Effect of Different Welding Wires on Microstructure and Mechanical Properties of Spray-Formed 7055 Aluminum Alloy Welds

Literature Overview

The paper by Qian Xu, Li Xiaoping, and Yang Zhenxing from Jiangsu University of Science and Technology, published in Hot Working Technology in 2015 (Vol. 44, No. 21, pp. 17–20), investigates the effect of different welding wires (ER4043 and ER5356) on the microstructure and mechanical properties of MIG welds in spray-formed 7055 aluminum alloy. The study was funded by the National Natural Science Foundation of China (51341010). The research addresses the important question of filler metal selection for welding advanced aluminum alloys produced by spray forming, a relatively novel manufacturing process.

Background on Spray-Formed 7055 Aluminum Alloy

AA7055 is a high-strength Al-Zn-Mg-Cu alloy belonging to the 7xxx series. It is used in aerospace applications where high strength-to-weight ratio is critical. The alloy typically contains approximately 5.5–6.5 wt% Zn, 2.0–2.8 wt% Mg, and 0.2–0.5 wt% Cu. In the T73 temper condition, 7055 exhibits tensile strengths exceeding 500 MPa.

Spray forming is a near-net-shape manufacturing process that produces solid or semi-solid metallic parts with a fine, equiaxed microstructure. The process involves atomizing molten or semi-solid metal into fine droplets that are deposited onto a substrate to build up a component. The resulting microstructure typically features:

• Fine equiaxed grains (typically 10–50 μm)
• Uniform distribution of precipitates
• Low porosity and high density
• Improved mechanical properties compared to cast or wrought counterparts

The spray-formed microstructure is particularly advantageous because the fine grain size and uniform precipitate distribution provide high strength and improved resistance to stress corrosion cracking (SCC) compared to conventional wrought 7055.

Welding Wire Selection and Metallurgical Considerations

The selection of filler wire for welding 7055 alloy is critical because the weld metal composition directly influences the microstructure, mechanical properties, and service behavior of the joint. The two wires evaluated in this study represent fundamentally different metallurgical approaches:

ER4043 (Al-5Si) Filler Wire

ER4043 is a silicon-based aluminum welding wire that produces weld metal with excellent fluidity and low hot cracking susceptibility. The silicon addition lowers the melting range of the weld metal, promoting complete solidification and reducing the tendency for hot cracks. However, the weld metal strength is relatively low (approximately 140 MPa), and the weld metal is not age-hardenable.

When used for welding 7055 alloy, ER4043 dilutes the Zn, Mg, and Cu content of the base metal in the weld zone. The resulting weld metal composition is a complex Al-Si-Zn-Mg-Cu alloy with a microstructure that differs significantly from both the base metal and the pure ER4043 weld metal. The dilution effect is particularly important because it affects the type and distribution of precipitates that form in the weld metal during solidification and subsequent aging.

ER5356 (Al-5Mg) Filler Wire

ER5356 is a magnesium-based aluminum welding wire that produces weld metal with higher strength than ER4043. The magnesium addition promotes the formation of AlMg2 precipitates, which contribute to the weld metal strength. However, ER5356 has a higher melting range than ER4043, which increases the hot cracking susceptibility of the weld metal.

When used for welding 7055 alloy, ER5356 adds magnesium to the dilution mixture while diluting Zn and Cu. The resulting weld metal is an Al-Mg-Zn-Cu alloy with a different precipitate chemistry than ER4043 welds. The presence of both Mg and Zn in the weld metal can promote the formation of complex precipitates (such as η-AlZnMg) that may contribute to strength but also increase cracking susceptibility.

Microstructural Analysis

The microstructural differences between the two welds are substantial and have direct implications for mechanical performance:

ER4043 Weld Microstructure

The ER4043 weld joint exhibits:

• Weld zone: Predominantly equiaxed grains with some dendritic structures. The equiaxed grain morphology is attributed to the high fluidity of the Al-Si weld metal and the relatively rapid solidification rate in MIG welding.
• Fusion boundary: Primarily equiaxed grains, indicating complete melting and re-solidification of the base metal at the fusion boundary.
• Heat-affected zone (HAZ): Equiaxed grain structure, with grain size slightly larger than the base metal due to partial recrystallization.
• Fracture mode: Cleavage fracture, indicating brittle failure with limited plastic deformation.

