Impact of Welding Parameters on Weld Seam Formation and Process Stability

Impact of Welding Parameters on Welding Process and Formation – Two Welding Modes and Three Welding Processes

Impact of Focus Position

The focus position (or focal length) △f refers to the distance between the laser beam focus and the workpiece surface. When the focus is on the workpiece surface, △f = 0 is defined; when the focus is below the workpiece surface (in-focus), it is positive (△f>0); when the focus is above the workpiece surface (out-of-focus), it is negative (△f<0), as shown in Figure 2-2.

The focus position affects the size of the laser spot on the workpiece surface, changing the irradiance on the workpiece surface and thereby affecting the depth of melting and weld formation. Therefore, the focus position is a critical process parameter in laser processing. Figure 2-3 shows the relationship curve between the weld penetration depth and width, and the focus position.

The curve reflects the following rules:

1) When the focus is at a certain position (around △f=0.5mm in Figure 2-3), the maximum penetration depth is achieved. Deviating from this range results in a decrease in penetration depth. This point is called the Optimal Focal Point Position (OFPP).

2) When the focus position is within the range of △f=-2.2mm (out-of-focus) to △f=3.0mm (in-focus), welding always follows the deep penetration welding mode, with relatively large penetration depth and width. This is a stable laser deep penetration welding process.

3) Increasing the in-focus (or out-of-focus) amount, when in-focus is △f=3.0~3.5mm and out-of-focus is △f=-2.2~-2.5mm, deep penetration welding and conduction welding modes appear randomly, and the penetration depth and width correspondingly jump between two levels, referred to as the mode unstable welding process. This is an unstable welding process that occurs under stable welding parameters.

4) Continuing to increase the in-focus (or out-of-focus) amount, when in-focus is △f>3.5mm and out-of-focus is △f<-2.5mm, welding always follows the conduction welding mode, and both penetration depth and width are small. This is a stable conduction deep penetration welding process.

Figure 2-2: Diagram of Focus Position
Figure 2-3: Relationship curve between weld penetration depth and width, and focus position

Note: 5mm thick low carbon steel, P=2000W, v=2.0m/min
P – Stable Deep Penetration Welding
H – Stable Conduction Welding
U – Mode Unstable Welding

As can be seen from Figure 2-3, changes in the focus position can cause changes in the welding mode. There are two critical points of mode transition: one is the critical point of transition from stable deep penetration welding to mode unstable welding (△f1, △f11), the other is the critical point of transition from mode unstable welding to stable conduction welding (△f2, △f21), and the two critical points are not coincident.

Theoretical proof of this will be provided in the next section. Therefore, the transition of the welding mode does not occur as previously understood – from stable conduction welding suddenly transitioning to stable deep penetration welding, or from stable deep penetration welding suddenly transitioning to stable conduction welding.

Instead, a transition zone must be passed through, which is the mode unstable welding zone represented by the dashed line range in Figure 2-3.

The physical phenomena and weld formation of the three welding processes have the following fundamental differences:

1) Stable laser deep penetration welding process: during welding, a uniform blue flame can be seen, and a sharp explosion sound can be heard, indicating that laser plasma is generated from beginning to end, and metal vapor is continuously sprayed out from the keyhole. Both penetration depth and width are large, and the weld is uniformly formed along its entire length, as shown in Figure 2-4.

Figure 2.4: Weld Formation of Stable Deep Penetration Welding

a) Weld surface
b) Weld cross-section
Note: P=1500W,
v=1.0m/min,
Δf=0.5mm

2) Stable Laser Conduction Welding Process: The welding process produces an orange flame accompanied by a slight “hissing” sound, indicating no photo-induced plasma is generated during the welding process. Both the penetration depth and width are very small, but the entire length of the weld is uniformly formed, as shown in Figure 2-5.

