The use of modern thermal spraying techniques for filler metal application may offer new solutions for brazing complex metal components without fluxing agents, when the spraying processes are fitted to the requirements of the following brazing process. The necessary coating parameters always depend on the kind of base material, the geometry of the joint, the used filler metal, and the chosen heating process for brazing (heating in vacuum or protective atmosphere furnaces, inductive or flame heating etc.). Copper and nickel based alloys are typical filler metals for brazing complex components made of stainless steel (heat exchanger, fuel pipe systems, exhaust systems, catalytic devices etc.). Using these examples, the results of brazing experiments and technical aspects of thermally sprayed braze coatings compared with conventional filler metal application techniques are discussed.

The cold spray process is interesting for brazing technology due to its special properties: low oxidation of spray materials and rupture of oxide scales on the substrate and particles surfaces during deposition. In the present study, two brazing alloys AlSi 12 and AlSi 10 Cu 4 were deposited onto 6063 and 3003 aluminium alloys using the cold spray process. The influence of the spray parameters on the particle velocity was investigated by means of DPV 2000. The influence of spray materials and parameters on coating build up and on coating microstructure was investigated. Some of the AlSi 12 coated samples were heat-treated at 500 and 600 °C to investigate the effect of the rupture of oxide scales on the diffusion processes. Some of the AlSi 12 deposited samples were brazed under argon atmosphere using a flux or without any fluxes. The results show that the process gas temperature influenced the particle velocity and the deposition behaviour of the powders significantly. The AlSi 12 powder showed a much better deposition behaviour than the AlSi 10 Cu 4 powder. Due to the rupture of oxide scales, silicon in the brazing alloy coating could diffuse into the substrate. The brazed samples show a very good bond between the substrate and the brazing alloy.

Application of cold gas spraying for deposition of braze filler coatings is investigated. Different light weight alloy substrates, i.e. aluminum AA1050, AA3005, AA5754 and AA7022, magnesium AZ91 and titanium TiAl6V4, are used. Filler coating materials depend on the substrate melting temperature. So for aluminum alloys Al12Si and zinc based fillers, for AZ91 pure zinc and for Ti6Al4V different Cu-Ni blends are applied. CGS process parameters are varied with regard to process gas (nitrogen) temperature and pressure, powder feed rate and spray distance. Correlation to process characteristics and economical aspects are given. The usability of the produced filler coatings is shown by different optimized brazing/soldering processes. In case of aluminum braze joints a full metallographical investigation is carried out by optical and scanning electron microscopy as well as EDXS analyses. The gathered results are compared with those of conventional filler material addition, i.e. wire, roll plating and foil.

In principle, coatings made by brazing is a process variant of brazing which can be classified as high-temperature brazing. It is a thermal process which is carried out either without flux in a vacuum or under inert gas with tapes/slurries/pastes/powders with a liquidus temperature generally above 900 °C. In this process, no components are firmly connected to each other but brazeable materials are applied which, after the heat treatment process, produce a metallic, material-tight coating. The most commonly used filler matrix materials are nickel-based ones, cobalt, iron, copper-based ones or corresponding alloys. Hard materials are mixed in depending on the coating function. Carbides, silicides, borides, oxides, diamonds, CBn or hard material mixtures can be used as hard materials. Common industrial used hard materials are WC, CrC or NbC. Hard material proportions in the coating can be up to 80 Vol.%. Actual developments show contents up to 90 Vol.% and more. Depending on the application, layer matrix hardnesses are flexibly adjustable from 20-30 HRC to 62-65 HRC. The coating produced can therefore perform various functions. For this reason, they are also called functional coatings. For example, hard material particles introduced into the matrix can be firmly brazed onto the surface of the component and thus take on a wear protection or gripping function. Alternatively, worn components such as moulds or turbine blades can be recontoured by brazing suitable materials as tapes or slurries into the wear areas and then reworking them. The coatings are very dense and crack-free and are therefore also very suitable for corrosion protection, even at high temperatures. In contrast to deposit welding, the deposit-brazed coatings are relatively smooth and often do not need to be reworked or ground. The strength of high-temperature brazed hard material coatings can reach the strength of the base materials. This results in a highly stressable layered composite. 2D and 3D geometries can be coated both internally and externally. Coating thicknesses are usually ranging from 1.0 up to 4.0 mm. Minimum layer thicknesses of 0.05-0.1 mm up to 10 mm and more can be achieved. Recent developments also show the possibility of locally brazing on applied tapes or suspensions using laser energy without having to heat the entire component. By selecting appropriate morphologies of the starting powders for braze matrix materials and hard materials, the coating system can be specifically optimised and adapted for the respective application. In addition, work is being carried out on systems in which "signaling elements" are incorporated into the coating in order to record the condition of a surface during operation. For example, forces, wear or temperature. The lecture gives an overview of selected processes, materials and applications and an outlook on new developments.

