Additive Manufacturing of PH 13-8 Mo Family: A Review

Abstract

The PH 13-8 Mo family of steels belong to the martensitic precipitation hardening stainless steels (MPHSSs) category, which exhibits a good combination of mechanical properties and corrosion resistance. Additive manufacturing (AM) offers advantages, including reduced material waste and the capability to produce complex, near-net-shape parts. Consequently, the application of AM techniques to the PH 13-8 Mo family is being increasingly explored across various industries. This review paper presents the existing literature on the topic and provides an overview. The review starts by presenting information about the PH 13-8 Mo family, including microstructure, chemical compositions, heat treatments, and mechanical properties. Afterwards, the work focuses on presenting the microstructure and resulting properties of PH 13-8 Mo family processed by three different additive manufacturing processes: Powder Bed Fusion using a Laser Beam (PBF-LB), Directed Energy Deposition using an Electric Arc (DED-Arc), and Directed Energy Deposition using a Laser Beam (DED-LB), both in their as-built condition and following post-processing heat treatments. The review concludes with a summary and outlook that highlights existing knowledge gaps and underscores the need for further research to tailor the microstructural evolution and enhance the properties. The findings indicate that AM of the PH 13-8 Mo family has the potential for industrial applications, yet further studies are necessary to optimize its performance.

1. Introduction

Martensitic precipitation hardening stainless steels (MPHSSs) have a good combination of mechanical properties and corrosion resistance, which makes them suited for aerospace, petrochemical, and molding applications [1,2,3]. Their strength and hardness result from the precipitation hardening mechanism and their martensitic matrix [1]. The first MPHSS was developed in the 1940s [4,5], and Armco Steel Corporation developed the well-known alloys 17-4 PH, 15-5 PH, and PH 13-8 Mo [6,7,8,9,10], being the numbers representing the weight percentage of Chromium and Nickel, respectively [11]. Copper was the hardening element used for the MPHSS alloys developed as 17-4 PH and 15-5 PH [12]. Afterwards, PH 13-8 Mo was produced by adding Aluminium as a hardening element, and its chemical composition was tailored to avoid the formation of δ-ferrite [13,14], as δ-ferrite is known to reduce the impact toughness of these materials [15,16]. PH 13-8 Mo exhibits higher ultimate tensile strength (UTS) and resistance to stress corrosion cracking compared to 17-4 PH and 15-5 PH alloys [17,18,19,20]. The chemical composition of PH 13-8 Mo can be found in Table 1.

The primary solidification phase for alloy PH 13-8 Mo is δ-ferrite, and δ-ferrite transforms to austenite, which subsequently transforms to martensite during cooling [21,22]. The resulting microstructure consists of a martensitic matrix that can contain some amounts of retained austenite and residual δ-ferrite depending on the cooling rate [15,22,23]. It is strengthened by the formation of β-NiAl (B2 structure) precipitates via its precipitation hardening mechanism [24,25,26]. Heat treatment (solution treatment and aging) and the resulting mechanical properties of PH 13-8 Mo according to the ASTM A693 standard [27] are shown in Table 2. The microstructure of solution-treated PH 13-8 Mo can be found in Figure 1 [20].

Table 1. Chemical composition of PH 13-8 Mo family used during AM processes (wt.%).

PH 13-8 Mo Family	Fe	C	Si	Mn	Cr	Ni	Mo	Al	S	N	Reference (Data from)
PH 13-8 Mo	Bal.	0.05	0.1	0.1	12.25–13.25	7.5–8.5	2–2.5	0.9–1.35	0.01	0.01	Ghaffari et al. [28]
EOS StainlessSteel CX	Bal.	≤0.05	≤0.40	≤0.40	11.00–13.00	8.40–10.00	1.10–1.70	1.20–2.00	-	-	Chang et al. [29]
Uddeholm AM Corrax®	Bal.	0.03	0.3	0.3	12	9.2	1.4	1.6	-	-	Krakhmalev et al. [30]

Table 1. Chemical composition of PH 13-8 Mo family used during AM processes (wt.%).

PH 13-8 Mo Family	Fe	C	Si	Mn	Cr	Ni	Mo	Al	S	N	Reference (Data from)
PH 13-8 Mo	Bal.	0.05	0.1	0.1	12.25–13.25	7.5–8.5	2–2.5	0.9–1.35	0.01	0.01	Ghaffari et al. [28]
EOS StainlessSteel CX	Bal.	≤0.05	≤0.40	≤0.40	11.00–13.00	8.40–10.00	1.10–1.70	1.20–2.00	-	-	Chang et al. [29]
Uddeholm AM Corrax®	Bal.	0.03	0.3	0.3	12	9.2	1.4	1.6	-	-	Krakhmalev et al. [30]

Table 2. Heat treatment procedures and resulting mechanical properties of PH 13-8 Mo family from ASTM A693 standard and manufacturers.

Material	Ref. (Data from)	Material Form	Solution Treatment (ST) + Aging (A)		Cooling	Building Direction	Ultimate Tensile Strength (MPa)	Yield Strength (MPa)	Elongation (%)	Hardness
PH 13-8 Mo	[27]	Wrought	ST: 927 ± 15 °C	A: 510 ± 6 °C for 4 h	ST: Cool to below 60 °C A: Air cooling	-	1515	1410	10	45 HRC
EOS StainlessSteel CX	[31]	Additive Manufacturing	ST: 850 ± 10 °C for 30 min	A: 525 ± 10 °C for 2 h	ST: Rapid air cooling below 32 °C A: Air cooling	Vertical	1740	1670	6	50 HRC
						Horizontal	1720	1650	7
Uddeholm AM Corrax®	[32]	Additive Manufacturing	ST: 850 °C for 30 min	A: 525 °C for 4 h	ST: Cool in air A: Cool in air to room temperature	-	1700	1600	10	50 HRC

Table 2. Heat treatment procedures and resulting mechanical properties of PH 13-8 Mo family from ASTM A693 standard and manufacturers.

Material	Ref. (Data from)	Material Form	Solution Treatment (ST) + Aging (A)		Cooling	Building Direction	Ultimate Tensile Strength (MPa)	Yield Strength (MPa)	Elongation (%)	Hardness
PH 13-8 Mo	[27]	Wrought	ST: 927 ± 15 °C	A: 510 ± 6 °C for 4 h	ST: Cool to below 60 °C A: Air cooling	-	1515	1410	10	45 HRC
EOS StainlessSteel CX	[31]	Additive Manufacturing	ST: 850 ± 10 °C for 30 min	A: 525 ± 10 °C for 2 h	ST: Rapid air cooling below 32 °C A: Air cooling	Vertical	1740	1670	6	50 HRC
						Horizontal	1720	1650	7
Uddeholm AM Corrax®	[32]	Additive Manufacturing	ST: 850 °C for 30 min	A: 525 °C for 4 h	ST: Cool in air A: Cool in air to room temperature	-	1700	1600	10	50 HRC

Additive manufacturing (AM) is a production method to manufacture parts in layers [33]. The commonly used AM processes for metals are powder bed fusion (PBF) and directed energy deposition (DED) [34]. The PBF processes use an energy source (electron beam or laser) to selectively melt powder layer upon layer according to a 3D model in the powder bed, and it is well known to be used to manufacture complex-shaped parts [33,34]. DED processes use an energy source (arc, electron beam, or laser) to create a melt pool, and the feedstock, such as powder and wire, is deposited layer upon layer [33,35]. High deposition rates can be achievable with DED using wire as a feedstock compared to powder additive manufacturing processes, while the powder deposition process is preferred for finer features and repair applications [34,35,36,37].

Nowadays, the AM of metals has garnered significant attention owing to its numerous advantages. These include a reduction in lead times and minimizing material waste [38,39,40]. In addition, AM can be used to repair damaged and worn-out parts [38]. The family of MPHSSs are suitable candidates for AM due to good printability [38,41]. Therefore, different powder manufacturers (EOS GmbH, Krailing, Germany and Uddeholms AB, Hagfors, Sweden) have recently developed MPHSS alloys (EOS StainlessSteel CX (EOS GmbH, Krailing, Germany) and Uddeholm AM Corrax^® (Uddeholms AB, Hagfors, Sweden)) for the AM market that could be considered under the PH 13-8 Mo family due to their similarities in chemical composition [31,32]. Their chemical compositions are shown in Table 1, and the suggested heat treatment regime (solution treatment and aging) for the highest hardness and strength established by the manufacturers, as well as the resulting mechanical properties of those, are shown in Table 2.

This work aims to identify and review the existing literature on the AM of the PH 13-8 Mo family, with a focus on its microstructure and properties. As detailed below, both PBF and DED techniques have been employed to process these alloys. To date, most research has focused on PBF using a laser beam (PBF-LB) and DED using an electric arc (DED-Arc). These studies predominantly focus on understanding these alloys’ microstructures and mechanical properties for the as-built and heat-treated conditions. However, only a few studies have addressed the use of DED with a laser beam and powder (DED-LB/p). The paper concludes by summarizing the findings and identifying potential research directions for further exploration of this alloy family.

2. PBF-LB of PH 13-8 Mo Family

2.1. As-Built Microstructure

In 2018, Asgari et al. [42] published the first paper about the microstructure of an almost entirely dense build of CX by PBF-LB. The microstructure consisted of martensite and austenite phases, which was confirmed later by different researchers [42,43,44,45,46,47,48,49]. In addition to austenite and martensite, aluminium oxide inclusions were also found in the as-built microstructure of PBF-LB/CX by Shahriari et al. [43]. Oikonomou et al. [50] also identified aluminium oxide inclusions in Uddeholm AM Corrax^® produced through PBF-LB. Further research demonstrates that precipitation hardening can occur during the PBF-LB process [29,46,47,51]. Transmission Electron Microscopy (TEM) revealed nano-precipitates ranging from 3–25 nm in the as-built state of PBF-LB/CX [29]. Subsequent analysis classified these nano-precipitates as NiAl, which were also observed in the as-built microstructures of both PBF-LB/CX (average diameter = 3.7 nm) and Corrax (average diameter = 4.3 nm) [46,51]. The texture evaluation of the PBF-LB/CX was reported by Pirgazi et al. [45]. Electron backscattering diffraction (EBSD) confirmed the dominant texture in martensite (<111> || BD) and austenite (<011> || BD). On the other hand, as-built PBF-LB/Corrax exhibits a weak crystal texture, and an obvious preferred orientation was not found [46].

