In honor of Henry, I present to you a wall of original text.
Two typical causes of subsurface porosity in aluminum, hydrogen bubbles and oxides at fusion lines.
Since your porosity you say is at the end of each weld it's more likely hydrogen is your issue.
Molten aluminum is especially soluble to hydrogen pickup... When the weld begins to freeze the hydrogen is forced out. However if there is too much Hydrogen in the solution, some may remain trapped and appear as very fine porosity.. very fine.
As heat increases and weld pools expand, more hydrogen may enter the weld pool... This is why GTAW of aluminum must be performed hot and fast... This is also why preheat is not recommended if the welding power supply can produce a weld pool quickly (your Dynasty can do this)... The initial arc strike and high thermal conductivity of aluminum will eliminate any residual moisture on a properly prepped workpiece, by the time a weld pool is formed the work piece and weld area simply cannot have any moisture on it.
Often the welder in manual GTAW will "slow down" toward the end of the assembly. Slower travel speed equals greater heat input, add to this the fact that aluminum is a SUPER FAST thermal conductor and you have a scenario where heat is building as the weld progresses (naturally) and the operatory may slow down travel speed and exacerbate the situation. The heat runs in the direction of travel and continues to build, at the end of the weldment there is no place for it to go, so it grows hotter and hotter. Meaning a greater amount of hydrogen may be absorbed, possibly so much that when the weld solidifies it is not driven out. This is why you often see very good GTAW welders greatly reduce the weld current via foot pedal at the end of the weld while keeping up or even increasing the travel speed !
Another factor may be associated with operator technique, this is the often typical change in torch angle from almost perpendicular to the work to an exaggerated "push" angle. Meaning the welder may turn the wrist to finish the last inch or two, thus changing the work angle of the torch. When the angle becomes exaggerated, the weld pool becomes tear-drop shaped rather than round and offers more area for hydrogen absorption. The exaggerated angle may also reduce the efficiency of gas coverage allowing for humidity and increased oxides to be absorbed into the weld pool.
A positive welder technique is to use the largest filler wire diameter the welder is comfortable with. The larger mass of filler will help notably to cool the weld pool... Keeping a very low dip angle when adding the wire is as important as keeping the tip of the filler constantly under the inert gas shield. Pulling the wire out of the inert shield will add oxides every dip... Having a radical angle on wire feed often causes the end of the filler rod to ball up and can cause problems as well.
I'm skeptical about any advantage to that very large cup. As Terry mentioned, a typical size has been proven effective over time. I see that supersized cup as a stumbling block to keeping a consistent electrode angle for the entire length of a long weld. Plus:: I'm not sure about the latest revision, but if nozzle/cup size is an essential variable, you are stuck with the big cup for tight spots
Lastly... AC waveforms available with Dynasty open the door to some problems if misunderstood. Operators who select a balance greater than about 70% EN take a risk of reducing the "cleaning action" to the point that oxides are not efficiently removed. Very careful surface prep and cleaning is required to use greater. A frequency of 200hz is sufficient to push the arc to the tip of the tungsten and produce a nice columnar arc. Frequencies above that have little benefit for manual GTAW. All this to say, if you have a balance setting that reduces cleaning action, oxides may become a problem. If your problem were oxides, the porosity indication on your RT report would likely be along the entire length of the weld.
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