Robotic GMAW Additive Manufacturing of Impeller Blades Based on Dual NURBS Curve Path Planning
1. Definition and Fundamental Principles
1.1 Technology Definition
Robotic GMAW (Gas Metal Arc Welding) additive manufacturing of impeller blades based on dual NURBS (Non-Uniform Rational B-Spline) curves is a directed-energy deposition technology that employs industrial robotic systems equipped with GMAW torches to build complex, thin-walled impeller blade geometries layer-by-layer. The dual NURBS curve methodology provides a parametric mathematical framework for generating both the deposition toolpath and the support structure path, enabling precise geometric control over the complex airfoil profiles inherent to hydraulic and gas turbine impeller blades.
1.2 Underlying Principles
The technology integrates three core principles:
- Directed Energy Deposition (DED): The GMAW arc serves as both the heat source and the wire feed mechanism, simultaneously melting the consumable electrode wire and the substrate/base material to form a continuous weld bead. Each deposited bead represents one discrete layer in the additive build sequence.
- Dual NURBS Curve Parametric Modeling: Two independent NURBS curves are employed—one defining the deposition trajectory (toolpath) and the other defining the boundary or support contour. NURBS curves offer exact representation of conic sections (circles, ellipses, parabolas, hyperbolas) and provide C² continuity, which is essential for smooth impeller blade surface profiles without geometric discontinuities.
- Robotic Motion Control: A 6-axis industrial robot (typically a 6-axis articulated robot with payload capacity of 25–500 kg) executes the programmed GMAW deposition path with positional accuracy of ±0.05 mm to ±0.1 mm, synchronized with wire feed speed and shielding gas flow.
1.3 Mathematical Foundation of Dual NURBS Curves
A NURBS curve is defined by the parametric equation:
N(u) = Σᵢ₌₀ⁿ Nᵢ,ₚ(u) · Pᵢ
where Nᵢ,ₚ(u) represents the B-spline basis functions of degree p, Pᵢ are the control points, and u is the parametric variable. In the dual NURBS approach:
- Primary NURBS curve (C₁): Defines the deposition toolpath—the actual trajectory followed by the GMAW torch along the blade surface.
- Secondary NURBS curve (C₂): Defines the boundary contour, support structure geometry, or compensation path for thermal distortion correction.
The dual-curve architecture allows independent optimization of deposition quality (via C₁ parameterization) and geometric accuracy (via C₂ boundary definition), which is critical for achieving the tight dimensional tolerances required in impeller blade applications.
2. Category and Business Positioning
2.1 Technology Classification
This technology falls under the Wire Arc Additive Manufacturing (WAAM) category within the broader additive manufacturing taxonomy defined by ASTM F2792 and ISO/ASTM 52900. Specifically, it belongs to the GMAW variant of WAAM, distinguished from FCAW and SAW variants by its open-arc configuration, use of solid or flux-cored wire, and higher deposition rates achievable with appropriate shielding gas configurations.
2.2 Business Positioning within Cladding Technology Shanxi Co., Ltd.
While the company's primary technology routes encompass TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding for clad plate and pipe fabrication, this robotic GMAW additive manufacturing capability represents a strategic extension into complex component manufacturing and repair. The positioning is as follows:
- Complementary to Weld Overlay: Shares common welding metallurgy knowledge (WPS qualification, heat input management, dilution control) with the company's established TIG/MIG overlay capabilities but extends into 3D additive geometries.
- Distinct from Bonding Routes: Unlike hydraulic explosive bonding and explosion welding which produce bonded clad interfaces through kinetic energy transfer, this technology relies on fusion welding principles for material deposition.
- Value-Added Service: Enables the company to offer impeller blade manufacturing, repair, and remanufacturing services to customers in power generation, mining, marine, and chemical processing industries.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
- Complex Geometry Realization: Produce impeller blades with complex 3D airfoil profiles, thin sections (wall thickness 2–8 mm), and tight dimensional tolerances (±0.15 mm to ±0.25 mm) that are impractical through conventional casting or machining alone.
- Material Tailoring: Enable functionally graded material deposition—transitioning from a structural base material to a corrosion/wear-resistant surface layer within the same component.
