T. Zacharia

New analytical procedure to determine stress-strain curve from spherical indentation data

Abstract Spherical-indentation process was analyzed by finite element (FE) method. A systematic analysis of relationship between indentation parameters and true stress/plastic-strain ( σ 1 - e p ) curve was performed for a range of material properties. An existing method relates the ratio or residual contact diameter, d , and indenter diameter, D , to e p by the well-known Tabors empirical equation e p = 0.2 d / D . The method is based on parameters of residual indentation, where a loading-unloading cycle needs to be made in order to calculate a point on ( σ 1 - e p ) curve. A new analytical approach is presented which relates the indentation data continuously measured during loading to ( σ 1 - e p ) curve. e p calculated by the new method is in the range from yield strain to a strain between 0.3 and 1.6, depending on materials strain hardening properties. In addition, different measures of indentation diameter are discussed and their influence on the resulting ( σ 1 - e p ) curve analyzed. Experimental work was performed by an instrumented spherical-indentation technique in order to verify the FE analysis results. A good agreement between the FE and experimental results was obtained.

Numerical analysis of GTA welding process with emphasis on post-solidification phase transformation effects on residual stresses

The objective of this work was to analyze the residual stress state in spot welds made in an HY-100 steel disk by an autogenous gas tungsten arc (GTA) welding process. An uncoupled thermal-mechanical finite element (FE) model was developed that took into account the effects of liquid-to-solid and solid-state phase transformations. Effects of variations in mechanical properties due to solid-state phase transformations on residual stresses in the weld were studied. Extensive experimental testing was carried out to determine the mechanical properties of HY-100 steel. The residual stresses in the disk with the spot weld were measured by a neutron diffraction (ND) technique. The FE results are in good agreement with the ND measurements. The results show that the volumetric changes associated with the austenite to martensite phase transformation in HY-100 steel significantly affect residual stresses in the weld fusion zone and the heat affected zone.

Modeling the kinetics and microstructural evolution during static recrystallization : Monte Carlo simulation of recrystallization

The kinetics of microstructure and texture evolution during static recrystallization of a cold-rolled and annealed f.c.c. material is simulated by coupling a finite element model of microstructural deformation with a Monte Carlo simulation of recrystallization. The salient features of the simulations include a nucleation model for recrystallization based on subgrain growth and the modeling of simultaneous recovery during recrystallization. The simulation results quantify the effects of non-uniform stored energy distribution and orientation gradients present in the cold-worked microstructure on recovery by subgrain growth, and hence on the spatial distribution of nuclei and their orientations. The growth of these recrystallized nuclei in the presence of continued recovery of the substructure has been simulated for initial cold-work levels of e=0.7 and 1.1 obtained by plane strain compression. The simulations are shown to be potentially capable of capturing the formation and evolution of cube texture commonly observed in cold-rolled and annealed f.c.c. materials.

Effect of evaporation and temperature-dependent material properties on weld pool development

This paper evaluates the effect of weld pool evaporation and thermophysical properties on the development of the weld pool. An existing computational model was modified to include vaporization and temperature-dependent thermophysical properties. Transient, convective heat transfer during gas tungsten arc (GTA) welding with and without vaporization effects and variable properties was studied. The present analysis differs from earlier studies that assumed no vaporization and constant values for all of the physical properties throughout the range of temperature of interest. The results indicate that consideration of weld pool vaporization effects and variable physical properties produce significantly different weld model predictions. The calculated results are consistent with previously published experimental findings.

Simulation of curvature-driven grain growth by using a modified monte carlo algorithm

The Monte Carlo (MC) algorithm that currently exists in the literature for simulating curvature-driven grain growth has been modified. The modified algorithm results in an acceleration of the simulated grain growth and an early estimate of the grain growth exponent that is close to the theoretical value of 0.5. The upper limit of grain size distributions obtained with the new algorithm is significantly lower than that obtained with the old, because the new algorithm eliminates grain coalescence during grain growth. The log-normal function provides an excellent fit to the grain size distribution data obtained with the new algorithm, after taking into account the anisotropy in grain boundary energy.

