Introduction
The manufacturing of automotive engine block represents a pinnacle of industrial engineering. Among advanced surface treatment technologies, Atmospheric Plasma Spraying (APS) has gained prominence in applications such as turbine blades, combustion chambers, and cylinder bores. APS technology involves depositing a high-hardness iron-based coating approximately 0.3 mm thick onto the aluminum substrate of an engine block. This coating replaces traditional cast iron liners, offering superior heat dissipation, reduced weight, enhanced wear resistance, and prolonged engine lifespan.
However, APS processes often introduce defects such as porosity, particle adhesion, and coating discontinuities, leading to scrap rates as high as 1.5%. To address these challenges, this study focuses on optimizing secondary spray coating techniques to improve the yield and quality of engine block production.
APS Spraying Principles and Common Defects
1.1 APS Spraying Mechanism
APS is a thermal spray process where powdered material is injected into a plasma flame, melted, and propelled onto the substrate. The molten particles solidify upon impact, forming a dense, adherent coating. The key steps in APS processing for engine block include:
- Rough Machining: Initial shaping of the cylinder bore.
- Laser Texturing: Creating a micro-roughened surface using high-energy lasers to enhance coating adhesion.
- Plasma Spraying: Depositing the iron-based coating.
- Rough Grinding: Removing excess coating thickness.
- Finish Grinding: Achieving final dimensional accuracy and surface finish.
The critical parameters influencing coating quality include plasma voltage, powder feed rate, and surface roughness post-laser texturing.
1.2 Common Defects in APS Coatings
Defects observed during engine block production include:
- Coating Discontinuities: Caused by unstable plasma voltage (<36 V) or clogged powder feeders.
- Surface Porosity: Resulting from entrapped gases or particle spallation during grinding.
- Large Particles: Oxide buildup on spray gun nozzles leads to uneven particle distribution.
A statistical analysis of defects revealed that porosity accounts for over 70% of scrap cases.
Theoretical and Process Optimization for Secondary Spraying
2.1 Porosity Classification and Acceptance Criteria
Porosity in APS coatings was classified into five types based on size, morphology, and post-grinding behavior (Table 1).
Table 1: Classification of Porosity in APS Coatings
| Type | Description | Post-Grinding Outcome |
|---|---|---|
| 1 | Micro-pores embedded in coating | Visible pores after grinding |
| 2 | Uniform-sized pores at coating-substrate interface | Visible pores after grinding |
| 3 | Pores with increasing diameter | Visible pores after grinding |
| 4 | Pores with decreasing diameter | Acceptable if < threshold |
| 5 | Superficial particles | No visible pores after grinding |
Based on this classification, acceptance criteria were established:
- Upper Cylinder Region (Depth >105 mm): Max pore diameter ≤5 mm.
- Lower Cylinder Region (Depth ≤105 mm): Max pore diameter ≤X mm (proprietary value).
2.2 Secondary Spraying Process Flow
For engine block failing initial porosity checks, a secondary spraying process was designed:
- Initial APS Coating: Deposit ~250 μm coating.
- Rough Grinding: Remove ~70 μm to expose defective regions.
- Secondary Spraying: Apply ~120 μm additional coating using optimized parameters.
- Secondary Rough Grinding: Adjust grinding parameters to retain ~10 μm excess.
- Finish Grinding: Achieve final dimensions and surface quality.
Key adjustments included:
- Reduced powder feed rate to minimize oxide formation.
- High-pressure air cleaning pre-spraying.
- Modified spray gun nozzle geometry.
Quality Control in Secondary Spraying
3.1 Batch Validation Protocols
Three validation stages ensured process reliability:
Table 2: Batch Validation Strategy
| Stage | Sample Size | Key Tests |
|---|---|---|
| 1 | 5 units | Visual inspection, metallography, pull-off strength |
| 2 | 100+ units | Full dimensional checks, engine dyno tests |
| 3 | 168 units | Statistical analysis of porosity reduction |
3.2 Performance Metrics
Post-secondary spraying, critical quality parameters were evaluated:
Table 3: Pull-Off Strength Measurements
| Sample | Pull-Off Strength (MPa) | Fracture Mode (C+B%) |
|---|---|---|
| 90° | 45.94 (avg) | 95% |
| 270° | 40.21 (avg) | 85% |
C = Cohesive failure, B = Adhesive failure
Metallographic analysis confirmed uniform coating layers without delamination. Engine dyno tests demonstrated no abnormal wear or coating spallation after 5 hours of operation.
Results and Discussion
4.1 Porosity Reduction Efficiency
Secondary spraying reduced scrap rates from 1.5% to 0.3% in pilot batches. Post-process inspections showed:
- Surface Porosity: 87% of defects were eliminated.
- Coating Adhesion: Pull-off strength exceeded 30 MPa (spec: ≥30 MPa).
Table 4: Batch Production Results
| Batch Size | Passed Units | Yield (%) |
|---|---|---|
| 168 | 147 | 87.5 |
4.2 Cost-Benefit Analysis
Implementing secondary spraying reduced material waste and reprocessing costs by 22%. The ROI period for process upgrades was calculated as:
ROI=Annual SavingsImplementation Cost×100%ROI=Implementation CostAnnual Savings×100%
Conclusion
This study successfully developed a secondary spray coating process for engine block, addressing critical porosity defects in APS coatings. Key achievements include:
- Classification System: Enabled targeted defect remediation.
- Process Optimization: Achieved 87.5% yield in batch production.
- Quality Assurance: Validated through metallography, pull-off tests, and engine dyno trials.
Future work will focus on scaling the process for high-volume manufacturing and integrating AI-based defect detection systems.