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Article

Thermal and Mechanical Properties of Nano-TiC-Reinforced 18Ni300 Maraging Steel Fabricated by Selective Laser Melting

by
Francisco F. Leite
1,2,3,4,
Indrani Coondoo
1,2,*,
João S. Vieira
3,4,
José M. Oliveira
1,3,4 and
Georgina Miranda
1,2,*
1
CICECO, Aveiro Institute of Materials, University of Aveiro, 3810-193 Aveiro, Portugal
2
Department of Materials and Ceramic Engineering (DEMaC), University of Aveiro, 3810-193 Aveiro, Portugal
3
EMaRT, Emerging Materials and Research Technologies, University of Aveiro, 3720-511 Oliveira de Azeméis, Portugal
4
School of Design, Management and Production Technologies Northern Aveiro (ESAN), University of Aveiro, 3720-511 Oliveira de Azeméis, Portugal
*
Authors to whom correspondence should be addressed.
J. Manuf. Mater. Process. 2024, 8(6), 268; https://doi.org/10.3390/jmmp8060268
Submission received: 23 September 2024 / Revised: 12 November 2024 / Accepted: 22 November 2024 / Published: 28 November 2024
Browse Figures
Figure 1
<p>SEM images of (<b>a</b>) as-received 18Ni300 maraging steel powder, (<b>b</b>) TiC-18Ni300 nanocomposite feedstock (after high-energy ball milling), (<b>c</b>) TEM image of the TiC nanoparticles, where inset shows the statistical average size of the nanoparticles, and (<b>d</b>) TEM-EDS profile of the TiC nanoparticles in the TiC-18Ni300 composite powder from (<b>b</b>). Inset (1) shows the HRTEM image of the TiC nanoparticles of the TiC-18Ni300 composite powder, while inset (2) illustrates the lattice fringes.</p> "> Figure 2
<p>Cross-sections (XY and ZZ) of specimens from experiments 5, 10 and 13 (see <a href="#jmmp-08-00268-t004" class="html-table">Table 4</a> for details of the experimental groups).</p> "> Figure 3
<p>XRD patterns of 18Ni300 powder, nano-TiC powder, and produced specimens from experiments 3 and 10, before (as-built) and after heat treatment.</p> "> Figure 4
<p>SEM images of specimens from experiment 10: (<b>a</b>) as-built and (<b>b</b>) after aging.</p> "> Figure 5
<p>SEM image (<b>a</b>) and EDS mapping (<b>b</b>–<b>f</b>) of the specimen from experiment 10, after aging.</p> "> Figure 6
<p>SEM and EDS of specimen from experiment 10 after aging, for TiC particle and steel matrix.</p> "> Figure 7
<p>Average hardness for the nanocomposite produced under different experiments, for as-built and heat-treated conditions.</p> ">
Versions Notes

Abstract

:
Additive manufacturing (AM) has brought new possibilities to the moulding industry, particularly regarding the use of high-performance materials as maraging steels. This work explores 18Ni300 maraging steel reinforced with 4.5 vol.% TiC nanoparticles, fabricated by Selective Laser Melting (SLM), addressing the effect of post-fabrication aging treatment on both thermal and mechanical properties. Design of Experiments (DoE) was used to generate twenty-five experimental groups, in which laser power, scanning speed, and hatch distance were varied across five levels, with the aim of generating conclusions on optimal fabrication conditions. A comprehensive analysis was performed, starting with the nanocomposite feedstock and then involving the microstructural, mechanical, and thermal characterisation of SLM-fabricated nanocomposites. Nanocomposite relative density varied between 92.84% and 99.73%, and the presence of martensite, austenite, and TiC was confirmed in the as-built and heat-treated conditions. Results demonstrated a hardness of 411 HV for the as-built 18Ni300-TiC nanocomposite, higher than that of the non-reinforced steel, and this was further increased by performing aging treatment, achieving a hardness of 673 HV. Thermal conductivity results showed an improvement from ~12 W/m·K to ~19 W/m·K for nano-TiC-reinforced 18Ni300 when comparing values before and after heat treatment, respectively. Results showed that the addition of TiC nanoparticles to 18Ni300 maraging steel led to a combined thermal and mechanical performance suited for applications in which heat extraction is required, as in injection moulding.

