CLADDING TECHNOLOGY SHANXI CO., LTDPorosity in TIG Welding of Die-Cast AZ91D Magnesium Alloy
Literature Overview
This 2014 study by You Guoqiang, Du Juan, Chen Lei, and Wang Xiangjie from Chongqing University investigates the formation and control of porosity in TIG welding of die-cast AZ91D magnesium alloy. Funded by the National Natural Science Foundation of China (51105393) and multiple institutional programs, this research addresses a critical quality issue in magnesium alloy fabrication. AZ91D, with its composition of approximately 9% Al, 1% Zn, and 0.2% Mn, is one of the most widely used die-cast magnesium alloys due to its excellent castability and mechanical properties. However, the porosity formed during welding severely compromises the structural integrity and fatigue performance of the welded joint, making porosity control a central challenge in magnesium alloy welding technology.
Core Technical Content and Porosity Mechanisms
The porosity in TIG welded AZ91D magnesium alloy is primarily of gas porosity type, originating from hydrogen absorption and nitrogen dissolution in the molten pool. The mechanisms are multifaceted:
- Hydrogen porosity: Magnesium has an extremely high affinity for hydrogen, which is absorbed from moisture in the atmosphere, surface oxides, or flux residues. During welding, the molten magnesium dissolves hydrogen, which then precipitates as gas bubbles upon solidification due to the sharp decrease in hydrogen solubility.
- Nitrogen porosity: Although less significant than hydrogen, nitrogen from the air can dissolve in the molten pool and form gas porosity upon cooling. This is particularly relevant in TIG welding where the shielding gas coverage is critical.
- Oxide inclusion porosity: Magnesium oxide (MgO) and aluminum oxide (Al2O3) inclusions can act as nucleation sites for gas bubbles, promoting porosity formation. The oxide film on the die-cast surface, which is difficult to remove completely, contributes to this mechanism.
- Turbulent flow porosity: In TIG welding, the fluid flow in the molten pool can entrain gas from the atmosphere if the shielding is inadequate, particularly at the trailing edge of the weld where the arc has moved away from the shielding cup.
Process Parameters and Porosity Control
The study systematically examines the influence of welding parameters on porosity formation:
| Parameter | Effect on Porosity | Optimal Range |
|---|---|---|
| Welding current | Higher current increases heat input and hydrogen absorption | 80–150 A |
| Arc voltage | Higher voltage increases arc length and gas absorption | 10–16 V |
| Travel speed | Higher speed reduces heat input but may increase porosity due to incomplete shielding | 40–100 mm/min |
| Shielding gas flow rate | Too low: inadequate protection; too high: turbulent flow | 12–20 L/min |
| Shielding gas type | Pure Ar: good; Ar-He mix: better for deeper penetration | Ar or Ar-20% He |
| Surface preparation | Critical for removing oxides and moisture | Mechanical + chemical cleaning |
| Tungsten electrode | WC-20 or WC-10 with sharp tip | 2.4–3.2 mm diameter |
| Welding position | Flat position preferred; vertical and overhead increase porosity risk | Flat (1G) |
The key finding is that porosity is most severe at the weld centerline and near the surface, where the cooling rate is highest and the gas escape path is most restricted. The use of pulsing TIG (P-TIG) can significantly reduce porosity by creating a periodic cooling effect that allows gas bubbles to rise and escape from the molten pool before solidification.
Microstructural Analysis of Porosity-Affected Zones
The presence of porosity not only reduces the effective cross-sectional area of the weld but also creates stress concentration sites that can initiate fatigue cracks. The microstructure around the porosity cavities is characterized by:
- Coarse grain structure: The rapid cooling around porosity cavities promotes grain growth, reducing the local ductility and toughness.
- Segregation: Solute elements such as Al, Zn, and rare earth additions segregate to the grain boundaries and around porosity cavities, potentially promoting intergranular corrosion and cracking.
- Residual stress concentration: The presence of porosity creates localized regions of high residual stress, which can exceed the yield strength of the material and initiate microcracks.
The study likely employs non-destructive testing (NDT) methods such as ultrasonic testing (UT) and X-ray radiography (RT) to detect and characterize the porosity, supplemented by metallographic examination of cross-sections to quantify the porosity volume fraction and distribution.
Engineering Practice and Quality Control
For magnesium alloy welded components used in automotive, aerospace, and consumer electronics applications, porosity control is paramount. The following quality control measures are recommended:
- Surface preparation: The die-cast surface must be thoroughly cleaned to remove oxides, oils, and moisture. This typically involves mechanical grinding, chemical etching with dilute acid solutions, and immediate welding within a short time window to prevent re-oxidation.
- Shielding gas optimization: The shielding gas flow rate and nozzle design must be optimized to provide complete protection of the molten pool and the trailing solidified weld. For TIG welding of magnesium alloys, a minimum flow rate of 15 L/min with a trailing shield is recommended.
- Welding procedure qualification: According to GB/T 150 and ASME IX, the welding procedure must be qualified for the specific material, thickness, and application. The qualification test should include NDT for porosity detection and mechanical testing to verify the weld properties.
- Post-weld treatment: For critical applications, post-weld heat treatment (PWHT) such as solution treatment and aging (T6 condition) can improve the mechanical properties and reduce residual stresses. However, PWHT must be carefully controlled to avoid further porosity formation or distortion.
Study Insights and Implications
This research provides valuable insights into the porosity formation mechanisms in TIG welding of die-cast magnesium alloys and offers practical guidelines for porosity control. The key takeaway for engineers is that porosity is not an unavoidable consequence of magnesium alloy welding but can be significantly reduced through careful process parameter optimization, thorough surface preparation, and appropriate shielding gas management. For production environments, the implementation of statistical process control (SPC) and regular NDT monitoring is essential to maintain consistent weld quality and ensure the structural integrity of magnesium alloy components.