Diffusion welding of commercially pure aluminum (JIS A1200) to mild steel (JIS S20C) was performed using various insert materials such as Ni and Al-Cu-Mg alloy (JIS A2024) foils. A Ni foil was at first diffusion welded to steel at 850°C for 30mn at 0.065kg/mm
2 in welding pressure. This joint results in tensile strength higher than that of the Al base metal. Diffusion welding of Al to the overlayed steel with Ni foil was then accomplished employing an insert material of 2024 alloy foil. The joint welded at partially melting temperature (520°C) of 2024 alloy for 30mn at 0.065kg/mm
2 in welding pressure shows 85% of tensile strength to the Al base metal and the best Charpy impact value. The interlayer which consists of intermetallic compounds of Al
3Ni (β-phase) and Al
3Ni
2 (γ-phase) is formed at 2024 alloy side and Ni side in the bonding interface respectively. The mechanical strength lowers with increasing the thickness of this interlayer.
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