Effect of Accumulative Roll Bonding on Microstructure and Mechanical Properties of AA1060-AA7075 Alloy Composite Plates

Literature Overview and Technical Significance

This study examines the microstructural evolution and mechanical property development in AA1060-AA7075 aluminum alloy composite plates fabricated through the accumulative roll bonding (ARB) process. The combination of AA1060 (a commercial pure aluminum with excellent formability) and AA7075 (a high-strength aluminum-zinc-magnesium-copper alloy) creates a composite that leverages the formability of the pure aluminum and the strength of the high-strength alloy. This is directly relevant to bimetal manufacturing because it demonstrates how the ARB process can create functional composites from materials with vastly different mechanical properties, similar to the concept of cladding a ductile base material with a high-strength or corrosion-resistant overlay.

The AA1060-AA7075 system presents unique challenges due to the large difference in strength, ductility, and work hardening rate between the two alloys. AA1060 has a tensile strength of approximately 80-100 MPa and elongation of 25-40%, while AA7075 has a tensile strength of 500-570 MPa and elongation of only 11-14%. This mismatch creates asymmetric deformation during rolling that must be carefully managed to achieve uniform bonding and avoid defects.

Core Technical Content and Key Findings

Asymmetric Deformation Behavior

The fundamental challenge in ARB processing of AA1060-AA7075 composites is the asymmetric deformation caused by the large strength difference between the two alloys. During rolling:

• The AA1060 layers deform much more easily than the AA7075 layers, leading to non-uniform strain distribution across the composite thickness.
• The softer AA1060 layers tend to spread laterally and may develop wrinkles or folds at the interface, particularly at higher reduction ratios.
• The harder AA7075 layers experience less deformation and may retain larger grain structures compared to the heavily deformed AA1060 layers.
• The interface between the two alloys is subject to shear deformation, which can either promote bonding or cause delamination, depending on the process parameters.

Microstructural Evolution with ARB Cycles

The microstructure of the AA1060-AA7075 composite evolves significantly with increasing ARB cycle number:

After 1 cycle: The initial bonding is achieved with a relatively clean interface. The AA1060 layers show moderate grain refinement, while the AA7075 layers retain much of their original grain structure. The layer thickness is approximately 500 micrometers each.

After 3 cycles: The layer thickness reduces to approximately 60 micrometers. The AA1060 layers show significant grain refinement with grain sizes below 5 micrometers. The AA7075 layers show partial recrystallization with grain sizes of 10-20 micrometers. The interface shows evidence of diffusion bonding with limited intermetallic formation.

After 5 cycles: The layer thickness reduces to approximately 15 micrometers. Both alloys show extensive grain refinement, with AA1060 grain sizes below 2 micrometers and AA7075 grain sizes of 5-10 micrometers. The interface is well-bonded with a thin diffusion zone of 0.5-2 micrometers.

Mechanical Property Enhancement

The mechanical properties of the ARB composite improve with increasing cycle number, but the improvement is asymmetric due to the different deformation behaviors of the two alloys:

The composite achieves a tensile strength that is approximately 3-4 times higher than the pure aluminum and 60-70% of the high-strength alloy, while maintaining better ductility than the monolithic AA7075. This represents a significant improvement in the strength-to-ductility ratio compared to either constituent material alone.

Process Optimization and Defect Prevention

Critical Process Parameters

The optimization of ARB parameters for AA1060-AA7075 composites requires careful consideration of the asymmetric deformation behavior:

1. Rolling temperature: A temperature range of 150-250 °C is typically used. Lower temperatures may not achieve sufficient bonding in the AA1060 layers, while higher temperatures may cause excessive softening of AA7075 and loss of its strength advantage.
2. Reduction ratio: A reduction of 40-50% per pass is recommended to avoid excessive spreading of the AA1060 layers. Higher reductions can be used for the first cycle to ensure initial bonding, but should be reduced in subsequent cycles.
3. Backing plates: The use of hard backing plates (such as steel or tungsten carbide) helps to constrain the deformation of the AA1060 layers and promote more uniform strain distribution across the composite.
4. Cycle number: Three to five cycles are typically sufficient to achieve uniform properties and adequate bonding. Additional cycles provide diminishing returns and may introduce defects.