- Cost Reduction: Eliminate or minimize expensive investment tooling (molds, patterns) required for traditional impeller blade casting, reducing lead times from 8–12 weeks to 5–10 days for prototype or low-volume production.
- Repair and Remanufacturing: Restore worn or damaged impeller blades in situ or ex situ, extending asset life by 50–200% compared to replacement.
3.2 Economic Value
| Value Driver | Quantified Benefit | Industry Application |
|---|---|---|
| Tooling elimination | 50,000–200,000 RMB saved per design iteration | Hydropower, mining dewatering pumps |
| Lead time reduction | 6–8 weeks vs. conventional casting | Emergency repair, spare parts |
| Material utilization | 95–98% vs. 40–60% for subtractive machining | Superalloy impeller blades |
| Design flexibility | Unlimited geometric complexity without additional cost | Custom impeller optimization |
4. Key Process and Implementation Points
4.1 Process Flow Overview
- Geometric Modeling: Import or generate the impeller blade CAD model; extract surface mesh; apply dual NURBS curve fitting to define deposition paths and boundary contours.
- Build Orientation and Slicing: Determine optimal build orientation to minimize support requirements and maximize deposition quality; slice the geometry into discrete layers at intervals of 0.8–2.0 mm.
- Process Parameter Selection: Select wire material, shielding gas composition, current/voltage parameters, wire feed speed, travel speed, and inter-pass temperature limits based on material system and geometric requirements.
- Robotic Path Programming: Convert the NURBS-defined toolpaths into robot kinematic trajectories using offline programming software (e.g., RoboDK, KUKA Sim, ABB RobotStudio); perform collision detection and reachability analysis.
- Pre-heat and Substrate Preparation: Clean and degrease the substrate; apply pre-heat to 150–400°C depending on base material (carbon steel: 200–300°C; stainless steel: 150–250°C; nickel alloys: 300–400°C).
- Additive Deposition: Execute the robotic GMAW deposition sequence layer-by-layer with synchronized wire feed, torch motion, and shielding gas delivery.
- Post-Processing: Stress relief heat treatment; CNC machining of critical surfaces; dimensional verification; non-destructive testing.
4.2 Key Process Parameters
| Parameter | Typical Range (Carbon Steel) | Typical Range (Stainless Steel 316L) | Typical Range (Nickel Alloy Hastelloy C-276) |
|---|---|---|---|
| Wire Diameter | 1.0–1.6 mm | 1.0–1.2 mm | 1.0–1.2 mm |
| Wire Feed Speed | 4–8 m/min | 3–6 m/min | 2.5–5 m/min |
| Travel Speed | 300–800 mm/min | 250–600 mm/min | 200–500 mm/min |
| Current (DC) | 150–250 A | 120–200 A | 100–180 A |
| Voltage | 18–24 V | 18–22 V | 18–22 V |
| Heat Input | 0.8–2.0 kJ/mm | 0.6–1.5 kJ/mm | 0.5–1.2 kJ/mm |
| Shielding Gas | Ar + 2% O₂ or Ar + 5% CO₂ | Pure Ar or Ar + 2% O₂ | Pure Ar |
| Gas Flow Rate | 15–25 L/min | 18–25 L/min | 20–30 L/min |
| Layer Thickness | 1.0–2.0 mm | 0.8–1.5 mm | 0.8–1.5 mm |
| Inter-pass Temperature | < 250°C | < 200°C | < 150°C |
| Pre-heat Temperature | 200–300°C | 150–250°C | 300–400°C |
4.3 Dual NURBS Curve Implementation Details
4.3.1 Curve Fitting Strategy
The dual NURBS curve approach involves the following implementation steps:
- Surface Discretization: The impeller blade surface is discretized into a point cloud or mesh; control points for the NURBS curves are extracted from the geometric model at strategically spaced intervals.
- Weight Function Optimization: The rational weights in the NURBS formulation are optimized to minimize deviation between the curve and the target geometry, with particular attention to the leading edge (LE) and trailing edge (TE) regions where curvature changes are most severe.
