Today, laser cladding technology is studied in the repair of aerospace parts and components to replace chromium plating. Through experiments, it is verified that the cladding layer has high hardness and feasibility of subsequent processing. Finally, laser cladding is compared with traditional chromium plating technology.
01
Application background
Aerospace manufacturing tools, such as fixtures, rings, and support structures, have long faced wear issues caused by the processing of high-strength materials (such as nickel-based alloys and titanium alloys). Traditional repair methods primarily use hard chrome plating, but this approach has significant drawbacks:
① Environmental risks: chromic acid solutions are carcinogenic and strictly regulated under EU REACH regulations;
② Process defects: the coating is prone to peeling and bubbling, requiring multiple rework cycles;
③ Thickness limitations: the coating typically exceeds 1 mm, leaving insufficient machining allowance.
To address these issues, a new repair solution centered on laser cladding (Laser Cladding, LC) technology is proposed. This method uses environmentally friendly, high-precision additive manufacturing processes to regenerate tool surfaces and enhance their performance. The technical features are as follows:
Excellent environmental performance
① Completely eliminates chromic acid, using metal powders as coating materials, aligning with green manufacturing trends;
② The process has no harmful emissions, meeting EU regulatory requirements.
Metallurgical bonding
① The coating forms a metallurgical bond with the substrate through diffusion mechanisms, ensuring no defects such as bubbles or peeling at the interface.
Adaptability to complex structures
① Capable of multi-dimensional repairs on flat surfaces, external cylindrical surfaces, and internal cylindrical surfaces, covering typical tool structures;
② Through robot collaborative control and inclined powder feeding (10°-30°), it can solve the challenge of cladding in confined spaces.
Machining allowance
① Multi-layer cladding (e.g., 2 mm thick) ensures machining allowance, avoiding the rework issues caused by overly thin traditional coatings.
02
Laser cladding: materials and methods
Features: hardness 28-32 HRC, widely used in the manufacture of aero-engine machining tools, can meet the requirements of high strength and high wear resistance.
Selection basis: its heat treatment performance (quenching + tempering) and the compatibility of laser cladding heat input to ensure that the substrate does not deform or crack during the cladding process.
Coating material: NiCrBSi alloy powder
Composition: Ni base (Cr 17%, B 3.5%, Si 4%, C 1%, Fe 4%), particle size distribution 15-53µm. Brand name: Swiss Oerlikon Metco Metco 15F.
① Self-melting: B and Si can reduce the melting point, promote the flow of the melt pool, and reduce the un-melted particles.
② High hardness: Cr and C form hard carbides, such as Cr₇C₃, Cr₃C₂, to improve wear resistance.
③ Crack resistance: Ni matrix alleviates thermal stress and avoids cracking of the cladding layer.
Product requirements for laser cladding process
1. The thickness of the cladding layer is greater than or equal to 1.5 μm
2. The hardness of the cladding layer is more than 38 HRC
* Product physical (left), technical drawings (right)
Laser cladding system
Laser: Laserline, model LDF 4000-30, wavelength 940-980nm.
Powder feeding system: GTV PF Powder feeder.
Cladding head: Fraunhofer IWS Coaxial cladding head, spot diameter 3.5 mm.
Robot: Reis RV60-40 robot + RDK-05 rotating table, which can realize complex trajectory control.
Process parameter optimization
· Logic: maximize the height and hardness of the cladding layer, minimize the depth of fusion and thermal affected zone, and avoid overheating and softening of the substrate.
· Optimal parameters: laser power 1000W + powder feeding rate 17.4g/min, high hardness (> 700 HV 1) and low dilution rate (<10%).
* Cladding process parameters
* Schematic diagram of single channel cladding layer measurement
Multi-pass multi-layer cladding strategy
Path planning
Planar surface (Clad A): parallel scan path, overlap rate 50%, tilt 10° to avoid powder accumulation.
Outer cylindrical surface (Clad B): spiral scan path, synchronous control of the rotating table, inclined 10°.
Inner cylindrical surface (Clad C): 30° inclined in the confined space, adjust the powder feeding Angle to ensure the stability of the molten pool.
