Applications of powder interlayers for large gap joining

Abstract

Large gap joints are frequently encountered in the manufacturing of massive components as well as in the repairing of damaged parts. Several methods using powder interlayers to produce large clearance transient liquid phase (TLP) joints have been developed and investigated in this work. One of the methods was using a mixture of powders of the melting point depressant (MPD) and the base material, from which a great amount of MPD (>30 vol.%) usually has to be used to eliminate residual porosity. To reduce the MPD in the joint, the liquid infiltrated powder interlayer bonding (LIPB) process was developed. For a material system that has a large mutual solubility between the liquid and the solid at the bonding temperature, a protective coating on the particles of the base material was applied to avoid excessive dissolution and inhibit early diffusional solidification, which can block the infiltration paths and prevent full infiltration. Direct coating of the MPD on the particles of the base material proved highly effective in producing tough and strong joints for certain material systems. The classic liquid phase sintering (LPS) theory was adopted to explain the physical process as occurring in the powder interlayer during joining. Despite the general applicability of the theory, there are several other important factors have to be considered as well. For example, the reaction rate between the MPD and the base material can markedly affect the densification of a mixed powder interlayer. Fast growth of the intermetallic compounds as a result of reaction can significantly retard the liquid flow. For the infiltration process, kinetics of dissolution and diffusional solidification largely depend on the mutual solubility between the liquid (infiltrant) and the solid (powder interlayer). Dissolution is needed to open up closed pores in the powder interlayer. However, excessive dissolution is undesirable due to fast liquid saturation and subsequent diffusional solidification, which may prevent complete infiltration of the interlayer. A protective coating on the particles of the base material provides a way of reducing the dissolution rate, which facilitates full infiltration of the interlayer. The solubility factor is also crucial for direct coating of the MPD on the particles of the base material. Higher solubility of the base material in the MPD is preferred to maintain enough liquid for complete densification. Experiments were performed on joining the materials including titanium alloy, Ti- 6A1-4V, nickel base superalloy, Inconel 625, stainless steel, SS304, and commercially pure copper. Application of the particular joining process depends on careful choice of the MPD, the base powders as well as the geometry of the interlayers.