SUMMARY
The reuse of waste in investment casting processes is a potentially risky practice: if the waste is not adequately cleaned, it is possible to find in the alloy the presence of impurities such as oxides, or in any case of elements that contribute to the embrittlement of the piece.
PROBLEM
In the case in question, the customer complained, in a silver alloy containing a grain refiner, of a high fragility during the shutdown of the cylinders. This type of breakage seemed very strange, since the type of mother alloy used, thanks to the presence of the grain refiner, usually has great mechanical resistance. In fact, the wheat refiner is an element that helps to avoid or at least reduce this type of fragility problem.
Analyses were carried out on the samples sent by the customer (Figure 1): specifically, the pieces were observed at the SEM and subsequently analyzed with the C-S-O combustion machine to check for any impurities (see the enlargement at the SEM in figure 2)
The analyses showed that the fracture surfaces had a typical dendritic morphology, with dark portions at the edges of the dendrites. A consistent presence of carbon and sulfur was also detected: these two elements have a marked tendency to place themselves at the grain border, with consequent embrittlement phenomena since they do not have very high mechanical characteristics. So, when the pieces suffered the thermal shock due to the shutdown, breakage occurred.
However, the problem only occurred when the customer was using scrap and not when he started from pure material. This was probably because the waste had not been adequately damaged by the gypsum residues due to the investment casting process.
SOLUTION
To avoid embrittlement of the parts, it is therefore very important to clean the materials. We therefore recommend that you:
- limit the percentage of waste used as much as possible;
- decrease the number of material remelts
- clean the waste well from the plaster that may remain after the investment casting process; This is possible by acid pickling or sandblasting.