Additive manufacturing (AM) has rapidly become a mainstream method of industrial production. Due to the involved layer-by-layer deposition, the amount of waste material can be limited, drastically reducing overall cost and conserving resources, unlike subtractive methods of manufacturing [1]. Capable of producing complex, quality components in a matter of hours, metal 3D printing is ideal for rapid prototyping [2]. As AM continues to develop, many sectors seek to scale up their use for end-use component manufacturing. In this regard, metal AM is an ideal method due to its accessibility, low-cost materials, and minimal waste. Furthermore, a vast range of materials, such as titanium-aluminum alloys, stainless steels, and nickel superalloys, can be used to form complex components that could not be made with traditional techniques [2,3]. Titanium, steels, nickel, copper, aluminum alloys, and even gold have all been successfully used in AM processes [4,5,6,7,8,9,10,11,12,13,14]. Aside from the material choice, metal AM processes also allow for improving and tailoring mechanical properties due to the innate optimization opportunities regarding the topology or microstructure design offered by the method [15,16]. As such, metal AM can be utilized to create components with superior mechanical properties compared to traditionally manufactured metal parts [15,17].

The use of metal AM to produce complex and crucial metallic parts becomes increasingly popular due to the unique characteristics of metal AM components. Among these AM-specific attributes are the reduced weight, reduced carbon-footprint, and propensity for part consolidation [18,19]. Also notable is metal AM’s potential to create highly complex parts [20]. In traditional metal manufacturing, parts are produced via casting and forging. When utilizing these methods for the fabrication of metal components, the formation of thin walled and irregular shapes becomes particularly difficult [21].

Additionally, AM offers advantages such as design flexibility and the minimization of waste [22]. The most well-known implementation of metal AM in an industrial setting relates to additively manufactured fuel nozzles in GE’s Leading Edge Aviation Propulsion jet engine. In this application, powder-based metal AM made the production of a complex component faster while reducing weight by 75% and multiplying the durability five-fold [23].

The most apparent property of AM is a direct result of the layer-by-layer process. Each layer can be as thin as 20 microns, allowing for the fabrication of highly detailed, complex CAD models, a feat difficult to accomplish via the traditional methods of casting or forging [24,25,26]. According to a study conducted by Khaing et al., powder-based AM methods can produce complex metal components, often with finer details than that of traditionally manufactured parts [27]. The strength of metal AM components is also advantageous. It should be noted that the choice of the raw material and the printing parameters have a significant impact on the mechanical properties of metal AM components.

Consequently, metal AM becomes commonplace in industries such as aerospace, architecture, automotive, biomedical, and fuel-cell manufacturing, where its application spans from joint implant fabrication to architectural modeling [28,29]. Though the technology is notable in its current applications, there remains much untapped potential. AM has the potential to reshape the manufacturing supply chain. Components can be printed on demand and the production and distribution of material components can be de-globalized. The carbon footprint of manufacturing could be drastically reduced [30]. While metal AM offers substantial advantages over traditional manufacturing, the layered surface quality presents an obstacle to industry adoption.

In all metal AM processes, material addition is achieved through repeated melting and solidification of raw materials. As a result of the complex cyclic thermal history from heat extraction, melting, and rapid solidification, the surfaces of AM parts feature rather irregular, stochastic surface roughness [18,31]. This stochastic surface topography results in qualities inferior to parts produced via traditional methods [32,33], which currently hinders the efficiency of mechanical AM components.

Significant proportions of energy are lost due to friction in mechanical components, and the lifespan of these components is substantially reduced due to wear [34]. Minimizing friction and wear of metal AM components through tribological assessment is necessary for long-term optimization of the technology for practical use. In practice, consistent mechanical properties, including coefficient of friction (COF), shear strength, and ductility, of AM components are prerequisites for functional application. However, the lack of understanding of the tribological properties of metal AM components, as well as the innately anisotropic nature of layer-by-layer manufacturing, presents a limitation [35,36,37]. Until the surface quality of AM components can be optimized with varying methods, material choices, and post processing technique, traditional techniques cannot be replaced. Typically, as-fabricated metal AM components tend to have a higher COF and are more susceptible to wear due to their irregular surface topography [38].

2. Tribological Behavior of AM Components