1. Introduction
1.1 Background and Significance
The lining plate of a concrete mixer is a critical component that protects the mixer barrel, significantly extending its service life and improving mixing efficiency. However, due to harsh working conditions involving abrasive wear and corrosive environments, the lining plate becomes one of the most severely worn parts. Frequent failures necessitate costly shutdowns and replacements, directly impacting productivity. Globally, wear-related losses account for over 30% of energy consumption in industrialized nations. For instance, the U.S. suffers annual losses exceeding $100 billion due to friction, wear, and corrosion[1]. Enhancing the wear resistance of the lining plate is thus vital for reducing maintenance costs and improving operational efficiency.
Laser cladding, an advanced surface modification technology, offers a promising solution. It enables the deposition of high-performance coatings on the lining plate surface through metallurgical bonding, minimizing thermal impact on the substrate while enhancing hardness, wear resistance, and corrosion resistance. This study explores laser cladding parameters, gradient composite coatings with WC particles, and laser remelting techniques to optimize the performance of concrete mixer lining plates.
2. Experimental System and Methodology
2.1 Materials
2.1.1 Substrate Material
The lining plate is made of chromium alloy steel. Its chemical composition is detailed in Table 1.
Table 1: Chemical Composition of Chromium Alloy Steel (wt.%)
| Element | C | Si | Mn | P | Ni | Mo | S | Fe |
|---|---|---|---|---|---|---|---|---|
| Content | 3.76 | 0.53 | 0.35 | 0.018 | 0.012 | <0.01 | 0.027 | Bal. |
2.1.2 Laser Cladding Material
Fe60 alloy powder was selected due to its cost-effectiveness, compatibility with the substrate, and superior mechanical properties. Table 2 summarizes its composition.
Table 2: Chemical Composition of Fe60 Alloy Powder (wt.%)
| Element | C | Si | B | Cr | Ni | Fe |
|---|---|---|---|---|---|---|
| Content | 0.8–1.2 | 1.0–2.0 | 3.8–4.2 | 16–18 | 9.0–12 | Bal. |
2.2 Laser Cladding System
The experiment utilized a Han’s Laser HL-WM-4000 system, with key parameters listed in Table 3.
Table 3: Key Parameters of Laser Cladding System
| Parameter | Value |
|---|---|
| Wavelength | 800–1100 nm |
| Laser Power | 0–4000 W |
| Powder Feed Rate | 0–20 g/min |
| Scanning Speed | 0–2000 mm/min |
2.3 Testing Methods
2.3.1 Microstructural Analysis
- Optical Microscopy (OM): DM2700M metallurgical microscope.
- Scanning Electron Microscopy (SEM): Hitachi SU8010 with EDS.
- X-ray Diffraction (XRD): Cu-Kα radiation, 40 kV, 300 mA.
2.3.2 Mechanical Property Evaluation
- Microhardness: HMV-10S Vickers tester (1000 g load, 10 s dwell time).
- Wear Resistance: RTEC-MFT5000 tribometer (10 N load, Al₂O₃ counterpart).
- Crack Detection: DPT-5 penetrant inspection.
3. Optimization of Laser Cladding Parameters
3.1 Orthogonal Experiment Design
A three-factor, three-level orthogonal experiment (L9(34)L9(34)) was conducted to optimize laser power, scanning speed, and powder feed rate. Table 4 shows the parameter combinations.
