Ultrasonic Impact Treatment Effects on 7A52 Aluminum Alloy VPPA-MIG Composite Welded Joints

Overview and Significance

The study by Chen Furong, Liu Chenghao, and Li Nan from Inner Mongolia University of Technology (Welding Journal, 2020, Vol. 41, No. 9, pp. 39-43) investigates the influence of ultrasonic impact treatment (UIT) time on the microstructure and mechanical properties of 7A52 aluminum alloy joints produced by VPPA-MIG composite welding. This research is significant for two reasons: first, 7A52 is a high-strength Al-Zn-Mg-Cu alloy widely used in aerospace structural components; second, VPPA-MIG composite welding represents an advanced hybrid welding process that combines the deep penetration of plasma arc with the high deposition rate of MIG.

Process Description and Experimental Design

VPPA-MIG (Variable Polarity Plasma Arc - Metal Inert Gas) composite welding operates by using the plasma arc as the primary heat source for base metal melting and penetration, while the MIG arc provides filler metal deposition. The variable polarity feature allows for cathodic cleaning of oxide films and anodic melting of the filler wire, optimizing the arc characteristics for aluminum welding. The experimental design systematically varied UIT treatment time at six levels: 2.5, 5, 10, 15, 30, and 75 minutes per unit area. This range spans from light peening to intensive treatment, covering the practical window for post-weld stress relief and surface strengthening.

The UIT process uses high-frequency (typically 20-25 kHz) ultrasonic vibration transmitted through a conical tool (indent) that impacts the weld surface at high velocity. Each impact creates a localized plastic deformation zone, introducing residual compressive stresses, refining the near-surface microstructure, and introducing dislocation structures that impede crack propagation.

Key Findings on Microstructure Evolution

The microstructural analysis revealed a clear progression with increasing UIT time. At 15 minutes of treatment, the weld surface grain size reached 26.28 nm — an extraordinary refinement from the typical grain sizes in the as-welded condition (often in the range of 100-500 nm in the weld metal of aluminum alloys). This nanocrystalline structure results from the severe plastic deformation induced by repeated ultrasonic impacts, which breaks down larger grains and creates high-density dislocation networks.

The hardness increase of 36.3% at 75 minutes (reaching 138.8 HV) is substantial for aluminum alloy welds, which typically exhibit lower hardness than the base metal due to grain coarsening and precipitate dissolution during welding. The UIT process effectively counteracts this softening through mechanical strengthening mechanisms — work hardening, grain refinement, and dislocation density increase.

Residual Stress Behavior

The residual stress results show a non-monotonic relationship with UIT time. The maximum compressive stress of 243.3 MPa was achieved at 30 minutes, not at the longest treatment time of 75 minutes. This finding is physically intuitive: at low UIT times, insufficient plastic deformation is induced to generate substantial compressive stresses. At intermediate times (around 30 minutes), the balance between plastic deformation and elastic recovery is optimal for compressive stress development. At extended times (75 minutes), the accumulated plastic deformation may lead to strain hardening of the surface layer, reducing the incremental plastic strain per impact and consequently diminishing the compressive stress increment. Additionally, over-treatment may introduce surface damage or micro-cracking that compromises the beneficial stress state.

This non-monotonic behavior has direct implications for engineering practice. The optimal UIT time for residual stress relief is not the longest possible time — it must be determined through systematic testing for each specific weld configuration and material.

Engineering Implications for High-Strength Aluminum Welding

For aerospace and high-performance structural applications where 7A52 or similar Al-Zn-Mg-Cu alloys are used, the UIT post-weld treatment offers a practical solution to the persistent problem of weld softening and residual tensile stresses. These alloys are particularly susceptible to stress corrosion cracking (SCC) in the HAZ and weld metal, where precipitate-free zones and residual tensile stresses combine to create a vulnerable microstructure. The introduction of residual compressive stresses through UIT can significantly improve SCC resistance, as compressive stresses oppose crack opening and reduce the driving force for crack propagation.

The VPPA-MIG composite welding process itself is noteworthy for aluminum alloy welding. Compared to conventional TIG welding, it offers higher deposition rates while maintaining good penetration. Compared to conventional MIG welding, it provides better arc stability and reduced spatter. The variable polarity feature is particularly beneficial for aluminum welding, as it enables periodic cathodic cleaning of the oxide film without requiring excessive gas flow rates.

Critical Assessment and Practical Considerations

While the results are encouraging, several practical considerations must be addressed for industrial implementation. First, the UIT process is inherently slow — treating a large weld surface at 15-30 minutes per unit area is impractical for large-scale production. The unit area basis of the experiments does not translate directly to production throughput. Second, the study does not address the effect of UIT on the fatigue properties of the joint, which is the primary criterion for aerospace structural qualification. Third, the influence of UIT on the weld geometry (bead profile, reinforcement) is not discussed — intensive UIT can flatten or distort the weld bead surface.

The study also does not compare the UIT-treated joints with other post-weld treatments such as T6 tempering, laser peening, or shot peening. A comparative analysis would provide context for the relative advantages of UIT in terms of cost, equipment requirements, and achievable property improvements.

Summary

This study provides valuable quantitative data on the effects of UIT time on 7A52 aluminum alloy VPPA-MIG welded joints, demonstrating significant grain refinement, hardness improvement, and residual compressive stress development. The optimal UIT time for compressive stress generation (30 minutes) differs from that for maximum hardness (75 minutes), highlighting the need for property-specific optimization. For aerospace and high-performance aluminum structures, UIT represents a promising post-weld treatment to mitigate weld softening and improve stress corrosion resistance, though production scalability and fatigue performance validation remain open challenges.