In recent years, major advances have taken place in our understanding of welding processes and welded materials Because of the complexity of fusion welding processes, solution of many important contemporary problems in fusion welding requires an interdisciplinary approach Current problems and issues in fusion welding are reviewed Solution of these problems, apart from being a contribution to the advancement of science, is also necessary for science-based tailoring of composition, structure, and properties of the welded materials

Physical processes in fusion welding

A model of the mechanical alloying process, applicable to a single collision event involving ductile species, is developed. Simple physical models are constructed that allow development of analytical expressions for particle shape and hardness changes during a collision and stipulation of criteria for particle fracture and cold welding. These provide a“snapshot” of that which transpires during a single collision event. The model also accounts for the heterogeneity of deformation within the powder trapped between colliding media. This heterogeneity, together with the model criteria, can then be incorporated within a computational scheme capable of predictive description of the evolution of powder morphology and properties during mechanical alloying, as will be described in a subsequent article.

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Modeling of mechanical alloying: Part I. deformation, coalescence, bdand fragmentation mechanisms

Ladle steel deoxidation reactions are reviewed and the principles are extended to inclusion formation in steel weld metal. The dissolution of oxygen, the stability of various oxides, and the nucleation and growth of inclusions are discussed. Theoretical time–temperature transformation (TTT) diagrams are calculated for various oxide inclusions based on an overall kinetics approach using nucleation and growth rate expressions. These concepts are then extended to understand the development of weld metal inclusion characteristics. A strong correlation between the published inclusion composition and the stability of the oxides was found. An analysis of the TTT diagrams indicates that, during weld cooling, sequential oxidation of dissolved deoxidizing elements takes place, which agrees with the reported layered morphology of inclusions. The analysis indicates that the inclusion characteristics are quite sensitive to the oxygen content, the deoxidising element concentrations, the presence of preformed in...

Development of macro- and microstructures of carbon–manganese low alloy steel welds: inclusion formation

A critical review of current thermodynamic theories of slag-metal reactions is presented. A series of preliminary experiments indicates that the previously proposed droplet theory is incorrect and the primary reactions controlling Mn, Si, and Cr content occur in the weld pool. In addition, these experiments show that the net transfer of oxygen is independent of the transfer of Mn and Si.

Slag-metal reactions during welding: Part I. Evaluation and reassessment of existing theories

A previously developed kinetic model of alloy transfer (Part II)[1] is tested experimentally for transfer of Mn, Si, Cr, P, S, Ni, Cu, and Mo. The results show very good agreement between theory and experiment. The transfer of carbon and oxygen is also discussed. It is shown that the transfer of oxygen into the weld metal occurs in the zone of droplet reactions, whereas oxygen is lost by formation and separation of inclusions in the solidifying weld pool. Methods of applying this analysis to multipass welds and active fluxes containing ferroalloy additions are also described.

Slag-metal reactions during welding: Part III. Verification of the Theory

A kinetic model is developed to describe the transfer of alloying elements between the slag and the metal during flux-shielded welding. The model accounts for changes in alloy recovery based on the geometry of the resulting weld bead. It also distinguishes compositional differences between single-pass and multiple-pass weld beads. It is further shown that the final weld metal oxygen content is directly related to the weld solidification time as well as the type of flux used.

T. W. Eagar

Papers

Slag-metal reactions during welding: Part II. Theory