Just a couple of points worth mentioning, 'friction' occurs when two surfaces move in relation to each other. If two objects are fixed together, like a wheel and a hub for example, there is no friction. Secondly, when dissimilar metals like aluminium and steel are cleanly fitted together in a dry environment this is not normally a problem. However, if an electrolyte like dirty or more especially salty water is present then this can result in galvanic corrosion. This is what causes alloy wheels to become 'stuck' on the hub. A barrier such as a film of grease helps prevent the reaction known as galvanic corrosion.