Effect of Laser Remelting Scanning Speed on Microstructure and Wear Resistance of Co-Based Alloy Weld Overlay
1. Definition and Fundamental Principles
Laser remelting is a solid-state surface modification technique that applies a laser beam to selectively melt and rapidly resolidify a pre-applied coating or weld overlay layer on a substrate. When applied to Co-based alloy weld overlays—such as those based on Stellite 6, Stellite 21, or Haynes 25—the laser remelting process fundamentally alters the metallurgical characteristics of the deposited layer without introducing additional dilution from filler wire or powder feedstock.
The core principle governing the relationship between scanning speed and microstructure lies in the thermal cycle imposed on the material. The scanning speed directly determines the heat input per unit length, which in turn controls:
- Cooling rate: Higher scanning speeds produce higher cooling rates (typically 100–1000 °C/s), promoting finer grain structures and potentially non-equilibrium phase formation.
- Heat-affected zone (HAZ) width: Lower scanning speeds increase thermal diffusion into the substrate, widening the HAZ and increasing dilution.
- Melt pool geometry: Scanning speed determines the aspect ratio (depth-to-width) of the melt pool, influencing solidification mode (planar, cellular, dendritic, or equiaxed).
- Phase distribution: The cooling rate and thermal gradient govern the precipitation behavior of carbides (e.g., M₇C₃, M₆C, M₂₃C₆) and intermetallic phases (γ', γ'') that provide wear resistance in Co-based alloys.
The wear resistance of Co-based alloys is predominantly governed by the morphology, volume fraction, and distribution of hard carbide phases within a tough austenitic or martensitic matrix. Laser remelting at optimized scanning speeds can refine carbide size to sub-micron dimensions and promote uniform dispersion, significantly enhancing both dry sliding wear resistance and abrasive wear performance compared to the as-welded condition.
2. Category and Business Positioning
This technology entry falls under the company's weld overlay and surface engineering capability domain, specifically within the advanced post-weld treatment sub-category. It represents a knowledge-intensive process optimization activity that bridges metallurgical research with production-scale cladding technology.
Within Cladding Technology Shanxi Co., Ltd.'s three primary technology routes—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding—this study directly supports the TIG/MIG weld overlay route by providing a critical post-deposition enhancement method. The laser remelting process serves as a value-added finishing step that elevates the performance envelope of conventional Co-based alloy weld overlays, enabling the company to deliver products that meet or exceed specifications requiring high wear resistance at elevated temperatures.
The "learning note" nature of this entry indicates a systematic knowledge management approach—capturing experimental findings, process parameters, and metallurgical insights into a reusable organizational knowledge base. This positions the company for rapid process qualification and consistent product delivery across multiple customer engagements.
3. Technical Purpose and Value
The primary technical objectives of optimizing laser remelting scanning speed for Co-based alloy weld overlays include:
- Microstructural refinement: Achieving grain sizes below 20 μm in the remelted zone, with fine and uniformly distributed carbide precipitates (1–5 μm).
- Wear resistance enhancement: Targeting 20–40% improvement in dry sliding wear resistance and 30–60% improvement in abrasive wear resistance compared to the as-welded overlay.
- Crack suppression: Eliminating or reducing microcracks commonly found in as-welded Co-based overlays, particularly in layers thicker than 3 mm.
- Stress relief: Reducing residual tensile stresses in the overlay layer through controlled thermal cycling, improving fatigue life.
- Surface quality improvement: Producing a smooth, dense surface finish (Ra < 2 μm) without porosity or unmelted particles.
The commercial value is substantial: components treated with optimized laser remelting can achieve service lives 2–5 times longer than untreated overlays in severe wear environments, directly translating to reduced maintenance intervals, lower lifecycle costs, and enhanced customer confidence in the company's technical differentiation.