It is known that additively manufactured metals can exhibit anisotropic microstructures [38,52]. Therefore, Shahriari et al. [43] studied the anisotropy of as-built CX microstructure using PBF-LB. Dislocation density, lath size, grain boundary density, and residual stresses were found to be different when comparing the side and the top of the sample, resulting from different thermal histories.

The influence of built direction on the microstructure of the PBF-LB/CX was unfolded by Sanjari et al. [53,54]. It was observed that the different built orientations caused different thermal histories and resulted in differences between the microstructures of the horizontal and vertical samples. The vertical specimen has lower retained austenite compared to the horizontal specimen due to the different cooling rates and different prior austenite grain (PAG) sizes. The vertical specimen had a bigger PAG size than the horizontal specimen. It is known that when the PAG size reduces, while austenite strengthens. Therefore, the transformation from austenite to martensite can be suppressed, which explains the higher retained austenite in horizontal material. Similar results were observed by Afkhami et al. [55] and Wu et al. [56] in terms of lower retained austenite and larger grain size in the vertical samples compared to the horizontal PBF-LB/CX and Corrax samples. The morphology of the grains changed due to the building orientation. Vertical material has mostly cellular–columnar grains. On the other hand, equiaxed–columnar grains were the majority in the horizontal material [53]. The different morphologies can be seen in Figure 2. Hadadzadeh et al. [48] also observed fine equiaxed cells in the microstructure of as-built PBF-LB/CX due to high solidification rates. The microstructural evolution process was explained in three states by Chang et al. [29] and Dong et al. [51] during the PBF-LB/CX: the cell state (1), the cellular dendritic state (2), and the blocky state (3).

The influence of linear energy density on the as-built CX by PBF-LB was studied by Dong et al. [51]. CX powder was processed with a linear energy density in the range of 167–438 J/m, where 245 J/m was found to be the optimal value, as it resulted in the best relative density and surface roughness. Fang et al. [46] investigated the influence of the laser energy density (volumetric) on the as-built PBF-LB/Corrax. It was also concluded that when the laser energy increased in a range of between 42.73 J·mm⁻³ and 78.19 J·mm⁻³, the relative density rose and then declined. The powders might not be completely melted when processed with low energy density. On the other hand, the powder might be overmelting under high energy density, which results in defects. The volumetric energy density of 63.97 J·mm⁻³ was chosen as optimum, because it resulted in the highest relative density. Zhang et al. [49] also conducted similar studies and processed the CX with different volumetric energy densities between 57.42 J·mm⁻³ and 88.89 J·mm⁻³. It was confirmed that the number of pores decreased with increasing volumetric energy density. However, when the volumetric energy density was too high, the number of pores increased due to the gas production by ablation. In this case, an 80 J·mm⁻³ volumetric laser energy successfully achieved the highest relative density.

2.2. Post-Processing Heat Treatments: Microstructure

Post-processing heat treatments are very important to improve the microstructure and properties of the PH 13-8 Mo family. Therefore, several authors have investigated the relationship between different heat treatments and the resulting microstructure and tried to find the optimum post-processing heat treatment. Post-processing heat treatment procedures after the PBF-LB process from the literature can be found in Table 3.

The influence of three different heat treatments on the microstructure of PBF-LB/CX was investigated by Yan et al. [47]. These heat treatments were solution treatment at 900 °C for 1 h, aging at 530 °C for 3 h, and solution treatment at 900 °C for 1 h + aging at 530 °C for 3 h. The as-built sample had only 0.3% austenite, and after solution treatment, the austenite was not detected in the sample, which suggested that the solution heat treatment successfully eliminated the austenite. However, the solution annealed microstructure was coarser. Aged and solution-aged samples had higher austenite content (4.5% and 3.5%, respectively). It was considered that during aging treatment, a reverse transformation from martensite to austenite was promoted, which increased the austenite content. NiAl precipitates (3–25 nm) were observed via TEM in the as-built microstructure. On the other hand, larger NiAl precipitates (7–30 nm) at a higher intensity were found in the solution-aged specimen. Those precipitates were rod-like. Zhang et al. [49] used the same heat treatment procedures and achieved similar results regarding the effectiveness of solution-aging heat treatment. In addition, Hadadzadeh et al. [48] conducted experiments with the same heat treatment procedure for PBF-LB/CX. It was confirmed that solution annealing increased the average size of martensite, and reverted austenite was present after the aging treatment. It was added that the aging treatment refined the grains due to the recovery process compared to solution-annealed and as-built conditions. However, solution-aging heat treatment suppressed the formation of NiAl precipitates. Direct aging heat treatment was more successful in promoting the formation of precipitates.

Krakhmalev et al. [30] studied the microstructure of Corrax in three different conditions: heat-treated wrought, heat-treated after PBF-LB, and heat-treated in the HIP (hot isostatic pressing) after PBF-LB. In that study, the recommended heat treatment procedure (solution treatment at 850 °C for 30 min + aging at 525 °C for 4 h) was used. Mostly spherical B2 NiAl precipitates (1–9 nm) were observed using Atom Probe Tomography (APT) in all conditions, as shown in Figure 3. The size distribution of precipitates can be found in Figure 3d. The precipitates of heat-treated PBF-LB/Corrax (d50 = 4.3 nm) were slightly higher than the other conditions. The HIPed microstructure was coarser and presented the highest austenite amount compared to the other conditions investigated.

Chadha et al. [57] heat-treated the PBF-LB/Corrax with solution heat treatment at 900 °C for 1 h and aging at 530 °C for 3 h. However, NiAl precipitates were detected in the same range (1–10 nm) and had a similar average size (4.27 ± 1.20 nm) when comparing the microstructure in both conditions. Similarly, Chang et al. [58] investigated the influence of different heat treatments on the microstructure of PBF-LB/Corrax. When only HIP treatment was applied after the PBF-LB process, the amount of austenite increased. The reason was most probably that the austenization temperature was lower than the HIP temperature used (1300 °C). On the other hand, when HIP + Solution treatment + Aging was applied, the austenite amount was very low. Therefore, the latter was chosen to achieve the optimum properties.

Turnier Trottier et al. [59] compared the microstructure of wrought Corrax and PBF-LB/CX. The materials were solution-treated at 850 °C for 30 min and then subjected to different aging times (525 °C for 1, 2, 3, 4, and 6 h). The highest hardness was achieved for both alloys at 525 °C for 2 h. Therefore, those samples were chosen for further investigation. It was observed that due to the higher cooling rates experienced with PBF-LB, the microstructure was finer than the wrought sample. The as-built microstructure contained more retained austenite (~6%) than the wrought sample (~3%), which was measured using EBSD. However, after the aging treatment (525 °C for 2 h), the retained austenite decreased to ~1% for the PBF-LB/CX and increased to ~4% for the wrought Corrax. As previously observed by Yan et al. [47], during aging, austenite can be reverted from martensite, and the austenite content could be increased. However, the reason was unknown as to why the austenite content decreased after aging treatment.

2.3. Material Properties

2.3.1. Tensile Properties and Hardness

Table 3 shows the collection of tensile properties and hardness measurements for as-built and heat-treated PH 13-8 Mo processed using PBF-LB from the literature.

Asgari et al. [42] published the first paper about the mechanical properties of CX using PBF-LB. The ultimate tensile strength (UTS = 1113 MPa) of the horizontally manufactured CX using PBF-LB was higher compared to the PBF-LB of other stainless steels (PH 17-4 and 316L) and compared to the CX data values from the manufacturer. The hardness of the as-built CX was 35 ± 1 HRC.

The influence of the built direction on the tensile properties and hardness of the as-built PBF-LB/CX was unfolded by Sanjari et al. [53,54]. It was observed that the different built orientations caused different thermal histories, which led to different hardness values. The vertically built sample had lower hardness compared to the horizontal sample due to the slower cooling rate, which resulted in in situ tempering. In addition, the vertical sample had lower UTS and elongation (1085 MPa—5.7%) than the horizontal sample (1092 MPa—21.2%), which was attributed to a higher austenite content, with higher hardening capacity due to transformation-induced plasticity (TRIP) effect in the horizontally built sample. Afkhami et al. [55,60] investigated the influence of the built orientation properties not only for as-built PBF-LB/CX but also for aged PBF-LB/CX. The elongation values of the vertical samples were lower compared to horizontal ones for the as-built and heat-treated conditions. This could be due to the lower austenite content of the vertical samples. On the other hand, the hardness, surface quality, and notch toughness were not significantly influenced by the building orientation. Wu et al. [56] explored the influence of the heat treatment and the built direction on the tensile properties of the PBF-LB/Corrax. The fracture mode (ductile) of the samples was not really affected by the building direction or the heat treatment. Heat-treated vertical samples had lower elongations than horizontal ones because of their higher grain size. It is known that a finer grain size increases crack resistance and enhances elongation.

Dong et al. [51] studied the influence of the linear energy density on the properties of the as-built PBF-LB/CX, and 245 J/m was the value that resulted in the highest UTS (1068 ± 5.9 MPa), total elongation (15.7 ± 0.26%), and hardness (351 ± 4.8 HV0.05). Fang et al. [46] chose 63.97 J·mm⁻³ as the optimum process parameter, which resulted in the highest mechanical properties such as UTS (1084 ± 3 MPa), yield strength (946 ± 7.3 MPa), total elongation (17.64 ± 0.18%), and hardness (374.2 ± 6.5 HV0.05). The optimal tensile specimens showed dimples and necking, which represents ductile fracture.

Strengthening mechanisms of PBF-LB/CX and Corrax in as-built conditions were also investigated by Dong et al. [51] and Fang et al. [46]. According to Dong et al. [51], dislocation hardening (274.37 MPa) was found to be the dominant contributor to the yield strength, which was followed by grain boundary (236.50 MPa), texture (205.72 MPa), and precipitation hardening (164.53 MPa). In addition, the calculated and experimental yield strength were comparable. Fang et al. [46] and Dong et al. [51] confirmed that dislocation hardening has a significant role in strengthening, which is followed by grain boundary strengthening and precipitation hardening. The strengthening mechanisms of aged PBF-LB/CX were explored by Hadadzadeh et al. [48]. It was revealed that the yield strength of the aged PBF-LB/CX is provided by three different strengthening mechanisms such as sub-boundary (247 MPa), dislocation (679 MPa), and precipitation (602 MPa) hardening.

