Aluminum Plate Vs Extruded

[d1ulookin4]

Hi all,

Sorry if this is a silly question, but I'm kind of naieve, so I'll try to get away with it...

I was at a local metal supplier a while ago, looking through the drop pile, and found a chunk of 1/4" plate. It was roughly 8" x 36", and I decided that it'd be nice for inventory. When I took it in, the sales guy looked at it like it was gold. So the question is... why is plate so much more expensive than extruded (all 6061), and when would I want to use plate instead of extrude?

thanks,
-Eric

[Millman]

OK, my prediction is you will get 37.489 responses to your question. If anyone says it's junk, feel free to ship it to me. I'll even pay shipping charges. I,m sure I could use it for something!

[Evan]

What you have/had is probably what is known as tooling plate. It's surface ground to better than .001" accuracy on overall thickness. Extruded flat material can vary +-.010 or more.

[Millman]

Thanks alot, Evan. Now he'll raise the price on shipping.

[d1ulookin4]

Thanks Evan.

The material seems to have a brush finish and it's kind of nicked and scratched. This place mainly deals with construction type materials, 12 or 25 foot lengths, very little scrap in the drop pile. So I wasn't sure what their normal customer would use this material for.

Millman,

The material was never for sale, sorry.

-Eric

[Evan]

Sounds like tooling plate. It will have a sort of matte finish with no indication of linear draw or rolling marks. It will also be the same thickness all around. Try a mic on it.

[topct]

Without a stencil mark of some kind it could be anything from 7075, 2024, 6061. Or one of several other numbers. And 6061 is very common in plate.

I have seen all these alloys run through a surfacing machine and it is impossible to tell what they might be by looking at them.

Without a number on it I consider it scrap. And I hope you didn't pay the per pound rate for the big dollar stuff and got something that may or may not give you fits when you try to make something out of it.

Just read your description. Brushed surface almost sounds like normal roll marks. The rolls have a ground surface that transfers itself to the metal. Again any of the above alloys would look like that further adding to the confusion.

[ERBenoit]

See here:

http://www.alcoa.com/industrial/en/p...s/overview.asp

Pick a product, pick the alloy and you can get the technical specifications/tolerances for that product.

As an example, look at the wrought tooling plate (6061-T651) as opposed to the cast tooling plate (Mic-6).

Notice the differences in tolerances.

Suppose you have a 2 pieces of 6061 1" thick 6" wide, 6' long. One being an extruded bar, the other cut from a section of plate. They are not subject to the same manufacturing tolerances.

Any particular alloy, depending upon method of manufacture, wether it is an extrusion, cold formed, or wrought/rolled plate have different tolerances for the same alloy.

Sorry if I sort of repeated myself in the above.

May shed some light on your question as to why the "same thing" costs more in one form as opposed to another.

[d1ulookin4]

Thanks for the links. I knew it wasn't exactly the same thing, or else there wouldn't be a price difference (unlike consumer marketing gimicks ) I just wasn't exactly sure what the difference was. Judging from a couple of mail-order metal sites, I paid pretty close to extruded price for the plate. It is marked 6061.

As for applications. For me, I'm just tinkering in the shop, looking for interesting projects and building a few... This was just for "inventory" stock. I'm still at the point where my ability and projects are below the tolerance of the material. So when would I pick one over the other?

A hypothetical example... If I have two pieces 4x4x1/2 one plate and one extruded; and I skim 0.010 of one surface (of each) to clean them up. From the tolerances in the material, the actual cut on the plate could range from 0.009 in some places to 0.011 in others (maybe not on a 4x4, but for illustration); whereas the extruded could vary from 0 in some spots to 0.020 in others? Is that correct?

Is the extruded more likely to warp because of the internal stresses developed during the manufacturing process? So in this case, I would be better off to use the plate, assuming I wasn't taking any other cuts?

thanks,
-Eric

[Evan]

The tolerances given for the tooling plate don't tell the entire story. While it may not be exactly the specified thickness (I didn't say it would be) it will usually always be the SAME thickness all over the plate to within a thou or so.

Extruded material can be all over the place. It will often be thicker at the edges than at the center. It will often be bowed to one side of the width dimension, sometimes quite a bit. The wider the extruded stock is the more likely it is to be bowed.

