Optimizing the additive manufacturing of parts like actuators, regulators and valves will result in faster delivery, lower costs and improved performance for these crucial equipment components

Additive manufacturing (AM) technologies are benefiting the chemical process industries (CPI) in two main ways. First, existing parts that have been in service for decades and require replacement can often be produced using AM, with much faster lead times and at a lower cost than with conventional manufacturing techniques. Second, entirely new equipment designs are now possible — for example, unique or more complex valve trims — leading to improved performance (Figure 1).

Laser bed powder fusion (LBPF), a type of AM process, has opened pathways for producing complex actuators, regulators, valves and other related parts — each optimized for chemical processing applications — by removing many of the constraints associated with traditional manufacturing. Before LBPF, it was either impossible or impractical to produce such parts, but this has changed due to the freedom of design provided by AM. One example of how AM can lower costs is that new internal structures in regulator bodies and intricate geometries for valve internals are formed in one processing step. However, in some cases, parts produced using AM still require post-process machining due to the rough surface-finish inherent to LBPF techniques. This article details the optimization of LBPF parameters, including scanning speed, laser spot size, layer thickness, scan patterns and laser power, aiming to overcome such challenges related to quality for AM parts. The new parameters were developed through a selective parameter matrix of laser power, hatch spacing (the separation between laser beams) and laser speed for 20-µm layers to improve the surface finish of parts made from 316L stainless steel and to reduce machining time from previously used 40-µm layers. Tensile bars were made to quantify the mechanical properties and surface finish of the 20-µm layer samples. The changes in physical properties were then correlated with the processing parameters to create a predictive trend analysis for future builds.  

AM challenges

As previously mentioned, AM of metals offers a wide breadth of benefits compared to traditional manufacturing, such as faster production and the ability to create complex geometries, among many others. This makes AM a worthwhile investment for a range of industries, particularly as the technology becomes more affordable [1– 3]. But many parts produced using LBPF still require machining when tight tolerances are an issue, such as with threads and bores, due to the nature of the rough surface finish of these parts [4]. As a result, there have been various attempts to try to shorten the overall manufacturing process of the finished part, either by the creation of hybrid AM systems with built-in computer numerical control capabilities [5], or by improving the properties of the material itself to make it more amenable to machining [6].

The preferred approach in many cases is changing the processing parameters during LBPF, namely the layer height, to improve the surface finish of parts printed with 316L material. This is effective because a shorter layer height corresponds with a decrease in surface roughness, at the cost of an increased build time [7]. However, when changing the layer height of a build, the effect on the overall energy density of the build must be considered, because this is the parameter typically used to quantify the quality of solidification of a part [2, 3]. This is shown in Equation (1) for volume energy density (VED), where P is the laser power (W), v is laser speed (mm/s), h is the layer height (µm) and d is the hatch spacing (mm):

 

VED = P / v d h (1)

  Laser scanning speed is related to the laser energy density applied to the molten pool of the material processed, which is specified as points per second as the beam travels across the print bed. These parameters can be modified to keep the volume energy density high. Through a trial-and-error process, the best combination of properties can be selected to create 316L stainless-steel parts that are mechanically strong, yet with low surface roughness of 20-µm, and with faster manufacturing time.

Part production details

For this exercise, parts were printed using 316L powder with a particle distribution of 5–50µm. The samples were printed on an AM machine using a hard, recoated blade with a spot size of 80µm and a 37-deg rotating scan pattern. Three cylindrical bars (A, B and C) of 18 mm x 85 mm were printed as shown in Figure 2 using the parameters shown in Table 1. The bars were then machined on a lathe to meet ASTM E8 standards governing the tensile testing methods for metallic materials.

Tensile tests were conducted using a load cell at room temperature and normal laboratory atmosphere, also following the ASTM E8 standard, with speed of 0.05 mm/mm. Stress-strain plots were created from the data to characterize the mechanical properties of each bar. Microstructural analysis was performed on a cross-section taken from tensile bars for each specimen, and samples were analyzed along their length for microstructure anomalies. Vickers hardness tests were conducted at room temperature using a hardness tester with a load of 500 g.  

The impact of printing parameters

The objective of this work was to evaluate the predominant effect of varying AM printing parameters, namely scanning speed, laser spot size, layer thickness, scan patterns and laser power. These parameters — individually or combined — can influence the final microstructure of the product or part. It was also necessary to evaluate the manifestation of defects, such as dislocation, lack of fusion, cracks, pores (common in high laser energy density), anomalous microstructures (martensite formation) and balling effects.

Balling effects can be distinguished from unmelted particles only by metallography, which reveals solidification structure layers. This effect will occur when the molten layer does not wet the substrate due to surface tension. This leads to spheroidization of the liquid, resulting in a bead-shaped surface and preventing smooth layer deposition, thereby decreasing the density of the final part build [3, 8].

The laser power and scan speed used in this investigation were suitable to prevent balling. Another method to alleviate the balling effect is decreasing powder layer thickness, as suggested by other researchers [9– 11].

