CLADDING TECHNOLOGY SHANXI CO., LTDLaser Scanning Welding versus TIG Welding of Zinc Aluminum Magnesium Coated Steel
Literature Overview
The comparative study of galvo-scanning laser welding and TIG welding for zinc-aluminum-magnesium coated steel plates addresses a practical manufacturing challenge in the automotive and structural steel industries. Zinc-aluminum-magnesium (ZAM) coatings, such as those conforming to ASTM A1048 or JIS H3405, offer superior corrosion resistance compared to conventional zinc or zinc-iron alloy coatings due to the synergistic effect of zinc, aluminum, and magnesium in the coating microstructure. However, the presence of these reactive elements in the coating creates unique welding challenges, including excessive spatter, coating burn-off, and zinc vapor-induced porosity.
The galvo-scanning laser welding process, utilizing a two-axis galvanometer mirror system for rapid beam deflection, offers significantly higher welding speeds (typically 3 to 15 m/min for 2 mm plate) compared to conventional TIG welding (0.5 to 2 m/min for the same thickness). The study evaluates whether the superior productivity of laser welding can be achieved without compromising joint mechanical properties, corrosion resistance, or weld appearance.
Core Technical Parameters and Process Windows
The key welding parameters compared in the study are summarized below:
| Parameter | Galvo-Scanning Laser Welding | TIG Welding |
|---|---|---|
| Heat source | Fiber laser, 6-10 kW | Tungsten electrode, AC/DC |
| Welding speed | 4-12 m/min | 0.5-2.0 m/min |
| Heat input | 0.1-0.5 kJ/mm | 1.5-4.0 kJ/mm |
| Shielding gas | Argon or Ar/CO2 mix | Pure argon |
| Gas flow rate | 15-30 L/min | 15-20 L/min |
| Weld penetration | Full penetration (keyhole mode) | Partial or full penetration |
| HAZ width | 0.5-2.0 mm | 3.0-8.0 mm |
| Spatter level | Moderate (Zn vapor) | Low |
| Coating burn-off | 60-80% in weld zone | 40-60% in weld zone |
The galvo-scanning laser process operates in keyhole mode for 2 mm plate thickness, where the laser power density exceeds 10^6 W/cm^2, creating a deep, narrow weld with a depth-to-width ratio of 5:1 to 10:1. The rapid scanning pattern (typically hatching or contour scanning) distributes the heat input more uniformly, reducing the overall heat-affected zone compared to a single-pass continuous beam.
Weld Formation and Mechanical Properties
The weld bead morphology differs significantly between the two processes. Laser welding produces a narrow, deep weld with minimal reinforcement on the upper surface, while TIG welding produces a wider, more convex bead with greater reinforcement. The laser weld cross-section typically shows a single keyhole cavity, whereas TIG welds exhibit a more uniform fusion zone with possible multiple passes for thicker plates.
Mechanical property evaluation reveals that laser welds generally exhibit higher tensile strength (yield strength 450-550 MPa for S350-grade steel) due to the reduced heat input and finer grain structure in the HAZ. However, the laser weld HAZ may exhibit reduced impact toughness at low temperatures due to the formation of martensitic structures in the rapid cooling zone. TIG welds, with their higher heat input, produce a more tempered microstructure in the HAZ with improved ductility but lower strength.
| Test Method | Laser Weld | TIG Weld | Base Metal |
|---|---|---|---|
| Tensile strength (MPa) | 580-620 | 480-520 | 500-550 |
| Yield strength (MPa) | 450-510 | 370-420 | 355 |
| Elongation (%) | 18-22 | 24-28 | 26-30 |
| Charpy V-notch (J, -40°C) | 35-55 | 60-90 | 80-120 |
| Microhardness (HV) | 280-350 | 200-260 | 180-220 |
The corrosion resistance of the weld joints is a critical concern for ZAM-coated steels. The loss of the protective coating in the weld zone creates galvanic coupling between the bare steel weld and the remaining intact coating on the base metal. The study demonstrates that laser welds, with their narrower heat-affected zone, retain more of the original coating in the immediate vicinity of the weld, resulting in marginally better corrosion resistance in salt spray testing (ASTM B117). After 1000 hours of 5% NaCl spray testing, the laser weld joint showed slightly less coating degradation compared to the TIG weld joint.
Coating Behavior During Welding
The zinc-aluminum-magnesium coating undergoes complex phase transformations during welding. At temperatures above 420 degrees Celsius, the aluminum and magnesium in the coating begin to diffuse into the molten weld pool, while zinc evaporates at temperatures above 907 degrees Celsius. The evaporation of zinc creates porosity in the weld metal and generates zinc oxide fumes that must be adequately controlled for operator safety.
The scanning pattern of the galvo laser affects the coating burn-off distribution. A hatching pattern with multiple passes over the same area results in more complete coating removal but also more uniform heat distribution. A single-pass contour scan preserves more coating at the edges of the weld but creates a sharp transition in coating thickness from intact to completely removed.
From a practical standpoint, the post-weld coating repair strategy differs between the two processes. Laser welds require a thinner coating repair layer due to the narrower weld zone, while TIG welds require more extensive coating repair over the wider HAZ. Hot-dip re-coating is the most reliable method for restoring corrosion protection, but it is only feasible for batch production or small components.
Engineering Practice and Quality Control
For structural applications where ZAM-coated steel is used, the welding procedure qualification per NB/T 47014 or ASME IX must address the specific challenges of the coating. The welding procedure specification should include:
- Pre-weld cleaning requirements for the coating removal zone (minimum 20 mm from the weld edge for laser welding, minimum 40 mm for TIG welding).
- Shielding gas composition and flow rate to minimize zinc vapor entrainment in the weld pool.
- Post-weld coating inspection requirements, including the minimum acceptable coating thickness in the repair zone.
- Corrosion resistance testing protocols specific to the ZAM coating system.
The galvo-scanning laser welding process offers significant productivity advantages for automotive body-in-white production, where welding speeds of 8 to 12 m/min are achievable with acceptable joint quality. However, the equipment investment for a galvo-scanning laser system (typically 1.5 to 3 million RMB for a 10 kW fiber laser with galvo head) is substantially higher than for TIG welding equipment.
Study Insights and Reflections
The comparative study highlights a fundamental trade-off in welding technology selection: productivity versus joint quality predictability. Laser welding delivers superior mechanical properties and productivity but requires more sophisticated process control and equipment investment. TIG welding, while slower, offers more forgiving process parameters and easier visual inspection of weld quality.
For pressure vessel fabrication involving ZAM-coated steel components, the selection between laser and TIG welding should be guided by the criticality of the joint, the required production volume, and the available inspection resources. For high-volume automotive applications, galvo-scanning laser welding is clearly advantageous. For pressure vessel fabrication where weld quality documentation and traceability are paramount, TIG welding with its slower speed and more uniform heat input may be preferable.
A key insight from this study is the importance of considering the coating system as an integral part of the welded joint, not merely as a surface treatment to be repaired after welding. The interaction between the welding process and the coating system determines the long-term corrosion performance of the joint, and this interaction must be systematically evaluated during welding procedure qualification.