Both of these rods are for welding aluminum, 4043 being the most commomly used. I really do not know what the difference is, but having welded alot of aluminum, I have never used 4047. Really, the best rod to use is 5356 as the mechanical properties of this rod are supposedly better.
For what it is worth......Aluminum is one of the few alloys where the weld is NOT as strong as the base metal.
Some difference exists between these two.
First, if my memory is any good, 4043 is a 5% silicon alloy and 4047 is a 12% silicon alloy.
What does this do??-silicon increases the "wetting" of an alloy as well as somewhat lowering melting temp. The increased silicon content of 4047 often allows it's use as a brazing alloy. Another advantage to the 4000 series fillers is that they will respond to subsequent heat treatment. This is something that 5000 series fillers do not.
Also, keep in mind that many aluminum alloys are not compatible, metallurgically. As a rule, 5000 series alloys should never be welded with 4000 series alloys as mag/silicide can be produced and underbead cracking can be a result. There are some, but, relatively few exceptions to this.
The published reduced "as welded" strength between 4000 and 5000 series fillers is often academic. A few thousand pounds of tensile one way or the other often will not guarantee the success/failure of a weld, as most welds are over-engineered. The exception to this, of course, is when parameters are dictated by print or code.
Hope this helps
Good Luck
brande
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