3D metal printing propeller test for US Navy

Stainless steel propeller deposition trial. Surface tension drove a defect on the edge to propagate and grow. Used to illustrate why metal surface tension (10× higher than other materials) makes 3D printing of metals hard.

I told you we were trying to make propellers for the Navy so they wouldn't have to buy castings and let them sit in a warehouse for a year in case the propeller broke. We tried some stainless steel because it was easy — simple geometries. You can see on the edge we started to get some imperfect melting, and that defect propagated and got larger because of surface tension. The first part gets a little wavy, but it didn't look too bad. All of that is due to the surface tension of metals, which is ten times higher than the surface tension of other materials.