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

Overview of the Study

This research investigates the weldability and joint performance of joining selective laser melting (SLM) fabricated AlSi10Mg alloy to conventionally heat-treated 6061-T6 aluminum alloy using gas tungsten arc welding (GTAW). The study is particularly significant because it bridges the gap between additive manufacturing (AM) substrates and traditional wrought or cast components, a scenario increasingly encountered in modern aerospace and automotive manufacturing where repair and hybrid assembly are becoming routine engineering practices. The authors examined the macrostructure, microstructure, hardness distribution, tensile strength, and fracture behavior of the weld joints, providing valuable data for process parameter optimization and quality assurance protocols.

Key Technical Findings

The SLM-processed AlSi10Mg base material exhibits a fine, equiaxed cellular dendrite structure with grain sizes typically in the range of 20 to 80 micrometers, resulting from the rapid solidification rates inherent to the SLM process (cooling rates on the order of 10^4 to 10^6 K/s). In contrast, the 6061-T6 substrate retains its characteristic elongated grains and fine precipitate dispersion (beta'' Mg2Si phase), which has been hardened through the solution treatment and aging sequence. Upon TIG welding, the weld zone experiences significant thermal cycling that leads to coarsening of the SLM cellular structure near the fusion boundary, while the 6061-T6 side undergoes partial dissolution and re-precipitation of strengthening phases, causing a measurable softening in the heat-affected zone (HAZ).

The weld metal composition is a mixture of the two base materials, enriched in silicon from the AlSi10Mg side and magnesium from the 6061-T6 side. This compositional asymmetry results in a non-uniform distribution of second-phase particles (AlSi and Mg2Si) within the weld nugget, which directly influences crack susceptibility and mechanical performance. The hardness profile across the joint shows a characteristic "U" shape, with the lowest hardness typically occurring at the HAZ boundary on the 6061-T6 side, where the loss of precipitate strengthening is most pronounced.

Process Parameter Analysis

The following table summarizes the typical GTAW parameters used and their effects on joint quality:

A critical observation is that the dilution ratio (the proportion of base metal melted into the weld pool) is highly asymmetric between the two substrates due to their different thermal conductivities and melting behaviors. The SLM-AlSi10Mg side tends to melt more readily because of its lower melting point (approximately 577°C) compared to 6061-T6 (approximately 650°C), leading to a weld metal composition that is richer in silicon than expected from a simple linear dilution calculation. This asymmetry must be accounted for when predicting joint performance and designing filler metal selection strategies.

Defect Analysis and Countermeasures

Porosity is the most frequently encountered defect in this dissimilar aluminum alloy joint. Gas porosity originates from hydrogen absorption during the welding process, while shrinkage porosity develops at the weld centerline due to the high solidification shrinkage of the Al-Si-Mg alloy system. The following countermeasures were identified:

• Ensuring thorough surface cleaning to remove oxide layers and adsorbed moisture
• Using a slightly pulsating current waveform to promote gas escape during solidification
• Applying a narrow gap joint preparation to reduce the solidification volume and shrinkage stress
• Maintaining stable shielding gas coverage, particularly at the trailing edge of the arc

Cracking susceptibility is another concern, particularly hot cracking in the weld metal due to the formation of low-melting-point eutectic phases at the interdendritic regions. The presence of both silicon and magnesium creates a complex solidification sequence that can promote centerline cracking if the cooling rate is too slow or if the composition falls within a susceptible range.

Engineering Practice Implications

From a quality assurance perspective, this study underscores the necessity of developing dedicated welding procedure specifications (WPS) for dissimilar aluminum alloy joints, particularly when one substrate is AM-fabricated. The conventional approach of treating both sides identically during welding parameter selection is inadequate. Engineers should consider the following practices:

1. Perform a thorough metallurgical characterization of the AM substrate before welding, including grain size, porosity content, and residual stress state.
2. Develop a dedicated filler metal selection strategy that accounts for the asymmetric dilution behavior.
3. Implement multi-layer welding sequences that allow the first layer to establish a favorable composition profile before subsequent layers are deposited.
4. Conduct post-weld heat treatment (PWHT) carefully, as the SLM substrate may not respond to conventional aging cycles in the same manner as wrought 6061-T6 material.

Study Insights and Reflections

This research highlights a fundamental challenge in the integration of additive manufacturing into conventional fabrication workflows: the microstructural and mechanical properties of AM substrates are fundamentally different from their wrought or cast counterparts, and this difference must be respected during subsequent joining operations. The rapid solidification of SLM produces a material that is inherently stronger and finer-grained than conventional AlSi10Mg, but this advantage can be partially or completely lost if the welding thermal cycle is not carefully controlled.

A particularly thought-provoking aspect is the question of how to best exploit the superior properties of the AM substrate while protecting it from welding-induced degradation. One promising strategy, not fully explored in this study, would be to apply a thin overlay layer of compatible material on the SLM substrate before welding, creating a buffer zone that shields the fine AM microstructure from excessive thermal exposure. This approach is analogous to the cladding techniques used in bimetallic pressure vessel fabrication, where a corrosion-resistant overlay protects the structural base material from process degradation.

The practical significance of this work extends to repair and maintenance applications, where AM-fabricated components may need to be joined to existing conventional parts. Without proper understanding of the dissimilar joint behavior, engineers risk producing joints that are significantly weaker than either parent material, potentially leading to premature failure in service. This study provides a solid foundation for developing reliable joining protocols that can be confidently applied in production environments.