June 2009 / Volume 61 / Issue 6|
Minimizing titanium part warpage
By Edward F. Rossman, Ph.D.
Large titanium alloy parts often warp beyond specified dimensions, and a lot of time is expended deciding whether to scrap the part, live with bad dimensions or attempt to straighten the part. Warped parts are costly to fix or replace and cause schedules to slide. Unless the process is repaired, a shop will continue to produce bad parts. The following proposed solutions to warpage are not perfect—there is still much to learn in this area—but they are tried and proven in production.
Based on my experience with warped parts and considerable research of technical papers on stress and warpage, stresses can be grouped into three areas: inherited, clamped-in and machined-in.
Inherited stresses exist in the raw material. They are caused by such practices as rapid cooling during thermal processing or cold straightening of the raw plate or forgings. These stresses show up when initial skin cuts are made, releasing some of the stresses.
An example of inherited stress was found in 10 '-long forgings for titanium alloy spars that had been “stress relieved” prior to delivery. After visiting the supplier, the cause appeared to be that the cooling following stress relief was too fast. Rapid cooling in the final process of stress relief on this part with thick sections next to thin sections put stress back into the part. The forging supplier denied that it was cooling too fast. After our visit, however, we didn’t receive warped forgings.
Clamped-in stresses are best described by an example. Here a part does not fit into the fixture until “Big Charlie” stands on the part while the operator tightens the clamps. Later, when the part is unclamped, the part springs out of the fixture. If a supervisor is watching, the operator exclaims, “I wonder why that happened!”
Heat from milling can cause machined-in stresses and a warped part. Dull cutters and lack of coolant exaggerate this stress by producing even more heat.
The obvious short-term solution to warpage is to straighten the part. Cold straightening below 1,200° F works fine for many metals, but cannot be used on titanium alloys when fatigue life is important. Notes on the part drawing that allow an interim stress relief operation usually cover hot or creep straightening. Support fixturing that will function at 1,350° F and weights are used to creep-straighten parts made of Ti-6Al-4V-EL1. This is usually done in a vacuum furnace to minimize the formation of alpha-case.
Straightening by shot peening is another short-term solution. A trial part about 3 ' long, forged from Ti-6Al-2Sn-2Zr-2Cr-2Mo-STA was shot peened on a rotating table. Typical part cross sections were 0.50 ". There was distortion up to 0.010 ". The conclusions reached were that the titanium alloy distorts from the stresses induced by shot peening, that distortion can be controlled by the process and that shot peening can be used as a straightening or shape-altering process.
Long-term solutions to warpage require investigation to determine the source category of the specific warpage stresses. The resolution of inherited stress was implied in the previously presented example.
Two approaches exist to resolving clamped-in stresses. One solution is to force the part into the fixture and gamble that the stresses of milling on the opposite side will offset the original milling stresses and produce a straight part. The straight part will contain stresses that balance out to yield a straight part. The other solution is to shim the part so it is not forced into the fixture. This solution works if there is enough cover material to allow a full clean up of the part in final milling, and if additional milling warpage is not expected.
The long-range solution to machined-in stresses is often solved by changing the sequence of various roughing and finish milling media (flip-flopping the part in a different sequence). Also, apply sharp cutting tools and a large volume of coolant to reduce part warpage. CTEAbout the Author: The late Edward F. Rossman, Ph.D., was an associate technical fellow in manufacturing R&D with Boeing Integrated Defense Systems, Seattle. Rossman’s Shop Operations column is adapted from information in his book, “Creating and Maintaining a World-Class Machine Shop: A Guide to General and Titanium Machine Shop Practices,” published by Industrial Press Inc., New York. The publisher can be reached by calling (212) 889-6330 or visiting www.industrialpress.com.
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