查看更多>>摘要：Ultrasonic vibration cutting (UVC) has proven its effectiveness in suppressing the severe chemical wear of diamond tools in machining of metals with strong chemical affinity to carbon, and the formation of inert oxide film on the freshly cut metal surface is expected to further extend the tool life due to the oxide's passivation effect. Based on the existing tool wear suppression mechanism to slow the diamond graphitization, this paper proposed ultraviolet-ozone assisted UVC to further enhance the oxide formation and improve the diamond tool life. A numerical model for calculating the oxide formation rate is developed to study the influence of supplying gases with different oxidizing capabilities. Meanwhile, an in-situ ultraviolet-ozone assisted UVC system is in-house built, and the diamond tool wear for cutting of stainless steel and tungsten are studied. Experimental results show that, compared to air, oxygen and ozone, ultraviolet-ozone assisted UVC demonstrates the highest oxide growth rate and the longest tool life accordingly, which further verifies the theory that metal oxidation performs an important role in suppressing the chemical wear of diamond tools.
查看更多>>摘要：Cutting tool geometries play an important role in tool performance, material flow, cutting force, and cutting temperature distribution. The most common edge geometries of commercial tools are hone, chamfer, and chamfer with a hone. A novel method for designing tool edge geometry that combines experimental and simulation results is investigated in this paper. An uncoated carbide tool was used to orthogonally cut AISI 1045 and AISI 4140 steel. By observing how the tool geometry changed during the machining process with white light interferometry, a new tool wear geometry model could be proposed. Furthermore, temperature and stress distribution modeling were carried out with Finite Element Analysis (FEA) based on the newly designed tool geometry under various cutting parameters. A comparison of the machining results of the new and conventional tool geometries has confirmed that the newly designed tool is capable of achieving a lower wear rate, thereby extending the tool life.
查看更多>>摘要：Al-Mn-Sc based alloys present potential in manufacturing high-performance aluminum alloy by selective laser melting (SLM) technology. However, the solidification and solid-state phase transformation behaviors induced by SLM layer-by-layer deposition and the interaction between the Al-Mn and Al-Sc systems result in complexity in the precipitation of Mn-bearing phases and Al3Sc. In this work, ternary Al-5Mn-xSc (x = 0, 0.7, 1.5 wt%) alloys were fabricated via SLM, and their microstructure and mechanical behavior were investigated. The results indicated that the Sc/Mn ratio and the precipitation sequence were critical in controlling the grain and precipitate characteristics of the Al-Mn-Sc alloy deposits. The precipitation of primary Al3Sc was inhibited by the pre-precipitated Mn-bearing particles in Al-5Mn-0.7Sc alloy, resulting in columnar grain structure similar to that of Al-5Mn. In the case of Al-5Mn-1.5Sc alloy, the preferentially precipitated Al3Sc induced a bimodal grain structure. However, with the mismatch in plastic deformation capacity between different grain regions, micron-sized Al6(Mn,Fe) particles formed around the equiaxed/columnar grain region interface tended to induce cracking.
查看更多>>摘要：With the rapid progress of science and technology, complex optical surfaces have been widely used in various devices. Due to the complexity of their geometric features, machining with a single trajectory inevitably has an impact on the accuracy of the machined surface. Sub-regional milling has been studied intensively by researchers worldwide. However, few studies have focused on sub-region polishing. Therefore, sub-regional polishing is not widely used in the machining field. This paper investigates the method of sub-regional polishing of complex surfaces and focuses on the selection of polishing trajectories for different processing areas. The method takes into account the curvature characteristics of each point on the surface and combines the Freeman code method to divide the surface into multiple processing regions. The selection of polishing trajectories for each processing area is analyzed from the viewpoint of surface roughness and power spectrum density analysis. The correctness of the trajectory selection is verified by polishing experiments on K9 optical glass. The results show that all three trajectories used in the experiments can reduce the surface roughness of the workpiece by more than 90%. Using the Hilbert trajectory can effectively reduce the medium-frequency and high-frequency errors of the machined surface and improve the optical properties of the surface. This work is expected to provide significant guidance for efficient and high-quality processing of complex optical surfaces compared with traditional machining methods using a single trajectory.
查看更多>>摘要：Selective laser melting (SLM), which is an advanced manufacturing method developed in recent years, produces parts with complex shapes and critical dimensions using metallic powder and laser beam. In this study, copper based conductive materials with silver networks were fabricated by electroless silver coating and SLM. The effects of laser power, scanning speed, hatch spacing, build orientation and post treatments on the mechanical and tribological properties of SLM compacts manufactured from core-shell particles were investigated. After the mechanical tests, higher stress and elongation values were obtained in the sample positioned horizontally on the build platform with a laser power of 100 W, scanning speed of 250 mm/s and hatch spacing value of 45%. After that SLM process, the post-processes were applied to the samples and the physical and mechanical properties improved significantly. The highest density value was obtained by the rolling process at 900 °C, and an increase of approximately 35.6% was obtained compared to the sample without post-processing. The lowest wear rate among all samples was obtained in the hot-pressing process at 750 °C.
