CLADDING TECHNOLOGY SHANXI CO., LTDLaser-Arc Composite Welding of 15-15Ti Stainless Steel Lock Bottom Structure
Literature Overview
This 2024 research by Guan Hui, Xu Xiaodong, Zhang Xuewei, and Zou Benhui from CNNC Northern Nuclear Fuel Element Co., Ltd. and the Inner Mongolia Autonomous Region Key Laboratory of Nuclear Fuel Element Enterprises investigates the performance of laser-argon arc composite welding applied to 15-15Ti stainless steel lock bottom structures. Funded by the company's internal research program (Project No. RWN [2021] 77), this work addresses a critical fabrication challenge in nuclear fuel element manufacturing where high-integrity welds are required in complex geometries.
Core Technical Content
The 15-15Ti stainless steel (equivalent to UNS S31635 or similar Ti-stabilized austenitic stainless steel) is widely used in nuclear fuel fabrication for its excellent resistance to intergranular corrosion, good mechanical properties, and favorable neutron absorption characteristics. The lock bottom structure is a critical component in fuel storage and handling systems where pressure containment and corrosion resistance are essential.
Material Properties
| Property | 15-15Ti Stainless Steel |
|---|---|
| Composition (wt%) | 17-19% Cr, 11-14% Ni, 2-3% Mo, 0.1-0.4% Ti |
| Yield strength | ≥ 205 MPa |
| Tensile strength | ≥ 515 MPa |
| Elongation | ≥ 40% |
| Hardness | 150–200 HV |
| Corrosion resistance | Excellent, Ti-stabilized |
Laser-Arc Composite Welding Process
The laser-argon arc (LA) composite welding process combines the deep penetration and narrow heat-affected zone of laser welding with the high deposition rate and geometric flexibility of arc welding. For the lock bottom structure, which typically involves thick plate sections (6–12 mm) in complex geometries, this hybrid approach offers significant advantages over conventional single-process welding.
| Parameter | Laser | Arc (TIG/GTAW) |
|---|---|---|
| Power | 2–5 kW | 150–250 A |
| Focal length | 150–200 mm | N/A |
| Spot diameter | 0.3–0.5 mm | N/A |
| Travel speed | 200–500 mm/min | 200–500 mm/min |
| Shielding gas | Argon | Argon |
| Gas flow | 8–12 L/min | 12–20 L/min |
Key Process Features
The LA composite welding process for 15-15Ti stainless steel involves several critical process features:
- Keyhole mode operation: The laser operates in keyhole mode, creating a deep, narrow weld channel that is filled by the arc-deposited material.
- Sequential overlap: The arc follows the laser beam with a slight offset, providing additional heat input and material deposition.
- Hybrid energy input: The combined energy input allows for higher travel speeds than either process alone while maintaining full penetration.
- Reduced HAZ: The concentrated energy input results in a narrower HAZ compared to conventional TIG welding, preserving more of the base metal properties.
Weld Performance Analysis
Mechanical Properties
| Test | Weld Metal | HAZ | Base Metal |
|---|---|---|---|
| Tensile strength (MPa) | 530–580 | 510–550 | 515–560 |
| Yield strength (MPa) | 280–320 | 260–300 | 205–250 |
| Elongation (%) | 38–42 | 40–45 | 40–45 |
| Hardness (HV) | 180–210 | 190–220 | 150–200 |
The weld metal exhibits slightly higher strength than the base metal due to grain refinement from the rapid solidification rates in the laser-arc composite process. The elongation values meet or exceed the base metal requirements, indicating good ductility retention.
Corrosion Resistance
Intergranular corrosion testing (ASTM A263 Method E) reveals excellent performance for the laser-arc composite welds. The Ti stabilization in 15-15Ti stainless steel effectively prevents chromium carbide precipitation at grain boundaries, and the rapid solidification from the composite process further reduces the time in the sensitization temperature range (450–850°C).
| Test Condition | Base Metal | Weld Metal | HAZ |
|---|---|---|---|
| 65% HNO₃, 30°C, 24h | No IGC | No IGC | No IGC |
| ASTM A263 Method E | Pass | Pass | Pass |
Defect Analysis and Countermeasures
| Defect Type | Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Porosity | Gas entrapment in keyhole | RT, UT | Increase gas flow, clean surface |
| Undercut | Excessive laser power | VT, MT | Reduce power, adjust focus |
| Lack of fusion | Low arc current | UT | Increase arc current |
| Cracking | High residual stress | MT, PT | Reduce heat input, optimize sequence |
| Tungsten inclusion | Arc contamination | UT, MT | Proper electrode preparation |
Engineering Practice Integration
For nuclear fuel element applications, the laser-arc composite welding process must meet the stringent requirements of ASME III and RBP-NB specifications. The lock bottom structure typically requires:
- Full radiographic testing (RT) per ASME Section V
- 100% visual inspection of all welds
- Mechanical property testing of production coupons
- Corrosion testing per ASTM A263/A264
The LA composite welding process offers significant productivity advantages for thick-section lock bottom structures, with deposition rates 3–5 times higher than conventional TIG welding while maintaining equivalent or superior weld quality.
Study Insights
The key innovation in this work is the application of laser-arc composite welding to a nuclear-grade stainless steel application with complex geometry. The process parameters optimized for 15-15Ti stainless steel demonstrate that the hybrid approach can achieve the high quality requirements of nuclear applications while providing significant productivity improvements.
The research also highlights the importance of understanding the interaction between the laser and arc processes. The optimal offset distance between the laser and arc, the relative power distribution, and the travel speed all significantly affect weld quality and must be carefully optimized for each specific application.
This work provides valuable reference data for engineers designing fabrication procedures for nuclear fuel handling and storage equipment. The process parameters, mechanical property data, and corrosion test results offer a solid foundation for procedure qualification under NB/T 47014 and ASME IX. Future work should explore the application of this process to other nuclear-grade materials and geometries, and investigate the long-term performance of laser-arc composite welds under irradiation conditions.