CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Different Welding Wires on Microstructure and Properties of 7075 Aluminum Alloy Double-Pulse MIG Welds
Literature Overview
The research by Liu Changjun, Sun Jia, Zhang Wei, Zhang Tianming, and Liu Zhengjun from Shenyang University of Technology, published in Thermal Processing Technology in 2016, investigates the influence of different welding wire compositions on the microstructure and mechanical properties of 7075 aluminum alloy welds produced by double-pulse MIG welding. 7075 aluminum alloy is a high-strength precipitation-hardened alloy widely used in aerospace structures, and its welding is challenging due to its susceptibility to hot cracking, significant HAZ softening, and the difficulty of achieving full-strength welds. This study addresses the critical role of filler metal selection in optimizing weld quality for this demanding alloy.
Core Technical Viewpoints
The authors examined several welding wires including 4043, 5183, and 5356 aluminum alloys, as well as a custom-designed filler wire, and compared their effects on the weld microstructure, mechanical properties, and crack susceptibility of 7075-T6 aluminum alloy welds produced by double-pulse MIG welding. The central finding is that the filler metal composition has a profound effect on the weld properties, with 5356 aluminum alloy wire providing the best combination of mechanical properties and crack resistance among the commercial wires tested. The custom-designed wire, which incorporated specific alloying additions to control solidification behavior, showed even better performance in some respects but required careful parameter optimization to avoid porosity.
A particularly important insight from this work is the role of the double-pulse MIG welding process in controlling the weld pool dynamics and solidification behavior. The double-pulse waveform, which uses two distinct current pulses within each cycle, allows independent control of the droplet transfer and the weld pool stirring, resulting in a more stable arc and improved weld quality compared to conventional MIG welding. The authors demonstrated that the double-pulse waveform reduces the peak current and minimizes the heat input, which is beneficial for minimizing HAZ softening in 7075 aluminum alloy.
Welding Wire Comparison
The following table summarizes the composition and properties of the welding wires tested in this study:
| Wire Type | Composition (wt%) | Tensile Strength (MPa) | Crack Susceptibility | HAZ Softening |
|---|---|---|---|---|
| 4043 | Si 5.0, Mn 0.5 | 200 | Low | Moderate |
| 5183 | Mn 1.0, Mg 0.3 | 220 | Low | Moderate |
| 5356 | Mg 4.5 | 280 | Low | Severe |
| Custom Wire | Mg 4.5, Zn 1.0, Zr 0.1 | 300 | Low | Moderate |
The table shows that 4043 aluminum alloy wire, which contains silicon as the primary alloying element, produces welds with low tensile strength but excellent crack resistance due to its low solidification cracking susceptibility. 5183 aluminum alloy wire, containing manganese and magnesium, offers slightly higher strength with comparable crack resistance. 5356 aluminum alloy wire, which is the closest in composition to 7075 aluminum alloy, provides the highest weld strength but also causes the most severe HAZ softening due to the high magnesium content promoting precipitate dissolution in the base metal. The custom-designed wire, which incorporates zinc and zirconium additions, achieves a strength level comparable to 5356 wire while reducing HAZ softening through the grain refining effect of zirconium and the precipitation strengthening effect of zinc.
Microstructural Analysis
The metallographic examination revealed significant differences in the microstructure of welds produced with different wires. Welds produced with 4043 wire exhibited a fine equiaxed grain structure with a high volume fraction of fine Si particles, which contributed to their excellent crack resistance but limited their strength. Welds produced with 5356 wire showed a coarse columnar grain structure with a high density of Mg2Si precipitates, which provided high strength but also increased the susceptibility to stress corrosion cracking due to the formation of a continuous network of precipitates along the grain boundaries.
The custom-designed wire produced welds with a refined grain structure and a more uniform precipitate distribution, resulting in improved mechanical properties and reduced stress corrosion cracking susceptibility. The authors attributed this improvement to the grain refining effect of zirconium, which promotes heterogeneous nucleation during solidification, and the precipitation strengthening effect of zinc, which forms fine Zn-rich precipitates that strengthen the weld matrix without promoting grain boundary precipitation.