The material properties are clearly related to laser power and speed. An increase in laser power results in elevated temperature, leading to increased metal vapor pressure formed on the molten metal, thereby dislocating the material and creating pores (also called keyholes). Molten metal will try to fill the pores created after being melted again when the laser passes (Figure 3). No spatter effects or hot cracks were found within the material at the investigated power, scanning speed and hatch spacing.

Laser energy input directly determines the melt condition of metal powders and the flow of molten metal, both of which have a significant impact on the type and size of the defects in selective laser melting (SLM) process. The energy input to the material can be related to the main process parameters, such as laser power, scan speed, hatch spacing and layer thickness. At a low scan speed and a high laser power, the energy input is high, so more powders are melted at an elevated temperature, thus creating porosity defects.

Deformation twinning (Figure 4) and induced plasticity were revealed from metallography samples after tensile testing. It is believed that deformation twinning is responsible for the elevated plasticity, a positive effect, of the tested samples, regardless of the level of porosity found [12– 15]. Deformation twinning, on the other hand, can have an adverse effect on ductility in other alloy systems [16].

As laser speed increases, the depth of penetration is reduced, along with the interlayer bonding between the layers, thereby increasing porosity [12]. Porosity reduction benefits the material fatigue life, as well as mechanical properties [17, 18]. The subsequent tensile testing results (Figure 5) show that the build with the fastest print speed had the largest ductility (around 90%) before failure.

 

Findings

The influence of varying SLM parameters — namely hatch distance, scan speed and power — on the internal structure and porosity of the samples was investigated. Defects encountered included spherical porosities, irregularly incomplete fusion holes and cracks (indicating a lack of fusion). Spherical porosities were randomly distributed, while the lack of fusion was found to be between layers. All these issues are clearly affected by layer thickness, hatch spacing, scan speed and laser power. Deformation twinning was found to enhance plasticity, despite the presence of pores. It was shown that processing at the selected high scanning speed is the most appropriate for minimizing porosity at the optimum hatch distance of 0.05 mm. Mechanical properties of the samples fabricated using varying strategies did not show significant differences in the yield strength and ultimate tensile strength values. These results will be used to complement upcoming work, which will facilitate improved quality and delivery time of AM parts, while maintaining strength, ductility and hardness. For CPI companies, these benefits will result in improved performance of valves and related components, along with lower prices and faster delivery times. Some of these benefits are already being realized due to AM, but further improvements will result when parts produced with AM require less machining and other processing. ■

Edited by Mary Page Bailey

 

References

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2. Ferro, P. and others, A modified volumetric energy density-based approach for porosity assessment in additive manufacturing process design, The International Journal of Advanced Manufacturing Technology, Vol. 110, September 2020.

3. Rombouts, M. and others., Fundamentals of selective laser melting of alloyed steel powders, CIRP Annals — Manufacturing Technology, Vol. 55, 2006.

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6. Pérez, J. and others, Machining stability improvement in LPBF, CIRP Annals, Vol. 72, 2023.

7. Patil, D., Effects of Increasing Layer Thickness in the Laser Powder Bed Fusion of Inconel 718, Arizona State University, Tempe, Ariz., Vol. 8, 2019.

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11. Antony K. and Rakeshnath T., Study on selective laser melting of commercially pure titanium powder, Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture, Vol. 233, September 2018.

12. Penn, R., 3D printing of 316L stainless steel and its effect on microstructure and mechanical properties, Montana Tech University, 2017.

13. Dengcui Y. and others, Twinning behavior in deformation of SLM 316L stainless steel, Materials Research Express, Vol. 9, September 2022.

14. Pham, M., Dovgyy, B. and Hooper, P., Twinning induced plasticity in austenitic stainless steel 316 L made by additive manufacturing, Materials Science and Engineering: A, Vol. 704, pp. 102–111, September 2017.

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16. Shiyang L. and others, Effects of deformation twinning on the mechanical properties of biodegradable Zn-Mg alloys, Bioactive Materials, Vol. 4, pp 8–16, 2019.

17. Cherry, J. and others, Investigation into the effect of process parameters on microstructural and physical properties of 316L stainless steel parts by selective laser melting, Int. J. Adv. Manuf. Technol., pp. 869–879, September 2014.

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^{All figures courtesy of Emerson}

  Authors

Ali Babakr is a senior principal materials engineer for research and development at Emerson ([email protected]). Babakr previously held various positions with Emerson and other companies as a subject matter expert dealing with metallurgical and failure evaluation, material selection and corrosion studies. He has field experience in the petroleum and petrochemical industries, and he participates in various industry subcommittees. Babakr holds M.S. and Ph.D. degrees in metallurgy from the University of Idaho and a B.S. degree in chemistry from Huston-Tillotson University in Austin, Tex.

 

Gerardo Gamboa is a materials engineer with the research and development team at Emerson ([email protected]). He started his professional career with Emerson as an additive manufacturing co-op student, while also attending the University of North Texas (UNT) in Denton, Tex. to obtain his M.S. degree in materials science and engineering. Gamboa has previous research experience as an undergrad and graduate researcher at UNT in semiconductor materials, batteries and composite materials. He is currently pursuing his Ph.D. at UNT in materials science and engineering.