Fill Wire Laser Welding: Techniques & Applications

Figure 6-1 depicts the principle of laser welding with filler wire. During the process, the wire feeding mechanism drives the filler wire at a certain angle into the focal spot of the laser beam in the welding zone. Part of the wire is directly melted by the laser beam, while another portion is heated and melted by the plasma induced by the laser. The melted wire, along with the molten parent material from both sides of the groove, forms a weld pool.

Figure 6-1: Schematic of the filler wire laser welding process.

To protect the welding zone and suppress the plasma induced by light, a shielding gas is blown towards the laser beam, the wire interaction spot, and the molten pool at a certain angle. Sometimes, the shielding gas delivery tube and the wire feeding tube are integrated, with the wire and gas output through a combined nozzle, as shown in Figure 6-2.

Considering the wire feeding relative to the position of the weld pool, there are two methods: leading wire feeding (or drag wire feeding) and trailing wire feeding (or insertion wire feeding), as shown in Figure 6-2. The leading wire feeding method is commonly used because of its high reliability and its guiding effect on the wire to the groove. The trailing wire feeding method results in finer ripples on the weld surface, providing a better appearance.

Figure 6-2: Two methods of wire feeding

a) Leading wire feeding method
b) Trailing wire feeding method

However, a slight decrease in wire feeding accuracy may result in the wire sticking to the cooling weld pool. Nonetheless, if a sensor is needed to detect the groove in front of the weld pool for real-time control of weld formation, only the trailing wire feeding method can be used. Details on groove detection and weld formation control will be discussed later.

The wire feed angle, i.e., the angle between the filler wire and the workpiece surface, is an essential parameter in filler wire laser welding. Typically, this angle ranges between 35° and 45°. Angles too small or too large (outside the range of 20° to 60°) are detrimental to the accuracy and stability of wire feeding.

Because at this time, the wire feeding nozzle can only be installed far from the focus of the light beam (to avoid interference from the workpiece or the light beam), resulting in a non-guiding segment of the wire protruding too long from the nozzle, which affects the directionality of the wire feeding. This is a crucial issue in filler wire laser welding.

Compared with non-filler wire laser welding, filler wire laser welding has the following main advantages:

1) It can reduce the precision requirements for the workpiece’s groove processing and assembly, improve the quality of weld formation (preventing undercut, sinkage, etc.), and broaden the application scope of laser welding.

Because conventional non-filler wire laser welding requires very high precision in workpiece processing and assembly, which is often difficult to achieve in actual industrial production, this greatly limits the application range of laser welding. However, the use of filler wire laser welding technology loosens the requirements for workpiece assembly precision and seam gap due to the addition of the wire. This has great practical value.

Lasers butt welding without filler wire, the gap between the two butt plates must not exceed 1/10 of the plate thickness, otherwise defects such as undercut, sinkage, and incomplete fusion on the side walls will occur. Even worse, the laser beam may leak through the gap between the plates, making it impossible to melt and connect the parent material, as shown in Figure 6-3.

Figure 6-3: Intermittent weld formation defects caused by gaps (non-filler wire laser welding)

Note: Laser power is 2000W.
a) Gap of 0.1mm
b) Gap of 0.5mm

Figure 6-4 shows the case of butting welding of 2mm thick aluminum alloy plates. If filler wire is not used, the butt gap must be zero to obtain good weld formation. However, when welding with a 1.6mm diameter filler wire, the maximum gap can reach 1mm. Except for a slight lack of excess height, the fusion through and both sides are well fused.

Figure 6-4: Formed butt joint with a gap completed by filler wire laser welding

2) Thick plate welding (multi-layer welding) can be achieved with lower laser power. Figure 6-5 shows the case of butting welding of a 15mm thick low-carbon steel plate.

If an I-shaped groove is opened for non-filler wire laser welding, using a 6kW laser power, the welding speed is as low as 0.2m/min, and the welding fusion depth is only 10mm, and both the fusion width and the heat-affected zone are wide (Figure 6-5a); if the workpiece is opened into a step-shaped groove and three-layer filler wire laser welding is used, the butt of the 15mm thick steel plate can be perfectly completed.

Moreover, both the weld and the heat-affected zone become narrower, and the ratio of plate thickness to fusion width reaches 7:1 (Figure 6-5b).

Figure 6-5: Thick plate multi-layer welding achieved by filler wire laser welding

a) Single-pass welding without filler wire: P=6kW, v=0.2m/min
b) Multi-layer welding with filler wire: P=6kW, v=0.7m/min, vf=4m/min

3) By choosing the filler wire material, the composition of the weld can be conveniently adjusted, improving the weldability of the material (such as suppressing cracks, pores, etc.), and improving the performance of the weld (such as strength, toughness, corrosion resistance, etc.).

For example, when welding some aluminum alloys with non-filler wire laser welding, there is a serious tendency for hot cracking. By adding filler wire to adjust the chemical composition and structure of the weld, cracking can be effectively prevented. Figure 6-6 is a typical example of this. In Figure 6-6, the parent material is AlMgSi0.7 aluminum alloy.

Figure 6-6: The role of filler wire in eliminating welding cracks

If non-filler wire laser welding is used, the tendency to crack is very severe, and the number of cracks in the weld sharply increases with the increase in welding speed. After adding AlMg4.5MnZr filler wire, due to the grain refining effect of Zr, the crack resistance of the weld is significantly improved, and no cracks are produced at all welding speeds.

In dissimilar metal laser welding, the filler wire is often used to adjust the composition and structure of the weld, prevent welding defects, and ensure good weld performance. For example, in the laser welding of 13CrMo44 ferritic low alloy steel and AISI347 austenitic stainless steel, if no filler wire is added, the weld is a dual-phase structure of martensite and austenite.

The formation of the martensite structure reduces the corrosion resistance of the weld. If EniCrMo-3 nickel-based wire with a diameter of 1.2mm is used for filler wire laser welding, the beneficial supplement of nickel alloy elements facilitates the formation of a single-phase austenitic weld structure.

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