Hey, Akhil!

IELTS is an English proficiency exam which a student from non native English speaking country has to give to prove that they have sufficient command over the language to study a course fully/ partially taught in English. A mechanical engineer can choose any relevant stream and apply to it. However, to apply to such Masters programs one has to give GRE (Graduate Record Examinations), for studying in the USA or in some other countries apart from that. 

So, you can't apply for a course in mechanical engineering or/and related fields just by giving the IELTS exam; it is just one of the scores required by the colleges/ universities. For more information regarding IELTS and applying to Masters programs, refer to the follwing links:

https://studyabroad.careers360.com/articles/ielts

https://studyabroad.careers360.com/articles/how-apply-for-ms-in-usa

https://studyabroad.careers360.com/articles/gre

Hope this helps. All the best!