Approaches and considerations of Pure Laser Welding of Thick Sections

Laser welding of thick sections differs fundamentally from conventional arc-welding processes in both thermal behavior and defect-formation mechanisms. The process is characterized by significantly lower heat input per unit length and much higher energy density, producing a narrow heat-affected zone, reduced residual distortion, and diminished need for post-weld correction. However, the highly concentrated and dynamic laser–material interaction introduces additional physical challenges—particularly related to keyhole stability and molten-pool force balance—that must be precisely controlled to achieve defect-free welds in thick materials.

In this context, pure laser welding denotes root welding performed solely by a laser beam, with or without filler-wire addition. Successful implementation therefore requires careful optimization of component design, joint configuration, and process control. This article presents the key considerations and technological capabilities associated with pure laser welding of thick sections.

1.     Joint type and Design Considerations

A wide range of joint geometries traditionally associated with arc welding can also be realized using laser welding, while benefiting from the process’s deep penetration capability and localized thermal input. In square-groove butt joints, where no bevel is required, sufficient laser power and beam stability enable single-pass full penetration in thick materials. Demonstrations using high-power dynamic beam lasers have shown single-pass full penetration in approximately 35 mm-thick mild steel, including procedure qualification according to international standards. For even greater thickness, double-sided laser welding may be applied.

T-joints may be welded autogenously or with filler-wire addition, and from one or both sides. Autogenous laser welding minimizes heat input and distortion, whereas filler wire enables reinforcement formation and metallurgical tailoring. Overlap joints (lap joint in AWS terminology) are particularly compatible with laser processing because the beam can penetrate multiple stacked plates, achieving penetration depths exceeding 25 mm, depending on material and process conditions. Overlap welds may be designed for full penetration or partial penetration. Because the load-bearing capacity of overlap joints is governed primarily by the interface width, this region can be increased through multiple welding passes, increased spot size or by tailored beam-shape parameters.

Figure 1: Cross-sectional macrographs of representative laser-welded configurations: (a) 25 mm mild steel butt joint; (b) 45 mm mild steel double-sided butt weld; (c) 4 mm Al 5083 T-joint without filler wire; (d) 8 mm mild steel T-joint with filler wire; and (e–g) mild steel lap (overlap) joints exhibiting penetration depths from 7 mm to 25 mm.

2.     Mechanical Integrity and Metallurgical Considerations

Laser welding is characterized by significantly higher cooling rates than arc welding, yet welds in stainless steel, mild steel, and aluminum alloys can still meet international qualification standards such as ISO 15614-11. The highly localized thermal cycle limits grain coarsening in the heat-affected zone, while rapid solidification promotes fine microstructures that contribute to strength. When additional metallurgical control or reinforcement is required, filler wire may be introduced within the same laser pass to enable uniform penetration, alloy modification, and geometry correction.

The relatively high travel speeds typical of laser welding further support weld quality when properly optimized. Faster welding shortens the molten-pool lifetime and reduces heat input, helping to suppress keyhole overexpansion, vapor entrapment, porosity formation, and shrinkage-induced cracking. At the same time, appropriate speed contributes to a stable balance between recoil pressure, surface tension, and hydrostatic forces within the keyhole, enabling consistent deep-penetration welding. These advantages, however, exist only within an optimal process window, since excessively high speeds may instead lead to lack of penetration or humping instabilities.

Table 1: Mechanical test results for representative laser-welded joints in S355J0, 316L stainless steel, and aluminum 5083, demonstrating compliance with international qualification standards (ISO 15614-11 and AWS D1.6). The measured values confirm the capability of laser welding to achieve high mechanical performance across different materials and thicknesses. 

