Figure 1.39 shows linear friction welding (LFW). It was developed to use linear reciprocating motion for non‐round parts. One part is set in linear reciprocating motion and the other part, stationary, is forced against it. Again, friction at the interface generates heat to plasticize the material near the faying surfaces, and the applied force squeezes it out as flash (not shown). As an example, the amplitude of vibration is 1–3 mm, the frequency 25–125 Hz, and the axial force 150 kN. By reducing the amplitude of reciprocating motion at the end of the weld cycle to terminate the friction phase, instead of slowing down the frequency, the two parts can be accurately aligned with respect to each other. A well‐known application of linear FSW is the joining of aero‐engine compressor blades to compressor disks, to form blisks [29].
Figure 1.39 Linear friction welding in which a stationary member is forced against a reciprocating member to generate frictional heat to plasticize and remove the material and oxide films near the faying surfaces to cause bonding.
1.6.3 Explosion and Magnetic‐Pulse Welding
Figure 1.40 illustrates a solid‐state welding process in which a large force is suddenly generated to fast accelerate (e.g. to >500 m/s) one member (called flyer plate) of the workpiece to make it collide with the other member (called the base plate) at an angle (e.g. 15°). As the collision front moves forward fast, the materials near the interface, along with their surface oxide films, are expelled as a jet, allowing the fresh, oxide‐free materials to bond together firmly. At the interface between the two members, the material of the flyer plate moves forward very fast, but the material of the base plate is stationary. The mismatch in the forward moving speed causes a flow instability at the interface, resulting in a wavy interface. The wavy interface is clear if the two materials are similar in strength. However, it may not be as clear if one material is much harder than the other such as steel and Al.
Figure 1.40 Schematic illustration of solid‐state joining by making one member (flyer) of the workpiece fast collide with a stationary member (base), such as explosion welding or magnetic impulse welding.
In the explosion welding (EXW) process, the sudden force is caused by an explosive. In the magnetic‐pulse welding (MPW) process, on the other hand, an instant electromagnetic force is generated by a fast discharge of capacitors into a coil (e.g. 500 kA) to cause a high pulsed current (e.g. 500 kA and 15 kHz). The pulsed current produces a high‐density magnetic field, creating an eddy current in one member of the workpiece and a repulsive force to accelerate it to collide with the other member. The high collision velocity (e.g. 500 m/s) causes the two members to be welded together upon impact.
EXW or MPW have two main advantages:
1 Excellent welds can be made between different metals, with hardly any brittle intermetallic compounds.
2 The joint strength can be close to that of the base metal.
They have three main disadvantages:
1 Explosion welding is expensive and requires a special license.
2 Magnetic pulse welding equipment is expensive.
3 Limited to joint designs with overlapping between two members of the workpiece.
1.6.4 Diffusion Welding
Diffusion welding is a joining process in which the main mechanism for joint formation is solid‐state diffusion. It is often called diffusion bonding. Coalescence of the faying surfaces is accomplished by applying pressure at elevated temperature as illustrated in Figure 1.41 [1]. The faying surfaces need to be machined very smooth and clean. However, microscopically the faying surfaces are still rough and need to be brought into close contact under pressure at elevated temperature. The elevated temperature softens the materials, allowing close contact under pressure and increasing the diffusion coefficient.
Figure 1.41 Diffusion welding between an upper piece and a lower piece showing the microstructure in the vertical cross‐section near the faying surfaces [1].
Source: Welding Handbook, Volume 3, 1980, © American Welding Society.
There are two main advantages of diffusion welding:
1 High‐quality joints can be produced with microstructure and properties close to those of the base metal.
2 Dissimilar metals that are not weldable by fusion welding can be joined.
Disadvantages include the following:
1 High equipment cost and long joining time are often required.
2 Great care is required in preparing the faying surfaces, and the fit‐up of mating parts often need extra care.
Examples
Example 1.1 A 1100 Al (commercially pure Al) sheet was welded by GTAW with a 1100 Al filler wire, as shown in Figure E1.1. (a) What is wrong with the weld? (b) Were the filler metal droplets able to enter the weld pool properly? Why or why not? (c) Which polarity of GTAW was used for welding? (d) How can the problem be overcome?
Figure E1.1 GTAW process used to weld A 1100 Al sheet with a 1100 Al filler wire.
Answer:
1 (a) The weld is covered with heavy oxide films.
2 (b) No, the oxide films on the weld pool surface kept the filler metal droplets from entering the pool properly.
3 (c) DC electrode negative was used.
4 (d) Use AC of DC electrode positive to keep the weld pool surface clean.
Example