Heat transfer during Nd: Yag pulsed laser welding and its effect on solidification structure of austenitic stainless steels

Theoretical and experimental investigations were carried out to determine the effect of process parameters on weld metal microstructures of austenitic stainless steels during pulsed laser welding. Laser welds made on four austenitic stainless steels at different power levels and scanning speeds were considered. A transient heat transfer model that takes into account fluid flow in the weld pool was employed to simulate thermal cycles and cooling rates experienced by the material under various welding conditions. The weld metal thermal cycles and cooling rates are related to features of the solidification structure. For the conditions investigated, the observed fusion zone structure ranged from duplex austenite (γ)+ferrite (δ) to fully austenitic or fully ferritic. Unlike welding with a continuous wave laser, pulsed laser welding results in thermal cycling from multiple melting and solidification cycles in the fusion zone, causing significant post-solidification solid-state transformation to occur. There was microstructural evidence of significant recrystallization in the fusion zone structure that can be explained on the basis of the thermal cycles. The present investigation clearly demonstrated the potential of the computational model to provide detailed information regarding the heat transfer conditions experienced during welding.

Modeling of fundamental phenomena in welds

Recent advances in the mathematical modeling of fundamental phenomena in welds are summarized. State of the art mathematical models, advances in computational techniques, emerging high-performance computers, and experimental validation techniques have provided significant insight into the fundamental factors that control the development of the weldment. The current status and scientific issues in the areas of heat and fluid flow in welds, heat source-metal interaction, solidification microstructure, and phase transformations are assessed. Future research areas of major importance for understanding the fundamental phenomena in weld behaviour are identified.

Three-dimensional transient model for arc welding process

A direct computer simulation technique, discrete element analysis (DEA), was utilized in the development of a transient multidimensional (2-D and 3-D) mathematical model for investi-gating coupled conduction and convection heat transfer problems associated with stationary and moving arc welding processes. The mathematical formulation considers buoyancy, electro-magnetic, and surface tension driving forces in the solution of the overall heat transfer conditions in the specimen. Furthermore, the formulation of the model allows realistic consideration of the geometrical variations in the workpiece. The model treats the -weld pool surface as a truly deformable free surface, allowing for the prediction of the weld surface deformations such as the “weld crown.≓ A marked element formulation was employed to monitor the transient de-velopment of the weld pool as determined by the latent heat considerations and the calculated velocities in the weld pool. The model was utilized to simulate the heat and fluid flows in the weld pool that occur during stationary (spot) and moving (linear) gas tungsten-arc welding. Also, the present analysis considers a simple rectangular specimen and a geometrically complex specimen to demonstrate the capability of the model to simulate realistic 3-D arc welding prob-lems. The results of the present investigation clearly demonstrate the significant influence of the heat and fluid flows and the specimen geometry on the development of the weld. Comparison of the predicted and the experimentally observed fusion zone and heat-affected zone (HAZ) geometries indicate good agreement.

Finite element simulations of cold deformation at the mesoscale

Abstract The deformation of polycrystalline aggregates is modeled using the finite element method. Explicit discretization at the single crystal level is employed to study the inhomogeneous deformations of individual crystals. Plastic deformation by crystallographic slip is treated using a constitutive model based on crystal plasticity. The formulation is used to predict the non-uniform nature of strain hardening and texture evolution in the crystals subjected to plane strain compression. The capability of the simulations to capture the inhomogeneous deformation of individual grains during plastic deformation of polycrystals is demonstrated. The hardness and orientation values of elements from the same grain evolve to different final values due to local inhomogeneities and interactions with neighbors. The simulations provide a means to obtain quantitative information on the inhomogeneous distributions of stored energy and orientations among the different crystals comprising the microstructure.

Computational modeling of stationary gastungsten-arc weld pools and comparison to stainless steel 304 experimental results

A systematic study was carried out to verify the predictions of a transient multidimensional computational model by comparing the numerical results with the results of an experimental study. The welding parameters were chosen such that the predictions of the model could be correlated with the results of an earlier experimental investigation of the weld pool surface temperatures during spot gas-tungsten-arc (GTA) welding of Type 304 stainless steel (SS). This study represents the first time that such a comprehensive attempt has been made to experimentally verify the predictions of a numerical study of weld pool fluid flow and heat flow. The computational model considers buoyancy and electromagnetic and surface tension forces in the solution of convective heat transfer in the weld pool. In addition, the model treats the weld pool surface as a truly deformable surface. Theoretical predictions of the weld pool surface temperature distributions, the cross-sectional weld pool size and shape, and the weld pool surface topology were compared with corresponding experimental measurements. Comparison of the theoretically predicted and the experimentally obtained surface temperature profiles indicated agreement within ±8 pct for the best theoretical models. The predicted surface profiles were found to agree within ±20 pct on dome height and ±8 pct on weld pool diameter for the best theoretical models. The predicted weld cross-sectional profiles were overlaid on macrographs of the actual weld cross sections, and they were found to agree very well for the best theoretical models.