If it is using Lollipop, there is no way to actually make it use the card as the Internal rather than External. Without going through all the craziness of "rooting" that is. Not something I would recommend. However, if you can upgrade your OS to Marshmallow, then you can actually make the change. Marshmallow allows for setting a card as Internal rather than External.
NOTE: To do this, the card will be formatted to work correctly. You should really move anything personal off the card (to say a computer or cloud account) prior to doing this. Also I would suggest making a back up copy of what you have on your phone prior to the Marshmallow update.
How to turn external SD card into internal storage on Marshmallow.
1. Go to device “Settings”, then select “Storage”.
2. Select your "SD Card", then tap the “three-dot menu“ (top-right), now select “Settings” from in there.
3. Now select “Format as internal”, and then “Erase & Format”.
4. Your SD Card will now be formatted as internal storage.
5. Reboot your phone.
NOTE: If you don't reboot the phone, many things may not work correctly, so make sure you do.
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