Joining Strategies for Open Porous Metallic Foams on Iron and Nickel Base Materials

Abstract

Capacitor discharge welding and laser beam welding/ -brazing are suitable methods for the joining of metal foams into a compound structure. Both methods are characterised by a minimum input of energy and, connected with this, a small joining zone. The capacitor discharge welding method has less impact onto the foam structure than i.e. cold pressure welding or friction welding. Both methods guarantee that the influence on the mechano-technological properties of the base material is slight and that only little distortion of the components occurs. [1] Excessive input of energy destroys, moreover, the porous structure of the foams; i.e. the foam starts sintering by diffusion-induced volume decrease and the high energy input causes liquefaction of the foam and drop development through the surface tension of the molten metal. For the application field of aluminum foam sandwich structures, the ultrasonic spot welding has been used success. [2] Figure 1(a) shows the scheme of the multi-layer cooling system consisting of thermal barrier coating with bond coat, an open porous metallic foam and the substrate with integrated ducts for the cooling air. The detail in Figure 1(b) shows a micrograph of the joining area of foam and solid material by laser welding. The joining area connects the foam with the substrate. The prevailing temperature level reaches there the temperature of the cooling gas of approximately 420°C. [3] The final goal is to determine an appropriate joining strategy to connect the foam structure to a solid material, that offers a low energy initiation into the foam structure to avoid temperature damages in the foam structure. Further the method should allow to connect a foam to foam structure, to combine the single coated foam plates in lateral direction. It will therefore be necessary to find an appropriate joining additive, that slightly penetrates into the foam structure. Experimental Setup Foam ProcessingA promising method to produce open porous metallic foams is the SlipReactionFoamSintering (SRFS)-process. [4] This process, that bases on powder metallurgy, generates a porous cell structure with the aid of a chemical reaction. In comparison to other production routes for metallic foams, the SRFS-method provides several advantages: it allows working at room temperature, needs lean machinery and results in foams with a great variety of densities, pore size and pore form distributions. Structural and compound materials as well as graded structures can be supplied.The base of the technique is a metal suspension. Fine metallic powders are mixed with a dispersant, solvent and concentrated phosphoric acid as binder (Fig. 2). The phosphoric acid and the solvent wet the metallic particles and two reactions take place: hydrogen evolves by the reaction between the acid and the metallic particles and foams the slip. Metal phosphate is formed and freezes the hydrogen bubbles in the slip. The big hydrogen bubbles are called primary pores and can reach up to 3.5 mm. A carbonyl iron powder is added for the production o...

Ultrasonic Spot Welding of Nickel Foam Sheet and Aluminum Solid Sheet

“…According to Chen, in some given range, the indentation size is controlled by plastic deformation, and changes only slightly with variations in the energy. Compared with fusion welding, USW is capable of joining foam metals with solid metals without excessively damaging the foam structure and inducing large deformation.…”

“…Jarvis joined nickel foam and Ti6Al4V sheet via brazing. However, these welding methods are unsuitable for producing light‐weight foam metal joints with small deformation and constant physicochemical properties produced an aluminum foam sandwich structure via ultrasonic welding, with negligible deformation, and the resulting joints exhibited excellent performance.…”

A Review on Welding Techniques of Metallic Foams

“…This indicates that the melting property is related to the thermal conductivity difference caused by the pore anisotropy. Longerich et al (2007), aimed to find the appropriate welding method by welding the metallic foam to a composite structure by capacitor discharge and laser method to be used in a cooling system in their study. Iron and nickel-based metallic foams were more difficult to weld by capacitor discharge welding which is a resistance welding method.…”

Brazing filler metals

“…For many joining methods, thin sections, which easily melt or undergo shape change can pose challenges. Brazing does not suffer from this limitation, and is frequently used to connect faces to honeycomb cores in sandwich panels (Figure 18) (honeycomb foils can have thicknesses of ∼100 µm [167]), and has been found to be effective for metallic foams [168,169]. In these investigations, it is frequently found that the brazed joint area, which is more massive than the thin honeycomb cores, is stronger than the core itself as the stresses developed in the joint are lower [169][170][171].…”