Compressive strength and microstructural characterization of porous nickel brazed to copper and stainless steel / Ramizah Rozaimay

Ramizah , Rozaimay (2024) Compressive strength and microstructural characterization of porous nickel brazed to copper and stainless steel / Ramizah Rozaimay. Masters thesis, Universiti Malaya.

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Abstract

Heat exchangers are fascinating devices with an important role in many aspects of daily lives and widely used in heat supply systems. It is suggested to design heat exchangers with porous metals to enhance the heat efficiency in the application of heat exchangers. However, utilizing porous metal, is challenging job, since it involves joining of base metals with porous metal. The way of fabrication is essential to ensure high thermal conduction at the interfaces. Since base metals and porous metals are dissimilar materials, an appropriate method must be adopted to create a tight bond for reducing the resistance at the contact surface. Thus, brazing has been regarded as an effective technique for joining dissimilar materials because it neither distorts base metal nor porous metal. In this study, brazing process of joining porous nickel to copper and stainless steel using copper-based brazing filler metal (VZ2250) was investigated. The brazing process parameters (brazing temperatures and times) as well as different brazing filler metals were assessed. A high vacuum furnace is used to braze the samples. The resultant joints were evaluated for compressive strengths (Instron Universal Testing machine) to determine the rigidity and extent of diffusion of VZ2250 brazing filler metal into the porous nickel. The characteristics of the joint interface were investigated to evaluate the performance of the brazed samples by using scanning electron microscope (SEM) and energy-dispersive X-Ray spectroscope (EDS). It was found that the compressive strength declined linearly with the increases in the brazing temperature from 680 °C to 740 °C. The maximum compressive strength of 27.64 MPa is achieved at the brazing condition of 680 °C for 15 minutes of brazing time. This means that the molten VZ2250 brazing filler metal has effectively and evenly diffused into the struts at the specified brazing condition. SEM images revealed microstructure flaws such as cracks and voids started to appear with the increasing brazing time. EDS analysis have shown that nickel and phosphorus elements are able to diffuse at brazing temperature of 680 °C for 15 minutes. These elements contribute to high compressive strength of porous nickel after brazing. Therefore, from the investigated parameters, including the analysis from the structure of porous nickel, brazed joint strength and the microstructure; it can concluded that using VZ2250 brazing filler metal at 680 °C for 15 minutes is the most suitable parameter for brazing porous nickel to copper and stainless steel.

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