What I know is that when the basic material doesn't change , we use made of . Am I right ? Yes.
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Anonymous .What I know is that when the basic material doesn't change , we use made of . Am I right ?Yes. In the case you give, us, the material (actually, the 'one piece') has changed. However, you will find that native speakers do not follow that rule strictly anyway.
Anonymous How has the material changed ?It is no longer one piece: that is the change. It is made 'from one piece', not 'from metal'.