metals Review AReview Review of Factors Affecting the Mechanical PropertiesA Review ofof MaragingFactors Affecting Steel 300 the Fabricated Mechanical via Laser PowderProperties Bed of Fusion Maraging Steel 300 Fabricated via Laser Powder Bed Fusion Barry Mooney and Kyriakos I. Kourousis * Barry Mooney and Kyriakos I. Kourousis * School of Engineering, University of Limerick, V94 T9PX Limerick, Ireland * SchoolCorrespondence: of Engineering, [email protected]; University of Limerick, Tel.: V94+353-61202217 T9PX Limerick, Ireland * Correspondence: [email protected]; Tel.: +353-61202217 Received: 29 August 2020; Accepted: 20 September 2020; Published: 22 September 2020 Received: 29 August 2020; Accepted: 20 September 2020; Published: 22 September 2020 Abstract: Maraging steel is an engineering alloy which has been widely employed in metal additive Abstract: Maraging steel is an engineering alloy which has been widely employed in metal additive manufacturing. This paper examines manufacturing and post-processing factors affecting the manufacturing. This paper examines manufacturing and post-processing factors affecting the properties of maraging steel fabricated via laser powder bed fusion (L-PBF). It covers the review of properties of maraging steel fabricated via laser powder bed fusion (L-PBF). It covers the review of published research findings on how powder quality feedstock, processing parameters, laser scan published research findings on how powder quality feedstock, processing parameters, laser scan strategy, build orientation and heat treatment can influence the microstructure, density and porosity, strategy, build orientation and heat treatment can influence the microstructure, density and defects and residual stresses developed on L-PBF maraging steel, with a focus on the maraging steel porosity, defects and residual stresses developed on L-PBF maraging steel, with a focus on the 300 alloy. This review offers an evaluation of the resulting mechanical properties of the as-built and maraging steel 300 alloy. This review offers an evaluation of the resulting mechanical properties of heat-treated maraging steel 300, with a focus on anisotropic characteristics. Possible directions for the as-built and heat-treated maraging steel 300, with a focus on anisotropic characteristics. Possible further research are also identified. directions for further research are also identified. Keywords: maraging steel; additive manufacturing; anisotropy; powder bed fusion Keywords: maraging steel; additive manufacturing; anisotropy; powder bed fusion 1. Introduction 1. Introduction Laser powder bed fusion (L-PBF) is a technology used in metal additive manufacturing (AM). ControlledLaser bypowder existing bed computer-aided fusion (L-PBF) is design a technology/manufacturing used in know-how,metal additive L-PBF manufacturing systems drive (AM). a high-wattageControlled by laser existing energy computer-aided source to selectively design/ meltmanufacturing successive thin know-how, layers of metallic L-PBF powder systems particles, drive a whichhigh-wattage in turn produces laser energy high-density source functionalto selectively components, melt successive though the thin majority layers of of these metallic parts requirepowder additionalparticles, finishing.which in turn A typical produces L-PBF high-density process configuration functional iscomponents, illustrated inthough Figure the1. majority of these parts require additional finishing. A typical L-PBF process configuration is illustrated in Figure 1. Figure 1. Typical Laser Powder Bed Fusion (L-PBF) process configuration. Figure 1. Typical Laser Powder Bed Fusion (L-PBF) process configuration. Since the mid-1990s, there have been major advancements in controlling the L-PBF technology’s Since the mid-1990s, there have been major advancements in controlling the L-PBF technology’s underlying physics, and as a result, it became less complicated to implement in practice, and has become underlying physics, and as a result, it became less complicated to implement in practice, and has become much more prevalent in recent years [1–3]. Metal L-PBF systems are well suited to serial Metals 2020, 10, 1273; doi:10.3390/met10091273 www.mdpi.com/journal/metals Metals 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/metals Metals 2020, 10, 1273 2 of 22 Metals 2020, 10, x FOR PEER REVIEW 2 of 21 much more prevalent in recent years [1–3]. Metal L-PBF systems are well suited to serial production as theyproduction provide as consistent they provide and repeatableconsistent and outputs repeatab andle are outputs capable and of producingare capable metallic of producing components metallic withcomponents the durability, with the quality, durability, and precision quality, and suitable