Additive manufacturing of platinum alloys - practical aspects during LPBF of jewellery items

a speech by Ulrich Klotz

Abstract

In the present paper, optimum process parameters were determined for typical 950Pt jewellery alloy. Optimum densities of <99.9% were reached for a wide range of processing parameters. However, the resulting density was found to depend significantly on the part geometry and on the chosen support structure. The supports have to take into account for the geometrical orientation of the part relative to the laser build direction and the orientation on the build plate. Local overheating is responsible for porosity in these areas. Therefore, the supports play an important role in the thermal management have to be optimized for each part. The design of suitable supports was successfully demonstrated for a typical jewellery ring sample.

1 Introduction

The additive manufacturing of platinum alloys jewellery items found increasing interest in the last years. However, so far most work focused on gold alloys [1–4] and publications presented results on platinum alloys [5–7] Two aspects promote such interest: on one hand, the investment casting process of platinum alloys is rather challenging and struggles with casting defects such as shrinkage porosity, micro porosity or investment reactions [8–10]. On the other hand the physical properties of platinum alloys, particularly the reflectivity of the infrared laser light, are much more similar to steel or titanium alloys [11]. This makes the laser powder bed fusion (LPBF) process much easier compared to gold or silver alloys. In the past, the LPBF process of different 950Pt alloys has been successfully demonstrated for several alloys on different machines [5,12].
In the present work LPBF trials were conducted with a commercially available 950Pt-Au-In alloy (alloy 951Pt P1, C. Hafner, Germany). The LPBF process parameter were optimized concerning minimum porosity. The effect of support structures was studied and effects on alloy chemistry and defects are described in detail.

2 Experimental

2.1 Laser Powder Bed Fusion experiments

A MLab R LaserCUSING machine (ConceptLaser/GE Additive, Lichtenfels, Germany) with a laser power of max. 100 W was used in this study. The laser power was set to 95 W and kept constant for all tests. Despite the relatively low laser power a sufficient energy density could be achieved because of the small spot size (30 µm) of the machine. A two-step scanning routine was applied where the contour scan was made prior to the hatch scan [1]. The contour scan speed was 600 mm/s for all tests. The hatch scan parameters were varied to find optimum parameters with minimum porosity in a test part. The hatch distance and the laser speed were changed from 27 – 63 µm and 100 – 600 mm/s, respectively. The powder was provided as alloyed powder. It had a size distribution of 5 -30 µm (d10/d90 value) and was applied with rubber lip wiper in layers of 20 µm. The test part has an angular shape with wires and plates of different diameters similar to the one described in [1]. The support structure and the slicing of the model was done with the software AutoFab. The part was oriented in a 45° angle relative to the movement of the wiper.

2.2 Microstructure investigation and porosity measurement

The test parts were embedded in epoxy (EPO Fix) and metallographically prepared. Grinding was done with grit P320, P600, P1200 paper followed by subsequent polishing with 9 µm and 3 µm diamond paste The last polishing step was made with 0,04 µm OPS suspension. Scanning electron microscopy (SEM) images were obtained by a ZEISS Gemini instrument that is equipped with an energy dispersive x-ray (EDX) instrument for local chemical analysis. The porosity measurement was conducted by image analysis with the software AxioVision (ZEISS, Germany) on a stitched light optical image recorded at 5x optical magnification (Figure 1). The horizontal part of the sample was selected as region of interest (ROI). In order to determine the porosity the image was binarized using a threshold value at the minimum of the histogram. The porosity value is given as the percentage of black pixels inside the ROI.

3 Results

3.1 Process parameter optimization

Figure 1 shows two examples of test samples that were produced by different sets of laser parameters. The right part shows the optimum process parameters with minimum porosity. Figure 2 illustrates the effect of the laser parameters on the porosity. The lowest porosity values of about 0,1 % (99,9% density) were achieved for a hatch distance of 63 µm. At this hatch distance, the porosity levels are nearly independent of the laser speed. The porosity increases with decreasing hatch distance and increasing laser speed. Both, decreasing hatch distance and increasing laser speed result in lack-of-fusion porosity. A hatch distance of 63 µm and a laser sped of 500 mm/s were selected as optimum parameters throughout this study.

Figure 1: Typical metallographic section of the test samples. Left: hatch distance 27 µm, laser speed 600 mm/s. Right: hatch distance 63 µm, laser speed 500 mm/s
Figure 2: Effect of hatch distance and laser speed on the porosity. Laser power 95W.

3.2 Design of support structures for jewellery parts

Two jewellery items, typical engagement rings with three or seven stones, were provided by project partners for additive manufacturing. The rings were supported by columnar hollow supports in areas less than 45° to the build plate (Figure 3). The supports were symmetrical on both sides of the ring. The wiper applied the powder on the build plate perpendicular to the plane of the ring shank. The laser direction was from right to left. Defects occurred on the right side in unsupported areas of the ring shank. Along a certain length of the ring shank, material is missing, but only on the right, outer side of the ring shank. The problem starts at the end of the support structure and it ends at a build angle of 90°.

Figure 3: As manufactured rings with standard support structure. The arrow marks defects on one side of the ring shank.

In order to understand the problem, the AM process was interrupted at a height of 7 mm, which is in the problematic region of the ring shank. An SEM investigation of the last built layer (Figure 4) indicates a perfect surface on the left side of ring. On the right side however, the surface appears highly porous. The surface is uneven with significant balling of the melt pool. The view on the outer surface of the ring shank and a metallographic section through the centre of the ring shank (Figure 5) show powder particles that stick to the surface. The powder particles itself show a layer of much finer particles that appears like a kind of condensate on the surface. Local chemical analysis using EDX showed an enrichment of the condensate in the alloying Au and In compared to the ring shank material.

