# Additive Manufacturing of PH 13-8 Mo Family: A Review
## 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 Armoc 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 \(\delta\)-ferrite [13; 14], as \(\delta\)-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 \(\delta\)-ferrite, and \(\delta\)-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 \(\delta\)-ferrite depending on the cooling rate [15; 22; 23]. It is strengthened by the formation of \(\beta\)-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]. 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 \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**PH 13-8 Mo Family** & **Fe** & **C** & **Si** & **Mn** & **Cr** & **Ni** & **Mo** & **Al** & **S** & **N** & **Reference (Data from)** \\ \hline PH 13-8 Mo & Bal. & 0.05 & 0.1 & 0.1 & 12.25-13.25 & 7.5–8.5 & 2–2.5 & 0.9–13.5 & 0.01 & 0.01 & [PERSON] et al. [28] \\ EOS Stainless Steel CX & Bal. & \(\leq\)0.05 & \(\leq\)0.40 & \(\leq\)0.40 & 11.00-13.00 & 8.40-10.00 & 1.10-170 & 1.20-2.00 & - & [PERSON] et al. [29] \\ Uddeholm AM Corraa\({}^{\text{\textregistered}}\) & Bal. & 0.03 & 0.3 & 0.3 & 12 & 9.2 & 1.4 & 1.6 & - & - & [PERSON] et al. [30] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of PH 13-8 Mo family used during AM processes (wt.%). Figure 1: Microstructure of solution-treated PH 13-8 Mo (Reprinted from Ref. [20], with permission from Springer Nature).