Effects of Different Welding Wires on Pulse MIG Weld Microstructure and Mechanical Properties of Al-Zn-Mg Aluminum Alloys

Literature Overview

This 2018 research from Hebei University of Science and Technology, funded by the Hebei Provincial Science and Technology Program (Project No. 15214705D), systematically investigates the influence of different filler metals on the weld microstructure and mechanical properties of Al-Zn-Mg series aluminum alloys when using pulse MIG welding. The study by Diao Guangyun, Wang Dianlong, Li Haichuan, and Liang Zhimin addresses a practical engineering challenge: selecting the optimal filler metal to balance weldability, mechanical strength, and corrosion resistance in high-strength aluminum alloy applications.

Core Technical Points

Filler Metal Selection Matrix

The study evaluates multiple filler metal options for Al-Zn-Mg alloys (primarily AA7075 and AA7050):

Microstructural Analysis

The pulse MIG welding process, with its controlled heat input and droplet transfer characteristics, produces distinct microstructural zones that vary significantly with filler metal selection:

• Weld metal: ER7075 produces a fine dendritic microstructure with retained strengthening precipitates (η' and T'), while ER5356 forms a coarser equiaxed structure with β-phase (Mg2Al3) networks at grain boundaries.
• HAZ: The thermal cycle of pulse MIG welding (peak temperature ~500-600°C, cooling rate ~10-30°C/s) causes significant precipitate dissolution in the T6 base metal. The extent of HAZ softening depends on the thermal cycle parameters rather than the filler metal, but the residual stress state is influenced by the coefficient of thermal expansion mismatch at the weld/HAZ interface.
• Interface zone: The Al2Cu intermetallic phase formation at the fusion boundary is most pronounced with ER4043 (Si-containing filler), creating a brittle layer that significantly reduces fatigue performance.

Mechanical Property Results

Process-Property Relationships

The pulse MIG parameters used in this study were carefully controlled to minimize thermal distortion while ensuring complete fusion:

• Peak current: 180-220 A
• Background current: 60-80 A
• Pulse frequency: 100-150 Hz
• On-time: 0.12-0.18 s
• Travel speed: 350-450 mm/min
• Shielding gas: 100% Ar at 18-22 L/min

The pulsed transfer mode offers superior control over droplet detachment compared to short-circuit or spray transfer, resulting in reduced spatter, narrower weld beads, and more uniform heat input. This is particularly important for thin-walled pressure vessel components where excessive heat input can cause distortion and residual stress.

Engineering Application and Reflections

For pressure vessel applications involving Al-Zn-Mg alloys, the selection of filler metal represents a fundamental trade-off between strength matching and crack resistance. In high-pressure applications where strength retention is critical (such as aerospace pressure vessels), ER7075 may be preferred despite its cracking susceptibility, provided that adequate preheating and post-weld heat treatment are implemented. In corrosion-critical environments (such as marine or chemical processing), ER5183 offers the best combination of adequate strength and superior corrosion resistance.

The key insight from this study is that there is no universally optimal filler metal for Al-Zn-Mg alloy welding—the selection must be driven by the specific service requirements, environmental conditions, and applicable codes. This parallels the decision-making process in bimetal pressure vessel fabrication, where the selection of overlay material must balance corrosion resistance, bonding strength, and mechanical compatibility with the base material.