查看更多>>摘要：To further understand metadynamic recrystallization, various measurement techniques were performed with two materials: SUS316 austenitic steel and A5083 aluminum alloy, which were selected to represent materials with low to medium and high stacking fault energies, respectively. Microstructure characterization using electron backscatter diffraction with the grain orientation spread approach was utilized as a direct measurement technique to study metadynamic recrystallization. Additionally, indirect measurement techniques, including double pass compression, stress relaxation, and microhardness tests, were conducted. A new approach, including its simplified version, is proposed to determine the recrystallized fraction from the stress relaxation test, which is effective even in the case of incomplete recrystallization. A comparison between various approaches for determining the recrystallized fraction shows their advantages and disadvantages. A further understanding of the metadynamic recrystallization is achieved. First, with careful setup of water cooling, unlike the conventional understanding that metadynamic recrystallization does not require an incubation time, the incubation time is approximately 1 s in both materials and can be reduced to 0 s at high temperatures in SUS316. Second, the retardation effect of external stress on recrystallization in both materials is different because on the one hand recovery competes with recrystallization, while on the other hand recovery promotes the nucleation of recrystallization.
查看更多>>摘要：Low-frequency vibration-assisted metal plastic forming is a novel promising technology, adopting a highpressure-capability device compared to the ultrasonic vibration system. This work focuses on the lowfrequency vibration assisted upsetting (LFVAU) process of Ti45Nb, to reduce the flow stress and crack initialization in the cold deformation of titanium alloy. The results show that the flow stress is significantly reduced by applying the vibration, and this stress reduction phenomenon becomes more obvious with larger frequency and amplitude. The vibration promotes the dislocation movement and grain rotation, resulting in dislocation annihilation in the shear band but dislocation concentration and low-angle grain boundary formation near the grain boundary in the large deformation zone. Therefore, more homogenous deformation occurred in LFVAU compared to the conventional quasi static upset (QUS) specimen, though the grain size is similar in the cylinders after QSU and LFVAU. Based on the above research results, the application of low-frequency vibration assisted processing technology in the field of aerospace will help to improve the plastic deformation quality, especially the riveting quality for Ti45Nb.
查看更多>>摘要：This article presents a new method to suppress the chatter vibrations in the milling process of the weakly-rigid workpiece through a moving device. The basic idea is to design a moving fixture to instantaneously and continuously provide additional stiffness and damping to the instantaneous contact position between the moving fixture and the workpiece, and as the device moves, the chatter vibrations of the workpiece during the whole process are suppressed without the need of any other additional hardware. Two theoretical means, i.e. the structure split-assembly technology and the fractal contact theory, are combined to mathematically model the underlying dynamic behavior of the workpiece under the support provided by the moving fixture. The workpiece is divided into three parts, i.e. the machined part, the machining part and the part to be machined. The dynamic response of each part is separately analyzed and then theoretically assembled to calculate the whole workpiece's varying dynamic characteristics, which involve the support influence induced by the moving fixture. The effect of material removal on the workpiece's dynamic characteristics is also integrated into the analyzing procedure. Based on the above analysis, the dynamic governing equation of the thin-wall milling process with the moving fixture is finally established and solved to predict the stability lobe diagrams (SLDs). The validity and reliability of the proposed method are verified by conducting a series of actual milling tests with and without the support of the designed moving fixture.
查看更多>>摘要：Machining-induced white layers and severely deformed layers are undesirable surface integrity features which can be formed when machining high-strength aerospace alloys. An orthogonal milling process has been designed and performed to assess the impact of cutting speeds, tool wear, cutting edge radius and climb vs conventional milling on white layer formation and plastic strain distribution. The plastic deformation in the machined surface associated with the formation of white layers in Ti-6Al-4V has been quantified using micro-grids of different length scales printed using the electron beam lithography technique. It was found that white layers formed via the severe plastic deformation mechanism, at equivalent plastic strain values in excess of 1.2 and in regions of the cutting arc with the instantaneous chip thickness of less than the cutting-edge radius and ploughing and rubbing being the dominant mechanisms. The results indicated that the magnitude of the measured strains and the depth of plastically deformed material was greater at lower cutting speeds, during climb milling and when machining with a larger cutting edge radius and tool flank wear land.
查看更多>>摘要：An alternating magnetic field was designed to increase weld depth of laser-MIG hybrid welded 316 stainless steel joint. The influencing mechanism of alternating magnetic field on improving penetration ability of hybrid heat source of laser-MIG hybrid welding was revealed. The results suggested that the difference of weld depth under different magnetic flux density was attributed to the change of characteristics of arc plasma and laser-induced plasma, droplet transition and keyhole behavior. More compressed arc shape under lower magnetic flux density (LMFD) with 20 m T and 30 m T promoted laser-induced plasma to move along the channel of laser beam, leading to decrease of weld depth. However, the laser-induced plasmas could be dispersed away from the channel of laser beam under higher magnetic flux density (HMFD) of 60 m T and 90 m T, which was the main reason for the increased penetration ability of hybrid heat source. Besides, the droplet transferred to behind the keyhole instead of dropping into the keyhole and more stable keyhole was obtained under HMFD, which were also beneficial for deeper penetration depth of laser energy. Moreover, the motion behavior of charged particles of arc plasma and laser-induced plasma and forces condition of droplet transition were discussed in magnetic field assisted laser-MIG hybrid welding.
