This is just a quick post on how I sintered some bronze. This may be of interest to people who are thinking of moving into metal fabrication. I’m interested in sintering as a way to 3d print metal.
Bronze seems like the ideal metal to start with, for a few reasons
- It’s not very dangerous. The FDA allows it to be used in cosmetics
- It has a low sintering temperature, between 800°C & 950°C
- It’s quite accessible. I bought mine from ebay. In particular, the types sold as makeup have very small grain size (40 micron), which is easier to sinter
- It’s quite inert, so I coudl mix it with water to form a paste without causing a chemical reaction (e.g., oxidation)
- I prepared a mix of water, isopropyl alchohol and poly-vinyl alchohol. The ratio was roughly 45:45:10, respectively. The water & isopropyl alchohol were to form the bronze into a paste, while the PVA was a binder that would remain once the solvents had evaporated
- I mixed a small amount of the liquid mixture with the bronze powder, as little as possible to form a workeable paste
- I pushed this mixture into a small 4mm nylon standoff tube I had lying around, and used two 4mm pins to compress the mixture into a pellet. This compression was done by hand, and assuming I pushed with approx. 10kg of force, concentrated over a 4mm cylinder, it would have reached 2Mpa.
- I let the mixture dry for 30 mins or so
- I pushed the mixture out into a cruicble filled with previously dried sand.
- I put the whole thing into a furnace and shot for 820°C.
- Once the furnace reached temperature, I let it bake for 10 mins or so, then turned off the heat.
- After an hour or so the system had cooled to ~300°C or so, and I took the item out of the crucible, and found it had sintered well, losing about 3% of its volume.
Sintering outcomes are apparently driven by density; specifically:
- The denser the input, the denser the output
- The hotter & longer the sintering, the denser the output
- The denser the output, the better the pysical strength. So the more you can compress the item in step 3, the better
Top temperature & sintering time
I found a lot of different sintering recipes on the internet. The one I used was based on a paper. However, more research suggests
- Bronze’s melting point is very dependent on percentage of tin, so definitely work out what your bronze alloy is and infer a temperature from the phase diagram. Mine was 90/10 bronze; 10% tin.
- The link above has a 10 minute soak time, but many sources suggest longer and hotter. Probably something to experiment with
- See the note on density - you should probably be measuring the density of your items after sintering to validate your time & temp.
I made a little cylinder. It’s a bit useless. The next step is to try:
- Extrueable shapes, where I’m easily able to compress the object
- 3d printed paste, without compression.
My main interest is in the 3d printing 😀