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1.
Material flow and microstructure in the friction stir butt welds of the same and dissimilar aluminum alloys 总被引:20,自引:0,他引:20
The material flow and microstructural evolution in the friction stir welds of a 6061-Al alloy to itself and of a 6061-Al alloy
to 2024-Al alloy plates of 12.7 mm in thickness were studied under different welding conditions. The results showed that plastic
deformation, flow, and mechanical mixing of the material exhibit distinct asymmetry characteristics at both sides of the same
and dissimilar welds. The microstructure in dissimilar 6061-Al/2024-Al welds is significantly different from that in the welds
of a 6061-Al alloy to itself. Vortex-like structures featured by the concentric flow lines for a weld of 6061-Al alloy to
itself, and alternative lamellae with different alloy constituents for a weld of 6061-Al to 2024-Al alloy, are attributed
to the stirring action of the threaded tool, in situ extrusion, and traverse motion along the welding direction. The mutual
mixing in the dissimilar metal welds is intimate and far from complete. However, the bonding between the two Al-alloys is
clearly complete. Three different regions in the nugget zone of dissimilar 6061-Al/2024-Al welds are classified by the mechanically
mixed region (MMR) characterized by the relatively dispersed particles of different alloy constituents, the stirring-induced
plastic flow region (SPFR) consisting of alternative vortex-like lamellae of the two Al-alloys, and the unmixed region (UMR)
consisting of fine equiaxed grains of the 6061-Al alloy. Within all of these three regions, the material is able to withstand
a very high degree of plastic deformation due to the presence of dynamic recovery or recrystallization of the microstructure.
The degree of material mixing, the thickness of the deformed Al-alloy lamellae, and the material flow patterns depend on the
related positions in the nugget zone and the processing parameters. Distinct fluctuations of hardness are found to correspond
to the microstructural changes throughout the nugget zone of dissimilar welds. 相似文献
2.
Understanding the Influence of Copper Nanoparticles on Thermal Characteristics and Microstructural Development of a Tin-Silver Solder 总被引:1,自引:0,他引:1
D.C. Lin T.S. Srivatsan G-X. Wang R. Kovacevic 《Journal of Materials Engineering and Performance》2007,16(5):647-654
This paper presents and discusses issues relevant to solidification of a chosen lead-free solder, the eutectic Sn-3.5%Ag,
and its composite counterparts. Direct temperature recordings for the no-clean solder paste during the simulated reflow process
revealed a significant amount of undercooling to occur prior to the initiation of solidification of the eutectic Sn-3.5%Ag
solder, which is 6.5 °C, and for the composite counterparts, it is dependent on the percentage of copper nanopowder. Temperature
recordings revealed the same temperature level of 221 °C for both melting (from solid to liquid) and final solidification
(after recalescence) of the Sn-3.5%Ag solder. Addition of copper nanoparticles was observed to have no appreciable influence
on melting temperature of the composite solder. However, it does influence solidification of the composite solder. The addition
of 0.5 wt.% copper nanoparticles lowered the solidification temperature to 219.5 °C, while addition of 1.0 wt.% copper nanoparticles
lowered the solidification temperature to 217.5 °C, which is close to the melting point of the ternary eutectic Sn-Ag-Cu solder
alloy, Sn-3.7Ag-0.9Cu. This indicates the copper nanoparticles are completely dissolved in the eutectic Sn-3.5%Ag solder and
precipitate as the Cu6Sn5, which reinforces the eutectic solder. Optical microscopy observations revealed the addition of 1.0 wt.% of copper nanoparticles
to the Sn-3.5%Ag solder results in the formation and presence of the intermetallic compound Cu6Sn5. These particles are polygonal in morphology and dispersed randomly through the solder matrix. Addition of microsized copper
particles cannot completely dissolve in the eutectic solder and projects a sunflower morphology with the solid copper particle
surrounded by the Cu6Sn5 intermetallic compound coupled with residual porosity present in the solder sample. Microhardness measurements revealed the
addition of copper nanopowder to the eutectic Sn-3.5%Ag solder resulted in higher hardness. 相似文献
3.