The influence of three different heat treatments on the properties of PBF-LB/CX was investigated by Yan et al. [47]. These heat treatments were solution treatment, aging, and solution treatment + aging (900 °C for 1 h + 530 °C for 3 h). The as-built sample only showed 0.3% austenite. After solution treatment, the austenite was not detected in the sample, which suggested that solution heat treatment was successful in eliminating austenite. Aging and solution-aging heat treatment increased the hardness and UTS, while only solution heat treatment decreased the hardness and UTS of PBF-LB/CX due to the coarser martensitic microstructure. The solution-aging heat treatment was the best combination, because it resulted in the highest hardness (520 HV0.2) and UTS (1610 MPa) values. Similarly, Zhang et al. [49] concluded that solutionizing at 900 °C for 1 h and aging at 530 °C for 3 h was the most suitable procedure in terms of the UTS (1683 MPa) and hardness (50.4 HRC). On the contrary, the same solution-aging heat treatment used by Hadadzadeh et al. [48] did not increase the hardness of the PBF-LB/CX. Solution-aging heat treatment was not found suitable to increase the hardness. Solution heat treatment caused the martensite laths to grow, and this resulted in the impediment of precipitation hardening during aging treatment. However, direct aging treatment resulted in the highest hardness (502 ± 15 HV0.3).

Zhou et al. [61] tried different solutions (850–950 °C for 1 h and 900 °C for 0.5–1.5 h) and aging (430–630 °C for 3 h) heat treatments to find the optimum heat treatment for the PBF-LB/CX. Solution treatment at 900 °C for 1 h followed by aging at 530 °C for 3 h was chosen as the optimum due to the achievement of the highest UTS (1598 MPa), YS (1486 MPa), and hardness (526.3 HV0.2).

By using DSC, but also the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model, for the PBF-LB/CX, Fabian et al. [62] tried to establish a suitable heat treatment to strengthen the material. Direct aging heat treatment at 550 °C for 115 min was chosen as the optimal heat treatment, because the highest hardness was achieved after this treatment. After heat treatment, the tensile strength increased (1032 MPa to 1738 MPa), and the elongation declined slightly (15.3% to 14.7%). The heat treatment was successful in increasing the strength and retaining the dislocation network at the same time, which resulted in maintaining the ductility.

The tensile strength of heat-treated PBF-LB/Corrax was found to be comparable to conventional Corrax by Oikonomou et al. [50]. Corrax was built in two different PBF-LB machines (TruPrint1000 and EOS M290) with different process parameters. It was concluded that the defect density and elongation were different between the specimens processed with two different machines, although the strength values were similar. The elongation of Corrax produced by (TruPrint1000) was lower for the heat-treated samples due to the higher amounts of defects.

Turnier Trottier et al. [59] compared the mechanical properties of wrought Corrax and PBF-LB/CX. The materials were solution-treated at 850 °C for 30 min and then subjected to different aging times (525 °C for 1, 2, 3, 4, and 6 h). Afterwards, the hardness values were compared. Samples that aged at 525 °C for 2 h achieved the highest hardness (XY: 50.7 ± 0.8 HRC, XZ: 50.4 ± 1.0 HRC); therefore, these samples were tensile tested. The UTS, YS, and elongation values of the PBF-LB/CX were higher than in wrought conditions. In addition, anisotropic mechanical properties for the PBF-LB/CX samples were observed, especially in terms of elongation. Horizontal specimens had better elongation compared to vertical ones, because the pores were generally located vertically to the built direction and between the layers. Afkhami et al. [63] also used heat treatment (850 °C for 30 min + 525 °C for 2 h), which is the recommendation from the powder manufacturer for PBF-LB/CX. As expected, the hardness increased after heat treatment from 320 HV3 to 460 HV3.

The influence of the URQ-HIP (Uniform Rapid Quenching–Hot Isostatic Pressing) on the properties of the PBF-LB/Corrax (optimized parameters) was investigated by Maistro et al. [64]. The hardness was increased after URQ-HIP at 850 °C and URQ-HIP at 1140 °C. Krakhmalev et al. [30] measured the hardness of Corrax in three different conditions: heat-treated wrought, heat-treated after PBF-LB, and heat-treated in HIP after PBF-LB. The sample that underwent HIP exhibited the lowest hardness compared to the other two conditions due to the higher amount of austenite and coarser grains. This value was under the recommended hardness range. Therefore, it was concluded that to ensure higher hardness and properties, new heat treatment recipes need to be developed for AM processes. Chang et al. [58] explored the optimization of mechanical properties through changing laser scanning pitch and with different post-processing series for the PBF-LB of Corrax. It was possible to increase the relative density by around 1% after the HIP process (>99%). With the sequence of laser scanning pitch (170 μm)—followed by HIP, solid treatment, and aging—the optimal hardness and UTS were achieved. The values can be found in Table 3.

Table 3. Post-processing heat treatments, tensile properties, and hardness measurements of as-built and heat-treated PBF-LB/PH 13-8 Mo family documented in the literature.

