Diesel engines, despite their emissions and noise concerns, remain a critical choice for powering mobile vehicles due to their high torque, excellent fuel economy, and widespread applicability. As industrialization accelerates, there is an increasing demand for diesel engines that are more efficient and lightweight. However, high-performance diesel engine cylinder blocks pose significant challenges during casting, often resulting in defects such as porosity, inclusions, cold shuts, shrinkage porosity, and dimensional deviations. These defects not only elevate production costs but also contribute to resource waste, hindering the growth of the diesel engine industry. This comprehensive article delves into the casting process design and quality enhancement strategies for high-power diesel engine cylinder blocks.
1. Technical Specifications and Quality Requirements
The diesel engine cylinder block discussed in this article is made of HT300 material, weighing up to 720 kg with maximum dimensions of 1650 mm x 700 mm x 500 mm. The block features 8 to 20 cylinder bores with substantial wall thickness variations, ranging from a maximum of 75.13 mm to a minimum of 8 mm. The casting process must overcome challenges posed by these variations to avoid defects like shrinkage porosity, gas porosity, and cold shuts.
Key Mechanical and Metallurgical Properties:
- Tensile strength: ≥ 276 MPa
- Hardness: 179-255 HBW
- Graphite types: A-type ≥ 80%, B-type ≤ 10%, D+E-type ≤ 10%
- Microstructure: Pearlite ≥ 95%, Ferrite ≤ 5%, Carbide ≤ 5%
- Graphite size: 2-6 grades
Post-casting, the cylinder block undergoes rigorous quality checks, including visual inspection of internal cavities, magnetic particle testing (MT) of external surfaces, and a 1.5 MPa hydrostatic pressure test for 30 minutes to ensure no leakage.
2. Casting Process Design
The casting process for the diesel engine cylinder block involves meticulous planning and execution to ensure defect-free production. This section details the crucial aspects of the casting process, including melting and pouring, molding, and gating system design.
2.1 Melting and Pouring Process
The cylinder block’s intricate geometry and varied wall thicknesses necessitate a high-carbon equivalent, low-alloy melt strategy. The furnace charge comprises 60-70% scrap iron and 30-40% return iron to maintain optimal chemical compositions (Table 1).
Table 1: Chemical Composition of the Melt (Mass Fraction, %)
| Element | Range |
|---|---|
| C | 3.15 |
| Si | 1.80 – 1.90 |
| Mn | 0.70 – 0.80 |
| P | < 0.05 |
| S | < 0.1 |
| Cu | 0.85 |
| Cr | 0.25 – 0.28 |
| Ti | < 0.025 |
Melting and Pouring Parameters:
- Furnace charge temperature: 1420°C ± 10°C
- Pouring temperature: 1370°C ± 10°C
- Pouring time: 25-30 seconds
- Ladle capacity: 3 cylinder blocks per ladle
Inoculation: In-stream inoculation is performed using granular silicon-barium inoculant to refine the graphite structure and enhance fluidity, resulting in fewer shrinkage defects and improved mechanical properties.
2.2 Molding Process
The complex internal passages and walls of the cylinder block necessitate the use of furan resin sand for both external and internal cores. The parting line is strategically placed along the large “V”-shaped section to facilitate easy molding and core removal.
To prevent sand adhesion, a zirconium-based high-strength anti-stick coating is applied. The cylinder block’s significant dimensional variations necessitate varying casting shrinkages of 0.91%, 0.85%, and 0.85% in the length, width, and height directions, respectively.
Gating System Design:
A semi-closed “U”-shaped gating system is employed, with cross-sectional area ratios of ΣS直:ΣS横:ΣS内 = 1:1.65:1.34. Filter plates (120 mm x 75 mm, 20 PPI) are inserted into the runner to purify the melt and regulate flow velocity, minimizing turbulence and inclusion formation.
3. Initial Production Challenges
Initial production runs revealed significant issues, with porosity, sand inclusion, and leakage defects accounting for over 50% of rejections. Leakage primarily occurred at the interface between the U-shaped water jacket core and the tappet core, where wall thickness is merely 8 mm.