Dongming Hu Radovan Kovacevic 《International Journal of Machine Tools and Manufacture》2003,43(1):419
Laser-Based Additive Manufacturing (LBAM) is a promising manufacturing technology that can be widely applied to part preparation, surface modification, and Solid Freeform Fabrication (SFF). A large number of parameters govern the LBAM process. These parameters are sensitive to the environmental variations, and they also influence each other. This paper introduces the research work in RCAM on improving the performance of the LBAM process. Metal powder delivery real-time sensing and control is studied to achieve a controllable powder delivery for fabrication of functionally graded material. A closed-loop control system based on infrared image sensing is built for control of the heat input and size of the molten pool in the LBAM process. The closed-loop control results show a great improvement in the geometrical accuracy of the built features. A three-dimensional finite element model is also established to explore the thermal behavior of the molten pool in the closed-loop controlled LBAM process. 相似文献
4.
C. -F. Li D. B. Johnson R. Kovacevic 《International Journal of Machine Tools and Manufacture》2003,43(9):325
Waterjet guided laser processing is an internationally patented technique based on guiding a laser inside a thin, high-speed waterjet. The process combines the advantages of laser processing with those of waterjet cutting and offers promise as a method for processing thin and heat sensitive materials with a high degree of precision. An improved understanding of the complex interaction between laser, waterjet, and workpiece is required to enable the process to achieve its potential. A model for waterjet guided laser grooving of silicon is presented that treats the energy input of the laser, the cooling effect of the waterjet, and the melting and removal of the silicon. The thermal process is represented in detail in the new method. The model is validated by comparisons of simulation and experimental results, and the simulation provides insight regarding the details of the interactions among laser, waterjet, and workpiece. 相似文献
5.
Subband coding systems incorporating quantizer models 总被引:1,自引:0,他引:1
A new method for dealing with the effects of quantization in a subband system is proposed. It uses the "gain plus additive noise" linear model for the Lloyd-Max quantizer. Based on this, it is demonstrated how, by an appropriate choice of synthesis filters, one can cancel all signal-dependent errors at the output of the system. The only remaining error is random in nature and not correlated with the input signal. We therefore have a tradeoff between the error being only random or having signal-dependent components as well (since the error variances in both cases are comparable). As a result of having only a random error, it is possible to reduce this error using, for example, a noise removal technique. The result is then extended to the case where the input is a multidimensional signal, and arbitrary sampling lattices are used, as well as to the QMF (alias cancellation) case. To demonstrate the validity of the proposed approach, two types of experiments on images are carried out: In a toy example, it is shown that using noise removal could be beneficial. For a more realistic coding scheme, however, it is demonstrated that even in the case when the model is no longer valid (when some of the subbands are discarded), the output error is still much less correlated with the input signal as opposed to the commonly used subband system, while visually, the reconstructed images look very similar. 相似文献
6.
A closed-loop system is developed to control the weld fusion, which Is specified by the top-side and back-side bead widths of the weld pool. Because in many applications only a top-side sensor is allowed, which is attached to and moves with the welding torch, an image processing algorithm and neurofuzzy model have been incorporated to measure and estimate the top-side and back-side bead widths based on an advanced top-side vision sensor. The welding current and speed are selected as the control variables. It is found that the correlation between any output and input depends on the value of another input. This cross coupling implies that a nonlinearity exists in the process being controlled. A neurofuzzy model is used to model this nonlinear dynamic process. Based on the dynamic fuzzy model, a predictive control system has been developed to control the welding process. Experiments confirmed that the developed control system is effective in achieving the desired fusion state despite the different disturbances 相似文献
7.