Material	Reference (Data from)	Process/Machine	Process Parameters LP * (W) HS * (μm) SS * (mm/s) LT * (μm)		Condition			Building Direction	Tensile Test Direction	Ultimate Tensile Strength (MPa)	Yield Strength (MPa)	Elongation (%)	Hardness		Cooling
EOS Stainless Steel CX	Asgari et al. [42]	PBF-LB/EOS M290	LP: 258.7 HS: 100 SS: 1066.7 LT: 30		As-built			H *	-	1113	1036	21.7	35 ± 1 HRC		-
	Yan et al. [47]	PBF-LB/EOS M290	LP: 260 HS: 100 SS: 1060 LT: 30		As-built			-	-	1043 ± 7	694 ± 6.5	16.3 ± 0.06	350 ± 8.5 HV0.2		-
					ST *: 900 °C for 1 h					926 ± 5.7	784 ± 5.3	14.4 ± 0.07	325 ± 12.7 HV0.2		Air cooling
					A *: 530 °C for 3 h					1510 ± 8.5	1395 ± 5.8	9.3 ± 0.11	500 ± 9.8 HV0.2
					ST: 900 °C for 1 h	A: 530 °C for 3 h				1601 ± 5.2	1528 ± 8.1	7.3 ± 0.18	510 ± 10.5 HV0.2
	Chang et al. [29]	PBF-LB/EOS M290	LP: 260 HS: 100 SS: 1060 LT: 30		As-built			-	-	-	-	-	357 HV0.2		-
					ST: 900 °C for 1 h					-	-	-	326 HV0.2
					A: 530 °C for 3 h					-	-	-	504 HV0.2
					ST: 900 °C for 1 h	A: 530 °C for 3 h				-	-	-	514 HV0.2
	Hadadzadeh et al. [48]	PBF-LB/EOS M290	LP: 258.7 HS: 100 SS: 1066.7 LT: 30		As-built			-	-	-	-	-	336 ± 5 HV0.3		-
					ST: 900 °C for 1 h					-	-	-	296 ± 6 HV0.3		Rapid air cooling
					A: 530 °C for 3 h					-	-	-	502 ± 15 HV0.3		-
					ST: 900 °C for 1 h	A: 530 °C for 3 h				-	-	-	301 ± 3 HV0.3		ST: Rapid air cooling
	Dong et al. [51]	PBF-LB/EOS M290	HS: 100 LT: 30	LED * = 182 J/m	As-built			-	-	1033 ± 4.4	892 ± 10.7	11.1 ± 0.55	325 ± 3.5 HV0.05		-
				LED = 245 J/m	As-built					1068 ± 5.9	889 ± 7.3	15.7 ± 0.26	351 ± 4.8 HV0.05		-
				LED = 333 J/m	As-built					1059 ± 2.8	886 ± 12.3	14.7 ± 0.26	342 ± 4.2 HV0.05		-
	Zhang et al. [49]	PBF-LB/HBD-280	LP: 340 HS: 100 SS: 850 LT: 50		As-built			H	-	1058 ± 2	992 ± 6	18.4 ± 0.9	38.1 ± 0.5 HRC		-
					ST: 900 °C for 1 h					982 ± 9	819 ± 11	12.7 ± 0.4	29.7 ± 0.6 HRC		Air cooling
					A: 530 °C for 3 h					1648 ± 1	1503 ± 21	11.6 ± 0.5	49.4 ± 0.2 HRC		Furnace cooling
					ST: 900 °C for 1 h	A: 530 °C for 3 h				1683 ± 10	1563 ± 56	8.4 ± 1.6	50.4 ± 0.5 HRC		ST: Air cooling A: Furnace cooling
	Turnier Trottier et al. [59]	PBF-LB/EOS M280	LP: 260 HS: 100 SS: 1067 LT: 60		As-built			H	P *	1098.3 ± 2.5	874.8 ± 23.6	16.06 ± 0.19	XY: 36 ± 0.7 HRC	XZ: 33.3 ± 1.9 HRC	-
								V *	PL *	1079.5 ± 1.2	793.7 ± 62.3	10.76 ± 0.51
					ST: 850 °C for 30 min			H	P	1029.3 ± 1.0	781.7 ± 33.8	12.93 ± 0.26	XY: 31.5 ± 0.7 HRC	XZ: 30.6 ± 0.7 HRC	Forced air cooling
								V	PL	1051.8 ± 5.8	853.4 ± 6.6	10.99 ± 0.76
					ST: 850 °C for 30 min	A: 525 °C for 1 h		-	-	-	-	-	XY: 47.8 ± 0.5 HRC	XZ: 48.7 ± 0.3 HRC
					ST: 850 °C for 30 min	A: 525 °C for 2 h		H	P	1719.5 ± 15.0	1647.3 ± 8.3	8.22 ± 0.54	XY: 50.7 ± 0.8 HRC	XZ: 50.4 ± 1.0 HRC
								V	PL	1698.1 ± 10.4	1617.2 ± 45.0	6.31 ± 0.22
					ST: 850 °C for 30 min	A: 525 °C for 3 h		-	-	-	-	-	XY: 49.1 ± 0.2 HRC	XZ: 49.1 ± 0.2 HRC
					ST: 850 °C for 30 min	A: 525 °C for 4 h		-	-	-	-	-	XY: 48.7 ± 0.3 HRC	XZ: 48.7 ± 0.2 HRC
					ST: 850 °C for 30 min	A: 525 °C for 6 h		-	-	-	-	-	XY: 48.6 ± 0.2 HRC	XZ: 48.1 ± 0.3 HRC
	Fang et al. [46]	PBF-LB/FARSOON FS121M	LP: 190 HS: 90 SS: 1100 LT: 30		As-built			-	-	1084 ± 3	946 ± 7.3	17.64 ± 0.18	374.2 ± 6.5 HV0.2		-
	Ćirić-Kostić et al. [65]	PBF-LB/EOSINT M290	LT: 30		As-built			-	-	-	-	-	31.0 ± 1.5 HRC		-
					ST: 900 °C for 45 min	A: 530 °C for 3 h		-	-	-	-	-	47.0 ± 0.9 HRC		ST: Rapid cooling in air A: Cooling in a furnace
	Chang et al. [66]	PBF-LB/EOS M290	LP: 260 HS: 100 SS: 1060 LT: 30		As-built			-	-	-	-	-	354.9 ± 8.8 HV0.2		-
					ST: 900 °C for 1 h	A: 530 °C for 3 h		-	-	-	-	-	533.4 ± 9.7 HV0.2		Argon cooled
	Chang et al. [67]	PBF-LB/EOS M290	LP: 260 HS: 100 SS: 1060 LT: 30		As-built			-	-	-	-	-	356 ± 14.3 HV0.2		-
					AS *: 900 °C for 1 h			-	-	-	-	-	320 ± 7.2 HV0.2		Argon cooled
					T *: 530 °C for 3 h			-	-	-	-	-	515 ± 6.8 HV0.2
					AS: 900 °C for 1 h	T: 530 °C for 3 h		-	-	-	-	-	527 ± 5.2 HV0.2
	Afkhami et al. [55,60]	PBF-LB/EOS M290-Not machined	LP: 260 HS: 100 SS: 1000 LT: 30		As-built			V	PL	1081.2	899.3	13.3	-		-
					ST: 850 °C for 30 min	A: 525 °C for 2 h		V	PL	1641.5	1556.3	8.1	-		Air cooled
		PBF-LB/EOS M290-Machined			As-built			H	P	1170.4	1006.0	16.6	BP *: 318 HV3	SP *: 336 HV3	-
					ST: 850 °C for 30 min	A: 525 °C for 2 h		H	P	1680.1	1533.4	11.2	BP: 467 HV3	SP: 462 HV3	Air cooled
					As-built			V	PL	1090.2	919.2	10.5	BP: 324 HV3	SP: 322 HV3	-
					ST: 850 °C for 30 min	A: 525 °C for 2 h		V	PL	1682.7	1600.9	5.9	BP: 477 HV3	SP: 472 HV3	Air cooled
	Afkhami et al. [63]	PBF-LB/EOS M290	LP: 260 HS: 100 SS: 1000 LT: 30		As-built			-	-	-	-	-	320 HV3		-
					ST: 850 °C for 30 min	A: 525 °C for 2 h		-	-	-	-	-	460 HV3		Air cooled
	Sanjari et al. [53]	PBF-LB/EOS M290	LP: 258.7 HS: 100 SS: 1066.7 LT: 30		As-built			H	P	1092 ±	807 ±	21.2 ±	-		-
								V	PL	1085 ±	1036 ±	5.7 ±	-		-
	Fabian et al. [62]	PBF-LB/EOS M290	LP: 258.7 HS: 100 SS: 1067 LT: 30		As-built			-	-	1032	929	15.3	39.9 ± 2.9 HRC		-
					A: 400 °C for 1667 min					-	-	-	48.6 ± 0.1 HRC		-
					A: 460 °C for 1078 min					-	-	-	50.8 ± 0.3 HRC		-
					A: 495 °C for 424 min					-	-	-	50.9 ± 0.2 HRC		-
					A: 530 °C for 180 min					-	-	-	50.4 ± 0.5 HRC		-
					A: 550 °C for 115 min					1738	1659	14.7	51.2 ± 1.9 HRC		-
					A: 600 °C for 40 min					-	-	-	43.4 ± 0.4 HRC		-
	Zhou et al. [61]	PBF-LB/FARSOON FS121M	LP: 170 HS: 90 SS: 900 LT: 30		As-built			-	-	1084 ± 3	946 ± 7	14.6 ± 0.9	362 ± 7 HV0.2		-
					ST: 850 °C for 1 h			-	-	875 ± 10	784 ± 13	12.2 ± 0.5	339.2 HV0.2		Air cooling
					ST: 875 °C for 1 h			-	-	808 ± 13	781± 16	11.8 ± 0.8	330.0 HV0.2
					ST: 900 °C for 1 h			-	-	893 ± 8	832 ± 11	11.1 ± 0.5	325.6 HV0.2
					ST: 925 °C for 1 h			-	-	836 ± 12	722 ± 15	11.9 ± 0.7	316.2 HV0.2
					ST: 950 °C for 1 h			-	-	672 ± 13	511 ± 17	11.0 ± 0.7	307.9 HV0.2
					ST: 900 °C for 0.5 h			-	-	870 ± 10	717 ± 13	11.6 ± 0.5	327.8 HV0.2
					ST: 900 °C for 0.75 h			-	-	779 ± 13	600 ± 16	11.8 ± 0.8	326.3 HV0.2
ST: 900 °C for 1.25 h					-	-	788 ± 12	612 ± 15	11.9 ± 0.7	321.2 HV0.2
ST: 900 °C for 1.5 h					-	-	806 ± 9	710 ± 17	11.7 ± 0.6	318.2 HV0.2
ST: 900 °C for 1 h	A: 430 °C for 3 h				-	-	1500 ± 21	1398 ± 43	10.9 ± 1.3	389.3 HV0.2
ST: 900 °C for 1 h	A: 480 °C for 3 h				-	-	1493 ± 15	1400 ± 36	10.6 ± 0.9	454.9 HV0.2
ST: 900 °C for 1 h	A: 530 °C for 3 h				-	-	1598 ± 10	1486 ± 19	9.6 ± 0.8	526.3 HV0.2
ST: 900 °C for 1 h	A: 580 °C for 3 h				-	-	1305 ± 12	1196 ± 24	11.1 ± 1.0	432.6 HV0.2
ST: 900 °C for 1 h	A: 630 °C for 3 h				-	-	1132 ± 13	912 ± 33	17.4 ± 1.6	396.4 HV0.2
Uddeholm AM Corrax®	Oikonomou et al. [50]	PBF-LB/TruPrint 1000	LP: 175 HS: 50 SS: 800 LT: 20		As-built			-	-	-	-	-	35 HRC		-
					ST: 850 °C for 30 min	A: 525 °C for 4 h		V	-	1667 ± 7	1595 ± 12	1.7 ± 0.6	50 HRC		-
					ST: 850 °C for 30 min	A: 525 °C for 4 h		H	-	1624 ± 13.7	1518 ± 14.2	3.68 ± 1.3			-
		PBF-LB/EOS M290	LP: 170 HS: 100 SS: 1250 LT: 30		As-built			-	-	-	-	-	35 HRC		-
					ST: 850 °C for 30 min	A: 525 °C for 4 h		V	-	1701 ± 4	1640 ± 12	8.6 ± 0.3	50 HRC		-
					ST: 850 °C for 30 min	A: 525 °C for 4 h		H	-	1653 ± 6	1560 ± 18	9.7 ± 0.3			-
	Krakhmalev et al. [30]	PBF-LB/EOS M290	LP: 170 HS: 100 SS: 1250 LT: 30		ST: 850 °C for 30 min	A: 525 °C for 4 h		-	-	-	-	-	49.4 HRC		Air cooling
					HIP-ST: 850 °C for 30 min, 150 MPa	HIP-A: 525 °C for 4 h, 150 MPa		-	-	-	-	-	45.9 HRC		After ST cooled down from 850 °C to 60 °C in 60 s
	Chang et al. [58]	PBF-LB/EOS M290	LSP *: 170 μm		As-built			-	-	1065.7 ± 6.7	911.0 ± 9.2	8.4 ± 0.2	31.6 ±1.4 HRC		-
					HIP *: 1300 °C, 100 min, 175 MPa			-	-	928.8 ± 38.9	844.5 ± 57.1	3.3 ± 0.1	30.7 ± 2.1 HRC		Cooling in N2 gas
					HIP: 1300 °C, 100 min, 175 MPa	A: 525 °C for 4 h		-	-	1330.4 ± 104.2	1150.7 ± 42.3	0.8 ± 0.2	47.5 ± 1.3 HRC
					HIP: 1300 °C, 100 min, 175 MPa	ST: 850 °C for 30 min	A: 525 °C for 4 h	-	-	1245.3 ± 198.1	1052.5 ± 25.9	0.5 ± 0.1	48.8 ± 0.5 HRC
	Maistro et al. [64]	PBF-LB/EOS M290	Non-optimized parameters		ST: 850 °C for 30 min in vacuum	A: 525 °C for 4 h in vacuum		-	-	1711 ± 2	1633 ± 9	-	49 HRC		-
					URQ-HIP *: 850 °C, 30 min	URQ-HIP: 525 °C for 4 h		-	-	-	-	-	43 HRC		-
		PBF-LB	Optimized parameters		ST: 850 °C for 30 min in vacuum	A: 525 °C for 4 h		-	-	-	-	-	49.9 HRC		-
					URQ-HIP: 850 °C, 30 min	A: 525 °C for 4 h		-	-	-	-	-	51.4 HRC		A: Air cooled
	Chadha et al. [57]	PBF-LB/EOS M290	-		ST: 900 °C for 1 h	A: 530 °C for 3 h		-	PL	1724 ± 14	1666 ± 12	6 ± 1	50 ± 1 HRC		-
								-	P	1702 ± 16	1628 ± 13	7 ± 1	50 ± 1 HRC		-
	Wu et al. [56]	PBF-LB/EOS M290	LP: 170 HS: 100 SS: 1250 LT: 30		As-built			H	-	1168 ± 16	476 ± 8	16.6 ± 0.1	-		-
					As-built			V	-	1145 ± 8	728 ± 45	14.2 ± 0.2
					ST: 850 °C for 30 min			H	-	1118 ± 21	664 ± 13	13.5 ± 0.2	-		ST: Quenched in N2
					ST: 850 °C for 30 min			V	-	1129 ± 14	717 ± 16	12.9 ± 0.2
					ST: 850 °C for 30 min	A: 525 °C for 4 h		H	-	1589 ± 24	1335 ± 127	12.4 ± 1.2	-		ST: Quenched in N2 A: Air cooled
					ST: 850 °C for 30 min	A: 525 °C for 4 h		V	-	1578 ± 47	1386 ± 73	11.7 ± 1.5

* A: Aging treatment, AS: Austenizing, BP: Building Plane, H: Horizontal, HIP: Hot Isostatic Pressing, HS: Hatch Spacing, LED: Linear Energy Density, LP: Laser Power, LSP: Laser Scanning Pitch, LT: Layer Thickness, P: Perpendicular, PL: Parallel, SP: Scanning Plane, SS: Scanning Speed, ST: Solution Treatment, T: Tempering, URQ-HIP: Uniform Rapid Quenching–Hot Isostatic Pressing, V: Vertical.

