Effect of Filler Material Selection on Microstructure and Properties of MIG Welded Joints of 10 percent SiCp/6061Al Composite

Literature Overview

This paper by Ba Xianli, Gao Zeng, and Niu Jitai from Henan University of Science and Technology and Harbin Institute of Technology was published in the International Journal of Heat Treatment (Hot Working Technology) in 2020, Volume 49, Issue 17, pages 33-37. The study investigates the influence of two different filler wire materials, ER4043 and ER5356, on the microstructure and mechanical properties of MIG welded joints of 10% SiC particle reinforced 6061 aluminum matrix composite plates with a thickness of 4 mm.

Core Technical Content and Key Findings

The primary challenge in welding aluminum matrix composites (AMCs) is the formation of brittle intermetallic phases, particularly Al4C3, which forms when the SiC reinforcement reacts with molten aluminum. This phase is thermodynamically unstable and can further decompose, leading to porosity and significant strength reduction in the weld zone. The authors address this challenge by comparing two commonly used aluminum welding wires: ER4043 (Al-Si, approximately 5% Si) and ER5356 (Al-Mg-Si, approximately 5% Mg and 0.5% Si).

The experimental results reveal distinct differences between the two filler materials. The ER4043 filler produced joints with a higher average tensile strength of 136.23 MPa, which corresponds to 37.84% of the base material strength. In contrast, the ER5356 filler produced joints with lower tensile strength. The hardness distribution in both joints showed a symmetric profile with the minimum values occurring at the weld center, measuring 33.4 HV for ER4043 and 71.6 HV for ER5356.

The most critical metallurgical finding is that brittle Al4C3 phase formed in both welds, but with significant differences in morphology and quantity. In the ER5356 weld, Al4C3 appeared more abundantly in a fine needle-like morphology, while in the ER4043 weld, it was less abundant and appeared as short rod-like structures. The silicon content in ER4043 plays a crucial role in suppressing Al4C3 formation by preferentially reacting with carbon from SiC decomposition, thereby reducing the carbon availability for Al4C3 nucleation.

Microstructural Analysis and Phase Formation Mechanisms

The following table summarizes the comparative microstructural and mechanical characteristics of the two filler materials:

The XRD analysis confirmed the presence of Al4C3 in both welds, validating the metallographic observations. The scanning electron microscopy of fracture surfaces revealed that both joints exhibited ductile fracture characteristics, indicating that despite the presence of brittle phases, the overall fracture behavior was governed by the ductile aluminum matrix. This is an important finding because it suggests that the joints, while weakened, do not exhibit catastrophic brittle failure under tensile loading.

The formation mechanism of Al4C3 can be explained by the reaction: 4Al + 3C → Al4C3, where carbon is released from the decomposition of SiC particles during the high-temperature welding process. The silicon in ER4043 acts as a carbon scavenger, reacting preferentially to form Al4SiC4 or other Si-C compounds, thereby reducing the carbon activity available for Al4C3 formation. The magnesium in ER5356 does not provide this scavenging effect, leading to more extensive Al4C3 formation.

Engineering Practice Implications

For engineers working with aluminum matrix composites, this study provides clear guidance on filler material selection. ER4043 is the preferred choice for MIG welding of SiC particle reinforced aluminum composites due to its superior ability to suppress Al4C3 formation and its resulting better mechanical properties. However, the tensile strength of only 37.84% of base material strength highlights the fundamental challenge of welding AMCs, where the SiC reinforcement is inevitably lost or degraded in the weld zone.

The following practical recommendations emerge from this study:

• ER4043 filler wire should be the first choice for MIG welding of SiC/Al composites to minimize brittle phase formation.
• Welding parameters should be optimized to minimize the time the base material spends above the SiC decomposition temperature, thereby reducing carbon release.
• Post-weld heat treatment may be considered to improve joint properties, although this was not investigated in the present study.
• The joint strength limitation means that design considerations must account for the weakened weld zone, potentially requiring increased section thickness or alternative joint designs.

Key Questions and Reflections

An important unanswered question from this study is whether alternative filler materials, such as Al-Si-Mg-Sn or other modified compositions, could further suppress Al4C3 formation and improve joint properties. The study also does not address the effect of welding parameters such as heat input, travel speed, and shielding gas composition on the extent of SiC degradation and Al4C3 formation. Furthermore, the long-term performance of these joints under cyclic loading or in corrosive environments, which are relevant for many AMC applications, remains uninvestigated.

The finding that ER5356 produces higher hardness (71.6 HV vs. 33.4 HV) at the weld center despite lower tensile strength is somewhat counterintuitive and warrants further investigation. This may be attributed to the different precipitate distributions and grain structures resulting from the different alloying elements in the two fillers.

Study Insights and Conclusions

This paper makes a valuable contribution to the understanding of filler material effects on the weldability of SiC particle reinforced aluminum composites. The clear demonstration that ER4043 outperforms ER5356 in suppressing Al4C3 formation and improving mechanical properties provides actionable guidance for engineers. The combination of tensile testing, hardness mapping, optical microscopy, XRD, and SEM fracture analysis offers a comprehensive characterization approach. The fundamental limitation of AMC weldability due to SiC degradation remains a challenge, but the selection of appropriate filler materials is a critical factor in optimizing joint quality, and ER4043 represents the best available option among conventional aluminum welding wires for this application.