- Parametric Continuity Enforcement: C² continuity is maintained between adjacent NURBS curve segments to ensure smooth deposition transitions and avoid geometric discontinuities that could induce residual stress concentrations.
- Thermal Compensation Integration: The secondary NURBS curve incorporates predicted thermal distortion offsets (derived from finite element analysis or empirical models) to pre-compensate for shrinkage and warping during deposition.
4.3.2 Deposition Pattern Strategies
| Pattern Type | Description | Applicability | Advantages |
|---|---|---|---|
| W-pattern (zigzag) | Alternating direction deposition with overlap | Flat surfaces, thick sections | High deposition rate; good bead fusion |
| Single-bead contour | Single pass along NURBS-defined contour | Thin walls (2–4 mm), complex profiles | Low heat input; good geometric accuracy |
| Spiral/contour | Continuous spiral path following surface normals | Curved surfaces, impeller shrouds | Uniform layer thickness; no start/stop defects |
| Hybrid (contour + fill) | Contour pass followed by fill passes | Structural sections, thick areas | Good surface quality; high build rate |
4.4 Critical Implementation Challenges
- Thin Wall Deposition: Impeller blades often have minimum wall thicknesses of 2–3 mm. Achieving this requires precise control of heat input, travel speed, and bead width to prevent burn-through or collapse. The dual NURBS approach enables adaptive parameter modulation along the path to maintain thermal balance.
- Support Structure Management: Complex blade geometries with overhangs require support structures. The secondary NURBS curve can define support geometry that is geometrically compatible with the primary deposition path, enabling efficient support removal.
- Residual Stress Control: Layer-by-layer deposition creates significant residual stresses. Inter-pass temperature monitoring (using infrared thermography or embedded thermocouples) and adaptive inter-pass cooling are essential to prevent cracking, particularly in high-strength alloys.
- Geometric Accuracy: Accumulated errors from robot positioning, thermal distortion, and bead width variation must be controlled to achieve the required ±0.15–0.25 mm dimensional tolerance. In-process monitoring with optical tracking systems enables real-time correction.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard | Scope | Relevance to Impeller Blade AM |
|---|---|---|
| ISO/ASTM 52900 | Additive Manufacturing — General Principles | Terminology, classification, and general requirements |
| ASTM F2792 | Standard Classification System for Additive Manufacturing | Technology classification and naming conventions |
| ISO/ASTM 52915 | AM — General Requirements for DED | Process qualification and validation requirements |
| ASTM F3001 | Standard Practice for Designing Parts for AM | Design for additive manufacturing guidelines |
| NACE MR0175/ISO 15156 | Materials for Use in H₂S Environments | Material selection for corrosion-resistant impeller blades |
| ASME BPV Section VIII Div. 2 | Pressure Vessel — Alternative Rules | Acceptance criteria for AM components in pressure equipment |
| API 610 | Centrifugal Pumps for Petroleum Industry | Impeller performance and dimensional requirements |
| ISO 5199 | Centrifugal Pumps for Pumping Water | Dimensional and performance standards for pump impellers |
| GB/T 19001 | Quality Management Systems | Quality management framework for AM production |
| NB/T 20003 | Welding Procedure Specification Requirements | WPS qualification for GMAW processes |
| ASME Section IX | Welding, Brazing, and Fusing Qualifications | PQR and WPS qualification for GMAW overlay/deposition |
5.2 Acceptance Criteria
- Dimensional Accuracy: Overall dimensional tolerance ±0.25 mm; blade profile tolerance ±0.15 mm; balance quality per ISO 21940-11 (G2.5 or better for high-speed applications).
- Surface Quality: As-deposited surface roughness Ra ≤ 15 μm (before machining); machined surface Ra ≤ 3.2 μm for hydraulic surfaces; Ra ≤ 1.6 μm for critical bearing surfaces.
- Mechanical Properties: Tensile strength ≥ 95% of base material specification; elongation ≥ 90% of base material specification; hardness within ±10% of specified range.
- Microstructural Quality: No intergranular cracking; grain size ≤ ASTM No. 5 (for austenitic stainless steels); no excessive carbide precipitation at bead boundaries.
