查看更多>>摘要：Cold spraying (CS) is one of the most recently used additive manufacturing (AM) processes that allow forming large 3D objects efficiently without severe thermal effects. However, due to some limitations of the CSAM process, it is difficult to achieve the desired geometry with high precision. As a potential solution to this issue, CSAM process coupled machining, that is, hybrid addition and subtraction manufacturing, is proposed to achieve desired accuracy and reliable shape. Therefore, it is necessary to assess the mechanical reliability of multilayer deposits deposited by CSAM + machining processes before that. In this work, different bi-deposits were obtained via hybrid CSAM. The adhesion performance at the bonding interface of bi-deposits was assessed by mechanical measurements including tensile and shear failure tests. The morphology of fracture surfaces was observed by scanning electron microscope. The results showed that CSAM bi-deposits with different materials and different interface states had different adhesion effects: the adhesion effect of soft/soft bi-deposits was the best, followed by hard/hard bi-deposits, and hard/soft bi-deposits were the worst. This is due to the fact that hard CS deposits are difficult to deform, making it difficult to embed with particles and build a good bonding; The bonding interface without any treatment had the best adhesion effect, followed by the milling + grit-blasting surfaces, and the milling surfaces performed the worst. The interface's roughness is the major influencing factor. Roughening of bonding interface promotes more embedding and interlocking effect, which improves bonding strength.
查看更多>>摘要：The precision forming of thin-walled hollow components with the complex surface is difficult due to the excessive thinning of small radius circular arcs and the springback of small curvature surfaces. The difficulty is because the pressure required for forming a small radius circular arc is larger than that needed for other regions, while the pressure supplied inside the hollow blank in a conventional hydroforming process is equal everywhere causing the forming pressure cannot meet the requirements of different deformation regions. To solve this problem, the thin-walled hollow component with the complex surface is formed precisely by employing selfadaptive non-uniform pressure. Herein, theoretical analysis of the self-adaptive non-uniform pressure of viscous medium acting on the blank and the stress distribution of blank during the circular arc filling process is conducted. Then, different viscous medium parameters are employed in the forming of hollow components with different circular arc radii by numerical simulation. The results indicate that the non-uniform pressure is formed during the forming process, and is affected by the mechanical properties of viscous medium. The stress and strain distribution of blank are uniform, as well as the wall thickness thinning ratio and springback are small when the appropriate viscous medium is selected. As an application, the thin-walled hollow turbine blade is manufactured. The wall thickness is uniform and the dimensional deviation is kept at a lower level.
查看更多>>摘要：Plastic flow of hollow cylinders under combining axial compression with torsion was examined in upsetting and in lateral extrusion. The hollow cylinders with initial height/outer diameter of 0.5-2.5 and initial inner/outer diameter of 0.3-0.9 were compressed/extruded and simultaneously twisted with respect to the axisymmetric forging axis in one-way or cyclic alternating. Due to superposition of torsional shear stress, the lateral metal flow was found to be assisted. Barreling of the workpiece was reduced by approximately 10 % in upsetting with torsion/compression speed of 6 degrees /mm. The enhancement of the plastic flow in upsetting was applied to fill up tooth cavity in lateral extrusion. The extruded tooth depth of the workpiece was approximately 5-7% larger in lateral extrusion with torsion/extrusion speed of 30 degrees /mm. In addition, cyclic alternating torsion was effective to extrude evenly in the tooth thickness direction due to the periodic reversal of die rotation in peripheral direction.