Figure 4: Interrupted build job at a height of ca. 7mm. Surface of the left and right side of the ring shank according to Figure 3.
Figure 5: Detail of the surface area of the right ring shank as marked in Figure 4.

4 Discussion

4.1 Parameter optimization

Suitable LPBF parameters of the chosen 950Pt platinum could be determined to obtain a porosity below 0,1 %. The porosity is a factor of 10 lower compared to surface treated 18k 3N gold alloys that were produced using the same machine [1]. Staiger [13] investigated the width and depth of laser tracks on metallic sheet material and found that the width and depth of a 950 platinum alloy was comparable to austenitic stainless steel and grade 5 titanium. 18k palladium white gold showed slightly higher width and depth compared to 950 platinum, while 18k yellow and red gold showed much lower width and depth of the laser lines. The similarity of 950 platinum to austenitic stainless steel and grade 5 titanium is due to its similar reflectivity for the infrared light and the similar thermal conductivity.
The porosity of parts produced by LPBF is a function of scan speed [14,15]. At low scan speed, i.e. high energy tendency the porosity is relatively high due to keyhole porosity. Keyholing could be achieved on 950 platinum sheet only at extremely low scan speed (25-50 mm/s at 95W) [13]. According to Tang et al. [15] the lowest porosity is a achieved in a range of medium scan speed. For 950 platinum alloys, this was achieved in the present study at 100-600 mm/s (hatch distance 63 µm, laser power of 95 W, Figure 2). If the scan speed is further increased, Tang et al. [15] describe an increase of porosity due to lack of fusion. This work showed lack of fusion in 950 platinum for hatch distances below 63 µm. The previous study on 18K yellow gold [1] found lack of fusion for the complete range of process parameters. Fully dense gold parts could be achieved at much higher laser powers of 375 W [4].

4.2 Optimization of the support structure

The condensation of material that was observed on the defective ring shank (Figure 5) requires an initial evaporation of alloying elements. This is a clear indication of localised excessive heating of the material. In order to identify the reason for such overheating the process condition were analysed in detail. It appears that the laser is working from the right to the left during the hatch scan. The different curvature of the ring on left and right side relative to the laser direction results in insufficient heat dissipation on the right side of the ring shank. The laser is scanning from right to left. Therefore, it first encounters an unsupported powder bed with limited heat dissipation on the right side of the ring shank. As a consequence, about 50% of ring shank cross section (Figure 4) is locally overheated, which results in the evaporation of the lower melting elements (Au, In) of the alloys. The left ring shank however does not sufer from such overheating because the laser starts on a well supported powder bed. On the very left side of the left ring shank the already lasered layer provides sufficient heat dissipation to prevent overheating.
In order to prevent the defective ring shank, sufficient heat dissipation has to be provided on the right side of the ring shank by additional supports. The critical angle that requires additional supports was determined to be ca. 61° and 72° on the left and the right side of the ring shank, respectively. The critical angles on either side were determined by an optical quality control of the rings.The supports should reach up to 6 mm and 8 mm on the left and the right side of the ring, respectively. Such regions are marked in red in Figure 6. These angles are much larger than a conventional rule of thumb that only surfaces with an angle below 45° should require supports.

Figure 6: Critical regions with possible excessive heating are shown in red.

Finally, all regions with smaller angles relative to the building plate were supported. Figure 7 shows the support structures before and after optimization. With the optimized supports the ring could be manufactured without defects Figure 8. Polishing and stone setting resulted in perfect finish.

Figure 7: CAD-Model of the ring with the original support structure (left) and the optimized support structure (right). Arrows indicate the additional supports.
Figure 8: Final ring (outer diameter 23 mm) with optimized support structure after the LPBF process (left) and after polishing and stone setting (right).

5 Summary and Conclusions

The additive manufacturing of 950 platinum alloys was successfully demonstrated by the laser powder bed fusion technology. The optimum process parameters were a hatch distance of 63 µm and a laser speed of 100-600 mm/s at a laser power of 95 W (Nd-YAG laser with 1064nm wavelength and a spot size of 30 µm). For such parameters a residual porosity below 0,1 % could be reached. A smaller hatch distance resulted in lack of fusion porosity. 950 platinum alloys can be processed with similar parameters like austenitic stainless steel (316L).
Jewellery ring samples were prepared with conventional support structures. However, it appeared that the supports have to take into account machine-specific laser scanning procedures. Additional supports were required at positions were the laser encounters an unsupported powder bed, if the orientation of the parts was below ca. 72° relative to the build plate. Otherwise, excessive heating resulted in the evaporation of material and defective surfaces. Therefore, a careful design of the supports structures has to be considered as part of the LPBF process optimization.

6 Acknowledgements

This research project was supported by the Federal Ministry for Economic Affairs and Energy (BMWi) through the AiF (IGF no. 20670N) based on a decision taken by the German Bundestag. We kindly acknowledge the support of the members of the users committee, in particular, the provision of 950Pt alloy powder and 3D CAD models by C. Hafner GmbH+Co.KG and Christian Bauer Schmuck GmbH+Co.KG, respectively. We thank the colleagues at fem for their contribution, namely Dario Tiberto, Daniel Blessing and for the additive manufacturing trials, metallography and SEM.

7 References

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