The equiaxed grain structure in the weld zone is beneficial for ductility and resistance to cracking, but the low strength of the weld metal (145 MPa) limits the overall joint performance.

ER5356 Weld Microstructure

The ER5356 weld joint exhibits:

• Weld zone: Predominantly columnar grains. The columnar grain morphology is attributed to the directional solidification pattern in the MIG weld pool, where grains nucleate at the fusion boundary and grow toward the weld center.
• Fusion boundary: Columnar grains extending from the fusion boundary into the weld zone.
• HAZ: Coarse equiaxed grains, indicating significant grain growth due to the higher heat input associated with the Al-Mg weld metal.
• Fracture mode: Fatigue fracture, indicating a more ductile failure mode with evidence of crack propagation.

The columnar grain structure in the weld zone can be detrimental because it creates a preferred path for crack propagation along the columnar grain boundaries. The coarse equiaxed grains in the HAZ indicate significant softening due to precipitate dissolution and coarsening.

Mechanical Property Comparison

The mechanical property data reveals several important observations:

1. ER5356 provides higher joint strength: The 190 MPa tensile strength of the ER5356 weld is 31% higher than the 145 MPa strength of the ER4043 weld. This is attributed to the higher strength of the Al-Mg weld metal and the different precipitate chemistry in the dilution zone.
2. ER5356 provides better ductility: The 2.92% elongation of the ER5356 weld is 31% higher than the 2.23% elongation of the ER4043 weld. The fatigue fracture mode of the ER5356 weld indicates better crack resistance compared to the cleavage fracture of the ER4043 weld.
3. Both welds exhibit significant strength loss: The strength retention of both welds is well below 50% of the base metal strength. This is a fundamental limitation of welding precipitation-hardened aluminum alloys, as the HAZ softening cannot be avoided by filler metal selection alone.
4. Hardness anomaly: The ER4043 weld shows a higher hardness (200 HV) than the ER5356 weld (90 HV), which is counterintuitive given the lower tensile strength. This discrepancy may be attributed to the measurement location (hardness may have been measured in the HAZ rather than the weld zone) or the different precipitate distributions in the two welds.

Engineering Practice Implications

For engineering applications involving spray-formed 7055 aluminum alloy, the filler metal selection must be guided by the specific service requirements:

• Strength-critical applications: ER5356 is preferred due to its higher joint strength and better ductility. However, the increased hot cracking susceptibility requires careful process parameter control and possibly preheat.
• Cracking-sensitive applications: ER4043 is preferred due to its lower hot cracking susceptibility. The lower joint strength may be acceptable for non-critical structural components.
• Post-weld heat treatment: Both welds would benefit from post-weld aging to restore some of the lost strength. However, the aging response of the weld metal depends on the filler wire composition and dilution ratio, and must be optimized for each filler wire.
• Welding process parameters: The welding current, voltage, and travel speed must be optimized to minimize the HAZ width and reduce the extent of softening. Lower heat input generally preserves more of the base metal properties but may increase the risk of incomplete penetration.

Study Insights and Reflections

This research highlights the critical importance of filler metal selection in welding advanced aluminum alloys. The differences between ER4043 and ER5356 are not merely quantitative but qualitative, affecting the microstructure, fracture behavior, and service performance of the joint.

The spray-formed microstructure of 7055 provides a unique starting point for welding studies because the fine, equiaxed grain structure and uniform precipitate distribution differ from conventional wrought or cast alloys. The welding process inevitably disrupts this beneficial microstructure, and the filler metal selection determines the extent and nature of the disruption.

Future research should explore alternative filler metals, such as Al-Zn-Mg-Cu wires that more closely match the base metal composition, or advanced filler metals with tailored precipitate chemistry. Additionally, the development of welding process parameters that minimize the HAZ width and preserve the spray-formed microstructure outside the weld zone would be valuable.