查看更多>>摘要：The conventional single point incremental forming (SPIF) process is unable to perform high geometrical accuracy and formability for the Ti-6AI-4V alloy sheet. In response, this article has proposed a reliable high-frequency induction heating-assisted SPIF system. Rapid localised heating (600 degrees C and 700 degrees C) was integrated with a synchronized Inconel 625 Nickel alloy ball-roller forming tool to achieve high geometric accuracy and surface quality. This article also produced new insights into correlating mechanical and microstructural properties in SPIF at 600 degrees C and 700 degrees C. By investigating the mechanical properties (forming force, geometric accuracy, thickness profile), an explicit finite element (FE) simulation was established to predict the results. The output strain history from the FE simulations was used as input and integrated with electron backscatter diffraction (EBSD) and micro-hardness characterisations, to form a constitutive model (Arrhenius model) to calculate the Zener-Hollomon parameter (Z-parameter). The grain size and micro-hardness experimental results were corre-lated with Z-parameter calculation to predict the microstructural development at the initial, middle, and final stages of the deformation process. The mechanical results revealed that the 700 degrees C experiment performed enhanced geometric accuracy and thickness profile, with a reduced forming force. However, the surface quality is reduced as the lubricant dissipated rapidly, while the ball-roller tool effectively compensated for this behaviour by reducing the friction. At the microstructural level, 600 degrees C revealed strong strain hardening and grain deformation, and 700 degrees C revealed better grain refinement by dynamic recovery (DRV) and dynamic recrystal-lisation (DRX). A proportional relationship between Z parameters and grain size and a low-high-low micro-hardness profile was proposed.
查看更多>>摘要：Conventionally, the main goal of process optimization in selective laser melting is to achieve the highest relative density. However, for Inconel 718, this study has demonstrated that the highest relative density does not correspond to the best mechanical properties. Moreover, similar relative densities can result in significant differences in mechanical properties. This phenomenon arises from the presence of cracks in the microstructures. The research was carried out to study the problem systematically using combinations of 2 layer thicknesses (40 and 50 mu m) and 2 laser energy densities (3.17 and 3.47 J/mm(2)). Microcracks were observed near the melt pool boundaries and within the heat-affected zones of the newly deposited layer, occurring along the grain boundaries and interdendritic regions. Evidence was obtained to show that the cracking was associated with remelting of Laves phase; therefore, it was identified as liquation cracking. It is interesting to observe that layer thickness has a much greater influence on crack formation than laser energy density, owing to the significant change in the melt pool shape and grain boundary morphology when the layer thickness was changed. The influence of laser energy density was only observed at the larger layer thickness as the severity of cracking was amplified when laser energy density was increased due to microstructural coarsening. Thus, this presents a unique problem in additive manufacturing (AM) regarding liquation cracking in Inconel 718 as one of the key differences from conventional manufacturing is the successive heating and reheating of multiple layers of material in AM.
查看更多>>摘要：High-efficient welding technology with rotating arc assisted by laser under shielding gases with different He content was investigated for the application of 9% Ni steel in LNG storage. High-speed image and electrical signal acquisition were adopted to investigate arc stability and metal transfer behavior during hybrid laser arc welding. The results indicated that with increasing content of He, the weld formation became better first and then worse. Ar+30 %He improved the formation of welding seam. The He content had a significant influence on the electric signal waveforms. With the addition of 30 %He, the waveforms had the minimal fluctuation. It was rotating spray transfer with pure Ar, Ar+30 %He and Ar+50 %He. The difference was that with the He content increase, transfer mode shifted from unstable rotating spray transfer to stable rotating spray transfer, to irregular rotating spray transfer. With pure He, it was large droplet transfer, resulting in the formation of large spatters. The research of rotating spray transfer trajectory showed that the rotation velocity was relatively constant and one rotation time was 3.19 ms in one pulse period under Ar+30 %He. The trajectory equation was obtained for describing the rotating spray transfer. This indicated that the addition of 30 %He improved the coupling effect of laser and arc, enhanced the matching degree between rotating spray transfer and arc pulse period. It was meaningful to understand the effects of shielding gases on 9% Ni steel hybrid laser arc welding, providing theoretical basis for LNG storage efficient and reliable manufacturing.
