This question relates to solidification and addition processes.

$($a$)$ Die casting of a magnesium alloy was carried out by a researcher in Nanyang Technological University. Microstructures of the magnesium alloy were examined after the casting process and some defects were observed, as shown in Figure $2.$ What is the name of the defects? What are the major causes for the defects? Do you expect to see the same type of defects if the casting process is sand casting?

$(7$ marks$)$