Investigation of Microstructural Evolution in Copper Short-Circuit Melt Marks After Fire Re-Exposure

Copper melt marks recovered from fire debris may record both an initial short circuit and subsequent fire heating. This study examined how flame re-exposure changes the microstructure of primary short-circuit marks using electron backscatter diffraction (EBSD). Marks were generated at 500 A on solid copper conductors (1.5 and 2.5 mm2) from heat-resistant polyvinyl chloride-insulated building wire. Half were then exposed to a butane flame until a visually defined metal-reflow endpoint, giving a 2 × 2 design with five independent specimens per group (n = 20). Five descriptors were assessed using specimen-level summaries, 1-Wasserstein distances, permutational multivariate analysis of variance (PERMANOVA), and supporting dispersion and sensitivity analyses. Remelting shifted the area-weighted grain-diameter distribution toward coarser grains (thermal-history R2 = 0.464) and increased the area-weighted aspect ratio in both conductor sizes (η2 = 0.618). Circularity changed in opposite directions between the conductor sizes, whereas three intragranular misorientation metrics showed no supported thermal-history effect. The grain-diameter result persisted under a common post-reconstruction small-grain cutoff, whereas the ASTM grain-size contrast depended on small-grain inclusion. These findings indicate that EBSD interpretation of fire-recovered melt marks should consider subsequent remelting and conductor size rather than rely on any single descriptor as an origin-classification criterion.