During the handheld laser welding process, the laser beam is focused by the optical system to form a high energy density region on the surface of the workpiece or inside the material. The focus position determines the energy distribution per unit area, molten pool morphology, and weld seam formation quality, and is one of the key process parameters affecting welding stability and weld quality.
The focus position is usually referenced to the workpiece surface as the zero point and can be classified into positive focus, negative focus, and defocused states.
1. Welding Characteristics with the Focus on the Workpiece Surface (Zero Focus)
When the laser focus is located on the workpiece surface, the laser spot diameter is minimal and the energy density is at its maximum. The coupling efficiency between the laser and the material is high.
The welding characteristics are as follows:
Rapid molten pool initiation and stable welding process
Narrow weld seam with clear boundaries
Concentrated penetration depth and high energy utilization efficiency
Suitable for thin materials and precision welding applications
In handheld laser welding, maintaining the focus on the surface requires high operational stability. Variations in torch height during welding can easily cause fluctuations in penetration depth and weld morphology.
2. Welding Phenomena under Positive Focus Conditions
Positive focus refers to the condition where the laser focus is positioned below the workpiece surface, meaning the focal point is located inside the material.
The welding phenomena include:
Peak laser energy density occurring inside the material
Increased penetration depth and higher depth-to-width ratio
Pronounced keyhole effect with enhanced longitudinal weld penetration
Relatively constricted surface molten pool and a narrower weld appearance
Positive focus is suitable for medium to thick plate welding or applications requiring greater penetration. In handheld welding, positive focus improves process tolerance; however, excessive focus depth may lead to insufficient surface fusion or incomplete weld formation.
3. Welding Phenomena under Negative Focus Conditions
Negative focus refers to the condition where the laser focus is located above the workpiece surface, meaning the laser beam begins to diverge before reaching the material.
The welding phenomena include:
Enlarged spot diameter and reduced energy density
Increased weld width and reduced penetration depth
Weakened or eliminated keyhole effect
Improved surface smoothness of the weld seam
Under negative focus conditions, laser energy distribution is more uniform, making it suitable for thin plate lap welding and applications with high surface appearance requirements. In handheld laser welding, a moderate negative focus helps reduce spatter and suppress burn-through.
4. Welding Characteristics under Defocused Conditions
Defocusing refers to the condition where the focus is positioned far from the workpiece surface, regardless of whether it is in the positive or negative direction, resulting in a significant reduction in energy density.
The welding characteristics include:
Difficulty in forming or maintaining a stable molten pool
Insufficient penetration and a high risk of lack of fusion
Wide and shallow weld seams with loose formation
High sensitivity of the welding process to travel speed variations
Defocused conditions are generally unsuitable for stable welding and are mainly used in special low heat input or surface heating processes. In handheld laser welding, excessive defocusing significantly reduces welding consistency.
5. Comparison of Welding Effects under Different Focus States
Compared with the focus located on the workpiece surface:
Positive focus improves penetration and welding capability, suitable for applications requiring high structural strength
Negative focus increases weld width, improves surface appearance, and reduces the risk of excessive energy concentration
Defocusing lowers overall energy density and is unfavorable for stable welding
In practical applications, the focus position should be adjusted in combination with material type, plate thickness, welding speed, and laser power.
6. Process Significance of Focus Position in Handheld Laser Welding
Handheld laser welding places high demands on operational stability and process tolerance. Proper selection of the focus position plays a critical role in the following aspects:
Improving welding stability and reducing the impact of operator movement
Balancing penetration depth and weld seam formation
Reducing spatter and welding defect occurrence
Enhancing adaptability in complex welding conditions
Focus position adjustment is typically achieved through control of torch height or adjustment of the focusing system and is a core step in handheld laser welding process optimization.