| Summary: |
Nanostructured reactive multilayered foils are materials that show potential for use in joining processes. These reactive foils consist of alternating nanoscale layers of materials with large negative heats of mixing. Application of a small thermal pulse initiates local atomic mixing, resulting in a rapid exothermic reaction that self-propagates. Although the reaction temperature can be quite high, the total heat evolved is small because the foil is thin. Recent interest in reactive foils arises from their potential use as a local heat source for joining temperature-sensitive materials.
The main goal of this project is to optimize the joining of titanium aluminides to nickel superalloys and steels, using what we call a nano-braze foil; an assemblage of Me/Al (Me = Ni or Pt) nanostructured reactive multilayered foils with brazes alloys (Ti-Cu-Ni, Ti-Ni and Ag-Cu based). This technique, that combines the advantages of diffusion brazing with low temperatures, presents a number of significant advantages compared to more conventional joining techniques: no external heat sources (except to activate the reaction) are required; joining can be performed in any atmosphere or under vacuum; and the temperatures of the joined components are never raised significantly. Therefore temperature sensitive materials and components with very different coefficients of thermal expansion can be joined.
The joining parameters depend on the physical properties of the materials to be joined and on the nano-braze foil characteristics. The Me/Al multilayer foils will be deposited using magnetron sputtering tailored to meet the requirements of the joining. The question to be answered is if the rapid formation reaction of the foil, which causes the temperature to increase and decrease rapidly, transfer enough heat at a sufficient rate into the braze alloy to enable melting. As the heat generated depends on the foil parameters, such as the layer period, the ratio of the Al to the Me laye  |
Summary
Nanostructured reactive multilayered foils are materials that show potential for use in joining processes. These reactive foils consist of alternating nanoscale layers of materials with large negative heats of mixing. Application of a small thermal pulse initiates local atomic mixing, resulting in a rapid exothermic reaction that self-propagates. Although the reaction temperature can be quite high, the total heat evolved is small because the foil is thin. Recent interest in reactive foils arises from their potential use as a local heat source for joining temperature-sensitive materials.
The main goal of this project is to optimize the joining of titanium aluminides to nickel superalloys and steels, using what we call a nano-braze foil; an assemblage of Me/Al (Me = Ni or Pt) nanostructured reactive multilayered foils with brazes alloys (Ti-Cu-Ni, Ti-Ni and Ag-Cu based). This technique, that combines the advantages of diffusion brazing with low temperatures, presents a number of significant advantages compared to more conventional joining techniques: no external heat sources (except to activate the reaction) are required; joining can be performed in any atmosphere or under vacuum; and the temperatures of the joined components are never raised significantly. Therefore temperature sensitive materials and components with very different coefficients of thermal expansion can be joined.
The joining parameters depend on the physical properties of the materials to be joined and on the nano-braze foil characteristics. The Me/Al multilayer foils will be deposited using magnetron sputtering tailored to meet the requirements of the joining. The question to be answered is if the rapid formation reaction of the foil, which causes the temperature to increase and decrease rapidly, transfer enough heat at a sufficient rate into the braze alloy to enable melting. As the heat generated depends on the foil parameters, such as the layer period, the ratio of the Al to the Me layer thickness, and the number of bilayers, these characteristics will be optimized. The individual layers of aluminium and nickel will be deposited on different substrates (braze alloy or components to join).
The pressure applied to the assemblage, the processing atmosphere and the temperature are the joining procedure parameters to optimize. Heating the foil up to self-ignition will be the method used to trigger reaction.
The joints obtained will be fully characterised. This characterisation aims at understanding the reactive diffusion processes and the formation of reaction products. The mechanical properties of the joints will also be evaluated.
The main objective of this work is to obtain reliable joints between advanced materials. The long experience of the research team in the domain of thin films production and diffusion bonding and brazing of TiAl alloys is a guarantee of success. |