查看更多>>摘要：Although 2-d (two-dimensional) beam oscillation can reduce the porosity defects during laser welding of aluminum alloys, it distributes the heat input, resulting in a higher power required to form a capillary. Therefore, this research draws attention to an alternative approach, which is considered to be beneficial to laser absorption in oscillation laser beam welding, 3-d (three-dimensional) oscillation of the laser beam. (3-d oscillation means that the laser spot moves on the two-dimensional plane and the defocus change simultaneously, causing the heat input to change) An experiment was designed where the variation of oscillation frequency leads to significant changes in capillary behavior. The result shows that the increase of vertical oscillation can promote the formation of a capillary. When the oscillation frequency reaches some special values, the capillary appears and disappears periodically, such as the vertical oscillating frequency is 5 Hz, rotation oscillating frequency is 200 Hz. Meanwhile, welding pores are generally distributed at the position where the capillary begins to disappear. The reason for promoting capillary formation during 3-d laser beam oscillation, and the change of normalized temperature, which is meaningful for understanding the welding mode transition, is illustrated by a simplified model. Compared with traditional linear welding, the experimental results show that it is possible to obtain lower porosity seam during 3-d laser beam oscillation welding in low feed rate, and the mechanism of stabilizing the molten pool is discussed qualitatively.
查看更多>>摘要：Interrupted in situ tests at high temperature characterized with X-ray microtomography were employed to study porosity evolution under loading conditions representative of aluminum hot rolling. Coupling digital volume correlation and simulations, it is possible to access, for each pore, its morphological evolution, and the mechanical fields it experienced. The closure kinetics of hundreds of pores were studied, which is described as the pore volume as a function of the hydrostatic integration. Closure kinetics are a function of the shape of the pore. Focus is put on pores with a complex shape which are more detrimental to mechanical properties. Their closure kinetics are exponential due to the pore fragmentation during closure. The closure of complex pores is fostered by an initial orientation perpendicular to the loading direction and a low initial fragmentation. Based on the closure kinetics of 243 complex pores, a new pore closure model is proposed.
查看更多>>摘要：Vertical rolling has been widely applied to control slab widths in hot plate rolling plants. In this research, due to the high ratio of width/thickness of the slab in vertical rolling, a symmetric cubic function model is established to depict the three-dimensional dog bone shape of the slab. The three-dimensional kinematically admissible velocity and strain rate fields are innovatively established based on the dual-stream function (DSF) method and the boundary conditions. According to the first variational principle of rigid-plastic, the numerical results of the force and shape parameters are obtained. To decrease the computation load and time, the Geometrical Approximation (GA) yield criterion, the Pavlov projection principle, and the mean value theorem are utilized to calculate the plastic deformation, shear, and friction power terms separately, and the analytical results of the slab shape and rolling force are received later. To certify the effectiveness of the submitted model, the computed slab shapes are compared with the results of other models and the rolling forces are compared with the measurements in a plate plant. Good agreement is found. Furthermore, the effects of key rolling process parameters, such as width reduction, thickness, and vertical roll radius, on the slab shape and stress state are illustrated. The variation mechanisms of the force and shape parameters are used to process parameters setting and adjusting.
查看更多>>摘要：Pores are often present in large aluminum ingots after casting. To ensure the mechanical reliability of the final thick plates, these pores must be closed during the forming process, hot rolling in the present case. This study aims at understanding the effects of rolling parameters on the volume evolution of pores. To do so, X-ray microtomography is used to track real casting pores during deformation. Nevertheless, thick plates are too large to enable a fine characterization of the evolution of pores during the process. The size and shape of the samples as well as the mechanical boundary conditions must be optimized to meet imaging constraints. This paper focuses on the reproduction of complex loading paths. Multi-scale FE simulations are used to reproduce the loading conditions of thick plate rolling with uniaxial tests on samples of a few millimeters. Uniaxial tests are then characterized with tomography on a synchrotron X-ray beamline. In each sample, tens of pores are individually tracked, giving access to their volume evolution. The local loading path experienced by each pore is determined with FE simulation. It is shown that the volume evolution of a real pore correlates with the hydrostatic integration which is the integral of stress triaxiality along cumulated strain. This confirms that the closure of complex casting pores is enhanced by a high relative reduction and high values of L/H ratio during rolling similarly to what was observed numerically on simpler shapes.
