Kazuaki Yokoi

Effect of residual stress on the strength of alumina-steel joint with Al-Si interlayer

The effect of residual stress on the strength of an alumina-steel joint with the Al-Si interlayer was studied. Alumina rods, 32 mm in diameter and 9 mm in length and steel pipes were diffusion bonded at 873 K and at a contact pressure of about 5 M Pa for 30 min in a vacuum of 2∼4×10−2Pa. The interlayer of aluminium sheet clad with Al-10% Si alloy on both sides was used. The tensile strength of the joints is influenced by the thickness of the interlayer or the intermetallic compound formed between the interlayer and the steel. The strength increases with increasing interlayer thickness and with decreasing intermetallic compound thickness. It is found that the residual stress measured by Sachs method is much lower than that by the elastic calculation. The stress decreases with increasing interlayer thickness. Increase in thickness of the aluminium core of the interlayer is effective in improving the joint strength. This improvement can be explained by considering the stress of the joint.

Residual stress estimation of a silicon carbide-Kovar joint

Residual stress of silicon carbide and Kovar is calculated using the elasto-plastic method and its validity is checked by the four-point bending test. Silicon carbide and Kovar (Fe-27% Ni-7% Co) are diffusion bonded using Al-10% Si alloy clad on a pure aluminium sheet at 883 K and 4.9 MPa under a vacuum condition. The non-linear structure analysis program (ADINA) is used for the stress estimation. It is found by the calculation that the maximum tensile stress, about 170 MPa, is generated in the silicon carbide close to the interlayer. In the bending test, the fracture of the joint is found to occur from the point where the maximum calculated tensile stress is generated, and the bending strength of the joint is 113 MPa. It becomes clear that the calculated stress and the measured strength of the joint is nearly equal to the strength of the silicon carbide itself (280 MPa). Therefore, it can be concluded that the stress estimation in this method indicates a good approximation of the practical residual stress of the joint.