查看更多>>摘要：This study investigates flank face friction while micro machining commercially pure titanium (cp-Ti grade 2) work material considering size effects. It is important to understand friction phenomena at the tool flank and work material surface since they affect the surface integrity of the machined parts. A single crystal diamond grooving tool is used in machining experiments to reduce the influence of cutting edge radius. In addition, plunging type of cutting experiments were performed to investigate the influence of flank face contact on the machined surface. A friction model which is based on work and tool material properties is proposed to model the contribution of adhesion and deformation of the flank face coefficient of friction. The results show that for the cpTi and diamond tool pair, adhesion seems to be the dominant model of friction and also contributes to the size effect. The deformation friction becomes more dominant during the chip formation stage. When cutting edge effect is eliminated, the influences of flank and rake face friction on the size effect are shown.
查看更多>>摘要：Laser texturing of cutting tool is a promising surface modification method for improving the machinability and form accuracy of the machined workpieces in the precision machining of difficult-to-machine material. However, the blocking of micro-textures by the cutting chips, i.e., derivative cutting, usually occurs because of the severe friction between the tool and chip, which can hinder the effectiveness of micro-textures prepared on the rake face of the cutting tool. To address this issue, an external magnetic field was superimposed on the micro-textured tools in order to promote the infiltration of magnetic nanofluid into the micro-textured tool-chip interface, and the possible mechanisms for the effect of magnetic field on the inhibition of derivative cutting caused by the micro textures was for the first time revealed in this research. Herein, the cutting of 316 L stainless steel with the micro textured tools was performed under the magnetic field, and the effect of different parameters of magnetic fields on the machining characteristic of micro-textured tools (TT) under Fe3O4 nanofluid (MNF) lubrication condition was investigated. The results reveal that the MNF could be effectively migrated into the micro-textured tool-chip interface in presence of the external magnetic field, and consequently the degree of derivative cutting caused by the micro-textured tools was effectively suppressed. Thus, the machining characteristic of TT tools was gradually improved with increasing the magnetic field strength from 300 to 1200 Gs. Besides, infiltration mechanism of MNF at the micro-textured tool-chip interface under different parameters of magnetic fields were discussed.
查看更多>>摘要：To broaden the applications of Hastelloy X superalloy manufactured by laser directed energy deposition (DED-L), ultrasonic micro-forge treatment (UMT) assisted DED-L was developed to print Hastelloy X components with improved mechanical properties. In this study, the residual stress and mechanical properties of the non-UMTand UMT-treated samples are compared, and the microstructure was examined to reveal the root cause. During additive manufacturing, the sample structure recrystallized from columnar grains to equiaxial grains, which exhibited much finer structures after UMT. The impact toughness, ductility, tensile strength, and microhardness of the superalloy increased after UMT. During the UMT process, the dislocation pileups appeared and offered hinderance to the dislocations moving within it, thereby improving the mechanical strengthening of the Hastelloy X superalloy.
查看更多>>摘要：Ultrasonic vibration has been widely studied because of its excellent utility and performance in the plastic forming of metals. The mechanism and effects of ultrasonic vibration on material flow and deformation have therefore become a focus for current research. The mechanisms of ultrasonic vibration as it acts on materials include the volume effect and the surface effect. Double Cup Extrusion (DCE) testing is an ideal method for studying the surface effect but there have been few studies and some conclusions from those studies have been contradictory. For the present study, a new ultrasonic vibration-assisted DCE device was specifically designed and used for the experiment. In the tests, friction factors were calibrated and analyzed using the finite element method (FEM). The results show that ultrasonic vibration can effectively improve interface friction conditions in DCE. By changing ultrasonic loadings, an explanation was found for the 'illusion' that ultrasonic vibration increased interface friction, as had previously been reported. It was also shown that ultrasonic vibration exhibits a significant size effect, which can efficiently offset the negative size effect of friction in the micro-forming process. Further, it was found that the ultrasonic effect is influenced by the wall thickness of the formed part-the smaller the wall thickness, the more significant the effect of ultrasonic vibration.
查看更多>>摘要：ABSTR A C T Ultrasonic weld (USW) techniques for thermoplastics are used to be limited identical polymers, which obstruct the increasingly development of dissimilar polymers joining. In this study, as a direct-joining method without using environmentally unfriendly adhesives, ultrasonic thermal welding by the third phase (TWTP) was suc-cessfully proposed to join the immiscible thermoplastics. Poly(methyl methacrylate) (PMMA) and poly-formaldehyde (POM) were chosen as the work piece (WP) materials. The third phase of functionally graded material (FGM) were prepared from PMMA, POM and polylactic acid (PLA). The optimal welded strength of 47 MPa (approximately 80 % of tensile strength of POM matrix) was achieved, which is satisfied to the light-weight demand in automotive industry. Meanwhile, the welded interfacial morphology can be divided into four parts, which is related to the diffusion depths of interposed sheet (IPS) and WP molecules, respectively. Based on the interdiffusion analysis, the interdiffusion chains can give the similar configuration as that of the IPS due to the steric hindrance and Van der Waals force.
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Elsevier