查看更多>>摘要：Additive manufacturing (AM) commonly utilizes slicing techniques to create layers of a model, in which material is deposited layer-by-layer. However, slicing method directly affects the mechanical properties of the printed parts. This paper introduces a new AM technique (named as Helical5AM), which employs print-paths having helical geometry for five-axis AM. Given an object to be printed, a base (supporting helical print-paths) with a center curve and helix parameters (such as lead angle and turn direction), a complete volumetric coverage using helical print-paths is obtained. A collision-free tool orientation is then generated using a probabilistic roadmap algorithm for depositing material along the helical print-paths by avoiding tool interference with obstacles. As a proof of concept, print-paths (of models) with orientation information obtained using the proposed algorithms are simulated using a five-axis AM simulation software, and material deposition process in Helical5AM is demonstrated using a five-axis AM machine. Furthermore, compression tests are performed on the printed parts for evaluating the effects of helix lead angles of the helical print-paths on mechanical properties of the printed parts. It has been confirmed that mechanical behavior of a printed part is predictable and tunable according to the helix lead angles of the print-paths. Helical5AM can potentially empower engineers to obtain AM parts with desirable mechanical properties.
查看更多>>摘要：Cf/SiC composites are continuous fiber-reinforced ceramic matrix composites with excellent properties, such as high-temperature resistance, high specific strength, and low coefficient of thermal expansion. However, Cf/SiC composites often cause severe tool wear and quality problems. In this paper, tool wear is studied during the machining of Cf/SiC materials. The cutting force model is applied to understand the correlation between cutting force and tool wear. Polycrystalline Diamond (PCD) tools with various point and clearance angles are designed to carry out drilling experiments on Cf/SiC composites with different machining parameters. During experimental investigations, force and acoustic emission signals are analyzed, and PCD tool wear and material machining quality are observed using scanning electron microscopy. Results indicate that PCD tools with a point angle of 140° and a clearance angle of 20° demonstrate the lowest tool wear during machining. Moreover, low-speed and feed machining parameters can effectively reduce tool wear and prolong its life. By observing fracture patterns of hole wall fibers and chip shapes at different stages of tool wear, and combined with the frequency domain distribution of AE signal, the effects of tool wear on the material removal mechanism are summarized. Before tool wear, fiber fracture is flatter, the matrix is mainly removed via crushing. After tool wear, although fiber fracture is more uneven, the matrix is changed from crushing to friction removal, leading to lower surface roughness of the hole wall.
查看更多>>摘要：An ultrasonic frequency pulse high-current (UFPHC) was utilized to improve the stability of underwater wet flux-cored arc welding on Q235 steel. Based on the studies of the bubble evolution, arc behavior, droplet geometry, weld formation, and mechanical properties of the joint, the arc extinguishing phenomenon was suppressed effectively by the additional UFPHC, which decreased the amount of spatter on the workpiece and improved the appearance of the weld. The additional UFPHC reduced the average droplet radius by 40% and increased the average transition frequency by 99%, compared with the same value of additional welding current. The UFPHC enabled to decrease the hardness of the heat-affected zone by 15% due to the production of less martensite compared to results obtained by simply increasing the welding current. The vibration effect of the UFPHC in the welding pool was indirectly confirmed by the significantly decreased porosities in the final joint. The morphology of the impact fracture of different welding cases showed that the UFPHC was helpful to decrease the brittle fracture area in the weld joint and increase the value of the impact toughness for the weld joint.
查看更多>>摘要：Ultrasonic-assisted grinding (UAG) is a suitable machining solution for hard and brittle materials, effectively reducing cutting forces and improving the machined surface quality. With the development of UAG system, multi-dimensional vibration devices have emerged and been applied in the machining of advanced materials. In this study, a discrete numerical model is proposed for two-dimensional ultrasonic-assisted grinding (2D-UAG) of silicon carbide (SiC) to describe the dynamic cutting behaviors and a grinding force model considering the material removal mechanism is further established. In the model development, the three-dimensional surface topography of grinding wheel is simulated and adopted to determine the cutting state of individual grit. Meanwhile, a workpiece profile update procedure and a novel approach to realize the decomposition and synthesis of grinding forces are proposed. Experimental validation demonstrates that the prediction error of this proposed model is limited within 11.14% with an average value of 8.16% under conventional machining parameters. Furthermore, the tool amplitude and workpiece amplitude affect the grinding force in a similar manner. As the ultrasonic amplitude increases from 1 μm to 9 μm, the grinding force declines first and then goes up and the inflection point appears around 5 μm. The inherent characteristics of grinding forces in 2D-UAG of hard and brittle materials can be explained by obtaining the variation trends of the average cutting depth of individual grit and the number of active grains. The modeling methodology presented in this study provides a reference for studying the 2D-UAG process of brittle materials.
查看更多>>摘要：The steady magnetic fields were utilized to decrease the porosity of aluminum alloy laser welded joints, and the effect is related to the magnetic field orientation. The influence of the magnetic field orientation on the suppression of porosity was investigated by establishing a three-dimensional heat flow model coupled with magnetohydrodynamics (MHD) and free surface of a keyhole. The Lorentz force decreased the backflow velocity toward the keyhole wall when the magnetic fields were horizontally parallel to the welding direction (magnetic field X-axis, MFX) and perpendicular to the welding direction (magnetic field Y-axis, MFY), thereby clearly improving the keyhole stability. Additionally, the melt flows in the rear region of the molten pool were suppressed downward and forward under the MFX and MFY, respectively, leading to additional heat accumulation in the rear periphery of the molten pool. As a result, a long molten pool in the longitudinal section and a small penetration depth were observed under the MFX and MFY. Therefore, the porosity defects decreased from 9.4% to 3.5% and 3.2% under the MFX and MFY, respectively. However, many pores were observed when the magnetic field orientation was vertical (magnetic field Z-axis, MFZ), owing to the high penetration depth and poor keyhole stability. This study provides a deeper understanding of the effect of magnetic fields on the laser welding of aluminum alloys and potentially lays a foundation for the adjustment of suitable magnetic field parameters toward obtaining the desired effect.
NSTL
Elsevier