Stir Friction Welding Equipment Basics

Friction Stir Welding (FSW) equipment is mainly composed of a main body and auxiliary parts. The main body is divided into mechanical and electrical control parts, where the mechanical part mainly includes the bed, column, crossbeam, worktable, spindle head, and transmission system. The auxiliary parts mainly refer to the stir welding head, workpiece fixture, and heating system.

1. Stir Welding Head

The stir welding head is the key and core component of FSW, and it mainly consists of a shoulder and a stirring pin, as shown in Figure 4-25. The stir welding head generally needs to possess the following characteristics: thermal strength, wear resistance, creep resistance, impact resistance, material inertia, easy machinability, good friction effect, and reasonable heat conduction performance.

During the welding process, the stir welding head generates heat by friction with the material being welded, causing the material to thermoplastically deform. This process shatters and disperses the oxide layer on the joint surface, enabling the thermoplastic material to exhibit good plastic flow and transfer.

The stir welding head applies forging pressure to the welding area metal, causing the material under pressure to form a solid-phase joint. The structural design of the stir welding head is one of the core technologies of FSW.

Its shape determines the heating and formation of the plastic fluid; its size determines the seam size, welding speed, and tool strength; its material determines the friction heating rate, tool strength, working temperature, and the type of material being welded. Therefore, only by pairing the appropriate stir welding head with optimized welding parameters can a high-quality weld seam be achieved.

Figure 4-25: Stir Welding Head and Stirring Pin Types

a) Cylindrical smooth surface
b) Cylindrical threaded surface
c) Conical smooth surface
d) Conical threaded surface

(1) Shoulder

The primary function of the shoulder is to generate heat through friction, encapsulate the plastic zone of the metal as much as possible, create a sealed welding environment, and drive the plastic flow of the surrounding material to form a joint. The shoulder can take various forms, such as flat, concave, concentric ring grooves, or helical lines.

The surface of the shoulder, shaped as a circular groove or a helical line, ensures the thermoplastic material experiences an inward force. This helps in gathering the thermoplastic material under the end of the shoulder to the center of the shoulder’s end face, filling the cavity formed behind the stirring pin. It can also reduce the stress concentration inside the stir welding head during the welding process.

(2) Stir Pin

The function of the stir pin is to generate the heat required for welding through rotational friction and improve the flow path of the thermoplastic material, enhancing its performance.

Currently, stir pins come in various forms such as cylindrical smooth surface stir pins, cylindrical threaded stir pins, conical threaded stir pins, tri-groove conical threaded stir pins, eccentric circular stir pins, eccentric circular threaded stir pins, asymmetrical stir pins, and retractable stir pins.

Figure 4-26 shows two types of stir welding heads developed by the British Welding Institute: the Tri-flute and the Whorl.

Figure 4-26: Two Types of Typical Stir Welding Heads

a) Tri-Chair Conical Threaded Stir Welding Head b) Conical Threaded Stir Welding Head

The Tri-flute has three spiral grooves on the conical surface of the welding pin, reducing the volume of the stir pin, increasing the flow of the softened material, disrupting and dispersing the oxides attached to the surface of the workpiece, and expanding the thickness range of the welded material.

After friction stir welding, a keyhole remains at the end of the seam. To address this issue, the retractable stir welding head was invented, available in both automatic and manual retractable forms.

The retractable stir welding head can weld materials of different thicknesses and achieve connections between varying thickness plates by adjusting the length of the welding pin. It can also gradually retract the stir welding pin into the shoulder as welding nears completion, allowing the keyhole to heal, thus avoiding the formation of keyhole defects.

2. Friction Stir Welding Machine

So far, the manufacturing of friction stir welding machines has moved from the experimental research stage to the industrial application period. World-renowned welding equipment manufacturer ESAB has produced various types of friction stir welding machines. In 2001, ESAB manufactured a large gantry-style friction stir welding machine for the British Welding Institute (TWI).

This machine is equipped with a vacuum clamping workbench and can weld non-linear joints. It can weld aluminum plates with thicknesses from 1 to 25mm, the workspace is 8m (L) x 5m (W) x1m (H), with a maximum clamping force of 60kN and a maximum rotation speed of 5000r/min.

ESAB has also designed and manufactured a commercial friction stir welding machine that can weld a 16m long seam. This machine has passed the acceptance of the Norwegian Ship Classification Society and is in use. Based on this, ESAB has developed a smaller and lighter device based on CNC technology with five degrees of freedom.

This machine can reach a welding speed of 750mm/min when welding a 5mm thick 6000 series aluminum plate.

China has developed C-type, gantry, and cantilever series of friction stir welding equipment for welding different specification products. Figure 4-27 shows the friction stir welding equipment produced by China’s Friction Stir Welding Center (Beijing Safe Fast Technology Co., Ltd.) for welding aluminum alloy fin radiators, aluminum alloy hubs, and other types of aluminum alloy structures.

Figure 4-27: Friction Stir Welding Equipment

Table 4-17 lists the models and specifications of the series of friction stir welding equipment designed and manufactured by China’s Friction Stir Welding Center.

Table 4-17 Models and Specifications of Friction Stir Welding Equipment

Equipment ModelNameModel and Specifications [Worktable Travel (X Direction × Y Direction) × Maximum Welding Thickness]Notes
C-type EquipmentFSW-4CX-006                   500mm×300mm×6mmFor welding longitudinal and circular seams
FSW-4CX-012                         500mm×300mm×12mm
FSW-4CX-020                            500mm×300mm×20mm
Gantry TypeFixed Gantry Type (Short distance from the spindle end face to the worktable)FSW-3LM-M06                            1500mm×500mm×6mmFor welding longitudinal seams
FSW-3LM-M12                          1500mm×500mm×12mm
FSW-3LM-M20                           1500mm×500mm×20mm
FSW-3LM-L06                               2600mm×600mm×6mm
FSW-3LM-L12                            2600mm×600mm×12mm
FSW-3LM-L20                       2600mm×600mm×20mm
Fixed Gantry Type (Large distance from the spindle end face to the worktable)FSW-3LM-012                      1500mm×500mm×12mmFor welding longitudinal seams
FSW-3LM-020                              1500mm×500mm×20mm
Cantilever TypeCantilever External Longitudinal Seam Welding MachineFSW-2XBW-006                      Φ1200mm×1500mm×6mmFor welding both longitudinal and circular seams
FSW-2XBW-012                           Φ1200mm×1500mm×12mm
FSW-2XBW-020                                Φ1200mm×1500mm×20mm
Cantilever Internal Longitudinal Seam Welding MachineFSW-2XBN-006                 Diameter greater than 2000mm, length of 1500mm, wall thickness of 6mmFor welding large-diameter short simple longitudinal seams
FSW-2XBN-012                  Diameter greater than 2000mm, length of 3000mm, wall thickness of 12mmFor welding large-diameter long simple longitudinal seams
OtherVertical Longitudinal Seam Welding MachineFSW-2LS-006For welding long simple longitudinal seams
Floor-standing Circular Seam Welding MachineFSW-2LD-006For welding simple circular seams with a diameter no less than 1000mm
Horizontal Transverse Seam Welding MachineFSW-2HH-006                              Specifications to be determinedFor welding large-scale wall plates
Portable Welding MachineFSW-2BX-006                                Specifications to be determinedFor field operations

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