Influence of Salt Support Structures on Material Jetted Aluminum Parts

This introduction paper is based on the paper "Influence of Salt Support Structures on Material Jetted Aluminum Parts" published by "Materials (MDPI)".

Figure 1. Phase diagram of KCl and NaCl according to Bale et al. [20]. The melting temperature of the eutectic mixture is 657 °C. There is a solid solution miscibility gap up to temperatures of approximately 500 °C [19].
Figure 1. Phase diagram of KCl and NaCl according to Bale et al. [20]. The melting temperature of the eutectic mixture is 657 °C. There is a solid solution miscibility gap up to temperatures of approximately 500 °C [19].

1. Overview:

  • Title: Influence of Salt Support Structures on Material Jetted Aluminum Parts
  • Author: Benedikt Kirchebner, Maximilian Ploetz, Christoph Rehekampff, Philipp Lechner and Wolfram Volk
  • Year of publication: 2021
  • Journal/academic society of publication: Materials (MDPI)
  • Keywords: additive manufacturing; material jetting; support structure

2. Abstract:

Like most additive manufacturing processes for metals, material jetting processes require support structures in order to attain full 3D capability. The support structures have to be removed in subsequent operations, which increases costs and slows down the manufacturing process. One approach to this issue is the use of water-soluble support structures made from salts that allow a fast and economic support removal. In this paper, we analyze the influence of salt support structures on material jetted aluminum parts. The salt is applied in its molten state, and because molten salts are typically corrosive substances, it is important to investigate the interaction between support and build material. Other characteristic properties of salts are high melting temperatures and low thermal conductivity, which could potentially lead to remelting of already printed structures and might influence the microstructure of aluminum that is printed on top of the salt due to low cooling rates. Three different sample geometries have been examined using optical microscopy, confocal laser scanning microscopy, energy-dispersive X-ray spectroscopy and micro-hardness testing. The results indicate that there is no distinct influence on the process with respect to remelting, micro-hardness and chemical reactions. However, a larger dendrite arm spacing is observed in aluminum that is printed on salt.

3. Introduction:

Material jetting (MJT) is an additive manufacturing process involving the controlled droplet-wise deposition of material. While commercially established for polymers and waxes [1], MJT of molten metals [2] and salts [3] has also been demonstrated. Metal MJT offers high part strength and design freedom [4–6]. However, like many additive processes, it requires support structures for full 3D capability, especially for overhangs and complex geometries. Removing these supports, often made of the same material as the part, increases cost and processing time [7]. Strategies to mitigate this include optimizing support structures (e.g., fine lattices [8]) or using different, easily removable support materials [9]. This study explores the use of water-soluble salt (a KCl-NaCl mixture) as a support material for aluminum MJT, drawing parallels with soluble cores used in foundry processes [10–12]. Key considerations when using molten salt supports include potential corrosion of the metal part [13] and the salt's low thermal conductivity [14], which might affect the solidification and microstructure of the aluminum printed on it.

4. Summary of the study:

Background of the research topic:

Material Jetting (MJT) of metals requires support structures for complex geometries, but their removal adds cost and time. Water-soluble salts offer a potential solution for faster and more economical support removal compared to conventional metal supports.

Status of previous research:

Previous work demonstrated MJT for AlSi12(a) [16] and explored salt processing via MJT, identifying KCl-NaCl as a suitable candidate due to its processability [3]. The use of soluble salt cores is known in die casting [10]. Potential issues with molten salts include corrosiveness [13] and low thermal conductivity [14], which could impact the build material. Interface studies between dissimilar materials in additive manufacturing exist [15].

Purpose of the study:

The study aimed to answer the research question: "How will the introduction of salt as support material influence our MJT process?". Specifically, it investigated the potential negative effects of using a eutectic KCl-NaCl salt mixture as a support structure for material jetted AlSi12(a) aluminum parts. The investigation focused on identifying:

  • Visual signs of corrosion at the aluminum-salt interface.
  • Surface structure changes (remelting) due to heat from molten salt.
  • Salt residues after cleaning.
  • Changes in aluminum microstructure (dendrite arm spacing) due to potentially slower cooling on salt.
  • Changes in micro-hardness due to altered microstructure or thermal history.

