Joining composite materials by means of reactive microparticles
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Reactive systems generate high temperatures within a few seconds through exothermic reactions. Researchers of the Technical University of Munich use the energy from the reaction of the so-called reactive microparticles to create joints in plastics, metals or ceramics.
Joining Different Types of Material
Resource-efficient lightweight structures are a result of the targeted use of plastics, fiber-reinforced plastics or metals. However, joining materials of the same type or of different types is a key challenge in production engineering, as the joint partners can only be exposed to thermal or mechanical stress to a limited extent. Diverging thermophysical properties in particular, such as different coefficients of thermal expansion, make it more difficult to use conventional methods and demand innovative and adaptable joining technologies. Reactive systems, which react in an exothermic combustion reaction,enable a specific amount of heat to be released into the joint zone within a short period of time.
Reactive systems release a specific amount of heat into the joint zone within a short period of time.
Reactive Systems as Heat Source
Reactive systems consist of at least two components, such as nickel and aluminum (Ni+ Al), titanium and boron (Ti + 2B), or platinum and aluminum (Pt + Al), that are capable of reacting with each other. In the field of combustion synthesis, reactive systems are used above all in the production of high-performance materials. Remarkably, sufficient thermal energy is released after the reactive system is activated to permit the reaction to continue self-sustainingly without additional energy. In academic literature, this is referred to as self-propagating high-temperature synthesis. What is crucial for this self-propagation is the stoichiometric ratio of the educts. Furthermore, the release of large amounts of energy leads to higher combustion temperatures, which in turn results in impurities being vaporized and thus in very pure alloys.
Generally, reactive systems can be divided up into multilayer systems and particles. Reactive multi-layer systems consist of alternating layers in the range of between 10 and 100 nm of at least two metallic components. The overall thickness of the reactive multilayer system varies between 20 and 100 μm. It is possible to further divide particles up into lamellar particles, particles with a core-shell structure, and homogeneous powder mixtures of reactive educts. It should be noted that reactive particles include lamellar and core-shell structures, and that each particle is reactive in its own right. In contrast to that, homogeneous powder mixtures are only reactive in their entirety. The intrinsic structure of a reactive system influences, for example, the rate of reaction, the activation and maximum combustion temperature, and can thus be used to generate a tailored heat source for the joining process.
Joining with Reactive Systems
Although several hundred reactive systems and possible combinations of educts exist, great importance is given to the system nickel and aluminum. Nickel aluminides are not only suitable for applications in aerospace, but also for performance lightweight materials, owing to their low density, high melting temperatures, relatively good strength values and resistance to oxidation and corrosion . Temperatures of up to 2083 K  are reached, depending on the stoichiometric ratio of nickel to aluminum, which underlines the added value of their use as a heat source in the joining process.
Depending on the intrinsic layer structure and the educts, the reaction can reach speeds of up to 100 m/s . The maximum combustion temperature accordingly occurs for a few milliseconds, which means that the joint partners are, firstly, only exposed to minimal ther-mal effects. Secondly, the energy that is briefly released in refractory metals is not sufficient to melt the base material. Prior to the joining process, the joining surfaces of the components being combined must therefore be coated with a soldering material that melts during the exothermic reaction to become part of the firmly bonded joint. Examples of successful joining using multilayer systems can be found in the field of semiconductors, bodywork construction and electromobility, with composite materials made from metals, plastics and ceramics .
Nevertheless, multilayer systems do also feature certain disadvantages. The complex production by means of vapor deposition is costly. Furthermore, the systems also show brittle material behavior and are therefore only suitable for flat or slightly curved joints. Moreover, it has not yet been possible to automate the process .
Reactive Microparticles as Heat Source
A more flexible approach with similarly customized heat sources is offered by joining with reactive particles. Compared to multilayer systems it offers the possibility of adjusting the released heat through the intrinsic structure of the individual particles, and of allowing the particles to be adapted to the shape of the surfaces being joined.
