1. Principle of friction welding
I. The Principle and Classification of Friction Welding
(1) The Principle of Friction Welding
Friction welding: It is a pressure welding method that uses the heat generated by mutual friction in the relative movement of the contact surfaces of welding parts to obtain a reliable material connection.
The welding process takes place under pressure, where the materials to be welded generate friction due to relative movement, causing the interface and nearby temperatures to rise and reach a thermoplastic state.
As the disturbing force comes into play, the interface oxide film is broken, the material undergoes plastic deformation and flow, and a joint is formed through metallurgical reactions of diffusion and recrystallization of the interface element.
The welding process does not add filler metal, requires no flux and does not use shielding gas. The entire welding process takes just a few seconds.
The high-speed relative friction under pressure between the joint surfaces of the two welds produces two effects:
1) Destroys the oxide film or other contamination layer on the joint surface, exposing the clean metal;
2) Generates heat, quickly forming a thermoplastic layer on the joint surface. Under the subsequent friction torque and axial pressure, these broken oxides and part of the plastic layer are extruded out of the joint surface to form burrs, and the metal deformed by the remaining plastic constitutes the weld metal. The final turning causes the weld metal to be forged further, forming a good quality welding joint.
From the welding process, it can be seen that the friction stir welding joint is formed below the melting point of the welded metal, therefore, friction welding belongs to the solid state welding method.
(2) Classification of friction welding:
Friction welding can be classified into three types: part rotation, stationary part and other movements. These three can be divided as follows:
1. Part rotation:
- Continuous Friction Welding
- Inertia Friction Welding
2. Stationary part:
- Radial Friction Welding
- Stir Friction Welding
3. Other movements:
- Friction surface
- Linear Friction Welding
- Orbital Friction Welding
2. Classification of Friction Welding
(1) Continuous Friction Welding
This is a common type of friction welding. During the welding process, the workpiece is continuously driven by the spindle motor to rotate at a constant speed until it reaches the specified friction time or amount of frictional deformation. The part then immediately stops rotating and forge welding begins.
- 1. Rotation
- 2. Braking
- 3a. Rotating device
- 3b. Non-rotating device
- 4a. Rotating part
- 4b. Non-rotating part
- 5. Workpiece Cylinder
(2) Inertia Friction Welding
The rotating end of the workpiece is fixed to the handwheel. At the beginning of the welding process, the flywheel and the rotating end of the part are accelerated to a certain speed, and then the flywheel is disengaged from the main motor.
At the same time, the moving end of the part moves forward. After the workpiece contacts, it begins to rub heat. During the heating process of friction welding, the flywheel is braked by the friction torque and the speed gradually decreases. When the speed reaches zero, the welding process ends.
- 1- Rotation
- 2- Adjustable inertial body
- 3a- Rotating clamp
- 3b- Non-rotating clamp
- 4a- Rotating part
- 4b- Non-rotating part
- 5- Part cylinder
(3) Radial friction welding
An annular ring with a beveled surface is mounted on the end surface of a tube with a split opening. During friction welding, the ring is rotated and radial friction forces are applied to both ends of the pipe. When friction ends, ring rotation stops and disturbance pressure is applied.
- 1 – Rotating Ring
- 2 – Pipe to be welded
- n – Ring Speed
- Powder – Forging Axial Pressure
- P – Radial Pressure
(4) Friction and Mixing Welding
The working principle of friction stir welding is as follows: A stirring needle of a certain shape made of high temperature resistant hard material is rotated and inserted deep into the edge of two materials to be welded.
The shaking head adjusts the rotation, generating a large amount of frictional heat at the edges of the two welds, thus forming a metal-plastic softening zone at the connection.
This plastic softening zone is stirred and compressed under the action of the stirring head, and flows backward along the weld seam with the rotation of the stirring head, forming a plastic metal flow. In the cooling process, after the stirring head comes out, it is compressed to form a solid phase welding joint.
(5) Friction Surface
The surface metal rod rotates at high speed and applies frictional pressure to the parent metal. Due to the large volume of the parent material, good heat conduction and fast cooling speed, the friction surface moves from the interface between the surface metal and the parent material to the surface metal side.
At the same time, the surface metal solidifies and transitions to the parent metal to form a surface weld. When the base material rotates or moves relative to the surface metal rod, a surface weld seam is formed in the parent material.
- 1- Surface metal rod
- 2- Surface Piece
- 3- Surface welding seam
(6) Linear Friction Welding
One of the two parts to be welded is fixed and the other moves back and forth at a certain speed, or the two parts move relative to each other. Under the action of pressure, the interface of the two pieces generates heat due to friction, thus achieving welding.
(7) Orbital Friction Welding
Orbital friction welding is a recently developed welding method mainly used for welding non-circular cross-section parts.
For straight-line orbital friction welding, the workpiece moves along a straight-line track, with a certain amplitude and frequency ensuring that the vibration speed reaches the required value, causing the welding surface to make relative repetitive vibrational friction .
For circular orbital friction welding, each mass point of the workpiece moves with the same radius and speed, moving along the circular orbit to make the welding surface make relative frictional movement. After the joint is heated to welding temperature, the frictional movement of the part is stopped and upsetting is performed.