In order to overcome the disadvantage of local carburizing of steel components in contact with light-weight graphite or carbon fiber reinforced ceramic racks alumina based thermal spray coatings are produced as diffusion barriers with improved life time compared to rapidly degrading alumina or boron nitride pastes. The powder flame sprayed coatings are also capable to prevent damage by excess filler material in high temperature brazing processes effectively. Besides graphite also C/C racks are coated with pure alumina, Al 2 O 3 -TiO 2 and Al 2 O 3 -Cr 2 O 3 . Conventional powder flame spraying is applied in order to provide a low-cost solution for realization of diffusion barriers. Coatings are characterized by means of optical microscopy and SEM with regard to the interface to the substrates and their porosity. Coated racks are used in field tests for case hardening of steel components. The life time of thermal spray coatings is compared to alumina and boron nitride based pastes. Comparative liquid metal corrosion tests are carried out with NiCr7Si4.5B3.1Fe3 filler at 1,050 °C.

The aim of the research project is to combine repair brazing with protective coating against hot-gas corrosion into a common integrated process. Both the braze-metal as well as the hot-gas corrosion protection coating is applied by means of thermal spraying. The material layout is to be realized as far as possible to the near net shape by using thermal spraying. The processes are to be performed in such a way that the brazing is integrated into the CVD diffusion annealing process as a transient liquid phase bonding (TLP bonding) process which, as a consequence, can then be eliminated as a separate processing step. The thermal spraying processes of atmospheric plasma spraying (APS), high velocity oxygen fuel spraying (HVOF) and cold gas spraying (CGS) are to be qualified for this purpose. Thus the project working hypothesis is to be able to transform thermal coating and joining processes into a common integrated hybrid process and, in doing so, obtain both high-quality and economic advantages. The importance of combining these processes lies in reducing the effort of grinding as well as economizing on the vacuum brazing, which is currently a separate process step, and consequently lowering the production costs.

SOFCs for mobile applications require short starting times and capability of withstanding several and severe cycles. For such applications metallic cassette type cells with low weight and thermal capacity are beneficial where the active cell part is set in interconnects consisting of two sheets of ferritic steel. These cells are stacked serially to get higher voltage and power. This approach needs interconnect sheets that are electrically insulated from each other to prevent electrical short circuit. The technology discussed here is to use brazed metals, as sealants, and ceramic layers, as electrical insulators, which are vacuum plasma sprayed on the cassette rims. For reliable insulating layers, a variety of deposits were developed, starting from cermet-spinel multilayers with various compositions and constituents, where reactive metals (such as Ti, Zr) were part of the coatings, to pure ceramic layers. The qualities and characteristics of these coatings were investigated which included electric insulation at room temperature and at 800 °C (SOFC operating temperature), wettability of different brazes towards these deposits, phase stability and peeling strength. The single steps of development, characteristics of the insulating layers for SOFCs as well as some challenges that have to be taken into account in the process are described.

This study investigates the use of cold gas spraying (CGS) for depositing braze filler coatings. In the experiments, pure Cu layers were sprayed onto Mg alloy substrates, which were then joined to AlSi steel by contact reaction brazing in a vacuum furnace. The bonding temperature influenced the dissolution of Cu as well as the eutectic reaction between the coating and substrate. The thickness of the brazed seam was found to be 300 μm although the initial thickness of the Cu layer was just 50 μm. The shear strength of the joint peaked at 37 MPa, corresponding to a brazing temperature of 530 °C. Intermetallic phases and interfacial defects of various types were responsible for the low strength of the joints.

Depending on the size and type defects of nickel-based alloy turbine blades two procedures are used mainly: cladding and high temperature brazing. The repair brazing of turbine blades is used to regenerate cracks and surface defects and is the focus of this work. In this contribution a two stage hybrid repair brazing process is presented which allows reducing the current process chain for repair brazing turbine blades. In the first stage of this process the filler metal (NiCrSi) then the hot gas corrosion protective coating (NiCoCrAlY) and finally the aluminium are applied in this order by atmospheric plasma spraying. In the second stage of this hybrid technology the applied coating system undergoes a heat treatment in which brazing and aluminising are combined. The temperature-time regime has an influence on the microstructure of the coating which is investigated in this work.

