I was kicking about the Autodesk Discussion Groups and found this question:

Default Weldment Material [How to change it?]

Your drawing Parts lists show the material for all welded assemblies (weldments) as Welded Aluminum-6061. Using the assembly iProperties in the weldment gets you nowhere, as the material is deactivated. This frustrated me for some time too, and the solution is sadly quite logical: It’s a weld bead, so go to the Welding Bead section!

From the Ribbon, navigate to the Weld tab. Under the Process panel, pick Welds to go to the area that you would add your welds.

In the Assembly Browser, pick Welds -> Beads. From the context menu, pick iProperties.

Inventor 2015 Weldment Bead Material

The Physical tab will now allow you to change the Material. This makes perfect sense since your assembly does not have a material, but you are adding material in the welding process. (You can actually make the change one level up from the Welds collection header.)

After the change, the material will update in your Inventor Assembly Parts Lists.