CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Transition Alloy Wire Feeding on Microstructure and Properties of Laser Multi-Layer Cladding
Literature Overview
This 2005 study by Luo Fang and Yao Jianhua from Zhejiang University of Technology investigates the influence of transition alloy wire feeding on the microstructure and mechanical properties of laser multi-layer cladding. The work addresses a critical practical challenge in laser cladding technology: achieving a metallurgically sound bond between dissimilar materials while maintaining the functional properties of the final overlay. The research is particularly relevant to the manufacturing of bimetallic components where the base material (typically carbon steel or low-alloy steel) must be joined to a corrosion-resistant or wear-resistant overlay layer.
Core Technical Content
Laser multi-layer cladding involves the sequential deposition of multiple layers using a laser beam as the heat source. The transition alloy wire serves as an intermediate material between the base substrate and the final cladding alloy, controlling dilution, reducing residual stress, and preventing cracking at the interface. The study examines how different transition alloy compositions and feeding strategies affect:
| Parameter | Transition Alloy Effect | Final Overlay Effect |
|---|---|---|
| Dilution ratio | Reduced from 40–60% to 15–25% | Improved purity of functional layer |
| Cracking susceptibility | Significantly reduced | Crack-free overlay achievable |
| Hardness gradient | Smooth transition (100–400 HV) | Uniform hardness in final layer |
| Bond strength | Enhanced from 150–200 MPa to >300 MPa | Meets API 934 / ASTM A263 requirements |
| Residual stress | Reduced by 30–50% | Lower risk of delamination |
Microstructural Evolution
The transition layer microstructure typically consists of a mixed matrix of ferrite, austenite, and carbide phases, depending on the specific alloy composition. Common transition alloys include:
- Type A — Nickel-based (e.g., Ni-20Cr-10Mo): Produces austenitic matrix, excellent ductility, suitable for stainless steel overlays on carbon steel.
- Type B — Iron-nickel (e.g., Ni-35Fe-5Cr-2Mo): Balanced dilution control, good for moderate-alloy overlays.
- Type C — Low-alloy with boron and titanium additions: Promotes fine grain structure and improved toughness.
Process Parameters and Multi-Layer Strategy
The multi-layer cladding process parameters studied include:
- Laser power — 2.0–4.0 kW for typical industrial fiber or CO₂ lasers
- Scanning speed — 0.3–0.8 m/min depending on wire diameter and alloy melting behavior
- Wire feeding rate — 0.5–1.5 kg/h, synchronized with laser power
- Powder/wire standoff distance — 8–15 mm from nozzle to substrate
- Layer thickness — 0.5–1.5 mm per pass, with 70–90% overlap between adjacent tracks
The critical finding is that the transition layer thickness should be at least 1.5–2 times the final overlay layer thickness to ensure adequate dilution buffering. For a final overlay of 3 mm, a transition layer of 5–6 mm is recommended.
Defect Analysis
| Defect | Cause | Solution |
|---|---|---|
| Porosity in overlay | Incomplete melting of wire; gas entrapment | Increase laser power; optimize shielding gas flow |
| Cracking at interface | Thermal mismatch; high carbon in base | Thicker transition layer; preheating to 150–200 °C |
| Uneven layer thickness | Wire feeding instability | Closed-loop feeding control; regular nozzle cleaning |
| Poor track overlap | Scanning speed variation | Laser tracking system; calibrated traverse table |
Engineering Practice Integration
In bimetal pressure vessel fabrication, this transition layer concept directly applies to weld overlay of hydrogenation reactor shells and heat exchanger tubesheets. For example, when overlaying Inconel 625 on SA-516 Gr.70 carbon steel, a transition layer of Ni-Fe-Cr (similar to Stellite 6 or a custom Ni-30Fe-10Cr composition) is deposited in 2–3 passes before the final Inconel 625 layer. This approach satisfies ASME Section IX QW-462 qualification requirements and achieves bond strength exceeding 300 MPa, well above the minimum 170 MPa specified in ASTM A263.
Study Insights
The research demonstrates that the transition alloy is not merely a "filler" but a critical engineering design element that determines the metallurgical compatibility of the entire cladding system. The ability to independently control dilution, residual stress, and crack susceptibility through transition layer design represents a significant advance over conventional single-layer overlay approaches. For laser cladding specifically, the rapid solidification rates (10³–10⁵ °C/s) create unique microstructural features—including cellular dendrites, nanoscale carbides, and refined grain structures—that cannot be achieved by conventional welding methods. These features, combined with the dilution control provided by the transition layer, enable the production of overlay layers with hardness values exceeding 600 HV while maintaining acceptable toughness—a combination that is essential for demanding service conditions such as those in petroleum refining, chemical processing, and power generation.