Welding is one of the most complicated fabrication processes used to assemble engineering structures. The process involves heating and cooling rates that are significantly higher than most manufacturing processes, and the materials are subjected to a wide range of peak temperatures that span from ambient to above the liquidus temperature. As a result, the base materials can undergo a wide range of solidification and solid state phase transformations that will induce significant changes to the pre-existing microstructure and associated properties. Due to these extreme conditions, the phase transformations usually occur under non-equilibrium conditions and often produce unexpected microstructures that have detrimental effects on properties.
Microstructures of the base metal (left) and fusion zone (right) of IN740H after creep testing.
As an example, the figure above compares the microstructures of the base metal (left) and fusion zone (FZ, right) of the γ’ precipitation strengthened nickel base superalloy IN740H after creep testing. This alloy was recently developed as the leading candidate material for advanced power generation plants that are designed to operate at high temperatures and pressures for improved efficiency. The base metal exhibits a uniform distribution of γ’ precipitates throughout the matrix, which provides among the highest creep strength of any candidate alloy developed to date for this application. In contrast, the weld metal possesses a reduction in precipitates at the dendrite core region, excess precipitates near the dendrite boundaries, and a precipitate free zone (PFZ) on the grain boundaries. The PFZ is driven by a discontinuous coarsening mechanism that is caused by the non-equilibrium segregation that occurs under the fast solidification rates. Formation of the PFZ leads to local grain boundary softening and premature failure in the FZ by creep cavitation This microstructural modification is met with a sharp reduction in creep strength by over 30% and is currently a major challenge to full scale use of this alloy. This is one of many examples in which the complicated phase transformations that occur during fusion welding can severely limit the use of new materials that are developed for advanced engineering applications. Similar problems have been observed with reduced corrosion in welds of stainless steels, premature creep failure in the heat affected zones (HAZ) of ferritic steels, corrosion-fatigue of weld overlays in high temperature service, dissimilar weld failures, and weld repair attempts in single crystal nickel alloys, just to name a few.
Safe and effective use of welded components requires integration of many disciplines, including heat/fluid flow, solidification, process control and optimization, solid state phase transformations, residual stress, and structure-property relations. In addition, modern materials require the development and use of sophisticated modeling tools that can be used to manipulate these processes in an integrated manner for the ultimate purpose of microstructure control in the FZ and HAZ. This is a critical step in controlling the final weldment properties for the intended service. Prof. DuPont and his students utilize advanced modeling tools and microstructural characterization techniques to understand and control microstructural evolution and associate properties in welds of complex alloy systems.