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Welding Techniques of Metallic Foams

Metallic foams are characterized as solid metallic materials that possess pores within their structure. These foams can be manufactured with porosity ranging from 20% to 97%, and their distinctive properties are contingent upon the nature and quantity of the pores present.

Metallic foams are categorized into three types based on their pore characteristics: open-pores, semi-open-pores, and closed-pores. Open-pores are interconnected with each other and with the surrounding environment. The presence of porosity is essential for allowing the passage of liquids or gases through the metallic foam, making it suitable for applications such as heat exchangers, filtration systems, separation processes, and catalyst utilization. Consequently, metallic foams with open-pores are preferred for these specific applications. On the other hand, closed-pores are isolated from one another and from the external environment. Metal foams with closed-pores possess absorption properties and can bear loads due to the strength provided by the pore walls. As a result, they find application in load-bearing components and energy absorption systems. Therefore, metallic foams serve a wide range of functional and structural purposes in various industries, including automotive, aerospace, biomedical, and construction sectors.

Nickel Foam (6)

Metal foams have garnered significant attention as an emerging class of metallic material, particularly in lightweight construction applications. These foams possess a unique combination of properties that intersect the characteristic features of both metals and foams. In addition to their lightweight nature, they exhibit exceptional absorption properties (e.g., sound, vibration, thermal) and possess high strength-to-weight ratios. Consequently, they have gained widespread usage in the automotive industry, particularly in the body of automobiles. The utilization of metallic foams in the automotive sector began in 1996 with Karmann Car’s research on the application of aluminum foam sandwich (AFS) structures in automotive body parts. Today, prominent automotive brands such as BMW, Audi, and Ferrari incorporate metallic foams in certain components. However, the commercialization of metallic foams faces challenges, with welding being a significant obstacle. Efficiently welding metallic foam to either foam or bulk materials has become a pressing need due to the expanding range of applications. Welding is a commonly preferred, cost-effective joining technique in industrial settings, offering practical solutions for joining metallic foams. However, the porous structure of metal foams poses difficulties in the welding process. Consequently, research on this issue remains limited. Given the lack of a single technique yielding clear success, the combination of metallic foams produced using different techniques has become necessary. Furthermore, the production techniques for porous parts often impose limitations on the size and shape of the final product, further complicating the welding process. Furthermore, the presence of impurities trapped within the pores of metallic foam can significantly hinder the weldability of the component. Porosity, in particular, plays a crucial role in the welding process of metallic foams due to its susceptibility to cracking within the heat-affected zone (HAZ) during welding. This can result in reduced ductility near the joints, especially in areas where porosity is limited, such as the interparticle bonding zone. Moreover, the foam structure is prone to collapse at elevated temperatures, necessitating the welding process to be conducted without compromising the integrity of the foam structure. The welding of metal foam is a highly intricate process that involves the consideration of numerous factors. One of the primary challenges lies in preserving the integrity and structure of the porous framework. Consequently, in order to achieve satisfactory weldability in metallic foams, it is imperative to comprehend the impact of porosity, chemical composition, and contamination level of the weld metal on various weld properties, including cracking, ductility, residual stresses, distortion, and toughness. Regrettably, the existing literature lacks sufficient studies on the welding of metallic foams. The majority of the conducted studies primarily focus on assessing the suitability of different welding methods.

Roll Type Ultra-Thin Foam Copper (6)

  1. MIG/TIG Welding

The Metal Inert Gas (MIG) and Tungsten Inert Gas (TIG) welding methods are classified as gas shielded arc welding techniques. These methods involve the creation of an arc between the electrode and the workpiece to perform welding. The welding process is safeguarded by an inert shielding gas (such as Ar or He) to protect the electrode, arc, and welding zone from the detrimental effects of the environment. In the MIG welding process, a metal electrode is melted and deposited onto the joint, and the materials are welded through the solidification of the droplets. The short-circuiting or dip metal transfer variants of MIG welding are suitable for use as they require low energy input, which reduces the heat-affected zone (HAZ) and minimizes distortion.

In the Tungsten Inert Gas (TIG) welding process, the electrode used is composed of non-melting tungsten. Additionally, an inert gas is employed to safeguard the welding zone. These techniques are commonly favored in industrial settings due to their ease of application and cost-effectiveness. However, when TIG welding foam metals, the inclusion of filler metal becomes necessary to prevent shrinkage and condensation resulting from the melting process within the weld area. It should be noted that these techniques are not suitable for highly porous metallic foams, but are appropriate for welding AFS parts together or joining AFS parts with aluminum dense sheets. The thermal degradation of AFS parts is minimal due to their high hardness and low thermal conductivity properties. The literature reveals successful instances of welding AFS parts together. The TIG welding process offers greater control, thereby enabling the production of desired outcomes in most cases.

