Dear R:
Probably the best way, and I'm taking a risk saying it because someone will disagree with me, is to simply nail knot the leader directly to the fly line and put a tiny drop of superglue (cyanoacrilate adhesive) on the knot. Then tug on both lines so the excess superglue flicks off and hold it straight for about 30 seconds while the glue hardens. Now you have a perfect transmission of forces between fly line and leader with no hinge. By the way, I would never use a soldering iron on a fly line.
Migs