IMPACT OF HEAT TREATMENTS ON THE CORROSION RESISTANCE OF A BIOCOMPATIBLE CO–CR–W ALLOY FABRICATED BY SLM

Additive manufacturing (AM) enables the rapid fabrication of parts with complex geometries that cannot be easily manufactured with traditional methods. While originally limited to rapid prototyping, recent advances in AM technology also enable direct fabrication of functional end-use parts in, e.g., aerospace, medical devices, and military applications. However, the transition from rapid prototyping to fabricating end-use parts has also revealed technology barriers, including surface quality, accuracy, part variability, and uncertainty about the process–structure–property relationship, to name a few. Crucially, fundamental questions about friction, wear, and lubrication of AM parts have led to substantial research interest in the tribology community. This Special Issue provides significant value to the tribology community by highlighting recent advances of tribology research related to AM, defining the state-of-the-art of tribology knowledge, and framing the challenges and opportunities for future tribology research in this exciting field. It is a collection of 17 research/review papers covering a wide range of state-of-the-art topics in the tribology of additive manufacturing. All the papers have undergone a rigorous and anonymous peer-review process.Additive manufacturing technology is rapidly progressing and the future may bring many new printing methodologies. However, at present, the AM technology can be broadly grouped into seven categories: binder jetting (BJ), direct energy deposition (DED), material extrusion (ME), material jetting (MJ), powder bed fusion (PBF), sheet lamination, and vat polymerization (VP). Of particular interest is the understanding of the Process–Microstructure–Tribology (PMT) "research hotspot." Table 1 summarizes the topics covered in this Special Issue, in addition to the AM technology and the materials. Furthermore, we categorize the papers into PMT, tribology design, and surface characterization, based on the main topic of the paper. To set the stage, we summarize the contents of the papers per Table 1.Renner et al. presented a review paper focusing on the corrosion and wear properties of AM-fabricated alloys including steel, titanium, and aluminum. The paper points out that AM-fabricated alloys have better corrosion and wear properties than the casted parts, while the influence of process parameters on the microstructures does not hold true across different additive manufacturing processes and materials. Many other challenges—e.g., anisotropic behaviors, effects of heat treatments, the role of nano-particles, and failure analysis—are recommended for future studies in the field. Also noteworthy is the AM-fabricated metal parts (including PBF and DED) often end up with unique microstructures due to the rapid and repeated heating/cooling cycles and extremely large thermal gradient. Indeed, melting and solidification are highly time dependent and complex processes, making it difficult to simulate and predict. These are areas where more research is needed.Sharma et al. presented a literature review on hybrid surface metal matrix composites produced by friction stir processing and provided insight into the PMT relationship. Kang et al. studied the microstructure on the surface, sub-surface, and inner region of a commercial pure Ti part fabricated using laser PBF (LPBF). They indicated that the friction and wear behavior of the three regions are distinct. This is thought to be the consequence of the intrinsic heat treatment induced by the LPBF process. The remelting/heating and recrystallization cause microstructure coarsening and refinement between the three regions.Thasleem et al. studied the influence of various post-processing methods such as heat treatment and electric discharge alloying (EDA) on ambient and elevated temperature wear behavior of LPBF AlSi10Mg alloy and compared with the cast parts. Their results indicated that an EDA-treated part has the least wear-rate and coefficient of friction at both ambient and elevated temperatures due to its higher hardness than other samples. Thus, EDA-treating can be considered as a potential post-processing technique.Microstructure reinforcement is also an efficient way to increase wear resistance. Wang et al. studied the effect of TiB2 content on the microstructure and wear behavior of nano-TiB2p/2024Al composites fabricated by laser DED. Their results revealed that the wear-rate of an 8 wt% TiB2p/2024Al matrix composite with full equiaxed grains is almost 20 times lower than that of the unreinforced alloy due to the grain morphology-induced wear mechanism. Li et al. fabricated a dense Al–Fe–Cr