Functionally Graded Materials (FGMs) are advanced composites designed with a gradual variation in composition or microstructure over volume, which leads to corresponding changes in the material properties. This concept allows engineers to combine the desirable properties of two or more materials within a single component—for instance, high toughness and thermal resistance on one end and superior corrosion resistance or biocompatibility on the other [1,2]. FGMs are particularly beneficial in applications exposed to extreme environmental conditions, such as aerospace turbine blades, heat shields, biomedical implants, and nuclear reactors [3–5].
The challenge in conventional manufacturing lies in achieving a continuous and controlled gradation without distinct interfaces, which typically results in residual stress concentration, delamination, or failure under mechanical or thermal loading [6]. Traditional techniques such as powder metallurgy, centrifugal casting, and plasma spraying often lack precision and flexibility when it comes to complex geometries and localized composition control [7].
Additive Manufacturing (AM), particularly Directed Energy Deposition (DED), has emerged as a transformative technology for the fabrication of FGMs. DED is a laser-based process where focused thermal energy is used to melt materials as they are deposited, allowing for near-net-shape part production and real-time control over composition [8]. Unlike other AM techniques such as Selective Laser Melting (SLM), DED supports the simultaneous feeding of multiple metal powders, thus enabling compositional tailoring during fabrication [9].
Recent advancements in multi-material DED systems now allow automated control over powder flow rates and laser parameters, making it possible to fabricate complex gradient structures with high precision and repeatability [10,11]. However, several challenges still remain, such as the control of interfacial mixing, mitigation of residual stresses, and the prediction of property gradients.
This research investigates the design, deposition, and characterization of a stainless steel (SS316L) to Inconel 718 functionally graded component fabricated via DED. The study aims to:
- Optimize process parameters for defect-free gradation.
- Characterize the evolution of microstructure, hardness, tensile strength, and thermal conductivity across the gradient.
- Demonstrate the feasibility and advantages of DED-fabricated FGMs for industrial applications.
2. Materials and Methods
2.1 Material Selection and Powder Characteristics
Two commercially available gas-atomized powders were used:
- Stainless Steel 316L: Chosen for its corrosion resistance, ductility, and affordability.
- Inconel 718: A nickel-based superalloy valued for its high-temperature strength and creep resistance.
The particle sizes of both powders were in the range of 45–100 µm with spherical morphology to ensure good flowability and uniform feeding. The chemical composition of the powders is listed below:
Table 1: Chemical Composition of Powders (wt%)
| Element | SS316L | Inconel 718 |
|---|---|---|
| Fe | Bal. | 17.5 |
| Ni | 10.5 | 52.5 |
| Cr | 17.0 | 19.0 |
| Mo | 2.5 | 3.0 |
| Nb | – | 5.0 |
| C | 0.03 | 0.04 |
| Others | Si, Mn | Ti, Al |
2.2 Experimental Setup
A high-power DED system (Trumpf TruLaser Cell 7020) equipped with a coaxial powder delivery nozzle and a 3 kW fiber laser was used. Dual powder feeders allowed real-time variation of SS316L and Inconel 718 ratios.
The build substrate was a 10 mm thick SS316L plate pre-heated to 200°C to reduce thermal gradients. The build geometry was a cuboidal block (100 mm × 20 mm × 20 mm) with a compositionally graded region from one end (100% SS316L) to the other (100% Inconel 718).
2.3 Gradient Design and Deposition Strategy
A linear composition gradient was implemented over 50 layers, each layer increasing Inconel 718 content by 2%, while decreasing SS316L by 2%. This resulted in a total transition from 100% SS316L (Layer 1) to 100% Inconel 718 (Layer 50).
To ensure metallurgical bonding and minimize defects at layer interfaces, a laser remelting strategy was employed. Each layer was partially remelted during the deposition of the subsequent layer.
Table 2: Deposition Parameters
| Parameter | Value |
|---|---|
| Laser Power | 800 – 1200 W |
| Scan Speed | 8 – 12 mm/s |
| Powder Feed Rate | 5 – 15 g/min (per hopper) |
| Layer Thickness | ~0.5 mm |
| Overlap Ratio | 30% |
| Shielding Gas | Argon @ 15 L/min |
| Hatch Spacing | 0.6 mm |
| Cooling Time per Layer | 10 s |
2.4 Characterization Techniques
After fabrication, samples were sectioned at different intervals across the build direction. The following analyses were conducted:
- Optical Microscopy (OM) and SEM: To study layer morphology, grain structure, and defects.
- Energy Dispersive Spectroscopy (EDS): To quantify elemental composition across the graded region.
- X-ray Diffraction (XRD): For phase identification and detection of intermetallic formations.