Layer control: 2 layers of cladding, total thickness of 2mm, to avoid cracks caused by multiple thermal cycles.
Matrix pretreatment:
Surface polishing: sandpaper polishing to Ra<1.6 µm, remove the oxide layer and oil contamination.
Cleaning: ultrasonic cleaning with isopropanol to ensure no oil residue.
postprocessing
Turning: flat and external cylindrical surfaces are turned on CNC lathes.
Grinding: use center hole grinding machine for flat and external cylindrical surfaces.
Milling: milling of internal cylindrical surfaces on a special milling machine.
03
Laser cladding: process parameters
The effect of laser power
High power leads to the expansion of the melt pool and the aggravation of the base body melting, but the dilution rate may exceed 20%, reducing the purity of the coating composition.
A) The height of the cladding layer, b) the width of the cladding layer, c) the depth of fusion, d) the HAZ depth varies with the laser power and powder feeding rate
Hardness and dilution rate
① When the laser power is 1000W and the powder feeding rate is 10.4g/min, the hardness reaches the peak of 680 HV0.3. At this time, the dilution rate is low (~10%), and the proportion of hard phase (Cr₇C₃, Cr₃C₂) in the coating is high.
② High dilution rate (>20%) leads to the infiltration of matrix iron into the coating, forming Fe-Cr solid solution, which weakens the effect of hard phase strengthening.
* Influence of process parameters on hardness and dilution rate: a) hardness, b) dilution rate
The effect of powder feeding rate
Excessive powder feeding rate (>17.4g/min) will lead to more un-melted particles and decrease the density of coating.
* Relationship between powder feeding rate and single channel cladding height: when the laser power is less than 1000W, the powder feeding rate increases and the cladding height increases logarithmically
Multi-layer cladding strategy
With a 50% overlap rate and two layers of cladding, the total thickness is 2 mm. Although the height of a single layer is limited and multiple layers can meet the machining allowance requirements, thermal input must be controlled to avoid matrix softening (HAZ depth <200 μm).
* Surface coating thickness: the coating thickness of plane, outer cylindrical surface and inner cylindrical surface is 2 mm
* Local defects on the surface of the product after cladding: a) convex and concave starting and ending points of the outer surface cladding, b) powder adhesion phenomenon on the inner surface
04
Mechanical processing and defect analysis
abrasive machining
Surface quality: The surface roughness Ra = 0.272μm after grinding, which meets the requirements of aerospace tools Ra <1.25μm. No cracks were found when the grinding depth was 0.4 mm.
Advantages: Grinding removes material through micro-cutting, avoiding impact loads on high hardness coatings (~750 HV1) and reducing the risk of cracking.
Turning and milling
Tool wear: When turning the outer cylindrical surface, the cutting edge of the hard alloy tool will crack after cutting 0.3 mm. The reason is that the coating hardness is high, resulting in excessive shear stress.
Surface defects: When milling the inner cylindrical surface, local cracks appear in the coating. The main reason is related to the coupling effect of residual stress in the cladding layer and cutting vibration.
* The plane and the outer cylindrical surface after turning: coating cracking and irregular chips
* Tool wear: a) external cylindrical surface after turning, b) hard alloy blade edge fracture
* Polished outer cylindrical surface: Surface roughness improved, but still visible micro scratches
* Milled inner cylindrical surface: local crack of coating, milling vibration and residual stress coupling action
Processing parameter suggestions
Turning: higher red hardness tools such as CBN or diamond coatings are required, supplemented by coolant to reduce thermal stress.
Milling: reduce the feed per tooth and use high speed milling strategy to suppress vibration.
05
Microstructure and phase analysis
Interface metallurgical bonding
SEM: There are no pores or cracks at the interface between the cladding layer and the substrate, showing continuous transition. The substrate 40HM steel forms plate martensite due to rapid cooling, while the area away from the interface is tempered martensite.
Diffusion mechanism: Ni and Cr elements in the melt pool diffuse to the matrix, forming a mutual diffusion zone about 5μm thick, which enhances the interfacial bonding strength.
