Laser Assisted Friction Stir Welding (LAFSW) is a new and improved Friction Stir Welding (FSW) technology. The development of this manufacturing technology has matured in the last 10 years. Since the welding heat energy is generated by the friction between the rotating tool and the joining workpiece during the friction stir welding process, that is, a large working load needs to be applied, the equipment structure of the conventional friction stir welding system is not only complicated but also manufactured. The cost is also more expensive. However, laser-assisted friction stir welding uses laser energy to heat the workpiece, and then uses a welding probe to perform agitation to engage the workpiece. Therefore, the laser assisted friction stir welding system can achieve the advantages of simple structure and low manufacturing cost.

This paper mainly describes the preliminary research results of laser assisted friction stir welding technology on AZ91D magnesium alloy sheet, and discusses the superiority of this manufacturing technology.

First, the traditional friction stir welding

Friction stir welding is a fairly new joint manufacturing technology that is currently attributable to The Welding Institute [1], which ESAB AB developed into a commercial manufacturing technology. This technology has been derived from many common friction welding methods, mainly used in the joint technology of aluminum and its alloys [2]. However, in recent years, friction stir welding has been widely used in the joining of other light metals and their alloys [3-7].

In the friction stir welding process, the workpiece to be joined is first clamped and fixed on a backing plate using a strong and powerful jig, and the rotary tool having a special contour shape is moved along the workpiece to be engaged. Since the material is plasticized by the action of pressure and frictional heat, the welding probe can be used to transfer the heat energy required for the joint to complete the joining of the workpiece.

When the thickness of the aluminum alloy weldment is 40 mm, it can be manufactured by unilateral friction stir welding. If the weldment thickness is 75 mm, it needs to be manufactured by bilateral friction stir welding [4, 6]. According to the research [3, 4, 6], when aluminum and magnesium alloys are manufactured by friction stir welding, they will have better welding performance than conventional fusion welding.

Friction stir welding can eliminate the crack defects generated during the welding process, and the alloying elements of the base metal will not be lost due to evaporation. In addition, the formation of the bead area is mainly due to the intense agitation of the probe, so that a fine crystal structure can be obtained and pore defects are hardly generated, thereby improving the mechanical properties of the weldment.

As mentioned below, friction stir welding has many advantages:

1. The entire welding process is quite simple and does not require the use of consumables such as welding rods or packing.

2. The edge of the joint does not need to be processed beforehand.

3. It is not necessary to remove the oxide on the surface of the workpiece before soldering.

4. The entire process can be automated and can be used in all positions.

5. For aluminum and magnesium alloy weldments, friction stir welding can achieve high joint strength performance requirements.

6. Friction stir welding can be used for alloy materials with high sensitivity to weld bead cracks.

Disadvantages of friction stir welding include the need for a strong grip to hold the workpiece to the welding table. In addition, the welding probe generates a high wear rate due to the need to apply a large workload to drive the rotating tool to move.

In the patent application filed by Midling et al. [8], an improved design for friction stir welding is used, which includes using a moving induction coil as a heat source in front of the rotary tool to provide a material under the welding probe. The heat needed. When the material is subjected to thermal energy, plasticization will occur. As for the welding probe, the main function of the welding probe is to control the plastic flow pattern of the preheating material and to destroy the oxide film existing on the surface of the workpiece.

However, there are still some shortcomings in the improved friction stir welding manufacturing. For example, it is not possible to accurately heat a specific position using an induction coil. All conductive materials, including the soldering probe and the clamping device, are heated by the thermal energy generated by the current in the induction coil. In addition, the induced current may traverse the soldering path to form a poor discharge spark. Of course, the method of heating by induction coil can only be applied to conductive materials. As for other non-metallic and non-conductive materials, this heating method cannot be applied.

Second, laser assisted friction stir welding

In order to overcome the lack of traditional friction stir welding, a laser-assisted friction stir welding system [9] was developed. This system combines milling machines for industrial use with a set of Nd : YAG laser systems. The main function of the laser energy is to preheat the workpiece (a local area in front of the rotating tool), which will cause plasticization of the workpiece in front of the rotating tool (as shown in Figure 1).

The workpiece is then joined as is conventional friction stir welding. Since the high temperature action of the laser energy will soften the workpiece located in front of the rotary tool, there is no need for a strong clamp to clamp the workpiece to be joined to the welding table. In addition, since only a small workload is required to drive the rotary tool to move, the wear of the welding probe can be reduced. The laser energy in this new manufacturing has another advantage: even in the high-speed welding state, it does not cause excessive wear of the rotating tool.

Combining a commercially available beam manipulator with a high-precision controller that determines the laser power assists the operator in accurately controlling the amount of energy delivered to the workpiece. In this way, the area where the workpiece is heated and the total amount of laser energy transmitted to the workpiece can be effectively controlled to prevent the welding probe and the clamping device from being heated by the thermal energy generated by the laser beam.

Figure 1 Schematic diagram of the principle of laser assisted friction stir welding

Third, experimental methods and equipment

The welding test system consists of a 700W multimode modal Nd:YAG laser system with a wavelength of 1064nm and a 3hp conventional vertical milling machine.

The experimental test piece was fixed on the milling machine table using four 23 mm bolts. The rotating tool is a high-speed steel cylinder with a diameter of 20mm. Below it is a probe with a diameter of 9 mm and a length of 4 mm. The main function of this probe is to weld the experimental test piece. The laser beam was transmitted to the experimental test piece using a step-angle optical fiber having a core of 800 μm and a length of 5 m. The laser beam is completely focused in front of the rotating tool into a spot of 1 cm diameter (as shown in Figure 1). The thermoelectric junction was attached to the surface of the test piece by spot welding to correct the weldment temperature. The laser power was set to 200 W, and the experimental test piece was heated to about 320 °C using this energy. As for the rotational speed of the rotary tool, it was 1700 rpm, and the welding speed was 5 cm/min.

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