Laser processing of metal foam

Metallic foams are categorized intoopen-cell and closed-cell foams. Closed-cell foams have membranes that separate neighboring cells or voids, whereas open-cell foams have a network of randomly distributed voids that are interconnected as shown in Figures (a) and (b), respectively. The categorization is somehow very important as it distinguishes metallic foams according to their applications. Closed-cell metal foams have high stiffness and are mainly used in structural applications. On the other hand, open-cell foams are mainly used in functional applications such as heat exchangers due to their large surface area and internally connected porous structure.

Therefore, processing methods are mainly responsible for customizing metal foamsto their applications by defiNing their anatomical structure or pore distribution. The processing methods are broadly categorized into liquid and solid processes. Liquid processing is the most popular in commercial production due to high foaming rates and high yields. The process creates pores in liquid or semi-solid metals by either of two main routes, direct and indirect foaming. Direct foaming can be accomplished by introducing a gas (air, nitrogen, or argon) or by introducing secondary substances such as TiH2, ZrH2, MgH2, CaCO3, etc. directly into the molten metal matrix. In both cases, bubbles are formed on the surface of the melt and they are trapped and cooled to create voids.

When the indirect foaming method is used, the molten metal is not directly foamed; instead, certain precursors such as polymer foams or inorganic particles and low-density hollow spheres are used to cast the molten metal into a foam. Liquid processing, while allowing for the mass production of metal foams in sheet form, lacks stability and controllability. As a result, near net shapes are difficult to produce. The diversity of materials that can be used to form foams by this method is also very limited.

Solid-state foaming can be defined as a process for manufacturing metal foams that keeps the processing temperature within the melting temperature range of the base material. Metal foams are prepared either by introducing the blowing agent directly into the mixed solid metal (intrinsic porosity) or by integrating the metal powder around a pre-treated temporary template with voids (extrinsic porosity). Although solid-state processes are more reliably controlled, they are costly. Current manufacturing industries are very critical in selecting products for manufacturability.

Metal foams are a new type of porous lightweight material consisting of pores or voids within a solid metal. The pores are usually created inside the base material in an uncontrolled manner, following different process routes to form a highly randomized porous metallic structure. The volume fraction of the resulting pores is very high compared to the base material, which makes the material lightweight. In addition to its light weight, the material offers a range of other outstanding structural properties: a high strength-to-weight ratio and excellent vibration damping and noise absorption potential. To incorporate metal foam in any industry, it must be closely integrated with a reliable manufacturing process. Foamed metals are characterized by high stiffness and low tensile strength and are prone to fracture under load. Therefore, conventional manufacturing processes are not convenient for processing the general shape of metal foam fabrication. As a result, the widespread adoption of metal foams is hampered. The laser processing method is a method that utilizes a precise heat source, a laser, to process the material instead of a mechanical load.