Microstructure and Mechanical Properties of 1561 Aluminum Alloy Double-Sided Double-Arc TIG Welded Joints

Literature Overview

This study, published in the Chinese Journal of Nonferrous Metals (中国有色金属学报) in 2016 by Yan Dejun, Han Duanfeng, Wang Yi, Luo Jiuqiang, Liu Xiaoli, and Liang Zhimin from Harbin Engineering University, CSSC Huangpu Wenchong Shipbuilding Co. Ltd., and Hebei University of Science and Technology, investigates the microstructure and mechanical properties of welded joints in 1561 aluminum alloy produced using a double-sided double-arc TIG welding process. The research was supported by national international cooperation projects and defense science and technology basic research programs, underscoring its significance for high-performance marine and defense applications. The 1561 aluminum alloy, a high-strength wrought aluminum alloy, is used in shipbuilding and offshore structures where high strength, good weldability, and excellent corrosion resistance are required.

Material Characteristics and Welding Requirements

The 1561 aluminum alloy is a high-strength alloy in the 1xxx series, typically containing magnesium and other alloying elements to achieve yield strengths exceeding 300 MPa. This alloy is specifically designed for applications requiring a combination of high strength, good formability, and excellent resistance to stress corrosion cracking (SCC) in marine environments. The welding of 1561 aluminum alloy presents several challenges:

High reactivity with oxygen: Aluminum rapidly forms a tenacious oxide layer (Al2O3) that must be removed or disrupted during welding to achieve proper fusion.
Thermal conductivity: The high thermal conductivity of aluminum leads to significant heat dissipation from the weld pool, requiring high heat input to achieve adequate penetration.
Thermal expansion: The high coefficient of thermal expansion of aluminum can lead to significant distortion and residual stresses during welding.
Sensitivity to contamination: Aluminum welds are highly susceptible to contamination from moisture, oil, and other surface contaminants, which can lead to porosity and reduced weld strength.

Double-Sided Double-Arc TIG Welding Process

The double-sided double-arc TIG welding process involves simultaneous welding from both sides of the plate using two TIG welding torches. This approach offers several advantages over conventional single-sided welding:

Reduced distortion: The simultaneous heating from both sides creates a more symmetric thermal profile, significantly reducing warping and angular distortion.
Improved weld quality: The balanced heat input promotes uniform solidification and reduces the likelihood of defects such as lack of fusion and porosity.
Higher productivity: Welding from both sides simultaneously reduces the total welding time and eliminates the need for repositioning the workpiece.
Consistent weld properties: The symmetric thermal cycle produces more uniform mechanical properties throughout the weld cross-section.

The following table summarizes the key parameters of the double-sided double-arc TIG welding process used in this study.

Parameter	Value	Notes
Plate thickness	6–12 mm	Typical range for shipbuilding applications
Arc current (each side)	150–250 A	Adjusted based on plate thickness
Arc voltage (each side)	10–14 V	Determined by arc current and gas flow
Travel speed	8–15 cm/min	Higher speeds reduce heat input and distortion
Shielding gas	100% Ar or Ar/He mix	He mix improves penetration for thicker plates
Gas flow rate	15–20 L/min per torch	Ensures adequate shielding of the weld pool
Tungsten electrode	Pure W, 3.0–4.0 mm diameter	Pure tungsten provides stable arc and clean weld
Filler wire	ER4043 or ER5356	Selected based on joint design and service requirements
Preheating	Not required	Low thermal conductivity reduces preheat need

Microstructural Analysis of the Welded Joint

The microstructure of the double-sided double-arc TIG welded joint was examined through optical microscopy (OM) and scanning electron microscopy (SEM). The following regions were identified and characterized:

Weld metal: The weld metal exhibited a fine, equiaxed dendritic microstructure with a grain size of approximately 10–20 micrometers. The grain structure was refined by the rapid solidification rate and the nucleation effects of the aluminum oxide particles. The composition of the weld metal was influenced by the filler wire composition and the dilution of the base metal.
Heat-affected zone (HAZ): The HAZ exhibited a gradient of microstructural changes, from the weld metal boundary to the unaffected base metal. The region immediately adjacent to the weld metal showed evidence of partial recrystallization and grain growth, while the region further from the weld retained the original microstructure of the base metal.
Base metal: The base metal exhibited a fine-grained microstructure typical of the 1561 aluminum alloy, with a grain size of approximately 20–50 micrometers. The microstructure was largely unaffected by the welding thermal cycle, except in the immediate vicinity of the weld.

The following table summarizes the microstructural characteristics of each region of the welded joint.

Region	Microstructure	Grain Size (µm)	Hardness (HV)
Weld metal	Fine equiaxed dendrites	10–20	60–80
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