Dear Atik Islam ,

welocme!

It is so that the addition of elements to a metal contributes to changing its thermal and mechanical properties. The addition of Silicon to aluminum decreases it melting temperature and makes it more hard and less ductile.

Your question is answered in the education material given in the link:http://www.alcotec.com/us/en/education/knowledge/qa/How-and-why-alloying-elements-are-added-to-aluminum.cfm

Best wishes