Friction Stir Welding Basics Explained

Case Study

Currently, MIG welding is the primary method for welding aluminum alloy car bodies domestically, which easily leads to defects such as cracks, pores, and slag inclusions during welding. The quality of the weld is greatly influenced by the operator’s skill level. The advent of friction stir welding offers a new solution for aluminum alloy welding.

Research results show that using friction stir welding to weld a 6082-T6 aluminum alloy results in a tensile strength 10% higher than MIG welding, a yield strength 20% higher than MIG welding, an increase in micro-hardness by 21.7%, a reduction in joint softening zone width by 42.6%, and a fatigue strength 50% higher than MIG welding.

Moreover, the welding process is pollution-free, without arc radiation, can be automated, has low energy consumption, and can easily guarantee welding quality. Therefore, friction stir welding technology has broad application prospects.

Traditional Friction Welding Methods and Principles

Friction Welding (FRW) is a solid phase welding method that uses frictional heat generated from the relative movement between the contact ends of the welding parts, causing the ends to reach a thermoplastic state, then rapidly forging, and completing the welding.

Friction welding is valued by manufacturing industries for its high quality, efficiency, energy saving, and pollution-free advantages, especially new technologies developed in recent years such as friction stir welding and superplastic friction welding, which are being used more and more extensively in technology fields such as aerospace, nuclear energy, marine development and industrial sectors such as electricity, machinery, petrochemistry, and automobile manufacturing.

Classification of Friction Welding

There are many types of friction welding. The classification methods are usually two: one is based on the relative motion form of the welding parts; the second is based on the process characteristics of the welding process, as shown in Figure 4-1.

Figure 4-1 Classification of Friction Welding

(1) Classification by the relative motion form of the welding parts

1) Welding parts rotate around the axis. This includes continuous drive friction welding, inertia friction welding, hybrid rotary friction welding, and phase control friction welding, etc.

2) Welding parts do not move. This includes radial friction welding and friction stir welding.

3) Other forms of motion. This includes friction surfacing, linear friction welding, and orbital friction welding, etc.

(2) Classification by welding process characteristics

1) Based on interface temperature, it can be divided into high-temperature friction welding, low-temperature friction welding, superplastic friction welding, and gas-shielded friction welding.

2) Based on process measures, it can be divided into induction-heated friction welding, conductive-heated friction welding, and enclosed friction welding.

3) Based on composite motion methods, it can be divided into brazing layer friction welding, embedded friction welding, and third body friction welding.

4) Based on welding environment, it can be divided into space friction welding and underwater friction welding.

The so-called friction welding mainly refers to continuous drive friction welding, inertia friction welding, phase control friction welding, and orbital friction welding, collectively known as traditional friction welding. Their common feature is that they generate heat energy through relative friction motion between the two parts to be welded.

Whereas, friction stir welding, embedded friction welding, third body friction welding, and friction surfacing, generate heat through relative friction movement between the stir tool and the parts to be welded.

Through a close combination with related disciplines and high-tech technologies, friction welding methods have developed from traditional methods to the current 20+ kinds, expanding the application field of friction welding. The shape of the welded parts has expanded from a typical round cross-section to non-round cross-sections.

In actual production, continuous drive friction welding, phase control friction welding, and inertia friction welding are commonly used. Friction stir welding, which has been developed in recent years, has shown strong advantages in non-ferrous metal welding and has a broad application and development space.

Traditional Friction Welding Methods and Principles

(1) Continuous Drive Friction Welding

This is the most commonly used friction welding method. As shown in Figure 4-2, the welding part is continuously driven by the main shaft motor to rotate at a constant speed, and then axial force is applied to heat the parts through friction. When the predetermined friction time or axial shortening amount is reached, the connecting part of the weldment is in a high-temperature plastic state.

Figure 4-2 Schematic Diagram of Continuous Drive Friction Welding

At this point, the rotation of the weldment is immediately stopped, axial forging pressure is applied, and it is held under pressure for a period of time, firmly connecting both sides of the metal.