Prabu Balu Perry Leggett Radovan Kovacevic 《Journal of Materials Processing Technology》2012,212(7):1598-1610
The manner with which the composite powder particles injected into the laser formed molten pool decides the deposition quality in a typical laser-based powder deposition of composite material. Since, the morphology and physical properties of nickel (Ni) and tungsten carbide (WC) are different their powder flow characteristics such as the powder particles stream structure, maximum concentration at the converging spot, and the powder particles velocity are noticeably different. In the current study, a computational fluid dynamics (CFD) based powder flow model is established to characterize the coaxial powder flow behavior of Ni–WC composite powders. The key powder flow characteristics such as the stand-off distance, the diameter of the powder stream at the stand-off distance, and the velocity of the powder particles are measured using three different vision based techniques. Both the numerical and experimental results reveal the exact stand-off distance where the substrate needs to be placed, the diameter of the concentration spot of powder at the stand-off distance, and a combination of suitable nozzle angle, diameter, and carrier gas flow rate to obtain a maximum powder concentration at the stand-off distance with a stable composite powder flow. 相似文献
8.
Wei Liu Junjie MaGuang Yang Radovan Kovacevic 《Journal of Materials Processing Technology》2014,214(12):2823-2833
This study investigated the synergetic effect between laser beam and electrical arc during hybrid welding by using a spectral diagnostic technique. The synergetic effect increased the energy density in the keyhole and deepened the weld penetration, resulting in a lower plasma electron temperature. The metal transfer mode was a globular one at a small offset distance while a spray mode was achieved with an increase in the offset distance. The decrease in the arc voltage and arc current due to the synergetic effect caused this transition from spray to globular modes. Globular transfer mode destabilized the molten pool and keyhole with the large droplet impingement, leading to the formation of porosity in the corresponding weld bead. The presence of porosity was on-line detected by identifying serious fluctuations in the Fe I electron temperature signals based on the fact that the instability of the molten pool and keyhole is strongly related to the signals coming from the plasma. 相似文献
9.
Jovana Kovacevic Jason Sagert Anna Wozniak Matthew W. Gilmour Kevin J. Allen 《Food microbiology》2013
Antimicrobial resistance (AMR), co-selection phenomenon, and the relationship between reduced susceptibility (RSC) to ciprofloxacin (CIP) and resistance to other antimicrobials in Listeria spp. (n = 103) recovered from food processing environments (FPE) and food were investigated. Resistance of Listeria monocytogenes and other listeriae, respectively, to cefoxitin (FOX; 98% vs. 88%), CIP (7% vs. 4%), clindamycin (CLI; 33% vs. 59%) and tetracycline (6% vs. 8%) was observed, as was RSC to CIP (67% vs. 57%) and CLI (65% vs. 41%). L. monocytogenes also possessed RSC to linezolid (LZD; 6%), rifampicin (2%) and streptomycin (6%), with other listeriae displaying RSC to chloramphenicol (4%). L. monocytogenes serotype 1/2a (90%) isolates were more frequently resistant or possessed RSC to CIP compared to serotype 4b (55%) (p = 0.015). When eight strains were experimentally adapted to high concentrations of CIP, co-selection occurred as MICs to benzalkonium chloride (BAC) increased (n = 5), gentamicin MICs remained the same (n = 6) or increased 2-fold (n = 2), and led to RSC to LZD (n = 1) and resistance to CLI (n = 8). Overall, levels of resistance/RSC to CIP in food chain isolates, particularly 1/2a, are concerning. Further, reduced sensitivity to disparate antimicrobials following CIP exposure highlights the need for increased knowledge of co-selection phenomenon linked with antimicrobial agents. 相似文献
10.
S. Djordjevic S. Kovacevic LJ. Nikolic M. Miljkovic D. Djordjevic 《Journal of Natural Fibers》2014,11(3):212-224
Synthesis and characterization of copolymer obtained by grafting acrylamide onto hydrolyzed potato starch and its application in cotton yarn sizing were presented in this work. Some process parameters of starch hydrolysis and grafting, like as hydrolysis yield, grafting yield, grafting percentage, graft efficiency, and monomer to polymer conversion were determinated. Molar mass values of hydrolyzed starch and copolymer samples are located in a narrow range of 1 × 105 to 1 × 107 and 1 × 102 to 1 × 104. Grafted hydrolyzed starch, as a sizing agent, gives better results than nongrafted, particularly in terms of sizing uniformity, yarn mechanical parameters, and easier removal in subsequent desizing process. 相似文献