  1. Diffusion welding

Diffusion welding is a solid-state welding technique employed to join two parts together. This process involves the application of heat and pressure for a sufficient duration to facilitate the diffusion and formation of a joint. The parts to be welded are brought into contact with each other either in the presence of an inert gas or within a liquid medium. A schematic representation of the diffusion welding process for metal foams is depicted. It is preferable to conduct the welding without the use of filler material, relying solely on the application of heat and pressure. The welding occurs through the diffusion of atoms at the contact surfaces over a period of time. However, it is important to note that the formation of reaction products, such as oxides, during diffusion welding can potentially diminish the strength of the bond. Conversely, the addition of suitable elements, such as Cu, to iron-based metallic foams can enhance the bond strength by activating the diffusion process.

  1. Ultrasonic welding

The ultrasonic welding technique involves the application of both normal and vibratory forces through the use of moderate pressure between the two components to be joined, along with oscillating motion at ultrasonic frequencies in the direction parallel to the contact surfaces. This combination results in the generation of shear stresses, which effectively eliminate surface films and promote atomic bonding between the surfaces, leading to successful welding. This method is particularly well-suited for welding metallic foams and metal sheets due to its relatively low welding temperature, time, and energy requirements. Ultrasonic metal welding systems can be classified into spot, seam, and torsion variations.

  1. Friction stir welding

Friction stir welding (FSW) is a solid-state welding technique that involves the use of a rotating mandrel-like tool to join two workpieces along a welding line. This method has gained recent attention in the welding of metallic foams and the production of foam panels due to its ability to provide high quality welds with low energy consumption. FSW is particularly suitable for welding aluminum foam panels because it is a solid-state welding process that minimizes heat input. This process promotes pore closure, resulting in a non-porous weld interface, and also facilitates microstructural refinement in aluminum foams. Additionally, FSW can break down any oxide layer that may form on the particles within the bond area through shear deformation, leading to improved bond strength. However, it is important to note that FSW can also introduce changes in the microstructure of the welded metallic foam, which may create potential weak zones and reduce the fatigue performance of the joint.

  1. Laser welding

Laser welding is a specific welding technique that utilizes laser energy as the primary energy source. Various types of lasers, including Nd:YAG, CO2, and diode lasers, can be employed for this purpose. Laser welding exhibits a significantly higher energy density, approximately 1011 W/m2, compared to conventional welding methods such as TIG (approximately 108 W/m2), as well as other high energy density welding techniques like plasma welding (approximately 1010 W/m2) and electron beam welding (approximately 1013 W/m2). This high energy density enables the welding process to occur rapidly, resulting in reduced deformation energy and minimized undesired deformations after welding.

One notable advantage of laser welding, particularly for metal foams, is its ability to provide localized and limited energy input, thereby minimizing the heat-affected zone. This characteristic makes it suitable for welding foam panels, as it allows for the utilization of an energy density that is appropriate for both the sheet and foam components. Additionally, the highly automated nature of laser welding offers advantages in terms of precision and control. By increasing the depth of the molten metal in the laser-applied area, the formation of bubbles in the weld region can be prevented, leading to improved welding strength.

Roll Type Ultra-Thin Foam Copper (3)

The advancements and future trends demonstrated by metallic foams exemplify the extensive array of research avenues for further advancement. Despite possessing unique properties, metallic foams have not yet fully realized their anticipated potential in terms of practical applications. One contributing factor to this limitation is the insufficiency of joining techniques. The joining of metallic foams has been associated with challenges related to inherent characteristics, such as porosity, contamination, and inclusions, which tend to impact the properties of a welded joint. The welding of metallic foams(nickel foam,copper foam, )is constrained by the ability of the heat-affected zone (HAZ) region to withstand stress. Consequently, it is recommended to employ low heat input techniques for welding porous metals, as this reduces the stresses generated in the HAZ region and minimizes the risk of shear adjacent to the weld, provided that these techniques are compatible with the composition of the metallic foam. The selection of a suitable welding method for metallic foams should consider several key factors, including low energy input, prevention of foam damage during welding, and control over the heat-affected zone. While traditional welding methods such as TIG and MIG were previously deemed suitable for welding metallic foams, recent studies have indicated that laser welding is more appropriate, leading to a significant focus on this method in current research.

 

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Lu

Our materials research team from Tsinghua University postdoctoral researcher lin and Harbin Institute of Technology researcher Mu, Nanjing University of Technology researcher Wei, they share their expertise in foam metal materials article.

About HGP

WE were established in 2003, located in the Gaoxin Zone of Guangdong-Guangxi Cooperation Special Experimental Zone, covering an area of 70 mu, with a plant of about 30,000 square meters, with more than 170 employees, is an advanced new material technology enterprise integrating research and development, production and sales.

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