3.     Incompletely Filled Groove (Face Underfill)

During thick-section laser welding, an incompletely filled groove may occur as a surface imperfection resulting from the combined effects of solidification shrinkage, melt-pool flow, hydrostatic pressure, and surface-tension-driven (Marangoni) convection within the large molten volume characteristic of deep-penetration welding. These mechanisms can prevent sufficient liquid metal from remaining at the weld face during final solidification, producing a depression in the reinforcement profile. Partial mitigation may be achieved by increasing travel speed to reduce heat input and molten-pool volume. In addition, welding in a vertical position may in certain configurations assist molten-metal redistribution toward the weld face, thereby reducing the tendency for surface depression. However, when available laser power is limited, full penetration cannot be achieved, or beam shaping cannot adequately redistribute energy, additional corrective measures are required. Practical solutions include a secondary surface-filling pass—performed either by laser with filler wire or by GMAW—where added material compensates for shrinkage and restores the required surface geometry, or a single-pass laser weld with filler wire that prevents material deficiency from occurring. In some production environments, post-weld machining of the reinforcement is applied instead of filler addition, making minor surface depression acceptable within the manufacturing route. Regardless of the selected approach, effective control of an incompletely filled groove is essential for compliance with international weld-quality standards such as ISO 13919.

Figure 2: Examples of incompletely filled groove mitigation strategies in thick-section laser welding: (a) cross-sectional macrograph of a 35 mm thick stainless steel weld exhibiting face underfill; (b) schematic illustration of post-weld machining (milling) of the top and bottom surfaces; (c) radiographic image of the machined 35 mm stainless steel weld meeting acceptance criteria; (d) cross-sectional macrograph of a 20 mm thick stainless steel joint welded in two passes—laser root pass followed by GMAW surface-filling pass to eliminate underfill; and (e) cross-sectional macrograph of a 15 mm thick mild steel joint welded in a single laser pass with filler wire, showing complete surface reinforcement without underfill.

4.     Welding Position, Beam Incidence, and Melt-Pool Stability

Laser beams can be delivered to the workpiece at various incidence angles, enabling welding in all standard positions, including 1G (flat), 2G (horizontal), 3G (vertical), and 4G (overhead). Although positional accessibility is therefore not a fundamental limitation of laser welding, each welding orientation significantly influences keyhole stability, melt-pool geometry, and the relative effects of surface tension, recoil-pressure, hydrodynamic, and hydrostatic forces.

Vertical and overhead welding positions introduce a destabilizing gravitational component along the direction of molten-metal flow, increasing melt-pool asymmetry, reducing effective surface-tension support, and raising the likelihood of droplet ejection, humping, or keyhole instability. As a result, the stable processing window becomes narrower and more sensitive to parameter variation, requiring enhanced keyhole stabilization and precise control of thermal distribution and melt-pool fluid flow.

Dynamic beam-shaping technology provides this control by enabling dynamic beam shaping for redistribution of energy density and recoil pressure, thereby regulating melt-pool dimensions, temperature gradients, and flow behavior to restore the force balance necessary for stable deep-penetration welding—an effect that is particularly critical in thick-section applications where minor instabilities can rapidly evolve into significant weld defects.

Figure 3: Positional laser welding examples: (a) schematic illustration of the vertical welding position; (b) cross-sectional macrograph of a 35 mm thick mild steel weld produced in the vertical position; (c) photograph of the overhead welding setup; and (d) cross-sectional macrograph of a 25 mm thick mild steel weld produced in the overhead position, where partial penetration was specified as the requirement.  

To conclude, Pure Laser Welding of thick sections—particularly when enabled by dynamic beam-shaping control—significantly expands the practical design and manufacturing envelope compared with conventional arc-based processes. The combination of high energy density, rapid thermal cycles, and controllable keyhole–melt-pool dynamics enables welds with narrow heat-affected zones, low distortion, and mechanical performance consistent with international qualification standards. However, the process remains sensitive to force balance, heat input, and fluid-flow stability, especially in thick materials and positional welding, making precise control of travel speed, energy distribution, and reinforcement formation essential. When these factors are properly managed, pure laser welding becomes a robust, efficient, and industrially scalable solution for high-integrity thick-section fabrication across diverse materials, joint configurations, and welding positions.