On Tue, 09 Aug 2016 16:32:23 -0400, Ed Huntress
wrote:

On Tue, 09 Aug 2016 16:11:45 -0400, wrote:

On Tue, 9 Aug 2016 06:37:42 -0400, "Jim Wilkins"
wrote:

wrote in message
...
...
All of the aluminum on my plane is 6061T6 - rivetted.
All flight surfaces and flight structures.

Aluminum boat trailers are routinely welded.

What's the difference?
--jsw

The stuff on the plane is .016 to .030" thick. The trailer is minimum
3/16 inch - mostly 1/4 inch wall.

It's very difficult to weld any aluminum alloy above the 5000 series
without creating a crack-sensitive joint. And 5000 series and below
are not very strong. Besides that, 5052, which is used a lot in boats,
has a "cracking peak" at 2.5% magnesium, so you need to use filler
that will supply plenty of extra magnesium to be sure your weld
doesn't hit the cracking peak after welding.

6061 is very crack sensitive if you use insufficient filler metal.
Welds in 6061 are tricky because welding exceeds the artificial ageing
temperature; your heat-affected zone will be a mess of varying
hardness and ductility, and it will change over time.

It is almost impossible to produce a weld in aluminum that is as
strong as the base metal. In contrast, mild-steel welds generally are
close in strength and ductility to the base metal.

Thus, they don't weld aluminum on aircraft. They aren't welding the
new aluminum car chassis, either.
The only welded aluminum parts on our plane are the fuel tanks. I
believe also the only parts that are not 6061.