CLADDING TECHNOLOGY SHANXI CO., LTDWeld High-Entropy Alloying Analysis of TA2 Titanium and 0Cr18Ni9 Stainless Steel TIG Welding
Literature Overview
Published in 2021 by researchers from Xi'an University of Technology and Xi'an Younait Container Manufacturing, this study investigates the microstructural evolution and mechanical properties of dissimilar TIG welds between TA2 titanium and 0Cr18Ni9 (304) austenitic stainless steel, with particular focus on the high-entropy alloy (HEA) phenomenon occurring in the weld zone. This research is directly relevant to bimetal pressure vessel fabrication, where titanium and stainless steel interfaces are critical for corrosion resistance in aggressive chemical environments.
Core Technical Analysis
The welding of titanium to stainless steel is inherently challenging due to significant differences in thermal expansion coefficients (Ti: ~8.7 × 10⁻⁶/K, 304 SS: ~17.3 × 10⁻⁶/K), melting points (Ti: 1668 °C, 304 SS: ~1400 °C), and metallurgical compatibility. The study reveals that during TIG welding, the melt pool creates a complex multi-component system where Ti, Fe, Cr, and Ni atoms interact to form regions with high configurational entropy, leading to high-entropy alloy-like microstructures in specific zones of the weld.
Weld Zone Microstructural Zones
| Zone | Composition Characteristic | Microstructure | Mechanical Properties |
|---|---|---|---|
| Pure Ti side | >90% Ti | Wrought α-Ti, fine grains | High ductility, low yield strength |
| Dilution zone | Ti-Fe-Cr-Ni mixture | BCC α-Ti with FCC austenite | Moderate strength, reduced ductility |
| HEA-like zone | Near-equimolar Ti-Fe-Cr-Ni | BCC + FCC dual-phase | Highest hardness, potential brittleness |
| Dilution zone (SS side) | 304 with Ti dilution | Austenite + martensite transformation | Elevated strength, reduced corrosion resistance |
| Pure SS side | ~304 composition | Recrystallized austenite | Standard 304 properties |
Key Findings on High-Entropy Alloying
The researchers identified that the weld centerline, where dilution ratios approach approximately 50:50, exhibits characteristics consistent with high-entropy alloy formation. The mixing entropy in this region exceeds 1.5R (where R is the gas constant), which is the conventional threshold for HEA classification. This results in:
- Solid solution strengthening from severe lattice distortion due to atomic size mismatch
- Synergistic strengthening from the BCC/FCC dual-phase microstructure
- Hardness values reaching 350–420 HV in the HEA-like zone, compared to 180 HV for pure Ti and 200 HV for 304 SS
- Reduced ductility in the HEA zone, with elongation dropping to 8–12% versus 20–25% for the base metals
Process Optimization for Dissimilar Welding
| Process Parameter | Recommended Value | Rationale |
|---|---|---|
| Shielding Gas | 99.99% Ar (Ti side), Ar + 5% CO₂ (SS side) | Prevent Ti oxidation, control SS weld pool |
| Preheat | Ti side: 100–150 °C, SS side: 200–250 °C | Reduce thermal gradient at interface |
| Welding Current | 120–180 A (DCEN) | Control dilution ratio |
| Travel Speed | 8–15 cm/min | Ensure proper fusion without excessive dilution |
| Fillers | ERNiCrMo-3 (Inconel 625) or pure Ti | Buffer dissimilar interface |
| Back Purge | High-purity Ar, flow 20–30 L/min | Critical for Ti side oxide prevention |
Engineering Practice and Pressure Vessel Relevance
In bimetal pressure vessel fabrication, titanium-to-stainless steel welds are commonly encountered in hydrogenation reactors, chlor-alkali equipment, and acid processing vessels. The findings from this study directly inform WPS qualification under NB/T 47014 and ASME IX Section IX. The HEA formation in the weld zone, while providing enhanced strength, introduces potential concerns for:
- Stress corrosion cracking susceptibility in chloride environments
- Hydrogen embrittlement resistance at the HEA zone
- Long-term thermal stability under cyclic loading conditions
- NDE sensitivity due to heterogeneous microstructure
Study Insights and Reflections
The concept of high-entropy alloying in conventional dissimilar welds is a paradigm-shifting observation. Rather than viewing the dilution zone as a defect region to be minimized, this research suggests that controlled HEA formation can be leveraged to create functionally graded interfaces with tailored mechanical properties. For pressure vessel engineers, this opens possibilities for designing dissimilar welds where the interface itself serves as a strengthening layer, provided that long-term environmental stability is verified through dedicated testing protocols.