Aluminium Stick of Rock?

A strange "core" right along a length of bar.

Various quality-control discussions on here reminded me of my time years back as the metals-stores keeper in a company making precision screen-printing machines.

These used a lot of aluminium alloy (HE30 - I don't know its modern number) bar, and my task was cutting the blanks from stock bar of appropriate sizes.

The well-built but crudely-designed hacksawing-machine naturally left very rough sawn faces. So we were all surprised when the milling, if not the subsequent anodising, revealed a peculiar elliptical "core" or ring right through the centres of one particular batch of parts perhaps 300mm long, made from bar about 1.5 inch square.

They moaned at me until I pointed out no-one could have known until the parts were finished.

Anyone any idea what may have caused this effect, which none of us saw in anything else? Or has encountered it personally?

I see it quite often when sawing steel. Just put it down to the way it was drawn/rolled

Ah yes ! Lettered rock, I was one of a team of 4 making rock in 1967 ! a stick of rock would start off 12" dia and be drawn down to about .75" dia, very interesting ! 3 spoke norfolk, I spoke english ! Possibly the same idea with ali bar. Noel.

A lot of bar stock starts off as continuously cast billets. There are tun dishes at the top of the moulds and they are topped up from large ladles. It is possible that when there is a topping up operation dross on the surface is pushed down towards the mould by the flow of the fresh liquid metal being poured. I can imagine this dross getting into the centre of the cast billet. I don't know if this is used for aluminium since it starts off as slabs that are formed by an electrolysis process but it is certainly used for steel. Plenty of YouTube videos about the process.

Martin C

Thank you.

I've never noticed the effect in steel, and I don't recall any other than that one aluminium bar being affected.

It was a consistent pattern as I recall, so probably not a slag inclusion.

Very odd. I forget if we let the parts pass or made new ones.

++++

It wasn't the first time I'd had problems with the place, but the first was a personal not metallurgical error.

We made some work-holding vacuum-tables a bit larger than A4 size I think, from mild-steel plate ordered as cut blanks. At some point the designers switched to gauge-plate. Fine: I worked from the issued drawings and would send purchase-requests to the buyers, accordingly. So asking for so-many pieces of gauge-plate instead of mild-steel was easy enough.

One day I was handed a drawing-set in which the vacuum-tables were mild-steel again. I was surprised but duly ordered the plates and when they arrived issued them with the drawing as normal - putting them on a rack in the middle of the machine-shop.

Next day, following a query by the miller who'd picked them up, one very concerned supervisor and buyer demanded to know why not the gauge-plate we'd been using for quite some time by now. I stood my ground and pointed out it's my job to issue to the drawings I was given, not to question the drawing-office! They had to admit I had a point and left me in peace to order gauge-plate pieces in line with the correctly re-issued drawing. Oh fun and games.

I did occasionally question the drawing-office when studying a drawing to select the material, I'd spot a mis-dimension wiould result in some feature like a hole, being in fresh air.

As Martin mentioned the core of the billet can be contaminated and end up at one end or other of the extruded rod. It depends on the quality of quality control when they cut their losses and discard the bad material. Possibly like so many 'seconds' from factories they get sold off cheaper nowadays and get sold on as full spec by less reputable intermediaries.
In a semi-continuous production like this I would expect some detector, ultrasonic or x-ray perhaps to detect the change in composition as it passes.

Nigel

These used a lot of aluminium alloy (HE30 - I don't know its modern number) bar,

In case you need some more it's 6082T6

Emgee

Used to have to check for "Pipes and Laminates" in carbon steel strip supplied to make my avatar name as part of the laboratory jobs for all incoming material. Caused by not cropping enough from the end of cast steel billets before rolling. Wonder if the core of the Allumininium is due to chilling when being extruded but subsequent heat treatment to turn it into"T6" should have sorted that.

Regards Ian.

Interesting ideas to mull over...

Of them I am inclined to think Circlip's the most likely explanation - a heating and cooling effect.

That would explain its consistency by length and concentricity with the bar section. The section was a rectangle, as I remember 1.5 X 1.24 inches (aluminum-alloy was still being sold in Imperial sections), which difference could have influenced the core to be rather elliptical in section by depth from surface. Might be an interesting geometry and spreadsheet exercise to plot constant radial depths from a similar rectangle, to see if it gives an ellipse. That lots of trig though...

The "core" effect was uniform throughout the length, and may have been in other bars from the same batch - I have long forgotten the number of components but they were some 300+ mm long, from stock I think supplied in 4m lengths.

It was invisible on the sawn ends, hidden by the cut marks from both the metal-manufacturer's circular-saw, and certainly by the 'Kasto' hacksawing machine I operated.

An inclusion would be expected irregular in section, inconsistent in length, and not necessarily concentric with the bar, from end to end.

'

Engee - Thankyou for the reference number for the alloy.

'

The company's drawing-office did not usually bother with such niceties, often putting in the "Material" box only Alum or Mild Steel, or Stainless; leaving me as store-keeper to judge the sort from the drawing and trade catalogues. That one brush with an incorrect-edition drawing apart, no-one queried it so I must have been doing something right.

It won't be me having to worry about alloy grades though. I left that company long ago, firstly for better work and now by retiring anyway!

Just as well as it turned out, for a year later its management abruptly closed the machine-shop, with no notice; merely calling everyone into the workshop-manager's office on Friday and telling them not to come in on Monday - here's your P45s, gentlemen.

Oh, and as we've seen with many other companies behaving in that way, only just before Christmas!

A former colleague told me that the MD didn't have the guts to announce it, but left it to the Production Director. At least that was before universality of portable 'phones and e-posts so they could not use merely a cold, cowardly text message. Luckily most of the machinists found local employment with sub-contract engineering firms; ironically, making the same printing-machine parts.

Thr firm changed hands subsequently, is thriving, and rather better run. Their main customers? Computer and portable 'phone manufacturers....

My book 'The Properties of Engineering Materials', Higgins, Hodder&Stoughton, London, 1986 explains these lines in terms of what happens when liquids freeze.

All being well, metals freeze from molten to a regular crystalline structure. Unfortunately things are rarely perfect when metals are mass produced - billets are often cooled too quickly or at irregular rates.

Ideally molten metal releases it's latent heat smoothly as it solidifies and the resulting crystals lay down in regular formation. In practice, the lattice is likely to be disturbed by metal remelting at the boundary, by liquid metal super-cooling, and by crystals forming lumps inside the fluid around impurities such as slag. These discontinuities cause interface lines between regularly formed solid metal and the joint at which the crystal structure is malformed. The effect is rather like tree rings.

Interface lines are usually invisible: they always exist in steel forgings, and the book describes how to make them visible by sectioning the forging, putting a high polish on it, and then bringing out the lines by boiling in 80% Hydrochloric Acid for 30 minutes. Not always necessary to go to such extremes - ordinary cutting and polishing processes can highlight the lines.

Do discontinuities matter? Possibly if they were caused by excessive slag. There's always some slag in metals because furnace linings erode. Too much slag is bad, and all furnace linings eventually disintegrate. A negligently managed or prematurely failing lining could produce poor metal containing excessive slag. More usually, discontinuities don't alter the metal's properties enough to matter.

Always exceptions! In production, metal is often chilled quickly to get it out of the way. Cast-iron was notoriously hosed down to cool it enough to get it off the moulding floor ready for the next pour. The ends of billets chopped from a continuous casting process are best avoided. And a modern controlled cooling process would accidentally alter the metal if it went wrong! (Faulty control system, mechanical failure, or Friday afternoon operators...)

Dave