Experimental Study of Build Orientation in Direct Metal Laser Sintering of 17-4PH Stainless Steel

Introduction

Numerous techniques such as selective laser sintering (SLS), selective laser melting, direct metal laser sintering (DMLS), and fused deposition modeling are 3D printing techniques that fall under the umbrella of additive manufacturing (AM).^1–5 This is due to the common manufacturing method that allows the stock material to be applied in an additive manner. Three-dimensional objects are constructed in a layer-by-layer manner through computer-aided design (CAD) and other modeling software that transform the 3D models into a sliced stereolithography file, which contains the object to be built in a format that can be read by the printer.^6,7 AM utilizes almost all of the stock material powder in the reservoir, which allows it to conserve material and reduce waste in contrast to conventional means of manufacturing or what we might refer to as subtractive manufacturing.^8,9

Various manufacturing industries have recently resorted to the use of AM mainly for the purpose of rapid prototyping (RP) as the first step of utilizing AM. Following that, numerous disciplines, such as transportation, defense, energy, medicine, and aerospace, have incorporated the use of AM in their industrial development to streamline their manufacturing processes and enhance the quality of their products. Utilizing the many advantages of DMLS, 17-4PH stainless steel has been molded into different shapes for different purposes to benchmark the integrity of the product as is, without any postmanufacturing treatment. This will allow us to realize the maximum potential of such a material when it is manufactured using this technology. ¹⁰

DMLS has the ability to fuse stock metal powder without the use of any binders. The same material (17-4PH) (0.07% C, 1% Mn, 0.04% P, 0.03% S, 1% Si, 16.25% Cr, 4% Ni, 4% Cu, and 0.3% Nb), ¹¹ which has been manufactured using DMLS, is compared with the same types of tensile specimens that are manufactured using rolling. Normally, parts produced by AM are subject to postprocessing or heat treatment such as hot isostatic pressing to minimize the difference in density levels between the AM part and the wrought part, yielding 80% or higher for density levels and 92% for surface densities. ¹⁰

Therefore, the purpose of this research is to provide a solid analysis through examining both mechanical and microstructural integrities of the products of DMLS by benchmarking the quality of materials manufactured as is, without any postprocessing, and comparing them with the quality achieved using conventional means of manufacturing (rolling).

Direct metal laser sintering

DMLS was developed jointly by Rapid Product Innovations and EOS GmbH as the first commercial RP method to produce metal parts in a unified process back in 1994. With DMLS, 20 μm metal powders that are free of fluxing agent are melted completely by a high-power laser beam to build the part with properties of the original metal powder material. The burn-off and infiltration steps are avoided in DMLS by elimination of the polymer binder, thus producing a 95% dense steel part compared with ∼70% density with SLS. A schematic of the process is shown here in Figure 1, while an excellent literature review on the development history of DMLS is very well presented by Shellabear and Nyrhilä. ¹²

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FIG. 1.

DMLS rapid prototyping schematic. DMLS, direct metal laser sintering.

Materials and Methods

The stock powder is obtained through a method called powder atomization; a process that has been made available for many industries not just AM through producing large amounts of ready for sintering metal powder. ¹³ Powder atomization occurs through melting of the precursor, which is followed by breakage of molten metal. The particle sizes ranged from 2 to 42 μm with the 17-4PH alloyed powder consisting of mainly a martensitic microstructure that is fused with other particles to yield maximum mechanical properties. It consisted of large amounts of chromium, nickel, and copper to generate good corrosion resistance.

To facilitate this process, the melt feed was passed through an induction arc, which produced three different phases; rudimentary, alloyed metal, or waste. At this stage, homogeneity of the molten metal is very crucial, which means no disturbances should occur during the heating. The molten metal was then passed through high-speed jets that usually comprise an inert gas on its way to the tundish. This allowed the molten metal to harden and get broken down into tiny droplets as they cascade to the base of the tank. ¹⁴

The CAD file is fed into the 3D printer software, Phenix, where only the orientation and supports for the parts could be adjusted according to the part parameters in relevance to the substrate material boundaries. Both gridded and solid supports were applied to the parts; gridded supports were usually intended for ease of removal from the substrate material; however, it sometimes crashed the build as it was too brittle and would often catch on to the scraper as it layered the next layer of powder. The DMLS parts were manufactured at different orientations using this stock metal powder that was provided by 3D systems to realize the effect of orientation on both microstructural and mechanical integrities.

The printer used here was the Pro300 X by 3D systems with laser specifications: power of 500 W, spot size of 100 μm, and wavelength of 1070 mm, with a scan speed of 0.09 m/s. Nitrogen gas was used as 17-4PH stainless steel is not a reactive metal, although argon gas could have been used instead, as other work in literature suggests. ¹¹ The build volume was 25 × 25 × 28 cm³; taking this size in consideration, the designer must watch out for the 5-to-1 rule stating that the height of the part built cannot exceed the width of the base by five times, which at times caused a problem due to the height of supports that were added to the specimens.

