Influence of Salt Support Structures on Material Jetted Aluminum Parts

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.

Keywords: additive manufacturing; material jetting; support structure.

Figure 1

Phase diagram of KCl and NaCl according to Bale et al. [20]. The melting temperature of the eutectic mixture is 657 $°\mathrm{C}$. There is a solid solution miscibility gap up to temperatures of approximately 500 $°\mathrm{C}$ [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). For the SA-sample, the aluminum layer is printed first and then analyzed using confocal laser scanning microscopy. Confocal laser scanning microscopy is performed again on the aluminum layer after it has been printed with molten salt. Then, energy-dispersive X-ray spectroscopy is performed. For the AS-sample, the salt and aluminum layers are printed successively and the aluminum surface that came into contact with salt is analyzed using energy-dispersive X-ray spectroscopy. All layers of the UL-samples are also printed successively. 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. In the horizontal direction, the distance between the measuring points and the samples’ edges is maintained in accordance with DIN EN ISO 6507-1. 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. The numbers above the result points show the number of measurements performed. Based on the measurements, 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]. 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). 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. The three spectra differ in peak height, the peak positions do not change. 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. 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.