The localized corrosion behavior of laser spot welded UNS N08367 superaustenitic stainless steel (SASS) in 0.6 M NaCl solution at ambient and elevated temperatures was investigated via a variety of approaches. Localized corrosion behavior was connected with laser weld microstructure using macro- and micro-electrochemical characterization and exposure experiments conducted on laser welded sheet over a range of weld input energies. Corrosion experiments were augmented by studies on closely related materials such as UNS N08366, UNS N08904, and UNS N08367 with varying Cr, Mo, or N contents, used to mimic the Mo rich and depleted regions of the laser weld microstructure of UNS N08367. In addition, complimentary studies were conducted on furnace heat treated, resistance spot welded, and laser welded UNS N08367 SASS sandwich structures. Long term furnace heat treatments resulted in subsequent degradation of corrosion resistance of the alloy. Good corrosion resistance was maintained in rapidly cooled laser welds owing to retention of an FCC solid solution, minimization of Mo partitioning between dendritic and interdendritic regions in the solidified weld microstructure, and avoidance of detrimental phase precipitation. Significant dendritic undercooling and small dendritic tip radius are rationalized to contribute to formation of weld dendrite cores and interdendritic regions with minimal Mo partitioning. High energy laser spot welded and bonded face sheet and truss core structures fabricated from UNS N08367 maintained good localized corrosion resistance owing to a minimization of Mo segregation and speculated minimal loss of N loss in the weld region.
Cellular Metal Structures and Joining Issues Throughout industry there is a growing driving force for using materials solutions to produce structures that are lightweight without compromising their strength. One materials solution is the use of cellular metal structures, which represent the state of the art in ultra-light weight structures. Compared to competing materials, these multifunctional structures have many advantageous properties such as impact energy absorption, heat dissipation and vibration dampening [1-3]. Successfully implementing cellular metals in a given application requires selecting a suitable candidate, which is largely determined by the cell topology used and the specific properties of the base metal [1-3]. One material candidate for use as a base material in marine environments is a superaustenitic stainless steel (SASS). SASS has the advantage of being intrinsically resistant to corrosion in marine environments at ambient temperature [4]. Laser welding is one processing technique available for the construction of cellular metals. In this method, the face sheet is joined to the truss core structure at periodic nodes via an autogenous weld made with a laser beam which rapidly heats and cools the structure. However, this joining strategy raises concerns over altered metallurgy at the fusion zone, which will also be the site of a crevice formed between the face sheet and truss core of the sandwich structure. AL-6XN(1) or UNS N08367, is a SASS with a composition of Fe-24Ni-20.5Cr-6.3Mo-.22N. The resistance of UNS N08367 to different forms of corrosion is provided by the relatively high levels of Cr, Ni, Mo, and N in the alloy.