Table 2: Defect Statistics from Initial Production Runs (2018-2022)
| Production Run | Castings | Scrap Rate (%) | Leakage | Sand Inclusion | Porosity | Dross | Others |
|---|---|---|---|---|---|---|---|
| First Run | 42 | 42.9 | 9 | 2 | 4 | 1 | 2 |
| Second Run | 116 | 40.5 | 21 | 8 | 11 | 2 | 5 |
| Third Run | 131 | 20.6 | 13 | 3 | 4 | 5 | 2 |
4. Defect Analysis and Diagnosis
A multi-faceted approach was adopted to diagnose the root causes of the observed defects, including visual inspection, metallographic analysis, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and numerical simulations.
4.1 Visual and Microscopic Inspection
Visual inspection revealed porosities and inclusions with oxidized interior surfaces. Metallographic analysis showed a mix of A-type and minor C-type graphite in sound regions, contrasting with coarse, irregular graphite in defective areas.
4.2 SEM and EDS Analysis
SEM images and EDS spectra confirmed the presence of impurities, including Al, Na, Ca, Si, and Mn, within pores. These elements, combined with oxygen, form oxides contributing to porosity and slag inclusion defects.
4.3 Numerical Simulations
Numerical simulations using MAGMA software were conducted to visualize gas entrapment risks, particle tracking, and temperature distributions during pouring and solidification. The simulations highlighted significant gas generation and entrapment in the U-shaped water jacket and tappet core regions due to binder decomposition and oxidation.
5. Improvement Measures and Verification
Based on the diagnostic findings, several corrective measures were implemented to address the identified defects.
5.1 Core Assembly Optimization
- Pre-assembly of Cores Outside the Mold: To prevent sand inclusion, cores were pre-assembled and locked outside the mold before insertion.
- Replacement of Binder with Self-Tapping Screws: Self-tapping screws replaced adhesive binders to eliminate prolonged gas generation.
5.2 Coating Modification
- Mixed or Graphite-Based Coatings: Alkaline coatings were replaced with mixed or graphite-based coatings to minimize slag inclusion and porosity.
5.3 Core Production Process Enhancement
- Cold Box Core Production: Cold box technology replaced hand lay-up for the U-shaped water jacket and tappet cores, improving core strength and reducing defects.
5.4 Melt Composition Adjustment
- Optimized Furnace Charge: The furnace charge was revised to include 10-15% pig iron and 85-90% scrap iron, eliminating return iron to reduce tramp elements.
- High-Temperature Graphitized Carburizer and Inoculants: High-temperature graphitized carburizers and inoculants improved melt cleanliness and graphite morphology.
5.5 Verification Results
The revised processes were validated through progressive production batches (4, 8, 16, and larger-scale runs). Defect rates drastically declined, with leakage rates falling below 2% (Table 3).
Table 3: Defect Statistics Post-Improvement
| Production Batch | Castings | Scrap Rate (%) | Leakage | Sand Inclusion | Porosity | Dross | Others |
|---|---|---|---|---|---|---|---|
| First Batch | 400 | 0 | 0 | 0 | 0 | 0 | 0 |
| Second Batch | 800 | 0 | 0 | 0 | 0 | 0 | 0 |
| Third Batch | 16 | 6.25 | 0 | 0 | 0 | 1 | 0 |
| Fourth Batch | 73 | 4.1 | 1 | 1 | 1 | 0 | 0 |
6. Conclusion
By addressing the root causes of porosity, inclusion, and leakage defects in high-power diesel engine cylinder block casting, significant quality improvements were achieved. The optimized casting process, including core assembly modifications, coating changes, core production enhancements, and melt composition adjustments, led to a drastic reduction in defect rates. The cylinder block’s leakage rate decreased to below 2%, and overall scrap rates were controlled within 4-6%, meeting the targeted quality enhancement goals.