quasicrystal reinforced Al matrix composite using DED. The reinforcement phases contributed to the mechanical mixing layer formation that significantly reduced the coefficient of friction and improved the wear resistance. Luo et al. fabricated short carbon fiber-reinforced nylon using ME and reported that the tribological performance improved.Rolling contact fatigue (RCF) is another critical performance for many tribological applications. Xie et al. and Fasihi et al. used laser cladding to enhance the railway rail materials. They showed that by carefully selecting cladding materials, both wear and RCF performance can be improved. However, micro-cracks may initiate from the interface between clad and unclad regions. Jalalahmadi et al. presented a predictive platform for fatigue prediction and AM-fabricated metallic parts qualification. They reported developing an integrated computational materials engineering tool that includes models of crack initiation and damage progression, exploring the design space across geometries and materials.Additive manufacturing can also be used to design and process unique functional structures, which may create some breakthroughs in tribological design. Suh highlighted the importance of design in improving the performance of all tribological systems. AM was mentioned as an innovative way to produce a part that is very difficult or even impossible to manufacture using conventional manufacturing while at the same time improve the design quality.There is a large body of tribology literature on the use of surface texturing to reduce the friction and wear characteristics of conventional materials. Surface texturing appears to offer viable flexibility for improving the tribological behavior of AM parts as well. Luo et al. reported that by designing specific surface textures—such as convex squares and triangles, processed via ME—they were able to improve the tribological performance. Hoskins and Zou designed and fabricated a micro-texture inspired by Ocellated Skink using two-photon polymerization (TPP), a VP technique producing nanostructures. They reported that wear was substantially reduced due to the texture through the controlled formation of microcracking. Maddox et al. also used TPP to design and fabricate surfaces inspired by frog toes and applied in the piston ring and liner interface. These designs reduce surface friction by an average of 18% and up to 39%, compared to a flat control. Zhang et al. used VP technology to produce various polygonal three-dimensional patterns inspired by dragonfly wings to identify how the polygonal patterns of the samples with bionic wing veins affected the skin friction. Their study provides insight into the mechanism of flow separation of the dragonfly wing and further improves the structure design. Murashima et al. used VP to design and produce a novel morphing surface that selectively performs as a low-friction or break-like surface. By applying air pressure, the surface switches between a convex and a concave shape, giving a different coefficient of friction. It is worth noting that AM of a part often tends to change a "continuous surface" into many discrete layer boundaries, inducing staircase effects due to the layer-on-layer nature. Narasimharaju et al. systematically investigated the impact of varying surface inclination angles on the build direction on the resultant surface textures. The areal surface texture characterization and particle analysis indicated that the resulted surface topographies are strongly correlated with the surface inclination angles.Modeling and simulations of the hydrodynamic effects associated with AM processes are also worth investigating. Wagner and Higgs studied the capillary and hydrodynamic effects of the interfacial flow responsible for primitive formation when the binder spreads into the powder bed and forms a bound network of wetted particles in the BJ process.The collection of articles in this Special Issue represents the active and diverse research efforts in the tribology of additive manufacturing. However, there is still a long way to go in this journey. Fundamental material research, application-oriented research, and novel tribological design are extremely worthwhile and exciting to pursue. We hope this Special Issue on the latest advancements in tribology of additive manufacturing provides insights and stimulates the generation of novel ideas with industrial applications on system diagnosis and machine design for years to come.Finally, we wish to take this opportunity to sincerely thank all the authors for their scientific contributions. Special thanks also to reviewers for their constructive and insightful comments on all the papers published in this issue.

