I do weld HDpE , and even the best welds stay a tad more brittle than the base material.
The kit I use ain't cheap in any respect. Leinster Triac hot air gun with 5mm handfed wire nozzle.


The big difference with metal welding is that you don't want the HDPE to melt, but get hot enough to easely deform, so the welding rod can be pressed in the base material.

Oh well.