Compared to directional energy deposition, selective laser melting has been less studied for the fabrication of functionally graded materials, and the post-processing window remains unclear.
Our researchers used SLM technology to prepare 316L/IN718 functionally graded materials and systematically evaluated the effects of representative heat treatment processes on phase evolution and tensile properties
1.SLM preparation of 316L/IN718 functionally graded materials
2.Heat treatment process
Based on the analysis of the above figure, a gradient heat treatment scheme was designed. Two solution temperatures were selected: 980°C (below the solution temperature) and 1040°C (above the solution temperature), combined with two aging strategies: single aging at 720°C and double aging at 720°C + 620°C. Based on this, five sets of control experiments were set up:
AD group (deposited state): maintained in its original preparation state;
HT1 group: 1040°C solution treatment for 1 hour (water quenching) + 720°C single aging for 8 hours (air cooling);
HT2 group: 1040°C solution treatment for 1 hour (water quenching) + 720°C aging for 8 hours followed by 620°C aging for 8 hours (furnace cooling);
HT3 group: 980°C solution treatment for 1 hour (water quenching) + 720°C single aging for 8 hours (air cooling);
HT4 group: 980°C solution treatment for 1 hour (water quenching) + 720°C aging for 8 hours followed by 620°C aging for 8 hours (furnace cooling).
3.Phase transformation after heat treatment
Five sets of X-ray diffraction (XRD) patterns in the Y-Z plane under different heat treatment conditions, with test areas covering: region 1 (IN718 content 70-100%), region 2 (IN718 content 40-70%), and region 3 (IN718 content 0-30%).
The diffraction peak intensities under the five heat treatment conditions did not show significant differences; the Bragg reflection of the austenitic phase-especially the strong (111) and (200) peaks of the face-centered cubic (FCC) structure-dominated the diffraction pattern.
In the HT1-treated sample from region 1, the intensities of peaks (111) and (220) were higher than those of the deposited state (AD). In addition, all heat-treated groups showed a diffraction peak (311), indicating that an additional reinforcing phase was formed after heat treatment.
Under HT1 conditions, the diffraction peaks in region 2 are wider and have lower intensity, suggesting that the phase stability in this region is weaker.
In region 3, the intensity of the (111) peak in the HT3-treated sample was significantly enhanced. Notably, γ' and γ" strengthening phases were detected in the XRD pattern of region 1. Rapid cooling during high-throughput SLM preparation is not conducive to the precipitation of γ' and γ" phases, while heat treatment provides sufficient time for the precipitation of these strengthening phases, which explains the increase in the intensity of the (200) and (220) crystal plane peaks and the appearance of the (311) peak after heat treatment.
After heat treatment with HT2 and HT4, (311) diffraction peaks of the γ' and γ" phases were also detected in the XRD patterns. However, compared with the (311) diffraction peaks after solution treatment and single aging, the diffraction peaks after double aging were more intense, indicating that the double aging process further promoted the formation of the γ' and γ" strengthening phases. The intensity of the strengthening phase diffraction peaks was particularly significant under the HT2 treatment condition, indicating that this heat treatment promoted the precipitation of more γ' and γ" phases. The precipitation effect of the strengthening phase is expected to have a positive impact on the mechanical properties of the HT2-treated state. However, the crystal orientation of the main peak (111) did not change significantly, indicating that the heat treatment did not change the preferred orientation of the 316L/IN718 functionally graded material.
4.Microstructure after heat treatment
Under depositional (AD) conditions, long-chain Laves phases exist in region 1. Due to the high IN718 content in this region, a large amount of Nb-rich phase precipitates in the intergranular region, with a composition of (Ni, Fe, Cr)2(Nb, Mo, Ti). Under HT1 treatment, most of the Laves phase undergoes dissolution and fracture, and the residual phase transforms into a granular morphology. In HT3 treatment, the Laves phase also transforms into a granular form through a dissolution process, accompanied by the precipitation of needle-like/rod-like δ-Ni3Nb phases. This indicates that both HT1 and HT3 samples induced diffusion segregation of elements (Ni, Nb, C, Mo) in region 1, a phenomenon consistent with the results of in-situ statistical distribution measurements of metals in depositional and heat-treated samples using high-resolution microbeam X-ray fluorescence spectroscopy.
Multiscale analysis results confirm that by controlling the solubility of the Laves phase through solution temperature and controlling the morphology of the δ-Ni3Nb phase through aging time, the synergistic optimization of strength and plasticity of gradient materials can be achieved. This provides key phase engineering guiding principles for the development of novel gradient heat treatment processes.
The microstructure evolution of Region 3 under different heat treatment regimes reveals the phase transformation kinetics driven by the coupling effect of compositional gradient and thermal history. The cross-scale microstructure evolution mechanism of this region is summarized, and the correlation mechanism between heat treatment, grain boundary engineering, and mechanical properties is established. Under depositional (AD) conditions, the 316L-dominant region (Cr/Ni = 1.82) follows a ferrite-austenite (FA) dual-phase solidification path, forming a cellular dendritic structure. After HT1 heat treatment, the Cr/Ni ratio decreases to 1.35. This compositional transformation promotes the solidification path from a ferrite-austenite dual-phase to a fully austenitic single-phase structure, significantly reducing the interdendritic ferrite content. Phase identification confirms this transformation: the FCC phase is a γ-austenite matrix, the BCC phase is δ-ferrite, and Ni3Al corresponds to the γ' precipitate phase. Region 3 is dominated by austenite, containing a small amount of dispersed ferrite. The volume fractions of ferrite measured by quantitative image analysis were 3.5% (AD), 0.7% (HT1), 0.2% (HT2), 1.5% (HT3), and 0.8% (HT4), respectively, confirming that the ferrite content in all heat-treated states was lower than that in the deposited state.
