What is a friction stir welder?

Friction stir welding machine

Friction Stir Welding (FSW), a solid-phase bonding technique, was invented by The Welding Institute (TWI) in 1991. Compared with traditional fusion welding,

FSW does not need to add welding wire, no protective gas, no pollution, no smoke, no radiation, welding joint residual stress is low, so it has a series of advantages

such as high welding efficiency, small welding deformation, low energy consumption, simple equipment, welding process safety and so on. After more than 20 years

of development, FSW has been widely used in aerospace, rail transit, ships and other fields.

The principle of friction stir welding is shown in Figure 1. The high-speed rotating stirring head is inserted into the workpiece to be welded, the rotating stirring

needle fricts with the material to be welded and plasticizes it, and the shoulder friction with the surface of the workpiece generates heat and is used to prevent the

material in the plastic state from spilling. In the welding process, the workpiece should be rigidly fixed on the back plate, and the stirring head side will rotate at high

speed along the seam of the workpiece and the workpiece

Rocket fuel storage tank friction stir welding machine

3.2 Robot friction stir welding equipment

Traditional gantry friction stir welding equipment can only complete the linear or planar two-dimensional welding requirements, can not meet the welding of

complex structural parts, industrial robots have high flexibility, can achieve complex trajectory movement, making the welding of complex structural parts possible.

Robot friction stir welding technology can improve the degree of welding automation and production efficiency, and its technical advantages and social economic

benefits are significant.

Robot friction stir welding equipment has been applied more and more in recent years because of its strong welding adaptability, easy to realize space all-position

welding, high degree of automation and high production efficiency.

Robot friction stir welding technology and equipment has become a hot spot in the international research and development of friction stir welding technology,

and has been widely concerned by welding engineers and industrial users at home and abroad.

At present, the robot friction stir welding technology needs to solve the following key technologies:

1) Heavy-duty industrial robot body design and manufacturing technology suitable for friction stir welding. The basic condition of robot friction stir welding is that

the carrying capacity of the robot body must be high, usually > 500kg, and high requirements are put forward for the working stability, repeated positioning accuracy,

spatial position and attitude planning of the robot under high load operation conditions.

2) The design and manufacture of the robot friction stir welding head is the key to the integration and function realization of the robot friction stir welding system.

The robot friction stir welding head needs to integrate complex measurement and control system, including: pressure measurement and control system, temperature

measurement and control system, weld tracking system, three-dimensional laser positioning system and welding process cooling and lubrication system;

At the same time, in order to improve the flexibility and accessibility of the robot movement, it is necessary to reduce the volume of the welding head as much as possible.

3) The robot friction stir welding control module is an important factor to ensure the stability of the welding process and an important guarantee for the reliability

of robot friction stir welding of complex curved surfaces. The robot friction stir welding process control sensor mainly includes three-dimensional positioning, pressure,

displacement, temperature, weld tracking, etc. The system is connected to the sensor through the communication module and bus, and the real-time data is collected

and analyzed by the system, and then the communication module is output to correct the welding trajectory, attitude and process parameters, and finally achieve

the purpose of accurate control.

4) By determining the standardization process of friction stir welding, organizing and effectively cooperating with subsystems such as path planning, calibration,

process simulation, process control and welding process module, realizing the integration of robot friction stir welding system, and developing robot software

packages suitable for friction stir welding.

5) The current industrial products mostly have the characteristics of small batch and multiple varieties, requiring the production line to have flexible manufacturing

capabilities. When the industrial robot is used for friction stir welding, the welding sequence and operation path must be planned, and interference detection and

instruction generation must be realized through off-line programming, and the subsequent processing must be synchronously adjusted according to the real-time

information of the pre-production process, so as to give full play to the advantages of the robot in the industrial efficient automated production.

3.3 Heavy-duty robot friction stir welding system

Guangdong Institute of Welding Technology (Guangdong Sino-Uzbekistan Research Institute) based on the 1t load KUKA industrial robot, integrated with independent

intellectual property rights of the heavy-duty robot friction stir welding system. Through the design and manufacture of the head, the development of the application

software system, the research on the stability of the welding process and the research on the welding process, the goals of product development and standard formulation

are realized. The developed friction stir welding system is used for the processing and production of the product, and the industrial application is successfully realized.