- NDT Acceptance: Per ASTM E164 (Ultrasonic Testing): no indications exceeding 50% of reference reflector; per ASTM E94 (Magnetic Particle Testing): no linear indications; per ASTM E165 (Liquid Penetrant Testing): no indications exceeding 0.5 mm length for surface-breaking defects.
- Corrosion Resistance: Salt spray test per ASTM B117: ≥ 500 hours without red rust for stainless steel impellers; per NACE TM0169 for H₂S environments.
6. Common Risks and Controls
6.1 Technical Risks
| Risk | Cause | Detection Method | Mitigation Control |
|---|---|---|---|
| Cracking (hot/cold) | Excessive cooling rate; high carbon equivalent; hydrogen embrittlement | MT, PT, UT inspection | Pre-heat control; post-weld heat treatment; low-hydrogen consumables |
| Porosity | Insufficient shielding; wire surface contamination; high travel speed | RT, UT, visual inspection | Gas flow monitoring; wire cleaning; travel speed optimization |
| Delamination | Insufficient inter-layer fusion; thermal shock; residual stress | UT (phased array), thermal imaging | Inter-pass temperature control; overlap optimization; stress relief |
| Geometric deviation | Thermal distortion; robot calibration error; cumulative bead width error | 3D scanning (laser/CMM); in-process optical monitoring | In-process compensation; robot recalibration; adaptive NURBS curve adjustment |
| Material segregation | Non-uniform mixing of alloying elements in multi-material deposition | EDS, optical microscopy, XRD | Deposition sequence optimization; stirring techniques; post-build homogenization treatment |
| Warping/distortion | Asymmetric heat input; constrained boundary conditions | In-process displacement monitoring; post-build CMM | Symmetric deposition sequences; fixture design; thermal compensation in NURBS paths |
6.2 Quality Assurance Controls
- Input Controls: Wire material certification (mill test reports per ASTM A396/A554); shielding gas purity verification (O₂ < 0.1% for inert gas applications); substrate material verification.
- In-Process Controls: Real-time arc voltage/current monitoring; wire feed speed verification; robot position feedback; inter-pass temperature monitoring with automated shutdown above limits; gas flow rate verification.
- Post-Build Controls: Dimensional verification via 3D laser scanning (accuracy ≤ 0.05 mm); mechanical property testing (tensile, hardness, impact); microstructural examination (optical microscopy, SEM); NDT per applicable standard.
- Traceability: Complete build log including all process parameters, material lot numbers, operator identification, and environmental conditions (temperature, humidity, contamination levels).
7. Application Scenarios Across Technology Routes
7.1 Synergy with TIG/MIG Weld Overlay Route
The robotic GMAW additive manufacturing capability shares fundamental metallurgical knowledge with the company's established TIG/MIG weld overlay technology:
- Shared WPS Qualification: GMAW process parameters (current, voltage, travel speed, gas composition) are directly transferable to overlay WPS development. A qualified GMAW PQR per ASME Section IX can be adapted for both overlay and additive deposition applications.
- Material Compatibility Database: The extensive material compatibility data developed through overlay work (dilution rates, transition layer requirements, microstructural evolution) directly informs GMAW AM material selection and process optimization.
- NDT Methodology: The NDT techniques and acceptance criteria established for overlay inspection (UT, MT, PT, RT) are directly applicable to AM component inspection, with appropriate adjustments for multi-layer geometry.
- Heat Treatment Knowledge: Post-weld heat treatment procedures developed for overlay applications (stress relief, solution treatment, aging) are applicable to AM components, with consideration for different thermal histories.
7.2 Complementary Role to Hydraulic Explosive Bonding
While hydraulic explosive bonding produces clad plates and pipes through kinetic energy transfer, the GMAW AM technology provides complementary capabilities:
- Complex Geometry Cladding: Where explosive bonding is limited to relatively flat or simple curved geometries, GMAW AM can deposit corrosion-resistant layers onto complex 3D impeller blade surfaces, extending the company's cladding service to previously inaccessible geometries.
