A process for growing a substrate (24) as a melt pool (28) solidifies beneath a molten slag layer (30). An energy beam (36) is used to melt a powder (32) or a hollow feed wire (42) with a powdered alloy core (44) under the slag layer. The slag layer is at least partially transparent (37) to the energy beam, and it may be partially optically absorbent or translucent to the energy beam to absorb enough energy to remain molten. As with a conventional ESW process, the slag layer insulates the molten material and shields it from reaction with air. A composition of the powder may be changed across a solidification axis (A) of the resulting component (60) to provide a functionally graded directionally solidified product.