Numerical and experimental analysis for solidification and residual stress in the GMAW process for AISI 304 stainless steel

Gas Metal Arc Welding (GMAW) process was analyzed by combining a finite element thermomechanical model for temperature and stress with solidification model. Model prediction was compared with experimental data in order to validate the model. The effects of welding process parameters on these welding fields were analyzed and reported. The effort to correlate the residual stress and solidification was initiated, yielding some valuable results. The solidification process was simulated using the formulation based on the Hunt-Trivedi model. Based on the temperature history, solidification speed and primary dendrite arm spacing were predicted at given nodes of interest. Results show that the variation during solidification is usually within an order of magnitude. The temperature gradient was generally in the range of 10⁴–10⁵ K/m for the given welding conditions (welding power = 6 kW and welding speed = 3.39 to 7.62 mm/sec), while solidification speed appeared to slow down from an order of 10^–2 to 10^–3 m/sec during solidification. SEM images revealed that the Primary Dendrite Arm Spacing (PDAS) fell in the range of 10¹–10² m. The range of predicted sizes was in agreement with the experimental values. It was observed that the average size of the PDAS was dependent upon the welding speed. The PDAS fell between 7.5 to 20 m for columnar and 10 to 30 m for equiaxed dendrites, for welding speeds between 3.39 to 7.62 mm/sec. When the welding speed increased, it was observed that the average size of the PDAS decreased, as the model had predicted. For grain growth at the Heat Affected Zone (HAZ), Ashby's model was employed, and the prediction was in agreement with experimental results. For the residual stress calculation, the same mesh generation used in the heat transfer analysis was applied to make the simulation consistent. The analysis consisted of a transient heat analysis followed by a thermal stress analysis. An experimentally measured strain history was compared with the simulated result. The relationship between microstructure and the stress/strain field of welding was also obtained.     

An on-chip ECC circuit for correcting soft errors in DRAMs withtrench capacitors

A modified error-correcting code that can correct up to two soft errors on each row (word line) in a dynamic random-access memory (DRAM) chip is proposed. Double-bit soft errors frequently occur in DRAM cells with trench capacitors, when charged alpha particles impinge on the intervening space between two vertical capacitors causing plasma shorts between them. The conventional on-chip error-correcting codes (ECCs) cannot correct such double-bit word-line soft errors, which significantly increase the uncorrectable error rate (UER). An ECC circuit that uses an augmented rectangular product code to detect and correct double-bit soft errors is presented. The proposed circuit automatically corrects the addressed bit if it is faulty, and then quickly locates the other faulty bit. A comprehensive study is made to estimate improvements in soft error rate (SER) and mean time to failure (MTTF). The ability of the circuit to correct soft errors in the presence of multiple-bit errors has also been analyzed by combinatorial enumeration     

Electrostatic Microencapsulation Technique for Producing Composite Particles

The electrostatic microencapsulation process and the various means of achieving such encapsulation for developing composite particles with two or more powders is briefly reviewed, with a particular focus on the dual-function sorbent/scintillation particles for use in radionuclide selective sensing. In preparing the composite particles, two different types of particles, an ion-exchange resin and scintillating microbeads, were used. The sorbent particles capture and preconcentrate the radionuclide of interest (e.g., 99 Tc) from solution. The scintillating microbeads are used to convert the energy of radioactive decay into that detectable light that can be deducted by a photomultiplier tube arrangement. To simulate the electrostatic microencapsulation of resin and scintillators, several surrogate materials such as acrylic powder (mean particle diameter, d 50 =23 µm), red toner (d 50 =16 µm), resin (d 50 =134 µm), and fluorescent latex spheres (d 50 =2 µm) were used. A microencapsulation tower was constructed to use corona guns operating at high voltages of opposite polarity to charge the "host" and "guest" particles. Experimental arrangements and test results are presented. The results show that if polymer particles are used and if one of the two powders is smaller than 10 µm in diameter, the electrostatic and van der Waals forces provide enough interparticle adhesion to bond the guest and host particles through plastic deformation. Interparticles adhesion between resin (d 50 =133 µm) and toner (d 50 =15 µm) showed that the composite particles were stable in water suspension. For larger particles, such as resin and scintillators, the use of a binding agent is necessary to form stable composite particles.

electrostatic microencapsulation particle coating groundwater monitoring composite particles radionuclide detection     

laser cladding of Ni-Al bronze on Al alloy AA333

The present study investigates the effects of laser-processing parameters, such as laser power, traverse speed, powder-feed rate, and flow rate and species of assisting gas, and material prop-erties, such as substrate surface condition, on laser cladding of Ni-Al bronze on Al alloy AA333. The proper processing parameters were determined experimentally and are discussed in terms of their effects on laser-clad quality and microstructure as observed using optical and scanning electron microscopes. Despite a large difference in melting points between the cladding material, Ni-AI bronze (MP = 1063 °C), and the substrate, Al-alloy AA333 (MP = 577 °C), clads of thickness from 1.2 to 2.5 mm that are crack-free and had good fusion were achieved. The substrate surface condition and the flow rate and species of assisting gas were found to be important for clad formation. A sandpaper-polished substrate absorbs less energy at the molten pool front and facilities reducing dilution. A large flow rate of assisting gas, such as helium, also has an effect on reducing dilution. A laser-generated molten-pool model was developed to explain the preceding experimental results. Y. LIU formerly Research Associate, Center for Laser Aided Materials Processing, Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign.     