1. Introduction

In the last few decades, the moulding industry has mainly used subtractive techniques to produce moulding areas and components such as cores and inserts. However, the emergence of additive manufacturing (AM)—especially Selective Laser Melting (SLM)—has introduced new possibilities within this industry, such as the creation of conformal cooling channels that are impossible to produce using subtractive machining techniques [1,2]. While the main principle of subtractive techniques is based on removing material to form a final part, AM techniques are based on adding material, generally layer by layer, to form a final part. In the case of SLM, the feedstock is a metal powder that is subsequently thermally fused by a laser. Among the several advantages of AM, the following can be highlighted: high design freedom [3], low lead times [4], and contribution to a circular economy given that the powders can be reused [5].
Several steels are currently used to conventionally manufacture moulds for injection moulding, such as AISI D2 [6], H13, P20, 420 stainless steel [7], and H11 [8]. Furthermore, the literature shows that the AM processability of some of these steels has been addressed [9,10]. The fabrication of 18Ni300 maraging steel with SLM has been explored within the moulding industry, also due to its low carbon content (0.03 wt.% max.), which contributes to reducing crack susceptibility [11,12]. Furthermore, 18Ni300 maraging steel simultaneously possesses superlative mechanical properties, such as a good combination of ultra-high strength and fracture toughness [13]. Unlike other steels, the carbon content of maraging steels may be seen as an impurity rather than a hardening element, and it is kept as low as is commercially allowed [14]. Additionally, while in other steels it is common to attain a hardness increase due to carbide precipitation, maraging steels are hardened by the precipitation of intermetallic compounds from martensite during the age hardening of the low-carbon iron–nickel martensitic matrix, which explains the name maraging (mar (from martensite) + aging) [14,15].
Solution treatment (ST), aging treatment (AT), or a combination of ST and AT are the standard heat treatments used to improve the mechanical properties of 18Ni300 maraging steel.
ST corresponds to an annealing treatment, commonly performed at temperatures between 815 °C and 1020 °C for 1 h, followed by cooling in air [13,16,17,18]. ST provides a complete transformation of martensite in austenite and a homogeneous distribution of alloying elements [13]. For SLM-fabricated 18Ni300, ST can promote a complete removal of the as-built cellular structure, depending on the temperature and duration of the treatment, thus eliminating the segregation at cell boundaries [13,19,20].
Aging treatments performed between 440 and 650 °C for 3–8 h, followed by cooling to room temperature, lead to the precipitation of intermetallic compounds, the type and amount of which are dependent on the AT temperature and time [21,22]. These intermetallic compounds include Ni3Ti, Ni3Mo [23,24], Fe7Mo6, and Fe2Mo [23], which promote a significant increase in the mechanical properties of the aged SLM-fabricated 18Ni300 when compared to the as-built condition. For AT temperatures between 400 and 450 °C, it is expected that metastable phases, as well as Fe2Mo, Ni3 (Ti, Mo), and Ni3Mo, will be found, with the latter commonly found when AT is performed at 500 °C [22,25,26]. Mutua et al. [27] tested different heat treatments, all combining an ST at 820 °C for 1 h with subsequent AT using different temperatures between 460 and 600 °C for 0.5–24 h. Compared to the SLM as-built condition, it was reported that the steel that underwent ST (820 °C; 1 h) followed by AT (460 °C; 5 h) had an improved hardness of 618 HV [27]. Another study from Tekin et al. [28] on SLM-fabricated 18Ni300 assessed the difference between performing ST + AT and, alternatively, only AT. ST was performed at 820 °C for 1 h and AT at 490 °C for 6 h, followed by furnace cooling. It was verified that the steel that only underwent AT showed a much higher ductility (an increase of 70–80%) when compared to steels that were subjected to ST + AT. Simultaneously, in both conditions, very similar hardness and tensile strength were reported [28]. Similar findings were reported by Elangeswaran et al. [29] for SLM-fabricated 18Ni300, where a hardness of 605 ± 8 HV was found for specimens that exclusively underwent AT, and a hardness of 615 ± 19 HV was found for specimens that underwent ST + AT. In terms of microstructure, after AT, the typical SLM cellular structure is maintained, like the one found in the as-built condition [13,19].
Previous studies indicate that for SLM-fabricated 18Ni300, AT can be readily applied without prior ST [18,22]. This is explained by the nature of SLM fabrication, which, unlike conventional technologies, uses a laser, providing high cooling rates (103–108 K/s) that can be compared to an intrinsic solution treatment [19,30]. A study by Bai et al. [16] compared the hardness of 18Ni300 maraging steel subjected exclusively to ST (840 °C, 1 h) and the hardness of 18Ni300 maraging steel subjected to ST (840 °C for 1 h) followed by AT (480 °C, 6 h). These authors concluded that while the as-built material displayed a hardness of 381 HV, after ST, a decrease to 342 HV was verified. Finally, when ST and AT were sequentially combined, a hardness of 646 HV was reached.
The fabrication of 18Ni300 maraging steel by SLM has been reported [11,12,22], also including its use as a matrix for obtaining metal matrix composites (MMCs) [31,32] or even metal matrix nanocomposites (MMNCs), the latter more scarcely found in the literature [33]. One of the reasons for developing MMNCs is that they can improve strength and ductility due to the decreased size of reinforcing particles. It has been proven that mechanical properties can be enhanced when reducing the size of ceramic reinforcing particles from micron to nano size [34]. TiC is one of the materials gaining relevance as a reinforcement/nano-reinforcement in MMCs/MMNCs produced by AM. This carbide has low density, a very high melting point, high hardness, high fracture toughness, and high resistance to oxidation and wear [35,36]. Gu et al. [37] used SLM to fabricate a nano-TiC-reinforced titanium, proving that the addition of the nano reinforcement improves the tribological performance. Jia et al. [38] produced Inconel 718 reinforced with nano-TiC by SLM, having found a positive effect of the nanoreinforcement, to reduce the wear rate during dry sliding wear tests. Cao et al. [39] produced Inconel 625 reinforced with nano-TiC using Laser Metal Deposition, assessing the influence of the energy input on the densification, microstructure, microhardness and wear performance, concluding that for optimal fabrication conditions, nano-TiC contributes to elevating the microhardness and reducing the wear rate. Hu et al. [40] explored the addition of different contents of micron-sized TiC (0–10 wt.%) to 18Ni300 maraging steel fabricated using Direct Laser Deposition. The composite with optimal content (5 wt.%) was afterwards subjected to a solution treatment at 850 °C for 1h followed by aging at 500 °C for 3 h [40]. Consequently, precipitation of nano-sized Ni3Mo and Ni3Ti and the presence of austenite dispersed in the martensitic matrix occurred, which resulted in an improvement of the microhardness from 377 HV in the as-built condition to 444 HV after heat treatment. Moreover, after the heat treatment, the microhardness increased (576 HV) when compared to the non-reinforced steel (368 HV) [40].
In this work, nano-TiC composites based on 18Ni300 maraging steel were additively manufactured by Selective Laser Melting, assessing the most adequate parameters for obtaining high densification and hardness. It may be noted that, to date, there is only one article on nano-TiC-reinforced 18Ni300 composites fabricated using SLM, which was studied by our group [33]. This previous work addressed the mechanical and wear behaviour; however, in the present work, the effect of the aging treatment on TiC-reinforced 18Ni300 nanocomposites was studied and correlated with the inherent metallurgical changes. The effects of nano-TiC reinforcements on the microstructural evolution, hardness and thermal properties were explored, the latter, to our best knowledge, being reported for the first time.

