7075-T7351 Aluminum, Gummy? How to machine?

[labmachinist]

I am looking for some advice on machining 7075-T7351 Al (purchased from McMaster). 90% of my work is in 6061 Al, and I have not worked with 7075-T7351 alloy before. I am finding it quite gummy, especially compared to 6061. This is a surprise to me, because I was expecting it to be similar to 6061, if not harder and more brittle.

During facing, I tried slow and fast speeds and feeds on the vertical mill without excellent results. So far, running the tool slower (235 sfm) with faster feeds seems to work a little better, but I still get a little gumminess (about 1 thou) on the edges of the end mill pass or block. I also found if the end mill is not almost perfectly sharp/new, the results are really bad, whereas in 6061 you wouldn't even notice.

For reference, I am using YG-1 2-flute Alu-power end mills with Tap Magic Aluminum cutting fluid.

Any advice on what to try for better, cleaner results? Also, is -T6 or some other tempering better in this regard?

 

[EmGo]

7075 gummy ? No way, Jose. Something is seriously wrong here. In order of gumminess it's 6061 then 2024 then 7075. I've had some 7000 series forging stock that was gummy but they had to work at it to make it that way.

 

[Orange Vise]

How thick is the material?

The T73 temper provides better corrosion resistance but lower tensile strength compared to T6.

Thicker sections are often gummier than thinner sections due to a slower quench.

 

[labmachinist]

How thick is the material?

The T73 temper provides better corrosion resistance but lower tensile strength compared to T6.

Thicker sections are often gummier than thinner sections due to a slower quench.

It is a 1x2" bar stock from McMaster: McMaster-Carr

Interestingly, McMaster says "Aluminum Performance Properties Easy to Machine"

 

[EmGo]

Interestingly, McMaster says "Aluminum Performance Properties Easy to Machine"

It is. The biggest problem with 7075 is that it's fairly abrasive and will wear out HSS cutters faster than other aluminums. Something is wrong with your piece. Mislabeled seems most likely. Forging stock is awful but otherwise I've never run across any 7075 that wasn't really nice to cut.

 

[LKeithR]

...I've never run across any 7075 that wasn't really nice to cut...

I've only machined a bit of 7075 but I always felt it was really nice to cut--I would also suspect that you've gotten
some mis-marked material. If possible try a piece of 7075 from another supplier; I think you'll find that there's a
big difference...

 

[jz79]

you can do a very crude hardness test - take a similar weight piece (to your gummy 7075) of 6061 T6 you trust and pick a 90 degree corner on it that you don't care about, take your gummy 7075 and also pick a corner, hit the corners against each other and compare the dings, doesn't need to be a hard hit, just enough to deform the softer part, if both look the same, they'll be similar hardness

your impact vectors should point to mass centers ideally and be as perpendicular to the test corners, but not strictly necessary to get an idea

if you have a drain cleaner, or grill cleaner at hand, make a small puddle on each part and let it sit for a while, 7075 piece should become black under the puddle, 6061 may turn gray/brownish, parts need to be room temp warm at least, warmer will make the caustic reaction happen quicker

 

[rklopp]

The T7351 temper on 7075 is a more than 10% softer and weaker than the T651 temper. That might explain the issue. I had always assumed I was machining 7075-T651, and it does machine very nicely compared to 6061-T6 with respect of gumminess and "pickup" (chip welding).

 

[labmachinist]

The T7351 temper on 7075 is a more than 10% softer and weaker than the T651 temper. That might explain the issue. I had always assumed I was machining 7075-T651, and it does machine very nicely compared to 6061-T6 with respect of gumminess and "pickup" (chip welding).

OK folks, I made a very careful measurement of the density of a 1.600 x 2.000 x 0.450 block I machined from this "mystery" metal. This was done using Mitutoyo Quantumikes and a 100 g scale with precision calibration weights.

I got a density of 2.7995 g/cm^3, compared to a nominal reference value of 2.81 g/cm^3 for 7075. Also, the corners were deburred, so we can easily call this at least 2.800 g/cm^3.

Also, if you look at the properties sheet for 7075-T7351, it does state machinability as only "fair" and workability as "poor". https://www.gabrian.com/wp-content/uploads/2018/09/7075-Aluminum-Alloy-Properties.pdf Still, I can see why McMaster states "easy to machine", if you compare it to stainless, as opposed to just other Al alloys. Yes, it's a lot easier than super invar, for sure.

So I am going to call this the real deal and chalk up my difficulty to two things:
1. T7351 being more difficult than other treatments, and 7075 generally a little more difficult that 6061
2. Using an end mill instead of a face cutter for surfacing the block...need to use the right tool for this alloy/temper.

 

[EmGo]

So I am going to call this the real deal and chalk up my difficulty to two things:
1. T7351 being more difficult than other treatments, and 7075 generally a little more difficult that 6061
2. Using an end mill instead of a face cutter for surfacing the block...need to use the right tool for this alloy/temper.

