So my friend is traveling at Phoenix, AZ, USA. And I'm wondering can he bring grade 5 titanium alloy back to Taiwan without any additional process? Like putting a grade 5 titanium alloy right in the luggage, is it legal doing this? Not for commercial but just bringing a small alloy for personal crafting?
There's nothing really special from a customs perspective about Ti-6Al-4V. The skin of the airplane he flies in is made out of the stuff. But the monetary value of what he brings in could be a customs issue. This stuff isn't necessarily cheap, so depending on how much he brings in, there might be a customs duty owed.
The personal duty-free exemption is 20,000 TWD (about 648 USD today). If the amount of goods he purchased outside Taiwan and is bringing in exceeds this amount, he should declare these items on the customs declaration form and go through the red channel.