2. Experimental Procedure

2.1. Feedstock Preparation

The feedstock used to produce the nanocomposites consisted of a mixture of 18Ni300 maraging steel powder (Renishaw Ltd., Wotton-under-Edge, UK) and 4.5 vol.% (2.8 wt.%) TiC black nanoparticles (Nanografi, Ankara, Türkiye), as our previous work [33] confirmed that this nano-TiC content led to the highest YS and UTS among different contents tested to reinforce 18Ni300 maraging steel. The particle size distribution of the nanocomposite powders was analysed separately using a Partica LA-960V2 (Horiba, Kyoto, Japan), and in the case of the nano-TiC powders, a Polyethyleneimine (PEI) surfactant was used to reduce the typical agglomeration of the nanopowders. The size and shape of the TiC nanoparticles were assessed using Transmission Electron Microscopy (TEM) with a JEM-2200FS (JEOL, Tokyo, Japan). The powder mixture was produced as previously described [33]. The starting 18Ni300 maraging steel powders and the mixture of 18Ni300 and nano-TiC after high-energy ball milling were characterised regarding the powder morphology and assessing possible agglomeration in the nanocomposite feedstock. The morphological observations were carried out using Scanning Electron Microscopy (SEM) with a Hitachi SU-70 (Hitachi, Tokyo, Japan), equipped with Energy-Dispersive Spectroscopy (EDS) Quantax 400 (Bruker, Fremont, CA, USA).
The flowability of the 18Ni3000 powders and the nanocomposite feedstock was assessed by determining the flow rate obtained using the Hall funnel method according to ASTM B213-20 [41], where the amount of time, in seconds, taken for 50 g of each powder to entirely flow through the 2.5 mm opening of the hopper was timed. This test was performed seven times for each feedstock, thus obtaining an average result.
Bulk (ρbulk) and tapped (ρtapped) densities of the nanocomposite feedstock were measured using the tapped density method, according to ASTM B527 [42]. This method consists of several taps of the powder inside a graduated cylinder for some time, in which bulk density is the density before tapping and tapped density is the density after tapping. The densities ρbulk and ρtapped were calculated as explained in the formulas described in Equations (1) and (2), which follow ASTM B212 [43] and ASTM B527 [42], respectively.
ρ b u l k = m V b u l k   g / cm 3
ρ t a p p e d = m V t a p p e d   g / cm 3
where m is the mass of powder, in g, inside the cup, and Vbulk is the volume of powder before the test, in cm3. The frequency of this test was 40 taps per minute, which allowed to measure Vtapped, the compacted volume after tapping. Finally, to assess the flowability quality of the powder, the Compressibility Index (C) and Hausner ratio (H) were measured, as indicated in Equations (3) and (4).
C = V b u l k − V t a p p e d V b u l k × 100   %
H = ρ t a p p e d ρ b u l k

