查看更多>>摘要：By adjusting the initial delta-ferrite (delta) microstructure based on the cold-rolling, two types of unusual austenite (gamma) transformation were investigated in a medium-Mn steel during intercritical annealing. First, delta/alpha' (martensite) phase interfaces are considered advantageous nucleation sites for gamma transformation. When the cold-rolling reduction ratio reaches up to 70% (CR70 sample), delta phase morphology evolves from fibrous structure to small pieces, resulting in more delta/alpha' (martensite) phase interfaces. The results reveal that an increase in delta/alpha' phase interfaces promotes gamma formation ranging from 15.7% to 17.9% (in volume fraction) for samples subjected to quenching and annealing. Furthermore, the gamma nuclei are found to grow into both phases, accompanied by a gradual shrinking of local alpha' and delta phases due to gamma transformation. Secondly, precipitation of carbide particles from delta phase in cold-rolled samples also creates a condition for subsequent gamma nucleation within delta grains, and then the dissolution of the carbides accelerates gamma development. Besides, the thin piece structure of delta phase in the CR70 sample can transform into gamma phase at an elevated degree of austenitization. This study innovatively sheds light on the effect of delta phase on gamma transformation in medium-Mn steels.
查看更多>>摘要：The non-uniform grinding conditions due to the wheel-gear contact geometry lead to the changeable of grinding force in face gear grinding. As a result, the classical grinding force models for other gears are not applicable to the grinding process of face gear. In this work, a dynamic grinding force model for face gear was developed based on the wheel-face gear contact geometry. In this process, surface of a virtual pinion engaged with the face gear was divided into many micro-elements to calculate the undeformed chip thickness and number of effective abrasive grains. Considering the mechanical properties of the tool and workpiece materials, equations for the total normal and tangential force components were derived by using the grinding force model of single abrasive grain. For verifying the model, the measurement coordinate system was established to convert the spatial components of experimental grinding force into the normal grinding force and tangential grinding force. Experimental results show reasonable consistency with simulation results that validate the proposed model. Besides, main reason for the changeable of grinding forces from top to root of tooth surface can be summarized by the change of material removal rate caused by the interference grinding width of adjacent grinding trajectories. Characteristics of surface morphologies and surface roughness prove this reason as well. This work provides a method to obtain a uniform grinding force during the process of grinding face gear and can be utilized to optimize the grinding process for higher tooth surface integrity.
查看更多>>摘要：Although laminated metal composites fabricated by accumulative roll bonding (ARB) undergo kink-band formation, there are no guidelines for optimizing their fabrication to avoid necking and shear localization without post-annealing. The necking of the composite layers during rolling generates non-uniformities that may degrade the mechanical performance of the composite. This paper proposes a model for laminated metal composites that aims to quantitatively assess and predict necking based on the work-hardening of each layer. Cu-Al-, Cu-A5052-, and Cu-A5083-laminated metal composites were fabricated using ARB at room temperature (~300 K) without post-annealing, and they were subjected to mechanical and microstructural analyses. A normalized neck amplitude was introduced as a new parameter that accurately predicted the degree of necking observed experimentally. Another new parameter, an instability index, was introduced to frame the necking criterion of the hard layers. A plastic instability criterion framed based on the new parameters accurately predicted the degree of necking in the hard layers of the as-prepared laminated metal composites. These findings may be applied to guide the fabrication of metal composites of any composition using the ARB process.
查看更多>>摘要：Dynamic supporting friction stir welding (DSFSW) is proposed by developing a novel welding platform. The conventional backing plate is replaced by a small-size supporting column to reduce heat dissipation from weld back in the FSW of titanium alloy. TA5 alloy joints were produced via varied FSW techniques and examinations on macrostructure, microstructure and mechanical properties were carried out. Results reveal that DSFSW enhances material flow by greatly improving the effective heat input into weld, thereby eliminating the flowrelated cavity defect. Specifically, DSFSW enables defect-free joint at 250 r/min with a significantly increased welding speed of 55 mm/min (83.3% higher than the conventional FSW). Furthermore, the ductility of joint can be improved more than 20%. Consequently, DSFSW is feasible to obtain high-quality joint of titanium alloy under rather high welding speed.
查看更多>>摘要：The porosity with chain-like shape is found in high power laser welding of thick steel plate, which are periodically distributed at the lower and middle parts of the weld and parallel to bottom fusion line. A 3D numerical model is developed to reveal the effect of laser energy absorption and transfer, keyhole fluctuation, molten pool flow and solidification behavior on the formation process of chain-like pores. It is found that the chain-like pores are mainly determined by the unique profile of molten pool concaved inward at middle part on its rear wall. The periodic distribution of chain-like pores is resulted of the periodic variation of molten pool profile influenced by simultaneous periodic change of keyhole collapse frequency, heat accumulation at lower part, heat transfer through clockwise vortex as well as solidification latent heat accumulation at middle part. With the increasing of weld speed, the distribution of pores changes to form in the same height from the bottom fusion line due to the unchanged molten pool profile, which is resulted from insufficient solidification latent heat accumulation at middle part and less heat transferred by weaken upward flow in clockwise vortex. While the bubbles tend to reach a relatively higher position or escape from molten pool for more heat input, slower solidification speed and violent upward flow in clockwise vortex when weld speed decreases.
Cheng, JiwenSong, GangZhang, ZhaodongKhan, M. Shehryar...
