Weldment properties are affected by thegeometry, chemical composition, and structure of theweld metal. Recent research at Penn State aimed atpredicting these factors by numerical modeling of heatand mass transfer, fluid flow, and solid state phasetransformation kinetics are reviewed.For well over a decade, the lack of reproducibilityof the weld penetration has been attributed to the smalldifferences in the concentrations of surface activeelements, such as sulfur, that are known to improve weldpenetration. What has been baffling is that in many cases,their presence has not actually resulted in the expectedhigh depth of penetration. Recent research has shownthat unexpected, and even apparently anomalous, resultsof weld penetration can be explained from the detailedinsight achieved through the modeling of transport processes.During fusion welding, the weld metal compositionchanges owing to the evaporation of alloying elements,chemical reactions, and the dissolution of oxygen,nitrogen, and hydrogen in the weld metal from a plasmacontaining excited and ionized species of these gases.Experiments involving exposure of isothermal metaldrops to well-characterized plasmas have provided newinsight into the commonly observed high gas solubilityin the weld metal. As a result of these experiments andthe recent modeling of alloying element evaporation andgas dissolution, weld metal composition can now bepredicted, at least under controlled conditions, based on transport phenomena.Critical to the weld metal properties are the natureand amount of various metallic phases and inclusions.The inclusion composition and size distribution in theweld metal have been modeled considering heat transferand fluid flow in the weld pool and the kinetics ofnucleation and growth. A model has also been developedto quantitatively predict the microstructures of lowalloy steel weld metal by combining transient, threedimensional, calculation of heat transfer and fluid flowwith solid state phase transformation theory.
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