1 Introduction
2 Materials and methods
2.1 Experimental methods
Star 0 | Cube 0 | Centre | Cube 1 | Star 1 | |
---|---|---|---|---|---|
Laser power P in W | 90 | 110 | 130 | 150 | 170 |
Scanning speed v in mm/s | 500 | 750 | 1000 | 1250 | 1500 |
Hatch spacing h in µm | 20 | 40 | 60 | 80 | 100 |
2.2 Methods for analysis
3 Results and discussion
3.1 Empirical model
Parameter combination pc | Laser power P in W | Scanning speed v in mm/s | Hatch spacing h in µm | Relative density in % | Build-up rate Br in mm3/s | Volume energy Ev in J/mm3 |
---|---|---|---|---|---|---|
1 | 130 | 250 | 65 | 99.90 | 0.33 | 400.0 |
2 | 130 | 500 | 65 | 99.92 | 0.65 | 200.0 |
3 | 140 | 500 | 65 | 99.94 | 0.65 | 215.4 |
4* | 170 | 1250 | 65 | 99.94 | 1.63 | 104.6 |
5 | 130 | 500 | 70 | 99.94 | 0.70 | 185.7 |
6 | 150 | 750 | 70 | 99.92 | 1.05 | 142.9 |
7* | 110 | 250 | 75 | 99.95 | 0.38 | 293.3 |
8 | 120 | 250 | 75 | 99.94 | 0.38 | 320.0 |
9 | 130 | 250 | 75 | 99.94 | 0.38 | 346.7 |
10* | 130 | 500 | 75 | 99.97 | 0.75 | 173.3 |
11 | 150 | 750 | 75 | 99.92 | 1.13 | 133.3 |
12 | 110 | 250 | 80 | 99.94 | 0.40 | 275.0 |
13* | 120 | 250 | 80 | 99.96 | 0.40 | 300.0 |
14 | 140 | 500 | 80 | 99.92 | 0.80 | 175.0 |
15 | 150 | 750 | 80 | 99.93 | 1.20 | 125.0 |
16* | 110 | 250 | 85 | 99.94 | 0.43 | 258.8 |
17 | 120 | 250 | 85 | 99.91 | 0.43 | 282.4 |
18 | 130 | 250 | 85 | 99.91 | 0.43 | 305.9 |
3.2 Microstructure
3.3 Hardness tests
3.4 Tensile tests
Parameter combination pc | Build-up rate Br in mm3/s | Volume energy Ev in J/mm3 | Ultimate tensile strength in MPa | Yield strength in MPa | Elongation at break in % |
---|---|---|---|---|---|
7 | 0.38 | 293.3 | 489 ± 13 | 310 ± 10 | 11.1 ± 1.6 |
13 | 0.40 | 300.0 | 525 ± 19 | 316 ± 25 | 12.6 ± 0.4 |
16 | 0.43 | 258.8 | 500 ± 16 | 309 ± 4 | 11.1 ± 2.0 |
10 | 0.75 | 173.3 | 475 ± 11 | 309 ± 11 | 16.6 ± 0.8 |
4* | 1.63 | 104.6 | 444 | 324 | 8.3 |
3.5 Demonstrator part
4 Conclusion
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In a first step, a process field with 13 different pc has been investigated. Based on the results for the relative densities of the built specimens, an empirical model was fitted. Using this empirical model, 84 pc have been chosen, which should theoretically provide parts with relative densities > 99.9%. The second step, the validation run of the 84 pc, resulted in specimens with at least 97% relative density. Eighteen pc actually led to parts with relative densities > 99.9%. It can thus be assumed that the DoE approach is highly applicable to predict process results in this case.
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The build-up rates for the 18 pc with relative densities > 99.9% varied from 0.33 to 1.63 mm3/s. This provides a high potential for cost savings, especially in tightly packed build cycles with long laser scanning times in each layer.
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The microstructures of the manufactured parts depended on the direction of the scanning paths. Scanning paths nearly orthogonal to the cross-section resulted in vertically oriented columnar grains, while scanning paths nearly parallel to the section plane resulted in a triangular grain growth, caused by the direction of heat conduction.
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All tested specimens exhibited high Vickers hardness of approximately 150 HV0.1. This is most probably attributed to high cooling rates occurring in PBF-LB/M and the associated formation of special fine and long grained microstructures.
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There were no significant differences detectable in the mechanical properties of niobium manufactured using different pc. Regarding UTS, there was only a slight trend observable: UTS was decreasing with higher build up rates or with lower volume energy input. However, all pc led to high values for both UTS and YS between 444 and 525 MPa as well as 309 MPa and 324 MPa, respectively. It can therefore be assumed that an increase of oxygen content during the process or powder atomization increased these values. The elongation at break was lower compared with conventionally processed niobium; the determined values are in the range of approximately 8 to 16%. SEM investigations revealed the presence of both transgranular and ductile fracture. The simultaneous appearance of ductile and transgranular (brittle) fracture was most probably caused by the special morphology, i.e. relatively long and fine-shaped grains.
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The applicability of the investigated parameters has been shown through a design demonstrator of a navigation nozzle for satellites. No obvious defects were observed.