2.2. SLM Fabrication and Specimen Preparation

To assess the optimal processing window for SLM fabrication of TiC-18Ni300 nanocomposite, three of the most influential SLM parameters were selected (laser power, scanning speed, and hatch distance). Using Minitab and following the Taguchi method, a Design of Experiments (DoE) based on the levels found in Table 1 was performed, generating 25 different combinations, as seen in Table 2. The Volumetric Energy Density (VED), in J/mm3, for each combination and the exposure time, in µs, are also found in Table 2. The layer thickness (50 µm) and the point distance (70 µm) were kept constant.
Cuboid specimens (12 × 12 × 12 mm3) were fabricated in an AM 500Q machine (Renishaw Ltd., Wotton-under-Edge, UK), using its Reduced Build Volume (RBV), having a build volume of 78 × 78 × 55 mm3. A minimum of six specimens per experimental group were produced, using a stripe strategy, and between each layer, a 67° clockwise rotation was performed to minimise residual stresses. Additionally, part of the specimens experienced an aging heat treatment for 6 h at 510 °C, as reported in our previous work [33]. Vertical (ZZ) and horizontal (XY) cross-sections were analysed, for fabrication defect assessment (as lack of fusion and keyhole porosity), porosity evaluation, and microstructural analysis. After polishing, the specimens were etched with 5 vol.% Nital (HNO3 and ethanol) for 5 s.
For the thermal characterisation, cylindrical specimens of 18Ni300 maraging steel and 18Ni300 maraging steel reinforced with nano-TiC, were SLM-fabricated using the parameters of experiment 10. These cylindrical specimens with a diameter of 18 mm and a height of 16 mm were cut using electro-discharge machining (EDM) to obtain samples with a diameter of 18 mm and a height of 6 mm for thermal conductivity measurements.

2.3. Material Characterisation

High-resolution transmission electron microscopy (HRTEM) using a JEM-2200FS (JEOL, Tokyo, Japan) was performed on the ball-milled feedstock nanocomposite powder. The TEM data were analysed using the Digital-Micrograph software package (version 3.53.4137.0). Cross-section analysis was performed via optical microscopy, using a JENAPHOT2000 (Zeiss, Jena, Germany), and with SEM using a Hitachi SU-70 (Hitachi, Tokyo, Japan), equipped with EDS, Quantax 400 (Bruker, Fremont, CA, USA). The optical density was determined using optical microscopy images using a threshold filter in ImageJ software (version 1.54g). Vickers micro-hardness (HV) was assessed using a Wilson VH1102 micro-hardness tester (Buehler, Lake Bluff, IL, USA), applying a load of 2 kgf for 15 s, according to ASTM E92-17 [44]. Average results were obtained after indenting each specimen five times in both the vertical and horizontal cross-sections. The as-built and heat-treated conditions of the non-reinforced and nano-reinforced 18Ni300 maraging steel were assessed. X-ray diffraction (XRD) was performed using an X’Pert Pro3 (Panalytical, Almelo, The Netherlands) with Cu-Kα radiation (λ = 1.5406 Å) in 2θ range 20°–100°, step size of 0.02°. Thermal conductivity measurements were performed using a TCi-3-A thermal conductivity analyser (C-Therm, Fredericton, NB, Canada), with the average results being determined from six measurements per group.

3. Results and Discussion

3.1. Feedstock Analysis

Figure 1 shows images of the as-received 18Ni300 steel powders (Figure 1a) and of the TiC-18Ni300 composite powder after ball milling (Figure 1b). The TEM examination of the as-received TiC nanoparticles revealed, in general, hexahedron shape with particle sizes in the range of 20–200 nm (Figure 1c). The inset of Figure 1c illustrates the particle size distribution fitted with a log-normal distribution function, showing an average particle size of ~53.5 nm. As seen in Figure 1b, the morphology of the powders changed due to plastic deformation after high-energy ball milling. One can also observe that nano-TiC particles surround the maraging steel powders. Similar observations were noted by AlMangour et al. [45], who produced H13 steel reinforced with nano-TiC by SLM, which verified that the H13 powders were deformed plastically during high-energy ball milling, whereas the nano-TiC powders remained intact. Additionally, they concluded that the plastic deformation during milling led to cold welding of the powders, thus increasing the average particle size. To ascertain the morphology of the TiC nanoparticles after high-energy ball milling, the feedstock TiC-18Ni300 nanocomposite powder was investigated using HR-TEM, and the same is depicted in Figure 1d. As seen in the inset (1) of Figure 1d, the TiC particles retain their shape, which is similar to that of the as-received TiC nanoparticles (Figure 1c). Moreover, the TEM-EDS clearly shows the intense peaks of Ti and C along with smaller contributions from Fe, Ni and Co. The presence of the strong Cu peak is due to the TEM grid. The distinct lattice fringes of the TiC nanoparticles are obvious in the inset (2) of Figure 1d. The inter-planar spacings were estimated to be ~0.21 nm, which corresponds to the {200} planes of TiC.
Both the as-received 18Ni300 powder and the ball-milled TiC-18Ni300 feedstock were tested regarding their flowability using the Hall funnel method. The 18Ni300 powders displayed a flow rate of 20.62 ± 0.31 s/50 g, whereas for the nanocomposite powders, a 29.82 ± 0.76 s/50 g flow rate was obtained. These results show that the nanocomposite feedstock presents lower flowability than the as-received steel powder. The addition of nanoparticles to micron-sized metal powders has been explored by other authors, such as Zhai et al. [46], who mixed 316L stainless steel powders with Y2O3 nanoparticles, verifying that the addition of nanoparticles and the milling process made the particles’ surface rougher, contributing to lower flowability. A similar conclusion was found in this study, with both the change in morphology and the presence of nanoparticles contributing to a lower flowability, as proven by the obtained results. Table 3 details the bulk density (ρbulk), tapped density (ρtapped), Compressibility Index and the Hausner Ratio (H) of the 18Ni300 + 4.5 vol.% nano-TiC feedstock. These results confirm the quality of this nanocomposite feedstock for SLM fabrication.

