J. W. Elmer

Microstructural development during solidification of stainless steel alloys

The microstructures that develop during the solidification of stainless steel alloys are related to the solidification conditions and the specific alloy composition. The solidification conditions are determined by the processing method,i.e., casting, welding, or rapid solidification, and by parametric variations within each of these techniques. One variable that has been used to characterize the effects of different processing conditions is the cooling rate. This factor and the chemical composition of the alloy both influence (1) the primary mode of solidification, (2) solute redistribution and second-phase formation during solidification, and (3) the nucleation and growth behavior of the ferrite-to-austenite phase transformation during cooling. Consequently, the residual ferrite content and the microstructural morphology depend on the cooling rate and are governed by the solidification process. This paper investigates the influence of cooling rate on the microstructure of stainless steel alloys and describes the conditions that lead to the many microstructural morphologies that develop during solidification. Experiments were performed on a series of seven high-purity Fe-Ni-Cr alloys that spanned the line of twofold saturation along the 59 wt pct Fe isopleth of the ternary alloy system. High-speed electron-beam surface-glazing was used to melt and resolidify these alloys at scan speeds up to 5 m/s. The resulting cooling rates were shown to vary from 7°C/s to 7.5×106°C/s, and the resolidified melts were analyzed by optical metallographic methods. Five primary modes of solidification and 12 microstructural morphologies were characterized in the resolidified alloys, and these features appear to be a complete “set” of the possible microstructures for 300-series stainless steel alloys. The results of this study were used to create electron-beam scan speedvs composition diagrams, which can be used to predict the primary mode of solidification and the microstructural morphology for different processing conditions. Furthermore, changes in the primary solidification mode were observed in alloys that lie on the chromium-rich side of the line of twofold saturation when they are cooled at high rates. These changes were explained by the presence of metastable austenite, which grows epitaxially and can dominate the solidification microstructure throughout the resolidified zone at high cooling rates.

Heat transfer and fluid flow during keyhole mode laser welding of tantalum, Ti–6Al–4V, 304L stainless steel and vanadium

Because of the complexity of several simultaneous physical processes, most heat transfer models of keyhole mode laser welding require some simplifications to make the calculations tractable. The simplifications often limit the applicability of each model to the specific materials systems for which the model is developed. In this work, a rigorous, yet computationally efficient, keyhole model is developed and tested on tantalum, Ti–6Al–4V, 304L stainless steel and vanadium. Unlike previous models, this one combines an existing model to calculate keyhole shape and size with numerical fluid flow and heat transfer calculations in the weld pool. The calculations of the keyhole profile involved a point-by-point heat balance at the keyhole walls considering multiple reflections of the laser beam in the vapour cavity. The equations of conservation of mass, momentum and energy are then solved in three dimensions assuming that the temperatures at the keyhole wall reach the boiling point of the different metals or alloys. A turbulence model based on Prandtls mixing length hypothesis was used to estimate the effective viscosity and thermal conductivity in the liquid region. The calculated weld cross-sections agreed well with the experimental results for each metal and alloy system examined here. In each case, the weld pool geometry was affected by the thermal diffusivity, absorption coefficient, and the melting and boiling points, among the various physical properties of the alloy. The model was also used to better understand solidification phenomena and calculate the solidification parameters at the trailing edge of the weld pool. These calculations indicate that the solidification structure became less dendritic and coarser with decreasing weld velocities over the range of speeds investigated in this study. Overall, the keyhole weld model provides satisfactory simulations of the weld geometries and solidification sub-structures for diverse engineering metals and alloys.

Modeling of heat transfer and fluid flow during gas tungsten arc spot welding of low carbon steel

The evolution of temperature and velocity fields during gas tungsten arc spot welding of AISI 1005 steel was studied using a transient numerical model. The calculated geometry of the weld fusion zone and heat affected zone and the weld thermal cycles were in good agreement with the corresponding experimental results. Dimensional analysis was used to understand the importance of heat transfer by conduction and convection at various stages of the evolution of the weld pool and the role of various driving forces for convection in the liquid pool. The calculated cooling rates are found to be almost independent of position between the 1073 and 773 K (800 and 500 °C) temperature range, but vary significantly at the onset of solidification at different portions of the weld pool. During solidification, the mushy zone grew significantly with time until the pure liquid region vanished. The solidification rate of the mushy zone/solid interface was shown to increase while the temperature gradient in the mushy zone at...

