Difference in chips bt AL alloys?

[James K]

Hi,

I'm pretty new to this forum and machining in general and I've got a quick (hopefully) question I'm hoping you all can answer. I recently machined several fairly large parts from what was supposed to be AL 7075 on our shops SL-20. Fairly soon afterwords, I made several other parts similar in size from stock that was verified to be AL 7075 (as in, it had printing on it straight from the mill). I noticed that these later parts machined somewhat differently, mainly in the way the chips broke (surface quality came out pretty much the same). The first batch of parts had chips that were about 1 inch in length and nice and curly. The second patch of parts with the different material had very fine chips, I would say probably less than .1in in length. My feeds and speeds were exactly the same between the parts (.012/rev-2000RPM) and I used the same tool between parts with a fresh insert at the beginning of each batch.

My question is, does the difference in the way the chips broke indicate the two materials are different grades of aluminum? Im concerned because it is critical that all the parts are in fact made out of 7075. Is there any way to verify the alloy of aluminum?

Thanks

[Geof]

It is possible they were both 7075 but with a different temper. You should see on the labelled one 7075 Tsomething. The T something will be a number that indicates how much the alloy has been age hardened. Freshyl extruded aluminum alloys are soft and they are heat treated by holding them at an elevated temperature for a certain length of time to allow the crystal structure to change to a stronger age hardened form; the longer the time the more the hardening, the higher the temper it is called, and normally the smaller the chips.

But there is no guarantee that the unlabelled stuff was 7075 and I don't know of any easy test to determine the alloy composition.

[tahustvedt]

On this useful website you will find charts of machinability (and other properties) of various aluminium alloys. If 7075 has been tempered to T6 (common) it will be be very hard, fantastically strong and machines very well with small chips because it's more brittle. I have never tried to mill untempered 7075 before so I can't compare.: http://aluminium.matter.org.uk/aluse...ion_browse.asp

[James K]

Hmm, well thanks for the help. Im guessing that it was in fact a different temper...