"John" wrote in message
...
Joseph Gwinn wrote:
There was a recent article in Aviation Week saying that GE was using
Electric Discharge Machining (EDM) to rough out turbojet blisks
(combined rotor and blades) from Inconel 718, which is almost as hard as
carbide. Apparently, GE got the idea in 2001 from the Chinese, who were
using EDM for drilling of such alloys.

After roughing, the blisks are cut to final form using electro-chemical
machining, and also polished.

GE calls the technology "Blue Arc", the blue being the color of the
proprietary working fluid.

Joe Gwinn


Aviation Week, 9 May 2011, page 16, "Submerged Results - Novel technique
speeds GE's blisk output", by Michael Mecham, The Inside Track (column).

I have not found this on the web; I assume it's behind a paywall.

There are patents, however. Search for "electric discharge" and
inconel. I found applications 2010/0301016 and 0301017 for instance.



EDM was shunned on aircraft parts because it was thought that microcracks
that would form in the cut which would cause failure of the part. Now
with the final finishing of the parts EDM is becoming the choice for
machining blades.

John

EDM has been used for drilling the cooling passages in aircraft turbine
blades for around 40 years. And it was a pretty fierce burn done at high
speed, which leaves a lot of microcracks.

However, that was on the inside of the blades, an area which is not in
tension. They're close to the neutral axis, in fact.

With all due respect to _Aviation Week_ (published a couple of floors above
_American Machinist_ when I worked there, whose editors often came to us
when they needed to know something about metalworking g), the way those
lines above read suggests that the writer isn't familiar with the
technology. EDM has been used for drilling holes in Inconel, Hastelloy,
tungsten carbide, and other refractory metals for, again, around 40 years.
Making blisks with EDM probably is new. As you say, the microcracks would be
a problem on any surfaces that are in tension.

Electro-chemical machining (ECM) of aircraft parts goes back even further --
at least to the early '60s, and maybe the '50s. It, too, was used initially
for drilling cooling passages in turbine blades. It has an advantage over
EDM in that it doesn't damage the surface, either with microcracks or with a
heat-affected zone. It was largely abandoned for that work in the mid-'70s,
when EDM took over. ECM is a messy process that presents some difficult
tooling problems, and it needs its own room in a plant, with its own
ventilation, or it will rust everything in the building.

But roughing with EDM and finishing with ECM would make good sense from the
standpoint of roughing efficiency combined with finished-surface integrity.
The tooling for the ECM must be very interesting to handle all of that
geometry. Maybe they're using CNC positioning or something.

--
Ed Huntress