SEM of fracture surfaces [Fig. 15(a-c)] shows that Alloy 2 has the most porosity of all the alloys with massive colonies of interconnected micro-porosity. The BSE images show that there was in fact considerable bismuth segregation to the grain boundaries; this segregation was more prevalent in alloys test- ed at 250ºC. Since the melting point of pure bismuth is 271ºC, the bismuth phase may not be pure. With some copper present with the bismuth, the melting point would decrease, and partial melting at 250ºC is likely. This has been observed in previous work conducted at company A.3
The fractographs of Alloy 3
(C93700) showed that the morphology of lead was more globu- lar than bismuth, and the distribution is more random. The lead phase in this case is pure and, since lead has a melting point of 327ºC, no change in the morphology of the lead phase was seen between bars tested at room temperature and 250ºC.
Tribological Properties
A comparison of the highest recorded friction force for the three alloys is presented in Fig. 16. Alloy 1 gives the highest
friction force of 22 lb (0.22 friction coefficient) followed by Alloy 2 with 18 lb (friction coefficient of 0.18). The bis- muth-containing materials (Alloys 1 and 2) run with a light- ly higher friction coefficient. Sahu2
has also reported similar
trends in his study. It is important to mention that, for each of the three alloys, the friction force drops suddenly at the beginning of the transient rub and then recovers to a stable level. This could be due to the reaction force right after ap- plying the static load. In Fig. 17, a comparison of the block temperature is given for Alloys 1 to 3. This figure compares the temperature changes of the three alloys during rubbing. Alloy 1 seems to have run hotter than Alloys 2 and 3. This can be attributed to differences in the room temperature for each of the tests. Figure 18 gives the test block temperature increase (max-min) for each of the materials and tests. The increase for the three alloys is almost the same. The average increase for Alloy 1 is 28.5°C, Alloy 2 is 24.6°C, and Alloy 3 is 27.1°C. Alloys 1 and 3 run at almost the same tempera- ture, and the difference is within the thermocouple accuracy. Alloy 2 runs cooler than the other two.
(a) Alloy 1 (N6178-4S),100X
(b) Alloy 2 (N6183-3),100X Figure 13. Micrographs of some selected samples, FeCl3 International Journal of Metalcasting/Winter 10
(c) Alloy 3 (N6183-3), 100X etch, tensile tested at room temperature. 27
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