Written Aptitude test which includes Mechanical Aptitude, Quantitative aptitude, Logical reasoning After that Personal interview which includes geeneral questions about yourself, basic Mechanical engineering questions etc. After that Medical which for general purpose to just verify candidate is able to work in Mechanical industry for 8hrs a day and to check any disabilities or weskness.

They can ask from any subject you have studied in diploma from subject like material science so you should have good command in all subjects of diploma . So keep on revising the concepts of different subjects of diploma