The conventional manufacturing process of the automotive brazed heat exchanger includes complex preparation processes before brazing: aluminum brazing filler alloy is pre-claded on both sides of a fin by an extrusion method, and holed aluminum tubes are coated on both sides with Zn for corrosion protection by a wire arc spraying process. The intent of this study is to simplify the preparation process by kinetic spraying using all of the components, including Al-12%Si (for the brazing filler metal), Zn (for corrosion protection), and KAlF4 (flux powder). Four kinds of blended powder, with and without flux, were evaluated. The bond properties and composition distribution at the braze joint area were evaluated by SEM and an electron probe micro analyzer (EPMA). It was necessary to control the Zn content so that the corrosion resistance and brazeability of the aluminum heat exchanger would not be affected. An optimal kinetic spray condition was obtained, in order to fabricate the heat exchanger in this study. It was observed that the joints of the brazed specimens on each side of the brazing part were sounder than those achieved brazed by the conventional methods. Further, the kinetic sprayed heat exchanger showed acceptable corrosion protection.

Traditional brazing alloys do not wet ceramics and are therefore unusable for metal-ceramic bonding. One way to overcome the problem is by depositing a metal layer onto the ceramic prior to brazing. The approach taken in this paper was to plasma spray copper onto different ceramics (Al2O3, AlN, SiAlON) and then assess the wettability of potential brazing alloys (AgCu, AgCuTi). Interface analysis showed that silver and titanium segregation occurs at the ceramic surface and that, conversely, sprayed copper diffuses into the brazed joint.

In this research project a hybrid technology is developed to repair turbine blades. This technology incorporates procedural and manufacturing aspects like raising the degree of automation or lowering the effort of machining and includes materials mechanisms (e.g. diffusion processes) as well. Taking into account these aspects it is possible to shorten the process chain for regenerating turbine blades. In this study the turbine blades of the high pressure turbine are considered and therefore nickel-based alloys are regarded. To repair or regenerate turbine blades the following methods are employed: welding and brazing and a subsequent aluminizing CVD-process. The focus in this work lies on the brazing method and the required filler-metal is applied together with the hot-gas corrosion protective coating by means of thermal spraying and represents the first stage of this hybrid technology. In the second stage of this hybrid technology the brazing process is integrated into the aluminizing CVD-process and a first effort is presented here.

Solid oxide fuel cells (SOFCs) are one of the options as auxiliary power units (APU) in transportation, e.g. in vehicles or in aircraft. In particular, metal supported SOFCs consisting of metallic frames and substrates coated with plasma sprayed functional layers have shown an excellent stability concerning redox cycling. In order to provide sufficient power, these single cells have to be assembled to stacks. To prevent short-circuiting the frame of each cell has to be electrically insulated from the neighbouring one. For that purpose a ceramic coating is applied on each metal frame by thermal spraying before it is brazed to other stack components. Such layers should at one hand show good wetting and adhesion to the silver based brazing materials. On the other hand it should maintain sufficient electrical resistance even at the fuel cell operating temperature. As the applied solder, which connects the cells and seals the gas manifold simultaneously, is an excellent electrical conductor, it is mandatory to prevent the brazing material from penetrating into the deposit. In this paper a description of the design and experiences with these plasma sprayed insulating layers is given.

Both bonding strength of coating to substrate in low pressure plasma spraying and the effect of reverse transferred arc treating before spraying are studied in this paper. It is difficult to obtain the bonding strength precisely in low pressure plasma spraying by standard testing methods, such as, ASTM Standard C633-79 and JIS H8666-80. Therefore, for the bonding strength test rather than using a conventional adhesive, we believe a vacuum brazing process using Ag-Cu-In-Ti active filler metal at 1023 K should be used. We have also confirmed the practicality of this step. By the above test method, it has been proven that the bonding strength of low pressure plasma sprayed coating is over 100 MPa. Also, that reverse transferred arc treating after blasting enhances the bonding strength of low pressure plasma sprayed coating. It is also believed that the projections formed on the substrate surface by reverse transferred arc treating are buried into the coating and perform the pile effect.

Pratt & Whitney's upper stage rocket engine development program, designated "RL60", has incorporated cold-sprayed copper to improve the design and function of this new engine. Combustion chamber designs contain two stainless steel manifolds connected by a series of copper tubes. The manifold where the hydrogen fuel exits is located near the injector face. The combustion gases from the injector would cause over-heating of this manifold. Thick copper application was needed to actively cool this manifold by conducting the cold temperatures from the hydrogen fuel inside the copper tubes. Plating copper greater than 0.050-inch thick resulted in poor adhesion following a subsequent braze cycle and required 2 weeks to plate. Cold sprayed copper was attempted which has surpassed plated copper in its ability to adhere through this braze cycle and can be applied in a few hours. In addition, the hazardous chemicals associated with copper plating have now been eliminated.