Mechanical properties and corrosion resistance of cobalt-chrome alloy fabricated using additive manufacturing

Recently, Additive Manufacturing (AM) technology has been attracting interest because of its process to fabricate directly from CAD data. A laser powder bed fusion (L-PBF), one of the AM technologies, is able to form the metal three dimensional (3D) objects from metal powder by building it layer-by-layer. Generally, intensity distribution of the laser beam employed by L-PBF is Gaussian. Therefore, these beam shape has energy gradient. It is difficult to fabricate with high precision due to metal powder is aggregated at the lower energy area of beam spot. In this study, fabricated with L-PBF for the purpose to clarify the effects of energy gradient of beam spot on surface roughness Ra of 3D objects. As the evaluation method of 3D objects fabricated with L-PBF, it is employed to measuring surface roughness Ra and Vickers hardness.Recently, Additive Manufacturing (AM) technology has been attracting interest because of its process to fabricate directly from CAD data. A laser powder bed fusion (L-PBF), one of the AM technologies, is able to form the metal three dimensional (3D) objects from metal powder by building it layer-by-layer. Generally, intensity distribution of the laser beam employed by L-PBF is Gaussian. Therefore, these beam shape has energy gradient. It is difficult to fabricate with high precision due to metal powder is aggregated at the lower energy area of beam spot. In this study, fabricated with L-PBF for the purpose to clarify the effects of energy gradient of beam spot on surface roughness Ra of 3D objects. As the evaluation method of 3D objects fabricated with L-PBF, it is employed to measuring surface roughness Ra and Vickers hardness.

Additive Manufacturing (AM) is a well-known technology, first patented in 1984 by the French scientist Alain Le Mehaute [...]

Laser Powder Bed Fusion (LPBF) is a metal additive manufacturing technology (AM) that uses high power laser to melt powder layer by layer to create mechanical parts. LPBF several advantages, including short manufacturing time, freedom to design complex geometries and user-friendly customization. It is widely used in the manufacture of high-value parts in aerospace, automotive and biomedical industries. Due to the nature of the layer-by-layer process, partially melted powder is attached to the as-built part surface during LPBF, resulting in a significant increase in surface roughness. Furthermore, initial surface roughness of final part is different with locations since quantity of powder adhesion varies depending on building angle. Increase of surface roughness due to attached metal powder can cause out of dimensional tolerance that designed. Such dimensional inaccuracy can lead to failure or breakage of the mechanical parts. Therefore, surface post-treatment is essential to reduce surface roughness with minimizing dimensional change. Conventional mechanical and chemical treatments for surface finishing have limitations such as limited tooling range and surface damage due to the use of strong acids and long-time of processing. Electropolishing, based on electrochemical reactions, is suitable for improving the roughness of LPBF manufactured parts with complex geometries. Thickness reduction can be predicted by controlling the applied voltage and processing time. Also, surfaces in contact with the electrolyte is polished without geometric restrictions. Studies have been reported that analyze the change in surface roughness after electropolishing to improve the roughness of alloys manufactured with LPBF. However, for application to real parts, it is necessary to conduct basic research to optimize the electropolishing conditions to obtain satisfactory surface roughness with minimal thickness reduction by considering the effect of dimensional changes during electropolishing. In this study, we electropolished Hastelloy X fabricated by LPBF. Four types of electrolytes were selected for electropolishing. We measured surface roughness and thickness reduction with respect to applied voltage and processing time. With such results, optimized condition to reduce the surface roughness with minimal thickness changes are discussed. Then, these conditions were applied to LPBF specimen with different building angles. Surface roughness and weight changes were measured to compare polishing efficiency to each electrolyte.

Great advances have emerged in recent years around additive manufacturing techniques, with an increasing number of different materials (polymers, ceramics, metals). However, metal part manufacturing has always been one of the most demanded in engineering. That is due to its ability to create final functional parts with good mechanical properties. One of the most widely used technique is Selective Laser Melting (SLM). The SLM process uses a laser power source to selectively melt metal powder layer by layer. Typically, this manufacturing technique requires mechanical post-processing operations, not only to split the parts from the build-plate, but also to improve the mechanical properties and surface finish of parts or the dimensional accuracy of specific regions to ensure assembly and interchangeability. In particular, sandblasting is a method of mechanical abrasion cleaning commonly used and very useful for improving the surface topology of SLM printed parts. Besides, the laser scanning strategy used in this additive manufacturing process influences the surface quality of parts. Therefore, in this work, the sandblasting post-process has been optimized for surface roughness improving in parts printed using the most common laser scanning strategies (normal, hexagonal, concentric). The role that sandblasting pressure and time plays in the surface quality of parts, indispensable to optimize this SLM post-process, has been evaluated. Thus, surface roughness of different specimens subjected to different sandblasting parameters has been measured to optimize both values related to the laser scanning strategy used in SLM manufacturing. The material used is 17-4PH stainless steel, an alloy that presents an excellent combination of high strength and good corrosion resistance, high hardness, good thermal properties, as well as excellent mechanical properties at high temperatures. This precipitation-hardened steel has important applications in the aerospace sector, chemical and petrochemical industry, energy sector, surgical instruments, high wear components, and general metallurgy, among others.

