Filler Wire Laser Welding: Selecting the Right Parameters

The main parameters of typical laser welding without filler wire are the focal point position, laser power, and welding speed, which have been detailed in Chapter 2 for their impact on weld formation. For filler wire laser welding, in addition to these parameters, the impact of the filler wire on weld formation must be considered.

The earlier sections have clarified the scope and control requirements for wire feeding position, wire feeding method, and wire feeding angle. This section focuses on the matching of wire feeding speed and welding speed under a certain laser power to achieve good weld formation, as well as the permissible range for both.

Matching of Wire Feeding Speed and Welding Speed

To achieve good weld formation, the volume of melted filler wire should be able to fill the groove gap and form a certain amount of reinforcement. Therefore, there is a relationship between wire feeding speed, groove gap, and welding speed as shown in Equation (6-1):

Where:

  • v1 is the wire feeding speed (m/min);
  • K is the formation coefficient, which can be taken as 1.1~1.3;
  • δ is the thickness of the workpiece (mm);
  • g is the groove gap (mm);
  • df is the diameter of the welding wire (mm);
  • vw is the welding speed (m/min).

As seen from Equation (6-1), the wire feeding speed (vf) and welding speed (vw) have a linear relationship, and they increase proportionally with the increase of the groove gap (Δg), as depicted in Figure 6-13. Each shaded fan-shaped area corresponds to a certain size of groove gap.

Since the formation coefficient is allowed to vary within a certain range, a single size groove gap (Δg) on the vf-vw plane is not a straight line but a fan-shaped area with a certain width on both sides of the centerline.

Figure 6-14 presents the matching relationship between wire feeding speed and welding speed when welding beads on an aluminum alloy plate. Because it’s not a butt joint of plates, there is no groove gap. However, to form a certain amount of reinforcement, a certain amount of filler wire is still needed, and the wire feeding speed must match the welding speed.

As the welding speed increases, the wire feeding speed also needs to increase correspondingly. The matching of wire feeding speed and welding speed cannot directly use the relationship in Equation (6-1); Figure 6-14 shows the results determined by experiment.

The shaded area in Figure 6-14 indicates the region where good weld formation can be obtained by matching wire feeding speed and welding speed with welding speeds between 20~140mm/s. Outside this region is the range where the parameters of the two do not match, resulting in poor formation. In the shaded area, in the upper left, the wire feeding speed is high, and the welding speed is too low, leading to excessive weld reinforcement.

In contrast, in the lower right of the shaded area, due to the small wire feeding speed and excessively high welding speed, the amount of wire deposition is insufficient, resulting in uneven reinforcement and irregular weld formation.

Figure 6-13: Schematic Diagram of Matching Wire Feeding Speed with Welding Speed and Groove Gap
Figure 6-14: Matching Relationship of Wire Feeding Speed and Welding Speed for Weld Bead Deposition on Plate

Note:

  • Base material: AIMgSi0.7;
  • Welding wire: AIM4.5MnZr,
  • d1=1.0mm

Equation (6-1) and Figure 6-13 illustrate the necessary conditions for achieving a good weld formation when the welding wire and base material can be properly melted, in terms of the relationship between wire feeding speed, groove gap, and welding speed.

However, to achieve proper melting, under a certain laser power, both welding speed and wire feeding speed are subject to certain allowable range limitations. If these limits are exceeded, even if wire feeding speed and welding speed match as per Equation (6-1), a good weld formation cannot be achieved.

Allowable Range of Wire Feeding Speed and Welding Speed

Allowable range of welding speed

Similar to laser welding without filler wire, for filler wire laser welding with a certain laser power and base material thickness, there are maximum allowable welding speed (vwmax) and minimum allowable welding speed (vwmin) constraints.

If the welding speed is too high (vw > vwmax), due to insufficient heat input, the plate will not be fully melted, or the root of the weld will not fuse. If the welding speed is too low (vw < vwmin), due to excessive heat input, the weld will sink or even burn through.

Figure 6-15 shows the welding parameter coordinates (1), (2), (3), and (4) for welding with different wire feeding speeds and welding speeds to achieve the same deposited metal height, and their corresponding weld cross-sectional profiles.

As can be seen from Figure 6-15, welds with a welding speed not exceeding 50cm/min all have excellent fusion and satisfactory formation quality. But when the welding speed reaches above 60cm/min, due to insufficient heat input, a non-fusion defect occurs at the weld root.

