CO2 Laser-Arc Hybrid Welding of CCS-A Ship Steel T-Joints

Literature Overview

The research by Xiao Jiong, Huang Jian, Hu Lianhai, and Ni Huifeng (Hot Working Technology, Vol. 39, No. 19, 2010, pp. 159-162) presents a systematic investigation of full-penetration T-joint welding on 14 mm thick CCS-A ship steel using high-power CO₂ laser-arc hybrid welding. Conducted at the Shanghai Key Laboratory of Laser Manufacturing and Material Modification (Shanghai Jiao Tong University) in collaboration with Jiangnan Shipyard (Group) Corporation, this study addresses a practical challenge in shipbuilding: the efficient and high-quality welding of T-joints in ship hull structures. The work was published in a journal dedicated to thermal processing technology, reflecting its focus on process optimization and metallurgical characterization.

Process Configuration and Parameters

The laser-arc hybrid welding process combines the deep penetration capability of high-power CO₂ laser welding with the stability and fill capability of arc welding (typically MIG or MAG). For the 14 mm thick CCS-A steel T-joint, the authors achieved full penetration with only two welding passes, which is remarkable given the thickness. The following table summarizes the key process parameters and their effects:

One of the most notable findings is that the T-joint was welded without groove preparation (no beveling). This is a significant practical advantage because groove preparation on T-joints is labor-intensive and time-consuming in shipyard conditions. The no-groove approach is enabled by the deep penetration of the CO₂ laser, which can achieve penetration depths exceeding 10 mm in mild steel with sufficient power. The arc welding component provides the necessary fill metal and helps stabilize the keyhole, preventing its collapse.

The study also found that a smaller angle between the laser beam and the panel surface produced greater weld depth. This is attributed to the increased effective interaction length of the laser beam with the material surface, which enhances energy absorption and keyhole formation. In practical terms, this means that the laser head can be positioned at a slight tilt rather than being perfectly normal to the weld line, providing greater flexibility in fixture design and access to confined areas.

Microstructural Analysis

Metallographic examination revealed that martensitic microstructures formed in both the weld metal and the heat-affected zone. The fine-grained zone (FGZ) of the HAZ, which experiences peak temperatures just above the A₃ temperature, exhibited fine martensite due to rapid cooling and high carbon concentration in the austenite. The coarse-grained zone (CGHAZ) also contained martensite but with coarser morphology. The weld metal, depending on the dilution ratio and cooling rate, showed a mix of martensite and possibly bainite.

The following table summarizes the microstructural characteristics observed in different zones:

The maximum hardness was observed in the FGZ, which is a common finding in high-carbon or high-strength steels. The FGZ experiences the highest cooling rate relative to its carbon content because the peak temperature is just above A₃, limiting the time for carbon to diffuse out of the austenite. This results in a high-carbon austenite that transforms to hard, brittle martensite upon cooling.

Hardness Distribution and Second-Pass Tempering

A particularly important finding is the effect of the second welding pass on the hardness of the first-pass weld. The second pass acts as a tempering treatment for the first-pass weld metal and adjacent HAZ. This tempering effect reduces the hardness of the first-pass weld from potentially brittle levels to more acceptable values. The final maximum hardness of the welded joint was measured to be less than 380 HV, which is within acceptable limits for most structural applications.

This tempering effect is a natural consequence of the multi-pass welding process and is particularly beneficial in high-strength steels where excessive hardness can lead to delayed cracking. The second-pass thermal cycle raises the first-pass weld metal to approximately 300-500 °C, which is sufficient to cause tempering of the martensite, reducing hardness and improving ductility. This is an important practical consideration: multi-pass welding not only achieves full penetration but also provides an inherent tempering effect that improves the mechanical properties of the first-pass weld.

Engineering Practice and Defect Analysis

The no-groove welding approach significantly reduces preparation time and cost in shipyard production. However, it also introduces specific challenges. The absence of a groove means that the weld geometry is dictated entirely by the process parameters, and any deviation in laser power, travel speed, or arc current can result in incomplete penetration or excessive undercut. The study notes that the no-groove configuration reduces the tendency for undercut defects, likely because the laser energy creates a deep, narrow melt pool that is less susceptible to the surface tension-driven undercut mechanism common in conventional arc welding.

The following table presents common defects and countermeasures for laser-arc hybrid T-joint welding:

Study Insights and Implications

This study demonstrates the significant potential of laser-arc hybrid welding for shipbuilding applications. The ability to achieve full penetration of 14 mm thick steel with only two passes and without groove preparation represents a substantial productivity improvement over conventional submerged arc welding or manual arc welding. The metallurgical findings, particularly regarding martensite formation and the tempering effect of multi-pass welding, provide valuable guidance for procedure qualification. For shipyards considering the adoption of laser-arc hybrid welding, the key considerations include the capital investment in high-power CO₂ laser systems, the need for precise motion control, and the development of welding procedure specifications that account for the unique metallurgical behavior of the process. The study also highlights the importance of post-weld inspection, particularly hardness testing in the FGZ, to ensure that the final hardness remains within acceptable limits.