Heat exchangers play a vital role in ongoing efforts to conserve energy. Plate-type heat exchangers typically consist of two flat separated flow paths in which heat transfer enhancing matrices are inserted. The combined effects of small irregular hydraulic diameters along with elevated heat transfer areas results in highly-efficient heat transfer to the external fluid. This allows for very versatile and compact heat exchanger designs. Typical plate-type heat exchanger fabrication methods such as brazing are labour intensive and limit post-processing operations like welding. In this paper, a novel micro-heat exchanger fabrication method using recently patented technologies is presented. The approach uses thermal spray processes such as Pulsed Gas Dynamic Spraying (PGDS) as an alternative to brazing for the production of a pressure barrier and integration of flow headers. Mesh wafer surfaces sealed using PGDS

Carbon fibre reinforced carbon (CFC) composites have been more and more used in different industrial areas as high temperature materials. Some application examples are CFC components in modern furnaces for heat-treatment and brazing. Because CFC components can react with metallic materials when they contact each other, diffusion barrier coatings are essentially important for such CFC components. The aim of the project IGF 14.880 N “Thermally sprayed diffusion barrier coatings for CFC components in high temperature applications” is to develop diffusion barrier coatings by thermal spraying technology. In the project, different coating systems have been developed and investigated regarding the coating build-up, coating microstructure, bonding, thermal shock resistance and diffusion barrier function. The research results reveal that some developed coating systems are suitable for applications in furnaces. In the present paper, some research results of this project are reported.

Fusible coatings of Nickel-Chromium alloys with various amounts of Boron and Silicon commonly used for severe load applications. The coating is normally sprayed, then fused by heating to the point of liquation. The fusing process causes powder coalescence and increases density. At the same time, the high fusing temperatures creates a “brazed” bond which gives these coatings extremely high adhesive bond strengths. The improved bond strength is the result of the metallurgical bond as compared to the majority of thermal spray coatings which rely only on mechanical bonding mechanisms. The fusing operation is very sensitive, especially when a hand torch fuse is required. To circumvent these problems, a study was conducted to see if high density HVOF sprayed coatings might achieve fused quality by furnace heating to temperatures well below the liquation point. Various times and temperatures were surveyed. Bond strength tests of coatings sprayed to heavy thicknesses, hardness and impact tests, and metallography were used for evaluation. It was determined that heating as low as 1500° F for three hours could improve the properties of an as-sprayed HVOF coating to where it developed characteristics very similar to that of a fused coating.

The use of diamond-impregnated tools for the machining of any type of building material or natural stone is an established technology. Sintering and brazing are the key manufacturing technologies. The restricted flexibility of the geometry variation, the absence of the repair possibilities of damaged tool surfaces as well as difficulties in controlling the materials interfaces are some drawbacks. Therefore, manufacturing diamond tools through new technology concepts are required not only to avoid these restrictions mentioned but also to enhance the tools properties. In particular, thermal spraying technologies can act as a potential problem solver due to their special technological properties. Especially the new high-speed technologies such as the detonation-gun and high velocity oxygen fuel thermal spraying are promising approaches. Through these technologies, it is possible to control the kinetic energies of the diamond particles as well as their temperature during coating. In this report, a novel manufacturing technology of the diamond-impregnated tools based on these high-speed thermal spraying technologies will be reported. The layer properties will be evaluated with respect to the diamond quality after coating and with respect to the properties of the diamond composite coatings. Diamond spraying efficiency, diamond distribution in the layer as well as diamond-metal binder-interaction are analysed.

Superabrasive composite materials are typically used for grinding stone, minerals and concrete. Sintering and brazing are the key manufacturing technologies for grinding tools production. But restricted geometry-flexibility, absence of repair possibilities for damaged tool surfaces as well as difficulties in controlling materials interfaces are main weaknesses of these production processes. Thermal spraying offers the possibility to avoid these restrictions. In this research work a fabrication method based on the detonation flame spraying technology has been investigated to bond large superabrasive particles (150 – 600 µm, needed for grinding minerals and stones) in a metallic matrix. Layer morphology and bonding quality are evaluated with respect to superabrasive material, geometry, spraying- and powder-injection-parameters. Influences of process temperature and possibilities of thermal treatment of MMC-layers are analyzed.