On Thu, 26 May 2016 23:48:27 -0000 (UTC), John Doe
wrote:

Steven wrote:

The first digit is the alloy type second digit is the modification of
the alloy and the last two digits are the purity of the aluminum so
6061 means that in this case magnesium and silicon were added to the
aluminum and that it was not modified and it is 61 percent pure
aluminum

Thanks. How can it be "modified"? What does "not modified" mean?

The first number is the primary alloying element or combination of
elements (6 indicates the magnesium/silicon alloy group). Except for
the 1xxx series, the second number refers to a possible modification
of an earlier alloy of the same group. This "modification" can be a
variety of things, but often refers to a change in the limits of
alloying element percentages or llimits of impurities. One example is
7075 vs. 7175. The latter has the same basic alloying elements, but
somewhat lower levels of silicon and iron. It's better for forging
than 7075.

The last two digits are purely arbitrary for all but the 1xxx series.
In the 1xxx series, the last two numbers indicate the percentage of
aluminum, which, in this case, generally means the level of purity of
aluminum.

--
Ed Huntress