decrepit said..

Imax1 said.. >>>
There is much , much more to it than that . I would love , and could , crap on for hours , but I gave you the condensed version .

The question remains, what could it be in a block of alli?
Is that built up in layers as well?

Yes . Alloy also starts in a cast block and rolled several times down to size . Its a very slight change in density , grain or composition that produces those layers . Were not talking much , but enough to show up as above .
Id guess the black ooze that comes out of the fin and is at the bottom of the tank is carbon ???

fangman said..

Imax1 said..Id guess the black ooze that comes out of the fin and is at the bottom of the tank is carbon ???

I am quickly out of my depth talking about aluminium alloys and their manufacture so I need guidance on this one please. I can't figure out where the carbon comes from. If an alloy meets the ANSI composition 'standard', does that specify the only elements in the alloy, or does it mean that those elements are there in that %composition and any amount leftover could be anything? (eg carbon) Or does the carbon come from some other source/reaction post-manufacture? ( including something in anodising processes).

I know nothing about fin casting or billet or metallurgy. But I know a lot about polishing mag wheels and aluminum motorcycle crankcases. The polishing cloth is always black.

does it burn? if yes could be carbon, if no probably an oxide or some other biproduct of the chemical reaction.

What is the chemical process of anodising?

Wouldn't that be an oxide? Although I thought that was white.
So anodised al is an oxide.
And the H2 So4 is just an electrolyte? or catalyst.

The plot thickens . So why is it black ?

Imax1 said..
....Surely there is a metallurgist amongst us ?

Defo there will be - on the SB forums there is an expert on everything, or so it seems.

olskool said..

fangman said..

Imax1 said..
....Surely there is a metallurgist amongst us ?

Defo there will be - on the SB forums there is an expert on everything, or so it seems.

Know just the guy.
Oh Sa#dm&n where are you?
Hehehe.....

We've all been blocked , he can't see any of us .
Its so much quieter in here now . I kind of enjoyed him like a rank cup of coffee. OMG , I miss him

Cooked the cast FF24 again today using Caswell's LCD method and chemicals..to be fair this fin has had a hard life. Got it off Fangman 8 months ago tomorrow...has endured a few NaOH baths, been high current anodised, bead blasted and been in the field a lot. Resanded to 400 grit (didn't seem worth going further or even trying to get the deep scratches out as so much porosity). Gave it 20 minutes of torture in the Aluetch which was a black bubble bath, rinse and Deox/Desmut and still looked like s**t so gave it another 10 mins + rinse in the cleaner, then another 5 mins clean before a final rinse/deox/desmut....the clean bath is black as is the rinse...Interestingly, like the previous fin all the subsequent baths are clean. Not really sure why the commercial anodisers don't touch cast Al as it appears only the cleaning that makes the black goo and is only just more goo than normal Al. The anodising was text book and almost identical to the CNC fin with 12.5V at 5.1 amps (constant current) and the voltage only fluctuating right at the start from about 4V rising to 12.5V in a minute or so. The defects in the dye on the fin base are bits of silicon from previous gasket/brass nuts and me not bothering to clean this area at all. Weird how the completely neglected fin base is as good or better than the fin which has literally hrs spent fusing over...

Such a simple question Fangman and one I wanted to answer...after 5 days sanding and cooking I'm still not closer to definitive answer, but the guess is it makes no difference. Along the way I think I now understand what is happening and a possible solution.

The path to enlightenment began with deciding to put the FF28 through the process...the idea being it was the biggest with the most material to play with. Note, this is second time around anodising and fin has been in heavy use the last 8+ months... Started with 80 grit flapper disk for a good 2 hrs to remove the 'moon' porosity...then proceeded to 100 then 240 grit orbital sand then 400 grit. Fin starting to look nice, very little porosity. Moved to 600 grit and suddenly porosity came out of nowhere, switched to 800 grit and was worse....I was creating porosity...the more I sanded the more holes there were. Moved to 1200 grit and started seeing improvement...2000, 2500, 3000, 5000, 7000 all improved....this time decided to polish the 'almost' perfect surface, luckily I only polished one side...complete disaster....now had porosity exactly the same way back at 600 grit. It was it I'd been peeling the material off in chunks of a particular size. Arg. This porosity is always at the areas of maximum material thickness. So restarted sanding at 1200 and eventually got back to 7000 with a ok, but nowhere near as good finish. Had no time to cook so left it over night. Next morning to my horror the porosity had reappeared and maybe worse than before...decided to get the tanks ready to cook the fin regardless, do one last sand and see what happens. Result is a disaster (as far as hrs in versus result).

However its clear now what is happening...the areas that cool slower during the casting (the thicker bits) are softer and/or have different grain size...there is very little actual porosity, when you sand at a particular grit there is some kind of relationship between grit and grain size that either rips material from the surface or doesn't. My guess would be when grit size = grain size it is the worst which would suggest the grains on the soft parts are about 600-800 grit in size. If you sand harder like you might to remove a scratch it actually digs more material out and it appears the porosity is never ending so it it counter intuitive...By sanding super softly or with higher grit all the porosity goes away....really takes forever though but its all complete wasted effort as the surface is just too soft to endure anything. Even just sitting in air for a few hrs the surface breaks.

