Strange colour change in aluminium.

I was making a finishing cut on this 6082 T6 aluminium and there is a sudden colour change in it.

It is extruded rod, no lubricant was used, no speed / cut changes, 1000 RPM 28.6mm diameter, very slow feed, 0.1mm cut.

Move never seem this before, any ideas?

I do have some ideas but they can only be wild guesses without knowing what colour the aluminium changed to?

Ian P

Sorry forgot the picture!

It is a very sudden change which I doubt would happen in the properties of the rod material. Therefore I suggest a change in surface finish refracting light which occurred suddenly as a chip got stapped by the tool and scraped finer lines.

Perhaps a bit of aluminium has welded its self on the tip just at that point. Ian S C

I checked the tip and it appeared fine.

Its like a titanium colour. I'm anodising the parts today so any difference in the material will show up I expect.

I have a 2" solid extruded 6082 rod here that shows up what looks like weld lines (normally get it on extruded tubes) when anodised.

Chris initially it looks like a slight change in surface finish due to tool wear or a piece of aluminium welding itself to the tip.

Alternatively, depending upon your source of the aluminium bar, it may be the result of a weld during the extrusion process. As aluminium is extruded and they get to the end of eack billet in the press the process stops momentarily. The extrusion ram withdraws for a few seconds and a new billet is placed behind the last one. The process resumes and the presuure and heat causes the 2 billets to butt weld together. This will show as a sometimes visible weld mark around the circumference of the finished product.

Now normally comes the clever bit. As the length of each billet is accurately calculated before it is cut/sheared to length, this is designed to ensure that the press "knows" exactly where the weld will occur and when to add another billet. The extrusions are cut into 6.0 or 6.1 metre, or other custom lengths, on the press and when the weld approaches the saw the press adjusts the bar position to cut a piece containing the weld out of the process. This piece is scrapped. The good stock then goes into the ageing ovens.

However some low end extruders do not cut the weld out so it can appear at random in a stock length. Often it is not visible because no oxidation has occured at that stage to the outer surface. The scrap is sold commercially and some scrappies sell noggin ends of stock bar to the trade. It is often this that places welded noggins in the hands of small end users.

Sorry if that was a bit nerdish. HTH.

Interesting. Thanks for that.

The part was anodised today with no strange results. So most likely something caused it while turning.

By by weld lines on tubes I meant where the aluminium flows around the mandrel supports in the die. That always shows up when it's anodised!

Reminds me of an experiment we did at university. We made up 'billets' of coloured layers of plasticine and squeezed them out through dies. The resulting 'bar' was sectioned to show the flow lines from the colours. As you might expect the centre flows much faster so the joint line mentioned above would be like two very long cones one inserted into the other.

Chris, you need to find an extrusion plant that uses a piercer press for tube, these presses punch a hole right t

hrough the billet before it is extruded through the die, not many in U.K. now I'm afraid Neville.