Hi Lawrence
You are asking a bit much for me to remember all the details, but here is some idea:
Base metal thickness around 5mm
Single V prep. butt
Pulse duration around 2 - 3 seconds
Background current around 50% of peak
I am aware that this weld can be performed by a skilled Al welder without any pulsing, but pulsing gives a less skilled Al welder the ability to perform a full penetration weld without such a big chance of burn-through.
There are a number of problems with welding Al. There are two that tend to work together to make it a "difficult" material to weld. These are:
1) The high thermal conductivity of Al
2) The narrow melting range of Al
To get the Al to melt, you need to get as much heat into the material as soon as possible. If your amps are too low, the heat is conducted away as fast as you pour it into the material. This means no melting. So, you need to crank your amps up as much as possible. Once melting does however start, you go from a solid to a total liquid within a very short space of time. There is a very small liquid + solid zone. (Also called the "mushy" zone.) - This is obviously also very dependant on the alloy being welded. If you are welding commercially pure Al this becomes a big problem.
What therefore happens, is that as the material looks like it is just making a really nice puddle, it suddenly burns right through the base metal and your wleding puddle suddenly falls on the floor.
To ease this problem, you set the pulse on the machine so that you get melting for a second or two, followed by a "freezing" cycle. During the high current cycle, you feed filler metal and move forwards with the weld. As the lower current cycle starts, you stop moving forwards and allow the puddle to freeze momentarilly. The high current cycle then again starts etc.
I hope that this is what you had in mind.
Regards
Niekie Jooste