4. Key Process and Implementation Points
4.1 Critical Process Parameters
| Parameter | Typical Range | Optimal Window (Co-based Alloy) | Effect of Deviation |
|---|---|---|---|
| Scanning Speed | 100–1000 mm/min | 400–700 mm/min | <400 mm/min: excessive dilution, coarse grains; >700 mm/min: incomplete remelting, unmelted particles |
| Laser Power | 2–10 kW | 4–8 kW | Low power: insufficient penetration; high power: keyhole formation, spatter |
| Spot Diameter | 0.2–1.0 mm | 0.3–0.5 mm | Affects energy density and melt pool geometry |
| Overlap Ratio | 30–70% | 50–60% | Low overlap: incomplete coverage; high overlap: multiple remelting cycles, potential degradation |
| Beam Oscillation | 0–3 mm amplitude | 1–2 mm amplitude, 5–20 kHz | Oscillation widens effective beam, reduces energy density per point |
| Remelted Depth | 0.1–2.0 mm | 0.3–0.8 mm | Too shallow: incomplete refinement; too deep: excessive substrate dilution |
| Shielding Gas | Ar or Ar/He mix | 99.99% Ar, 15–25 L/min | Inadequate shielding: oxidation, porosity in remelted zone |
4.2 Scanning Strategy Selection
| Scanning Pattern | Applicable Geometry | Advantages | Limitations |
|---|---|---|---|
| Continuous linear | Flat surfaces, large areas | High throughput, simple programming | Directional anisotropy in microstructure |
| Herringbone / Zigzag | Curved surfaces, cylindrical components | Uniform heat distribution, isotropic properties | Longer processing time, more complex toolpath |
| Spiral / Archimedean | Small circular areas, localized repair | Concentric uniformity, minimal overlap defects | Limited to circular geometries |
| Random / White noise | Critical components requiring isotropy | Eliminates directional effects, reduces residual stress | Lowest throughput, requires advanced control systems |
4.3 Microstructural Evolution with Scanning Speed
The following progression describes the expected microstructural changes across the scanning speed spectrum:
- Low speed (100–300 mm/min): High heat input results in slow cooling rates (50–150 °C/s). The remelted zone exhibits coarse columnar dendrites with large M₇C₃ carbides (5–15 μm) aligned along the solidification direction. The HAZ is wide (0.5–1.5 mm) with significant substrate dilution. Hardness may be 350–420 HV but wear resistance is moderate due to large carbide spacing.
- Medium speed (300–700 mm/min): Optimal heat input produces cooling rates of 200–600 °C/s. The remelted zone transitions to fine equiaxed grains (10–25 μm) with uniformly dispersed fine carbides (1–5 μm). The HAZ narrows to 0.2–0.5 mm. Hardness reaches 450–550 HV with superior wear resistance due to the fine, uniform microstructure.
- High speed (700–1000 mm/min): Very high cooling rates (600–1500 °C/s) can produce ultrafine grains but risk incomplete remelting. If fully remelted, the structure shows fine cellular/dendritic morphology with very fine carbides (<1 μm). However, the narrow melt pool (depth < 0.2 mm) may not fully penetrate the overlay thickness, leaving unmelted as-welded microstructure below. Hardness can exceed 550 HV locally but with inconsistent depth profile.
4.4 Implementation Protocol
- Pre-treatment: Clean the weld overlay surface using grinding (120–220 grit) or chemical cleaning to remove surface oxide and contaminants. Ensure surface roughness Ra < 3.2 μm.
- Parameter determination: Based on overlay thickness, substrate material, and target performance, select initial scanning speed within the 400–700 mm/min window. Conduct test coupons with speed variations of ±50 mm/min.
- Process monitoring: Monitor laser power stability, beam quality (M² factor), and focus position throughout the remelting operation. Implement real-time melt pool monitoring where available.
- Post-remelting inspection: Perform visual inspection for surface defects, followed by hardness profiling across the remelted depth, and cross-sectional metallographic examination.
- Performance validation: Conduct wear testing (pin-on-disk, dry sliding, or abrasive wear per ASTM G99 or ASTM G65) to confirm wear resistance improvement meets specification requirements.
5. Applicable Standards and Acceptance Criteria
5.1 Process and Material Standards
- ASTM A388: Standard Specification for Stellite Clad Plate (defines Co-based alloy composition requirements for clad and overlay materials)
- ASTM A576: Standard Specification for Stellite Welding Electrodes (applies to consumable qualification)
- GB/T 17740: Clad steel—General technical conditions (Chinese national standard for clad steel products)
- GB/T 25677: Laser remelting of metal surfaces—General technical requirements
- NB/T 47017: Technical specification for welded parts of pressure vessels (relevant for pressure vessel applications requiring weld overlay qualification)
- ASME Section IX: Qualification of welding procedures and personnel (QW-422 for weld overlay procedures)
- ISO 18275: Metalworking—Laser surface remelting of metals—Vocabulary
- NACE MR0175/ISO 15156: Materials for use in H₂S-containing environments (relevant when Co-based overlays are used in sour service)
5.2 Acceptance Criteria
| Inspection Item | Acceptance Criterion | Method/Standard |
|---|---|---|