3.2. Densification Analysis

After SLM fabrication, the relative densities were determined for each experimental group, with the results being listed in Table 4, which also includes the corresponding VED.
The results in Table 4 show that experimental groups 1 to 5 display the lowest densification levels, with relative densities between 92.84 and 96.81%. Figure 2 shows a cross-section of a specimen from group 5, clearly evidencing a lack of fusion porosity. It is worth mentioning that in groups 1 to 5, a laser power of 150 W was used, the lowest power tested in this study. Previous studies on SLM-fabricated 18Ni300 have reported a VED between 70 and 185 J/mm3 as most adequate for high densification [47,48]. However, it has been previously demonstrated that it is necessary to look beyond the VED, considering the effect of each parameter and their interactions, to effectively optimize the fabrication of a given alloy [49]. In fact, when comparing groups 1 to 5, where very distinctive VEDs were used, namely 92.31, 50.00, 31.58, 21.82, and 16.00 J/mm3, similar insufficient densification was verified, albeit one of these groups (Exp.1) falls inside the previously mentioned optimal VED range. On the other hand, gas porosity and keyhole porosity, the latter typically related to excessive energy input [49], were also observed, as shown in Figure 2, for specimens fabricated in experiment 13. Figure 2 also presents cross-sections of specimens fabricated in experiment 10, having a relative density of 99.59%.

3.3. Phase and Microstructural Analysis

For phase analysis, specimens from experimental groups 3 and 10 were analysed and selected as examples of poor and high densification. The martensite peaks and the higher intensity peaks of nano-TiC powders, shown in their respective diffraction patterns, are also identified on all the SLM-fabricated specimens.
As depicted in Figure 3, peaks of martensite, austenite and TiC were detected for the heat-treated conditions. Regarding the as-built condition, only martensite and TiC were detected. Austenite reversion has been reported in SLM-fabricated 18Ni300 maraging steel, after aging treatment, either partial or total [28], as the amount of reverted austenite depends on the aging temperature and/or time [50]. Furthermore, during over-aging, the Ni enrichment in the matrix leads to the lowering of the martensite start temperature (Ms), in some cases to below room temperature, hampering the martensitic transformation [50].
Regarding the intermetallic nanoprecipitates due to the aging treatment of 18Ni300 maraging steel, their detection was not possible using X-ray diffraction. In fact, previous authors also reported difficulty in detecting any intermetallic precipitates, due to their low volume fraction [23,27]. However, their presence can be indirectly confirmed by analysing the mechanical properties after the heat treatment, which will be further analysed in Section 3.4.
Figure 4 depicts the microstructures found in the as-built and aged conditions, for specimens from experiment 10, showing the typical cellular microstructure after SLM fabrication, caused by the rapid cooling and solidification rates [47]. Previous studies on maraging steels fabricated by SLM have reported Ni segregation due to the non-equilibrium conditions, namely the extremely rapid solidification. This Ni segregation, but also Mo and Ti, has been consistently reported to occur in the intercellular regions of typical cellular structures found in as-built SLM-fabricated maraging steels [28]. It has been reported that only solution treatments (STs) are able to eradicate completely the abovementioned segregation, as this treatment eliminates the cellular structure [13].
In the present study, we performed exclusively an aging treatment (AT); thus, the cellular structure is also present in the heat-treated condition (Figure 4b). Despite being visible, the cellular structure after aging (Figure 4b) is not as evident as the one found in the as-built condition (Figure 4a). Bai et al. [51] and Yin et al. [52] noted that an increase in AT temperature and time gradually dissolved the cellular structures and their boundaries became discontinuous. Simultaneously, the lighter regions around cell boundaries became more visible. According to Casati et al. [19] and Mooney et al. [53], this represents an increased austenite content due to austenite reversion, here also confirmed by the XRDs depicted in Figure 3, showing austenite in the heat-treated condition.
Furthermore, to ascertain the chemical composition of the nanocomposite, EDS elemental mapping and spectrum were obtained after aging (Figure 5), showing the main constituent elements (Fe, Ni, Co, Mo and Ti) of the 18Ni300 maraging steel, in agreement with the datasheet provided by the supplier of the 18Ni300 powder [54]. At this magnification (Figure 5), it was not possible to ascertain the element segregation at the refined cellular structure level.
Despite its low content, a careful examination revealed the presence of TiC nanoparticles in the nanocomposite, as seen in Figure 6, by the encircled region identifying a cluster of TiC nanoparticles. Furthermore, the EDS profile and quantification results support this statement, indicating that the cluster region contains 12.30 wt.% of Ti and 20.88 wt.% of C, whereas the matrix presents 4.33 wt.% of Ti and 6.47 wt.% of C.