(1) Design and manufacture of the head design of the head. That is, the structural design of the spindle system. The system includes high-speed motorized spindle,

pressure sensor, laser tracker, cooling system, mixing head and connecting accessories such as tool holder and transfer plate. This is shown in Figure 12.

FIG. 12 Real spindle system

Note: ① mechanical sensor, ② high-speed motorized spindle, ③ laser tracker,

④ for the oil cooling system, ⑤ for the mixing head, ⑥ for the tool handle.

High-speed motorized spindle provides rotating friction power of mixing head, nominal maximum speed 6000 r/min, torque 55Nm; The pressure sensor is symmetrical

distribution of 120° circumference, which is used for real-time monitoring of welding pressure, with a measuring range of 0~20kN; The laser tracker is set at the front

end of the welding, which is used to scan and track the welding seam, and the detection accuracy is < 0.2mm; The cooling system ensures that the heat input in the

welding process is not too large, extending the service life of the mixing head and mixing tools; The mixing head is a friction stir welding tool, which is used for weld

forming and can be customized according to different welding requirements.

(2) Research on robot welding technology. After studying the stability of welding process, the related research work of robot welding process is carried out. It mainly

includes one-dimensional conventional FSW, one-dimensional static shoulder FSW, one-dimensional friction stir spot welding, two-dimensional FSW, three-dimensional

complex surface friction stir welding technology research.

The main research contents include: the determination and optimization of welding parameters, the observation of the macroscopic morphology and microstructure of

the joint, and the testing and analysis of mechanical properties.

The welding test of 4mm thick 1561 aluminum alloy was carried out with conventional stirring head. Speed 600~2200r/min, welding speed 200mm/min, fixed pressure

constant 7500N. The welding process is stable and the pressure vibration amplitude is < 8%. Well-formed welds were obtained, and the optimal joint tensile strength

reached 99% of the base material, as shown in Figure 13.

FIG. 13 Relation between tensile strength and speed of joint

The 4mm thick 2219-T6 aluminum alloy was tested by using the self-developed high speed static shoulder friction stir welding tool. Speed 2000~2600r/min,

welding speed 100mm/min. By observing the surface morphology of the weld and the macro morphology of the joint at different positions (see FIG. 14 and FIG. 15),

it can be seen that friction stir welding on the static shoulder significantly improves the surface quality of the weld.

FIG. 14 Weld surface topography

FIG. 15 Macroscopic morphology of joint

The friction stir spot welding test of (2+2) mm thick 6061-T6 aluminum alloy was carried out by constant pressure and constant position mode. When the rotation

speed is changed within 1000~1500r/min, good solder joint formation is obtained. The optimal joint tensile shear performance obtained by constant displacement

welding mode is 2400N, and the optimal joint tensile shear performance is increased to 4000N by constant pressure welding mode.

At the same time, the two-dimensional friction stir welding process is developed by using constant pressure and constant position mode. Two-dimensional angular

friction stir welding (see Figure 16) was successfully implemented on a 4mm thick 6061-T6 aluminum alloy plate, and it was found that the constant pressure mode

could significantly improve the welding stability.

a) constant pressure b) constant position

Figure 16 Constant pressure and constant position mode two-dimensional Angle friction stir welding

Furthermore, the robot friction stir welding system developed in this paper successfully realizes the high quality friction stir welding of complex surfaces.

The workpiece model and weld forming situation are shown in Figure 17.

Figure 17 Surface weld forming

After a lot of tests and research, the friction stir welding system developed has successfully processed and produced new energy vehicle battery tray, water-cooled

radiator and mobile phone shell, and has realized industrial application. Among them, for the battery tray, its comprehensive welding efficiency increased

from 65 PCS/8h of gantry friction stir welding equipment to 91 PCS/8h, an increase of about 40%, and there is room for further improvement.