T6 aluminium.

Well T6 only refers to the hardness of the alloy, but I can assume that you are talking about 6061, since that is most commonly seen in T6 hardness.

6061 can be welded with 5356, or 4043 filler. Both work fine.

4043 will give a better color match if it will be polished or anodized.

5356 is a bit stronger and has better corrosion resistance

6061 can NOT be autogenous welded, meaning you HAVE to add filler metal to the joint. If you weld it without filler metal it will crack.

Mind you if you are in fact talking about 7075 aluminum base metal, then forget the above since 7075 can't be welded in any practical way. It will just crack.

Hello I do TIG welding for my living and can offer some advice , 5356 weld wire is somewhat stronger than 4043 ,but it's also does not flow out as easy as