Characterization of microstructure in laser-surface-alloyed layers of aluminum on nickel

In order to obtain basic understanding of microstructure evolution in laser-surface-alloyed layers, aluminum was surface alloyed on a pure nickel substrate using a CO₂ laser. By varying the laser scanning speed, the composition of the surface layers can be systematically varied. The Ni content in the layer increases with increase in scanning speed. Detailed cross-sectional transmission electron microscopic study reveals complexities in solidification behavior with increased nickel content. It is shown that ordered B2 phase forms over a wide range of composition with subsequent precipitation of Ni₂Al, an ordered ω phase in the B2 matrix, during solid-state cooling. For nickel-rich alloys associated with higher laser scan speed, the fcc γ phase is invariably the first phase to grow from the liquid with solute trapping. The phase reorders in the solid state to yield γ′ Ni₃Al. The phase competes with β AlNi, which forms massively from the liquid. The β AlNi transforms martensitically to a 3R structure during cooling in solid state. The results can be rationalized in terms of a metastable phase diagram proposed earlier. However, the results are at variance with earlier studies of laser processing of nickel-rich alloys.     

Microstructure and Mechanical Properties of Laser Welded Titanium 6Al-4V

Laser butt welds were fabricated in a titanium alloy (Ti-6A1-4V, AMS 4911-Tal0 BSS, annealed) using a Control Laser 2 kW CW CO₂ laser. The relationships between the weld microstructure and mechanical properties are described and compared to the theoretical thermal history of the weld zone as calculated from a three-dimensional heat transfer model of the process. The structure of the weld zone was examined by radiography to detect any gross porosity as well as by both optical and electron microscopy in order to identify the microstructure. The oxygen pick-up during gas shielded laser welding was analyzed to correlate further with the observed mechanical properties. It was found that optimally fabricated laser welds have a very good combination of weld microstructure and mechanical properties, ranking this process as one which can produce high quality welds.     

The effect of convection on disorder in primary cellular and dendritic arrays

Directional solidification studies have been carried out to characterize the spatial disorder in the arrays of cells and dendrites. Different factors that cause array disorder are investigated experimentally and analyzed numerically. In addition to the disorder resulting from the fundamental selection of a range of primary spacings under given experimental conditions, a significant variation in primary spacings is shown to occur in bulk samples due to convection effects, especially at low growth velocities. The effect of convection on array disorder is examined through directional solidification studies in two different alloy systems, Pb-Sn and Al-Cu. A detailed analysis of the spacing distribution is carried out, which shows that the disorder in the spacing distribution is greater in the Al-Cu system than in Pb-Sn system. Numerical models are developed which show that fluid motion can occur in both these systems due to the negative axial density gradient or due the radial temperature gradient which is always present in Bridgman growth. The modes of convection have been found to be significantly different in these systems, due to the solute being heavier than the solvent in the Al-Cu system and lighter than it in the Pb-Sn system. The results of the model have been shown to explain experimental observations of higher disorder and greater solute segregation in a weakly convective Al-Cu system than those in a highly convective Pb-Sn system.     

An efficient built-in self testing for random-access memory

The authors propose a test algorithm for pattern-sensitive faults in large-size RAM with high circuit density. The algorithm tests an n-bit RAM in 195√n time to detect both static and dynamic pattern-sensitive faults over the 9-neighbourhood of every memory cell. A 4 Mb RAM can be tested by the proposed algorithm several thousand times faster than the conventional sequential algorithms for detecting pattern-sensitive faults. The test speedup has been achieved by writing a test data simultaneously over many cells, and the stored data are tested simultaneously by a parallel comparator and error detector in a read operation. The existing RAM architecture has been modified very little so that the proposed technique can be implemented very easily even in switched-capacitor DRAM (dynamic random-access memory) with low intercell pitch width. The test procedure has also been applied to built-in self-testing (BIST) and is compared with other BIST implementations     

Interface study and boundary smoothing on designed composite material microstructures for manufacturing purposes

The manufacturability of composite material microstructures designed by the homogenized topology optimization method has to be considered when bringing the design into reality. In this paper, numerical studies are conducted on multiphase material microstructures that have negative coefficients of thermal expansion, for the purpose of improving manufacturability. Realistic manufacturing factors are considered, including the diffusion interface between two constituent phases and vectorized toolpath design. The effect of the mixture interface between two solid phases is examined by homogenization analysis. An image-processing program is developed to smooth out the boundaries of the topological structure to facilitate toolpath design. Numerical results show that the diffusion interface between two solid material phases reduces the effective negative thermal expansion property of the whole microstructure. The designed material properties are retained and converge after boundary smoothing .     

Pore morphology in pressurized lignite coal: A small angle x-ray scattering investigation

The morphological changes in lignite coal at two different pressures, using small angle x-ray scattering, have been investigated. All the scattering profiles are interpreted in terms of surface fractal morphology. It is observed that the surface fractal dimension does not change appreciably under the experimental pressure range, while the volume of micro-cracks and inter-particle pore space is reduced with increasing pressure. By and large, the constant value of the surface fractal dimension of all the specimens, as calculated from the scattering profiles, indicates that the roughness on the pore coal interface remains almost unaffected under the experimental pressure range. The validity of single scattering approximation for this experiment has also been confirmed by the observation of the identical nature of the small angle scattering profiles with respect to change of wave lengths, namely, 0.154 nm. (Cu K) and 0.072 nm (Mo K), of the probing radiation.