Phase transformation dynamics during welding of Ti–6Al–4V

In situ time-resolved x-ray diffraction (TRXRD) experiments were used to track the evolution of the α→β→L→β→α/α′ phase transformation sequence during gas tungsten arc welding of Ti–6Al–4V. Synchrotron radiation was employed for the in situ measurements in both the fusion zone (FZ) and the heat-affected zone (HAZ) of the weld, providing information about transformation rates under rapid heating and cooling conditions. The TRXRD data were coupled with the results of computational thermodynamic predictions of phase equilibria, and numerical modeling of the weld temperatures. The results show that significant superheat is required above the β transus temperature to complete the α→β transformation during weld heating, and that the amount of superheat decreases with distance from the center of the weld where the heating rates are lower. A Johnson–Mehl–Avrami phase transformation model yielded a set of kinetic parameters for the prediction of the α→β phase transformation during weld heating. Corresponding TRXRD ...

Kinetic modeling of phase transformations occurring in the HAZ of C-Mn steel welds based on direct observations

Abstract In situ Spatially Resolved X-Ray Diffraction (SRXRD) experiments were performed in the heat-affected zone (HAZ) of gas tungsten arc (GTA) welds of AISI 1005 C-Mn steel to directly observe welding induced phase transformations. These real-time observations were semi-quantified using diffraction peak profile analysis to construct a phase transformation map revealing ferrite ( α ) and austenite ( γ ) phase concentration gradients in the HAZ. Weld thermal cycles were calculated using a three-dimensional heat transfer and fluid flow model and then combined with the SRXRD phase map to provide a complete description of the HAZ under actual welding conditions. Kinetic modelling of the α → γ phase transformation during heating was performed using a Johnson–Mehl–Avrami analysis, modified to take into account non-uniform weld heating and transformation in the α+γ two-phase field. The results provide the most accurate JMA kinetic parameters to date for this alloy, n=1.45 and 1n(ko)=12.2, for an activation energy Q=117.1 kJ/mole. Using this kinetic description of the α→γ phase transformation, time temperature transformation (TTT) and continuous heating transformation (CHT) diagrams for this alloy were constructed to illustrate how the combination of SRXRD experiments and numerical modeling from one weld can be used to predict phase transformations for a variety of welding and heat treating applications.

Heat transfer and fluid flow in laser microwelding

The evolution of temperature and velocity fields during linear and spot Nd-yttrium aluminum garnet laser microwelding of 304 stainless steel was simulated using a well-tested, three-dimensional, numerical heat transfer and fluid flow model. Dimensional analysis was used to understand both the importance of heat transfer by conduction and convection as well as the roles of various driving forces for convection in the weld pool. Compared with large welds, smaller weld pool size for laser microwelding restricts the liquid velocities, but convection still remains an important mechanism of heat transfer. On the other hand, the allowable range of laser power for laser microwelding is much narrower than that for macrowelding in order to avoid formation of a keyhole and significant contamination of the workpiece by metal vapors and particles. The computed weld dimensions agreed well with the corresponding independent experimental data. It was found that a particular weld attribute, such as the peak temperature or weld penetration, could be obtained via multiple paths involving different sets of welding variables. Linear and spot laser microwelds were compared, showing differences in the temperature and velocity fields, thermal cycles, temperature gradients, solidification rates, and cooling rates. It is shown that the temperature gradient in the liquid adjacent to the mushy zone and average cooling rate between 800 and 500 °C for laser spot microwelding are much higher than those in linear laser microwelding. The results demonstrate that the application of numerical transport phenomena can significantly improve current understanding of both spot and linear laser microwelding.