Figure 2-5: Short Weld Formation of the Shifted Conduction Welding

a) Weld Surface
b) Weld Cross-section
Note: P=1500W, v=1.0m/min, Δf=-3.5mm

3) Unstable Mode Welding Process: During welding, the blue plasma flame intermittently appears and disappears, and the sound of metal vapor ejection is sporadic. An orange flame and a slight “hissing” sound emerge at the instant the blue flame and sharp noise vanish, indicating that the entire welding process is an irregular random change between deep penetration welding mode and conduction welding mode.

Correspondingly, the weld penetration depth and width also fluctuate considerably, as shown in Figure 2-6.

Figure 2-6: Weld Formation of Unstable Mode Welding

a) Weld Surface
b) Weld Longitudinal Section (Partial)
Note: P=1500W, v=1.0m/min, Δf=-2.5mm

Impact of Laser Power

Figure 2-7 illustrates the curve showing the effect of laser power on weld penetration depth and width. As shown in Figure 2-7, with a fixed focal point and welding speed, the increase in laser power sequentially results in three entirely different welding processes: stable conduction welding, unstable mode welding, and stable deep penetration welding. Two critical points of welding mode transition exist here, namely, two power thresholds, Pe1 and Pe2.

When the laser power is less than Pe2, it is stable conduction welding, with uniform formation and very small penetration depth and width. When the laser power exceeds Pe1, it is stable deep penetration welding, with uniform weld formation, but significantly larger penetration depth and width than conduction welding, which increases with power.

Laser power between Pe1 and Pe2 results in unstable mode welding zone, with drastic fluctuations in penetration depth and width, hence, their sizes cannot be determined, represented with a dotted line in Figure 2-7.

The physical phenomena during these three welding processes and the situations encountered when changing the focal point are the same, and the weld formation characteristics of unstable mode welding are also the same as shown in Figure 2-6.

Figure 2-7 also shows that within the range of stable deep penetration welding, with the increase in laser power, both the weld penetration depth and width increase in a certain proportion. Many authors have measured a large amount of data on the quantitative relationship between penetration depth and laser power.

Figure 2-7: Curve Showing the Effect of Laser Power on Weld Penetration Depth and Width

Note: Low-carbon steel with a thickness of 5mm, v=1.5m/min, Δf=1.0mm.
P – Stable Deep Penetration Welding
H – Stable Conduction Welding
U – Unstable Mode Welding

Figure 2-8 shows a set of data groups that consolidates the test results of several scholars, indicating that the maximum welding penetration depth is directly proportional to the 0.7th power of laser power. Later, larger power welding tests believed that the maximum melt depth is proportional to the 0.8th power of laser power.

In fact, these data were obtained under the conditions of stable deep penetration welding. The relationship between penetration depth and laser power in the stable deep penetration welding zone in Figure 2-7 basically agrees with the data measured by the predecessors.

Figure 2-8: Curve of the Relationship between Maximum Melt Depth and Laser Power during Stable Deep Penetration Welding

Impact of Welding Speed

Figure 2-9 shows the relationship curve between welding speed and weld penetration depth and width when laser power and focal point remain unchanged. As seen in Figure 2-9, with fixed laser power and focal point, the increase in welding speed reduces the weld penetration depth and width, and sequentially results in stable deep penetration welding, unstable mode welding, and stable conduction welding.

Figure 2-9 Relationship curve between welding speed and weld penetration depth and width

The physical phenomena during the welding process and the characteristics of weld formation are the same as the results caused by the impact of focal point and laser power. This shows that welding speed is also an important welding parameter affecting the mode of laser welding.

However, because the impact of welding speed on the welding mode is not as significant as laser power and focal point, at higher laser radiation illuminance, very high welding speeds (ve1, ve2 respectively) are required to possibly result in unstable mode welding or stable conduction welding processes.

The author once raised the laser power to 1300W under the same focal point, and no unstable mode welding process occurred within the welding speed range of 0~8m/min.

To summarize, during high-power laser welding, at a certain moment in the welding process, depending on the presence of small holes, it can be divided into two modes: conduction welding and deep penetration welding. But when considering the entire welding process, due to the stability issue of the small holes, it can be a stable welding mode, or an unstable process where the two modes alternate.