Table 3. Post-processing heat treatments, tensile properties, and hardness measurements of as-built and heat-treated PBF-LB/PH 13-8 Mo family documented in the literature.

Material	Reference (Data from)	Process/Machine	Process Parameters LP * (W) HS * (μm) SS * (mm/s) LT * (μm)		Condition			Building Direction	Tensile Test Direction	Ultimate Tensile Strength (MPa)	Yield Strength (MPa)	Elongation (%)	Hardness		Cooling
EOS Stainless Steel CX	Asgari et al. [42]	PBF-LB/EOS M290	LP: 258.7 HS: 100 SS: 1066.7 LT: 30		As-built			H *	-	1113	1036	21.7	35 ± 1 HRC		-
	Yan et al. [47]	PBF-LB/EOS M290	LP: 260 HS: 100 SS: 1060 LT: 30		As-built			-	-	1043 ± 7	694 ± 6.5	16.3 ± 0.06	350 ± 8.5 HV0.2		-
					ST *: 900 °C for 1 h					926 ± 5.7	784 ± 5.3	14.4 ± 0.07	325 ± 12.7 HV0.2		Air cooling
					A *: 530 °C for 3 h					1510 ± 8.5	1395 ± 5.8	9.3 ± 0.11	500 ± 9.8 HV0.2
					ST: 900 °C for 1 h	A: 530 °C for 3 h				1601 ± 5.2	1528 ± 8.1	7.3 ± 0.18	510 ± 10.5 HV0.2
	Chang et al. [29]	PBF-LB/EOS M290	LP: 260 HS: 100 SS: 1060 LT: 30		As-built			-	-	-	-	-	357 HV0.2		-
					ST: 900 °C for 1 h					-	-	-	326 HV0.2
					A: 530 °C for 3 h					-	-	-	504 HV0.2
					ST: 900 °C for 1 h	A: 530 °C for 3 h				-	-	-	514 HV0.2
	Hadadzadeh et al. [48]	PBF-LB/EOS M290	LP: 258.7 HS: 100 SS: 1066.7 LT: 30		As-built			-	-	-	-	-	336 ± 5 HV0.3		-
					ST: 900 °C for 1 h					-	-	-	296 ± 6 HV0.3		Rapid air cooling
					A: 530 °C for 3 h					-	-	-	502 ± 15 HV0.3		-
					ST: 900 °C for 1 h	A: 530 °C for 3 h				-	-	-	301 ± 3 HV0.3		ST: Rapid air cooling
	Dong et al. [51]	PBF-LB/EOS M290	HS: 100 LT: 30	LED * = 182 J/m	As-built			-	-	1033 ± 4.4	892 ± 10.7	11.1 ± 0.55	325 ± 3.5 HV0.05		-
				LED = 245 J/m	As-built					1068 ± 5.9	889 ± 7.3	15.7 ± 0.26	351 ± 4.8 HV0.05		-
				LED = 333 J/m	As-built					1059 ± 2.8	886 ± 12.3	14.7 ± 0.26	342 ± 4.2 HV0.05		-
	Zhang et al. [49]	PBF-LB/HBD-280	LP: 340 HS: 100 SS: 850 LT: 50		As-built			H	-	1058 ± 2	992 ± 6	18.4 ± 0.9	38.1 ± 0.5 HRC		-
					ST: 900 °C for 1 h					982 ± 9	819 ± 11	12.7 ± 0.4	29.7 ± 0.6 HRC		Air cooling
					A: 530 °C for 3 h					1648 ± 1	1503 ± 21	11.6 ± 0.5	49.4 ± 0.2 HRC		Furnace cooling
					ST: 900 °C for 1 h	A: 530 °C for 3 h				1683 ± 10	1563 ± 56	8.4 ± 1.6	50.4 ± 0.5 HRC		ST: Air cooling A: Furnace cooling
	Turnier Trottier et al. [59]	PBF-LB/EOS M280	LP: 260 HS: 100 SS: 1067 LT: 60		As-built			H	P *	1098.3 ± 2.5	874.8 ± 23.6	16.06 ± 0.19	XY: 36 ± 0.7 HRC	XZ: 33.3 ± 1.9 HRC	-
								V *	PL *	1079.5 ± 1.2	793.7 ± 62.3	10.76 ± 0.51
					ST: 850 °C for 30 min			H	P	1029.3 ± 1.0	781.7 ± 33.8	12.93 ± 0.26	XY: 31.5 ± 0.7 HRC	XZ: 30.6 ± 0.7 HRC	Forced air cooling
								V	PL	1051.8 ± 5.8	853.4 ± 6.6	10.99 ± 0.76
					ST: 850 °C for 30 min	A: 525 °C for 1 h		-	-	-	-	-	XY: 47.8 ± 0.5 HRC	XZ: 48.7 ± 0.3 HRC
					ST: 850 °C for 30 min	A: 525 °C for 2 h		H	P	1719.5 ± 15.0	1647.3 ± 8.3	8.22 ± 0.54	XY: 50.7 ± 0.8 HRC	XZ: 50.4 ± 1.0 HRC
								V	PL	1698.1 ± 10.4	1617.2 ± 45.0	6.31 ± 0.22
					ST: 850 °C for 30 min	A: 525 °C for 3 h		-	-	-	-	-	XY: 49.1 ± 0.2 HRC	XZ: 49.1 ± 0.2 HRC
					ST: 850 °C for 30 min	A: 525 °C for 4 h		-	-	-	-	-	XY: 48.7 ± 0.3 HRC	XZ: 48.7 ± 0.2 HRC
					ST: 850 °C for 30 min	A: 525 °C for 6 h		-	-	-	-	-	XY: 48.6 ± 0.2 HRC	XZ: 48.1 ± 0.3 HRC
	Fang et al. [46]	PBF-LB/FARSOON FS121M	LP: 190 HS: 90 SS: 1100 LT: 30		As-built			-	-	1084 ± 3	946 ± 7.3	17.64 ± 0.18	374.2 ± 6.5 HV0.2		-
	Ćirić-Kostić et al. [65]	PBF-LB/EOSINT M290	LT: 30		As-built			-	-	-	-	-	31.0 ± 1.5 HRC		-
					ST: 900 °C for 45 min	A: 530 °C for 3 h		-	-	-	-	-	47.0 ± 0.9 HRC		ST: Rapid cooling in air A: Cooling in a furnace
	Chang et al. [66]	PBF-LB/EOS M290	LP: 260 HS: 100 SS: 1060 LT: 30		As-built			-	-	-	-	-	354.9 ± 8.8 HV0.2		-
					ST: 900 °C for 1 h	A: 530 °C for 3 h		-	-	-	-	-	533.4 ± 9.7 HV0.2		Argon cooled
	Chang et al. [67]	PBF-LB/EOS M290	LP: 260 HS: 100 SS: 1060 LT: 30		As-built			-	-	-	-	-	356 ± 14.3 HV0.2		-
					AS *: 900 °C for 1 h			-	-	-	-	-	320 ± 7.2 HV0.2		Argon cooled
					T *: 530 °C for 3 h			-	-	-	-	-	515 ± 6.8 HV0.2
					AS: 900 °C for 1 h	T: 530 °C for 3 h		-	-	-	-	-	527 ± 5.2 HV0.2
	Afkhami et al. [55,60]	PBF-LB/EOS M290-Not machined	LP: 260 HS: 100 SS: 1000 LT: 30		As-built			V	PL	1081.2	899.3	13.3	-		-
					ST: 850 °C for 30 min	A: 525 °C for 2 h		V	PL	1641.5	1556.3	8.1	-		Air cooled
		PBF-LB/EOS M290-Machined			As-built			H	P	1170.4	1006.0	16.6	BP *: 318 HV3	SP *: 336 HV3	-
					ST: 850 °C for 30 min	A: 525 °C for 2 h		H	P	1680.1	1533.4	11.2	BP: 467 HV3	SP: 462 HV3	Air cooled
					As-built			V	PL	1090.2	919.2	10.5	BP: 324 HV3	SP: 322 HV3	-
					ST: 850 °C for 30 min	A: 525 °C for 2 h		V	PL	1682.7	1600.9	5.9	BP: 477 HV3	SP: 472 HV3	Air cooled
	Afkhami et al. [63]	PBF-LB/EOS M290	LP: 260 HS: 100 SS: 1000 LT: 30		As-built			-	-	-	-	-	320 HV3		-
					ST: 850 °C for 30 min	A: 525 °C for 2 h		-	-	-	-	-	460 HV3		Air cooled
	Sanjari et al. [53]	PBF-LB/EOS M290	LP: 258.7 HS: 100 SS: 1066.7 LT: 30		As-built			H	P	1092 ±	807 ±	21.2 ±	-		-
								V	PL	1085 ±	1036 ±	5.7 ±	-		-
	Fabian et al. [62]	PBF-LB/EOS M290	LP: 258.7 HS: 100 SS: 1067 LT: 30		As-built			-	-	1032	929	15.3	39.9 ± 2.9 HRC		-
					A: 400 °C for 1667 min					-	-	-	48.6 ± 0.1 HRC		-
					A: 460 °C for 1078 min					-	-	-	50.8 ± 0.3 HRC		-
					A: 495 °C for 424 min					-	-	-	50.9 ± 0.2 HRC		-
					A: 530 °C for 180 min					-	-	-	50.4 ± 0.5 HRC		-
					A: 550 °C for 115 min					1738	1659	14.7	51.2 ± 1.9 HRC		-
					A: 600 °C for 40 min					-	-	-	43.4 ± 0.4 HRC		-
	Zhou et al. [61]	PBF-LB/FARSOON FS121M	LP: 170 HS: 90 SS: 900 LT: 30		As-built			-	-	1084 ± 3	946 ± 7	14.6 ± 0.9	362 ± 7 HV0.2		-
					ST: 850 °C for 1 h			-	-	875 ± 10	784 ± 13	12.2 ± 0.5	339.2 HV0.2		Air cooling
					ST: 875 °C for 1 h			-	-	808 ± 13	781± 16	11.8 ± 0.8	330.0 HV0.2
					ST: 900 °C for 1 h			-	-	893 ± 8	832 ± 11	11.1 ± 0.5	325.6 HV0.2
					ST: 925 °C for 1 h			-	-	836 ± 12	722 ± 15	11.9 ± 0.7	316.2 HV0.2
					ST: 950 °C for 1 h			-	-	672 ± 13	511 ± 17	11.0 ± 0.7	307.9 HV0.2
					ST: 900 °C for 0.5 h			-	-	870 ± 10	717 ± 13	11.6 ± 0.5	327.8 HV0.2
					ST: 900 °C for 0.75 h			-	-	779 ± 13	600 ± 16	11.8 ± 0.8	326.3 HV0.2
ST: 900 °C for 1.25 h					-	-	788 ± 12	612 ± 15	11.9 ± 0.7	321.2 HV0.2
ST: 900 °C for 1.5 h					-	-	806 ± 9	710 ± 17	11.7 ± 0.6	318.2 HV0.2
ST: 900 °C for 1 h	A: 430 °C for 3 h				-	-	1500 ± 21	1398 ± 43	10.9 ± 1.3	389.3 HV0.2
ST: 900 °C for 1 h	A: 480 °C for 3 h				-	-	1493 ± 15	1400 ± 36	10.6 ± 0.9	454.9 HV0.2
ST: 900 °C for 1 h	A: 530 °C for 3 h				-	-	1598 ± 10	1486 ± 19	9.6 ± 0.8	526.3 HV0.2
ST: 900 °C for 1 h	A: 580 °C for 3 h				-	-	1305 ± 12	1196 ± 24	11.1 ± 1.0	432.6 HV0.2
ST: 900 °C for 1 h	A: 630 °C for 3 h				-	-	1132 ± 13	912 ± 33	17.4 ± 1.6	396.4 HV0.2
Uddeholm AM Corrax®	Oikonomou et al. [50]	PBF-LB/TruPrint 1000	LP: 175 HS: 50 SS: 800 LT: 20		As-built			-	-	-	-	-	35 HRC		-
					ST: 850 °C for 30 min	A: 525 °C for 4 h		V	-	1667 ± 7	1595 ± 12	1.7 ± 0.6	50 HRC		-
					ST: 850 °C for 30 min	A: 525 °C for 4 h		H	-	1624 ± 13.7	1518 ± 14.2	3.68 ± 1.3			-
		PBF-LB/EOS M290	LP: 170 HS: 100 SS: 1250 LT: 30		As-built			-	-	-	-	-	35 HRC		-
					ST: 850 °C for 30 min	A: 525 °C for 4 h		V	-	1701 ± 4	1640 ± 12	8.6 ± 0.3	50 HRC		-
					ST: 850 °C for 30 min	A: 525 °C for 4 h		H	-	1653 ± 6	1560 ± 18	9.7 ± 0.3			-
	Krakhmalev et al. [30]	PBF-LB/EOS M290	LP: 170 HS: 100 SS: 1250 LT: 30		ST: 850 °C for 30 min	A: 525 °C for 4 h		-	-	-	-	-	49.4 HRC		Air cooling
					HIP-ST: 850 °C for 30 min, 150 MPa	HIP-A: 525 °C for 4 h, 150 MPa		-	-	-	-	-	45.9 HRC		After ST cooled down from 850 °C to 60 °C in 60 s
	Chang et al. [58]	PBF-LB/EOS M290	LSP *: 170 μm		As-built			-	-	1065.7 ± 6.7	911.0 ± 9.2	8.4 ± 0.2	31.6 ±1.4 HRC		-
					HIP *: 1300 °C, 100 min, 175 MPa			-	-	928.8 ± 38.9	844.5 ± 57.1	3.3 ± 0.1	30.7 ± 2.1 HRC		Cooling in N2 gas
					HIP: 1300 °C, 100 min, 175 MPa	A: 525 °C for 4 h		-	-	1330.4 ± 104.2	1150.7 ± 42.3	0.8 ± 0.2	47.5 ± 1.3 HRC
					HIP: 1300 °C, 100 min, 175 MPa	ST: 850 °C for 30 min	A: 525 °C for 4 h	-	-	1245.3 ± 198.1	1052.5 ± 25.9	0.5 ± 0.1	48.8 ± 0.5 HRC
	Maistro et al. [64]	PBF-LB/EOS M290	Non-optimized parameters		ST: 850 °C for 30 min in vacuum	A: 525 °C for 4 h in vacuum		-	-	1711 ± 2	1633 ± 9	-	49 HRC		-
					URQ-HIP *: 850 °C, 30 min	URQ-HIP: 525 °C for 4 h		-	-	-	-	-	43 HRC		-
		PBF-LB	Optimized parameters		ST: 850 °C for 30 min in vacuum	A: 525 °C for 4 h		-	-	-	-	-	49.9 HRC		-
					URQ-HIP: 850 °C, 30 min	A: 525 °C for 4 h		-	-	-	-	-	51.4 HRC		A: Air cooled
	Chadha et al. [57]	PBF-LB/EOS M290	-		ST: 900 °C for 1 h	A: 530 °C for 3 h		-	PL	1724 ± 14	1666 ± 12	6 ± 1	50 ± 1 HRC		-
								-	P	1702 ± 16	1628 ± 13	7 ± 1	50 ± 1 HRC		-
	Wu et al. [56]	PBF-LB/EOS M290	LP: 170 HS: 100 SS: 1250 LT: 30		As-built			H	-	1168 ± 16	476 ± 8	16.6 ± 0.1	-		-
					As-built			V	-	1145 ± 8	728 ± 45	14.2 ± 0.2
					ST: 850 °C for 30 min			H	-	1118 ± 21	664 ± 13	13.5 ± 0.2	-		ST: Quenched in N2
					ST: 850 °C for 30 min			V	-	1129 ± 14	717 ± 16	12.9 ± 0.2
					ST: 850 °C for 30 min	A: 525 °C for 4 h		H	-	1589 ± 24	1335 ± 127	12.4 ± 1.2	-		ST: Quenched in N2 A: Air cooled
					ST: 850 °C for 30 min	A: 525 °C for 4 h		V	-	1578 ± 47	1386 ± 73	11.7 ± 1.5

