Microstructure and Mechanical Properties of 4043 Aluminum Alloy Thin-Walled Parts by TIG Additive Manufacturing

Literature Overview

The paper by Bai Juyang, Wang Jihui, Shi Jianhang, Lin Sanbao, Yang Chunli, and Fan Chenglei, published in the journal Welding (2015, No. 10, pp. 23–26), investigates the microstructure and mechanical properties of 4043 aluminum alloy parts fabricated using TIG additive manufacturing (AM). The study examines the deposited material's grain structure, segregation characteristics, and tensile properties, providing insights into the metallurgical behavior of additively manufactured aluminum components.

Core Technical Points

Microstructure Characteristics

The deposited 4043 aluminum alloy exhibits a distinctive microstructure dominated by columnar dendritic crystals. The key observations are:

Feature	Description	Implication
Columnar dendrites	Extensive columnar dendritic growth	Indicates rapid solidification with directional heat extraction
Grain orientation	Grains grow perpendicular to the weld bead direction	Results from the temperature gradient in the solidification front
Layer boundary crossing	Grains can traverse interlayer stripe regions	Suggests incomplete melting or remelting at layer interfaces
Dendritic segregation	Present within individual dendrites	Due to solute redistribution during rapid solidification
Interlayer segregation	Present at layer boundaries	Due to cumulative thermal cycling and incomplete homogenization

Mechanical Properties

The tensile test results reveal the following characteristics:

Property	Value	Comment
Average tensile strength	146.7 MPa	Moderate strength for 4043 alloy
Elongation after fracture	19.11%	Good ductility
Strength stability	Good	Consistent across samples
Ductility stability	Poor	Significant variation across samples
Anisotropy	None detected	Properties are direction-independent

Segregation Phenomena

Two types of segregation are identified in the deposited material:

Dendritic segregation: Occurs within individual dendrites due to the rapid solidification rates inherent in additive manufacturing. As the solidification front advances, solute elements (Mg, Si in 4043 alloy) are rejected into the interdendritic liquid, creating localized compositional variations.
Interlayer segregation: Occurs at the boundaries between deposited layers. This is attributed to the cumulative thermal cycling effect, where each new layer is deposited on a previously solidified layer, creating a thermal gradient that promotes solute redistribution at the interface.

Process and Standards Analysis

TIG Additive Manufacturing Process Parameters

TIG additive manufacturing of aluminum alloys involves several critical process parameters that influence the final microstructure and properties:

Parameter	Typical Range	Effect on Microstructure
Welding current	150–300 A	Higher current → wider bead, coarser grains
Travel speed	5–20 mm/s	Higher speed → finer grains, thinner layers
Wire feed speed	3–8 m/min	Affects layer thickness and dilution
Shielding gas	Pure Ar or Ar/He mix	He addition increases heat input
Layer thickness	1–3 mm	Affects thermal gradient and solidification rate
Pulse frequency	50–200 Hz (if pulsed)	Controls heat input per cycle

Comparison with Conventional Cast 4043 Aluminum

Property	Conventional Cast 4043	TIG AM 4043 (Deposited)
Tensile strength	150–200 MPa (as-cast)	146.7 MPa
Elongation	5–10%	19.11%
Grain structure	Equiaxed or mixed	Columnar dendritic
Segregation	Macro and micro	Dendritic and interlayer
Anisotropy	Minimal	None detected

The TIG AM material exhibits lower strength but significantly higher ductility compared to conventional cast material. This is attributed to the finer grain size and the absence of casting porosity, which typically reduces ductility in cast aluminum alloys.

Key Questions and Reflections

Why Columnar Dendrites Persist Despite Multiple Thermal Cycles

In conventional welding, multiple passes can partially remelt previous layers, promoting grain refinement and breaking up columnar structures. However, in TIG AM of aluminum, the thermal input per layer is relatively low, and the interlayer cooling is insufficient to fully remelt the previous layer. This results in columnar grains that can traverse multiple layer boundaries. This observation has important implications for the mechanical properties of AM components: the columnar grain orientation can lead to preferential crack propagation along grain boundaries, particularly under transverse loading.

The Paradox of Higher Ductility Despite Coarser Microstructure

Conventionally, finer grains lead to higher strength and ductility (Hall-Petch relationship). However, the TIG AM material exhibits higher ductility than conventional cast material despite having a coarser columnar dendritic structure. This paradox can be explained by the absence of casting defects (porosity, shrinkage cavities) that typically act as crack initiation sites in cast aluminum. The cleaner microstructure of the AM material allows for more uniform plastic deformation, resulting in higher elongation.

Implications for Thin-Walled Part Fabrication

The study specifically addresses thin-walled parts, which are challenging to fabricate using conventional casting or machining methods. TIG AM offers a viable pathway for producing complex thin-walled aluminum components with good ductility. However, the poor ductility stability suggests that process parameter control is critical, and engineers must carefully optimize parameters for specific geometries to ensure consistent mechanical performance.

Study Insights and Implications

This study provides valuable metallurgical insights into the behavior of 4043 aluminum alloy under TIG additive manufacturing conditions. The key finding is that while the deposited material retains a columnar dendritic microstructure with segregation phenomena, it achieves acceptable mechanical properties with good ductility and no detectable anisotropy. For engineering applications involving aluminum components—such as heat exchangers, pressure vessels, or aerospace structures—TIG AM offers a promising fabrication route, particularly for thin-walled geometries that are difficult to produce by conventional methods. The poor ductility stability, however, highlights the need for rigorous process control and post-processing (e.g., solution treatment and aging) to homogenize the microstructure and improve property consistency. Future research should focus on optimizing heat treatment protocols for TIG AM aluminum alloys to fully exploit the potential of this fabrication technology.