查看更多>>摘要：Ultrasonic vibration has been applied in the incremental sheet forming process and shows the potential to reduce the forming force and increase the formability, but the constitutive behavior and deformation behaviors of material under the high-frequency vibration are still not completely explained. In the present work, a hybrid constitutive model which combines the phenomenological, thermal activation and dislocation annihilation models is established and experimentally calibrated. Then, a finite element model is developed for the ultrasonicassisted incremental sheet forming (UISF), in which the hybrid constitutive model and a user defined subroutine were incorporated to describe the ultrasonic effect and to simulate the high-frequency vibration, respectively. Compared with the experimental forming force, the FE analysis with the hybrid constitutive model shows good predictability with errors less than 10 %. Based on the simulation results, the improvement of formability was observed and the material deformation behavior during ultrasonic-assisted incremental sheet forming was analyzed in detail. Moreover, the material microstructural features were analyzed through Electron Back Scatter Diffraction (EBSD). The occurrence of relative larger grains and the increase of low angle grain boundaries (LAGBs) indicates the occurrence of ultrasonic-actuated dynamic recovery of the material.
查看更多>>摘要：Electrohydraulic sheet forming is a high-velocity manufacturing process, which utilizes a pressure pulse induced by underwater electrical discharge to plastically shape metals. It has attracted wide interest for its potential to shape metal materials with poor formability. Such advantages can be exploited by combining this process with the conventional stamping process or by using it solely. In both cases, the dominant deformation mode is stretching, implying substantial thinning of sheet metal. The goal of this paper is to improve the material flow control in electrohydraulic sheet forming. A controllable drawing deformation mode is introduced into the electrohydraulic sheet forming process by using a radial inward pulsed Lorentz force at sheet edge induced by an electromagnetic coil, which can substantially enhance the draw-in of the sheet flange. The flexible combination of the stretching deformation and the introduced drawing deformation allows active control of the high-velocity material flow behavior, thus enabling much better control of the final forming quality. About 20 % improvement of the forming height limit is observed from the experimental results. Furthermore, a numerical model is established to better understand the process mechanism, and the critical roles of the amplitude and the action timing of the radial inward Lorentz force are identified.
查看更多>>摘要：The present study addresses a thin-walled investment casting methodology of AZ91D-1 wt% CaO magnesium alloy for stent manufacturing. Additive manufacturing and Yttria-based coating were applied to optimize the process. An induction melting furnace assisted by vacuum was used to cast magnesium alloy stents with 0.4 mm and 0.8 mm wall thickness under an argon-protective atmosphere. According to the proposed methodology and the results obtained for coated models, no metal-mould interface reactions were found, revealing samples with a good surface finish and sane microstructure. The cast magnesium stents without coating were compromised, resulting in samples with a high degree of reactions and impurities. Microscopic examination and hardness measurements demonstrated the effectiveness of the process.
Speidel, AlistairWadge, Matthew D.Gargalis, LeonidasCooper, Timothy P....
13页
查看更多>>摘要：The high optical reflectance of Cu at near-infrared wavelengths narrows the process window to fabricate Cu parts by laser powder bed fusion (LPBF). Coating powders with optically absorptive materials has been investigated to improve processability and enhance part properties. However, given the intense heat localization and thin coating layers relative to the powder, the mechanisms of thin film coating interaction in LPBF remain unclear, despite recent work showing the importance of the near-track environment in deposition behavior. In this study, optically absorptive Zn-coated Cu powders were prepared by physical vapor deposition and characterized. Single LPBF tracks were fabricated to elucidate material incorporation phenomena influenced by the volatile Zn coating. It is shown that Zn-coated powder enhances accretion at fastest effective scan speed tested (100 mm/s), where mean track volumes are increased from 0.72 +/- 0.05 mm3 (as-received) to 0.91 +/- 0.01 mm3 (Zn-coated). This has been correlated to the stronger vapor jet from the volatile Zn-coating, which denudes the surrounding powder bed. This exhausts the powder bed at slower effective scan speeds, causing instability and balling when compared to the as-received powder. It is shown that Zn is localized at the track surface and is undetectable in the track bulk, indicating Zn vaporization on interaction with the incident beam. Zn present mainly occurs through secondary deposition mechanisms like spatter and condensation, rather than in-process alloying. Coating powder feedstocks for use in LPBF therefore affects composition, laser beam absorptivity, and the neartrack vapor environment that is known to influence material incorporation behavior.