* A: Aging treatment, AS: Austenizing, BP: Building Plane, H: Horizontal, HIP: Hot Isostatic Pressing, HS: Hatch Spacing, LED: Linear Energy Density, LP: Laser Power, LSP: Laser Scanning Pitch, LT: Layer Thickness, P: Perpendicular, PL: Parallel, SP: Scanning Plane, SS: Scanning Speed, ST: Solution Treatment, T: Tempering, URQ-HIP: Uniform Rapid Quenching–Hot Isostatic Pressing, V: Vertical.

2.3.2. Impact Toughness

Table 4. Impact toughness of as-built and heat-treated PBF-LB/CX and Corrax from the literature.

Material	Reference (Data from)	Process	Condition		Direction	Notched Impact Toughness (J)	Unnotched Impact Toughness (J)
EOS StainlessSteel CX	Chang et al. [29]	PBF-LB	As-built		-	83.8	-
			ST *: 900 °C for 1 h		-	73.5	-
			A *: 530 °C for 3 h		-	5.5	-
			ST: 900 °C for 1 h	A: 530 °C for 3 h	-	5.3	-
	Zhang et al. [49]	PBF-LB	As-built		-	57.7 ± 2.5	-
			ST: 900 °C for 1 h		-	66.3 ± 3.2	-
			A: 530 °C for 3 h		-	10.3 ± 0.6	-
			ST: 900 °C for 1 h	ST: 900 °C for 1 h	-	8.7 ± 0.6	-
	Afkhami et al. [55,60]	PBF-LB	As-built		Vertical	128	-
					Horizontal	139
			ST: 850 °C for 30 min	A: 525 °C for 2 h	Vertical	22	-
					Horizontal	29
Uddeholm AM Corrax®	Oikonomou et al. [50]	PBF-LB (TruPrint 1000)	ST: 850 °C for 30 min	A: 525 °C for 4 h	Vertical	6.22 ± 0.53	-
					Horizontal	11.17 ± 0.68	-
		PBF-LB (EOS M290)	ST: 850 °C for 30 min	A: 525 °C for 4 h	Vertical	18.70 ± 2.16	-
					Horizontal	22.06 ± 2.54	-
	Maistro et al. [64]	PBF-LB (non-optimized parameters)	ST: 850 °C for 30 min in vacuum	A: 525 °C for 4 h in vacuum	-	9.8 ± 1.1	-
			URQ-HIP *: 850 °C, 30 min	URQ-HIP: 525 °C for 4 h	-	11.4 ± 1.2	156 ± 26
		PBF-LB (optimized parameters)	ST: 850 °C for 30 min in vacuum	A: 525 °C for 4 h	-	-	~240
			URQ-HIP: 850 °C, 30 min	A: 525 °C for 4 h	-	-	~210
			URQ-HIP: 1140 °C, 30 min	A: 525 °C for 4 h	-	-	~175

* A: Aging, ST: Solution Treatment, URQ-HIP: Uniform Rapid Quenching–Hot Isostatic Pressing.

shows the collection of impact toughness values for the as-built and heat-treated PH 13-8 Mo family processed using PBF-LB from the literature.

The impact toughness of as-built and aged of the PBF-LB/CX was investigated by Chang et al. [29]. As-built impact toughness (83.8 J) was the highest compared to solution-treated, aged, and solution-aged specimens. The as-built sample exhibited deep dimples, which suggest a ductile fracture. On the other hand, the aged sample exhibited little dimples and crystalline features, which showed ductile and brittle fracture. Zhang et al. [49] also explored the impact toughness of as-built and heat-treated PBF-LB/CX. It was found that impact toughness increased with solution treatment, while it decreased with aging and solution-aged treatment compared to the as-built condition. Aging and solution-aging treatment caused precipitation hardening, which restrains dislocation movements. As a result, the brittleness increased, and the impact toughness decreased. Afkhami et al. [55,60] also confirmed that impact toughness decreased with heat treatment for horizontal and vertical PBF-LB/CX samples.

