Air Force blisk (bladed disc) welding development

1980s–1990s Air Force program to weld turbine blades directly to disc, eliminating half the rotating mass that lives in the "tree" mechanical attachment. Tried electron beam, laser, arc, linear friction welding. Economics failed at Six Sigma quality requirement.

I wanted to mention three other types of friction welding. One is linear friction welding. This is circular friction welding — it's nice and easy to build a machine that goes in circles, but they would love to do linear friction welding. If you want to do some friction welding, you can do it with your hands — you feel your hand warming up. They would love to be able to make turbine discs for jet engines, where they weld the turbine blade to the substrate. Here's a regular turbine blade. The jointing is a mechanical joint, and they call this the tree — turn it upside down, it looks like a tree. This is ground very precisely. This is the most expensive part of the turbine, I think — $10 million grinding machine to grind these flats. You mount the whole thing in Wood's metal, a low-melting metal, to fixture it, because you can't clamp this precisely enough to get the tolerances they want. It has to fit into the mechanical sleeve on the turbine disc within a few tenths of a thousandth of an inch, otherwise at the speed it's going you'll get vibration and fretting wear. You have to be careful what temperature you assemble these at — a few degrees of temperature, you basically have to assemble these in a controlled temperature environment.

"Tens, if not hundreds, of millions of dollars" spent trying to figure out how to weld blades to disk. No commercial product yet at large-engine scale.

One of these is called a bladed disk, or a BLISK. You'd love to be able to take the turbine disk, which right now has this complex geometry to join the turbine blade to the disk, and you have this heavy structure right in here. If you could weld these things together or cast them together, you could reduce a lot of weight. And that's exactly what has been done in some cases. The only commercial BLISK that I know of happens to be on the Detroit Diesel Allison, now Rolls Royce, M250 engine, which they've made something like 10,000 of. They go in helicopters and other things. They cast the blades to the superalloy, all cast as one unit. It's a small turbine engine for helicopters and small planes, but not for great big commercial planes. People can't cast with the quality they need in the big disk. People are trying welding. The Air Force has spent tens, if not hundreds, of millions of dollars trying to figure out how to weld blades to disk, because there's tremendous value.

Air Force has spent billions trying to weld blades directly to disc to eliminate Christmas-tree flange weight. 20 lb savings per disc, multiplied by 3–4 discs × 2–4 engines × collateral airframe weight = ~2,000 lb total savings; military aircraft bogey is $1,000/lb saved.

A better example that I like to use is turbine discs in aircraft. The Air Force has spent billions of dollars trying to make a blisk — a bladed disc. You know what a turbine engine looks like, and what the disc looks like with turbine blades sticking off around it. It sort of looks like a sun from elementary school painted yellow. They attach the blades — which are this fancy material that can't be welded generally — with what they call a Christmas tree structure. This flange and all this weight, this is actually probably half the weight or more of the blade. If I could weld the blade directly to the disc, I could get rid of this big heavy flange on the end of the disc. These discs, depending on the engine, could be traveling at 18,000 rpm, or maybe only two or three thousand depending on the size of the engine. If you can get rid of this big heavy rim, you can lighten the whole thing. The Air Force estimates you can get twenty pounds out of a single disc. There might be three or four discs in the engine, two engines, maybe four engines on an aircraft, so you could save a couple hundred pounds off the engines in a military aircraft.