Naval Research Lab laser welding of HY-80

30-year-old NRL result: 25 kW laser welding HY-80 produced *better* weld-metal properties than the base material, because the laser preferentially vaporized oxide and sulfide inclusions. Tom's preferred illustration of the "laser couples better to inclusions" principle.

The last point is that electron beams heat the sample uniformly. Everything gets heated the same, partly because the electrons have a certain depth of penetration. From 100 kV it might be a tenth of a millimeter; at a megavolt it might be a millimeter. So it averages the heat out over inclusions and other things. Whereas it turns out the inclusions in steels are oxides, and they couple with the laser energy better than the base material. So 30 years ago the Naval Research Lab had a 25 kilowatt laser to study laser welding, and they found they were welding HY-80, and they got better properties in the weld metal of the HY-80 than they did in the base material. The mechanical properties were better. They asked, well why is that? It turns out they were refining the steel. The laser was selectively vaporizing the oxide inclusions in the steel — oxides and sulfides — and just blasting them away as vapor, leaving behind a more refined weld metal. So there are certain advantages you can get in changing the chemistry of the weld bead.

Ed Metzbower found laser-welded HY-80 weld metal cleaner than the base plate — superheat vaporized inclusions, yielding better fatigue properties in weld than parent metal.

So they heat this thing and it superheats on the surface. What Ed Metzbower did at Naval Research Laboratory — he was welding HY-80 steel with a laser, and he found that the weld metal was cleaner than the base metal he was welding. He figured out that the superheating from the laser beam in the weld metal was vaporizing all the inclusions, and he got better properties in the weld metal than in the base plate because he was getting rid of those inclusion particles. So one of the things the Air Force does to make very expensive material is remelt the material and let it resolidify, but in vacuum. They can have another beam rotating on top of the pool to keep it liquid. It's a fairly expensive facility. They were building one of these for titanium about 20 years ago right over here in Worcester, Massachusetts at the Wyman-Gordon facility.

Early-1980s NRL finding that laser welding HY-80 vaporized inclusions and gave better toughness — used to argue that ultra-clean steel resists hydrogen cracking, but isn't producible at tonnage scale.

The Naval Research Lab found in the early 1980s that they could weld HY-80 with lasers and have better toughness and more resistance to hydrogen in the weld pool, because the laser actually vaporized away the inclusions in the steel. They ended up with a super-clean steel from the welding process. A scientific curiosity that proves the point: if you're ultra clean, you can resist this — but you can't make steel that clean on a tonnage basis. Maybe a hundred years from now we'll have some way to do it. Today we make steel thirty times cleaner than it was 50 years ago. That's why we don't have laminations, why we don't get crap when you put it in a stress-relief furnace.