The impact toughness of aged PBF-LB/Corrax investigated by Oikonomou et al. [50] and the impact toughness of Corrax produced by TruPrint1000 was lower for the heat-treated samples due to the higher number of defects. Maistro et al. [64] showed that when the URQ-HIP process was applied after being processed by PBF-LB with a non-optimized parameter, the Charpy impact toughness could be increased. On the other hand, it was concluded that URQ-HIP was not able to eliminate all the defects at 850 °C. The unnotched impact toughness of the sample dropped after URQ-HIP at 1140 °C due to the coarsening of the martensite microstructure and MnS inclusions. In addition, the unnotched impact toughness of the PBF-LB/Corrax (heat-treated in a vacuum) was comparable with conventional Corrax.

2.3.3. Corrosion Properties

In relation to the corrosion properties, Shahriari et al. [43] studied the corrosion behavior of CX built by PBF-LB and found an anisotropic corrosion response. The side of the as-built sample exhibited higher corrosion resistance compared to the top side of the sample due to differences in the microstructure, such as the level of dislocation density, lath size, grain boundary density, and residual stresses. It was observed that melt pool boundaries and pores in the microstructure are more susceptible to pitting corrosion.

Lu et al. [68] studied the influence of process parameters (laser power and scan speed) on the corrosion properties and microstructure of the as-built PBF-LB/CX. The sample (P = 170 W and V = 0.9 m·s⁻¹) that exhibited the best corrosion resistance was the one with the highest relative density and lowest average grain size. The influence of laser power (P) on corrosion resistance was found to be less significant than the speed (V).

2.3.4. Fatigue Properties

Fatigue properties of the PBF-LB/CX were investigated by Ćirić-Kostić et al. [65] and Afkhami et al. [63]. Ćirić-Kostić et al. [65] showed that surface roughness was very important for the fatigue properties and concluded that machining increased up to four times the fatigue strength of the PBF-LB/CX samples in the as-built condition. Heat treatment and machining as post-processing were found to be even more beneficial for the fatigue strength, which increased up to five times. Afkhami et al. [63] also confirmed the influence of surface roughness on fatigue performance. When the surface roughness was decreased, the fatigue limit of the PBF-LB/CX increased (from 170 MPa to 250 MPa). However, it was found that the heat treatment and building direction did not significantly affect the fatigue properties, even though the hardness was increased by the heat treatment. It was also concluded that the fatigue properties of machined PBF-LB/CX specimens were similar to conventional high-strength steels.

2.3.5. Tribological Performance

The wear resistance and residual stress of PBF-LB/CX were explored by Chang et al. [66]. The heat treatment procedure decreased the residual stresses by 65.9% for the longitudinal and 39.1% for the transverse direction compared to the as-built state. Also, the wear resistance was improved via heat treatment. The same authors [67] also investigated how the heat treatment affected the tribological properties and high-temperature oxidation resistance of PBF-LB/CX. The heat treatment procedure (austenization at 900 °C for 1 h and tempering at 530 °C for 3 h) was decided with the help of DSC (differential scanning calorimetry) curves. The as-built microstructure consisted of submicron grains (<1 μm: 50.87%), ultra-fine grains (1–2 μm: 31.55%), and coarse grains (2 μm: 17.58%). Heat treatment increased the grain size and caused grain coarsening, and the distribution of grain sizes was submicron grains (<1 μm: 29.45%), ultra-fine grains (1–2 μm: 32.31%), and coarse grains (2 μm: 38.24%). The amount of austenite (0.21%) also increased after austenization and tempering (4.9%). The high-temperature oxidation resistance was decreased due to the tempering heat treatment. On the other hand, tempering increased the hardness and wear resistance.

3. DED-Arc of PH 13-8 Mo Family

3.1. As-Built: Microstructure

In 2022, the first paper on the feasibility of DED-Arc of PH 13-8 Mo was published by Ghaffari et al. [28]. A thin wall was deposited, and the as-built microstructure consisted of a martensitic matrix with vermicular and lathy δ-ferrite, retained austenite, and Al-rich oxide inclusions. It was observed that the microstructure was not homogenous, and the amounts of δ-ferrite and retained austenite were different between the top and the bottom of the wall. The UTS of the wall was similar compared to CX stainless steel produced by PBF-LB. Vahedi Nemani et al. [69] also deposited a PH 13-8 Mo wall using DED-Arc. The characteristics of the as-built wall can be seen in Figure 4. It was confirmed that the microstructure consisted of a martensitic matrix, δ-ferrite, and retained austenite (Figure 4).

3.2. Post-Processing Heat Treatments: Microstructure

Vahedi Nemani et al. [70] explored the formation of secondary phases at 600 °C (aging temperature) in DED-Arc-manufactured PH 13-8 Mo. The solution-treated material for 1 h at 1050 °C was subsequently aged at 600 °C for different dwell times: 20 min, 1, 2, and 4 h. It was found that the aging process started with the precipitation of β-NiAl by the end of the 20 min, whereas M₂₃C₆ carbides were found in the martensitic microstructure by the end of the 1 h aging treatment. The formation of carbides increased the Ni content in the surrounding area of the carbide, which subsequently enabled the formation of reverted austenite. At the latest stage (4 h), new carbides were formed in the reverted austenite due to the higher carbon content in the FCC austenite. Vahedi Nemani et al. [69] also conducted solution heat treatment at three different soak temperatures: 900, 1050, and 1200 °C. The temperature of solution treatment (900 °C) was not enough to eliminate the columnar structure and the δ-ferrite. The temperature of solution treatment (1200 °C) was successful in removing the columnar structure; however, the temperature was too high, and it caused the δ-ferrite phase to re-form. The temperature of solution treatment (1050 °C) was the most suitable solutionizing temperature, which resulted in a fully martensitic microstructure free from δ-ferrite.

Ghaffari et al. [71] also explored the influence of different heat treatments on the microstructure PH 13-8 Mo manufactured using DED-Arc. The samples were aged at 400, 450, 500, 550, and 600 °C for 4 h. The illustrations of the microstructure after each treatment are shown in Figure 5. It was possible to remove the unwanted δ-ferrite with the solution heat treatment at 1050 °C, which resulted in a fully martensitic microstructure. There were no precipitates in the sample aged at 400 °C. However, β-NiAl precipitates were observed in the material aged at 500 °C and 450 °C. When the aging temperature was increased to 550 °C, not only β-NiAl precipitates but also Cr-rich M₂₃C₆ carbides were present in the microstructure. Even further increased aging temperature (600 °C) caused the formation of β-NiAl precipitates, carbides, and blocky and elongated reverted austenite (Figure 5).

Moniruzzaman et al. [72] developed a direct aging heat treatment procedure using DSC for the DED-Arc/PH 13-8 Mo. The sample aged at 495 °C for 45 min achieved the maximum hardness and was therefore chosen as the optimal post-processing heat treatment.

3.3. Material Properties

3.3.1. Tensile Properties and Hardness

Table 5. Tensile properties and hardnesses of as-built and heat-treated DED-Arc/PH 13-8 Mo.

Material	Ref. (Data from)	Process	Process Parameters	Condition		Building Direction	Ultimate Tensile Strength (MPa)	Yield Strength (MPa)	Elongation (%)	Hardness	Cooling
PH 13-8 Mo	Ghaffari et al. [28]	DED-Arc	AAC *: 135 A AV *: 28 V WFS *: 67 mm/s TS *: 4 mm/s	As-built		H *	1115 ± 17	-	11.4 ± 1.8	35.4 ± 0.8 HRC	-
						V *	1120 ± 15	-	6.1 ± 1.1
	Ghaffari et al. [71]	DED-Arc	AAC: 135 A AV: 28 V WFS: 67 mm/s TS: 4 mm/s	As-built		H	~1114	-	~11.4	326 ± 11 HV0.5	-
						V	~1121	-	~6.3
				ST *: 900 °C for 1 h		-	-	-	-	339 ± 8 HV0.5	Still-air-cooling
				ST: 950 °C for 1 h		-	-	-	-	351 ± 5 HV0.5
				ST: 1050 °C for 1 h		-	-	-	-	396 ± 2 HV0.5
				ST: 1150 °C for 1 h		-	-	-	-	311 ± 7 HV0.5
				ST: 1050 °C for 1 h	A *: 400 °C for 4 h	-	~1203	-	~9.9	373 ± 2 HV0.5	ST: Still-air cooling
				ST: 1050 °C for 1 h	A: 450 °C for 4 h	-	~1420	-	~9.3	507 ± 4 HV0.5
				ST: 1050 °C for 1 h	A: 500 °C for 4 h	-	~1510	-	~9	538 ± 3 HV0.5
				ST: 1050 °C for 1 h	A: 550 °C for 4 h	-	~1350	-	~9.7	474 ± 4 HV0.5
				ST: 1050 °C for 1 h	A: 600 °C for 4 h	-	~910	-	~12.1	294 ± 6 HV0.5
	Moniruzzaman et al. [72]	DED-Arc	AAC: 135 A AV: 28 V WFS: 67 mm/s TS: 4 mm/s	As-Built		V	1046	860	14	33.6 HRC	-
				A: 400 °C for 312 min		-	-	-	-	41.9 HRC	-
				A: 460 °C for 77 min		-	-	-	-	41.1 HRC
				A: 495 °C for 45 min		V	1363	911	16.3	46.4 HRC
				A: 510 °C for 25 min		-	-	-	-	42.4 HRC
				A: 530 °C for 10 min		-	-	-	-	39.8 HRC

* A: Aging, AAC: Average Arc Current, AV: Arc Voltage, H: Horizontal, ST: Solution Treatment, TS: Travel Speed, V: Vertical, WFS: Wire Feed Speed.

shows the tensile properties and hardnesses for as-built and heat-treated PH 13-8 Mo processed using DED-Arc from the literature.

Ghaffari et al. [28] investigated the as-built tensile properties and hardness of DED-Arc of PH 13-8 Mo. It was concluded that the UTS (Table 5) of the wall was similar compared to CX stainless steel produced using PBF-LB. The hardness of the as-built microstructure was 35.4 ± 0.8 HRC. Ghaffari et al. [71] also explored the influence of different heat treatments on the mechanical properties of PH 13-8 Mo manufactured using DED-Arc. The aging procedures were 400, 450, 500, 550, and 600 °C for 4 h. The specimen aged at 500 °C had the highest UTS and hardness due to the coherent precipitation hardening. Aging temperature (600 °C) resulted in the lowest hardness and UTS due to the formation of reverted austenite and Cr-rich carbides. It was apparent from elongation results that there was an anisotropic ductility in the as-built PH 13-8 Mo.