Figure 6-15: Cross-Sectional Morphology of Welds with Mismatched Wire Feeding Speed and Welding Speed

Note:

  • P=10kW, Δg=2.5mm,
  • dt=2.0mm; hD represents the height of the deposited metal.

Allowable range of wire feeding speed

For filler wire in filler wire laser welding, under certain laser power and wire diameter conditions, there are maximum allowable wire feeding speed (vfmax) and minimum allowable wire feeding speed (vfmin) constraints. If the wire feeding speed is too high (vf > vfmax), it will cause poor melting of the welding wire.

At the same time, due to the excessive wire feeding speed, both absorption and reflection of the laser by the wire significantly increase, thereby reducing the laser energy penetrating the wire and causing insufficient heating of the base material.

This can even lead to the third mode of wire melting as shown in Figure 6-11c, i.e., the laser is completely shielded by the wire and no longer transmitted to the area below the wire, and the base material below will not be melted.

All these will result in welding defects. If the wire feeding speed is too low (vf < vfmin), the wire will melt as shown in the first mode in Figure 6-11a, i.e., the melted wire transitions to the molten pool in the form of discontinuous droplets, causing irregular weld surface formation defects.

Figure 6-16 shows the changes in weld surface formation under different wire feeding speeds. The experimental material is low carbon steel.

Using the same base material thickness, laser power, and welding speed, the groove gap changes from 0.1mm to 0.5mm, and the wire feeding speed changes correspondingly from 1.15m/min to 3.43m/min according to the relationship in Equation (6-1), to meet the need of filling the groove gap with the melted wire volume and forming a certain reinforcement.

As can be seen from Figure 6-16, when the wire feeding speed is not more than 2.86m/min, the weld formation is good; if the wire feeding speed is as high as 3.43m/min or above, both the wire and the base material under the wire are not well melted, resulting in irregular weld formation.

Figure 6-17 is a schematic diagram of the matching and allowable range of wire feeding speed (vf) and welding speed (vw) for filler wire laser welding, considering the above two factors. The square area enclosed by the dashed line is the allowable range of vf and vw. Within this area, parameters are selected according to the matching relationship of v1 with vw and Δg.

Figure 6-16: Surface Formation of Welds at Different Wire Feeding Speeds
  • a) Δg=0.2mm, vf=1.15m/min
  • b) Δg=0.3mm, vf=1.8m/min
  • c) Δg=0.4mm, vf=2.3m/min
  • d) Δg=0.5mm, vf=2.86m/min
  • e) Δg=0.6mm, vf=3.43m/min Note: P=1.8kW, δ=2min, vw=1.0m/min, df=0.7mm.
Figure 6-17: Schematic Diagram of Parameter Matching and Permissible Range for Laser Welding with Filler Wire

Note: Groove gap: Δt1 > Δt2 > Δt3

Welding Parameter Adjustment with Groove Gap Variation

During the welding process, when the groove gap changes, the matching relationship expressed by formula (6-1) can be maintained by adjusting either wire feeding speed (vt) or welding speed (vw) to ensure good weld formation. However, it’s essential to ensure that the adjusted parameters remain within the permissible range. Otherwise, not only will good formation be impossible to maintain, but new formation defects may also occur.

Generally, if the wire feeding speed is already at a high level, when the groove gap increases, it’s not advisable to further increase the wire feeding speed. Instead, the welding speed should be reduced to increase the metal filling amount per unit length of the weld. If the welding speed is already high and the groove gap decreases, it’s not advisable to further increase the welding speed.

Instead, lower the wire feeding speed to reduce the metal filling amount. The adjustment process can be illustrated in Figure 6-18.

Figure 6-18: Adjustment of Welding Parameters during Groove Gap Variation

In Figure 6-18, suppose the current groove gap is Δg2, the working point is A2, with corresponding parameters vt0 and vw0, all within the permissible parameter range. If the groove gap increases from Δg2 to Δg1, the welding speed vw should be reduced to change the working point from A2 to A1. If instead, the wire feeding speed vt is increased, the working point changes to At2 and exceeds the permissible parameter range.

If the groove gap decreases from Δg2 to Δg3, the wire feeding speed vt should be reduced to change the working point from A2 to A3. If instead, the welding speed vw is increased, the working point changes to At3 and exceeds the permissible parameter range.

Considering dynamic response, adjusting the wire feeding speed responds faster than the welding speed. Therefore, adjusting the wire feeding speed should generally be considered first to adapt to changes in the groove gap. Only when the wire feeding speed is too fast and exceeds the permissible range should the welding speed be adjusted.

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