I thought I might have another aluminum welding project. While refurbing my bicycle, I noticed a big split in one of the bottom back axle frame members. It looked like it was exploded from the inside with about a 2 inch longitudinal open split. The crack is parallel to a weld. I have no idea when it happened or how long it has been split.
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I was talked out of trying to fix it by my local friendly welder who said welding would spoil the temper and weaken the tube, unless it could be re-tempered.
As it turned out, the frame on my bike is guaranteed for life, so the maker will send me a new frame or bike, depending on if they have a frame. It is about 10 years old. I'll know something this week. I cannot imagine what kind of stress would cause that tube to crack like that. It looked blown out from the inside.
The guys at the bike shop admonished me not to ride it, but I've been riding it for about 6 weeks. I don't think if it breaks it will be a catastrophic break. There are three other frame members holding the rear axle mount. I had a friend who had the same tube break in half on his bike and it didn't fall apart. He said it just got a little loose feeling.
It is good that the bike shop thinks the damage is so dangerous. It increases the likelyhood that they will make a good case to the maker to replace it.