As mentioned in the previous chapter, Moniruzzaman et al. [72] showed that direct aging heat treatment increased the hardness and UTS of DED-Arc/PH 13-8 Mo (Table 5).

3.3.2. Corrosion Resistance

The corrosion resistance of the as-built and aged PH 13-8 Mo manufactured using DED-Arc was investigated by Vahedi Nemani et al. [69]. The corrosion resistance of as-built, aged at 400, 500, and 600 °C samples was compared. The as-built microstructure had the lowest corrosion resistance due to the existence of the δ-ferrite phase. The Cr-rich δ-ferrite phase caused a depletion in Cr in its surrounding area, which contributed to sensitization. The specimen aged at 500 °C had a relatively higher corrosion resistance than the specimen aged at 400 °C because of the decreased dislocation density and residual stresses of the sample aged at 500 °C. Secondary phases (M₂₃C₆ carbides) were formed during the highest temperature (600 °C) aging process. These secondary phases are well known to be deleterious to corrosion resistance. It was unfolded that a suitable corrosion resistance can be maintained by eliminating the unwanted δ-ferrite and Cr-enriched carbides.

3.3.3. Tribological Performance

The tribological performance of as-built and heat-treated PH 13-8 Mo produced using DED-Arc was studied by Afshari et al. [73]. The as-built sample exhibited anisotropic tribological performance due to its anisotropic microstructure. However, the anisotropic performance was eliminated with the help of heat treatment. The highest scratch resistance, wear resistance, and hardness were achieved after heat treatment, which was solutionizing at 1050 °C for 1 h and aging at 500 °C for 4 h. It was concluded that the heat-treated DED-Arc/PH 13-8 Mo showed slightly better performance compared to the heat-treated wrought PH 13-8 Mo.

4. Comparison of PBF-LB and DED-Arc of PH 13-8 Mo Family

Researchers compared the effects of different AM methods (PBF-LB and DED-Arc) on the microstructure, properties, and post-processing procedure of the PH 13-8 Mo family.

Benoit et al. [74] investigated the influence of different AM methods on the hardness and the microstructure of PH 13-8 Mo. It was clear that the differences between the cooling rate and composition of feedstock material resulted in a different primary solidification phase. Therefore, the resulting as-built microstructure was different for the DED-Arc/PH 13-8 Mo and PBF-LB/PH 13-8 Mo. The DED-Arc/PH 13-8 Mo had a martensitic matrix, δ-ferrite, and retained austenite. On the other hand, the PBF-LB/PH 13-8 Mo consisted of a martensitic matrix and a very low amount of retained austenite. The as-built hardness value of the DED-Arc/PH 13-8 Mo was higher than the PBF-LB/PH 13-8 Mo due to the existence of β-NiAl nano-precipitates, which formed due to repeated thermal cycles during the DED-Arc process. It was also concluded that direct aging heat treatment (530 °C—3 h) was not applicable for the DED-Arc/PH 13-8 Mo (only 6% increase in hardness), whereas it was applicable for the PBF-LB/PH 13-8 Mo (46% increase in hardness) (

Table 6. Comparisons of tensile properties and hardness of PBF-LB and DED-Arc of PH 13-8 Mo family.

Material	Reference (Data from)	Process	Process Parameters	Condition	Ultimate Tensile Strength (MPa)	Yield Strength (MPa)	Elongation (%)	Hardness	Cooling
EOS Stainless Steel CX	Benoit et al. [74]	PBF-LB/EOS M290	* LP: 259 W * HS: 100 μm * SS: 1067 mm/s * LT: 30 μm	As-Built	-	-	-	347.7 HV1	-
				A *: 530 °C for 3 h	-	-	-	508.3 HV1	Water quenching
PH 13-8 Mo		DED-Arc	* AAC: 135 A * AV: 28 V * WFS: 67 mm/s * TS: 4 mm/s	As-Built	-	-	-	401 HV1	-
				A: 530 °C for 3 h	-	-	-	428.3 HV1	Water quenching
PH 13-8 Mo	Moniruzzaman et al. [75]	DED-Arc	AAC: 135 A AV: 28 V WFS: 67 mm/s TS: 4 mm/s	As-Built	1046	860	14	33.6 ± 1.2 HRC	-
				A: 495 °C for 45 min	1363	911	16.3	46.4 ± 0.5 HRC	-
		PBF-LB/EOS M290	LP: 258.7 W HS: 100 μm SS: 1067 mm/s LT: 30 μm	As-Built	1032	929	15.3	34.9 ± 2.9 HRC	-
				A: 550 °C for 115 min	1738	1659	14.7	51.2 ± 0.4 HRC	-

* A: Aging, AAC: Average Arc Current, AV: Arc Voltage, HS: Hatch Spacing, LP: Laser Power, LT: Layer Thickness, SS: Scanning Speed, TS: Travel Speed, WFS: Wire Feed Speed.

). δ-ferrite was still in the microstructure, and undesirable phases such as Cr₂₃C₆ particles and reverted austenite (7.2%) were formed during the aging of the DED-Arc sample.

Moniruzzaman et al. [75] studied the direct aging heat treatment of the PH 13-8 Mo manufactured using DED-Arc and compared it with the PH 13-8 Mo manufactured using LBF-LB. It was shown that the as-built microstructures of PH 13-8 Mo processed by two different methods had differences, such as martensite lath size, due to experiencing different cooling rates. The materials also exhibited different kinetics of precipitation hardening. Therefore, two different heat treatment procedures for each manufacturing method were recommended. The recommended aging temperature was 550 °C, and the aging time was 115 min for the PBF-LB/PH 13-8 Mo, which improved the strength and decreased the ductility compared to the as-built condition. On the other hand, the recommended aging heat treatment (495 °C—45 min) was successful in improving both the strength and the ductility of the DED-Arc/PH 13-8 Mo. The reason for the improvement in the ductility was stated as the formation of β-NiAl clusters instead of β-NiAl precipitates in the material. The obtained tensile properties and hardness according to recommended heat treatments can be found in Table 6.

5. DED-LB of PH 13-8 Mo Family

As a difference with PBF-LB and DED-Arc processes, fewer references are available in the literature about DED-LB of the PH 13-8 Mo family.

In terms of process parameters, Muslim et al. [76] studied the effects of laser energy density on the shape of the depositions of PH 13-8 Mo powder manufactured using DED-LB. It was observed that low energy density was not so efficient in depositing a suitable geometry due to the lack of heat input to melt all the powder. High walls were produced with higher energy densities; however, heat accumulation led to improper shapes.

Aydin et al. [77] reported the influence of the process parameters of DED-LB/p on the geometry of deposited, modified PH 13-8 Mo single tracks. Also, the as-built microstructure was investigated. With the help of the design of experiments (DoE), suitable process parameters in terms of the geometrical features of tracks were established. In addition, it was concluded that the height was significantly affected by the powder feeding rate and not significantly affected by the laser power. On the other hand, the width and depth were affected by the laser power. The speed influenced the depth, height, and width. The as-built microstructure consisted of a martensitic matrix, δ-ferrite, retained austenite, and aluminium-enriched inclusions. Aydin et al. [78] continued and used those suitable parameters to deposit a multi-track single layer of modified PH 13-8 Mo using DED-LB/p. The microstructure was heterogenous, and martensitic matrix, δ-ferrite, austenite, and AlN inclusions were observed in the single layer. Kas et al. [79] investigated the microstructure of DED-LB/p/PH 13-8 Mo. The microstructure consisted of cellular, fine dendritic and columnar regions, and a martensitic matrix with embedded δ-ferrite was observed.

In terms of mechanical testing, limited information is available. Zheng et al. [80] processed the PH 13-8 Mo powder with DED-LB and revealed that the microstructure consisted of a martensitic matrix and retained austenite. The UTS, YS, and elongation of the cube samples were 1089–1117 MPa, 565–634 MPa, and 10–13%, respectively, which were similar to aged wrought material. Dimples were observed on the fracture surfaces of the tensile samples, which were associated with ductile fracture mode. The defects, such as unmelted particles and porosity, led to low ductility. Kas et al. [79] also investigated the mechanical properties of PH 13-8 Mo processed using DED-LB/p. The UTS was 1148.5 MPa, which is similar to the PH 13-8 Mo family manufactured using AM processes such as DED-Arc and PBF-LB. The hardness values ranged between 331 HV and 354 HV for the samples produced with different process parameters.

Concerning repair purposes by using DED-LB/p, Rabiey et al. [81] deposited the Corrax powder on a hot work tool steel (1.2343). The strength of the deposited material was higher than the substrate, and it was concluded that repair could be applied. However, the hardness difference between the deposited metal and substrate needed to be taken care of via heat treatment.

6. Summary and Outlook

In this review, the overview of the existing literature on the AM of the PH 13-8 Mo family was presented, with a focus on the microstructure and properties.

It was shown that the as-built microstructure of PBF-LB and DED-Arc of the PH 13-8 Mo family were different, i.e., the presence of δ-ferrite. However, more in-depth investigations are needed to understand the solidification modes and the microstructural evolution during the AM of the PH 13-8 Mo family with the help of in situ measurements, computational thermodynamics and kinetics, and computational modeling methods. The acquired knowledge could be used to improve the understanding of microstructure–property relationships.

Most of the initial studies used the recommended heat treatments for conventional production. In recent studies, it was realized that to ensure optimum properties, new heat treatment recipes need to be developed not only for the additively manufactured parts but also for the specific AM processes.

Publications regarding the microstructure and mechanical properties of the DED-LB/PH 13-8 Mo family are scarce. To the best of our knowledge, there are no studies on the impact toughness, corrosion, and fatigue properties of the PH 13-8 Mo family manufactured using DED-LB.

Researchers showed that additively manufactured MPHSS can achieve comparable properties to conventionally manufactured ones. Therefore, additively manufactured PH 13-8 Mo stainless steel has the potential to be used in industrial applications. However, due to the complex thermal history associated with AM processes, heterogeneous microstructure could occur, resulting in anisotropic properties. Therefore, the solutions to control the anisotropy need to be established. In addition, more investigations should be conducted on the corrosion, fatigue, impact toughness, and wear properties of the PH 13-8 Mo family processed using AM.