Type of Aluminium.

[PeterM]

So I´ve read and understood that some alloys are better for anodizing than others. But...
How about the manufacturing method of the aluminium? Does that also matter?
Ill mention ways they make the aluminium. They can for example either:
*Extrude it or
*Cast it.

(i hope you understand what im talking about, my english isnt the best )

The reason im asking is cause: Some months ago on this forum i wrote a thread about "Strange appereance" that i got when anodizing. This was a "pattern" on the part, like long strokes in darker color in the same direcion like the parts lenght-axel (the part being made DOM).

Today i found out something intresting, if i would not drill a hole through the part, just let it have a cylinder shape, then the ends of the part didnt get those strokes, instead it had small dots, like perfectly matching the strokes placement.

So it seems like the strokes are from the making of the rod, (its made by stretching aluminium into a shape-often used when doing long aluminium rods).

What do you think? Could my "Strokes" on the part after anodizing simply come from the fact that the aluminium was once stretch-manufactured?
And have any of you used same type of aluminium (this kind is generally harder than other kinds from the process) and got goodlooking anodizing results?
Thank you
.

[Fibergeek]

Most any metal will show work hardening at the surface if it is extruded but not then annealed. The marks you describe are characteristic of this. The usual solution is to machine off the offending hard spots before anodizing. You may need to experiment to find out how much of the surface has to be removed, and size your raw stock accordingly.

Castings have their own set of potential problems, ranging from a rough porous surface to some of the alloying metals coming out of solution during the casting process. Castings also have a high silicon content to improve casting qualities, silicon won't anodize and its presence impedes but not prevents) anodizing. High quality castings have these issues minimized.

[PeterM]

Ok Thanks Fibergeek.
Im going to try to get a thicker aluminum rod as raw material. But just to be sure: Did you mean that some types of aluminium simply isnt suited than for anodozing (not cause of the alloy) but due to the way the manufacturer chose to make it?
Maybe i should change manufacturer then if i still get these problems after working down even further into the piece before anodizing?