Modeling and real time mapping of phases during GTA welding of 1005 steel

Abstract Evolution of the microstructure in AISI 1005 steel weldments was studied during gas tungsten arc (GTA) welding experimentally and theoretically. The experimental work involved real-time mapping of phases in the heat-affected zone (HAZ) using a synchrotron-based spatially resolved X-ray diffraction (SRXRD) technique and post weld microstructural characterization of the fusion zone (FZ). A three-dimensional heat transfer and fluid flow model was used to calculate the temperature and velocity fields, thermal cycles, and the geometry of the FZ and the HAZ. The experimental SRXRD phase map and the computed thermal cycles were used to determine the kinetic parameters in the Johnson–Mehl–Avrami (JMA) equation for the ferrite to austenite transformation during heating in the HAZ. Apart from providing a quantitative expression for the kinetics of this transformation, the results are consistent with a decreasing nucleation rate of austenite from a ferrite matrix with time. In the FZ, the volume fractions of microconstituents were calculated using an existing phase transformation model and the computed thermal cycles. Good agreement was found between the calculated and experimental volume fractions of allotriomorphic and Widmanstatten ferrites in the FZ. The results indicate significant promise for understanding microstructure evolution during GTA welding of AISI 1005 steel by a combination of real time phase mapping and modeling.

Analysis of heat-affected zone phase transformations using in situ spatially resolved x-ray diffraction with synchrotron radiation

Spatially resolved X-ray diffraction (SRXRD) consists of producing a submillimeter size X-ray beam from an intense synchrotron radiation source to perform real-time diffraction measurements on solid materials. This technique was used in this study to investigate the crystal phases surrounding a liquid weld pool in commercial purity titanium and to determine the location of the phase boundary separating the high-temperature body-centered-cubic (bcc) β phase from the low-temperature hexagonalclose-packed (hcp) α phase. The experiments were carried out at the Stanford Synchrotron Radiation Laboratory (SSRL) using a 0.25 × 0.50 mm X-ray probe that could be positioned with 10-µm precision on the surface of a quasistationary gas tungsten arc weld (GTAW). The SRXRD patterns were collected using a position-sensitive photodiode array in a φ-2φ geometry. For this probe size, integration times of 10 s/scan at each location on the specimen were found adequate to produce high signal-to-noise (S/N) ratios and quality diffraction patterns for phase identification, thus allowing real-time diffraction measurements to be made during welding. The SRXRD results showed characteristic hcp, bcc, and liquid diffraction patterns at various points along the sample, starting from the base metal through the heat-affected zone (HAZ) and into the weld pool, respectively. Analyses of the SRXRD data show the coexistence of bcc and hcp phases in the partially transformed (outer) region of the HAZ and single-phase bcc in the fully transformed (inner) region of the HAZ. Postweld metallographic examinations of the HAZ, combined with a conduction-based thermal model of the weld, were correlated with the SRXRD results. Finally, analysis of the diffraction intensities of the hcp and bcc phases was performed on the SRXRD data to provide additional information about the microstructural conditions that may exist in the HAZ at temperature during welding. This work represents the first directin situ mapping of phase boundaries in fusion welds.

In situ observations of phase transitions in Ti–6Al–4V alloy welds using spatially resolved x-ray diffraction

In situ spatially resolved x-ray diffraction (SRXRD) experiments were used to directly observe the heat-affected zone phases present during gas tungsten arc welding of a Ti–6Al–4V alloy. The experiments were performed at the Stanford Synchrotron Radiation Laboratory using a 250 μm diam x-ray beam to gather real-time experimental information about the α−Ti→β−Ti phase transition during weld heating. Six different welding conditions were investigated using SRXRD to experimentally determine the extent of the single phase β-Ti region surrounding the liquid weld pool. These data were compared to predicted locations of the β-Ti phase boundary determined by calculated weld thermal profiles and equilibrium thermodynamic relationships. The comparison shows differences between the experimentally measured and the calculated locations of the β-Ti boundary. The differences are attributed to kinetics of the α−Ti→β−Ti phase transition, which requires superheating above the β-Ti transus temperature to take place during no...

Kinetics of Ferrite to Austenite Transformation during Welding of 1005 Steel

Abstract The kinetics of ferrite to austenite phase transformation in 1005 steel during welding was quantitatively determined by a combination of phase mapping using X-ray diffraction and transport phenomena based numerical modeling. The results can be used to calculate the phase transformation rates under various thermal cycles for this steel.