Fragile alluminium casts? Why can I crack it in my hands?

[Kiel366]

Hi!

I started my smithing/smelting adventure not long ago. I've built a furnace, melt some alluminium, poured it into my clay cast, let it cool down (and used water to make the process faster). Then I wanted to make a sword out of it (as it was just a base stick cast). I heated it up, hit it with a hammer and... it just cracked. Later I discovered that I can even tear my casted metal pieces apart with my bare hands...

Please tell me guys, what am I doing wrong?

[Crazy Ivan]

I'm not much of a knife/ sword maker but I can tell you now that you can not make a sword out of aluminum. You can make something that might look like a sword but its nothing more then a decoration. We call those SSO or "sword shaped objects". Also don't know much about casting but my bets are on you expediting the cooling rate of the aluminum when you add water to cool it faster. Normally that would anneal it but it may be different form a molten state to semi-solid, then quenched. Plus, it is incredibly dangerous to have ANY water near molten metal. It could also be poor material. Are you melting cans or engine blocks or what? The more info you give will make it easier for folks to give you good answers. Also, put your general location in your profile. Someone on here may live nearby and could help. Read up on the subject. Not youtube videos, lots of terrible info and false theory on there.

[ThomasPowers]

Any signs of porosity in the breaks? How did you degas the melt before you poured? Even cast steel is not generally suitable for swords as the grain size of a casting tends to be large and weak (some alloys can deal with that.) What alloy of Al did you pour?

Root cause is overstepping your knowledge base; you may feel a disturbance in the force caused by folks shuddering at the possible injuries you are leaving yourself open to.

[Kiel366]

The ingots are very pourous and look like as they were built of styroform (a lot of little bubbles I think [really, a lot, like a tenth of mm in diameter]). I bought the Al on a scrapyard, but in cylindrical bars, so I thought it is clean. On the same scrapyard I bought the crucible (an iron pot) and bulit a brick furnace powered by coal. The clay I use I mine myself next to my house. I know it sounds catastrophic, but I possibly wanted to stick to the ancient methods of forging. In my opinion, gas in the problem and I have no idea about "degassing".

 

Some of it also sometimes turns into a kind of powder when I hit it with a stick just before melting. I only wonder, why, on low temp, I somehow managed to flattern an Al bar with my hammer, without breaking it. It became 2 times flatter but same dimensions. Then when I forged it, it was perfecly forgable and I even managed to create a simple spear head. However, after making a "sword base" case, I wanted to flattern it and it just cracked into pieces upon the forst hit. Conclusion: if I have a small enough cast, like an ingot and I manage to flatter it, decreasing the volume 2x it becomes forgable... WHAT IS GOING ON?

[ThomasPowers]

The Al could have been very clean when you bought it. However you melted it in an atmosphere and it absorbed gasses, you need to degass the melt before pouring. Note the more times it's melted without degassing the more it can absorb. Also look into what dross is.

I strongly suggest you spend some time at backyardmetalcasting.com as a forum dedicated to casting whereas these forums center on forging of steel.

[Nobody Special]

Gas, grain size, casting alloy instead of extruded type alloy, burned out some of whatever it was alloyed with. The impromptu quench. Reheating it too hot, not reheating it hot enough. Working it too cold for too long. So many options and so little info.......

 

Baaaaad material for swords. Knives occaisionally, and those are contraversial. Aluminum is prone to cracks and fractures that are basically invisible until stressed enough. Forgeable aluminum makes lousy castings, and vise versa.  So, say, swing an 2-3 ft long lever of aluminum (at 1/3 the weight of steel) SMACK with the outside edge moving at XX mph. What the hey, shrapnel makes life interesting, don't you think?

 

If you gotta cast a sword, I'd work on my casting for awhile and then work up to bronze. Aluminum......well, might make a wall hanger, but the second someone played with it......

 

If you were melting cans, you'd get a tiny amount of fairly pure aluminum (which is terrible for anything but jewelry btw), and a LOT of oxides and slag, which would be silvery grey, but crumble almost exactly as you're describing. Aluminum oxidizes incredibly easily, and cans are almost all thin sheet metal with a huge surface area.

[Frosty]

You MUST let aluminum cool in the mold, putting water on it at all will turn it into aluminum oxide, dark gray, large grain and crumbly. What you did by speeding the chill with water was ruin it.

 

You are lucky, Lucky, LUCKY the aluminum had already solidified in the mold before pouring the water on it or the steam explosion could've done you permanent injury, possibly killed you. At the least the scald burns would've probably left you screaming till help got you  to the hospital.