[ERBenoit]

<font face="Verdana, Arial" size="2"> Originally posted by d1ulookin4:
As for applications. So when would I pick one over the other? </font>

I use extrusions, wrought/rolled plate and cast plate on a regular basis.

ALL of the criteria of the finished part should be considered in your material selection process. (See story below)

First, the size of piece you need will dictate your options.

Say you want a piece 3/8"x6"x24". Options, extruded bar, or plate, rolled or cast.

Now, say you want a 3/8" 24" square.

You are not going to find a 3/8" x 24" extrusion. This leaves plate as your only choice.

The intended use of the part, should help giude in your selection. Where "flatness", thickness and warpage are critical, cast tooling plate is the best choice. If not so critical, extruded bar or wrought/rolled plate may suffice.

If "flatness", warpage, and thickess are critical to the application, cast plate is the best choice. Because it is cast, there are no stresses introduced as with rolling or extruding. Machining does not affect the "flatness" of the plate

The wrought/rolled tooling plate is IMO, the worst for warping, particulary if you are using a thin cross section (less than 1/2"), yet a relatively large piece. (6"x6" should not be as "warped" or become as warped as a piece 24"x24", simply because of it's size). Also when removing a lot of material, such as cutting a pocket, affects the internal stresses, creating or changing them, resulting in warped parts.

Straight from the supplier, I have seen as much .020" "dishing" in a piece of 1/4"x8"x12" wrought plate.

Machining will affect the "flatness" of the plate, as it is stressed before you do anything to it. The rolling of the material puts stress into the material. The stress is more pronounced in the direction in which it was rolled.

I have found that thin parts of the same size, made/cut from extruded bar are less likely to be warped to start with, or become warped from machining, than one made from plate. I beleive that extruded bar is not as stressed as plate to start with.

Example: I had a job where we were making a slide dispensing system. Aluminum Alloy 6061. (Sort of a business secret, can't say too much about it). 96 holes in a pocket which was cut into the upper half. The flatness of both top and bottom halves of assembly facing each other was critical.

Cast plate would have been the best, however since the parts were to be anodized, cast was the worst. Wrought/rolled plate, anodizable, yet, wide variation in initial tolerances, and subject to warping. I ended up using extruded bar skim cut opposing faces, cut the pocket to leave a specified "floor" thickness in the upper half, then just skimmed the top of the part to clean it up.

I chose the extruded bar, because 1.)The initial flatness tolerances were closer than I would have gotten from plate material. 2.)Less chance of warping, because it is not rolled. 3.)It accepts anodizing.

Sorry for the excessive yapping!



[This message has been edited by ERBenoit (edited 01-20-2006).]

[d1ulookin4]

Thanks for the excellent reply. It was not excessive yapping, it's pretty much exactly what I was looking for!

Thanks again,
-Eric

[topct]

6061 is good. If I where going to decide whether to use plate or extruded I would chose the plate.

In the short section you have, if you were to mike it, you will probably find it to vary less than a thou. anywhere.

There is a tolerance for plate thickness, always on the plus side of the stated gauge.

As for surfacing? Thinner plate would be subject to any distortion more than thick. I would think. And extruded? Maybe more? Maybe less. (yes i snuck this in)

That's what I should have waited for a proper informed answer. I was just puttin along on my fork lift again.







[This message has been edited by topct (edited 01-20-2006).]

[ERBenoit]

I see we share the same name.

[WJHartson]

Plate and extrusions are made by two different processes for the same alloy.

Both produces start with a heat in furnace and cast into either rolling ingots or billets. The rolling ingots will become the plate products and the billet will be used in extrusions.

The rolling ingots are scalped, ie milled on both sides to remove impurities in the as cast surface. The ingots are then put in soaking pit and heated to a specific temperature for a specific time, this is called homogenization. The ingot are then rolled to the desired thickness in a rolling mill. As stated by topct the rolls will imprint the scratch pattern of the roll onto the metal. The metal plate is then stretched and heat treated. Final sizing of the plate is done with a saw.

The billet used for extrusion is cast usually into long round billets called logs. These logs are then homogenized and allowed to cool. They are then cut into lengths to fit a specific extrusion press. The billet pieces are reheated to temperature and put into the extrusion press and forced through a die to produce the desired shape.

This gives you a general idea of how the different products are manufactures.

Joe