Minimizing titanium part warpage

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.