<p indent="0mm">The selective laser melting (SLM) is a common additive manufacturing technology. SLM formation involves complex multiphysical phenomena, such as heat transfer, melting, molten flow, and solidification. The development of a numerical method for modeling multitrack powder melting in SLM has important scientific significance and broad application prospects. In this paper, a powder-scale multiphysics numerical model of molten pool dynamics is established using the SPH method. For the first time, the three-dimensional SPH simulation of multilayer multitrack SLM has been realized. First, a key technique for transforming SPH surface particles into interface meshes is proposed, and the multilayer powder bed is realized. Then, the effects of energy density and laser hatch spacing on molten formation are discussed. The optimal simulation condition of a single layer is obtained and further used in multilayer simulation. This work makes up for the deficiency in SPH simulation of powder-scale metal additive manufacturing and can provide a reliable simulation tool for defect analysis, process optimization, and the correlation between process parameters and product performance.

Selective laser melting (SLM), also known as 3-D printing or additive manufacturing process, has been developed for over 2 decades. This technique exhibits the capability to fabricate three-dimensional metal objects of complex geometries & structures from metal powders used in aerospace or medical orthopedics. This additive manufacturing technology has received a lot of attention, because various metals and metal alloys can now be processed to manufacture metal parts or structures, which includes the technologically important metals of stainless steel, cobalt chrome, titanium, tungsten and various other alloys. Typically SLM selectively melts thin layers of fine metal powder above each other by using a focused laser beam with high thermal energy on a substrate plate under argon atmosphere, resulting in production of almost full density metal or alloy parts. In this fashion successive layers of material are formed according to the computer edited 3-D-CAD design. Thereby, SLM could be applied to make functional end-use and geometrically complex products with cost-effect process especially in industry. Due to the geometrically complex form of the SLM manufactured structures, atomic layer deposition (ALD) is uniquely suitable to control the composition, thickness and conformability of thin films in the large scale and complex structures. In this study we report ALD coatings of Al2O3 of SLM stainless steel with the objective of providing corrosion protection by an alumina diffusion barrier and ALD ZrO2 coatings. During the course of this work we found the parts fabricated by 3-D printers tend to be porous and exhibit a rough texture. Figures 1 (a) show FE-SEM micrographs at progressively larger magnification of the surface of the 3-D printed stainless steel samples. As can be seen, the surface is very rough and contain many beads or spheres produced during the SLM manufacturing process, which is a direct result of the laser melting of the metal powder and rapid quenching. In order to reduce the surface roughness of the 3-D printed samples, electropolishing was used to smoothen and streamline the rough surface of the 3-D samples. After electropolishing, the surface of the 3-D printed samples is bright, clean and microscopically smooth and featureless. There are no beads or spheres remaining on the samples as shown in Figures 1 (b). We used SLM 316 stainless steel samples in a cross-flow ALD reactor with TMA and H2O vapor to attempt ALD coatings, while TDMA-Zr and H2O were used as precursors for ALD ZrO2. The analysis of the ALD Al2O3 and ZrO2 films revealed satisfactory chemisorption on the 3-D printed stainless steel surface. The ALD Al2O3 and ZrO2 films were uniformly grown on the 3-D printed stainless steel surface as shown in Figure 2. Figure 1

