Mechanical Properties Comparison of TIG and CO2 Laser Welds in 08Al Steel

Literature Overview

This paper by Jiang Shichun, Liu Jia, Shi Yan, Zhang Hong, and Zhang Zhimin, published in Applied Laser (2015, Vol. 35, Issue 5, pp. 569-573), presents a systematic comparison of mechanical properties between TIG (GTAW) and CO2 laser welds in 2 mm thick 08Al steel. The study employed static tensile testing, bend testing, and microhardness profiling to characterize the weld joints and their heat-affected zones.

Core Technical Findings

Tensile Properties

Both CO2 laser welds and TIG welds of 08Al steel exhibited fracture at the base metal location, indicating that the weld metal and HAZ were not the weakest links in the joint. The tensile strength for both welding processes reached approximately 300 MPa, which is consistent with the base metal strength of this low-carbon, low-alloy steel. This result confirms that neither process produces weld metal that is significantly weaker than the parent material.

Bend Performance

The CO2 laser weld demonstrated superior bend performance, achieving angles greater than the TIG weld. When bent to 180° without the appearance of micro-cracks, the laser weld exhibited excellent ductility and strain tolerance.

Microhardness Distribution

Both welding processes showed an overall decreasing trend in microhardness from the fusion zone to the base metal. However, notable differences emerged:

• TIG weld: A slight increase in microhardness was observed in the HAZ near the base metal, attributed to the thermal cycle causing localized hardening through grain refinement or precipitation effects.
• CO2 laser weld: A slight increase in microhardness appeared in certain phase transformation zones, and the fusion zone exhibited greater hardness fluctuation compared to the TIG weld.

Interpretation of Technical Points

The fracture at the base metal in both cases is a positive indicator for weld quality. In low-carbon steels like 08Al, achieving weld metal strength comparable to or exceeding base metal strength is generally achievable because the base metal is relatively soft. The 300 MPa tensile strength represents a good match to the base metal properties.

The superior bend performance of the laser weld can be attributed to the significantly narrower heat-affected zone (HAZ) produced by the high power density of the laser beam. The reduced HAZ width means less material is subjected to grain coarsening and phase transformations that could compromise ductility. The laser weld's fusion zone, while exhibiting greater hardness fluctuation, maintains sufficient ductility to withstand severe plastic deformation without cracking.

The microhardness increase in the TIG HAZ near the base metal is likely related to the thermal cycle parameters. TIG welding produces a broader, more gradual thermal cycle compared to laser welding. In 08Al steel, which contains small amounts of aluminum as a deoxidizer, the thermal cycle may promote fine grain formation in the sub-critical HAZ through recrystallization, leading to localized hardening.

Engineering Practice Integration

For steel pipe manufacturing and pipe fitting fabrication, the choice between TIG and laser welding depends on several factors:

1. Wall thickness: Both processes are suitable for thin-wall applications (2 mm in this study). For thicker pipes, TIG remains the workhorse process while laser welding is increasingly viable for thin-wall pipe production.
2. Dimensional accuracy: Laser welding produces less distortion due to the concentrated heat input, making it particularly advantageous for thin-wall pipe manufacturing where ovality and dimensional tolerances are critical.
3. Production speed: Laser welding typically offers higher travel speeds than TIG welding, making it attractive for high-volume pipe production. However, TIG welding provides greater process flexibility for different geometries and joint configurations.
4. Equipment investment: Laser welding systems require significantly higher capital investment than conventional TIG equipment, which must be weighed against productivity gains.
5. Weld quality: The superior bend performance of laser welds suggests better suitability for applications involving forming operations after welding, such as pipe fitting manufacturing where cold forming is common.

Key Questions and Reflections

The study does not address fatigue properties, which are critical for pipe applications subject to cyclic loading. The narrower HAZ of laser welds might be expected to improve fatigue performance by reducing the volume of susceptible microstructure, but this requires dedicated fatigue testing to confirm.

Another important consideration is the effect of welding parameters on the observed mechanical properties. The study does not provide detailed information on welding current, travel speed, or shielding gas flow rates, which limits the ability to establish process windows for quality reproduction. For engineering application, parameter optimization studies would be necessary to define the full range of acceptable welding conditions.

The greater hardness fluctuation in the laser weld fusion zone warrants further investigation. While the bend test results are encouraging, hardness non-uniformity can lead to localized stress concentrations under service loading, potentially initiating cracks at hardness gradient boundaries.

Study Insights and Implications

This comparison study provides useful guidance for process selection in thin-wall steel pipe manufacturing. The CO2 laser weld's superior bend performance and reduced HAZ make it an attractive option for applications requiring post-weld forming or where minimal distortion is critical. However, the greater hardness fluctuation in the laser weld fusion zone suggests that careful parameter control is essential to ensure consistent weld metal quality. For engineers involved in pipe and fitting fabrication, the key takeaway is that laser welding can match or exceed TIG welding in mechanical properties for thin-gauge steels, provided that process parameters are carefully optimized to minimize microstructural heterogeneity in the fusion zone.