are these pin holes due to Hydrogen you think? If so, suggested degass methods that actually work? Raw material was melted dry wheel castings, most a bit dirty etc. Dross skimmed off 1380f pour temp (pretty sure) still don"t trust the probe yet into a fully dried plaster mold, whole mold submerged in water after about 15 min. then 395f for 4 hours, T5 based on an engineering paper. Machined nicely. But way too much porosity. Suggestions? Also, if you look in the pocket depression of the machined photo, the surface has this "ball peened" surface finish but the plaster mold is very smooth. Gating is on the other side of the top mold half that feeds the mold from the bottom to the top, the triangle shown is the riser John
Hi John Nice looking mold and part. What did you use for a positive while molding the plaster? Was the plaster baked at all before casting?
Hello John, That looks like steam pox to me. Probably generated from a wet mold. So, that brings up the question 'did the steam cause the porosity'? Maybe some of it but I see it in the fins and all the rest of the casting. That leads me to think you're running your burner too rich...really rich. What fuel are you using? I've tried home foundry de-gassing and none ever worked for me. The best practice is to prevent it in the first place and that is done by running a lean flame and getting it out of the furnace as soon as possible. Others that have plaster mould experience will hopefully chime in and give you something more definitive.
Plaster will not "Just dry" it has to be baked , a plaster expert will chime in Gassy metal? Maybe, you already got good advice on clean metal , I get really good results buy Covering aluminum with Clean Dry Sawdust and letting it burn off just before pouring( think 30 sec to a minute), I dont know how it works, but it seems to, possibly the hydrogen is drawn to the surface and burnes off with the sawdust V/r HT1 P.S. I really suspect the plaster still has some moisture
That is a lot of hydrogen porosity so it could be from overheating or rich mixture. If the plaster was the problem there would be a lot of bubbling at your sprue after you poured the mold.
Hey, thanks for chiming in so quickly! 1. the patterns were 3d resin printed in my Elegoo Jupiter to then create a cast urethane mold from which to pour the plaster molds. 2. the plaster molds were air dried for 24 hours at 130f, then baked over night at 350f. They really had the appearance of being bone dry throughout. 3. A356 recovered material was melted in an electric kiln. No degassing. 4. Metal was cast into preheated molds of 350f 5. About 15 min after the gate was hard I dropped the whole mold into a bucket of water. I know the best way to have clean gas free metal is to start with clean metal and keep it clean by not stirring in air or use damp tools etc. Melt as fast as possible to temp then pour quickly. Less time molten metal is exposed to air (H) Which I did not, dirty'ish wheel chunks also. Strike 1 & 2 I think the "peened" surface is from dunking the molds to early. The water in the bucket quickly began to bubble up steam for quite a while. I think the water immediately made its way to the interface between the molds pattern face and the part which was "green" IE at an elevated temperature where it was still "soft". The water in that space between of only the 1.3% shrink factor, say .01", the water flashed to steam in tiny local pockets, exploding and hammering the soft aluminum, that's my theory anyway. I know the metal should perfectly replicate the Plaster surface, I'm going to let the molds cool and hour before dunking next pour. I dunked them sooner because I wanted the thermal shock to help breakdown the plaster (which it did well). The other issue is the filling, the first pour that is the hottest fully filled with one cold hole. Every other part had incomplete filling of the fins and the cold hole. With preheated molds, I'm suspecting the metal temp was too low. Parts are not that big. The very first mold set I did still had some moisture in the plaster even though they got cooked at 230 f for 5 hours, the metal in the gate fizzed like soda. Parts looked like crap, oxidized grey with all sorts of pockets. Tapered gate is shared between mold sets, one pour two parts, the little triangle is the riser, I think it needed 3 or 4 of these Cold shot, I think the metal flows around and comes together in the thicker sections entrapping the cold shot in this thin section. Fins on top of pour tend to not fill, 1 out 4 filled completely. new individual mold set with short pour basin to try and eliminate time to fill and fill orientation for better thin section filling. detail captured of logo 3d resin masters for urethane molds urethane used for the plaster pours. Metal and plaster faithfully replicates the 50um layer striations of the 3d print master. Back to the question at hand. I guess I'll make a SS wand and bubble Argon for 10 min and do a pour. I don't have Nitrogen on hand. I should be getting cleaner charge metal soon.
