Combustion Joining of Dissimilar and Refractory Materials

Combustion synthesis (CS) or self-propagating high temperature synthesis (SHS) is an energy saving, effective technique for production of a variety of advanced micron and nano-scale materials with properties that are superior to those manufactured by conventional methods. Recent developments and important breakthroughs within the CS field have shown its high level of diversity. In this work special attention has been paid to the gasless heterogeneous combustion joining (CJ) (rapid reactive welding) and topics related to it such as mechanical activation (MA) of reactive systems, kinetics study of high temperature reactions and CS of anti-oxidation (AO) coatings.

Joining dissimilar materials is of increasing interest for a wide range of military and industrial applications. For example, in combat fields, joining materials has received great attention in recent years due to weight reduction of both vehicle components and structures. In particular, layered ceramic/metal-alloy (i.e., SiC/5083 Al-alloy) composites show considerable potential for lightweight armor applications. However, ceramics and metal-alloys exhibit very different physical, mechanical and metallurgical properties, which make joining process problematic.

To solve this problem a rapid gasless reactive CJ technique is developed. A novel scaled-up CJ apparatus was designed and built. A reactive powder mixture (Ti+C) was used as a joining layer to bond SiC to 5083 Al-alloy. It showed that bonding of SiC to 5083 Al-alloy was achieved with formation of a transitional interface that was compatible with those components to be joined. Several joining schemes were utilized and both micron-sized (up to 10 μm) and submicron-sized (500 nm) joints were accomplished by CJ.

A number of gasless heterogeneous powder mixtures were utilized as a joining medium. It is critical to control reactivity and structure of such mixtures. In this work the influence of mechanical activation on the microstructures and ignition characteristics of such systems were investigated. The experimental approach is based on a high energy ball milling (HEBM) technique. Results show that short-term HEBM is an effective tool to tailor the microstructure and ignition characteristics of reactive powder mixtures and the MA-systems can be further utilized for CJ of various materials. Another technique, known as electro-thermal analysis (ETA), which allows one to evaluate the kinetics of high temperature rapid reactions before and after MA processes (Ni-Al system), was also employed. It is shown that a short-term MA treatment decreased the apparent activation energy for reaction between Ni and Al from 38 ± 1 (kcal/mol) to 25 ± 3 (kcal/mol).

A decrease of ignition temperature of reactive mixtures was observed after HEBM. This feature allows initiation of joining process at a much lower temperature. Combustion joining of refractory materials (carbon/carbon, C/C, composites) was accomplished using mechanically activated systems. An important industrial application is to refurbish C/C composites by bonding a new C/C part to an old C/C core to produce high-performance aircraft brakes to meet special high temperature requirements. Joining C/C composites is challenging since carbon does not lend itself to welding, and other approaches (e.g. mechanical or adhesives) would not hold up to the harsh operational conditions. A modified low-temperature CJ technique, which used a mixture of titanium and mechanically activated Ni/Al powders as a reactive medium, was developed. A robust crack and pore-free joint layer (~75−100 μm in thickness), composed of NiAl_x and TiC_y(O_z) phases, was produced by this technique.A high-temperature treatment of materials during CJ in air may result in their oxidation. In this work it was demonstrated that combustion synthesis can be used to produce refractory anti-oxidation (AO) coatings. An improved electrically induced liquid infiltration (EILI) method was developed to produce multi-functional AO coatings in one-step on the surface of C/C composites. A significant decrease of C/C weight loss in both thermal and catalytic oxidation tests was observed.To summarize, combustion synthesis of gasless heterogeneous systems can be utilized in the field of materials joining and applied further to associated area of interests such as mechanical activation of reactive powders, kinetics study of high temperature reactions and development of anti-oxidation refractory coatings.