A creative application of modern joining technology to P/M parts fabrication may well be the key to broadening the variety of industrial applications of P/M parts. Among candidate joining processes, sinter joining and diffusion bonding, which take place simultaneously with sintering process, are the technologies of great interest in recent years. In solid state diffusion bonding, it is necessary to apply substantial pressure during bonding. However, for the parts with complex shape, it is not easy to apply substantial pressure during bonding process. So it is useful to apply a pressureless diffusion bonding process with transient presence of liquid phase between base materials. In this study, the specimens with graphite coating, layer between base metals such as pure iron powder compacts and low carbon mild steel blocks were prepared, and the assemblies were heated up to above eutectic temperature rapidly and sinter-joined. At the sintering temperature, transient presence of Fe-C eutectic liquid occurs first, and later isothermal solidification procedure by carbon diffusion follows to complete the bonding. The whole procedure of transient liquid phase bonding (TLPB) between graphite layer and base metal is controlled by diffusion of carbon.
The sequence of TLPB can be described as follows
1) Formation of eutectic liquid
2) Isothermal Solidification
3) Homogenization
Four experimental variables were selected and the effect of each of them on the bonding strength were investigated. They are thickness of graphite coating layer, time for heating up, sintering temperature and sintering time.
As a result, it was found that joining strength was proportional to the actual bonded area below 200 microns of graphite layer. At about 200 microns of graphite layer, the max. bonding strength was obtained, and the fraction of actual bonded area against total bonding area was 85 percent.
Some recommended guide lines for an optimum condition for TLPB are as follows.
1) Thickness of graphite coating layer should be below 170 microns, above which parallism of joining surfaces becomes a problem
2) The specimens should be heated up to sintering temperature in 20 min. Longer heating time results in the loss of graphite due to diffusion and CH4 gas forming reaction etc.
3) Recommended sintering time is 1170℃
4) Recommended holding time at sintering temperature is 20-30 min.
