S. A. David

Current Issues and Problems in Welding Science

Losses of life and property due to catastrophic failure of structures are often traced to defective welds. However, major advances have taken place in welding science and technology in the last few decades. With the development of new methodologies at the crossroad of basic and applied sciences, the promise of science-based tailoring of composition, structure, and properties of the weldments may be fulfilled. This will require resolution of several contemporary issues and problems concerning the structure and properties of the weldments as well as intelligent control and automation of the welding processes.

Analysis of solidification microstructures in Fe - Ni - Cr single crystal welds

A geometric analysis technique for the evaluation of the microstructures in autogenous single-crystal electron beam welds has been previously developed. In the present work, these analytical methods are further extended, and a general procedure for predicting the solidification microstructure of single-crystal welds with any arbitrary orientation is established. Examples of this general analysis are given for several welding orientations. It is shown that a nonsymmetric cell structure is expected in transverse micrographs for most welding geometries. The development of steady-state conditions in the weld pool is also examined in terms of the weld pool size, its shape (as revealed by the dendritic growth pattern), and the size of the dendritic cells. It is found that steady state is established within a few millimeters of the beginning of the weld. Furthermore, steady state is achieved faster in welds made at higher welding speeds. A general analysis of the three-dimensional (3-D) weld pool shape based on the dendritic structure as revealed in the two-dimensional (2-D) transverse micrographs is also developed. It is shown that in combination with information on the preferred growth direction as a function of the solidification front orientation, the entire dendritic growth pattern in single-crystal welds can be predicted. A comparison with the actual weld micrographs shows a reasonable agreement between the theory and experiment. Finally, the theoretical analysis of the dendrite tip radius is extended from binary systems to include the case of ternary systems. The theoretical dendrite trunk spacing in a ternary Fe-Ni-Cr alloy is calculated from the dendrite tip radius and is compared with the experimental values for several weld conditions. Good agreement between experiment and theory is found.

Development of microstructures in Fe−15Ni−15Cr single crystal electron beam welds

A detailed analysis of the microstructures produced in an autogenously welded single crystal of Fc−15Ni−15Cr was performed in order to investigate the relationship between growth crystallography and solidification behavior. Electron beam welds were made at various speeds on the (001) surface of single crystals in either the [100] or [110] directions. A geometrical analysis was carried out in order to relate the dendrite growth velocities in the three 〈100〉 directions to the weld velocities for the different crystallographic orientations examined. From this analysis, the preferred dendrite trunk directions were determined as a function of the solidification front orientation based upon a minimum velocity or minimum undercooling criterion. A thorough examination of the weld microstructures and a comparison with the geometrical relationships developed in this work permitted a three-dimensional reconstruction of the weld pool shape to be performed. In addition, the dendrite spacings were measured, and the variation in spacings as a function of growth velocity was compared with theoretical predictions. It was found that the range of velocities over which dendritic growth is expected agreed with the experimental findings, and, furthermore, the change in dendrite spacing with growth velocity varied as predicted by theory. These results clearly demonstrate the effect of crystallography on the micro-structural development during weld pool solidification. The results also show that the resultant microstructures and pool shapes can be explained by geometrical analysis in conjunction with existing solidification models.

Microstructural modification of austenitic stainless steels by rapid solidification

The microstructural modifications in three austenitic stainless steels (types 308, 310, and 312) were evaluated after rapid solidification. These three steels are commonly used weld filler metals. Two methods of rapid solidification were investigated, autogenous laser welding and arc-hammer splat quenching. The structure of 310 stainless steel was found to be 100 pct austenite, and did not vary over the range of conditions studied. On the contrary, the structures of types 308 and 312 steels were very sensitive to the cooling rates and solidification conditions. With the highest cooling rates, the type 308 structure was fully austenitic while the type 312 structure was fully ferritic. At lower cooling rates, the structures were duplex ferrite plus austenite. The results were interpreted in terms of faster kinetics of solidification of austenite compared to ferrite under the conditions examined. A comparison of the structures produced by the two rapid solidification techniques indicated the cooling rates are comparable.

