查看更多>>摘要：Artificial aging plays important roles on the final performances of the manufactured components of Al matrix composites, and it is of great importance to find the appropriate aging parameters to obtain a good combination of mechanical properties and corrosion resistance. In this study, the Si3N4 particle reinforced 6061Al (Si3N4/6061Al) composite was produced by sintering and hot extrusion, and both single and two-step aging were carried out to examine the precipitation behavior. It was found that the grain structure of Si3N4/6061Al and 6061Al did not show a notable change after solution treatment, while Mg element inclined to concentrate at the Si3N4/6061Al interface. The compounds containing Mg and Cu formed at Si3N4/6061Al interface during aging process, and beta " and beta' phases precipitated after peak and two-step aging, respectively. beta " phase contributed to improving the hardness, strength and wear resistance, while beta' phase caused an opposite trend. The Si3N4 particles could also enhance the mechanical properties due to the load transfer and dislocation strengthening. The samples treated by solution exhibited the best corrosion resistance, while the peak-aged ones were sensitive to intergranular corrosion owing to the continuously distributed precipitates and the precipitate free zones at grain boundaries, and two-step aging could alleviate the intergranular corrosion susceptibility. Due to the preferential dissolutions of Mg element in Mg-containing layers and Mg/Cu-containing phases, the Al matrix around Si3N4 particles was seriously corroded.
查看更多>>摘要：In this work, better mechanical properties and thermal conductivity of the high-pressure die-cast AlMg6Si2MnZr alloy were reported by replacing the conventional H13 steel shot sleeve with a new ceramic type shot sleeve (named LX shot sleeve). The work mainly concentrated on the effect of microstructure on the mechanical properties and thermal conductivity of two die-cast samples of AlMg6Si2MnZr alloy by different shot sleeves. The results showed that the LX shot sleeve produced smaller and fewer externally solidified crystals of alpha-Al (ESCs-alpha) as it dissipated heat more slowly than the H13 shot sleeve. Meanwhile, ESCs-alpha in the die-cast sample by the LX shot sleeve had lower dendritic branching extent. Due to the decrease in ESCs-alpha, the eutectic content and the number of the porosities, particularly shrinkage porosities, of the die-cast sample by the LX shot sleeve decreased. The mechanical properties of the die-cast sample by the LX shot sleeve were increased greatly, particularly the elongation, which was increased from 6.64% to 9.06% due to the decrease in the shrinkage porosities. The thermal conductivity of the die-cast sample by the LX shot sleeve was also increased from 112.2 W m(-1) K-1 to 120.6 W m(-1) K-1. In the die-cast sample by the LX shot sleeve, the decrease in the Mg solute concentration in alpha-Al solidified in the cavity (alpha) was the main factor to distinctly increase the electrical thermal conductivity while the reduction of the eutectic phase resulted in a slight decrease in the phonon thermal conductivity.
查看更多>>摘要：The objective for bubble refining is to control the production of plentiful, tiny and dispersed bubbles. For this, the rapid downward stream in the down-leg snorkel of an RH vessel can separate the injected gas into fine bubbles. In this work, the formation behavior of bubbles generated by argon injection through the down-leg is investigated by combining physical simulation experiments and multiphase flow simulations. A physical model was first established to investigate the formation behavior of argon bubbles. A Volume of Fluid model (VOF) was then developed and the results from the physical simulation used to validate the mathematical model. Subsequently, the validated VOF model was expanded to include the prediction of bubble formation behavior in liquid steel. From the results obtained, it was found that with a wetting orifice, separated bubbles are obtained under most conditions. When the orifice is non-wettable, the injected gas forms a curtain. When the separated bubbles are carried down into the ladle, they can be compressed to 70 % of their initial value, which enhances collision, promotes adherence to inclusions and facilitates their removal.
查看更多>>摘要：Thixotropic-core compound forging (TCF1) is a promising technology for the precision forming of aluminumsteel bimetal parts. In this paper, the hitherto unexplained mechanisms of the interface heterogeneity in TCFed aluminum-steel bimetal parts were investigated. Moreover, the effects of diffusion heat treatment on the interface heterogeneity and shear strength were studied. A thixotropic 7075 Al core and a 304L steel shell were used in TCF for the forming of a complex bimetal part. Results suggested that the interface heterogeneity in TCFed aluminum-steel bimetal parts is resulted from three aspects: (i) asynchronous material flow and the associated liquid segregation; (ii) tearing and separation of oxide films; and (iii) formation of liquid film at core surface. The local liquid segregation in distal areas of the bimetal part is helpful to improve the interface wettability and the separation of oxide films, thus accelerating the atom diffusion. In this case, reliable metallurgical bonding interface composed of Al-Fe intermetallic compounds (IMCs2) with satisfactory thickness is possible to be obtained. The diffusion heat treatment is helpful to the formation of reliable metallurgical bonding interface by improving the growth of IMCs. The optimal shear strength was tested to be 26.4 MPa after a heat treatment for 475 degrees C/2 h, indicating the formation of reliable metallurgical bonding. With the further increment in the heat treatment time, the interface heterogeneity will be diminished, whereas the shear strength may be deteriorated due to the overgrowth of IMCs.