Core study:

The core of the study involved printing three different sample geometries (AS-sample: Al on salt; SA-sample: salt on Al; UL-sample: Al partly on salt, partly on printing plate) using AlSi12(a) as the build material and a eutectic KCl-NaCl mixture as the support material. The interface and bulk properties were characterized using optical microscopy, confocal laser scanning microscopy (CLSM), energy-dispersive X-ray spectroscopy (EDX), and micro-hardness testing to assess the interaction between the aluminum and the salt support structure.

5. Research Methodology

Research Design:

An experimental approach was used, comparing AlSi12(a) aluminum structures printed under different conditions: aluminum printed on a solidified salt (KCl-NaCl) support structure (AS-sample), salt printed on solidified aluminum (SA-sample), and aluminum printed partly on a salt support and partly directly on the printing plate (UL-sample). This allowed for the investigation of interactions during different contact scenarios (molten Al on solid salt, molten salt on solid Al) and comparison with a reference condition (Al on printing plate).

Data Collection and Analysis Methods:

  • Material Printing: Samples were produced using a pneumatically actuated drop-on-demand MJT test stand with interchangeable print heads for aluminum and salt, operating within a nitrogen-purged chamber [3, 16]. The build material was AlSi12(a) wire, and the support was a eutectic KCl-NaCl mixture.
  • Sample Preparation: Support structures were removed manually and by dissolution in distilled water using an ultrasonic bath, followed by ethanol cleaning and drying. For cross-sectional analysis (UL-samples), samples were cold-mounted in epoxy resin, ground, polished, and etched using either 2% aqueous NaOH or a double etch with NaOH followed by alkaline potassium permanganate solution [22].
  • Analysis Techniques:
    • Optical Microscopy: Used a Zeiss Axioplan 2 to examine etched cross-sections of UL-samples for microstructure (dendrite arm spacing) and signs of corrosion.
    • Confocal Laser Scanning Microscopy (CLSM): Employed a Keyence VK-X100 to analyze the 3D surface topography of SA-samples before and after salt deposition to detect potential remelting.
    • Energy-Dispersive X-ray Spectroscopy (EDX): Performed using a JEOL JSM-7500F SEM with an Oxford Instruments detector (10 kV accelerating voltage) on SA, AS, and UL samples to identify elemental composition and potential salt residues or reaction products at the interface/surface.
    • Micro-Hardness Testing: Conducted Vickers micro-hardness tests (HV0.025) on polished cross-sections of UL-samples using a LECO LM100AT tester, following DIN EN ISO 6507-1 guidelines for measurement grid placement.

Research Topics and Scope:

The research focused specifically on the interaction between material jetted AlSi12(a) aluminum alloy and a eutectic KCl-NaCl water-soluble salt support structure. The scope included investigating potential corrosion, thermal effects (remelting, microstructure changes like dendrite arm spacing), chemical residues, and resulting mechanical property changes (micro-hardness) at or near the interface between the build and support materials.

6. Key Results:

Key Results:

  • Corrosion: Optical microscopy of etched UL-sample cross-sections revealed no clear visual signs of corrosion at the aluminum-salt interface where aluminum had contacted molten salt.
  • Remelting: Confocal laser scanning microscopy of the aluminum surface (SA-sample) before and after contact with molten salt showed no significant change in the surface structure (dendritic features), indicating that significant remelting of the aluminum did not occur.
  • Microstructure: Optical microscopy showed that the dendrite arm spacing in aluminum printed on the salt support structure was larger (average 4.68 µm across three UL-samples) compared to aluminum printed directly on the aluminum substrate/printing plate (average 4.04 µm). This suggests slower solidification on the salt, likely due to the salt's lower thermal conductivity compared to aluminum.
  • Chemical Interaction/Residues: EDX analysis detected primarily Al, Si, C, and O on the sample surfaces. Fe (an alloying element) was also detected. In one measurement on an AS-sample, a weak chlorine (Cl) signal was observed, potentially indicating minor salt residue after cleaning, but optical microscopy did not support corrosion as the source. Sodium (Na) and Potassium (K) from the salt mixture were not detected in any EDX spectra.
  • Micro-Hardness: Vickers micro-hardness tests (HV0.025) on UL-samples showed no significant difference or gradient between the areas printed on aluminum (mean 51.0-52.6 HV0.025) and those printed on the salt support structure (mean 51.0-52.6 HV0.025), despite the observed difference in dendrite arm spacing.
Figure 2. Schematic representation of the SA-sample (a), AS-sample (b) and UL-sample (c). Dark gray areas designate the aluminum part, light gray areas the support structure and black areas the heated nickel-plated steel printing plate.
Figure 2. Schematic representation of the SA-sample (a), AS-sample (b) and UL-sample (c). Dark gray areas designate the aluminum part, light gray areas the support structure and black areas the heated nickel-plated steel printing plate.
Figure 3. SA-sample (a) and AS-sample (b) with salt support structure and after support removal. The top row shows the samples viewed from the side, the bottom row viewed from above.
Figure 3. SA-sample (a) and AS-sample (b) with salt support structure and after support removal. The top row shows the samples viewed from the side, the bottom row viewed from above.
Figure 4. UL-sample with salt support structure and after support removal. The top row shows the sample viewed from the side, the bottom row viewed from above.
Figure 4. UL-sample with salt support structure and after support removal. The top row shows the sample viewed from the side, the bottom row viewed from above.
Figure 9. Cross-sectional area of UL-sample etched with two percent aqueous sodium hydroxide solution and alkaline potassium permanganate solution according to Weck et al. [22]. [...] A coarser dendrite structure can be seen in the area above the salt support structure.
Figure 9. Cross-sectional area of UL-sample etched with two percent aqueous sodium hydroxide solution and alkaline potassium permanganate solution according to Weck et al. [22]. [...] A coarser dendrite structure can be seen in the area above the salt support structure.
Figure 13. Surface of the SA-sample analyzed with the confocal laser scanning microscope. The top images show the sample’s surface before coming into contact with molten salt and the bottom two images show the surface after molten salt contact. Next to the laser images (a,c), the corresponding optical images are displayed (b,d). No significant change in the surface can be seen. The results of the confocal laser scanning microscopy for the other two droplets do not differ qualitatively.
Figure 13. Surface of the SA-sample analyzed with the confocal laser scanning microscope. The top images show the sample’s surface before coming into contact with molten salt and the bottom two images show the surface after molten salt contact. Next to the laser images (a,c), the corresponding optical images are displayed (b,d). No significant change in the surface can be seen. The results of the confocal laser scanning microscopy for the other two droplets do not differ qualitatively.
Figure 14. Vickers micro-hardness values in the aluminum part (UL-sample). The squares show the micro-hardness values in the area above the aluminum and the circles show the micro-hardness values above the salt support structure. The numbers above the result points show the number of measurements performed. Based on the measurements, no significant influence of the salt support structure on the micro-hardness occurring in the aluminum can be determined.
Figure 14. Vickers micro-hardness values in the aluminum part (UL-sample). The squares show the micro-hardness values in the area above the aluminum and the circles show the micro-hardness values above the salt support structure. The numbers above the result points show the number of measurements performed. Based on the measurements, no significant influence of the salt support structure on the micro-hardness occurring in the aluminum can be determined.