This study explores the corrosion behavior of a dental cobalt–chromium (Co–Cr) alloy fabricated by selective laser melting (SLM) under conditions simulating the oral environment. Specimens were fabricated using either SLM or traditional casting methods. Microstructure and surface characteristics were evaluated using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy and X-ray diffraction. In addition, sample corrosion characteristics were assessed using electrochemical impedance spectroscopy. Large microstructural differences were observed between SLM and traditional cast samples, with SLM exhibiting a more compact and homogeneous microstructure. The SLM samples also exhibited a slightly thicker surface oxide layer in comparison to traditional cast samples. In artificial saliva at pH 5, no differences were observed in the electrochemical corrosion properties or XPS surface characteristics of SLM and traditional cast specimens. However, in artificial saliva at pH 2.5, significant differences in electrochemical corrosion properties were observed, with SLM specimens exhibiting more corrosion resistance in comparison to traditional cast samples. The enhanced corrosion resistance of SLM samples in an acidic environment provides further support for their use in dental applications, where the oral environment can become temporarily acidic after meals.

In the field of materials engineering, physical properties such as mechanical properties are considered as the most important properties, however, chemical properties such as corrosion resistance are also critical properties that cannot be ignored. In particular, Japan is island country where metallic materials are frequently used in coastal areas. Therefore, the problems of failures and accidents caused by corrosion reaction of structural materials are more serious than in other countries.Recently, laser powder bed fusion (LPBF) process which is a type of additive manufacturing (AM), has been attracting attention as an advanced processing tool for fabrication of metallic materials in various industrial fields. The direct-forming process by AM with the applicability of intricate-structured materials is strong advantage for manufacturing value-added and relatively small products such as medical implants or aerospace assemblies.Our previous study demonstrated the excellent corrosion resistance of LPBF-processed type 316L austenitic stainless steel, focusing on its crystallographic planes and grain boundaries [1]. Therefore, in this study, we investigated the corrosion resistance of LPBF-processed type 420J2 martensitic stainless steel. Martensitic stainless steels have the lowest corrosion resistance among all kinds of stainless-steel types such as precipitation hardening, ferritic, austenitic, and duplex, because of minimum chromium content without other corrosion-resistant elements, and high content of carbon. The primary advantage of martensitic stainless steel is superior hardness, which can be achieved quenching heat treatment. In other words, corrosion resistance and hardness are conflicting properties for stainless steels, and it is difficult to realize coexistence.In this study, the electrochemical and the non-electrochemical corrosion tests were performed to evaluate the corrosion behavior of LPBF-processed and commercial 420J2 stainless steels. The microstructural characterization and hardness tests were also performed.Commercial 420J2 stainless steel powder was used as the primary material for specimen fabrication by LPBF in this study. The nominal composition of 420J2 stainless steel was Fe-12Cr-0.3C. The LPBF process was performed in an argon atmosphere using a 3D printer. To examine the evolved texture and planes of interest of the specimens, the z-axis was defined as the build direction, and the x- and y-axes were defined as the laser scanning directions. In this study, we fabricated cubic specimens with dimensions of 11 mm × 11 mm × 11 mm. The specimens were cut mechanically to expose their yz-, xz-, and xy-planes. No post-processing was performed on the LPBF specimens.Anodic polarization measurement (linear sweep voltammetry) was performed using a potentiostat (HABF-501G, Hokuto Denko, Japan) connected to a function generator (HB-111, Hokuto Denko, Japan) with an analog cable. A saturated calomel electrode (SCE) and platinum electrode were used as the reference and counter electrodes, respectively. The specimens were fixed in a polytetrafluoroethylene holder with an O-ring. The exposed area contacting the electrolyte was 0.35 cm2 (6.7 mm in diameter). After immersing the specimens in a simulated body fluid (physiological saline: 0.9 mass% NaCl aqueous solution, aerated) at 310K, their open circuit potentials (OCPs) were recorded for 10 min. Then, a gradient anodic potential was applied at a constant sweep rate of 1 mVs-1 from the initial potential of −50 mV from the OCP. The measurement was stopped when the current density limit of 1 mAcm-2 was recorded.Figure shows the polarization curves of LPBF-processed and commercial 420J2 stainless steel specimens in physiological saline. The pitting potentials of LPBF specimens were significantly higher than those of commercial specimens. These experimental results indicate that localized corrosion resistance was effectively improved as like 316L austenitic stainless steel in the previous study [1]. The results of the hardness test showed that LPBF specimens was 53.50 ± 0.25 HRC, regardless of the measuring position. The hardness of the commercial specimens varied from 54 to 27 HRC depending on the tempering treatment conditions after the primal quenching.Thus, LPBF is found to be an ideal process that can simultaneously enhance corrosion resistance and hardness of martensitic stainless steels. The experimental results of another corrosion resistance evaluation and inclusion extraction will be presented in the session.Reference:[1] Tsutsumi Y et al. Additive Manufacturing 45 (2021) 102066. Figure 1