* The substrate and coating are metallurgically bonded, and there are no pores or cracks at the interface
Microstructure: a) base martensite, b) dendrite growth in the transition zone, c) distribution of coating dendrites and hard phase
* Hardness distribution and matrix phase transformation: the hardness of the cladding zone is 754-762HV1, the hardness of the matrix near the interface is 605HV1 (martensite), and the hardness of the far away area is 402HV1 (tempered structure)
06
Summary of engineering applications
Process substitution
For products limited by regulations or high precision, priority is given to laser cladding and chromium plating replacement. Suitable powders are selected to take into account hardness and crack resistance.
parameter optimization
① Through single-channel experimental calibration, the dilution rate is controlled to be less than 10% to avoid matrix softening.
② When multi-layer cladding, reserve 0.3-0.5 mm grinding allowance.
Defect prevention and control
Grinding of the substrate, thoroughly removing surface oil stains, eliminating pores; powder pre-drying in humid environment.
This is for your reference only!
* Note: Comparison between laser cladding and traditional chromium plating
| Laser Cladding vs. Chrome Plating: Comparative Analysis | ||
| Part 1: Process Principle & Environmental Impact | ||
| Dimension | Traditional Chrome Plating | Laser Cladding (LC) |
| Process Principle | Electrochemical deposition: Cr³⁺ reduced to metallic chromium in chromic acid solution (thickness <1 mm). | Metallurgical bonding: Laser melts substrate and metal powder (e.g., NiCrBSi) to form a diffusion-bonded layer (thickness ≤2 mm). |
| Environmental Impact | Toxicity: Uses carcinogenic Cr⁶⁺ solutions. Waste: Complex neutralization/filtration required. |
Non-toxic: Metal powders (e.g., NiCrBSi). Zero liquid waste: Powder utilization >90%. |
| Regulatory Restrictions | EU restricts Cr⁶⁺ industrial use. | No restrictions; classified as “green remanufacturing” technology. |
| Laser Cladding vs. Chrome Plating: Comparative Analysis | ||
| Part 2: Coating Performance & Bonding Mechanism | ||
| Dimension | Traditional Chrome Plating | Laser Cladding (LC) |
| Bonding Mechanism | Mechanical bonding (physical adsorption); prone to delamination. | Metallurgical bonding with elemental diffusion; interfacial strength ≈ substrate material. |
| Hardness & Wear | Hardness: 800–1000 HV (brittle). Wear resistance depends on thickness. |
Hardness: 700–760 HV (NiCrBSi). Cr₇C₃/Cr₃C₂ phases enhance wear resistance. |
| Defect Types | Blistering (contamination). Delamination (stress). |
Porosity (uneven powder feeding). Microcracks (thermal accumulation; fixable via parameters). |
| Laser Cladding vs. Chrome Plating: Comparative Analysis | ||
| Part 3: Process Flexibility & Cost Efficiency | ||
| Dimension | Traditional Chrome Plating | Laser Cladding (LC) |
| Process Compatibility | Limited to grinding; turning/milling causes peeling. | Compatible with grinding/turning/milling (optimized tools like CBN). Repeatable repairs. |
| Cost Structure | Low per-unit cost for bulk (>5 pieces), but high waste treatment costs. | No mold fees; ideal for small batches. |
| Failure Modes | Delamination exposes substrate. | Localized wear; targeted repairs possible. |
| Laser Cladding vs. Chrome Plating: Comparative Analysis | ||
| Part 4: Practical Application Scenarios | ||
| Scenario | Traditional Chrome Plating | Laser Cladding (LC) |
| Simple Geometry | Suitable for flat surfaces (e.g., fixture planes). | Environmentally preferred alternative. |
| Complex Geometry | Limited (e.g., inner cavities/narrow gaps). | Robotic path planning enables cladding on complex surfaces. |
| High Precision | Post-grinding tolerance ±0.01 mm, limited by coating thickness. | Thickness control (±0.1 mm); sufficient machining allowance. |
| Extreme Environments | Coating fails at >300°C (oxidation/delamination). | NiCrBSi withstands ~800°C (e.g., engine components). |
Contact Person: Ms. Coco
Tel: +86 13377773809