Figure Name List:

  • Figure 1. Phase diagram of KCl and NaCl according to Bale et al. [20]. The melting temperature of the eutectic mixture is 657 °C. There is a solid solution miscibility gap up to temperatures of approximately 500 °C [19].
  • Figure 2. Schematic representation of the SA-sample (a), AS-sample (b) and UL-sample (c). Dark gray areas designate the aluminum part, light gray areas the support structure and black areas the heated nickel-plated steel printing plate.
  • Figure 3. SA-sample (a) and AS-sample (b) with salt support structure and after support removal. The top row shows the samples viewed from the side, the bottom row viewed from above.
  • Figure 4. UL-sample with salt support structure and after support removal. The top row shows the sample viewed from the side, the bottom row viewed from above.
  • Figure 5. Flow chart of the experimental procedure. Three sample geometries (SA-sample, AS-sample and UL-sample) are printed out of aluminum and salt via Material Jetting (MJT). […] Optical microscopy, energy-dispersive X-ray spectroscopy and micro-hardness testing are performed.
  • Figure 6. Measuring grid for micro-hardness measurement. The measuring points are located in the center of the sample in the vertical direction so that the hardness measurement is not distorted by possible edge influences. […] The distance between the measuring points is 0.5 mm.
  • Figure 7. Cross-sectional area of UL-sample etched with two percent aqueous sodium hydroxide solution. The images above the cross-sectional area show the detailed views of the material microstructure of the sample area above the aluminum (left side) and above the salt support structure (right side). In the area above the salt support structure, a coarser dendrite structure can be observed.
  • Figure 8. Dendrite arm spacing in the aluminum part (UL-sample). The squares show the dendrite arm spacing in the area above the aluminum and the circles show the dendrite arm spacing above the salt support structure. […] a larger dendrite arm spacing tends to be observed in the area above the salt support structure.
  • Figure 9. Cross-sectional area of UL-sample etched with two percent aqueous sodium hydroxide solution and alkaline potassium permanganate solution according to Weck et al. [22]. […] A coarser dendrite structure can be seen in the area above the salt support structure.
  • Figure 10. Three spectra for the SA-sample, superimposed in a single diagram. Aluminum, silicon, carbon and oxygen were detected. The three spectra differ in peak height, the peak positions do not change.
  • Figure 11. Three spectra for the AS-sample, superimposed in a single diagram. Aluminum, silicon, carbon, oxygen, iron and chlorine were detected. […] One of the three spectra shows a weak chlorine signal with all peaks < 150 counts.
  • Figure 12. Three spectra for the UL-sample, superimposed in a single diagram. Aluminum, silicon, carbon and oxygen were detected. The three spectra differ in peak height, the peak positions do not change.
  • Figure 13. Surface of the SA-sample analyzed with the confocal laser scanning microscope. The top images show the sample’s surface before coming into contact with molten salt and the bottom two images show the surface after molten salt contact. […] No significant change in the surface can be seen. […]
  • Figure 14. Vickers micro-hardness values in the aluminum part (UL-sample). The squares show the micro-hardness values in the area above the aluminum and the circles show the micro-hardness values above the salt support structure. […] no significant influence of the salt support structure on the micro-hardness occurring in the aluminum can be determined.

7. Conclusion:

The study investigated the effects of using a KCl-NaCl salt mixture as a water-soluble support structure for material jetted AlSi12(a) aluminum parts. Analysis via optical microscopy, CLSM, EDX, and micro-hardness testing revealed no distinct negative influences on the process or the final part properties. Specifically, there were no clear visual signs of corrosion at the aluminum-salt interface, no significant remelting of the aluminum surface upon contact with molten salt, and no significant change in micro-hardness between aluminum printed on salt versus aluminum printed on aluminum. A minor effect observed was a coarser dendrite structure in the aluminum printed on salt, attributed to slower cooling rates due to the salt's lower thermal conductivity. EDX detected a possible minor chlorine residue in one instance, but no significant chemical reaction products. The results suggest that KCl-NaCl is a potentially suitable water-soluble support material for MJT of aluminum, particularly for simple geometries. However, further research is needed to evaluate its performance with more complex geometries and larger support structures.

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9. Copyright:

  • This material is a paper by "Benedikt Kirchebner, Maximilian Ploetz, Christoph Rehekampff, Philipp Lechner and Wolfram Volk". Based on "Influence of Salt Support Structures on Material Jetted Aluminum Parts".
  • Source of the paper: https://doi.org/10.3390/ma14154072

This material is summarized based on the above paper, and unauthorized use for commercial purposes is prohibited.
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