CLADDING TECHNOLOGY SHANXI CO., LTDProcess Parameter Design and Testing for TA2/316L Composite Rod Explosive Welding
Overview of the Study
This literature presents a systematic investigation into the explosive welding process parameters for producing TA2 (commercially pure titanium grade 2) / 316L stainless steel composite rods. The study addresses the critical challenge of achieving reliable metallurgical bonding between titanium and stainless steel—a combination widely used in chemical processing, nuclear engineering, and marine applications where corrosion resistance and mechanical strength must coexist. The research employs both numerical simulation and experimental validation to optimize the explosive welding parameters that govern the quality of the titanium-steel interface.
Explosive Welding Process Principles
Explosive welding (also known as explosive bonding or impact welding) is a solid-state joining process that relies on the high-velocity collision of two metal surfaces to achieve metallurgical bonding without melting. The process involves the detonation of a shaped explosive charge that accelerates the flyer plate (in this case, the TA2 titanium) toward the base plate (316L stainless steel). At the moment of impact, the metal surfaces reach velocities exceeding 200 m/s, creating sufficient shear stress and localized heating to form a metallurgical bond.
Critical Process Parameters
The study identifies the following parameters as primary variables controlling the quality of the TA2/316L composite interface:
| Parameter | Symbol | Typical Range | Optimal Value |
|---|---|---|---|
| Explosive charge thickness | h_e | 20–60 mm | 35–45 mm |
| Standoff distance | h_s | 5–15 mm | 8–12 mm |
| Flyer plate thickness | h_f | 3–8 mm | 5–6 mm |
| Base plate thickness | h_b | 20–50 mm | 30–40 mm |
| Contact angle | α | 10°–25° | 15°–20° |
| Specific collision velocity | V_n | 200–350 m/s | 250–300 m/s |
| Specific collision angle | β | 15°–30° | 20°–25° |
Numerical Simulation and Experimental Validation
The study utilizes finite element analysis (FEA) to model the collision dynamics and predict the bonding conditions at the interface. The simulation captures the high-strain-rate deformation, temperature rise, and pressure development at the collision point. The key criterion for successful bonding is that the collision velocity must exceed the material-specific bonding threshold velocity (V_b) while remaining below the maximum allowable velocity (V_max) that would cause material ejection or spatter.
For the TA2/316L system, the simulation determines that the bonding window lies between approximately 250 m/s and 320 m/s normal collision velocity. Below this range, insufficient shear stress develops to remove surface oxides and achieve intimate contact. Above this range, the material experiences excessive temperature rise leading to spatter, delamination, or even partial melting at the interface.
Experimental Results
The experimental campaign produced composite rods with diameters ranging from 20 mm to 60 mm. Metallographic examination of the bonded interfaces revealed characteristic sinusoidal (wave-like) patterns typical of successful explosive welding. The wave amplitude and wavelength serve as direct indicators of the collision conditions:
- Wave amplitude: 0.5–2.5 mm (indicating proper collision velocity)
- Wave wavelength: 5–15 mm (related to collision angle and material properties)
- Interface roughness: Ra 1.2–3.5 μm (confirming intimate metallurgical contact)
Bond strength testing demonstrated that the TA2/316L interface achieves peel strengths of 12–18 MPa, exceeding the minimum requirements specified in ASTM A263 for explosive-clad plate applications. The shear strength measurements confirm values of 85–110 MPa, indicating strong metallurgical bonding rather than mere mechanical interlocking.
Defect Analysis and Countermeasures
The study identifies several common defects and their root causes:
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Insufficient bonding | Low collision velocity (<250 m/s) | Increase explosive charge thickness |
| Delamination/spatter | Excessive collision velocity (>320 m/s) | Reduce standoff distance or increase base plate mass |
| Uneven bonding | Asymmetric collision angle | Ensure parallel alignment and concentricity |
| Surface oxidation | Inadequate surface preparation | Acid cleaning and immediate welding (<15 min exposure) |
| Cracks in titanium layer | Excessive strain rate | Reduce flyer velocity or increase flyer thickness |
A critical finding is that the standoff distance (h_s) has a nonlinear effect on bonding quality. While increasing h_s generally improves collision velocity, beyond a certain threshold (approximately 12 mm for this material combination), the flyer plate begins to deform non-uniformly, leading to localized bonding defects. The study recommends a standoff-to-flyer-thickness ratio of 1.5:1 to 2.0:1 for optimal results.
Material Compatibility and Interface Chemistry
The TA2/316L combination presents unique challenges due to the significant difference in material properties between titanium and stainless steel. Titanium has a lower thermal conductivity (approximately 21 W/m·K) compared to 316L (approximately 16 W/m·K), but the greater concern is the potential for titanium oxidation during the high-energy collision event.
The study confirms that the explosive welding process is inherently a solid-state process where the interface temperature remains below the melting point of both materials. However, localized temperatures at the collision point can reach 800–1200 °C, which is sufficient to promote some interdiffusion and the formation of a thin reaction layer. For the TA2/316L system, this reaction layer is typically 5–20 μm thick and consists primarily of titanium oxides and iron-titanium intermetallics. While this layer does not compromise bond strength significantly, it may affect long-term corrosion performance if exposed to aggressive environments.
Engineering Applications and Practical Considerations
The TA2/316L composite rod produced through explosive welding finds applications in:
- Heat exchanger tubes for chemical processing plants
- Valve stems and shafts for corrosive service
- Nuclear reactor components requiring radiation resistance
- Marine propeller shafts and rudder stocks
- Petrochemical pump components
For pressure vessel applications, the composite rod must satisfy the requirements of relevant standards including ASTM A263 (explosive-clad plate), ASTM A264 (explosive-clad pipe and tube), and applicable sections of ASME VIII Div.1. The bond strength requirement per ASTM A263 mandates a minimum peel strength of 10 MPa for titanium-clad products, which the TA2/316L explosive welding process reliably exceeds.
Quality Assurance Requirements
The following NDT methods are recommended for quality verification:
- Magnetic particle testing (MT) of the steel surface for indication of bonding defects
- Ultrasonic testing (UT) with specific techniques for interface bond detection
- Visual inspection of the interface after cross-sectioning
- Peel strength testing per ASTM A263 Section 7
Summary and Conclusions
This study successfully demonstrates that TA2/316L composite rods can be reliably produced through explosive welding with optimized parameters of 250–300 m/s collision velocity, 15°–20° collision angle, and standoff-to-flyer-thickness ratio of 1.5:1 to 2.0:1. The resulting composite interfaces exhibit strong metallurgical bonding with peel strengths exceeding 12 MPa and characteristic sinusoidal bonding patterns confirming proper process conditions. The engineering value of this research lies in providing a validated parameter set that reduces trial-and-error costs and improves first-pass quality in production environments. For practitioners specifying explosive welding for titanium-steel composite components, the key takeaway is that precise control of standoff distance and explosive charge geometry is more critical than the absolute amount of explosive used, and that surface preparation must be completed within 15 minutes of welding to prevent titanium oxidation from degrading bond quality.