查看更多>>摘要：The stress state built in the tool-particle contact region plays a dominant role in governing the particle-tool interactions that strongly determine the machinability of SiCp/Al. In the present work, we evaluate the coupled influence of rake angle of cutting tool and tool-particle position on the ultra-precision diamond cutting of SiCp/Al by finite element simulations and corresponding experiments. Specifically, 2D finite element modeling of SiCp/Al cutting with the consideration of the real microstructural characteristics of SiC particles and the mechanical behavior of particle-matrix interface is caried out, and the accuracy of which is verified by corresponding cutting experiments. Simulation results and experimental data jointly reveal different failure modes of SiC particles, as well as their correlations with cutting force, chip profile and machined surface integrity. In particular, the rake angle of cutting tool significantly alters the built stress state in tool-particle contact region, thus leading to a strong coupled effect of rake angle of cutting tool and tool-particle position on the diamond cutting of SiCp/Al. The findings reported in this work provide a theoretical basis for the rational selection of geometrical parameters of cutting tool for promoting the machinability of SiCp/Al.
查看更多>>摘要：Laser shock peening (LSP) is one of the main anti-fatigue technologies in high-end manufacturing industries. However, guaranteeing its quality consistency is difficult due to its process complexity. Acoustic emission (AE) is a promising solution to accurately monitoring the surface quality of LSP, although, the large scale of monitoring data is still challenging for industrial applications. This paper studied the in-situ evaluation of surface residual compressive stress (SRCS) for 7075 aluminum alloy in LSP and the efficient sensing of AE based on deep learning methods. Firstly, a monitoring system of LSP with four types of AE sensors was developed in order to simultaneously acquire both the ultra-high and ultra-low amplitude of shock wave. The performance of those sensors is further quantitatively evaluated via long-short term memory (LSTM). Then, a new spatio-temporal parallel CNN-LSTM model for SRCS classification was proposed and experimentally validated to outperform CNN and LSTM after parameters optimization. Finally, aiming to efficient sensing of AE, the importance of different frames of AE signal was analyzed by means of the proposed dual circulation LSTM (DC-LSTM) model, which can accurately locate the key frame of time series signal. Besides, the effects of different frames of AE on classification performance was discussed. It was found that the broadband AE sensor without attenuator shows the highest classification accuracy, while the fast decay stage of AE signal has more contribution to the SRCS classification. This paper provides guidance for real-time non-destructive evaluation of residual stress via AE in laser manufacturing.
查看更多>>摘要：The application prospect of 2195 alloy in aerospace structure parts has received widespread attention. The high strain rate forming combined with heat treatment process is a potential way to produce the high performance Al-Cu-Li alloy parts. In this work, the mechanical properties and microstructure evolution of the as-annealed and solution treated 2195 alloy specimens under dynamic compression were studied and compared. The dynamic compression tests were performed at the strain rates of 1400 s?1, 2200 s?1, 3400 s?1, and 4500 s?1 by split Hopkinson pressure bar. The results indicate that the mechanical properties of the two kinds of heat-treated specimens have been improved by dynamic compression, an increase of 59.7 % and 63.1 % of hardness respectively. The main strengthening mechanisms of the specimen under dynamic compression conditions are fine grain strengthening, dislocation strengthening, and low angle grain boundaries strengthening, which differs depending on the heat-treated state and the strain rate. Besides, it is found that the stable-end orientation of grain rotation in the two kinds of heat-treated specimens after dynamic compression is <110 > //CD (CD: compression direction). Different from the as-annealed specimen, the solution treated specimen shows insufficient grain rotation behavior. The mechanism of insufficient grain rotation is revealed by the Schmid factor. The Schmid factor of as-annealed specimen increases by 7.5 % than that of solution treated specimen. The lower Schmid factor makes the plastic deformation resistance of solution treated specimen higher, which contributes to the insufficient grain roration behavior.
