As part of recent measures to combat global warming, automobiles are required to be electrified and their weight reduced, leading to the advancement of multi-material structures that include aluminum alloys and aluminum die castings. Conventional fusion welding methods for joining aluminum alloys and steel materials have poor joining performance due to differences in thermal conductivity between the materials and the presence of oxide films. Friction stir welding (FSW) has been attracting attention in recent years because they are solid-phase joining methods and can also be used to join dissimilar materials. In this study, three types of FSW overlay joints were fabricated:AA6111, an aluminum alloy, was used for the upper plate, and AA6111, AA6061, an aluminum alloy, and DX30, a die cast material, were used for the lower plate. Non-destructive testing was performed on each joint to instantly inspect and visualize joint defects. In the case of FSW joints, no difference was observed in the heat transfer process when the joint was heated directly, but the location of the hooking could be identified by heating from a distance from the joint. The results of the analysis of the temperature change at the defect location showed a difference in heat propagation.