Additive manufacturing (AM) is a three-dimensional manufacturing technique that uses diverse materials to fabricate intended applications to manufacture complicated parts at a cheaper cost and shortens the time it takes for fabrication compared to the conventional method [1–4]. The materials used for additive manufacturing are as diverse as the product that is manufactured from the process. AM is very flexible in terms of the product's design, shape, and texture; similarly, it is flexible enough in terms of material use. Materials like polymers, resins, carbon fiber, paper, metals, thermoplastics, ceramics, alloys, composites, etc. Other than polymers, metals are used widely. Metals are the long-established raw material form for load-bearing applications [2–5]. The rapidly developing paradigm of additive manufacturing has shown significant benefits in allowing the custom design of structural components with superior performance compared to conventional subtractive processing procedures [6].
Among the existing additive manufacturing methods, selective laser melting (SLM) or Laser powder bed fusion (LPBF) stands out as a highly promising technique that can directly transform metallic powders into fully denser parts [7]. During the LPBF process, the powder bed is selectively scanned layer-wise, utilizing a high-intensity laser beam for melting. The solidified layer then piles up, creating complex geometries with tailored mechanical properties. [8, 9]
Among the various materials used for SLM/LPBF fabrication, stainless steel SS316L is popular due to its suitability in widespread applications covering nuclear, automotive, aerospace, marine, and medical sectors. These can be attributed to its high strength, excellent corrosion resistance, good weldability, good formability, and biocompatibility[10, 11]. Stainless steel is well-suited for a variety of automotive, industrial, food processing, and medical applications due to its superior mechanical properties. These properties include hardness, tensile strength, formability, and impact resistance. The 316L steel is selected for its high strength, corrosion resistance, and weldability [5, 12]. Research indicates that the increased stability of the oxide layer produced on AM 316L stainless steel samples may help to reduce the likelihood of localized corrosion. Temperature, gravity, and capillary forces characterize the part build density in the LPBF process. It is difficult to be controlled, as no mechanical pressure is involved during the build stage, as in molding [13]. The overall density of the material depends on the process parameters and microstructure. The process parameters of importance in LPBF include laser power, scanning speed, layer thickness, scan spacing, and scanning strategies. These parameters can directly impact the build density, porosity, and surface roughness of the fabricated parts, which in turn influences the microstructure and mechanical properties. [14, 15]. Due to the distinct properties of materials, it is necessary to use optimized processing parameters to minimize defects such as unbelted particles, porosity, distortions, residual stresses, undesirable phases, segregation, and cracking [16]. The amount of porosity on the build surface directly affects the part density. Porosity in the parts fabricated by LPBF can be due to a lack of fusion, gas pores that develop inside the metallic powders during their production by gas-atomization, and gas entrapment during the fabrication stage. [17–19]. To ensure reliability in working conditions, additively manufactured SS316L parts must be rigorously analyzed. Researchers have taken up numerous strategies for controlling the process parameters of the SS316L parts fabricated using SLM to achieve the desired mechanical and tribological behaviors.
Hiren et al. [20] have found a significant impact of layer thickness on hardness. The hardness value increases when the layer thickness is maintained low, as it is relatively easier to melt the thin layer of powder particles. High-density samples from SLM can be produced in addition to reducing porosity defects with increased laser power and lesser scanning speeds. [21]. Sun et al. [22] reported that the wear rate in SLM samples increases with the rise in the volume porosity percentage. The pores in the SS316L parts serve as the sites for crack initiation and propagation, increasing the wear rate. Hua et al. [23] have found that there is no appreciable influence of the built-up directions on the friction and wear rates of the SLM-fabricated SS316L specimens. With the increase in temperature, COF values decreased, but the wear rate increased up to \({200}^{0}\)C and further decreased slightly. This decrease in wear rate is attributed to the protective oxide layer formed at high temperatures on the surface, which decelerated further wear.
Ketal et al.[24] reported that the SS316L parts fabricated by different scanning strategies can result in unidentical grain growth direction, leading to a variation in their mechanical properties. Salman et al.[15] investigated the influence of various scanning strategies in fabricating SS316L parts. The variation of the scanning strategies has no impact on the phase formation. However, the scanning strategy considerably influenced the grains and cell size at the microstructural level, affecting the built samples' overall bulk density. Fine-grained microstructures yield improved mechanical properties [25]. Taban et al.[26] reported the influence of scan speed, scan strategies, and energy density on the morphology and mechanical behaviors of SLM-fabricated SS316L. The scanning strategy with the highest cooling rate resulted in higher density and fine microstructure. Decreasing the scanning speed increased the dendrite width, which can be attributed to the decreased cooling rate. Ram et al.[1] studied the impact of scanning rotation in LPBF. A checkerboard hatch style is used to fabricate the SS316L specimen, with alternate layers in the build direction having a scan rotation angle of \({0}^{0}\) and \({90}^{0}\). The fabricated specimen showed lesser friction values and moderate wear rates from scratch and sliding tests. The adopted methodology of LPBF proved to be
From the previous research on SS316L fabrication using the LPBF/SLM technique [1, 22, 23], it can be perceived that only a few studies have been reported on its tribological performance with different scanning strategies. The effect of altering hatch styles is barely reported as it is challenging to change compared to the other processing parameters. The LPBF additive manufacturing process is utilized due to its processing advantages, such as fully melted metal powder and the formation of highly dense products with complex part geometry. It overcomes the processing problems present in other AM techniques (porosity, low density, low mechanical strength, etc.) for fabricating similar structures. The present work analyzes the effect of adjusting the scanning strategies on the load-bearing characteristics of SS316L in terms of friction and wear rates. It would pave the way for more applications of LPBF fabricated SS316L, where rolling and sliding contacts are involved. Future work can also be expanded to analyze the corrosion and fatigue behaviors.