**1. Introduction**

Additive manufacturing (AM), or 3D printing, is a computer-controlled process that creates three-dimensional objects by depositing materials, usually in layers, thus enabling the creation of lighter and stronger parts and systems. The benefits of AM are numerous, allowing the creation of parts with complex geometries and low material waste, therefore providing cost reduction for high-value components while reducing lead times. In addition, parts that previously required assembly from multiple components can be fabricated as a single object with improved strength and durability. Furthermore, AM can be used to fabricate unique objects or replacement pieces for parts that are no longer produced.

**Citation:** Kovacs, S.E.; Miko, T.; Troiani, E.; Markatos, D.; Petho, D.; Gergely, G.; Varga, L.; Gacsi, Z. Additive Manufacturing of 17-4PH Alloy: Tailoring the Printing Orientation for Enhanced Aerospace Application Performance. *Aerospace* **2023**, *10*, 619. https://doi.org/ 10.3390/aerospace10070619

Academic Editors: Rhys Jones and Spiros Pantelakis

Received: 28 February 2023 Revised: 23 June 2023 Accepted: 27 June 2023 Published: 7 July 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Having demanding standards in terms of performance and weight reduction, the aerospace industry has been one of the first to adopt additive manufacturing since the deployment of such techniques can advance part-making methods [1,2]. Common AM applications include environmental control system ducting, custom cosmetic aircraft interior components, rocket engine components and combustor liners [3,4]. Certification requirements for these installed parts in aircrafts are discussed in *USAF Structures Bulletin EZ-19-01*, mandating a linear elastic fracture assessment for all load-bearing AM parts [5]. These requirements are based on US MIL-STD-1530D, which mandates linear elastic fracture mechanics (LEFM) and does not allow the use of S–N curves in the design/assessment of a load-bearing component on USAF aircrafts. This standard applies to the entire structure, type or procurement strategy for the entire life cycle of the aircraft [6]. AM also helps deliver complex, consolidated parts with enhanced strength, which is a prerequisite in this industry. To justify these AM parts, there are several ongoing studies on durability and damage tolerance certification [7], with a focus on the aforementioned standards, while others are taking different approaches towards the same goal [8].

What AM contributes to the already existing manufacturing methods is a whole new perspective. The layer-by-layer deposition provides product designers with more freedom to create innovative, high-performance parts. These parts can be further optimized using finite element methods (FEM) aimed specifically at AM processes [9]. Due to the fact that a minimum amount of material is used, parts are generally more cost effective and produce less waste, making the product more sustainable. Conversely, the demanding requirements of raw materials are associated with high costs, as well as an inferior surface quality and dimensional precision, necessitating the application of surface finishes to fulfill quality standards and specifications. Furthermore, the layering and multiple interfaces of additive manufacturing can cause defects in the product, whereby post-processing is needed to rectify any quality issues. Among the many 3D printing processes, such as atomic diffusion additive manufacturing (ADAM) and metal fused filament fabrication (MFFF) [10], the LPBF technique, namely, selective laser melting (SLM), is one of the most promising [11]. This method uses a continuous powder bed melted with a high-precision energy source, which is usually a solid-state laser or an electron beam [3]. During the process, this energy source scans the cross-sections layer by layer until the part is finished. This bottom-up approach allows a variety of detailed modifications to existing components, where traditional machining technologies suffer from the limitations of their operation. The issues of low surface quality and high thermal residual stresses occurring in the final parts are most of the time related to the heat source [12]. Both issues can be mitigated with the mechanical machining of the surfaces or with more complex technologies, such as hot isostatic pressing [13]. Another important parameter is the build/printing orientation that can yield different mechanical properties along different directions [14]. Since numerous components are subjected to loading along specific directions, the LPBF material does not require isotropic behavior. Furthermore, the LPBF process can accommodate various types of raw materials. Although lightweight alloys are prevalent in commercial aircraft, steels are also widely used, particularly in turbine blades, missile and rocket fittings, undercarriage components and fasteners. Of all the steel varieties, high-performance stainless steels (SSs) are particularly significant due to their remarkable ability to attain exceptional strength throughout the process of aging. The 17-4PH SS material represents one of the most common types of martensitic precipitation-hardening SSs. Owing to its high chromium, nickel and copper content, it has good corrosion resistance and high fatigue resistance, and in the aged state, the copper-based precipitates ensure high strength and high toughness, which are requirements for SS alloy applications in the aerospace industry, even in critical components, such as fan or propeller blades [15]. Typically used where designers need more reliability in their products, this alloy is great for gate valves, chemical processing equipment, pump shafts, gears, ball bearings, bushings and even fasteners. It is also one of the most common types of stainless-steel powders in the powder metallurgy and AM industry. Despite using identical raw materials, the mechanical properties of

parts manufactured using different methods can vary significantly due to differences in the resulting microstructures. The properties of CP-17-4PH, as shown in Table 1, including yield strength (YS), ultimate tensile strength (UTS), hardness (HV), toughness (IE) and elongation (EL), are determined using the heat treatment process according to ASTM A564. It is worth noting that the properties can be affected by the high temperature used in the process.

**Table 1.** Typical mechanical properties of commercially manufactured 17-4PH SS according to ASTM A564.


Achieving a desired build orientation/direction is crucial for ensuring that a 3Dprinted part can withstand the forces that will act upon it. In this work, the desired orientation was achieved using selective laser melting (SLM) and 17-4PH SS powder, which is a widely used material in aeronautical applications. The properties of the produced specimens were subsequently compared with corresponding parts made using traditional manufacturing methods and other AM processes to evaluate their potential applicability in the aviation industry. To evaluate the mechanical properties of the 3D-printed parts, several tests were conducted. Hardness measurements were performed to assess the resistance of the material to indentation and penetration, and tensile testing was performed to determine the ultimate tensile strength (UTS), yield strength (YS) and strain at break values. Impact testing was also carried out to evaluate the material toughness under high-stress conditions. Finally, density measurements were conducted to evaluate the porosity of the samples, and the fractured surfaces of the samples were analyzed to determine the fracture mechanisms.