John, thanks for all of the details and pictures. The last pour where I let the metal get significantly (ahem) above 1380F/750C was much more porous than the one that followed where I minded the temps better with a probe. I am 'cooking with gas' so the porosity danger is increased. Electric heat should be more in your favor. Wondering if doing a controlled melt and skim and ingot pour would help clean the wheel metal? Also wondering if it would be worth testing a simple sand mold against the same pattern molded in plaster and cured as above, to see what difference can be attributed to the plaster.
I get the same effect casting in green sand with an oil fired furnace, if I let the aluminium get too hot in the furnace. I'd assumed it was hydrogen gas absorption or from remelting too much already melted material like runners.
John, It would have been nice to have that information in the first post. I don't think the porosity is coming from your furnace because there is not enough O2 in there to cause absorption. Why not just purge the kiln with Argon in the first place. Dirty metal doesn't cause it either because I've melted plenty of it, even painted castings, and don't see it. I produced plenty of "swiss cheese" when I started casting. When you re-melt the gassy castings, sprues, ingots, the macro bubbles will bubble to the top like an Alka Seltzer on a micro scale and you'll get clean metal again. I've seen it and done it plenty of times. Concentrate on your mold and I think you'll be fine.
Clean yes, but not degas... Specifics Smelting, Not melting is a refining process to remove unwanted (stuff) specific elements (say Lead from gold), contaminants like paint, sand, all stuff we would call slag or dross Smelting can be thought of as cleaning metal, but not degassing that's different , Melting Scrap, especially stuff with large surface area, like soda cans(which are also heavily contaminated with liquids, and the label, Paint or plastic, to make ingots will get the trash/slag/dross out . but it still may be gassy , so you need a separate appropriate degassing process, they can be done in the same melt! some degassing processes may create some small amount of dross, that gets skimmed off just before pouring, and discarded. dont confuse what you are trying to do Ingots can be gassy, ever seen an ingot that looks like a loaf of bread, thats GASSY, melting it could possibly remove that gas, if melting in a reducing atmosphere, but if it is melted in a rich atmosphere and not degassed, it might pick up even more gas, or at least get/remain completely saturated and yes you can get to a point where you have just created trash, it has so much gas in it you cannot remove it as a hobbiest, this is especially true of dirty/oily /painted aluminum melted in a slow weak furnace with a rich environment, in an area with lots humidity, note the above worse case. most of that is under your control, clean your metal and adjust your furnace, the faster you can melt the better(because of humidity you cannot control) straight to Tops' inquiry just making ingots is almost always a waste of fuel melt, your metal fast, skim off dross (with clean dry tools) degass and pour quickly in short good ingots are good castings , so skip a step and cast something. I recommend a nice plaque V/r HT1
Forgive me for leaving out the heating process up front . The charge metal was shredded wheel castings, some paint, some dirt film etc. nothing else. OK, so back to the next question, WHO has successfully degassed their melts and how was it accomplished? I've watched and read of so many DIY urban myths that have been both proven and debunked. So a real world home brew success would be good to know of! Could have been induced by turbulence from my gate strategy? I'll try this new gating and see if it makes a difference. I have Argon at my disposal and I've also made a SS degassing tablet plunger. I suppose I could fill the kiln with Ar, was thinking of filling the crucible only and placing a lid on it, Argon is heavier that air so should stay put basically. refresh no and then. could test the fill by dropping a lit match in it, if it goes out, no Oxy present. My kiln takes about 90 min to get to temp. Thanks everyone.
no b No bubbling during the pour, just after dunking it too early. The first pour the plaster was still imperceptibly damp and the gate did fiz
I'm sure that there's been a post about Kelly's argon degassing lance. but you just need to flow a bit of argon through the molten metal (carbon rod or ceramic disk with some small holes ). The flow of inert gas through the metal causes the hydrogen gas to join in and move out. With Aluminum it's not about keeping the oxygen out like it is with more reactive metals (titanium, Stainless, etc) it's getting the trapped hydrogen out. Melting it in a vacuum kiln or in a shielding gas will help keep atmospheric hydrogen out, but any contamination on/in the metal will still be there (Paint, oil, etc).