查看更多>>摘要：Global formability and local formability are critical in different metal forming processes. Edge cracking, controlled by the local formability, is a dominant factor limiting the application of advanced high strength steels (AHSS) in automotive industries. The local formability of a medium-Mn steel (MMnS), a promising candidate of the third generation of AHSS, is evaluated based on forming limit curves at fracture and compared with a dualphase DP1000 steel using the damage mechanics approach. The superior tensile properties of the investigated MMnS, high tensile strength, pronounced strain hardening, large uniform and total elongation, lead to a very good global formability, which is an indicator of necking resistance. However, the local formability of the investigated MMnS, which is an indicator of fracture resistance and quantified by the plastic strain at fracture under different stress states, is worse than the DP1000 steel. By comparing the local and global formability of the two AHSS, it is confirmed that ductile fracture is the dominant failure mode in the MMnS and the onset of localized necking occurs prior to ductile fracture in the DP1000 steel. To achieve an accurate determination of the local formability, the effects of stress states need to be considered, which cannot be derived explicitly from uniaxial tensile tests. In addition to tensile properties, more attention should be paid to the local formability of new AHSS to assess their potential application in automotive industries.
查看更多>>摘要：Recently aluminum matrix composites (AMCs) manufactured by selective laser melting (SLM) have attracted extensive attention in the lightweight application fields. In this work, pre-alloyed 6.5 wt.% TiB2/AlSi10Mg composite powder was initially densified via SLM. The result indicated that the material had a wide processing window, and the fully dense bulked samples were successfully fabricated when the laser volumetric energy density is 50-70 J/mm(3). We concluded that the morphology, size, and distribution of TiB2 particles in the as deposited condition were generally consistent with those of the powder. The fine equiaxed microstructures without apparent anisotropy were remarkably realized. Therefore, a superior combination of excellent tensile strength and elongation was evident in the horizontal (similar to 536.9 MPa and similar to 16.5 %) and vertical (similar to 517.3 MPa and similar to 15.4 %) directions due to the systematic processing optimization, accompany with the strong work hardening capability. Its tensile properties are higher than that of the reported SLM-ed matrix alloy and other particle reinforced Al-Si composites. Besides, the fracture behaviors were complicated, and the crack propagation paths were affected not only by the defect and eutectic Al-Si of the molten pool boundary (MPB) but also by the homogeneously distributed TiB2 particles. This study is expected to establish an important guiding significance for the practical engineering application of SLM-ed in-suit TiB2/AlSi10Mg composite.
查看更多>>摘要：The performance of the selective laser melting (SLM) parts was critically affected by the surface quality and internal defects that are closely related to process parameters. An in-depth understanding of the relationship between the formation and evolution of surface and internal defects and process parameters is needed to achieve defect-free and high-performance SLM parts. In this study, the influencing mechanism of laser power, scanning speed, hatch spacing and layer thickness on melt pool morphology, surface quality and internal hole defect of SLMed 18Ni300 maraging steel was investigated. The thermal and physical behaviour and instability of the molten pool, as well as the formation and distribution behaviour of internal hole defects, were also analyzed and discussed. Recoil pressure, the insufficient overlap between tracks and remelting between layers, Plateau-Rayleigh instability and material aggregation caused by the Marangoni effect were characterized as the main factors closely related to molten pool morphology and surface quality. Within the selected parameters in this study, the obtained surface roughness and tensile strength range from 9.08-26.40 mu m and 544.14-1246.24 MPa, respectively. The internal defect changes from irregular lack-of-fusion at low energy density to the keyhole-included spherical hole at high energy density. In addition, the volumetric energy density (VED) has a certain limitation in predicting surface quality and mechanical properties due to the complex physical characteristics of the molten pool.
NSTL
Elsevier