In-depth Analysis: What Affects Weld Wire Melting?

Influence of the Relative Position of the Welding Wire and the Beam

The positioning of the welding wire in relation to the beam is a crucial parameter in filler wire laser welding. It impacts not only the melting speed of the wire but also the quality of the weld seam formation. Ideally, the beam focus should lie on the workpiece surface, and the wire should be inserted into the center of the beam spot. At this point, the weld seam formation is optimal, and the wire melt volume is at its maximum.

If the end of the wire deviates from the focus point, the melting volume decreases regardless of the deviation direction, either vertically (Z-axis) or horizontally (Y-axis, perpendicular to the welding direction).

Figure 6-8 illustrates the relationship between the relative position of the wire and the beam focus and the wire melting volume. As seen in Figure 6-8, when the end of the wire aligns with the beam focus vertically (△Z=0), under a determined laser radiation illuminance, the critical melting volume of the wire can reach 350~360mm3/s.

Figure 6-8 Relationship between the relative position of the wire and the beam focus and wire melting

ΔZ—Vertical defocusing amount at the end of the wire
ΔY—Horizontal defocusing amount at the end of the wire
Note: Laser power is 10kW, beam focus diameter is 1.0mm, wire diameter is 1.6mm.

If the wire deviates from the focus vertically by △Z=2mm or △Z=4mm, the critical melting volume of the wire decreases. Due to the expansion of the beam spot at this point, if the wire deviates horizontally by △Y=(-0.2~+0.2)mm, the critical wire melting volume remains unchanged.

Figure 6-9 shows the relationship between the horizontal offset of the wire and the beam focus and the wire melting volume for different wire diameters. As seen in Figure 6-9, as the wire diameter decreases, the critical melting volume of the wire decreases. For all wire diameters, when the horizontal offset exceeds half of the beam spot radius (0.25mm), the wire melting volume drops drastically.

Figure 6-9 Relationship between the horizontal offset of the wire and the beam focus and wire melting volume

ΔY—Horizontal defocusing amount at the end of the wire
Note: Laser power is 10kW, beam focus spot diameter is 1.0mm.

Thin wires are prone to bending, causing lateral swing and uneven wire melting, resulting in unstable formation. This issue is worth noting. A large offset between the wire insertion point in the welding direction (X-axis) and the laser beam focus center also affects the stability of wire melting and the quality of the weld seam formation.

Figure 6-10 shows the relationship between the deviation of the wire and the laser focus in the X-axis direction and the weld seam formation during aluminum alloy filler wire CO2 laser welding. In Figure 6-10, the angle between the wire and the laser beam axis is 35°. When the wire and the laser focus deviation is less than ±0.8mm, the wire melts evenly, and the weld seam formation is good.

When the wire deviation exceeds this range, either large droplets form from the melted wire (position 2), or the wire doesn’t fully melt or even interrupts melting (position 3). Both situations affect process stability and lead to poor weld seam formation quality.

Figure 6-10 Relationship between the offset of the wire and laser focus and weld seam formation

Influence of Wire Feed Speed

The wire feed speed significantly impacts the wire melting behavior and the wire’s absorption and reflection of the laser. Research shows that as the wire feed speed changes, the filler wire exhibits three different melting modes (Figure 6-11).

Mode 1: Unshielded melting by the laser. At lower wire feed speeds, the wire melts before it directly contacts the laser beam. The melted wire forms a large droplet hanging vertically below the end of the wire.

The laser beam is unobstructed by the wire and can directly radiate the material below, but once the droplet contacts the laser beam due to its movement, a blue metallic plasma is produced and escapes at an angle of 85°~90° to the laser beam, as shown in Figure 6-11a.

Mode 2: Partially shielded melting by the laser. As the wire feed speed increases, part of the laser beam is absorbed and reflected by the melting zone at the front end of the wire, with the normal of the melting zone forming an 85° angle with the laser beam.

Some beams deviate or pass through the wire (both melted and unmelted) and radiate downwards, as shown in Figure 6-11b. With the increase in wire feed speed, the energy required to melt the wire also increases.

Mode 3: Fully shielded melting by the laser. As the wire feed speed continues to increase, less and less laser passes through the wire. When the wire feed speed reaches a certain level, the laser beam is completely shielded by the wire. The wire continues to melt, but the part below it remains solid, forming a groove above it, and the melted metal flows out along the groove.

At this point, the laser beam no longer reaches the area below the wire, and therefore, the base material underneath the wire will not melt, as shown in Figure 6-11c.

Figure 6-11 Three melting modes of filler wire

Among the three melting modes mentioned above, the second mode is most beneficial for obtaining good weld seam formation in filler wire laser welding and is generally the preferred wire melting mode.

Many researchers have studied the relationship between wire reflection of the laser and wire feed speed. All research indicates that as the wire feed speed increases, wire reflection of the laser intensifies. Table 6-1 shows the relationship between wire feed speed and the wire’s reflection ratio of the laser at different laser power levels.

As seen in Table 6-1, at a certain laser power level, the higher the wire feed speed, the greater the power of the laser reflected by the wire, and the larger the reflection ratio (the ratio of reflected power to total laser power). Conversely, at a fixed wire feed speed, the higher the laser power, the smaller the reflection ratio.

For instance, when the incident laser power is 1kW, if the wire feed speed increases from 1m/min to 5m/min, the reflection ratio of the wire to the laser increases from 12% to 47%. When the laser power is 5kW, the corresponding reflection ratio increases from 4% to 9%.

Table 6-1 Relationship between wire feed speed and laser reflectivity at different laser powers

Wire feed speed (m/min)Incident laser power
3kW4kW5kW
Reflected power (kW)Reflection ratio (%)Reflected power (kW)Reflection ratio (%)Reflected power (kW)Reflection ratio (%)
10.35120.3590.20 4
30.75250.60 150.30 6
51.40 471.10 280.459
81.70 571.30 330.70 14
Note: The focal length of the used focusing lens is 150mm.

In fact, as the wire feed speed increases, the energy absorbed and reflected by the wire grows simultaneously. The only decrease is in the energy that passes through the wire. The research results are shown in Figure 6-12. As seen in Figure 6-12, when the wire feed speed is very low, i.e., the wire melting volume is minimal, the energy of the laser absorbed and reflected by the wire is minimal, and most of it passes through the wire.

As the wire feed speed increases, the wire melting volume increases, and both the energy absorbed and reflected by the wire increase. When the wire melting volume reaches the maximum shown in Figure 6-12, 40%~50% of the radiant beam energy is absorbed, and approximately 35% is reflected.

Figure 6-12: Ratio of energy absorbed and reflected by the welding wire

Influence of Laser Power

Table 6-1 shows that at the same wire feed speed, the higher the laser power, the smaller the reflection ratio of the wire to the laser.

This phenomenon can be easily explained based on the basic theory of the interaction between infrared lasers and metal materials: under the same conditions, the higher the laser power, the higher the temperature the metal reaches, the greater its resistivity, and therefore, the higher its absorption ratio and the smaller its reflection ratio. By the same logic, the absorption ratio of the melted liquid metal wire is higher than that of the solid wire.

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