3.4. Mechanical Properties

Figure 7 shows average hardness values, considering vertical and horizontal cross-section measurements. The results in Figure 7 show that after the heat treatment, the hardness of the specimens remarkably increased. While for the as-built condition, the hardness varied between 355 HV2 and 418 HV2, after the aging treatment, it varied between 631.52 HV2 and 673.13 HV2. To sum up, after the heat treatment, the percentage growth in the hardness of the nanocomposite varied between 53 and 85%.
Among the different experiments, group 10 was found to display one of the highest hardness values together with the lowest percentage of microstructural defects. In this sense, the parameters used in experiment 10 are herein used for the ensuing comparative analysis.
Aiming to understand the effect of the nano-TiC incorporation on the hardness, non-reinforced 18Ni300 maraging steel specimens were produced by SLM (as-built and for subsequent heat treatment), using the parameters of experiment 10. In the as-built condition, the non-reinforced steel reached 312.9 HV2, while after heat treatment, it reached 579.9 HV2. The aging treatment reportedly leads to the precipitation of nano-intermetallic compounds, which are known to strengthen these steels via the Orowan strengthening effect and are due to the mismatch of the coefficients of thermal expansion of 18Ni300 maraging steel and nano-TiC, which in turn increase the dislocation density [33]. The results obtained in our study are in line with previously reported hardness for wrought 18Ni300 maraging steel, prior to and after aging [55,56]. When comparing the non-reinforced steel hardness with the results depicted in Figure 7, it is possible to perceive a very significant improvement, before and after heat treatment.
Some studies have explored the addition of micron-sized reinforcements to 18Ni300 steel fabricated by SLM. Li et al. [57] studied the effect of the addition of different contents of micron-sized WC to 18Ni300 maraging steel produced by SLM, concluding that it is possible to increase this steel hardness with WC additions, reporting a maximum hardness of approximately 47 HRC when adding 20 wt.% WC. Wei et al. [58] assessed the effect of 1 wt.% micron-size SiC addition to SLM-produced 18Ni300, finding an 8% increment in hardness when compared to the non-reinforced steel. Hu et al. [40] fabricated 18Ni300 maraging steel reinforced with micron-sized TiC via SLM and compared this composite with the non-reinforced steel. For the as-built condition, the addition of 5 wt.% TiC led to an increase of 18% in hardness, which in turn, after aging, led to an additional 29% increase in hardness. On the other hand, the use of nano-reinforcements in 18Ni300 is scarcely found in the literature, and to the authors’ knowledge, only one study, performed by our group, addressed the fabrication of these nanocomposites using SLM [33].

3.5. Thermal Properties

The cooling step during the injection moulding process is not just important because it should be as short as possible, but also because the quality of the final parts depends on it. For this reason, it is of great interest to have adequate thermal conductivity and to promote an adequate heat flow between the mould and the injected part. In fact, the lack of previous data and research on thermal conductivity in maraging steel composites further motivated to explore the thermal properties of this nano-TiC-reinforced 18Ni300 composite.
Table 5 shows the average results obtained for the thermal conductivity measurements of the non-reinforced and nano-TiC-reinforced 18Ni300 maraging steel in the as-built and heat-treated conditions.
The thermal conductivity obtained in 18Ni300 maraging steel is comparable to the values available in the material datasheets [54], which report 14.2 W/m·K in the as-built condition, and 21 W/m·K after age hardening of 18Ni300. It may be noted that the thermal conductivity of TiC (20 W/m·K) is similar to that of aged 18Ni300 [59,60].
As seen in Table 5, the thermal conductivity decreased after the addition of nano-TiC, both in as-built and heat-treated composites in comparison to the unreinforced 18Ni300. The decrease is ~24.5% and ~9% under as-built and heat-treated conditions, respectively. However, after the heat treatment, the thermal conductivity increased by 27.6% for 18Ni300, while an enhancement of ~54% was observed in the nano-TiC-reinforced 18Ni300. As far as thermal properties are concerned, a literature survey revealed only a few articles that studied the thermal conductivity in composites based on Inconel 718 and on 431 and 316L stainless steels.
In a recent work, Cho et al. [61] observed a thermal conductivity value of ~21 W/m·K (at room temperature) in SS431, which reduced to ~17.5 W/m·K in micron-TiC-reinforced SS431. In another work, Gu et al. performed [62] a numerical simulation to derive thermal properties in nano-TiC and Inconel 718; however, nothing was mentioned regarding the composite. On the other hand, remarkable enhancement in thermal conductivity was reported in the metal composite 316L-Cu system processed by the laser powder bed fusion process [63].
There are several classical empirical, theoretical, and semi-theoretical models for predicting thermal transport properties in a wide range of materials [64,65,66]. Nevertheless, the estimation of thermal conductivity is not straightforward since several factors and mechanisms are involved in the heat transfer process that may vary in materials depending on whether they are single-phase or particulate composites. The latter has added complexities arising from the type (metallic/non-metallic) and size of the inclusions (micro/nano). Particularly when the composites have embedded particles, it is important to consider the following influencing factors that affect the overall thermal conductivity of the composites: (i) the interfacial thermal resistance between the matrix and the inclusions (governed by the density, phonon velocity and specific heat capacity of the matrix and the reinforcements); (ii) the difference in thermal conductivity between the matrix and the embedded particles; (iii) scattering of energy carriers: electrons and phonons; (iv) volume fraction and relative size of the inclusions concerning the intrinsic mean free path associated with the scattering; and (v) temperature. In general, for composites with nano-sized inclusions, the heat conduction is expected to be strongly affected by the interface/surface effects. In the present work, the nanosized TiC reinforcement particles that present a large volume fraction of boundaries are expected to cause greater scattering of energy carriers, thereby increasing the thermal resistance, or in other words, decreasing the effective thermal conductivity [60,67].
Moreover, the difference in the specific heat capacity between TiC (568 J/kg·K) and 18Ni300 (470 J/kg·K) [59,68] may induce an interfacial thermal resistance between the matrix and the reinforcing particles, degrading the effective thermal conductivity of the composite [67,69,70]. This explains the reduction in the bulk thermal conductivity observed in the nano-TiC-reinforced 18Ni300 nanocomposites as compared to the 18Ni300 steel. On the other hand, after heat treatment, the improvement in thermal properties of the unreinforced 18Ni300 and TiC-18Ni300 nanocomposite in comparison to as-built material (see Table 5) can be attributed to the enhanced thermal transport phenomena owing to the intermetallic precipitates and the austenite phases that are formed after the heat treatment.

