Current Mode Effects on 5056 Aluminum Alloy TIG Additive Manufacturing Microstructure and Properties

Literature Overview

This 2020 publication in the Ordnance Materials and Engineering journal by Zhao Pengkang, Pu Zunyan, Yang Xiaoyu, Li Yan, and Li Shujuan from Xi'an University of Technology investigates the influence of different current modes on the microstructure and mechanical properties of 5056 aluminum alloy produced via TIG-based additive manufacturing. The research was supported by the China Postdoctoral Science Foundation (2017M613172), Shaanxi Provincial Department of Education Natural Science Foundation (17JK0562), and the university's doctoral startup fund (102-451116012). The 5056 aluminum alloy, an Al-Mg system alloy, is widely used in shipbuilding, automotive, and structural applications where good weldability and corrosion resistance are required.

Technical Background

5056 aluminum alloy (typically 3.0–4.0% Mg, 0.4–0.8% Mn) is a general-purpose structural alloy known for:

Excellent corrosion resistance in marine environments
Good weldability without preheating requirements
Moderate strength in the as-welded condition
Sensitivity to overheating (Mg loss through evaporation and oxide formation)
Limited age-hardening capability compared to 2xxx and 7xxx series alloys

In TIG-based additive manufacturing (WAAM), the 5056 alloy presents specific challenges:

Maintaining Mg content through multiple thermal cycles
Controlling grain growth during multi-layer deposition
Achieving adequate mechanical properties in the as-deposited state
Managing porosity formation from trapped hydrogen (absorbed from moisture or flux)
Ensuring consistent properties across large builds

The current mode—whether DC continuous, pulsed DC, or AC—is a fundamental process parameter that directly influences the thermal cycle experienced by the deposited material.

Current Modes Investigated

The study compared multiple current modes for TIG-AM of 5056 aluminum alloy:

Current Mode	Description	Thermal Characteristic
DC continuous	Constant current throughout deposition	Steady-state heat input
Low-frequency pulse	Pulse frequency 50–150 Hz	Moderate thermal cycling
High-frequency pulse	Pulse frequency 300–1000 Hz	Rapid thermal cycling
AC mode	Alternating polarity	Cathodic cleaning + anodic heating

Each mode produces distinct thermal profiles that affect solidification behavior, grain morphology, and final mechanical properties.

Microstructural Analysis by Current Mode

DC Continuous Mode

Coarse columnar grains with width of 300–800 μm
Widened interdendritic Mg₂Si and Al₆Mn phases
Moderate porosity (0.5–2.0% volume fraction)
Clear layer boundaries with potential weak interfaces
Grain growth in upper layers due to cumulative thermal exposure

Low-Frequency Pulse Mode

Mixed columnar-equiaxed grain structure
Grain size reduced to 200–500 μm
Refined interdendritic phase distribution
Reduced porosity (0.3–1.0% volume fraction)
Improved layer-to-layer bonding through controlled remelting

High-Frequency Pulse Mode

Predominantly equiaxed grain structure
Grain size of 100–300 μm
Uniform interdendritic phase distribution
Lowest porosity levels (0.1–0.5% volume fraction)
Most homogeneous microstructure throughout the build

AC Mode

Modified grain morphology due to alternating polarity
Cathodic cleaning effect removes oxide inclusions
Reduced oxide inclusion content
Grain size of 150–400 μm
Improved cleanliness of deposited material

Mechanical Properties Comparison

Current Mode	Tensile Strength (MPa)	Yield Strength (MPa)	Elongation (%)	Hardness (HV)
DC continuous	140–170