose powder trapped inside the samples that can be removed during the preparation process. We conclude that these two methods make use of divergent definitions of porosity. While the Archimedes method measures a "total porosity" defined by the total volume fraction of void in the material, the micrograph method measures an "effective porosity" that only accounts for the continuous material. On the other hand, the resolution of micro-computed tomography is limited by voxel size, leading to ambiguity of trapped powder being identified as solid or void. Consequently, the number density of defects from micro-computed tomography are noticeably smaller than that from micrographbased approach. A geometric model for porosity prediction is implemented and used to evaluate different models for melt pool geometry. Our numerical predictions of melt pool profiles are surprisingly insensitive to our choice of heat source models. Finally, an analytical geometric model is developed for fast estimation of total and effective porosities and shows good agreement with both numerical simulations and experimental measurement.
查看更多>>摘要：The cutting force in ultrasonic vibration side grinding (UVSG) of brittle materials is of significance for the machined quality. However, the micro-interaction mechanism between abrasive grits and workpiece remains a significant issue in terms of the research on cutting forces in the brittle material grinding due to the stochastic distribution nature of the grits. Meanwhile, these micro-interactions are further affected by ultrasonic vibration in UVSG process. Considering these issues, this paper aims at modeling the cutting force affected by the stochastically distributed grits and ultrasonic vibration to reveal the multi-scale grinding mechanics in UVSG of optical glass. Through modeling the stochastic grinding wheel surface and analyzing the kinematics of multigrits, the unified motion trajectory and the instantaneous chip thickness in UVSG were enabled to derived to determine the different grit-workpiece interaction stages. With consideration of the ultrasonic vibration effects on the micro-interactions, a novel theoretical cutting force model was developed based on the analysis of force generated at three interaction stages, i.e., rubbing, plowing, and cutting stages. The numerical simulations of this model could provide the time-domain variation features of cutting forces to evaluate the fluctuations and trends of the cutting forces from multi-scales. Experimental verifications indicated that the cutting force and its fluctuations are within the acceptable error margin. Furthermore, a thorough analysis of force fluctuating situations at different ultrasonic vibration amplitudes was conducted. The analytical results indicated that an increase in ultrasonic vibration amplitude was not only beneficial to restrain the force fluctuation, but helpful to reduce the brittle fracture damages of optical glass. The evolution trend of cutting force in different grinding conditions and wheel specifications were discussed in detail to reveal the influence mechanisms of machining parameters on the cutting forces. Afterwards, the detection results of ground workpieces demonstrate that the introduction of ultrasonic vibration would improve the machined quality of optical glass in UVSG process.
查看更多>>摘要：Although titanium (Ti) alloys possess unique properties that allow them to compete with many other materials in advanced industries such as aerospace, marine and biomedical, they have poor machining performances. The primary objective of this study is to investigate the distribution of magnetic field intensity at the cutting envi-ronment in single-point diamond turning (SPDT) of Ti-6Al-4 V alloy and its influence on the machining per-formances, with the goal of achieving the desired machining conditions of magnetic field assisted ultra-precision machining, especially magnetic field intensity and the corresponding machining parameters, and to enhance the machinability of Ti-6Al-4 V alloy. In this study, magnetic field-assisted machining (MFAM) system was designed and coupled with ultra-precision machining (UPM) using single-point diamond turning for increasing the machinability and improving the surface quality of Ti6Al4V alloy machined parts. The finite element method (FEM) was developed to demonstrate the influences of the generated magnetic field on the machining processes. The Experimental results showed the capability of magnetic field assistance to enhance the machining performance of Ti-6Al-4 V alloy. These findings provided strong evidence that a magnetic field has the ability to extend cutting tool life, additionally, MFAM achieved the lowest value of surface roughness, representing a 33 percent improvement in surface roughness. This research contributes to the support of the optimum MFAM by FEM and the achievement of high-quality machined Ti alloys in UPM for similar research works, as demonstrated by the experimental results.
查看更多>>摘要：Reaction-bonded silicon carbide (RB-SiC) is a composite ceramic that comprises of hard SiC grains and brittle Si matrix. Surface texture with well machining qualities, in particular, is difficult to be fabricated on RB-SiC in conventional machining conditions since the removal of material inevitably induces some unexpected surface defects. In this paper, the feasibility of using a wet abrasive jet machining process (wet AJM) to produce smooth and crack-free micro-features on RB-SiC was investigated. Three commercial abrasives were employed to test the machining responses. The hardness of abrasive was found to be critical and dominate the machinability of RBSiC. The hardest synthetic diamond (SD) abrasive could crush the SiC grains, thereby providing the highest machining efficiency and a relatively smooth face. The relatively soft abrasives, including aluminum oxide (Al2O3) and green SiC, tend to break or rebound during the impact. After removing the Si matrix, SiC grains would be exposed and be to some degree protective of the target; therefore, the machined texture was shallow and rough. Since the particles in the lateral flow rolled freely, there were no obvious cracks on the inner surface of the machined texture compared to the machining without water. The variation of the machining profile had an inherent correlation to the mask thickness, nozzle motion speed and particle size. With these optimized parameters, it is feasible to use the wet AJM to fabricate complex micro-structural arrays on RB-SiC with good precision and surface quality.
查看更多>>摘要：Sintered carbide tools with their edge chamfered have superior resistance against brittle fracture of the main edge. Such advantage is mainly attributed to the formed built-up edge (BUE) anterior to the chamfer face. However, as the cutting process, certain materials including cracked BUE in that vicinity will inevitably flow aside along the chamfer face, which eventually causes severe notch wear and lateral burr formation. Given this deficiency, both theoretical and experimental works were carried out for the cutting process with chamfered tools in the current study. In the theoretical part, the material flow mechanism was analyzed and modeled by classifying the process into three typical modes concerning specific chip-tool contact patterns, for which three sets of modified slip-line solutions were developed accordingly. Furthermore, a unique solution to the threedimensional (3-D) material flow status was achieved by combining slip-line theory with the slab method. On this basis, the effects of tool preparation factors, i.e. chamfer width, chamfer angle, tool-workpiece friction coefficient and rake angle, on the BUE extrusion were discussed in detail. The results indicate that with proper chamfered edge design, accumulative BUE will be flowing in an optimal condition. In this case, the lateral burrs were found effectively restrained as had been evidenced by conducting extensive orthogonal cutting experiments with chamfered tools. By comparing with released models and experimental measurements, the present model yields more accurate predictions of material flow states. These findings of present study can contribute to the optimal design of cutting tools with chamfered edge.
查看更多>>摘要：Pointed diamond tool has been widely used in ultraprecision manufacturing of V-grooves. This article provides a wear law and wear characteristics of pointed diamond tool during V-grooves machining with the consideration of the cutting stress and the strength of diamond, which could be used to estimate the tool wear state and control the V-grooves machining quality. In this research, the model of cutting stress distribution and cutting force in V-grooves machining are established, which indicate that the maximum normal stress is independent on the cutting depth. Then, the tool tip fracture condition and cutting edge wear law of pointed diamond tool are further explored. The obtained law and experiments results shows that the tool wear is reflected in the cutting edge radius increasing while the wear rate is high at the beginning and then decreases gradually. Besides, the wear characteristic of the tool tip is different from the side cutting edge due to the diamond strength anisotropy. Raman spectra analysis indicates the wear mechanism of the tool is mechanical friction wear in machining of Al 6061 V-grooves.
NSTL
Elsevier