Both of these are hypoeutectic aluminum alloys. It is necessary to modify the beta phase in these alloys and strontium is often recommended for this purpose butmischmetal (rare earth) is also sometimes used. It is essential to add the modifier to the melt before pouring as otherwise the mechanical properties will be very poor, coarse beta being primarily responsible for this. Both have excellent fluidity but in common with most aluminum casting alloys, are prone to microporosity. Attempts to produce pore-free castings have met with little success. Care in mold making and casting design are as important as in any casting, to avoid defects (caused by inadequate understanding of heat transfer, fluid mechanics and materials knowledge required in this field). Both the alloys are of great importance to the automotive industry and there are large number of research publications dealing with their fatigue behavior as influenced by casting defects. I tried to attach some for your study but coud not do so at this time. I will try to send the attachment in a separate answer.

The publications I mentioned in my last answer are now attached.

Dear Arvind Kumar Dixit 

I think the following information in attachments  can help you to identify  the major defects in Al-Alloy castings.

The most important casting defect in aluminum cast alloys is the double oxide films known as bifilms. Bifilms act as very little cracks causing catastrophic damages in  the alloys. The main reason for the formation of bifilms is to cast the molten metal with high turbulence. You can find very useful information by looking at Castings, 10 rules for Castings, or Complete Casting Handbook written by professor John Campbell. I promise you if you read each of these books, you will find the answer of your question :).

Hi

A356 and A319, both are hypoeutectic alloy, strontium can be used as a modifier to make the dendrites structure more fine. also can use one of rheocasting process to change the dendrites structure to globular structure which will improve the properties of these alloys.