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.

Time-resolved X-ray diffraction investigation of primary weld solidification in Fe-C-Al-Mn steel welds

In situ time-resolved X-ray diffraction (TRXRD) using synchrotron radiation was used to monitor the phase evolution during welding of Fe-C-Al-Mn steel with 0.05 s resolutions. The primary solidification phase under rapid- and slow-cooling rate conditions was examined. The results showed nonequilibrium austenite solidification during rapid cooling; in contrast to the equilibrium δ-ferrite solidification that occurs under slow cooling conditions. These experimental results were analyzed using computational thermodynamics and interface-response function models. Results of the interface response function calculations considering the effect of carbon alone, predicted a change from primary δ-ferrite to primary austenite solidification with an increase in solid-liquid interface velocity.

Low temperature aging behavior of type 308 stainless steel weld metal

Abstract The aging behavior of welded type 308 stainless steel was evaluated by mechanical property testing and microstructural examination. Aging was carried out at 475°C for up to 20,000 h. The initial material consisted of austenite with approximately 10% ferrite. Upon aging, the ferrite hardness increased up to 100%. This hardening was accompanied by a noticeable increase in the ductile—brittle transition temperature and a drop in the upper shelf energy, as measured by Charpy impact tests, and a degradation in fracture toughness, as determined by J-integral test. Tensile properties did not change significantly with aging. Microstructural analysis indicated that the ferrite decomposed spinodally into iron-rich α and chromium-enriched α′. In addition, abundant precipitation of nickel- and silicon-rich G-phase was found within the ferrite and M23C6 carbide formed along the austenite-ferrite interface. These effects are similar to the aging behavior of cast stainless steels. Occasionally, large G-phase or α precipitates were also found along the austenite-ferrite interface after aging more than 1000 h. After comparison of the mechanical property changes with the microstructural features, it was concluded that both spinodal decomposition as well as G-phase formation contribute to ferrite hardening. Spinodal decomposition results in embrittlement of the weld insofar as the ductile-brittle transition temperature is raised. G-phase formation and carbide precipitation are associated with a degradation in the ductile fracture properties, as shown by a drop in the upper shelf energy and a decrease in the fracture toughness.

Characterization of the microstructure evolution in a nickel base superalloy during continuous cooling conditions

The solidification characteristics of the γ phase from the liquid and the subsequent decomposition of the γ phase control the evolution of the microstructure in nickel–base superalloy welds. The precipitation of the γ′ phase from the γ phase during continuous cooling conditions (0.17–75 K s−1) from the solutionizing temperature was characterized in a directionally solidified CM247DS alloy with thermomechanical simulator, and by transmission electron microscopy, atom probe field ion microscopy and atom probe tomography. The number density increased; size decreased and morphology of the γ′ precipitates changed with an increase in cooling rate. Under rapid water-quenched conditions, complex partitioning of the alloying elements between γ and γ′ phases was observed. Atom probe tomography on samples subjected to slower cooling rates showed different partitioning behavior compared to that of water-quenched samples and the presence of secondary γ′ precipitates in the samples subjected to a cooling rate of 1 K s−1.

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.

Friction Stir Spot Welding of Advanced High-Strength Steels - A Feasibility Study

An exploratory study was conducted to investigate the feasibility of friction stir spot welding advanced high-strength steel sheet metals. The fixed pin approach was used to weld 600MPa dual phase steel and 1310MPa martensitic steel. A single tool, made of polycrystalline cubic boron nitride, survived over one hundred welding trials without noticeable degradation and wear. Solid-state metallurgical bonding was produced with welding time in the range of 2 to 3 seconds, although the bonding ligament width was relatively small. The microstructures and hardness variations in the weld regions are discussed. The results from tensile-shear and cross-tensile tests are also presented.