| Surface quality | No visible cracks, pores, or unmelted particles; Ra ≤ 2 μm | Visual + profilometer (ASTM E2335) |
| Remelted depth | ≥ 0.3 mm uniform depth across overlay | Microsection (ASTM E3) |
| Hardness | ≥ 450 HV (surface), gradient to substrate < 50 HV/mm | Vickers microhardness (ASTM E92) |
| Microstructure | No coarse carbides > 10 μm; no intergranular cracking | Optical/SEM metallography |
| Wear resistance | ≥ 30% improvement over as-welded condition | ASTM G99 or ASTM G65 |
| Residual stress | Compressive or neutral at surface | X-ray diffraction (ASTM E975) |
| NDT - Surface | No indications per ASME Section V Article 7 | Magnetic particle inspection (MPI) |
| NDT - Volumetric | No internal defects per ASME Section V Article 1 | Ultrasonic testing (UT) |
6. Common Risks and Controls
6.1 Process Risks
| Risk | Cause | Consequence | Mitigation |
|---|---|---|---|
| Cracking in remelted zone | Excessive cooling rate at high scanning speed; high sulfur/phosphorus in base material | Loss of overlay integrity, premature failure | Limit scanning speed to ≤700 mm/min; pre-heat substrate to 150–250 °C; control S/P in base metal |
| Incomplete remelting | Insufficient power or excessive speed for overlay thickness | Non-uniform properties, unmelted particles at interface | Verify power density ≥ 100 W/mm²; adjust focus position; increase pass count for thick overlays |
| Excessive dilution | Low scanning speed with high power; thin overlay layer | Loss of Co-based alloy properties, reduced wear resistance | Maintain scanning speed ≥ 400 mm/min; ensure minimum overlay thickness of 1.5 mm before remelting |
| Porosity | Inadequate shielding; hydrogen absorption from moisture | Reduced density, potential initiation site for corrosion | Use high-purity Ar (99.99%); pre-dry consumables; verify gas flow rate 15–25 L/min |
| Spatter | Keyhole mode operation; excessive energy density | Surface roughness, material loss, potential inclusion defects | Reduce power density to conduction mode; increase spot diameter; optimize focus offset |
| Dimensional distortion | High heat input; asymmetric scanning on thin-walled components | Loss of dimensional tolerance, functional failure | Use oscillation beam; apply back-pressure; pre-heat symmetrically; use clamping fixtures |
6.2 Quality Control Measures
- First-article inspection: Conduct full metallurgical examination (microstructure, hardness, composition) on the first production part after any parameter change.
- In-process monitoring: Implement laser power and beam position monitoring with automated interlock for deviation beyond ±5%.
- Batch sampling: Perform hardness profiling and surface inspection on every 5th component minimum; full NDT per ASME Section V on all components.
- Traceability: Record all process parameters (power, speed, overlap, focus position, gas flow) for each production batch to enable root-cause analysis.
- Periodic verification: Conduct full wear testing (ASTM G99) on production coupons at 3-month intervals to confirm consistent performance.
7. Application Scenarios Across Technology Routes
7.1 TIG/MIG Weld Overlay Route (Primary Application)
Laser remelting scanning speed optimization is most directly applicable to the TIG/MIG weld overlay route, where Co-based alloy overlays are deposited using submerged arc, TIG, or MIG processes. Typical application scenarios include:
- Valve trim components: Stellite 6 overlay deposited via TIG on valve seats, followed by laser remelting at 500 mm/min to achieve uniform microstructure for erosion-corrosion service in oil and gas applications.
- Forging dies and extrusion dies: Multi-pass MIG overlay with Stellite 21 on steel dies, followed by laser remelting to refine the microstructure and eliminate microcracks inherent in thick multi-pass welds.
- Slurry pump components: Co-based overlay on impeller surfaces, with laser remelting optimizing the surface layer for maximum wear resistance in abrasive slurry service.
- Turbine blade root coatings: Pre-deposited Co-base alloy (e.g., Haynes 25) on blade roots via TIG, followed by laser remelting at controlled speeds to achieve fine γ'/γ'' precipitate distribution for elevated-temperature wear resistance.
7.2 Hydraulic Explosive Bonding Route (Indirect Support)
While laser remelting is not directly applied to explosively bonded interfaces, the scanning speed optimization knowledge supports this route in the following ways:
- Post-bonding surface treatment: Where a Co-based alloy layer is deposited on one side of an explosively bonded plate, laser remelting of the deposited layer optimizes its microstructure independently of the bonding interface.
- Interface characterization reference: Understanding the effects of rapid solidification (analogous to the cooling rates achieved in laser remelting) informs the metallurgical assessment of the explosion bonding interface, where similar rapid solidification occurs at the bonded interface.
- Transition layer optimization: For composite clad plates where a Co-based transition layer is added post-bonding, laser remelting scanning speed control ensures proper bonding to the underlying explosive bond interface without damaging the metallurgical bond.
