In sci.physics JF Mezei wrote:
On 2017-07-04 14:19, wrote:

Metal powder deposition and laser or electron beam sintering.

BTW, metal powders tend to be explosive.

Thanks. Does this really produce parts that are on par with those
produced with conventional means when it comes to
force/strength/endurance (especially when it comes to re-usable engines) ?

It depends on what you mean by "on par".

It appears so as SpaceX uses it, but just wondering if the physics of
melting layers of powder yields the same strength a a solid block that
is machine tooled.

It doesn't matter if it is the same strength as long as it is strong
enough for the purpose.

Also, if it is powder, how do they make a "roof" over empty spaces (such
a the top of a horizontal cylendrical pipe inside the unit).

Make the part upside down so the "roof" is a "floor" during printing.

--
Jim Pennino