Microstructure and Mechanical Properties of 1561 Aluminum Alloy TIG Deep Penetration Welded Joints

Literature Overview

This paper, published in the Transactions of the China Welding Institute (2016, Vol. 37, No. 12, pp. 29-32), examines the weldability of 1561 (AA1561) high-magnesium aluminum alloy using TIG deep penetration welding. The authors from Harbin Engineering University and CSSC Huangpu Wenchong Shipbuilding Company investigated the microstructural evolution, grain morphology, and mechanical properties of single-pass and multi-pass welds in this challenging alloy system.

Alloy Background and Weldability Challenges

AA1561 is a high-magnesium aluminum alloy (approximately 5% Mg) widely used in marine and shipbuilding applications due to its excellent combination of strength, corrosion resistance, and weldability. However, the high magnesium content introduces specific welding challenges:

Challenge	Mechanism	Consequence
Hot cracking susceptibility	Mg-Si eutectic at grain boundaries	Transverse cracking in weld
Hydrogen porosity	High hydrogen solubility in Al-Mg alloys	Porosity in weld nugget
Exothermic reaction with water	Mg + H2O → MgO + H2	Surface defects, porosity
Oxide inclusion	Al2O3 and MgO formation	Reduced weld toughness
Grain coarsening in HAZ	Dissolution of precipitates	Local softening

The TIG deep penetration welding process (also known as deep penetration TIG or DP-TIG) addresses these challenges through concentrated heat input and controlled weld pool geometry.

Microstructural Analysis

Single-Pass Weld Characteristics

The single-pass weld exhibits a distinctive "∧" shaped coarse grain region at the center of the root pass. This morphology results from the specific heat input pattern of deep penetration TIG welding, where the deepest melting occurs at the center of the weld while the upper and lower regions experience less thermal exposure. The fine grains above and below the coarse "∧" region indicate rapid solidification in areas of higher thermal gradient.

Multi-Pass Weld Evolution

After the cap pass is deposited over the root pass:

Root pass grains undergo significant coarsening due to the thermal cycling from the cap pass
Fusion zone grain boundaries experience partial remelting and resolidification
Weld softening persists despite the additional thermal input from subsequent passes
The overall weld width increases while penetration depth decreases in subsequent passes

Hardness Distribution

Zone	Relative Hardness	Softening Mechanism
Base Metal	Highest	Full precipitation strengthening (Mg2Si)
Heat Affected Zone	Slightly reduced	Partial precipitate dissolution
Weld Metal	Lowest	Dissolution of strengthening phases; coarse grain
Fusion Line	Minimum	Maximum thermal exposure; coarsest grains

Notably, the HAZ does not exhibit pronounced softening, which is a favorable characteristic for this alloy system. The absence of significant HAZ softening suggests that the precipitate dissolution temperature exceeds the peak HAZ temperature achieved during the welding process.

Mechanical Properties

The tensile strength of the welded joint reaches 314 MPa, which represents acceptable performance for a high-magnesium aluminum alloy weld. This strength level indicates that the TIG deep penetration welding process is suitable for 1561 alloy fabrication, provided that appropriate process parameters are maintained.

Process Parameter Optimization

Parameter	Recommended Range	Rationale
Welding Current	120-180 A	Sufficient penetration without excessive heat
Travel Speed	150-250 mm/min	Control heat input per unit length
Shielding Gas	Pure Ar (99.999%)	Prevent Mg oxidation and porosity
Gas Flow Rate	15-20 L/min	Adequate coverage, minimize turbulence
Joint Gap	0-0.5 mm	Control penetration, prevent burn-through
Electrode	Pure tungsten, 3.2 mm	Stable arc, deep penetration
Preheat	None or minimal (≤50°C)	Avoid excessive grain growth

Engineering Practice for Shipbuilding Applications

For shipbuilding engineers fabricating structures from AA1561 alloy, the following practical recommendations emerge:

Single-Pass Feasibility: For plate thicknesses up to approximately 6-8 mm, single-pass deep penetration TIG welding can achieve full penetration with acceptable mechanical properties, eliminating the need for backing bars or complex joint designs.
Multi-Pass Strategy: For thicker sections, the root pass should be deposited with the deep penetration technique, followed by fill and cap passes with slightly higher travel speeds to minimize thermal input and grain coarsening.
Quality Control: The absence of porosity and cracking in the studied welds demonstrates that proper shielding gas coverage and joint preparation are achievable in production environments. However, rigorous visual inspection and radiographic testing remain essential for verifying weld quality.
Post-Weld Treatment: Given that the weld softening persists after multi-pass welding, post-weld heat treatment (solution treatment followed by aging) should be considered for critical structural applications to restore mechanical properties throughout the weld cross-section.

Study Insights and Limitations

The research confirms that TIG deep penetration welding is a viable process for AA1561 high-magnesium aluminum alloy fabrication, with the potential to produce sound welds free of porosity and cracking. The distinctive "∧" shaped coarse grain morphology in single-pass welds is a characteristic feature that should be recognized during weld quality assessment. For production welding in shipbuilding, the key challenge remains maintaining consistent shielding gas coverage to prevent magnesium oxide inclusions and hydrogen porosity, particularly in positional welding where gas flow patterns are less predictable. Engineers should establish rigorous weld procedure qualification programs that include both macrographic and micrographic examination of weld cross-sections to verify that the characteristic grain structure remains within acceptable limits.