No. You're full of shit. 7075 machines nicely and workability has to do with bending not cutting. 7075 is not weldable either (welds fine but then cracks). It's way better to cut than any 6000 series. You're not going to see any real difference between T's. It's all solutioned and aged. Machines very nicely.

Either you have a mislabeled piece (since it seems there's no markings ?) or you don't know how to use a milling machine.

 

[jz79]

mr. Engineer, no one is guessing these alloys based on density, that is about the worst option of them all

 

[labmachinist]

mr. Engineer, no one is guessing these alloys based on density, that is about the worst option of them all

Its at least as good of an option as any of the others listed, likely better. Put Drano on it and see if it turns black? Drop it on a corner in an uncontrolled way? Density at least gives me a good idea of whether they mixed it up with something completely different, like 6061, and its something that is easy to measure precisely. The only thing authoritative would be to perform a mass spec analysis, and I'm not doing that.

 

[labmachinist]

No. You're full of shit. 7075 machines nicely and workability has to do with bending not cutting. 7075 is not weldable either (welds fine but then cracks). It's way better to cut than any 6000 series. You're not going to see any real difference between T's. It's all solutioned and aged. Machines very nicely.

Either you have a mislabeled piece (since it seems there's no markings ?) or you don't know how to use a milling machine.

Do me a favor and keep your uneducated and insulting opinions to yourself. Your advice is not helpful.

 

[EmGo]

Do me a favor and keep your uneducated and insulting opinions to yourself. Your advice is not helpful.

Yeah, I've only run through literally tons of 7075 and 2024 and 6061, I couldn't possibly know anything compared to an Engineer such as yourself.

Don't forget to put the crossing gates down when your little choochoo with the puffing smoke goes past the little station.


The Drano trick may actually work, never tried. 7075 has a lot of zinc in it that may react with something. I know that when you hard anodize 7075 you cannot dye it any color, the material turns a dark moldy green that won't take the dyes, so it does react differently than other alloys.

 

[moonlight machine]

Yeah, I've only run through literally tons of 7075 and 2024 and 6061, I couldn't possibly know anything compared to an Engineer such as yourself.

Don't forget to put the crossing gates down when your little choochoo with the puffing smoke goes past the little station.

I love machining 7075, it's much nicer than 6061. probably my favorite material, good thing because I do a lot of it! It face mills great for me, sounds like you need to talk to a proper tooling dude. [email protected] will hook you up with the right stuff and speeds and feeds as well. Why did you not buy your material with certs? Do so from now on and you will have no question about what you have.

 

[tnmgcarbide]

7075 is the 4140 of aluminum . chips are crisp... doesn't sleaze the insert.. is there a better AL ...
2024 is good too... but 5000 series is gummy shit.

 

[EmGo]

is there a better AL ...

Yeah, there is. For machining, anyhow. Got a bar of (I think) 6269 marked "micro-chip" once ? Have only seen one bar of it but machined like butter, faster than mag, chips flew off like snow, and the finish was mirror-bright. For ornamental work it would be the bee's knees. Never seen it in a catalog, came from the scrap yard but neato stuff. If you ever see any, grab it.

 

[tnmgcarbide]

6269 alloy ? i shall await it with open arms... when i see that hunk of metal cross my desk , it will be
a little bit of heaven......unless

unless it was radioactve material from a nuclear disaster. did you have it tested for micro/nano sieverts ?
it only takes a particle to get in your body and stay there forever.

don't accept un-known metals from fucked-up places . it might just been straight from some scrapyard,
and the local gov't doesn't wan't to

 

[jz79]

Its at least as good of an option as any of the others listed, likely better. Put Drano on it and see if it turns black? Drop it on a corner in an uncontrolled way? Density at least gives me a good idea of whether they mixed it up with something completely different, like 6061, and its something that is easy to measure precisely. The only thing authoritative would be to perform a mass spec analysis, and I'm not doing that.

you're displaying quite a lack of common sense and obvious lack of basic science knowledge to not understand the 2 methods I wrote about if you think that measuring density is better, this in turn makes me think that your engineering diploma is nothing but a piece of paper, if you even have that much

 

[triumph406]

Its at least as good of an option as any of the others listed, likely better. Put Drano on it and see if it turns black? Drop it on a corner in an uncontrolled way? Density at least gives me a good idea of whether they mixed it up with something completely different, like 6061, and its something that is easy to measure precisely. The only thing authoritative would be to perform a mass spec analysis, and I'm not doing that.

You can also check the electrical resistance of aluminum. When i worked at Mcdonnel-Douglas in Long Beach they would check incoming parts with a device that would check electrical resistance between two electrodes. There is enough difference in electrical resistance to tell whether the material is 6061,2024 or 7075.

You can also tell ther difference between 6061 and 7075 with a file, the way the 2 alloys file is different, after you've machine a few parts you'll know the difference.