查看更多>>摘要：Cryogenic medium pressure forming has been proposed to fabricate aluminum alloy complex tubular components. It is urgently needed to evaluate and predict the cryogenic forming limit of aluminum alloy tubes for the novel process. Cryogenic bulging test using different length-diameter ratios was proposed to test the forming limit in the tension-tension zone. A modified M-K model was established to rapidly predict the cryogenic forming limit, based on the dynamic hardening with deformation. Theoretical analysis and fracture characterization were implemented to reveal the improvement mechanism of cryogenic forming limit from the macroscopic and microscopic perspectives, respectively. The forming limit of AA6061 tube at-196 ? is approximately double that at room temperature, especially in the tension-tension zone, which contributes significantly to the forming of complex features under biaxial tension. The cryogenic limiting deformation ability increases gradually as the strain state changes from plane strain to biaxial tension. Under the biaxial tension condition of lambda = 1.2, the limiting strain can reach 0.52 at-196 ?. The cryogenic forming limit can be successfully predicted by the modified M-K model with a prediction deviation of only 4.35% at-196 ?. The enhanced cryogenic hardening ability promotes the improvement of deformation uniformity and delays the occurrence of uncoordinated deformation. Ductile fracture is main fracture mechanism at cryogenic temperature. The significantly increased ductile fracture characteristics contribute to the improved cryogenic forming limit. This research can provide important guidance for the cryogenic forming of aluminum alloy tubular components.
查看更多>>摘要：In this study, a numerical/experimental analysis is proposed to investigate the possibility of reshaping sheet metal-based End-of-Life (EoL) components using sheet Hydroforming (SHF). Returned EoL components are challenging to be reformed, they are usually characterised by high heterogeneity as there are localised thinning areas (caused by the original forming processes), and the overall formability is reduced with respect to the original flat sheet material. The reshaping route was replicated: a deep drawing process was adopted to impart a square feature; subsequently, SHF was performed. The capability of remove the deep drawn feature was analysed with varying Blank-holder force, oil pressure profile and the location of the previous deep drawn feature. A 3D finite element model of the entire manufacturing route was used to analyse the strain paths of the reshaping process. The change in the strain paths when considering a component previously subjected to deep drawing was analysed and discussed in comparison with SHF using an undeformed blank. This study for the first time provides an insight on the reshaping process mechanics as well as attempts at quantifying the quality of the reshaped components. Results revealed that SHF can be successfully adopted for reshaping purposes as it performed well under two analysed aspects: capability of removing the existing feature and imparting a brand-new shape and thickness/strain path analyses (avoiding fracture and excessive thinning). Nevertheless, the developed analyses revealed that reshaping is more challenging than conventional forming and new design rules, identified in the present paper, need to be followed.
查看更多>>摘要：High heat input during layer-wise deposition in the Gas Metal Arc-based Wire-Arc Additive Manufacturing (GMA-WAAM) process leads to significant defects such as high residual stress and distortion. However, the choice of layer-wise heat input is constrained due to the requirement of an optimal set of weld parameters. It is essential to study the effect of heat input on the evolution and control of residual stress in the long-scale WAAM fabricated components. Therefore, this study investigates a novel multi-tier heat input strategy represented by four levels for a GMA-WAAM fabricated long-scale thin-walled component. Accordingly, a layer-wise single-, two-, three-, and four-tier heat input strategy in a twelve-layered long-scale thin-walled component is investigated for transient thermal and residual stress distributions through Finite Element Modelling (FEM). Further, a set of validation experiments was performed, which agreed with the predicted results. It is concluded that a reduced heat input during successive layer deposition greatly influences the cumulative heat input into the system. The twotier heat-input (case 2) system is found suitable among proposed case studies as an appropriate thermal management system with a variation of 6.98% in peak (melt pool) temperature and 198.4 ?C increase of minimum temperature during complete deposition. Also, the four-tier heat input (case 4) produced the component's lowest longitudinal and transverse stresses by 13.07% and 25.83%, respectively.
查看更多>>摘要：A novel method based on image signals is proposed to achieve the fast morphology quantification of single abrasive grain and precise wear monitoring of pyramid abrasive belt in a robotic grinding system. The Regions of Interest (ROI) of multiple abrasive grains are precisely and adaptively segmented based on the U-Net deep learning algorithm, and the Intersection over Union (IoU) of predictions reaches above 0.9. Identification of effective abrasive grains is achieved by the intensity distribution, region boundary, and nearest neighbor searching for centroids. In addition, the morphological parameters of effective abrasive grains are calculated to quantify the multiple worn forms such as continuity, non-uniform, and adhesion. The calculated dimensional parameters (area ratio and perimeter ratio) intuitively characterize the evolution of belt wear in the time domain, and the calculated shape parameters (aspect ratio and extent) greatly illustrate the effect of force and adhesion on abrasive grain wear. A comprehensive experimental study of the grinding performance shows that the material removal rate is mainly affected by the macroscopic morphological wear of the compact grains, and the machined surface quality depends more on the interaction between small abrasive agglomerates and workpiece. Furthermore, the thresholds of the mean area ratio and perimeter ratio with high independence for variable process parameters can be used as the judging criteria for determining the belt service life.
NSTL
Elsevier