1.How does the composition of the base material affect the selection decision?
For plain carbon steel (such as SPCC, SPCD, SPCE): Ar or Ar-He mixtures are preferred. Ensure weld purity and avoid nitride embrittlement.
For high-strength low-alloy steel (HSLA): Sensitive to heat input, performance degradation must be prevented. Ar-He mixtures are a safe choice; helium helps slow cooling and improve microstructure.
For interstitial atom-free steel (IF steel): Contains strong nitride-forming elements such as Ti and Nb. Ar or Ar-He mixtures can be used. Low-proportion Ar-N₂ mixtures (e.g., 95% Ar + 5% N₂) can be experimentally explored to fix Ti/Nb with nitrogen, but weld toughness and porosity must be verified.
2.How do welding process parameters affect selection decisions?
Welding speed: Higher welding speeds require greater resistance to shielding gas disturbances and improved weld pool stability. Ar-He mixtures, due to their higher energy and better venting properties, are more suitable for high-speed welding (e.g., > 6 m/min).
Laser power and plate thickness: Higher power and thicker plates require greater penetration depth. Increasing the helium proportion helps achieve greater penetration depth.
3.How do weld quality requirements affect selection decisions?
Appearance: A bright, silvery-white finish without oxidation is required; high-purity argon or an Ar-He mixture is the optimal choice.
Mechanical Properties: Weld strength, elongation, and cupping value must match the base metal, especially for parts subject to severe subsequent deformation (such as automotive structural components). This necessitates that the shielding gas completely prevent the intrusion of harmful gases (O₂, N₂), therefore a high-purity inert gas-based gas (Ar/He) is essential.
Defect Control: Prevent porosity, undercut, and collapse. Gas composition and gas delivery method directly affect molten pool fluidity and solidification behavior.
4.What are the key operational points?
Gas Purity: High-purity gas (≥99.995%) must be used, especially with strict control of oxygen and water content (dew point <-40°C). Impurities are the main culprits of porosity and embrittlement.
Gas Supply System:
Employ double-sided protection (pre-protection at the front of the welding torch, delayed protection with a trailing shield at the rear) to ensure the molten pool remains within the protection zone from melting to solidification.
The shielding shield should be as close as possible to the strip surface, and its shape should match the weld.
If necessary, apply shielding gas to the back of the weld as well (for certain critical materials).
Gas Flow Rate and Flow Pattern:
The flow rate should be moderate (typically 15-25 L/min on the front, 10-20 L/min with the trailing shield). Excessive flow will disturb the molten pool and cause porosity; insufficient flow will provide inadequate protection.
Ensure the gas flow is laminar to avoid turbulence that could entrain air. Gas diffusers or screens can be used.
5.How is welding process evaluated?
Welded specimens.
Perform macroscopic metallographic inspection (checking penetration, formation, and presence of internal defects).
Perform mechanical property tests (tensile, bending, cupping).
Inspect the weld microstructure and hardness distribution.
Conduct stability tests simulating the highest speed on the production line.