Microstructure and Properties of TIG Welded Joints Between SLM AlSi10Mg and 6061-T6 Aluminum Alloys

Literature Overview

This study, published in The Welding Journal (Vol. 47, No. 4, 2026, pp. 153–160), addresses a rapidly emerging challenge in aerospace manufacturing: the joining of selectively laser melted (SLM) AlSi10Mg aluminum alloy to conventional wrought 6061-T6 aluminum alloy using TIG welding with ER4043 filler wire. The research, conducted at the China Aviation Industry Corporation Jin Cheng Nanjing Electromechanical and Hydraulic Engineering Research Center and Nanchang Hangkong University, is funded by the National Natural Science Foundation of China (52461021) and reflects the growing need to integrate additively manufactured components into conventional aerospace structures.

The Challenge of Dissimilar Aluminum Alloy Welding

Joining SLM AlSi10Mg to wrought 6061-T6 presents unique metallurgical challenges. SLM AlSi10Mg has a fine, dendritic microstructure with high Si content (typically 7–10 wt%), while 6061-T6 is a heat-treated alloy with precipitate-strengthened microstructure (Mg₂Si precipitates). The significant differences in thermal conductivity, coefficient of thermal expansion, and solidification behavior between these two materials create a complex welding environment.

Welding Parameters and Their Effects

The study examined two peak current conditions: Ip = 140 A and Ip = 125 A, both producing acceptable weld appearance. The following trends were observed:

The consistent finding that lower current produces superior mechanical properties is attributed to reduced thermal input, which minimizes grain coarsening, limits Si diffusion into the weld, and preserves more of the original microstructural features in both the weld metal and HAZ.

Microstructural Analysis

Weld Metal Zone

The weld metal exhibits a distinct asymmetry: the side adjacent to SLM AlSi10Mg shows equiaxed grain morphology, while the side adjacent to 6061-T6 displays columnar grain structure. The center of the weld contains coarse equiaxed grains. This asymmetry is attributed to the different thermal gradients and solidification rates on either side of the joint, influenced by the differing thermal conductivities of SLM AlSi10Mg (which has a more porous, lower-density structure) and dense 6061-T6.

Porosity Distribution

A notable finding is the chain-like distribution of pores near the SLM AlSi10Mg side of the weld, while the 6061-T6 side exhibits lower porosity. This is likely due to the residual porosity inherent in SLM AlSi10Mg components (typically 0.5–2% porosity), which migrates and coalesces during welding. The chain morphology suggests that pores formed during solidification align along the dendrite arms and are subsequently stretched by solidification flow.

Heat-Affected Zone

The HAZ width increases with welding current on both sides. On the 6061-T6 side, the HAZ is particularly critical because the T6 temper is destroyed by the welding thermal cycle, resulting in over-aged or solution-treated microstructures with reduced precipitate density. The fracture analysis confirms that tensile failure consistently occurs in the 6061-T6 HAZ, where precipitate phases aggregate along grain boundaries, increasing brittleness. This is a classic problem in aluminum alloy welding — the HAZ is invariably the weakest zone.

Mechanical Performance and Limitations

The maximum tensile strength achieved (185 MPa at Ip = 125 A) is significantly lower than the typical yield strength of 6061-T6 (approximately 276 MPa) and the ultimate tensile strength of SLM AlSi10Mg (approximately 400 MPa). The elongation after fracture remains below 4% across all conditions, indicating inherently low ductility in the joint. These values are consistent with expectations for TIG-welded aluminum joints using ER4043 filler wire, which produces a Si-rich eutectic structure with limited ductility.

Engineering Practice and Mitigation Strategies

For aerospace engineers considering hybrid SLM/wrought structures, several mitigation strategies should be considered:

1. Post-weld heat treatment: A T4 or T6 re-aging treatment could restore precipitate strength in the 6061-T6 HAZ, potentially increasing tensile strength by 20–30%.
2. Filler wire selection: ER4043 produces a Si-rich weld; alternative fillers such as ER4047 (Mn-containing) or AlSi5Mg (custom) might improve weld ductility.
3. Current optimization: The study confirms that lower currents (125 A) produce superior results; further reduction in current while maintaining full penetration should be investigated.
4. Joint design: Avoiding stress concentrations at the SLM/wrought interface through generous fillet radii or transition geometry could improve fatigue performance.

Study Insights and Implications

This research highlights the fundamental challenge of integrating additively manufactured components into conventional aerospace structures through welding. The significant mechanical property disparity between the weld, HAZ, and base metals — with the 6061-T6 HAZ being the weakest link — suggests that simple TIG welding may not be sufficient for high-strength aerospace applications without post-weld heat treatment. The chain-like porosity on the SLM side is a concern for fatigue-critical applications and may require additional process optimization or post-weld consolidation. Nevertheless, the study provides a valuable baseline for understanding the metallurgical behavior of dissimilar aluminum alloy joints and establishes that, with careful parameter selection, acceptable weld quality is achievable. For the aerospace industry transitioning toward hybrid manufacturing, this work underscores the need for comprehensive process development that includes post-weld treatments, alternative filler metals, and potentially advanced welding processes such as friction stir welding or laser welding to achieve the mechanical properties required for structural applications.