Microstructures and Oxide Scale of Melted Mark on Copper Wire Solidified under Various Cooling Conditions and Subsequent Heat Exposure

Abstract

A primary melted mark (PMM) on copper wire has been expected to identify the fire origin. However, the solidification microstructure under various cooling conditions has not been extensively studied. In this research, the effects of cooling conditions on PMM microstructure before and after heat treatment were investigated. The results represent dendrite growth direction to the surface of the melted mark with copper dendrites surrounded by (Cu+Cu₂O) eutectic structure for air and water-cooling specimens. Moreover, the melted mark cooled in the air has more than twice of secondary dendrite arm spacing (SDAS) than the water-cooling sample in the case of pre-heat treatment. After heat treatment and cooled down in the furnace, air, and water: the dendritic structure disappears and Cu₂O precipitates on the copper matrix. The smaller crystallite size and oxide layer cracking could be found in a higher cooling rate case. Therefore, by connecting thermal history with solidification structure and surface oxidation morphology, the change in microstructures and physical parameters of PMM could be expected to be helpful for fire investigation.