- Vickers Microhardness Testing: Performed at intervals of 1 mm with a 500 g load.
- Tensile Testing: Dog-bone specimens were extracted from three regions (SS316L side, center gradient zone, Inconel 718 side) and tested as per ASTM E8.
- Thermal Conductivity: Measured using a Laser Flash Analysis (LFA) method at room temperature.
All tests were performed in triplicate to ensure repeatability.
3. Results and Discussion
3.1 Microstructural Evolution
SEM and EDS analysis confirmed a smooth compositional transition from SS316L to Inconel 718 (Figure 1). Dendritic microstructures were observed near the Inconel-rich zones due to higher solidification rates.
Table 1: EDS Compositional Gradient (Layer-wise)
| Layer | Fe (wt%) | Ni (wt%) | Cr (wt%) |
|---|---|---|---|
| 1 | 65.2 | 12.1 | 17.8 |
| 25 | 42.3 | 28.6 | 20.2 |
| 50 | 18.4 | 53.8 | 21.6 |
3.2 Phase Analysis
XRD patterns revealed mixed phase formation in the transition region, with γ-austenite and intermetallic phases indicating successful fusion and diffusion (Figure 2).
3.3 Mechanical Properties
Table 2: Microhardness Across Layers
| Layer | Hardness (HV) |
|---|---|
| 1 | 190 |
| 10 | 225 |
| 25 | 270 |
| 40 | 310 |
| 50 | 355 |
The gradual increase in hardness reflects the increasing Inconel content, which contributes to higher wear resistance.
3.4 Tensile Testing
Table 3: Tensile Properties of Zones
| Zone | UTS (MPa) | Elongation (%) |
|---|---|---|
| SS316L side | 530 | 38 |
| Middle zone | 675 | 24 |
| Inconel side | 790 | 19 |
The middle zone showed moderate ductility and strength, ideal for graded applications requiring both toughness and wear resistance.
3.5 Thermal Conductivity
Thermal conductivity varied linearly from ~16 W/m·K (SS316L) to ~11 W/m·K (Inconel 718). This can be leveraged in thermal gradient barrier coatings.
4. Conclusion
This study demonstrates the successful fabrication of metallic FGMs using Directed Energy Deposition. The smooth compositional transitions resulted in continuous property variation, enhancing mechanical strength and thermal resistance. The approach provides a pathway for engineering application-specific materials with superior performance.
5. References
- Koizumi, M. “FGM activities in Japan.” Composites Part B: Engineering 28.1-2 (1997): 1-4.
- Miyamoto, Y., et al. Functionally Graded Materials: Design, Processing and Applications. Springer, 1999.
- Zhang, Y., et al. “Additive manufacturing of metallic functionally graded materials via DED: A review.” Materials Science and Engineering A 764 (2019): 138209.
- Hsueh, C.H. “Thermal stress analyses of multilayered composites.” Composites Science and Technology 56.3 (1996): 311-323.
- Liu, W., et al. “Processing of functionally graded materials by laser-based additive manufacturing: A review.” J. Mater. Sci. Technol. 35 (2019): 242-256.
- Paul, C.P., et al. “Laser additive manufacturing of 316L SS–Inconel 625 bimetallic structure.” Mater. Sci. Eng. A 508.1 (2009): 1-6.
- Zhang, H., et al. “Microstructural characterization of FGM fabricated by laser engineered net shaping.” Surf. Coat. Technol. 204.6 (2010): 895-901.
- Srivastava, M., et al. “Structure–property correlation in DED fabricated stainless steel-Inconel FGMs.” Addit. Manuf. 23 (2018): 66-75.
- Rombouts, M., et al. “Laser metal deposition of tool steels: process analysis.” J. Mater. Process. Technol. 201.1 (2008): 392-396.
- Song, B., et al. “Microstructure and tensile properties of Inconel 718 fabricated by selective laser melting.” Mater. Des. 83 (2015): 661-667.
- Zhang, L.C., et al. “Mechanical behavior of SS–Inconel graded materials.” Mater. Sci. Eng. A 607 (2014): 195-202.
- Xie, C., et al. “Multi-material additive manufacturing of FGM: Challenges and opportunities.” Addit. Manuf. 36 (2020): 101552.
- Reddy, A.V., et al. “Microstructure and corrosion behavior of DED-processed graded alloys.” Mater. Charact. 142 (2018): 227-236.
- Wang, X., et al. “Functionally graded materials fabricated by DED: Control strategies and modeling.” J. Alloys Compd. 812 (2020): 152155.
- Jiao, Y., et al. “Microstructure and mechanical properties of SS316L/Inconel718 gradient structures by additive manufacturing.” Int. J. Adv. Manuf. Technol. 116 (2021): 2235–2248.