查看更多>>摘要：Vibratory-based manufacturing processes such as polishing, and peening are widely used for surface enhancement applications. Vibratory peening is an excellent process to introduce residual stresses, improve fatigue life, and enhance the surface quality of metallic components. This research aims to understand the combined peeningpolishing mechanism in the vibratory peening process of a gas turbine engine's 3-stage blisk using a novel horizontal vibratory peening method. The research was aimed to understand the common understanding that the deeper the component in the vibratory machine, the higher would be the media-component interaction effect and better would be the peening effect. However, contrary to the hypothesis, the experimental findings showed significant peening (60.0 % higher than bottom) in the coupons near the top surface of the media in the trough.
查看更多>>摘要：Heat straightening of welding-induced distortion, which has mostly relied on shipyard experience, was systemized for computation. The process of geometrically flattening a curved surface with fixed edges was formulated step by step based on thermal forming mechanisms. The proposed algorithm divides the measured surface into patches, connects them with virtual springs, and optimally arranges the patches on a plane in a direction that minimizes the potential energy within the weld lines. The overlaps between the geometrically calculated patches become the amount to shrink. The relationship between spot heating distance, curvature, and shrinkage was investigated through experiments and thermal elastic-plastic analysis to convert shrinkages into process parameters. It showed that the plastic shrinkage increases as the spot heating distance narrows, but the rate decreases. Also, the plastic shrinkage keeps constant regardless of the curvature, whereas the total shrinkage decreases as the curvature decreases. For validation of the proposed methodology, flame straightening experiments were carried out. It has been shown that the case using the plastic shrinkage produced fewer heating spots with higher efficiency than that using the total shrinkage.
查看更多>>摘要：Ultrasonic vibration has been applied to suppress the chemical tool wear in ultra-precision diamond cutting of steel and other alloys. The tool wear is found significantly reduced with enriched ambient oxygen concentration. In this study, instead of supplying costly and potentially hazardous high concentration oxygen to the machining zone, a greener alternative, high-pressure air coolant has been applied to enhance the oxygen content at the cutting area. The effect of air-blowing pressure on ultrasonic vibration cutting of stainless steel was investigated, using both polycrystalline and single-crystal diamond tools. It has been found that an increased air blowing pressure and a more closely positioned nozzle not only improve the cutting performance but also reduce tool wear, reflecting consequences from both chemical and fluidic dynamics aspects. A higher air blowing pressure will increase the instantaneous air pressure as well as the density of oxygen molecules around the cutting zone. This increases the passivation speed for the freshly cut steel surface caused by the formation of thin oxide layers. Experimental results have formalized the findings that sub-millisecond oxide formation is the key factor to suppress diamond tool wear.
查看更多>>摘要：More and more experiments have shown that particle-reinforced metal matrix composites (PMMC) exhibit obvious size effects. The aclassic plasticity theory does not contain internal length scale and cannot explain this size effect. In this paper, based on the Taylor relationship, the concept of "geometrically necessary dislocations" and the mechanism of dislocation multiplication, slippage and extinguishing, a constitutive equation for SiCp/Al composites related to strain gradient is established. The established equation is imported into Abaqus for simulation by writing a user subroutine Vumat. Combining the simulation and experimental results, the effects of stress, temperature, cutting force, strain gradient effect and its dimensional effect on cutting deformation during machining SiCp/Al composites are analyzed from the perspectives of material micro-plasticity mechanics and material dislocation theory. The results show that the presence of SiC particles changes the microstructure of the matrix material, and induce a high strain gradient in the matrix. This high strain gradient makes the material more prone to shear deformation localization. In the cutting process, the defects and breakage of the SiC particles themselves will lead to the formation of micro-cracks. The growth of micro-cracks in the shearing area is an important factor in the generation of chips. By comparing the simulation results with the experimental results, the modified constitutive model is closer to the experimental results, indicating that the established theoretical model based on the strain gradient can better reflect the cutting process of particle-reinforced metal matrix composites.