(2) Inertia Friction Welding

Also known as energy storage welding, its welding process is shown in Figure 4-3. Before welding, the flywheel, the main shaft system, and the welding part on the rotary chuck are accelerated to a predetermined speed, then the main shaft motor and flywheel are disengaged or powered off.

Figure 4-3 Process Diagram of Inertia Friction Welding

Simultaneously, another weldment moves forward, contacts, and applies axial pressure to start the heating process. During the heating process, the flywheel is braked by the friction torque, the speed decreases, and mechanical energy is generated, which is converted into heat energy through friction to heat the joint interface.

When the speed of the flywheel, the main shaft system, and the welding part on the rotating chuck is zero, the heating temperature and its distribution on the joint have also met the requirements. Finally, the welding process ends under the action of axial pressure.

In the continuous drive friction welding process, there is a clear heating stage before the start of forging, during which the speed of the weldment remains constant. If a strong specification with a large friction pressure and short friction time is adopted, its friction heating power will be output in a pulse form, with very high power peaks.

In inertia friction welding, the speed gradually decreases from the moment the two workpieces contact and friction begins, the heating and forging stages are mixed together, and finally the energy stored on the flywheel is suddenly input to the joint surface.

In actual production, the heating power can be changed by replacing the flywheel or combinations of different size flywheels, thereby changing the moment of inertia of the flywheel. The main shaft motor power required for inertia friction welding is small, saving electricity, and is suitable for welding large-section weldments and dissimilar metal joints.

(3) Phase Friction Welding

Phase friction welding is mainly used for workpieces that require relative positions, such as various square steels, car control rods, etc., requiring the edges of the weldments to align after welding, the direction is correct, or the phase meets the requirements.

In practical applications, there are mainly mechanical synchronous phase friction welding, pin-coupling friction welding, and synchronous drive friction welding.

1) Mechanical Synchronous Phase Friction Welding.

As shown in Figure 4-4, before welding, tighten the alignment cam 2, adjust the relative positions of the two weldments and clamp the workpiece, brake the stationary main shaft brake 3, open the alignment cam 2, and then start friction welding.

When friction welding ends, cut off the power and drive the main shaft brake 3. Before the main shaft is about to stop rotating, release the brake, allowing the main shaft to regain partial rotation ability. At this point, immediately tighten the alignment cam 2, phase-correct and forge the joint.

Figure 4-4 Schematic Diagram of Mechanical Synchronous Phase Friction Welding
  • 1, 8 – Hydraulic Cylinder
  • 2 – Alignment Cam
  • 3 – Brake
  • 4 – Drive Shaft
  • 5 – Chuck
  • 6 – Workpiece
  • 7 – Stationary Shaft
  • 9 – Motor
  • 10 – Conveyor Belt

2) Pin-coupling Friction Welding

As shown in Figure 4-5, pin-coupling friction welding is an additional set of phase-determining mechanisms on a continuous drive friction welding machine with a main shaft and a tailstock main shaft. The phase determination mechanism consists of a pin, pin hole, and control system.

The pin is located on the tailstock main shaft, the tailstock main shaft can rotate freely, and the brake fixes it during the heating process. When the heating process ends, the main shaft is braked. When the computer detects that the main shaft enters the last turn, it gives a signal, causing the pin to enter the pinhole.

At the same time, release the brake of the tailstock main shaft, allowing the tailstock main shaft to rotate with the main shaft. This not only ensures the phase but also prevents impact when the pin enters the pinhole.