precisio forn industrial suitable for applications industrial [applications4–9]. On account [4–9]. ofOn theaccount demand, of the the demand, range of the L-PBF range processable of L-PBF processa materialsble has materials been expanding has been expanding simultaneously, simultaneously, where so far,where the compositionsso far, the compositions have closely have followed closely that follow of standarded that of alloys. standard The logicalloys. being, The logic for structural being, for applications,structural applications, it is desirable it that is desirable the AM parts that dothe not AM deviate parts characteristicallydo not deviate characteristically from those conventionally from those manufacturedconventionally (CM) manufactured [9,10]. (CM) [9,10]. MaragingMaraging steels steels possess possess the the mechanical mechanical properties properties which which o ffofferer them them engineering engineering significance significance in in applicationsapplications where where high high strength, strength, toughness, toughness, and ductility,and ductility, at high at hardness, high ha andrdness, dimensional and dimensional stability arestability required are [11 required–13]. Maraging [11–13]. refers Maraging to martensitic refers to microstructure, martensitic microstructure, commonly found commonly in steels, found which in cansteels, be strengthened which can /behardened strengthened/hardened through an ageing through heat-treatment an ageing [14 ,15heat-treatment]. Maraging steel’s [14,15]. low-carbon Maraging softsteel’s martensitic low-carbon matrix soft is martensitic mostly free frommatrix interstitial is mostly alloying free from elements, interstitial which alloying rank it elements, as an excellent which candidaterank it as for an synthesisexcellent bycandidate AM and for welding synthesis [11 ,16by– AM18]. Itand is maragingwelding [11,16–18]. steel’s outstanding It is maraging properties steel’s andoutstanding conjunction properties with the L-PBFand conjunction process’s shaping with ethefficiency L-PBF that process’s give maraging shaping steel efficiency 300 such that appeal. give Themaraging ideal situation steel 300 is asuch stand-alone appeal. L-PBFThe ideal technology situation that is cana stand-alone produce parts L-PBF exhibiting technology predictable, that can repeatableproduce parts and reliable exhibiting mechanical predictable, properties repeatable with implicitand reliable material mechanical characteristics. properties Comprehensive with implicit metallurgicalmaterial characteristics. evaluations and Comprehensive assessments of metallurgical machine-specific evaluations behaviours and are assessments necessary in of the machine- pursuit specific behaviours are necessary in the pursuit of this goal. In the case of L-PBF maraging steel 300, of this goal. In the case of L-PBF maraging steel 300, however, the reality is that the AM system/feedstock however, the reality is that the AM system/feedstock suppliers provide very limited data on the suppliers provide very limited data on the output of the process in terms of the material’s performance. output of the process in terms of the material’s performance. Moreover, the metallurgical Moreover, the metallurgical observations in the literature are highly varied and the body of published observations in the literature are highly varied and the body of published research is currently research is currently inadequate to characterise material performance for engineering designs and inadequate to characterise material performance for engineering designs and products. The purpose products. The purpose of this review is to examine how the mechanical properties of the L-PBF of this review is to examine how the mechanical properties of the L-PBF maraging steel 300 are maraging steel 300 are influenced by the: (a) AM process, namely powder and fabrication (focusing on influenced by the: (a) AM process, namely powder and fabrication (focusing on processing processing parameters, laser scan strategy and build orientation) and (b) post-processing (focusing on parameters, laser scan strategy and build orientation) and (b) post-processing (focusing on heat heat treatment). The aspects covered in the review paper are illustrated in Figure2. treatment). The aspects covered in the review paper are illustrated in Figure 2. Figure 2. Overview of the aspects covered in the review paper. Figure 2. Overview of the aspects covered in the review paper. 2. Conventionally Manufactured Maraging Steel 2. Conventionally Manufactured Maraging Steel 2.1. Characteristics 2.1. Characteristics The 18% Ni 300 grade maraging steel (referred for simplicity as maraging steel 300 in this paper), or slightThe modifications 18% Ni 300 grade thereof, maraging has been steel