Abstract
In this study, I conducted a comprehensive comparison between the microstructure and mechanical properties of AlSi10Mg alloy specimens produced via two distinct manufacturing processes: selective laser melting (SLM) as a representative of additive manufacturing, and sand casting utilizing 3D-printed (3DP) sand molds. The research aimed to evaluate the influence of manufacturing orientation, post-processing heat treatments, and process-specific characteristics on the alloy’s performance. Key findings revealed that SLM-produced specimens exhibited superior mechanical strength and stability compared to sand casting, with directional anisotropy in SLM parts effectively mitigated through stress-relief annealing. This work underscores the potential of additive manufacturing in automotive applications, particularly for lightweight and structurally complex components.
1. Introduction
The automotive industry is increasingly adopting additive manufacturing (AM) technologies to overcome limitations of traditional methods like sand casting. While sand casting remains prevalent for producing intricate geometries, AM—especially SLM—offers unparalleled design freedom, reduced material waste, and enhanced mechanical properties. AlSi10Mg, a hypoeutectic Al-Si-Mg alloy, is widely used in both processes due to its excellent castability, thermal conductivity, and strength-to-weight ratio.
This study focuses on addressing two critical gaps:
- The directional dependency of mechanical properties in SLM-fabricated AlSi10Mg.
- The performance disparities between AM and sand casting under standardized heat treatments.
By systematically analyzing microstructure, tensile strength, elongation, and fracture mechanisms, this work provides actionable insights for optimizing manufacturing protocols in automotive applications.
2. Materials and Methods
2.1 Material Specifications
The AlSi10Mg alloy powder used for SLM and the resin-coated sand for 3DP sand casting were characterized as follows:
Table 1: Chemical composition of AlSi10Mg powder (wt.%)
| Si | Mg | Fe | Cu | Mn | Ni | Ti | Al |
|---|---|---|---|---|---|---|---|
| 10.20 | 0.31 | 0.06 | <0.01 | <0.01 | <0.01 | <0.01 | Bal. |
Table 2: Properties of AlSi10Mg powder
| Loose Density (g/cm³) | Tap Density (g/cm³) | Angle of Repose (°) | Hall Flow Rate (s/50g) | Moisture Content (%) |
|---|---|---|---|---|
| 1.34 | 1.61 | 36 | 97 | <0.10 |
Table 3: Properties of resin-coated sand for 3DP molds
| Compressive Strength (MPa) | Tensile Strength (MPa) | Gas Evolution (mL/g at 850°C) |
|---|---|---|
| 6.1 | 1.8 | 13 |
2.2 Manufacturing Processes
2.2.1 SLM Additive Manufacturing
Specimens were printed using an FS421M SLM machine under the following parameters:
Table 4: SLM process parameters
| Layer Thickness (mm) | Scan Speed (mm/s) | Laser Power (W) | Oxygen Content (ppm) | Hatch Spacing (mm) | Chamber Pressure (kPa) | Baseplate Temp. (°C) |
|---|---|---|---|---|---|---|
| 0.04 | 1300 | 400 | 0–1000 | 0.15 | 1–3 | 160 |
Three build orientations (0°, 45°, 90°) were tested, with supports added to 45° specimens to prevent sagging. Post-printing, select specimens underwent stress-relief annealing at 300°C for 2 hours.
2.2.2 3DP Sand Casting
Sand molds were printed using an AJ-1800 3DP binder jetting system. Cast specimens were subjected to T6 heat treatment:
- Solution treatment: 540°C for 6 hours, followed by water quenching.
- Artificial aging: 165°C for 8 hours.
Table 5: T6 heat treatment parameters
| Stage | Temperature (°C) | Time (h) | Cooling Method |
|---|---|---|---|
| Solution Treatment | 540 | 6 | Water Quench |
| Aging | 165 | 8 | Air Cooling |
2.3 Testing Protocols
- Microstructural Analysis: Optical microscopy (GX71) and SEM.
- Mechanical Testing: Tensile tests (Z050 Universal Testing Machine) and hardness measurements (Duravision 300 Brinell Hardness Tester).
- Fractography: SEM analysis of fracture surfaces.
3. Results and Discussion
3.1 Microstructural Characteristics
3.1.1 SLM Additive Manufacturing
- As-printed: Rapid solidification resulted in fine α-Al(Si) supersaturated solid solution with spherical Si particles.
- Post-annealing (300°C/2h): Si particles coarsened slightly but retained uniform distribution. Directional striations (0°) and spiral patterns (45°) were observed, while 90° specimens exhibited porosity.
3.1.2 3DP Sand Casting
- As-cast: Coarse α-Al(Si) matrix with needle-shaped eutectic Si.
- Post-T6: Partial spheroidization of Si phases; however, residual porosity and gas-induced defects persisted.
Table 6: Microstructural comparison
| Process | Matrix Phase | Si Morphology | Defects |
|---|---|---|---|
| SLM (Annealed) | α-Al(Si) | Spherical, 1–5 µm | Minor porosity |
| 3DP (T6) | α-Al(Si) | Needle/Blocky, 10–50 µm | Gas pores, inclusions |
3.2 Mechanical Performance
3.2.1 Tensile Strength and Anisotropy
SLM specimens exhibited significant directional anisotropy pre-annealing:
- Yield Strength (YS): 295 MPa (0°), 280 MPa (45°), 254 MPa (90°).
- Tensile Strength (UTS): 340 MPa (0°), 320 MPa (45°), 300 MPa (90°).
Post-annealing homogenized properties:
- YS: ~230–240 MPa across all orientations.
- UTS: ~290–300 MPa.
3DP sand-cast specimens showed lower and more variable performance:
- As-cast: YS = 140 MPa, UTS = 195 MPa.
- Post-T6: YS = 210 MPa, UTS = 240 MPa.
Table 7: Mechanical properties comparison
| Process | Condition | YS (MPa) | UTS (MPa) | Elongation (%) |
|---|---|---|---|---|
| SLM | As-printed | 254–295 | 300–340 | 3–5 |
| SLM | Annealed (300°C/2h) | 230–240 | 290–300 | 8–12 |
| 3DP Sand Casting | As-cast | 140 | 195 | 3.2 |
| 3DP Sand Casting | T6-treated | 210 | 240 | 1.5 |
3.2.2 Ductility and Fracture Behavior
- SLM: Elongation improved 2.5–3× after annealing, reaching >10% for 0° specimens. Fracture surfaces displayed cleavage facets and minor gas pores.
- 3DP Sand Casting: Brittle fracture with limited elongation (1.5%) due to coarse Si phases and casting defects.
Equation 1: Relationship between strength and orientation (SLM)Δσ=−20×θ+295Δσ=−20×θ+295
Where ΔσΔσ = yield strength (MPa), θθ = angular deviation from 0° (in increments of 45°).
4. Conclusion
- Additive manufacturing via SLM produces AlSi10Mg with refined microstructures and superior mechanical properties compared to sand casting.
- Directional anisotropy in SLM specimens can be eliminated through 300°C/2h annealing, achieving isotropic YS (~235 MPa) and UTS (~295 MPa).
- Sand casting suffers from inherent defects (e.g., gas porosity, coarse Si), resulting in lower strength and ductility despite T6 treatment.
- SLM’s capability to fabricate complex, lightweight components makes it ideal for automotive applications, whereas sand casting remains cost-effective for low-volume production.