Objective: To master the principles of selecting laser power and its impact on the quality of laser welding.
Read this article with the following questions:
- What is the weld forming factor? How to calculate?
- What is the basis for selecting laser output power during laser welding?
- What are the effect of decreasing laser output power on weld penetration and achievable welding speed?
1. Principle of double beam welding
Classification of laser welding
1) According to power density and weld formation characteristics:
- Heat conduction welding: power density (laser output power / focal spot area) ＜ 105W/cm2
- Deep penetration welding: power density > 106W/cm2
2) According to the laser output mode:
- Pulse laser welding
- Continuous laser welding
Fig. 1 Schematic diagram of pulsed laser welding and continuous laser welding
2. Selection of laser power P
The choice of P (laser power) is based on the thickness of the plate. P determines the power density of the spot and influences both the penetration depth and the welding speed.
Fig.2 Curve of power density and metal temperature with time
3. Effect of laser power on penetration
As illustrated in the figure, the greater the power (P/kw), the deeper the penetration, and vice versa.
Fig. 3 Relationship between laser penetration and power
4. Influence of laser power on welding speed
Supplementary knowledge points:
The form factor of a weld is defined in GB/T3375-1994 as the ratio of the weld width (B) to the calculated weld thickness (H) on the cross-section of a single weld (φ = B/H), as shown in Figure 4.
A small form factor results in a narrow and deep weld. However, the center of the weld tends to have higher concentrations of impurities and poor thermal crack resistance due to regional segregation. Therefore, the form factor should not be too small.
For example, the form factor of the weld should be greater than 1.3, meaning the width of the weld should be at least 1.3 times the calculated thickness of the weld.
Fig. 4 Laser power and speed determine the forming of weld
Fig. 5 Relationship between welding speed and laser power
5. Relationship between laser power, welding speed and penetration
Fig. 6 Changes of penetration and welding speed of low carbon steel under different power
The laser welding power determines the power density of the spot and controls the penetration depth and speed of welding. As the laser power increases, both the welding speed and penetration depth increase.