Double-Pulse MIG Welding Process Characteristics
The double-pulse MIG welding process used in this study employed a waveform with a high-amplitude pulse followed by a low-amplitude pulse within each cycle. The high-amplitude pulse provides the energy necessary for droplet detachment from the wire tip, while the low-amplitude pulse maintains the arc stability and provides additional heat input to the weld pool. The following table summarizes the key parameters of the double-pulse MIG welding process:
| Parameter | Typical Value | Function |
|---|---|---|
| Base Current | 150 to 200 A | Maintains arc stability |
| High Pulse Current | 300 to 400 A | Detaches droplets from wire tip |
| Low Pulse Current | 100 to 150 A | Provides additional heat input |
| Pulse Frequency | 50 to 200 Hz | Controls droplet transfer frequency |
| Travel Speed | 0.5 to 1.5 m/min | Controls heat input per unit length |
| Shielding Gas | Argon 100% or Argon/Helium mix | Protects weld pool from oxidation |
The double-pulse waveform offers several advantages over conventional MIG welding for 7075 aluminum alloy. The independent control of droplet transfer and heat input allows optimization of the weld pool dynamics for improved penetration and reduced spatter. The lower average current compared to conventional MIG welding reduces the heat input and minimizes HAZ softening, which is critical for maintaining the strength of the weld joint. The more stable arc produced by the double-pulse waveform also reduces the incidence of porosity and other defects that can compromise weld quality.
Engineering Applications and Relevance to Cladding
The findings of this study have direct implications for the welding of 7075 aluminum alloy in pressure vessel and structural applications. For example, in the fabrication of aerospace pressure vessels or cryogenic storage tanks, 7075 aluminum alloy is often used for its high strength-to-weight ratio, and the welding process must be carefully controlled to maintain the required mechanical properties. The selection of an appropriate filler metal, such as the custom-designed wire or 5356 aluminum alloy wire, is critical to achieving acceptable joint efficiency and crack resistance.
For cladding and overlay applications involving 7075 aluminum alloy, such as the application of corrosion-resistant coatings on aluminum pressure vessels, the principles demonstrated in this study apply directly to the selection of welding processes and filler metals. The double-pulse MIG welding process, with its ability to control heat input and droplet transfer independently, is particularly suitable for overlay welding where precise control of the overlay layer composition and thickness is required. The use of a custom-designed filler metal, tailored to the specific requirements of the overlay application, can optimize the balance between mechanical properties, crack resistance, and corrosion resistance of the overlay layer.
Key Questions and Reflections
One question that emerges from this work is the scalability of the double-pulse MIG welding process to thick-section 7075 aluminum alloy welding. While the process has been demonstrated to produce high-quality welds in thin to moderate thickness sections, its effectiveness in thick sections, where multi-pass welding is required, has not been fully established. The control of interpass temperature and the accumulation of heat input over multiple passes can significantly affect the final weld properties, and further research is needed to define the optimal welding procedure for thick-section applications.
Another consideration is the cost-effectiveness of using custom-designed filler wires for 7075 aluminum alloy welding. While the custom wire offers superior performance in terms of mechanical properties and crack resistance, its cost may be significantly higher than commercial wires such as 5356 aluminum alloy wire. Engineers must evaluate the total cost of ownership, including the cost of the filler metal, the cost of welding defects and repairs, and the cost of post-weld heat treatment, to determine whether the use of a custom wire is justified for a given application. For high-value aerospace applications where weld quality is critical, the additional cost of a custom wire may be justified, but for general industrial applications, commercial wires may offer an acceptable balance of performance and cost.
Study Insights and Implications
This research provides valuable guidance for engineers welding 7075 aluminum alloy, highlighting the critical role of filler metal selection and welding process optimization in achieving acceptable weld quality. The findings underscore the importance of matching the filler metal composition to the base metal composition to minimize HAZ softening and maximize joint efficiency, while also considering the crack resistance and corrosion resistance requirements of the application. For cladding and overlay applications involving 7075 aluminum alloy, the double-pulse MIG welding process offers a promising approach to achieving high-quality overlay layers with controlled dilution and excellent mechanical properties. Engineers should carefully evaluate the welding procedure, filler metal selection, and post-weld heat treatment requirements to ensure that the final weld joint meets the required performance specifications in accordance with the applicable standards for aerospace and pressure vessel applications.