Study on Diffusion Bonding of Cast Irons

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Spheroidal graphite cast irons which are a very difficult material to fusion weld were diffusion bonded. The joints were fabricated with Ni foil as an insert metal or without it at 820, 900 and 950 ﾟC for 1.8 ks. And then they were annealed to ferritize the cast iron matrix at 720 ﾟC for 10.8 ks. Impact and torsion characteristics of the diffusion bonds have been investigated. Main results obtained are as follows.(1) The bond toughness of the joints directly bonded was very low due to much precipitation of graphites at the bond interface. When the cast irons were bonded with Ni foil as an insert metal, however, the bond toughness was strongly improved. The absorbed energy of the joints with Ni foil of 50 mu m in thickness was nearly the same as that of the base metal.(2) The absorbed energy of joints of cast irons with lower number of graphite particles, the cast irons were directly bonded, decreased with an increase in bonding temperature. When the number of graphite particles was more than 250 particles/mm^2 in the cast iron, however, the absorbed energy of the joints increased with the increase in bonding temperature. This is considered as follow. As the ferritizing speed of the cast iron increased with an increase in number of graphite particles, precipitation of graphites during the ferritizing was mainly developed on the graphite particles in the cast iron rather than at the bond interface. As a result, the bond toughness was improved by a decrease of precipitation of graphite at the bond interface.(3) When the cast iron joints directly bonded were torsion tested, and endurance of the torsion fatigue was largely scattered at the same stress. This is due to the precipitation of graphites at the bond interface. The endurance of the joints with Ni foil of 10 mu m in thickness was hardly scattered, and the fatigue limit was 160 MPa.