This is the reason why the Institute for Machine Tools and Industrial Management at the Technical University of Munich uses an innovative and holistic research approach to investigate reactive particles of nickel and aluminum with a lamellar and with a core-shell structure. Since the intrinsic structure of reactive particles plays a crucial role in the joining process, it is important to first produce reactive particles. Subsequently, the activation and reaction behavior of the reactive particles is analyzed, which forms the basis for joining different materials in order to promote industrial use.
Synthesizing Reactive Particles
Production using a planetary ball mill brings with it two central challenges. Grinding processes in an oxygen atmosphere lead to the formation of an oxide layer, in particular with aluminum. There are, for example, methods for grinding in a protective atmosphere which, however, require complex systems technology. Furthermore, it must also be ensured that the oxide layer is removed prior to the actual grinding in order to obtain the maximum possible system energy of the reactive particles. However, reactive systems are generally preserved even without a protective atmosphere and preliminary treatment of the particles.
Reactive particles can be used as heat sources in soldering, welding or bonding processes.
The second challenge is caused by the reactive nature of powder materials. Impact and collision in particular can result in unintentional activation of the reactive particles in the grinding bowl. In addition, grinding for too long can lead to partial or complete formation of the product. These fractions of the product that have already reacted are then no longer available for the reaction.
As with the synthesis of lamellar particles, the formation of an oxide layer constitutes a major challenge. This is why the oxide film is removed in an initial step and replaced with a thin layer of nickel. In a second step, nickel is deposited onto the pretreated aluminum particles either currentlessly or galvanically until the desired stoichiometric ratio is attained.
Activation and Reaction Behavior
It is crucial to understand the interaction between the intrinsic structure of the particles and the resultant activation and reaction behavior for the subsequent use of the reactive particles in the joining process. The lamellar structure and the concomitant enlarged contact surface between nickel and aluminum compared with the core-shell structure results in a different type of exothermic reaction. For this reason, the manufactured reactive particles are activated, and the temperature-time profile are analyzed.
The special aspect here is how the energy is input. In order to avoid temperature gradients, and since even compacted powder mixtures may contain air barriers, energy must be applied evenly through the whole structure. Moreover, fast reaction speeds prevent any influence on the reaction, as the increase in temperature occurs extremely rapidly. It is therefore not possible to subsequently input energy into the system by means of a conventional oven. Electromagnetic radiation, however, presents innovative possibilities, since it allows energy to be input into the system even after the reaction begins, thus enabling continued control over the reaction. A test stand was therefore set up to enable studies to be performed specifically in the maximum E or H field of the microwaves in order to minimize the energy required for activation. Furthermore, the specific geometry of the test chamber allows a more precise determination of where the two maximums are located.
Joining Using Reactive Particles
Reactive particles can generally be used as heat sources in soldering, welding or bonding processes. The activation of the reactive particles can be realized through a locally confined or volumetric energy input. As in the case of multilayer systems, reactive particles react in a self-sustaining combustion synthesis, provided there is sufficient contact between the particles. The reactive particles are currently applied to the surfaces being joined, and the reaction is activated in different ways. It has already been possible to demonstrate that reactive particles are suitable for joining applications with metals and plastics. Furthermore, additional investigations are currently being performed based on the findings from the synthesis routes and the activation and reaction behavior.
Adaptable joining technologies are needed in particular due to the demands for resource efficiency, the need to realize lightweight structures with mixed composite materials and the increasing diversity of variants. The use of reactive particles as an innovative heat source for joining metals, plastics and ceramics represents a promising approach in production engineering. More research is currently being conducted in order to promote the industrial use of reactive particles.
Activating Reactive Systems
Generally, it can be distinguished between the self-propagating or propagation mode  and the thermal explosion  . If the exothermic reaction is initiated by a locally confined energy input, for example using a swing hammer, a laser beam or an electric spark, the reaction front will self-propagate from the point of activation through the entire system until the educts have been fully converted into products. If, however, the reactive system is evenly heated in an oven or exposed to electromagnetic radiation, the volume combustion synthesis mode can be observed.
This research and development project is funded by the German Federal Ministry of Education and Research (BMBF) within the Framework Concept “Research for Tomorrow’s Production” (funding number 02P16Z010—02P16Z014) and managed by the Project Management Agency Karlsruhe (PTKA). The author is responsible for the contents of this publication.
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