Therefore, there are actually three processes: stable conduction welding, unstable mode welding, and stable deep penetration welding, with two mode transition critical points. Focal point, laser power, and welding speed are the three main welding parameters affecting weld formation and process stability. Changing one of these parameters may cause changes in weld formation and welding process.

If the welding parameters only change within the range of stable conduction welding or stable deep penetration welding, it will only cause changes in the quantity of weld penetration depth and width. If the change in welding parameters reaches the mode transition critical point, it will cause a jump in penetration depth and width, a qualitative change in the welding process, and large changes in process stability and weld formation quality.

Combined Impact of Multiple Parameters on Welding Mode

Combined Impact of Focal Point Position and Laser Power – Double-U Laser Welding Mode Transition Curve

Figure 2-10 shows the mode transition curve determined by focal point position and laser power under a fixed welding speed.As the curve is shaped like a double U, the transition conditions for the welding mode can be determined. The author refers to this as the double-U laser welding mode transition curve. As shown in Figure 2-10:

1) For a certain focal point position, we can find the minimum laser power Pe1 required to produce stable deep penetration welding and the maximum power Pe2 allowed for stable conduction welding. When the focal point is at the optimal focal point position OFPP (around △f=0.5mm in the figure), Pe1 and Pe2 are the smallest and are the peak of the double-U mode transition curve. Deviating from this position, both Pe1 and Pe2 increase.

2) For a certain laser power, there is a range of focal point positions that can achieve stable deep penetration welding. The higher the laser power, the larger the range of focal point positions for stable deep penetration welding; vice versa, the smaller the range.

When the laser power is less than Pe1, no matter what focal point position is chosen, it’s impossible to form stable deep penetration welding, and can only be unstable mode welding or stable conduction welding; when the laser power is less than Pe2, only stable conduction welding is possible.

Combined Impact of Focal Point Position and Welding Speed – Inverted Double-U Laser Welding Mode Transition Curve

Figure 2-11 shows the mode transition curve determined by focal point position and welding speed under fixed laser power, referred to as the inverted double-U laser welding mode transition curve. As shown in Figure 2-11:

Figure 2-10: Mode Transition Curve with Fixed Focal Point and Laser Power under Certain Welding Speed

Note: Low-carbon steel with a thickness of 2mm, v=2.0m/min
P – Stable Deep Penetration Welding
H – Stable Conduction Welding
U – Unstable Mode Welding

Figure 2-11: Mode Transition Curve Determined by Focal Point Position and Welding Speed under Fixed Laser Power

Note: Low-carbon steel with a thickness of 2mm, P=1000W
P – Stable Deep Penetration Welding
H – Stable Conduction Welding
U – Unstable Mode Welding

1) For a certain focal point position, we can find the highest welding speed ve1 allowed for stable deep penetration welding and the lowest welding speed ve2 for stable conduction welding.

When the focal point is at the optimal focal point position OFPP (around Δf=0.5mm in the figure), the allowable welding speeds ve1 and ve2 are the largest, and they are the peak of the inverted double-U mode transition curve. Deviating from this position, both ve1 and ve2 decrease.

However, under the process conditions in Figure 2-11, the highest welding speed is 7 m/min, which has not yet reached the peak of the unstable mode welding zone (the peak of the inverted U-shaped curve).

2) For a certain welding speed, there is a range of focal point positions that can achieve stable deep penetration welding. The lower the welding speed, the larger the range of focal point positions for stable deep penetration welding; vice versa, the range is smaller.

When the welding speed is higher than ve1, no matter what focal point position is chosen, it’s impossible to form stable deep penetration welding, and can only be unstable mode welding or stable conduction welding.

In actual welding, the welding parameters should be chosen based on the required depth of fusion of the workpiece, ensuring that the working point is within the range of stable deep penetration welding or stable conduction welding, and absolutely avoiding falling into the unstable mode welding zone.

The double-U mode transition curve can be directly used to judge and predict the stability of laser welding under certain conditions, providing a reliable basis for correctly choosing welding parameters and achieving the required stable welding process.

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