9 replies to this topic

[AKAMetal]

I suppose I'll start with casting. We are primarily an investment/bronze casting facility but we have recently started casting aluminum too. Does anyone have any aluminum - lost wax - investment casting experience? We tend to experience porosity in our castings (not always and certainly not when I was casting it ) but it plays hell on our welder and then the plater. Any suggestions would be appreciated.

[ornametalsmith]

Maybe you could describe your aluminum melting process in detail. Source of aluminum and photos might help.

Porosity in casting is always a concern, with aluminum.....it LOVES hydrogen when it's molten.

Edited by ornametalsmith, 17 August 2012 - 08:29 AM.

[AKAMetal]

Yes of course, more info and I'll get some pictures up as soon as I can. We are melting in a silicon carbide crucible A12 size. Our furnace is a blown natural gas unit(I have measured it at close to 2400 degrees F)but we tame it down a lot for aluminum, tombasil and brass melts. The aluminum is 356 ingot from Cass, Inc. here in California. I have discovered that using at least 50% new metal in the melt helps quite a bit on finer finish pieces but we can use up to 90% scrap for heavily textured parts that get powder coated after finishing.

[Rich Waugh]

Are you using any sort of hydrogen/oxygen scavenging substance in your melt process? I have very little experience with aluminum, but in copper alloy castings it is often necessary to add a scavenger to eliminate porosity from absorbed oxygen. An induction furnace running an inert atmosphere purge is the typical production solution, but not everyone has that capability. I'm sure there must be a chemical you can add to the melt to address this issue, though. Just have to locate the source.

[AKAMetal]

Are you using any sort of hydrogen/oxygen scavenging substance in your melt process? I have very little experience with aluminum, but in copper alloy castings it is often necessary to add a scavenger to eliminate porosity from absorbed oxygen. An induction furnace running an inert atmosphere purge is the typical production solution, but not everyone has that capability. I'm sure there must be a chemical you can add to the melt to address this issue, though. Just have to locate the source.

We use borax in the bronze, brass and white brass melts to keep oxidation from occurring and I have something for the aluminum as well (can't remember what it is called though). We do have an electric melt induction furnace but it is less predictable than the glory hole so I kinda scraped it a few years ago. Plus we do production runs of up to 500 or 600 pieces with limited time so 1/2 hour melts for each flask just doesn't cut it.

[AKAMetal]

We got our act together, made a few changes with the aluminum melt ie. more clean metal and a little flux to keep oxidation down (thanks for the reminder on that!) and lowered the flask temp. a little with much better results. We are investment/lost wax casting globe style castings with approx. 1/8 to 3/16 wall thickness at a flask temp. of 550f. My next thread will be about aluminum welding, I don't think my welder is really getting it, he has been working with bronze for so long (spoiled) that he forgot that welding can be really difficult from time to time. Not to mention some of the processes are different for aluminum.

[Rich Waugh]

Glad to hear that helped you out.

Aluminum welding is very similar to welding bronze (copper alloys) in that you need to stomp the pedal at the start to overcome the high thermal conductivity of the metal and then back off to your welding power. One big difference, other than the need to use AC instead of DC for aluminum is that aluminum is going to try to oxidize much worse during welding than bronze will. Also the range between liquidus and fluidus is narrower on the aluminum so you have to watch out for burn-through. The modern inverter TIGs that have pulse control really help in this regard as you can fine tune the "on" times to get really nice stable welds. But even with an old transformer-type like my Syncrowave 200 you can get good welds on aluminum with a bit of practice.

One caveat - welding cast aluminum is three times trickier than welding billet or plate. Any difference in density on a casting, or any included detritus like flux or dross, will become immediately apparent when welding it - that's almost always where you'll get the burn-through and/or heavy oxidization. So the quality of your casting will influence the quality of the welds on it.

Edited by Rich Waugh, 21 August 2012 - 10:11 AM.

[Tony Mertens]

What Rich said. Also be sure to wire brush the oxide off the aluminum before you try welding it.

[PTsideshow]

What Rich said. Also be sure to wire brush the oxide off the aluminum before you try welding it.

Use only stainless steel brushes that are only used for aluminum. and no cross contamination from ferrous metals.

[AKAMetal]

Excellent, thank you. I have heard of/read that brushing the surface to be welded with a stainless steel wire brush right before welding is useful if not necessary. We will implement that and see how it goes.