4. Conclusions

In this work, the printability via Selective Laser Melting of nano-TiC-reinforced 18Ni300 maraging steel was studied, aiming for applications in the injection moulding industry. Design of Experiments (DoE) was used to generate twenty-five experimental groups, in which different processing parameters (laser power, scanning speed, and hatch distance) were varied among five levels. Ensuing fabrication by SLM, an aging treatment at 510 °C for 6 h was performed, and its effect was analysed.
The analysis of the nanocomposite feedstock revealed an adequate flowability that allowed efficient fabrication, with high densifications, above 99.5% being achieved in several groups. Regarding the mechanical properties, the hardness of the nanocomposite varied between 355 and 418 HV2 in the as-built condition and between 632 and 673 HV2 after the aging treatment. As for the thermal conductivity, results showed an improvement from ~12 W/m·K in as-built TiC-reinforced18Ni300 composite to ~19 W/m·K after heat treatment. These results show that the combined thermal and mechanical performance of this nanocomposite is suited for applications where heat extraction is required, such as moulds.

Author Contributions

F.F.L.: Writing—original draft, writing—review and editing, investigation, formal analysis, data curation. I.C.: Writing—review and editing, formal analysis. J.S.V.: Writing—review and editing, investigation, formal analysis, data curation. J.M.O.: Conceptualization, validation, supervision. G.M.: Conceptualization, validation, Writing—review and editing, supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020, UIDP/50011/2020 and LA/P/0006/2020, financed by national funds through the FCT/MCTES (PIDDAC). The author, I. Coondoo would like to acknowledge Fundação para a Ciência e a Tecnologia (FCT) I.P., through DL 57/2016/CP1482/CT0048 (https://doi.org/10.54499/DL57/2016/CP1482/CT0048).

Data Availability Statement

It is declared that the authors did not use any AI-assisted technologies. Dataset available on request from the authors.