The manufacturing procedure was carried out in a vacuum environment filled with nitrogen. The oxygen levels were maintained at 2000 ppm at all times inside the chamber to control the induced spark that is resulted from contact between the laser and the powdered layer that only measured up to 40 μm, nitrogen was kept at 90 psi at all times to control the densification rate of melt pools. To make sure that the laser was aligned with the substrate material at the appropriate focal length, every time a new material was used, a defocusing was carried out. The mark speed and jump speed were set to 2500 and 5000 mm/s, respectively, with a 3.9-mm focal length that was concluded to be ideal from this run.

After setting all the desired parameters, the powder reservoir is filled with the stock material very carefully and the process is commenced. The three orientations shown in Figure 2 were all carried out in the same run, keeping in mind that special parameters had to be adjusted for vertical orientation as the height-to-base ratio did not fit the 5-to-1 rule. To honor that rule, the base area had to be increased by stacking together all the tensile specimens. A tapered boxing was designed to protect the stack from the roller as it layered after each laser pass. The reason why the box had a tapered shape was to reduce the cycle time as well as conserve as much material as possible. Solid supports of 1-mm size were added to each specimen to facilitate ease of removal from the substrate material using electric discharge machining. The horizontal and 45° specimens were each fitted with two types of supports at the same time. The solid support was offset from the part by 2 mm to enable addition of gridded support in between.

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FIG. 2.

Build orientations for different specimen fabrications.

Results

The result of layering vertical specimens was disappointing as the roller caught on to the specimens in the reduced cross-section zone of the tensile specimen.

Figure 2 (vertical build) shows exactly where the roller caught onto the specimen in the reduced cross-sectional zone. It also shows how all the specimens are stacked together to increase the base area to accommodate the 5-to-1 rule. The box surrounding the specimens was also added to help increase the base area while still reducing the material used thanks to its tapered profile. The tapered box was removed after layering was successfully completed.

Two types of supports were attempted in manufacturing of horizontal and 45° tensile specimens. The first type of support comprised two layers; a solid and a gridded support for ease of removal from the substrate material, with an offset of 2 mm between them. The gridded support was the reason behind the first crash due to its brittleness, where an undeveloped support between the specimen and the solid support where the crash occurred, therefore crashing the entire build. From this point onward, the build was carried out using solid supports only to avoid any more crashes. Both horizontal and 45° oriented specimens were successfully built after addition of solid supports, as shown in Figure 2.

Due to the presence of protrusions hanging from the 45° specimens, additional support material was required or else the build would have crashed, even though it is not preferred to add more material as it is considered a waste. Table 1 illustrates a contrast between the different yields for each manufacturing technique for both the wrought and DMLS parts. The table provides information for ultimate tensile and yield strengths as well as elongation percentages.

For the sake of experimentation, random parts were manufactured to gauge the capability of the printer to successfully sinter different geometries. The parts manufactured included an oil sprocket, a solar cell lattice, a piping system manifold, and junctions for a fluid delivery system. Other parts were manufactured for the sake of maintenance as we only attempted to manufacture the broken part of the original object. All the parts where successfully manufactured, except for the lattice as it had an exceptionally thin structure. The build was salvaged by lowering the build plate and then relayering and resintering; however, this did manage to burn some areas due to double sintering.

The manufactured parts were printed with the purpose of exploring the full capacity of the DMLS technique. The main advantage of AM is that it can produce parts with intricate geometrical designs. That is why a thin lattice was sintered to prove that such geometries can be manufactured using DMLS. Tilted structures were also manufactured; however, smearing occurred on protruding parts, therefore concluding that parts with an incline of more than 45° cannot be sintered, even if properly supported. As mentioned before, protruding geometries cause the build to crash due to poor supporting. This happened with the tilted junction, where the protruding area caused the build to crash and also smearing is visible at the bottom of the tilted area. However, the rest of the part was successfully manufactured.

Post tensile testing, the fractured surfaces were inspected using optical microscopy and scanning electron microscopy (SEM). Moreover, X-ray diffraction (XRD) was carried out to display the constituents of the alloyed metal.

Figure 3a illustrates the 5 × magnification of the horizontally oriented tensile specimen. The image clearly shows grain coalescence. Pitting is also visible between each grain, which causes a degree of porosity. Dissolution of the molten powder is seen in the dark regions between the grains; this phenomenon also helps increase porosity. Figure 3b reveals semicircular formations, an indication of the direction of laser sintering, where voids are very clear in intergranular spaces. In Figure 3c, the 45° oriented tensile specimen clearly shows micropores in intergranular spaces. Finally, in Figure 3d, and for the same 45° built specimen, the martensitic structure caused by the cooling effect of the nitrogen-induced environment that accelerates the solidification process of melt pools is visible.

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FIG. 3.

Optical microscopy of fracture zones at 5x magnification (a) horizontal build orientation showing grain coalescence, (b) horizontal build orientation showing semi-circular formations, (c) 45° build orientation showing micropores, (d) 45° build orientation showing martensitic structure.

