This report describes some options for weld repairs to the outer shell of the dome on the U.S. Capitol. During the past 140 years, corrosion products have built up within many of the joints, leading to cracking of some of the iron castings that form the shell. Since the dome is a national landmark, the goal is to restore the structural integrity of the original castings, replacing as few of the components as is absolutely necessary. While mechanical joining of the fractures or filling of the gaps with epoxy are alternative procedures, a fused (leak-tight) and ductile joint is preferred. A major challenge is that the castings were produced with 1850s' technology, so the composition is far different from current castings that are designed for weld repair. Therefore, we chose to develop some alternative approaches, designed specifically for the dome castings. Of the various options, oxyacetylene braze welding (a flame repair process where the filler metal melts at a temperature below that of the casting) with low-fuming bronze (about 60Cu-40Zn) worked best. The bronze forms joints that are very similar in strength to the castings. A joining trial in July 2002 demonstrated the utility of this technique in the flat and vertical positions at four corner cracks on the dome.
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