Aluminium axle? I'd gauge the bore carefully, but it's likely to be a bit too far over the tap size for M6 to be safe and reliable. Steel axles are proven, but aluminium is half to a third of the strength and really should have full form threads.
There is alway the option to ream out what is likely to be a 5.5mm bore to 6.0 and then tap it M7×1.0, the taps and screws are readily available despite not being a preferred size.
Aluminium axle? I'd gauge the bore carefully, but it's likely to be a bit too far over the tap size for M6 to be safe and reliable. Steel axles are proven, but aluminium is half to a third of the strength and really should have full form threads.