Figure 4 depicts the SEM image of the horizontally oriented tensile specimen where pores and cleavages seem to be dominant in the terrain. Incomplete sintering also caused the visible balling phenomenon as seen in the pits within the surface. Microvoid coalescence is visible, which is a result of ductile failure of the specimen. Cleavages and porosity are also seen all over the structure, while evidence of dimpled rupture is evident in the 20 kX magnification shown in Figure 5. On the other hand, Figure 6 depicts the SEM image of the 45° oriented tensile specimen, showing intergranular cracking in the semicircular region of the structure, which indicates the aforementioned martensitic structure caused by the cooling effect of nitrogen. Finally, a very coarse surface due to a poor sintering balling phenomenon is also visible as well as stress corrosion cracks in the 3 kX magnification shown in Figure 7 of the same orientation build.

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FIG. 4.

SEM of horizontal build orientation (5 kX magnification). SEM, scanning electron microscopy.

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FIG. 5.

SEM of horizontal build orientation (20 kX magnification).

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FIG. 6.

SEM of 45° build orientation (1 kX magnification).

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FIG. 7.

SEM of 45° build orientation (3 kX magnification).

Figures 8 and 9 show the XRD testing of the 17-4PH of the DMLS manufactured parts of horizontal and 45° build orientations, respectively. XRD results were found to be fairly similar, showing comparable amounts of peaks of austenite (γ-Fe), copper (Cu), iron (Fe), nickel (Ni), and chromium (Cr).

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FIG. 8.

XRD pattern of horizontal build orientation. XRD, X-ray diffraction. Color images are available online.

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FIG. 9.

XRD pattern of 45° build orientation. Color images are available online.

Discussion

The advantage that DMLS provides over other types of 3D printing such as SLS is that it eliminates another huge factor that affects the integrity of the grown part, which is the addition of polymer-based binders and fillers. This allows for better formability and marginally less porosity. For instance, and when compared with SLS, DMLS will provide a higher quality 3D printed work piece and at a higher density. DMLS of high-strength stainless steel proved to be rather an experimental tool at this point compared with being a tool sufficient for industrial use; this is due to factors such as (1) the necessity of adding supports to separate between the substrate material and the object being manufactured, (2) the limited build volume and 5-to-1 rule that limit the growth of parts that have a high length-to-base size ratio, and (3) the smearing that occurred at the presence of any protruding part or unsupported parts, which could easily ruin an entire batch.

However, it did prove the capability of manufacturing parts with high geometrical intricacy, which would have required numerous manufacturing techniques if fabricated using normal means of manufacturing. The addition of in situ heaters that directly warm up the powder bed is an idea that has been floated in many experiments and proved to have positive influence on the overall integrity of parts grown with polymer-based binders and fillers; this would act as a fortifying agent to the metal powders being laser sintered. All this put together, DMLS is thought of as a powerful manufacturing tool that serves many advantages for parts built without any postprocessing or even machining, yet still provides quite some room for improvement.

It has been reported in a similar experimental study that Ti6Al4V as well as 17-4PH stainless steel has been sintered using DMLS. ¹⁴ The EOS printer build volume used to sinter the parts did not accommodate vertical specimens due to the shape and size (ASTM E8) of the 17-4PH specimens used, which is similar to the case with the Prox 300 that was encountered in the present work. The 17-4PH specimens built on the horizontal orientation also reported microstructural integrity similar to the ones achieved during experimentation; segregated melt pools, small voids, and a fine crystalline structure were observed, which indicate a martensitic structure similar to that achieved during our study. In the same material, when sintered on an inclined angle, a slightly dissimilar microstructural formation appeared, with larger amounts of voids that are bigger in size and much smaller and less visible melt pools as well as lamellar colonies arranged in a random manner.

Conclusions

The aim of this article is to benchmark the DMLS technology off-product sans any postprocessing in contrast to the off-product of the conventional means of manufacturing used throughout this experiment, which was rolling. Orientation change of tensile specimens was the main testing point throughout this article, experimenting on how the build direction would affect the overall integrity of the specimen. Three separate orientations were examined, of which only two were successfully manufactured, both the horizontal and 45° angled tensile specimens. The vertically oriented tensile specimens crashed due to violation of the 5-to-1 rule regardless of the efforts that were exerted to increase the base-to-height ratio. Other reasons such as smearing and poorly supported parts were also responsible for build crashes. The mechanical features of wrought specimens seemed to yield higher strengths than that of the DMLS parts; however, the 45° specimens had a better elongation percentage.

The heat-treated wrought parts yielded higher density levels than those of DMLS parts, which do not come as a surprise. However, comparing the horizontally oriented DMLS parts with the 45° oriented specimens, it was found that the horizontally oriented parts yielded higher density and mechanical features than that of 45°. Little to no difference in microstructural features was observed when comparing between the different orientations of DMLS parts. All parts showed almost equal amounts of porosity, balling, cleavages, and poorly sintered regions. Different cleavage and necking shapes were visible, which is evidence of ductile fracture in some of the microscopic images. The different imaging techniques applied showed the martensitic phase to be the dominant phase, which was attributed to the cooling effect of the nitrogen-induced environment.