15页
查看更多>>摘要：The welding of aluminum alloys (Al-alloys) is of great practical relevance in several industries, but the process can be problematic due to the prevalence of a well-known softening phenomenon in the fusion zone (FZ) and heat-affected zone (HAZ) surrounding the weld. This phenomenon has a unique gradient-like characteristic such that the HAZ near the FZ softens the most and the softening degree decreases as the distance from the FZ increases. To minimize the adverse effect of joint softening, this study proposes the use of a novel hybrid manufacturing process for joining AA5083-O and AA6061-T6 dissimilar Al-alloys, using tungsten inert gas (TIG) welding with filler wire under in-situ cooling conditions using specialized welding fixtures followed by the subsequent rolling of the weld reinforcement. The results show that welding under in-situ cooling conditions can improve the joint softening gradient, and the rolling of the weld reinforcement can introduce a strain hardening gradient, that greatly improves the integrity of the joint. By employing the proposed method, the fracture is successfully transferred from the HAZ of the AA6061-T6 side, where it normally occurs, to the base metal (BM) of the AA5083-O side. The proposed process has an extremely high impact potential as it can greatly increase the adoption of Al-alloys in the manufacturing of lighter automotive vehicles which will improve their efficiency and performance.
查看更多>>摘要：FeCrAl alloys have been widely developed as an alternative accident-tolerant cladding material in light water reactors since the Fukushima nuclear accident in 2011. A large thermal neutron cross-section is the most severe challenge for FeCrAl alloys in the final commercial use, and a thinner cladding was necessary to overcome this, which led to higher corrosion resistance requirements. To address this issue, a surface treatment process is described here to enhance the corrosion resistance of the FeCrAl alloy via a water-assisted laser modification. By varying the laser power and laser scanning speed, the relationship between corrosion and processing parameters was investigated to better understand the mechanism with various characterizations. A heat-transfer model with boiling heat transfer boundary conditions was built and simulated using a finite element method to reveal the change in temperature field during processing. A corrosion test was performed with a simulated light water reactor (LWR) hydro-thermal environment for 120 h. The corrosion rate and oxide film thickness of the waterassisted laser modified FeCrAl alloy sample decreased by 83 % and 50 %, respectively, compared to the asreceived one, and this was ascribed to the formation of protective oxide film during laser processing, along with grain refinement and elimination of holes and scratches.
查看更多>>摘要：This paper presents a ceramic based superhydrophobic coating fabricated on aluminum via plasma electrolytic oxidation and chemical vapor deposition methods. The morphology, chemical composition and wettability were characterized by scanning electron microscopy, energy dispersive spectrometer, and water contact angle (WCA). Water repellency, mechanical durability, anti-corrosion resistance and self-cleaning properties were investigated respectively. The results indicate that the prepared superhydrophobic coating featured net-like structured silica nanoparticles and concave pits with porous ceramic. Based on these characters, the prepared coating with a WCA of 160.5 exhibits excellent water repelling and self-cleaning properties. In addition, compared with bare aluminum, the corrosion current density of the prepared superhydrophobic coating is smaller by 3 orders of magnitude with a high inhibition efficiency of 99.76%. Furthermore, thanks to the ceramic based wear-resistant coating and protective micro-architectures, the prepared superhydrophobic coating demonstrates a superb mechanical durability. Therefore, the prepared superhydrophobic coating on aluminum is expected to offer potential opportunities for industrial applications in marine corrosive environments.
Du, WeitaoWang, LimingPeng, DingqiangShao, Yimin...
11页
查看更多>>摘要：The accurate prediction of dynamic cutting force has great significance for the quality control of the boring process for deep-cavity thin-walled parts. However, most of the classic and improved cutting force models commonly neglect the mechanism analysis of dynamic cutting force, resulting in the dynamic prediction of cutting force being almost impossible. To resolve this problem, a novel analytical dynamic cutting force prediction model for the boring process was proposed to consider the time-varying toolpath and chip fracture in the machining process. The generation mechanism of dynamic cutting force was analyzed by cutting theory and mathematical formula. The theory of maximum shear stress and the law of conservation of energy were applied to derive the dynamic cutting force for an entire rotation cutting cycle. Then, the proposed dynamic cutting force model was validated using a designed boring experiment and a classical cutting force model. The compared results showed that the dynamic cutting force model based on the proposed method had good agreement with the experimental data. Meanwhile, the advantage of accurate prediction was also further demonstrated by comparing the classic cutting force model. This paper presents an analytical dynamic cutting force prediction model for the boring process considering the toolpath and chip breakage. The underlying mechanism of the dynamic change of the cutting force was studied, and data supporting the improved quality of boring deep-cavity thin-walled parts was provided.
查看更多>>摘要：Scanning strategies can significantly affect temperature distribution and location-specific variation of solidification microstructure in laser-directed energy deposition by powder-feeding (LDED-PF). In this paper, a physics based analytical model of multi-track LDED-PF is developed to rapidly predict the temperature field of cuboidal geometries under four different scanning strategies: bidirectional, unidirectional, inward spiral, and S-pattern. Based on 2D thermal models, a new universal algorithm is developed to predict the geometrical profile of the overlapping beads, applied to all scanning strategies. The effects of four scanning strategies on the temperature field, geometrical profile, and microstructural behavior are investigated and tested. The linkages between the process and solidification microstructure are revealed and rationalized through temperature simulations and microstructural characterization. The developed model is tested for multi-track deposition of near-beta titanium (Ti5Al-5 V-5Mo-3Cr) alloy at different laser powers, scanning speeds, and step-over distances under different scanning strategies. The solidification maps are established for the alloy, and the microstructural evolution of columnar to equiaxed transition (CET) is predicted. The results indicate that the model can ideally predict the deposition dimensions, temperature field, and solidification microstructure. This modeling methodology is applicable to other metallic materials, and it can be used to manipulate and engineer the location-specific variation of solidification microstructure.
NSTL
Elsevier