查看更多>>摘要：Since various build defects in the Selective laser melting (SLM) process are found to be associated with the instability of melt pool, the melt pool monitoring is particularly important for the final product quality control. Previous studies focus on the geometric features to describe the changes of the size and shape of melt pool, which are not sufficient to characterize the dynamic variations of the melt pool during the build process monitoring. To solve this problem, a new motion feature is introduced to describe the moving melt pool. The melt pool and spatters are extracted by thresholds combined with the connected component analysis method. The distance between the centroid and the boundary of melt pool is calculated from the unfolded clockwise at a step angle, which constructs a high dimensional feature vector as the motion features. The k-means clustering algorithm is applied to cluster the motion features under varied process parameters, aiming at construct the link between the melt pool states and processing parameter for the quality control. The research results have shown that the extracted motion features can describe the variation of melt pool more accurately than the traditional geometric features, and they can distinguish the moving direction and melted states of over melting, partial melting and defects simultaneously. This research provides a new approach for intelligent online monitoring of the SLM process.
Koga G.Y.Ferreira T.de Bribean Guerra A.P.Bolfarini C....
20页
查看更多>>摘要：This paper presents the effect of air carbon arc gouging (ACAG) and submerged arc welding (SAW) on the risk of intergranular corrosion and on the mechanical properties of AISI 304 stainless steel plate, welded with AWS-ER308L wire, and AISI 347H stainless steel plate, welded with AWS-ER347 wire. A high risk of carbon uptake was found for as-gouged surfaces. Different mechanical surface cleaning procedures were evaluated. Brushing was ineffective to remove the carbon-contaminated layer, while grinding was more efficient, as assessed by double-loop electrochemical potentiokinetic reactivation (DL-EPR), but still indicating few remnants carbon-contaminated areas. DL-EPR and oxalic etching tests of gouged and welded 304/ER308L stainless steel indicated the risk of sensitization and suspect etch microstructures regarding intergranular corrosion, while low sensitization risk was found for 347H/ER347 stainless steel welds, highlighting the role of low alloying elements such as Nb, recognized to reduce the risk of intergranular corrosion. Thus, the 304/308L SS weld joints were sensitized, even if no gouging was employed. This indicates a higher susceptibility of sensitization and intergranular attack caused by the thermal cycle induced during the SAW of AISI 304 SS using AWS-ER308L SS wire. Acceptable mechanical behavior of welding onto gouged surfaces was found for both weld joints (i.e., Charpy energy > 90 J and Ultimate Tensile Strength ≥ 600 MPa), revealing that the major risk is the sensitization in the case of 304/ER308L SS welds.
查看更多>>摘要：The manufacturing of extra-large thin-wall castings using high pressure die casting is one of the most significant challenges for structural applications requiring excellent ductility. The present study aims to understand the effect of process parameters on the castability, defect formation and mechanical properties of aluminium alloys in extra-large thin-wall castings with a maximum flow length of 1230 mm in the 2.8 mm thick channel. Numerical simulation and experimental verification were carried out to tailor the process parameters in high pressure die casting. It is found that the process parameters can significantly affect the castability and mechanical properties of as-cast components. For a complete casting, the yield strength is slightly increased but the elongation is significantly decreased at the locations further away from runners. A new concept of effective flow length (EFL) is proposed and used to assess the castability in extra-large thin-wall high pressure die castings. Under the optimum casting condition, the EFL can reach 525 mm, at which the ratio of EFL to wall thickness is 187 and the yield strength and elongation are greater than 120 MPa and 10%, respectively. Although the extra-large thin-wall castings can be geometrically filled under several conditions, the heterogeneity of mechanical properties is the most significant concern, in which the variation of elongation is overwhelmingly important for the structural applications requiring excellent ductility under as-cast conditions. Therefore, the criteria of casting quality should consider both geometrical soundness and the homogeneity of mechanical properties in the casting body.
NSTL
Elsevier