Common Defects and Their Prevention

The following defects are commonly encountered in ARB processing of AA1060-AA7075 composites:

Quality Assurance and Inspection

For quality control of ARB AA1060-AA7075 composites, the following inspection methods are recommended:

• Ultrasonic testing (UT): Phased array UT is effective for detecting delaminations and bonding defects at the interfaces.
• Eddy current testing (ET): Suitable for surface and near-surface defect detection in both aluminum alloys.
• Microhardness mapping: Provides a rapid assessment of bonding quality and microstructural uniformity across the composite cross-section.
• Metallographic examination: Essential for evaluating interface quality, grain structure, and the presence of defects or intermetallic phases.

Engineering Practice Integration

Relevance to Bimetal Clad Plate Manufacturing

The findings from this ARB study have direct relevance to conventional bimetal clad plate manufacturing, particularly for aluminum-clad aluminum composites used in heat exchangers and chemical processing equipment:

• The principle of asymmetric deformation is applicable to clad plate rolling where a soft base material is bonded to a harder cladding material.
• The use of backing plates in ARB processing is analogous to the use of backing rolls in clad plate rolling mills to control the deformation of the softer layer.
• The optimization of rolling temperature and reduction ratio for ARB processing parallels the optimization of rolling parameters in clad plate mills.
• The quality control methods used for ARB composites (UT, ET, microhardness mapping) are the same methods used for clad plate inspection in production environments.

Comparison with Weld-Overlay Cladding

The ARB process for AA1060-AA7075 composites can be compared with weld-overlay cladding in terms of the resulting properties and process characteristics:

Study Reflections and Technical Insights

The ARB processing of AA1060-AA7075 composites provides valuable insights into the challenges of joining dissimilar materials with large property mismatches. The asymmetric deformation behavior observed in this system is directly analogous to the challenges encountered in clad plate rolling of dissimilar metal combinations, such as stainless steel on carbon steel or nickel-based alloy on steel. The strategies developed to manage asymmetric deformation in ARB processing—such as the use of backing plates, controlled reduction ratios, and optimized rolling temperatures—are directly transferable to clad plate rolling mill operations.

The study also highlights the importance of understanding the fundamental deformation mechanisms in solid-state joining processes. The grain refinement observed in both the AA1060 and AA7075 layers is driven by dynamic recrystallization, which is the same mechanism that operates during the hot rolling of clad plates. Understanding the relationship between processing parameters and microstructural evolution enables more effective process optimization and quality control in production environments.

Furthermore, the mechanical property results demonstrate that the ARB process can create composites with properties that are superior to either constituent material alone. This concept of property enhancement through composite fabrication is central to the value proposition of bimetal products, whether produced by ARB, explosion cladding, roll bonding, or weld overlay. The ability to combine the formability of pure aluminum with the strength of a high-strength alloy in a single composite plate opens up new design possibilities for lightweight structural components in aerospace, automotive, and marine applications.

Summary and Outlook

The ARB processing of AA1060-AA7075 composite plates demonstrates the effectiveness of solid-state deformation in creating functional composites from materials with large property mismatches. The systematic approach to process optimization, defect prevention, and quality control presented in this study is directly applicable to conventional bimetal clad plate manufacturing. Engineers working in the bimetal field can benefit from the understanding of asymmetric deformation behavior and the strategies developed to manage it, as these principles are fundamental to the successful fabrication of clad plates from dissimilar metal combinations. The continued development of ARB and related solid-state processing techniques offers promising opportunities for creating advanced bimetal materials with tailored properties for demanding engineering applications in the chemical processing, aerospace, and energy sectors.