Effect of Filler Wire Composition on Microstructure and Mechanical Properties of Dual-Pulse MIG Welded 6061-T6 Aluminum Alloy

Literature Overview

This research by Yi Jie and colleagues from the State Key Laboratory of Advanced Design and Manufacturing Technology of Vehicle Body at Hunan University, published in Ordnance Materials and Science and Engineering in 2015, investigates the influence of filler wire composition on the microstructure and mechanical properties of dual-pulse MIG welded 6061-T6 aluminum alloy thin sheet joints. The study compares two commercially available aluminum alloy filler wires: ER4043 and ER5356, examining their effects on weld metal microstructure, HAZ microstructure, mechanical properties, and weld joint strength ratios.

Filler Wire Composition and Selection Rationale

The selection of ER4043 and ER5356 for comparison is highly representative of common engineering practice in aluminum alloy welding. These two filler wires have fundamentally different compositions and strengthening mechanisms:

ER4043 contains silicon as the primary alloying element, which forms discrete Si particles that provide limited strengthening through particle dispersion. The silicon also promotes eutectic solidification, which helps to reduce hot cracking sensitivity. ER5356 contains magnesium, which combines with aluminum to form Mg2Si precipitates that provide significant strengthening through precipitation hardening.

Microstructural Analysis

The microstructural differences between the two weld metals are striking and directly reflect their compositional differences.

Weld Metal Microstructure:

• ER5356 weld: Fine cast structure with relatively fine dendrites, indicating rapid solidification with high nucleation rate
• ER4043 weld: Coarse cast structure with well-developed dendrites, indicating slower solidification with lower nucleation rate

The finer microstructure of ER5356 welds is attributed to the higher solidification rate promoted by the Mg content, which increases the liquidus temperature and promotes rapid dendrite growth. The finer dendrite spacing in ER5356 welds provides more uniform solidification and reduced segregation of impurities and inclusions.

Fusion Zone Microstructure:

Both filler wires produce columnar grains on the weld side of the fusion zone, with fine equiaxed grains near the HAZ side. This transition from columnar to equiaxed growth is a common feature of weld solidification, driven by the changing thermal gradient and growth rate conditions.

Heat-Affected Zone Microstructure:

The HAZ exhibits grain coarsening compared to the original base metal, which is expected given the elevated temperatures experienced during welding. The quench zone (rapidly cooled region near the fusion boundary) shows some reprecipitation of secondary phases in both cases. However, the secondary phases in ER5356-welded joints are finer and more uniformly distributed compared to ER4043-welded joints.

This difference in HAZ microstructure is significant because the quench zone represents the weakest region of the weld joint, where the original T6 temper has been disrupted by the welding thermal cycle. The finer and more uniformly distributed secondary phases in ER5356-welded joints suggest better property retention in the HAZ.

Mechanical Property Comparison

The mechanical property results clearly demonstrate the superiority of ER5356 filler wire for 6061-T6 aluminum alloy welding:

The welding coefficient (ratio of joint strength to base metal strength) of 0.72 for ER5356 is substantially better than 0.65 for ER4043. This represents a 10.8% improvement in joint strength efficiency, which is highly significant for structural applications where weight optimization is critical.

The microhardness distribution shows a symmetric pattern centered on the weld, with the minimum hardness occurring in the quench zone of the HAZ. This is consistent with the microstructural analysis, where the quench zone experiences the most severe property degradation due to precipitate dissolution without reprecipitation. The ER5356-welded joints exhibit higher overall hardness compared to ER4043-welded joints, with the hardness difference being most pronounced in the weld metal and HAZ regions.

Engineering Practice Considerations

The selection between ER4043 and ER5356 for welding 6061-T6 aluminum alloy involves several practical considerations:

• Strength requirement: ER5356 provides superior joint strength and is the preferred choice for structural applications where strength-to-weight ratio is critical.
• Cracking sensitivity: ER4043 has lower hot cracking sensitivity due to its eutectic solidification behavior, making it more forgiving in production environments with less controlled conditions.
• Weldability: ER5356 requires more careful process control to avoid hot cracking, particularly in thick sections or with high heat input processes.
• Cost: Both filler wires are commercially available at comparable costs, so cost is not a significant differentiator.

For dual-pulse MIG welding specifically, the process parameters should be optimized for each filler wire to take advantage of their respective characteristics. The dual-pulse technique provides additional control over droplet transfer and heat input, which can help mitigate the cracking sensitivity of ER5356 while maintaining its superior mechanical properties.

Study Insights and Recommendations

This study provides clear evidence that filler wire composition has a profound influence on the microstructure and mechanical properties of welded aluminum alloy joints. The 10.8% improvement in welding coefficient achieved by using ER5356 instead of ER4043 is substantial and demonstrates that filler metal selection is a critical design variable that should not be overlooked.

The microstructural analysis reveals that the finer microstructure of ER5356 welds is not merely a cosmetic difference but has direct implications for mechanical properties. Finer dendrite spacing promotes more uniform solidification, reduces segregation of impurities, and provides better resistance to cracking. The finer secondary phases in the HAZ quench zone of ER5356-welded joints suggest better property retention, which is crucial for joint strength.

However, engineers must recognize that the superior mechanical properties of ER5356 come with the trade-off of higher cracking sensitivity. In production environments with less controlled welding conditions, the risk of hot cracking may outweigh the strength benefits. Process development and validation should include careful evaluation of cracking susceptibility under actual production conditions.

In summary, this study demonstrates that ER5356 filler wire provides superior mechanical properties and microstructural characteristics for dual-pulse MIG welded 6061-T6 aluminum alloy joints compared to ER4043. The 10.8% improvement in welding coefficient and the finer, more uniform microstructure make ER5356 the preferred choice for structural applications. However, the higher cracking sensitivity of ER5356 requires careful process control and validation to ensure reliable production welding. Engineers should select filler wire based on the specific application requirements, balancing strength needs against cracking sensitivity and process controllability.