Acknowledgments

Special acknowledgement to Simoldes Aços, Lda. (Oliveira de Azeméis, Portugal) for the support and availability for the SLM fabrication.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Jmmp 08 00268 g001
Figure 1. SEM images of (a) as-received 18Ni300 maraging steel powder, (b) TiC-18Ni300 nanocomposite feedstock (after high-energy ball milling), (c) TEM image of the TiC nanoparticles, where inset shows the statistical average size of the nanoparticles, and (d) TEM-EDS profile of the TiC nanoparticles in the TiC-18Ni300 composite powder from (b). Inset (1) shows the HRTEM image of the TiC nanoparticles of the TiC-18Ni300 composite powder, while inset (2) illustrates the lattice fringes.
Figure 1. SEM images of (a) as-received 18Ni300 maraging steel powder, (b) TiC-18Ni300 nanocomposite feedstock (after high-energy ball milling), (c) TEM image of the TiC nanoparticles, where inset shows the statistical average size of the nanoparticles, and (d) TEM-EDS profile of the TiC nanoparticles in the TiC-18Ni300 composite powder from (b). Inset (1) shows the HRTEM image of the TiC nanoparticles of the TiC-18Ni300 composite powder, while inset (2) illustrates the lattice fringes.
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Figure 2. Cross-sections (XY and ZZ) of specimens from experiments 5, 10 and 13 (see Table 4 for details of the experimental groups).
Figure 2. Cross-sections (XY and ZZ) of specimens from experiments 5, 10 and 13 (see Table 4 for details of the experimental groups).
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Figure 3. XRD patterns of 18Ni300 powder, nano-TiC powder, and produced specimens from experiments 3 and 10, before (as-built) and after heat treatment.
Figure 3. XRD patterns of 18Ni300 powder, nano-TiC powder, and produced specimens from experiments 3 and 10, before (as-built) and after heat treatment.
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Figure 4. SEM images of specimens from experiment 10: (a) as-built and (b) after aging.
Figure 4. SEM images of specimens from experiment 10: (a) as-built and (b) after aging.
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Figure 5. SEM image (a) and EDS mapping (b–f) of the specimen from experiment 10, after aging.
Figure 5. SEM image (a) and EDS mapping (b–f) of the specimen from experiment 10, after aging.
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Figure 6. SEM and EDS of specimen from experiment 10 after aging, for TiC particle and steel matrix.
Figure 6. SEM and EDS of specimen from experiment 10 after aging, for TiC particle and steel matrix.
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Figure 7. Average hardness for the nanocomposite produced under different experiments, for as-built and heat-treated conditions.
Figure 7. Average hardness for the nanocomposite produced under different experiments, for as-built and heat-treated conditions.
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Table 1. Input parameters for the DoE.
Table 1. Input parameters for the DoE.
Level 1Level 2Level 3Level 4Level 5
Laser Power (W)150200250300350
Scanning Speed (mm/s)500750100012501500
Hatch Distance (µm)658095110125
Table 2. Plan of experiments for SLM fabrication.
Table 2. Plan of experiments for SLM fabrication.
Exp. N°Laser Power (W)Scanning Speed (mm/s)Hatch Distance (µm)Exposure Time (µs)VED (J/mm3)
115050065140.0092.31
21507508093.3350.00
315010009570.0031.58
4150125011056.0021.82
5150150012546.6716.00
620050080140.00100.00
72007509593.3356.14
8200100011070.0036.36
9200125012556.0025.60
1020015006546.6741.03
1125050095140.00105.26
1225075011093.3360.61
13250100012570.0040.00
1425012506556.0061.54
1525015008046.6741.67
16300500110140.00109.09
1730075012593.3364.00
1830010006570.0092.31
1930012508056.0060.00
2030015009546.6742.11
21350500125140.00112.00
223507506593.33143.59
2335010008070.0087.50
2435012509556.0058.95
25350150011046.6742.42
Table 3. Properties of the 18Ni300 + 4.5 vol.% nano-TiC feedstock.
Table 3. Properties of the 18Ni300 + 4.5 vol.% nano-TiC feedstock.
ρbulk (g/cm3)ρtapped (g/cm3)C (%)H
4.234.546.821.07
Table 4. Relative density and corresponding VED of the SLM-fabricated nanocomposites.
Table 4. Relative density and corresponding VED of the SLM-fabricated nanocomposites.
Exp. N°Relative Density (%)VED (J/mm3)
193.4592.31
293.9350.00
396.8131.58
492.8421.82
595.2716.00
699.73100.00
799.6056.14
898.8136.36
999.6325.60
1099.5941.03
1198.61105.26
1299.6560.61
1399.3840.00
1497.8861.54
1599.4041.67
1699.42109.09
1798.5664.00
1898.8892.31
1998.5960.00
2098.3242.11
2198.95112.00
2299.03143.59
2399.2387.50
2499.7158.95
2598.5042.42
Table 5. Thermal conductivity of 18Ni300 maraging steel and 18Ni300 + nano-TiC, in as-built and heat-treated conditions.
Table 5. Thermal conductivity of 18Ni300 maraging steel and 18Ni300 + nano-TiC, in as-built and heat-treated conditions.
Thermal Conductivity (W/m·K)
As-BuiltHeat-Treated
18Ni30018Ni300 + Nano-TiC18Ni30018Ni300 + Nano-TiC
16.30 ± 0.4212.31 ± 1.1020.80 ± 0.3718.94 ± 0.48
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MDPI and ACS Style

Leite, F.F.; Coondoo, I.; Vieira, J.S.; Oliveira, J.M.; Miranda, G. Thermal and Mechanical Properties of Nano-TiC-Reinforced 18Ni300 Maraging Steel Fabricated by Selective Laser Melting. J. Manuf. Mater. Process. 2024, 8, 268. https://doi.org/10.3390/jmmp8060268

AMA Style

Leite FF, Coondoo I, Vieira JS, Oliveira JM, Miranda G. Thermal and Mechanical Properties of Nano-TiC-Reinforced 18Ni300 Maraging Steel Fabricated by Selective Laser Melting. Journal of Manufacturing and Materials Processing. 2024; 8(6):268. https://doi.org/10.3390/jmmp8060268

Chicago/Turabian Style

Leite, Francisco F., Indrani Coondoo, João S. Vieira, José M. Oliveira, and Georgina Miranda. 2024. "Thermal and Mechanical Properties of Nano-TiC-Reinforced 18Ni300 Maraging Steel Fabricated by Selective Laser Melting" Journal of Manufacturing and Materials Processing 8, no. 6: 268. https://doi.org/10.3390/jmmp8060268

APA Style

Leite, F. F., Coondoo, I., Vieira, J. S., Oliveira, J. M., & Miranda, G. (2024). Thermal and Mechanical Properties of Nano-TiC-Reinforced 18Ni300 Maraging Steel Fabricated by Selective Laser Melting. Journal of Manufacturing and Materials Processing, 8(6), 268. https://doi.org/10.3390/jmmp8060268

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