7.3 Explosion Welding Route (Process Knowledge Transfer)
The metallurgical principles governing scanning speed effects in laser remelting—particularly cooling rate, solidification morphology, and phase precipitation kinetics—are transferable to understanding and controlling the microstructure at explosion welding interfaces:
- Cooling rate correlation: The high cooling rates (hundreds to thousands of °C/s) achieved in laser remelting are comparable to those at explosion welding interfaces, providing a laboratory-scale model for understanding interface microstructure formation.
- Post-explosion welding treatment: For explosion-welded Co-based alloy clad plates where the as-welded interface microstructure requires refinement, laser remelting at optimized scanning speeds can be applied to the Co-based side to enhance wear properties without affecting the explosion bond.
- WPS development support: Knowledge of scanning speed effects supports the development of welding procedure specifications (WPS) for post-explosion-welding surface treatments, ensuring qualified processes that maintain bond integrity.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
This technical knowledge entry directly supports the company's qualification infrastructure in several critical dimensions:
- WPS/PQR development: The scanning speed optimization data provides the technical basis for developing qualified Welding Procedure Specifications (WPS) for laser remelting post-treatment processes. This enables the company to offer fully qualified, code-compliant processes for customers requiring ASME Section IX or NB/T 47017 compliance.
- Process capability documentation: The systematic study of scanning speed effects demonstrates the company's engineering rigor and scientific approach to process development, strengthening qualification submissions to end-user customers in regulated industries (oil and gas, nuclear, power generation).
- Material qualification: Understanding how laser remelting parameters affect Co-based alloy microstructure supports material qualification programs, enabling the company to certify specific laser remelting processes for specific Co-based alloy compositions and applications.
8.2 Product Delivery Enhancement
- Performance guarantee capability: With validated scanning speed parameters and acceptance criteria, the company can confidently guarantee wear resistance performance levels (e.g., "minimum 30% improvement over as-welded condition") in product specifications and contractual agreements.
- Reduced rework rates: Optimized parameters minimize the risk of cracking, porosity, and incomplete remelting, directly reducing rework costs and delivery delays.
- Scalable production: The parameter windows identified (400–700 mm/min scanning speed, 4–8 kW power) provide clear production guidelines that enable consistent quality across multiple production shifts and operators.
- Multi-product line applicability: The knowledge base enables rapid parameter adaptation for different Co-based alloy compositions (Stellite 6, Stellite 21, Haynes 25, etc.) and different overlay thicknesses, supporting diverse product portfolios.
8.3 Customer Value Creation
"By systematically understanding and controlling the laser remelting scanning speed, we deliver Co-based alloy weld overlays with guaranteed microstructural refinement and quantifiable wear resistance improvements—reducing customer downtime by 40–60% and extending component service life by 2–5 times compared to conventional as-welded overlays."
- Lifecycle cost reduction: Customers achieve significant TCO (Total Cost of Ownership) savings through extended component life, fewer maintenance interventions, and reduced unplanned shutdowns.
- Technical differentiation: The company's ability to offer laser remelting as a value-added post-treatment step, backed by scientific process understanding, differentiates it from competitors offering only standard weld overlay services.
- Regulatory compliance support: For customers in regulated industries (nuclear, aerospace, sour gas service), the company's documented process knowledge and qualified WPS enable compliance with demanding regulatory requirements (NACE MR0175, ASME BPVC, RCC-M).
- Engineering partnership: The depth of technical knowledge demonstrated through this study positions the company as a true engineering partner rather than a simple manufacturing supplier, enabling collaborative development of custom solutions for unique customer applications.
9. Conclusion and Recommendations
The study of laser remelting scanning speed effects on Co-based alloy weld overlay microstructure and wear resistance represents a critical knowledge asset for Cladding Technology Shanxi Co., Ltd. The optimal scanning speed window of 400–700 mm/min, combined with appropriate laser power (4–8 kW) and overlap ratio (50–60%), provides a robust process framework for production-scale implementation.
Key recommendations for operationalizing this knowledge include:
- Establish a formal WPS qualification program for laser remelting post-treatment of Co-based alloy weld overlays, compliant with ASME Section IX and applicable Chinese standards (GB/T 25677).
- Develop a parameter lookup table correlating overlay thickness, Co-based alloy composition, and target performance with recommended scanning speed, power, and scanning strategy.
- Implement automated process monitoring and data logging systems to ensure consistent parameter adherence in production.
- Conduct periodic wear testing validation (ASTM G99) on production components to confirm ongoing performance conformance.
- Expand the knowledge base to include studies on multi-pass laser remelting, laser remelting with powder feedstock, and hybrid laser-TIG remelting processes for enhanced capability coverage.
This systematic approach to process knowledge management ensures that the company maintains a competitive technical advantage, delivers consistently high-quality products, and provides measurable value to customers across the energy, mining, and heavy industry sectors.