Explosive Welding of Pure Titanium with SUS304 Austenitic and SUS821L1 Duplex Stainless Steels A Comparative Study

Literature Overview

This literature presents a systematic comparison of explosive welding (EW) between pure titanium and two distinct stainless steel grades: SUS304 (austenitic) and SUS821L1 (duplex). The study addresses a critical engineering challenge in bimetal manufacturing—achieving reliable metallurgical bonding between titanium and stainless steels that possess vastly different thermal expansion coefficients, melting points, and chemical reactivities. The authors investigated the interface microstructure, bonding quality, mechanical properties, and potential interfacial reactions under controlled explosive welding parameters. The work is particularly relevant to engineers designing titanium-lined pressure vessels, heat exchangers, and marine components where titanium's corrosion resistance must be combined with the structural strength of stainless steel substrates.

Core Technical Findings on Interface Microstructure

The explosive welding process creates a characteristic wavy or sinusoidal interface pattern driven by the aerodynamic instability of the colliding surfaces. The key finding is that the interface morphology differs significantly between the titanium/SUS304 and titanium/SUS821L1 systems. In the titanium/SUS304 pair, the interfacial wave amplitude and wavelength are influenced by the lower density and higher thermal conductivity of austenitic stainless steel compared to duplex grades. The SUS821L1 duplex steel, with its higher strength and more complex microstructure comprising ferrite and austenite phases, produces a more pronounced interfacial reaction zone.

The formation of intermetallic compounds at the interface is an unavoidable consequence of the high strain rates and localized temperatures generated during explosive collision. The study demonstrates that FeTi and Fe2Ti phases form preferentially at the troughs of the interfacial waves where intimate contact is achieved. In the duplex steel system, chromium segregation at the interface can further modify the local chemistry, potentially forming Cr-rich phases that influence long-term bond stability.

Mechanical Properties and Bonding Assessment

Mechanical testing, including shear strength, peel strength, and microhardness profiling across the interface, reveals important differences between the two systems. The titanium/SUS304 system generally exhibits higher shear strength values, typically in the range of 180–240 MPa, owing to the relatively homogeneous interface without extensive intermetallic accumulation. The titanium/SUS821L1 system shows slightly lower shear strengths, approximately 150–210 MPa, attributed to the formation of brittle intermetallic phases that can act as preferential crack initiation sites.

Microhardness measurements across the interface reveal a gradient from the base titanium (approximately 120–140 HV) through the interfacial reaction zone (potentially reaching 600–900 HV due to hard intermetallics) to the stainless steel substrate (250–350 HV for SUS304, 300–400 HV for SUS821L1). This hardness gradient is critical for understanding the mechanical behavior of the bonded joint under service conditions, particularly under cyclic loading or thermal cycling.

Engineering Practice Implications and Process Optimization

From a practical standpoint, the selection between SUS304 and SUS821L1 as the backing material for titanium explosive welding depends on the intended service environment. SUS304 offers superior formability and weldability, making it suitable for pressure vessels that require extensive post-weld forming operations. SUS821L1, with its higher yield strength (typically 450–550 MPa versus 200–300 MPa for SUS304), is preferred for applications demanding higher structural integrity, such as hydrogenation reactors or high-pressure marine components.

Key process parameters that must be controlled include:

• Stand-off distance: Typically 3–8 mm, directly influencing collision velocity and interface quality.
• Explosive charge geometry: The shaped charge design determines the pressure distribution and wave formation characteristics.
• Surface preparation: Both titanium and stainless steel surfaces must be cleaned to remove oxide layers, oils, and contaminants to ensure intimate contact during collision.
• Post-weld heat treatment: A controlled stress-relief anneal (typically 350–450 °C for 1–2 hours) can reduce residual stresses without significantly altering the interfacial microstructure.

Key Questions and Reflections

A significant question raised by this literature is whether the interfacial intermetallics formed during explosive welding can be minimized or controlled through parameter optimization. While some degree of intermetallic formation is inevitable, excessive growth can compromise joint integrity. The study suggests that optimizing the collision velocity and impact angle can limit reaction zone thickness to below 5 μm, which is generally acceptable for engineering applications.

Another important consideration is the long-term stability of the bond under service conditions. In chloride-containing environments, the interfacial region may become susceptible to galvanic corrosion due to the potential difference between titanium and stainless steel. This is particularly relevant for marine and chemical processing applications where the bimetallic joint is exposed to aggressive electrolytes.

Study Insights and Outlook

This comparative study provides valuable engineering guidance for selecting appropriate stainless steel backing materials for titanium explosive welding applications. The findings suggest that both systems can achieve acceptable bonding quality, but the titanium/SUS304 combination offers a more favorable balance of bonding strength and interfacial stability. For high-strength applications, the SUS821L1 system is viable but requires careful attention to interfacial reaction control and post-weld inspection. Future research should focus on long-term aging behavior of the interfacial region and the development of advanced inspection techniques to assess bond quality non-destructively in production environments.