Q) Why domain rendering as solid cube?
Viewport image :
Rendered Image :
So many beginners face the same problem. In view port every thing working fine. but in rendering output the domain is visible as solid cube.
Ans. Some of your settings may be wrong. But if some one is using blender quick smoke :
step 1 : select the object.
Step 2 : Object -> Quick effects -> Quick smoke
then every thing should work fine. But not working because you have to do another step
Step 3 : Select the domain. Go to physics properties tab-> Cache option. then click on bake.
Now render everything will work fine.
But here another question may come for some persons.
Q) Blender Smoke simulation : Why my smoke cache option on physics properties tab is inactive?
Q) Blender Smoke simulation : Why my smoke cache option on physics properties tab is grayed out?
Ans. The cache option on physics properties tab is inactive or in grey color because you have not saved your blender file. Save the blender file. Then this option will active. bake the smoke simulation and enjoy.