1-Iron surface is not dense enough, there are many tiny holes, polyurethane glue can penetrate a little bit, adhesion is very strong.
2-Aluminum surface generally has a very thin layer of dense oxide film, polyurethane adhesive can not penetrate, so the adhesion is not good.
Aluminum wheel surface needs to be sanded or acidified after treatment, the adhesion will be much improved. Or use polyurethane primer with strong adhesion to aluminum alloy to coat the surface of aluminum alloy and then inject glue.
Test to see.