the only information i could pick up was on rock shox, and they were using 6061 on crowns, and 7k for stanchions with magnesium lowers. no one bothers to even list materials for the steerer and all simply put "aluminum" ... you'd suspect they'd be labeling it 7k if it was special.
im pretty confident this will be secure as the method my shop was suggesting was taking 1.125 aluminum stock, stepping it, and sliding that an inch or two down the steerer (with a half inch of 1.125 out the top) before welding it in place at the step. hell, they're not even hollowing it out, they're gonna tap the solid piece in place of a star nut. i dont have reservations after they described that... they just said it would be difficult to weld 7k series and thats why i was wondering.