These trains are part of the Hitachi A train family and Javelins (395) have been running for 9 years with seemingly no problem.
Lets hope the problem is not at Newton Aycliffe or factories in Italy.
I'm pretty sure that attaching this bolster part to the body floor panels has always been done at Kosado. A full body shell welding facility was commissioned at Newton Aycliffe in March; before that all their body shells arrived complete. Pistoia assembled 802 bodies from flat (ish) side, floor, and roof panels sent from Japan. And I've found a picture that shows that!
There are a couple of relevant articles that covered the start of production of 802s in Pistoia, in
Rail Technology Magazine (by Berry Sas of Hitachi, 9/5/2017) and
(Key) Modern railways (Keith Fender, 22/6/2017), and also
Rail Engineer (Andy Milne, 4/7/2017). the important picture is in the second of those (and is by Keith Fender):
Flatpack kit: parts for train seven (No 802006).These body panels are
stir-fryfriction stir welded at Kosado from extruded alloy sections made by Kobe Steel (see below). In the picture are sides for intermediate and driving trailer cars (the shorter ones), and floors. If you look at the floor panels you can just see the bolsters peeping out at the far end. Assuming the description is correct, and these are as supplied from Japan, their attachment wasn't done in Italy. What Ansaldo did have there already was a very
very long MIG welding rig, used to make the aluminium alloy bodies of ETR1000s. For more about this, and pictures, see the articles.
Looking at he near ends of those body panels, you can just about see the cell structure between the inner and outer skins. Kobe Steel have some pictures that show this more clearly:
And attached is an illustration from a Hitachi presentation on the class 395 project, though it's a generic A-train as you can see. These extrusions are all custom designed for their location in the body shell, and have two small projecting strips for welding to their neighbour. FSW can only really be used for nearly flat and straight butt joints, so presumably MIG is used for the rest - including attaching the bolsters (though that depends on what they are made of, which I still don't know).
MIG = Metal Inert Gas, which doesn't tell you it's arc welding and that the metal melts (the tungsten in TIG doesn't).
FSW = Friction Stir Welding, a British invention of 30 years ago, but a speciality of Hitachi (who make the machines that do it). See Wikipedia for details, but this picture does about 60% of the explaining:
Fricton stir welding (Wikipedia) By Anandwiki at English Wikipedia, Creative Commons license.