Effect of Filler Material on MIG Welded Joints of 10%SiCp/6061Al Composite

Literature Overview

The research by Ba Xianli, Gao Zeng, and Niu Jitai, published in "Hot Working Technology" in 2020 (Volume 49, Issue 17, pages 33-37), investigates the influence of filler material selection on the microstructure and mechanical properties of MIG welded joints in 4 mm thick 10%SiCp/6061Al aluminum matrix composites. Two filler wires, ER4043 and ER5356, were evaluated through tensile testing, hardness testing, metallographic examination, X-ray diffraction (XRD) phase analysis, and scanning electron microscopy (SEM) fractography. The classification number TG407 indicates this work falls within the domain of welding process and joint design.

Core Technical Findings

The study demonstrates that the choice of filler material has a profound effect on the weldability and joint properties of SiC particle-reinforced aluminum composites. The ER4043 filler wire produced joints with higher tensile strength compared to ER5356, with an average tensile strength of 136.23 MPa representing 37.84% of the base metal strength. Both joints exhibited symmetric hardness distributions with minimum values at the weld center, but the absolute hardness values differed significantly between the two filler materials.

Mechanical Property Comparison

The significant difference in weld hardness between the two joints (33.4 HV for ER4043 versus 71.6 HV for ER5356) is attributed to the different compositions of the filler metals and the resulting phase compositions in the weld zone. ER4043 contains approximately 5% silicon, while ER5356 contains approximately 5% magnesium. These compositional differences lead to fundamentally different solidification behaviors and phase formation in the weld metal.

Microstructural Analysis and Phase Formation

The most critical finding of this study is the formation of the brittle intermetallic compound Al4C3 in both joints, which results from the reaction between aluminum and the SiC reinforcement particles during the high-temperature welding process. However, the morphology and quantity of Al4C3 differ significantly between the two joints. In the ER5356 joint, Al4C3 appears as fine needle-like structures in relatively large quantities, while in the ER4043 joint, Al4C3 appears as short rod-like structures in smaller quantities.

Role of Silicon in Suppressing Al4C3 Formation

The silicon content in ER4043 plays a crucial role in suppressing the formation of Al4C3. The mechanism involves the preferential reaction of silicon with carbon from the decomposed SiC particles to form SiC or other silicon carbide compounds rather than allowing the carbon to react with aluminum to form Al4C3. This competitive reaction effectively reduces the amount of free carbon available for Al4C3 formation, thereby improving the weldability of the composite.

The presence of Al4C3 is detrimental to weld joint properties because it is a hard, brittle phase that can act as crack initiation sites and reduce ductility. The needle-like morphology of Al4C3 in the ER5356 joint is particularly concerning because sharp, elongated particles are more likely to initiate cracks under applied stress compared to the shorter, more equiaxed particles found in the ER4043 joint.

Engineering Practice and Recommendations

For practical welding of SiC particle-reinforced aluminum composites, the selection of ER4043 filler wire is strongly recommended over ER5356 based on the findings of this study. The higher tensile strength, lower Al4C3 content, and more favorable Al4C3 morphology all contribute to improved joint reliability. However, the overall strength retention of 37.84% remains relatively low, indicating that significant challenges remain in welding aluminum matrix composites.

Process Optimization Considerations

To further improve the weldability of SiCp/Al composites, several process optimization strategies should be considered. These include reducing the heat input to minimize SiC decomposition, using pulsed MIG welding to control the thermal cycle, and potentially employing filler wires with higher silicon content to further suppress Al4C3 formation. The application of preheating and post-weld heat treatment may also help to improve the microstructure and properties of the weld zone, although these must be balanced against the risk of further SiC decomposition.

For pipe and fitting applications involving aluminum matrix composites, the welding challenges identified in this study have direct implications for manufacturing and repair operations. The formation of brittle Al4C3 phases represents a fundamental limitation that must be managed through careful selection of filler materials and welding parameters. Engineers should conduct thorough qualification testing of welding procedures for composite materials, as the welding behavior may differ significantly from that of the unreinforced base alloy.