African Fusion June 2015

Laser aided additive manufacturing

Figure7: Secondary electron SEM images showing morphologies of IN100 after chemical etching; (a): low magnification showing elongated grain structures; (b): high magnification showing precipitates at grain boundaries (blocky and elongated shapes) and in the grains (globular shape).

Figure 9: EDS results showing different carbides in the post heat- treated IN100 after electrolytic etching. The globular particles are of the MC-type whereas the blocky and elongated ones are M23C6- type. Some of the carbides contain alumina cores.

into secondary carbides such as M23C6 and M6C, which tend to populate the grain boundaries [17]. Crystalline phases and chemical composition Figure 8 illustrates EDS results of elemental distribution and shapes and sizes of different types of carbides in the as- deposited IN100. Similarly, EDS results of post heat-treated IN100 samples are shown in Figure 9 and Figure 10 by different etching methods. Although both carbides and borides were expected to be present as minor phases, borides were not identified in this work due to low boron concentration and inherent difficulty in detecting boron by the EDS. Commonly observed carbides were typically found to contain elements like molybdenum, chromium, vanadium and titanium. For instance, refractory elements such as molybdenum are ca- pable of forming MC carbides, but the bonding with carbon is weak and hence, MoC can degrade to the more stable forms of M 23 C 6 and M 6 C after heat treatment or service [17]. MC-type carbides were seen as globular particles whereas the blocky Figure 8: EDS results showing different carbides in the as-deposited IN100. The globular particles are MC-type whereas the blocky and elongated ones are M23C6-type. Some of the carbides contain an alumina core.

and elongated ones were M23C6-type. Some of the carbides contain alumina core that has also been observed by other researchers [18]. Oxygen may be introduced in the alloy by interactions with the atmosphere during laser processing. Trace amounts of oxygen (above 50 ppm) and the subsequent formation of oxides should be avoided because they cause grain boundary embrittlement and can significantly reduce the stress-rupture life of the alloys [20]. Figure 10: EDS results showing different carbides in the post heat- treated IN100 after chemical etching. TiC was observed as black dots and often found in the grains, while MoC and CrC were seen at grain boundaries.

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June 2015

AFRICAN FUSION