magnetic pulse welding Archives

Solid State Welding

Introduction

Solid-State welding refers to a family of processes that produce welds without the requirement for molten metal. Solid-state welding theory emphasizes that the driving force for two pieces of metal to spontaneously weld (or form a metallic bond) to each other exists if the barriers (oxides, contaminants, and surface roughness) to welding can be eliminated. All solid-state welding processes are based on this concept, and use some combination of heat, pressure, and time to overcome the barriers. Approaches include friction, diffusion, explosion, and ultrasonic welding.

Since there is no melting, there is no chance of forming defects such as porosity or slag inclusions which are only associated with fusion welding processes. Solid-state welding processes also require no filler materials, and in some cases, can be quite effective at welding dissimilar metals that cannot be welded with conventional processes due to metallurgical incompatibilities. The equipment is typically very expensive, and some processes involve significant preparation time of the parts to be welded. Most of these processes are limited to certain joint designs, and some of them are not conducive to a production environment. Non Destructive Testing processes do not always work well with solid-state welding processes because of the difficulties of distinguishing a true metallurgical bond with these techniques.

In this section you’ll find examples of three types of solid state welding processes: Friction, High-Frequency for tubes and pipe and Magnetic Pulse. Generally these are characterized as follows. Click to the specific menu for details.

Friction Welding Processes

Figure 1: Basic steps of both inertia and CD friction welding.

This family of processes relies on significant plastic deformation or forging action to overcome the barriers to solid-state welding. Frictional heating dominates at the beginning of the process, followed by the heating due to plastic deformation once the forging action begins. The friction welding processes which use rotation of one part against another are inertia and continuous (or direct)-drive friction welding. These are the most common of the friction welding processes and are ideal for round bars or tubes. In both inertia and Continuous-Drive (CD) friction welding, one part is rotated at high speeds relative to the other part (Figure 1). They are then brought together under force creating frictional heating which softens (reduces the YS) the material at and near the joint to facilitate the forging action, which, in turn, produces further heating. Following a sufficient amount of time to properly heat the parts, a high upset force is applied which squeeze the softened hot metal out into the “flash”. Any contaminants are squeezed out as well as the weld is formed. The flash is usually removed immediately after welding while it is still hot.^{A-11, P-6}

High Frequency Tube/Pipe Welding

HF welding processes rely on the properties of HF electricity and thermal conduction, which determine the distribution of heat in the workpieces. HF contact welding and high-frequency induction welding are used to weld products made from coil, flat, or tubular stock with a constant joint symmetry throughout the length of the weld.

Magnetic Pulse Welding (MPW)

MPW is a solid-state process that uses electromagnetic pressure to accelerate one workpiece to produce an impact against another workpiece. The metallic bond created by this process is similar to the bond created by explosion welding. MPW, also known as electromagnetic pulse or magnetic impact welding, is highly regarded for the capability of joining dissimilar materials.

Joining Dissimilar Materials

Multi-material design approach involves using High-Strength Steels and low-density materials, such as aluminium. The key idea is to utilize the right material for the right application in such a way that it should fulfill the service requirements and achieve mass efficiency at the same time. However, it is challenging to fully utilize multi-material design concepts due to joining issues.

Joining is considered a backbone of any manufactured assembly, since the ultimate reliability and integrity of the manufactured product relies on these joints. Dissimilar materials are difficult to join due to the difference in the physical and thermal properties. For example, one of the most desired combinations is aluminium to steel joints, which is challenging due to the large mismatch in the mechanical properties as shown in Table 1. This Joining section is dedicated specifically to the key joining issues between aluminium and steel.

Table 1: Aluminium vs. Steel Material Mechanical Properties

Currently there are three approaches to joining dissimilar materials: solid-state joining, partial solid-state joining and arc joining. During solid state welding, the peak temperature of the process remains below the melting temperatures of the materials to be joined. Examples include diffusion bonding, ultrasonic welding, magnetic pulse welding, friction stir welding and vaporizing foil actuator welding. In partial solid-state welding processes, the peak temperature goes beyond the melting temperature of one of the materials to be joined (e.g. arc brazing & resistance spot welding). The last process (arc welding) has a peak temperature above the melting temperature of both materials to be joined. Practically it is not used anywhere however, it can be utilized in certain applications where the mismatch between the physical properties of the materials to be joined is little.

Among various joining processes, resistance spot welding (RSW) is one of the most widely used joining process in the automotive and aerospace industry due to its ease of automation and high productivity. Browse the topics below to learn more about joining dissimilar materials.