Al-Ni-Cu alloys are used in energy, automotive, and aerospace industries because of their excellent mechanical properties, corrosion resistance, and high-temperature stability. In this study, Al-Ni-Cu alloy powder was subjected to selective laser melting (SLM). The SLM Al-Ni-Cu alloy was manufactured using appropriate printing parameters, and its properties were investigated. The results revealed that the As-printed material exhibited a typical melting pool stack structure, with an ultimate tensile strength of 725 MPa but a high brittleness effect (low ductility). After traditional heat treatment, the melting pool structure did not completely disappear. The strengthening phase Al7Cu23Ni precipitated from the boundary of the melting pools; thus, the Al-Ni-Cu alloy maintained high strength (&gt;500 MPa) and considerably increased ductility (&gt;10%). The SLM Al-Ni-Cu alloy has considerable industrial application potential; therefore, increasing the heat treatment temperature or extending the heat treatment time in the future works can promote the decomposition of the melting pool boundary and solve the problem related to the aggregation behavior of the precipitation phase, thereby improving the fatigue life of the alloy.

Additive manufacturing of metallic and polymeric load-bearing biomaterials using laser powder bed fusion: A review

Powder bed fusion (PBF) is recognized as one of the most common additive manufacturing technologies because of its attractive capability of fabricating complex geometries using many possible materials. However, the quality and reliability of parts produced by this technology are observed to be crucial aspects. In addition, the challenges of PBF-produced parts are hot issues among stakeholders because parts are still insufficient to meet the strict requirements of high-tech industries. This paper discusses the present state of the art in PBF and technological challenges, with a focus on selective laser melting (SLM). The review work focuses mainly on articles that emphasize the status and challenges of PBF metal-based AM, and the study is primarily limited to open-access sources, with special attention given to the process parameters and flaws as a determining factor for printed part quality and reliability. Moreover, the common defects due to an unstrained process parameter of SLM and those needed to monitor and sustain the quality and reliability of components are encompassed. From this review work, it has been observed that there are several factors, such as laser parameters, powder characteristics, material properties of powder and the printing chamber environments, that affect the SLM printing process and the mechanical properties of printed parts. It is also concluded that the SLM process is not only expensive and slow compared with conventional manufacturing processes, but it also suffers from key drawbacks, such as its reliability and quality in terms of dimensional accuracy, mechanical strength and surface roughness.

Metallic elemental powder mixture and pre-alloyed metallic powder are both frequently used powder feedstock in the additive manufacturing process. However, little research has been conducted to compare the corrosion behavior of selective laser melting (SLM) alloys, fabricated by pre-alloyed metallic powder and mixed metallic powder. Hence, it is important to investigate the corrosion behavior of SLMed alloys, as well as the corresponding cast ingot, with the aim to better understand the feasibility of designing new materials. In this work, the SLM-produced Ti6Al4V3Cu alloys were manufactured using a metallic elemental powder mixture and pre-alloyed metallic powder, respectively. The corrosion behavior of the different Ti6Al4V3Cu alloys was investigated in following electrochemical tests and ion release measurements. The results showed that the Ti6Al4V3Cu alloy prepared by pre-alloyed metallic powder showed better corrosion resistance than that produced from mixed metallic powder. Moreover, the SLM-produced Ti6Al4V3Cu alloys performed significantly better in corrosion resistance than the cast Ti6Al4V3Cu. The results are expected to achieve a better understanding of the feasibility of designing new materials using mixed powders, contributing to reducing development costs and cycles.

Is high-speed powder spreading really unfavourable for the part quality of laser powder bed fusion additive manufacturing?