 

You are literally playing with things that can kill or cripple you, like leaving a child in a running car. Read, take classes, find someone who knows what they're doing to teach you. Just STOP messing with casting metals till you have a clue, better yet, actually KNOW what you're doing.

 

Frosty The Lucky.

[eggwelder]

when you heat aluminum, then try to forge it, it will crumble, crack, what have you. it is called by back yard casters, Hot shortness. we use this method to break up large pieces of scrap aluminum such as BBQ and transmission housings into manageable pieces. heat it up, smack it with a hammer, bust it up, then repeat. dropping a hot solidified ingot into water will just cool it faster. dropping water on to liquid aluminum, or any liquid metal at those temperatures will result in an explosion, sending molten metal and hot steam flying everywhere. it is not steel, or remotely like steel. it forges cold, and you anneal it for forging by, yes, applying light heat to bring the piece to maybe 100`C. then you must let it cool to the ambient temperature. all the above mentioned stuff can happen to cast aluminum, and with different alloys, you never know what you`ll end up with. 

as a hobbyist, I`ve never degassed a melt, and if i end up with a bubbly poruos casting, i toss it, and don`t reuse the metal. scrap aluminum is relatively easy to come by.

[Kiel366]

Thank you, guys, for your advice. It really helped. My casts are not gassy. I tried not putting water on a cast and it worked. I've successfully forged a spear head.

 

A slight problem I have is that I don't know how to connect two pieces of a cast (e.g when I mess up forging a bit and I end up with some stand-off pieces): at low temp it doesn't connect and at high temp it breaks (maybe because I have a lot of oxyde in there as I havn't used clear Al, but the best pieces of the bars I ruined with water). Either way, if oxyde is not the case, please help.

 

BTW: Is it possible I can poison myself with coal smoke?

[Nobody Special]

When you say connect two pieces....what are you trying to do?  Were they originally cast together? Got broken off?

 

Forge welding them I'm not sure is even possible. I wouldn't want to try it, far too dangerous. Using a welder....well, talk to a specialist. I'm given to understand they don't work on aluminum cheap. A low temp solder might be possible I suppose?

 

Short version on the gas is possibly, most likely from carbon monoxide.  Use GOOD ventilation. If you were exposed and have had trouble breathing or dizziness, seek medical aid, don't wait for it to pass on its own.

 

You know, if you get an extruded alloy,(one that was drawn out, not cast) you can work aluminum cold, right? That would probably work a LOT safer, and possibly better for whatever you're doing. In your place, I really would avoid further casting without experienced help.

 

In working aluminum cold, you have to anneal first to prevent cracking. Heat the aluminum slowly until it gets to the point where a small piece of wood rubbed on it leaves a mark and then quench immediately in water. It may take a little practice to learn how often to anneal, but it can be very rewarding because it moves so easily under the hammer.  Don't take your eye off of it while heating, or your work will melt and be gone.

[arftist]

Annealing temp for Al. 315-345C. At this temperature a pine stick will char, a mark from a magic marker will burn off, soot from burning acetylene without oxygen will burn off, a 650 degree (345 C) tempilstick will melt. 

 

You do not need to quench aluminum after heating to anneal it, only do so to save time. 

 

When looking for scrap to melt, consider motorcycle or car engine parts. These are the best alloys for casting. Never buy unknown ingots, there is no telling what you are getting. 

 

Always melt your scrap and pour small ingots for remelting. This improves the quality (cleans) the metal greatly. 

 

Using an iron vessel *works* but is poor practice since molten aluminum dissolves *some* iron and too much iron is considered a contaminant. 

 

Consider learning pattern making and mold making. The best part of casting is creating a nearly finished object, not making a poor substitute for what you should be using (a cold forging alloy (6063 is my favorite). 

 

 

You can add small amounts of copper by cutting up thin wire into 25mm long pieces and dropping it into the molten aluminum.

 

Most of us learn and practice hot forging with an iron alloy such as low carbon steel, which can usually be found for very low cost at a scrapyard. 

 

If you must continue with your present journey, aluminum can be bolted, riveted, soldered, gas welded and electric welded. Of the non-mechanical means of joining, electric welding is the most difficult and expensive, gas (preferably oxy-hydrogen) is the easiest, and solder (special solder and flux made just for aluminum is the cheapest but may not be easy. 

 

In comparison, steel can even be welded by heating two pieces very hot and hammering them together quickly. Oxides which form on the surface of aluminum instantaneously prevent such joining, and are the main difficulty to overcome in the other methods as well. 

 

Do as others said and join a casting forum.