Table 4: Orthogonal Experiment Design
| Sample | Laser Power (W) | Scanning Speed (mm/min) | Powder Feed Rate (g/min) |
|---|---|---|---|
| 1 | 800 | 400 | 7 |
| 2 | 800 | 500 | 9 |
| 3 | 800 | 600 | 11 |
| … | … | … | … |
3.2 Weighted Comprehensive Evaluation
A multi-objective optimization approach considered microhardness, dilution rate, and surface quality. Weight coefficients were assigned as follows:
- Microhardness: 5
- Dilution Rate: 3
- Surface Quality: 2
Table 5: Scoring Criteria for Evaluation
| Parameter | Score Range | Criteria |
|---|---|---|
| Microhardness (HV) | 0–10 | 550–800 HV, higher = better |
| Dilution Rate (%) | 0–10 | 0–60%, lower = better |
| Surface Quality | 0–10 | Smoothness, absence of defects |
The optimal parameters were determined as:
- Laser Power: 900 W
- Scanning Speed: 600 mm/min
- Powder Feed Rate: 7 g/min
3.3 Single-Layer Cladding Analysis
3.3.1 Microstructure
The Fe60 coating exhibited a gradient microstructure:
- Top: Equiaxed grains.
- Middle: Dendritic/cellular crystals.
- Bottom: Planar crystals.
3.3.2 Mechanical Properties
- Microhardness: 745 HV (1.8× substrate hardness).
- Wear Resistance: Friction coefficient = 0.57; wear mass = 0.5 mg (29% lower than substrate).
Table 6: Comparison of Wear Performance
| Material | Friction Coefficient | Wear Mass (mg) |
|---|---|---|
| Substrate | 0.64 | 0.7 |
| Fe60 Cladding | 0.57 | 0.5 |
4. Gradient Fe60/WC Composite Coating
4.1 Effect of WC Addition
Adding WC particles (5–15 wt.%) improved hardness but introduced cracks due to thermal stress (δTδT):δT=Ec(αm−αc)ΔT1−vcδT=1−vcEc(αm−αc)ΔT
Where:
- EcEc: Elastic modulus of cladding layer
- αm,αcαm,αc: Thermal expansion coefficients of substrate and coating
- ΔTΔT: Temperature difference
Table 7: Crack Analysis with WC Addition
| WC Content (wt.%) | Crack Density (cracks/mm²) |
|---|---|
| 0 | 0 |
| 5 | 2.1 |
| 10 | 4.5 |
| 15 | 7.8 |
4.2 Gradient Coating Design
A two-layer gradient structure was designed:
- Transition Layer: Pure Fe60.
- Top Layer: Fe60 + WC (5–15 wt.%).
4.2.1 Microstructure and Phase Analysis
- XRD Results: Peaks for α-Fe, γ-Fe, M₂₃C₆, and WC.
- EDS Mapping: W enrichment at grain boundaries.
4.2.2 Mechanical Performance
- Microhardness: 822.1 HV (2× substrate) at 10% WC.
- Wear Resistance: Friction coefficient = 0.518; wear mass = 0.3 mg (57% lower than substrate).
Table 8: Gradient Coating Performance
| WC Content (wt.%) | Microhardness (HV) | Wear Mass (mg) |
|---|---|---|
| 0 | 708 | 0.5 |
| 10 | 822 | 0.3 |
| 15 | 851 | 0.2 |
5. Laser Remelting of Gradient Coatings
5.1 Effect of Remelting Power
Laser remelting at 450 W eliminated porosity and refined grains.
5.1.1 Microhardness Enhancement
- Post-Remelting Hardness: 857.6 HV (2.1× substrate).
5.1.2 Wear Performance
- Friction Coefficient: 0.454 (32% reduction vs. substrate).
- Wear Mass: 0.1 mg (85.7% reduction vs. substrate).
Table 9: Remelted Coating Performance
| Parameter | Before Remelting | After Remelting |
|---|---|---|
| Microhardness (HV) | 822 | 857 |
| Wear Mass (mg) | 0.3 | 0.1 |
6. Conclusions
- Optimal laser cladding parameters for the lining plate were determined: 900 W laser power, 600 mm/min scanning speed, and 50% overlap ratio.
- Gradient Fe60/WC coatings reduced crack susceptibility, achieving 822 HV hardness at 10% WC.
- Laser remelting at 450 W enhanced density and hardness (857 HV), significantly improving wear resistance.