Figure 4-5 Schematic Diagram of Pin-coupling Friction Welding
  • 1 – Main Motor
  • 2 – Clutch
  • 3, 7 – Brake
  • 4 – Proximity Switch
  • 5 – Preset Computer
  • 6 – Loop Program
  • 8 – Main Shaft
  • 9 – Workpiece
  • 10 – Tailstock Main Shaft
  • 11 – Pin Hole
  • 12 – Pin

(4) Radial Friction Welding

Radial friction welding is shown in Figure 4-6. Before welding, insert the core rod into the two weld pipes with bevels, and then put on a machined ring with a bevel. During welding, the ring rotates and applies radial friction pressure to the two pipe ends. When the friction heating process ends, the ring stops rotating, applying forging pressure to the ring, firmly welding it to the two pipe ends.

Since the connected pipe itself does not rotate, no burrs are generated inside the pipe, and the entire welding process takes about 10s. Radial pressure friction welding is suitable for field assembly welding of pipes.

Figure 4-6 Schematic Diagram of Radial Friction Welding
  • 1 – Rotating Ring
  • 2 – Pipe to be Welded
  • n – Ring Speed
  • F0 – Axial Forging Pressure
  • F – Radial Pressure

(5) Friction Surfacing

As shown in Figure 4-7, during friction surfacing, a metal rod (1) rotates at high speed (n1), and the workpiece (parent material) also rotates at speed (n2). Under the action of pressure (F), the rod and the parent material generate heat through friction.

Due to the large volume, good thermal conductivity, and fast cooling speed of the parent material to be surfaced, the surfacing metal transitions onto the parent material, forming a weld seam when the parent material rotates or moves relative to the metal rod.

Figure 4-7 Schematic Diagram of Friction Surfacing
  • 1 – Metal Rod
  • 2 – Parent Material
  • 3 – Weld Seam

(6) Linear Friction Welding

As depicted in Figure 4-8, during the linear friction welding process, one workpiece is fixed, while the other one moves back and forth at a certain speed, or both workpieces perform relative reciprocating movements. Heat is generated through friction at the interface of the two workpieces under the action of pressure (F), thus accomplishing the welding.

The main advantage of this method is that welding can be performed regardless of whether the structure of the workpiece is symmetrical or not. In recent years, numerous studies have been conducted on linear friction welding, primarily used for welding turbine disks and blades of aircraft engines, as well as for on-site welding of large plastic pipes.

Figure 4-8 Schematic Diagram of Linear Friction Welding

(7) Embedding Friction Welding

Embedding friction welding uses the principle of friction welding to embed a relatively harder material into a softer material, as shown in Figure 4-9. The friction heat produced by the relative motion between the two workpieces causes local plastic deformation in the soft material, and the high-temperature plastic material flows into the recess of the pre-processed hard material.

A restraining shoulder forces the high-temperature plastic material to tightly wrap around the connection head of the hard material. Once the rotation stops and the workpiece cools down, a reliable joint is formed.

Figure 4-9 Schematic Diagram of Embedding Friction Welding

(8) Superplastic Friction Welding

Superplastic friction welding is a technique proposed by Soviet scholars in the late 1980s. Its core is to strictly control the friction welding process so that the welded metal is in a superplastic state. By leveraging the excellent properties of metal in a superplastic state, low-temperature, high-quality connections can be achieved.

(9) Third-Body Friction Welding

For hard-to-weld materials such as ceramics-to-ceramics, metal-to-ceramics, and thermoset plastics-to-thermoplastic composites, the third-body friction welding method can be employed to form high-strength joints, as depicted in Figure 4-10. The low-melting-point third substance, under axial pressure and torque, generates heat and plastic deformation in the gap between the bonding surfaces.

Relative friction movement can create sufficient cleaning effect, thus eliminating the need for flux and protective atmosphere. After cooling, the third material solidifies, locking the two components to form a reliable joint. Generally, there is no deformation in the components on either side. Since the third material does not melt, some solidification issues associated with fusion welding are avoided.

This method can provide a large cross-sectional area of the third body to withstand the required axial and torsional loads, with the joint strength potentially exceeding the tensile strength of the component material.

Figure 4-10 Schematic Diagram of Third-Body Friction Welding

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