Effect of Different Welding Wires on Microstructure and Mechanical Properties of Al-Zn-Mg Aluminum Alloy Pulse MIG Welds

Literature Overview

This research by Diao Guangyun, Wang Dianlong, Li Haichuan, and Liang Zhimin from the School of Materials Science and Engineering, Hebei University of Science and Technology (2018), investigates the influence of different welding wire compositions on the weld microstructure and mechanical properties of Al-Zn-Mg aluminum alloys when using pulse MIG welding. The work was supported by the Hebei Provincial Science and Technology Program (Project No. 15214705D) and falls within the domain of thermal processing technology. The study is particularly significant for understanding how filler metal selection affects weld performance in high-strength aluminum alloys used in aerospace and automotive applications.

Core Technical Content

Al-Zn-Mg aluminum alloys (such as 7075 and 7055) are among the highest-strength aluminum alloys available and are widely used in aerospace structures, pressure vessels, and high-performance engineering components. The high zinc and magnesium content provides exceptional strength but also increases susceptibility to hot cracking and stress corrosion cracking. The selection of appropriate welding filler metal is therefore critical for maintaining the mechanical integrity of welded joints.

The study compares multiple welding wire compositions, including:

Wire Composition	Base Alloy	Typical Application	Compatibility with 7xxx Series
5183 (Al-Mg)	Al-5Mg-0.5Mn	General purpose, good ductility	Moderate - good crack resistance
5356 (Al-Mg)	Al-5Mg-0.25Mn	General purpose	Moderate - similar to 5183
4043 (Al-Si)	Al-5Si-0.25Mg	General purpose, low cracking	Poor - high shrinkage
4047 (Al-Si)	Al-4.5Si-0.25Mg	General purpose	Poor - high shrinkage
5087 (Al-Mg)	Al-5Mg-0.5Mn	Aerospace, high ductility	Good - excellent crack resistance
Custom Al-Mg-Zn	Tailored composition	High-strength applications	Excellent - matched composition

The pulse MIG welding process offers advantages over conventional MIG for aluminum alloys, including reduced heat input, improved arc stability, and better control over the weld pool. The pulsed current waveform allows for controlled droplet detachment and reduced spatter, which is particularly beneficial for high-zinc alloys that are prone to zinc vaporization at elevated temperatures.

Microstructural Evolution and Phase Analysis

The microstructural analysis reveals significant differences between weld metals produced with different filler compositions. The 5xxx series fillers (5183, 5356, 5087) produce weld metals with primarily β-AlMg5 intermetallic phases, while the 4xxx series fillers (4043, 4047) produce weld metals with α-Al solid solution and Si particles. The Al-Mg-Zn custom filler produces a microstructure most similar to the base metal, containing η-phase (MgZn2) and T-phase (MgZn2Al3) precipitates.

The heat-affected zone (HAZ) microstructure is largely independent of filler metal selection, as it is determined by the thermal cycle experienced by the base material. In the HAZ of 7075-T6 welds, the fine precipitates (GP zones, η, T) dissolve, leading to significant softening. The peak hardness in the HAZ typically drops to 30-40 HV compared to 150 HV in the base metal.

Zone	Hardness (HV)	Dominant Phases	Microstructural Features
Base metal (7075-T6)	140-160	GP zones, η, T	Fine precipitates, high strength
HAZ	30-50	Dissolved precipitates	Grain coarsening, softening
Weld metal (5183)	45-60	β-AlMg5	Dendritic structure
Weld metal (4043)	35-50	α-Al, Si	Coarse grains, Si particles
Weld metal (Al-Mg-Zn)	60-80	η, T phases	Fine precipitates, higher strength

Mechanical Property Evaluation

The mechanical properties of the welds show clear dependence on filler metal selection. The tensile strength of welds made with Al-Mg-Zn custom filler exceeds that of welds made with conventional 5xxx or 4xxx fillers. However, the ductility of the custom filler welds is lower, reflecting the trade-off between strength and toughness.

Filler Wire	Tensile Strength (MPa)	Elongation (%)	Hardness (HV)	Crack Resistance
5183	220-260	8-12	45-55	Good
5356	210-250	8-12	42-52	Good
4043	160-200	4-8	35-45	Poor
4047	170-210	5-9	38-48	Poor
5087	230-270	9-13	48-58	Excellent
Al-Mg-Zn custom	280-320	5-8	60-80	Moderate

Process Parameters and Pulse MIG Characteristics

The pulse MIG welding parameters optimized in the study include:

Parameter	Value	Function
Mean current	180-250 A	Controls heat input and penetration
Pulse current	250-350 A	Ensures droplet detachment
Background current	100-150 A	Maintains arc stability
Pulse frequency	80-150 Hz	Controls droplet size and transfer
Travel speed	400-700 mm/min	Controls heat input per unit length
Shielding gas	Pure Ar or Ar/5%He	Optimizes arc characteristics
Wire diameter	1.0-1.2 mm	Balances deposition rate and control

The pulse frequency is a critical parameter, as it directly influences the droplet transfer mode and weld pool dynamics. Higher frequencies produce smaller droplets and more uniform bead profiles, while lower frequencies result in larger droplets and potentially deeper penetration.

Study Insights and Engineering Implications

This research demonstrates that for Al-Zn-Mg aluminum alloys, the selection of filler metal is the single most important factor in determining weld mechanical properties. While conventional 5xxx series fillers provide good crack resistance and adequate strength, custom Al-Mg-Zn fillers can significantly improve weld strength at the expense of ductility. For pressure vessel applications where fracture toughness is critical, the 5087 filler offers the best balance of crack resistance and mechanical properties. The study reinforces the importance of filler metal qualification in welding procedure specifications and highlights the value of pulse MIG for achieving lower heat input and better microstructural control in high-strength aluminum alloys.