Lots of people above are saying you can't weld aluminium ... that hasn't been my experience, however you do need to get someone who knows what they're doing.

fixing
 

Tension, compression or torque and Mailability are a concern with aluminum, welding can be done if static like a case cover has no stress
this Problem is a trunnion cap crack 4 bolts it secures the axle and the axle is none rotating
it is highly unlikely the axle will ever come off Look at the meat on this thing
You can't Weld Aluminum especially cast that has lower radius tolerances than the axle tolerance it is called a friction fit that make the trunnion grip the axle
dont weld buy new
look after your moto and it will look after you
this is typical manufacturing mistakes that pass quality control You have to look at the whole picture when working on bikes there are many defects that you are not aware of this one needed a shim or washer and its obvious to have a grip on the axle
bikes are not made well But there are exceptions You get what you pay for Try Aircraft maintenance it is in a another league

If that came to me for MOT I would slap a dangerous fail on it and make it illegal to ride home.

Replace that ASAP..

It could be welded. But you would have to grind down to the beginning of the crack and built up in layers.. And that would cost more than a new cap for it to be done properly.

Welding it - And then, would you REALLY trust it with your life, and your wife's life?
Totally agree with Ted, REPLACE with NEW - or there's a link to a better one that looks like a great idea since the OEM is clearly flawed.

Interestingly enough, I looked back at some old photos from this time in 2019 and the same cracks are there, albeit not as long. The bike had only been ridden a handful of times since then, however, as it'd had a prolonged stay at David Lambeth's for an engine replacement.

I was high up in the Pyrenees in Spain when I spotted them, two weeks ago.

I did chew over getting it welded or even whacking a load of JB on there, just to get me through the last 7 days of the trip and back to the UK.

I decided against both in the end because a) either option would mask the crack, meaning I couldn't see if it continued to increase and; b) it seemed like too great a risk.

I parked up in a nice spot and got a replacement part sent from OffRoadCentrum in Holland via DHL Express, which worked as I fancied a day's rest anyway! I was good to go 48 hours later.

This used OEM part is an interim replacement - I've now received 2 x aftermarket replacements from Titan, which I'm going to fit to both my XT600 34L and 3TB as a precaution.

They certainly look well machined and look the business!

Ed