As soon as you hit it with the cleaning solution (I'm sure Aluetch versus NaOH versus other stuff makes little difference) the 'soft' material just breaks away in great clumps and leaves the pox like mega porosity appearance on the surface and makes lots of nice black goo. Of course proceeding in the anodising process from here results in just amplifying the **** surface finish. Definitely harder but definitely not pretty.

So the solutions are pretty clear...
0. Forget the whole idea as the fins work just fine with a whole lot less effort
1. CNC billet as no soft spots. Very little finish sanding but more upfront $
2. Harden the material somehow, two methods:-

A) Sand then bead blast to cold work the surface, possibly even bead blast between sanding various grits..will get tedious, possibly not work as material is fundamentally weaker in thicker areas.
B) Homogenize the fin prior to sanding. Supposedly this is done a lot with cast Al as all cast suffers the basic problem on non uniform cooling leading to areas of various harness. The idea is you bring the material to just below melting point (900-1000 degF or 482-532 degC), hold it long enough to ensure ALL material is at that temp and then cool it slowly to ensure uniform cooling of entire material. Since there are areas on the Fangy fin that polish/anodise well it implies the alloy itself is ok, just needs to be cooled correctly to give a robust grain size.

Enter the Pizza oven...my wood fired pizza oven can easily attain the 532 degC and by closing the door will take a good 24 hrs to drop below 100 degC so will be slow enough cooling. Was gonna chuck my last fin in tonight but thought I'd better check with Fangman on actual melting point of this alloy. (plus my remaining fin is the super thin FF18 cast so may not exhibit these symptoms as badly)

Attached final results, no improvement (probably worse) on FF24 which I gave almost zero sanding treatment...

I would think shot peening may work after initial shaping with flapper wheel . It should close porosity, harden the surface and decrease corrosion . By how much and is it worth it , ???

hmm, all the info right there on the tin. Not sure I can hold the Pizza oven at 540 degC +/- 5 for 10 hrs so might have to build an oven. Luckily Bunnings sell AAC bricks 600x200x100mm for $2 ea, 5 bricks will make an oven big enough for a FF28.

Might need to try 6061 T6 or even a 7000 series??

I am stealing Flex's thunder. He consulted the Oracle of all things Aluminium and the wisdom is as follows:
Apparently, the black stuff is bauxite. I guess it must be mixed with other impurities from the casting process to make it black. (Bauxite is the name for the group of Aluminium oxides. They range in colour from white to red/brown).
The above fin is a marine grade aluminium 5083 and has been given the thumbs down for suitability. Ideally (as Mr Love noted) a 7000 series would fit the bill better, but it drives the material cost up substantially.

Flex2 said..
There's something important about the relationship between Titanium and Aluminium that I failed to grasp but Aluminium always contains Titanium and Titanium contains Aluminium.

Depends on the grade of each metal.

For Aluminum, there aren't any that that I'm aware of that have Ti as the primary alloying metal. They're (obviously) all primarily Aluminum, with a blend of:
Grade - Primary Alloying Metal
1xxx - 99.000% Minimum Aluminum
2xxx - Copper
3xxx - Manganese
4xxx - Silicon
5xxx - Magnesium
6xxx - Magnesium and Silicon
7xxx - Zinc

And of course a few other things, but usually upper limits (like there can be between 0-0.15% Ti in 6061)

5083 and 5086 are the two that I'd be using for optimal corrosion resistance with reasonable strength, but as we can see from the dent, this might not be adequate... 7075-T6 is the "standard" choice for when you need very high strength, but it's corrosion resistance isn't great, and it's more difficult to machine. 6061 is closer to the two 5xxx alloys, and is much cheaper. Pay attention to temper, it can make a big difference in strength!!

Titanium alloys are a totally different beast, and will cost a very large pretty penny, and maybe your arm or leg as well. My go-to here would be "Grade 5", which is a Titanium-Aluminum-Vanadium (6% Al and 4% V) alloy. It's very strong and extremely corrosion resistant, but a COMPLETELY TOTAL PITA to machine. I'm not as familiar with casting Ti alloys, but that'd probably be the way to go if you're going to pursue this route, just to keep fabrication costs reasonable!

A note of caution, just like strength and stiffness are separate measurements, hardness is another. AC603 (from the datasheet above) has a hardness of about 90-100 brinell. 7075 is about 150, and Ti is 330.

Well, built and fired up the Bunnings special Fangy Fin Furnace only to be told by Windxtasy yesterday, "why don't you just use my kiln?" Arg! At least have backup plan now, Didn't quite finish the control electrics today but a short test run seems to suggest it might work. In theory should be able to reach 1300 degC. Reality, like wind forecasts, might be substantially different. Need to slowly dry and cure the bricks/mortar over a few days plus there's a pile of wind com'n so will be a while before actually cooking a fin. Most expensive part of the build was the steel to make the frame to hold the door. So far cost is $200 AUD for everything (bricks, mortar, element, thermocouple, controller, high temp wires, SSR, porcelain terminal blocks, steel)....coulda saved and not bought so much high temp wire, bought 10m for $25 when really only need <0.5m