查看更多>>摘要：Micro diamond tool has been developed as an essential cutting tool to fabricate the functional microstructure arrays, and its waviness and sharpness directly influence the accuracy and quality of the microstructures. The dependency of waviness and sharpness on the cutting edge dynamic strength is discussed in present work. To fulfill this objective, firstly, an analytical model of cutting edge dynamic strength is established to predict the spatial distribution of dynamic strength on the whole cutting edge. Secondly, the influence mechanism of dy-namic strength on the waviness and sharpness is revealed, and two comprehensive parameters are further defined to evaluate the influences of cutting edge dynamic strength on the waviness and sharpness. Finally, according to the different machining demands, two criteria are proposed for designing micro diamond tools in light of the analytical model and comprehensive parameters.
查看更多>>摘要：The combination of adhesive and other joining processes has been an increasing interest in the transportation industry. In this paper, a hybrid joining process combining friction self-piercing riveting (F-SPR) and adhesive bonding was developed to join AA7075-T6 aluminum alloy sheets. The formation process, macro morphology, microstructure, microhardness, and mechanical performance of F-SPR bonded joints were investigated comparing with the F-SPR joints. It was concluded that the adhesive played a role of lubrication for reducing the contact stiffness between the aluminum sheets, which further reduced the interlocking amount but did not affect the solid-state welding between the aluminum sheets. After the F-SPR bonding process, the initial 0.3-mm-thick adhesive layer was squeezed to less than 0.06 mm under a maximum riveting force of about 20 kN. The subsequent baking treatment for the adhesive curing (30min@180celcius) re-precipitated the eta strengthening phase in the aluminum heat-affected zone, which further improved the aluminum hardness as well as the mechanical performance of the joint. The F-SPR bonded joint exhibited a combination of adhesive failure and rivet pull-out failure under quasi-static loading, which improved the tensile-shear strength by 128.7 % than the baked F-SPR joint but had no apparent effect on cross-tension performance. The F-SPR bonded joint also superposed the fatigue failure modes of the F-SPR joint and adhesive bonding joint, exhibiting the longest fatigue lives under the same cyclic load amplitude. The process provides a new method for aluminum alloy sheet joining in body-in white production.
查看更多>>摘要：Laser direct deposition (LDD) is a typical additive manufacture process for complex parts, while the highly complex thermal behavior during LDD results in the generation and randomness of parts' internal defects. Among these internal defects, the maximum defect-induced fatigue cracks initiation is the most influential factor of fatigue life for the in-service performance. Apparently, how to estimate the random maximum defect size of surface polished parts is critical for predicting the fatigue life. Therefore, according to extreme value statistic (EVS) theory, an extremum probabilistic estimation method from small sample size was proposed for parts' random maximum defect size from a sub-volume to the whole-volume. Subsequently, with the obtained maximum defect and being taken to be equivalent to cracks, a defect-related fatigue life prediction model was established based on the failure critical stress. Orthogonal experiment was carried out for obtaining the different maximum defect, and the hardness for each sample was measured as well. The results showed that: (1) The proposed method can reliably estimate the maximum defect size of LDD-316L parts under the small defect samples size and the error was within 10 %. (2) The established prediction model provided a process independent method for directly estimating the LDD-316L parts' fatigue life, with the accuracy being over 78 %. This research provides a novel methodology for estimating parts' maximum defect size and fatigue life, and offers a theoretical basis for reliability and economy of parts during manufacturing and servicing process.
NSTL
Elsevier