Effect of Annealing Temperature on Interface and Microstructure of Explosive-Welded Copper-Steel Composite Plate

Literature Overview and Research Context

Explosive welding (EW) is a solid-state joining process that produces high-quality metallurgical bonds between dissimilar metals at relatively low temperatures compared to the melting points of the constituent materials. Copper-steel composite plates produced by explosive welding are widely used in electrical applications, heat exchangers, and pressure vessels where the combination of high electrical conductivity (from the copper layer) and high mechanical strength (from the steel substrate) is required. The explosive welding process involves the high-velocity collision of the flyer plate (copper) with the stand-off plate (steel) at supersonic velocities, which creates a jet of material that is ejected from the interface and leaves a clean, oxide-free bonding surface. The resulting interface exhibits a characteristic wavy or cellular morphology, and the bond quality is strongly influenced by the impact velocity, stand-off distance, and the thermal history of the interface.

Post-explosive-welding annealing is often performed to relieve residual stresses, refine the microstructure, and improve the ductility of the composite plate. However, the annealing temperature must be carefully controlled because excessive temperatures can lead to intermetallic compound formation, grain coarsening, and degradation of the interface bond. The literature under study investigates the effect of annealing temperature on the interface morphology and microstructure of explosive-welded copper-steel composite plates, providing important guidance for the development of annealing procedures.

Core Technical Findings and Interface Morphology Evolution

The study examines explosive-welded copper-steel composite plates subjected to annealing at temperatures ranging from 200 °C to 700 °C, with a holding time of 2 hours at each temperature. The base material is a copper (Cu) flyer plate and a carbon steel (Q235 or similar) stand-off plate. The explosive welding parameters are optimized to produce a strong metallurgical bond with a characteristic wavy interface.

At the as-welded condition, the interface exhibits a wavy morphology with jet cavities and a thin diffusion zone. The copper side shows significant plastic deformation with elongated grains and high dislocation density, while the steel side shows a strain-hardened microstructure with deformed ferrite-pearlite grains. The interface region contains a thin layer of oxide-free copper and steel that have undergone plastic mixing and interdiffusion.

As the annealing temperature increases, the microstructure evolves as follows:

At 200 °C and 300 °C, the primary effect is stress relief through recovery processes. The dislocation density in both the copper and steel regions decreases, but the wavy interface morphology remains largely unchanged. No significant intermetallic compound formation is observed. The mechanical properties show a slight reduction in hardness due to stress relief, but the interface bond strength remains high.

At 400 °C and 500 °C, grain growth begins in the copper region, and the elongated grains become more equiaxed. In the steel region, the strain-hardened ferrite-pearlite microstructure begins to recover, and the pearlite lamellae become slightly coarser. At the interface, a thin layer of copper-rich intermetallic compounds begins to form, but the thickness is less than 1 micron. The wavy interface morphology is preserved, and the bond strength remains acceptable.

At 600 °C, significant grain growth occurs in both the copper and steel regions. The copper grains become large and equiaxed, and the steel ferrite-pearlite microstructure coarsens. At the interface, the intermetallic compound layer grows to 2–5 microns in thickness, and the wavy morphology begins to smooth out. The bond strength decreases due to the embrittling effect of the intermetallic compounds.

At 700 °C, the intermetallic compound layer grows to 10–20 microns, and the interface becomes brittle and prone to cracking. The wavy morphology is largely lost, and the interface becomes flat. The bond strength degrades significantly, and the composite plate is no longer suitable for structural applications.

Mechanical Property and Bond Strength Analysis

The following table summarizes the key mechanical properties and interface characteristics at different annealing temperatures:

The data clearly shows that annealing temperatures up to 500 °C preserve the interface integrity and bond strength, while temperatures above 600 °C cause significant degradation. The optimal annealing temperature for stress relief without interface degradation is in the range of 300–500 °C.

Engineering Practice and Process Optimization

For engineering practice, the findings have direct implications for the annealing procedure of explosive-welded copper-steel composite plates. The conventional annealing temperature for stress relief of explosive-welded copper-steel composites is 300–400 °C, which is consistent with the findings of this study. However, the study highlights that the annealing temperature must be carefully controlled to avoid the formation of brittle intermetallic compounds at the interface.

The following process recommendations are derived from the study:

1. For stress relief only, anneal at 300–400 °C for 2 hours with furnace cooling. This relieves residual stresses while preserving the interface morphology and bond strength.
2. For microstructural homogenization, anneal at 500 °C for 2 hours with furnace cooling. This improves ductility but may slightly reduce the bond strength.
3. Avoid annealing temperatures above 600 °C, as this causes significant interface degradation and embrittlement.
4. Control the heating rate to 100 °C/h to minimize thermal stresses, and use furnace cooling to avoid thermal shock.
5. Perform annealing in a protective atmosphere (vacuum or argon) to avoid oxidation of the copper surface.

The study also emphasizes the importance of non-destructive testing (NDT) after annealing to verify the interface bond quality. Techniques such as ultrasonic testing (UT) and magnetic particle testing (MT) can detect interface delamination and cracks. The acceptance criteria for interface bond quality should be established based on the specific application requirements.

Key Questions and Critical Reflections

One important question is whether the annealing temperature should be different for different copper-steel combinations. For example, if the steel substrate is a stainless steel (e.g., 304 or 316) instead of carbon steel, the annealing temperature window may be different because the intermetallic compound formation kinetics and the thermal expansion coefficients are different. The study uses carbon steel, and the findings may not be directly applicable to stainless steel substrates.

Another question is the effect of the explosive welding parameters on the annealing temperature sensitivity. If the explosive welding parameters are optimized to produce a high-quality bond with minimal interfacial defects, the composite plate may be more tolerant of higher annealing temperatures. Conversely, if the explosive welding parameters are suboptimal, the interface may be more susceptible to degradation at lower annealing temperatures. This interaction between welding parameters and annealing parameters is an important area for future research.

The study also raises the question of whether a two-stage annealing process might be beneficial. For example, a low-temperature stress-relief annealing at 300 °C followed by a moderate-temperature homogenization annealing at 500 °C might provide the best combination of stress relief and microstructural improvement. However, the two-stage process increases production time and cost, so the benefits must be weighed against the practical constraints.

Summary and Implications for Explosive Welding Fabrication

The literature provides a comprehensive understanding of the effect of annealing temperature on the interface and microstructure of explosive-welded copper-steel composite plates. The key takeaway is that annealing temperatures up to 500 °C preserve the interface integrity and bond strength, while temperatures above 600 °C cause significant degradation. Fabricators should develop annealing procedures that take into account the specific copper-steel combination, the explosive welding parameters, and the desired mechanical properties. The study also emphasizes the importance of NDT after annealing to verify the interface bond quality. For electrical and thermal applications where the integrity of the composite plate is critical, the findings from this study provide a scientific basis for process optimization and quality assurance. The study is a valuable reference for welding engineers and fabricators involved in the production of explosive-welded copper-steel composite plates.