Can you please advise regarding normal practise for shot peeners when parts of different material are peened. (For example aluminium then titanium)?
Is is normal practise to have a peening machine dedicated to peening materials of a particular group so as to avoid cross contamination?
Or do some people change shot loads when material group changes?

Is there any documented evidence to demonstrate either the effects of cross contamination or that it is insignificant?

If the titanium parts have been debured and cleaned before shot peen, the recovery system and dust collector should be anough to remove any particles in the machine.

Interesting question...one which I haven't had to answer, but a major aerospace OEM once told me that they peen structural parts of titanium, aluminum, and steel in the same peeners. He didn't mention any concerns about contamination.