Post-deposition heat exposure promotes static recrystallization, leading to grain coarsening and a significant reduction in dendrite spacing. The synergistic effect of the compositional gradient is also significant: along the forming direction (IN718 content increasing from 0 to 100 wt%), the decreasing local cooling rate induces gradual coarsening of the dendrite arms. The deposited sample in region 3 is characterized by fine equiaxed grains, with even smaller grain sizes (~8.4 μm) at the bottom of the melt pool due to laser remelting. In contrast, the heat-treated samples exhibit a more uniform grain size distribution, but grain coarsening occurs in region 3 after heat treatment-the average grain sizes of HT1 and HT3 samples are 10.40 μm and 11.64 μm, respectively. This coarsening is mainly attributed to the synergistic effect of heat accumulation and cooling rate: region 3 is located at the bottom of the gradient material, resulting in less heat accumulation during the high-energy SLM process and finer initial grains; while the slow cooling process after deposition heat treatment provides sufficient time for grain growth. In addition, the sample contains continuous columnar crystals that penetrate multiple layers. Due to the rapid directional solidification characteristics of the SLM process, the grain growth direction is usually consistent with the direction of the maximum temperature gradient (i.e., perpendicular to the bottom of the molten pool).
Solution treatment significantly reduces texture strength and improves uniformity, with HT2 showing the most significant effect: 1040°C solution treatment combined with double aging induces subgrain boundary formation, increasing the proportion of small-angle grain boundaries (LAGBs) to 39.1% (the highest among all heat treatments). This greatly improves the multi-scale coordinated deformation capability of the gradient structure and promotes isotropic behavior.
Post-solution heat treatment significantly reduces residual stress and promotes substantial dissolution of the Laves phase (the degree of dissolution increases monotonically with solution temperature); high-throughput SLM inherently refines the deposited microstructure due to its high cooling rate, but subsequent heat treatment induces significant grain coarsening. Notably, a small amount of δ-Ni3Nb phase remains after solution treatment at 980°C, indicating that this temperature is below the δ-Ni3Nb phase solution line.
5.Tensile properties
Tensile fracture was almost entirely concentrated in the compositional transition zone between the 30% IN718 + 70% 316L and 40% IN718 + 60% 316L regions, where elemental segregation was most pronounced. The only exception occurred in the HT2 heat-treated state, where fracture initiated at the 50% 316L + 50% IN718 region and was accompanied by significant necking. These findings quantitatively demonstrate that compositional gradient variations dominate the load-bearing capacity of 316L/IN718 functionally graded materials (FGMs).
When the solution temperature is 1040°C, both the strength and plasticity of the material are improved. Under single aging treatment, the HT1 process significantly improves the strength of 316L/IN718 functionally graded materials (FGMs) better than HT2, with a strengthening effect of 6.58%. The sample treated with HT2 showed the most significant increase in elongation at the 1040°C solution temperature, with an increase of approximately 62.99%. These results indicate that at the 1040°C solution temperature, single aging is more conducive to strength improvement, while double aging is more conducive to plasticity improvement.
When the solution treatment temperature drops to 980°C, the material strength increases (higher with double aging and better with single aging), but the plasticity decreases compared to the deposited state. The combined improvement in strength and plasticity indicates that HT2 is the optimal heat treatment for 316L/IN718 functionally graded materials.
6.in conclusion
(1)Solution temperature dominates the phase evolution path, while the effect of aging is negligible. A solution temperature ≥1040°C can significantly dissolve the Laves phase and inhibit the formation of the δ-Ni3Nb phase, thereby releasing Nb elements for the subsequent precipitation of the γ″/γ′ strengthening phase, providing a necessary prerequisite for obtaining a good balance between strength and plasticity.
(2)Aging methods allow for strength-plasticity control. Double aging after solution treatment at 1040°C can increase plasticity by approximately 30% without sacrificing strength, making it suitable for high-plasticity applications. Conversely, solution treatment at 980°C induces the precipitation of needle-like δ-Ni3Nb phases along grain boundaries; this leads to a significant decrease in plasticity under both single and double aging, and is therefore only recommended for applications where medium-temperature creep is dominant.
(3)Gradient components require a "high-temperature homogenization followed by low-temperature aging" strategy. The IN718 enriched region itself is rich in Nb and Mo elements, necessitating pre-solution treatment at ≥1040°C; otherwise, subsequent low-temperature aging will form a continuous needle-like δ-Ni3Nb phase network, resulting in a room-temperature toughness loss of ≥40%. This treatment sequence can serve as a general design principle for the heat treatment following selective laser melting (SLM) of similar functionally graded materials (FGMs).
(4)The characterization of gradient materials should follow a three-stage closed-loop process: First, macroscopic tensile pre-screening is performed to identify batch-to-batch differences; second, strain field distribution maps ε(x) are plotted using full-field digital image correlation (DIC) technology, and local stress-strain (σ-ε) constitutive relations are obtained through micro/nano-scale mechanical testing; finally, the gradient constitutive model embedded with finite element analysis (FEA) is calibrated. This verification chain can decouple the overall response into spatially resolved design allowable values, thereby enabling fine-tuning of the process and assessment of service reliability.
