DOE OSTI2018
Reactive brazing is a novel technique in the joining industry that uses reactive materials to produce substantial amounts of heat locally to join components. In this project, we created mechanically processed dilute thermite foils (Redox Foils) that upon ignition selfpropagate to produce molten metal capable of joining dissimilar metal combinations. Due to the transient nature of the reaction, there is limited thermal damage to the base materials. Also, novel component combinations, that cannot be joined via tradition furnace brazing (such as magnesium alloys to steel), can be produced. Redox Foils are produced by mechanically processing and consolidating constituent powders. By adjusting the fabrication pathway and chemistry, the reaction products can be tailored to join specific material combinations. In addition to producing mechanically processed foils, we produce vapor processed foils with controlled microstructures to understand some of the mechanisms of the reaction. Over the course of the program, we worked to identify and enhance the properties of the Redox Foils so that they could produce strong braze bonds of dissimilar metals (steel, Al, and Mg alloys). In a first step, the reactivity of the powders, from which the foils were made, was enhanced by ball milling Al and metal oxide powders before consolidation. In a second step, Redox Foils were made by physical vapor deposition to identify what level of dilution is needed to prevent the formation of Cu vapor during the reaction of the Redox Foils. In a third step, we incorporated diluent powders within the milling process to improve homogeneity within the final foils. In addition, finite element model simulations of heat transfer were also combined with detailed microstructure studies to identify what level of homogeneity is needed to avoid the generation of Cu vapor. Lastly, we include Ag powders as a diluent to form a lower melting temperature braze. Unfortunately, none of these steps led to Redox Foils that could produce bonds with shear strengths greater than 10 MPa. The generation of oxygen vapor appears to be generating porous bond lines that are still relatively week. To circumvent this vapor generation, we investigated Ti:2B reactive scaffolds that provide a heated framework to enhance braze flow and minimize porosity. However, BM Ti:2B powders suffered ~8% mass loss due to surface adsorbed water resulting in porous and weak bonds. As an alternative solution, we developed Ti-C-Cu based reactive braze foils that rely on TiC formation reactions to melt the Cu braze material. In this system, the foils did not produce gas or vapor upon reaction but did form the desired TiC product with molten Cu. Unfortunately, the reaction temperatures and the duration of Cu melting were insufficient to produce effective bonds. The goal of achieving bond strengths of 20 MPa or higher was not achieved.