- Hybrid Cladding Approach: For large impeller components, a hybrid approach can be employed: explosive bonding for the base clad plate, followed by GMAW AM for localized repair or feature addition (e.g., adding wear-resistant tips to blade edges).
- Material System Extension: The GMAW AM route enables deposition of materials that may not be compatible with explosive bonding (e.g., dissimilar metal combinations with significant density or strength mismatch).
7.3 Extension Beyond Explosion Welding Route
The GMAW AM technology extends the company's capabilities beyond the traditional explosion welding domain:
- Small Batch and Custom Production: While explosion welding requires significant setup and is most economical for large batches or continuous production, GMAW AM is well-suited for small-batch, custom, and prototype impeller blade production.
- Repair and Remanufacturing: Field repair of damaged impeller blades in remote locations (mining operations, offshore platforms) where explosion welding equipment is impractical. The portable robotic GMAW AM system enables on-site repair with minimal infrastructure.
- Functionally Graded Components: The layer-by-layer deposition capability enables creation of functionally graded material (FGM) impeller blades with gradually transitioning composition from the root (structural material) to the tip (wear/corrosion-resistant material), achieving performance levels not achievable through explosive bonding alone.
8. Qualification Building and Customer Value
8.1 Qualification Building Pathway
- Process Qualification: Develop and qualify GMAW AM procedures per ASME Section IX (QW-452 for GMAW) and ISO/ASTM 52915, establishing the company's formal AM process capability.
- Equipment Qualification: Document robot system capabilities, calibration procedures, and maintenance protocols to demonstrate consistent process performance.
- Material Qualification: Qualify wire materials (ER308L, ER316L, ERNiCrMo-3, etc.) for specific impeller blade applications with documented mechanical property and microstructural data.
- Design Qualification: Develop validated design guidelines for GMAW AM impeller blades, including support structure requirements, build orientation recommendations, and post-processing specifications.
- Customer-Specific Qualification: Develop application-specific qualification packages for major customers (e.g., power generation, mining, marine) demonstrating compliance with their specific requirements and codes.
8.2 Customer Value Proposition
- Rapid Prototyping: Reduce impeller blade design iteration cycles from weeks to days, enabling faster optimization of hydraulic performance.
- On-Demand Production: Eliminate inventory requirements for spare impeller blades; produce parts as needed with minimal lead time.
- Performance Optimization: Enable topology-optimized impeller designs that reduce weight while maintaining structural integrity, improving pump efficiency by 3–8%.
- Extended Asset Life: Provide cost-effective repair solutions that extend impeller blade service life by 50–200%, reducing unplanned downtime and maintenance costs.
- Customization: Offer fully customized impeller blade designs tailored to specific fluid properties, operating conditions, and performance requirements.
8.3 Integration with Company Quality Management System
The GMAW AM technology must be integrated into the company's existing quality management system (GB/T 19001 / ISO 9001) with specific attention to:
- Document Control: Maintain controlled WPS, PQR, and build procedure documents for each material system and application.
- Personnel Qualification: Certify operators in robotic GMAW AM per NB/T 47014 or equivalent, including both welding operator certification and robotic programming competency.
- Process Monitoring: Implement real-time process monitoring and data logging to ensure traceability and enable root cause analysis of non-conformances.
- Continuous Improvement: Establish a feedback loop from NDT results, customer feedback, and performance data to continuously optimize process parameters and build strategies.
9. Conclusion
The robotic GMAW additive manufacturing of impeller blades based on dual NURBS curves represents a sophisticated integration of parametric geometric modeling, robotic motion control, and arc welding metallurgy. This technology extends Cladding Technology Shanxi Co., Ltd.'s capabilities from traditional clad plate and pipe fabrication into complex component manufacturing, creating significant value through rapid prototyping, on-demand production, and asset repair services. By leveraging the company's established expertise in welding metallurgy, NDT, and quality management, this technology can be rapidly qualified and deployed to serve demanding markets in power generation, mining, marine, and chemical processing industries. The dual NURBS curve approach provides the geometric precision required for high-performance impeller blades, while the GMAW process delivers the deposition rates and material flexibility needed for practical industrial applications.