Nearly all metallic material types can be chemically milled. For what MIL Inc. does, it’s limited to decarb / alpha case removal of forgings and stock reduction of titanium aircraft skins.
Decarb removal of ferrous forgings may require up to 0.125” be removed. Verification of material removal is accomplished by use of “witness pads” in defined locations of the part and by hard dimensional reads before and after. Surface finish, usually 32 micro inches or better is required, because only a portion of the forging is machined and the remaining as-forged portion is essentially in its final blueprint condition.
Stock reduction of titanium sheet is required to selectively reduce material thickness in low stress areas of sheet material with the goal of weight reduction. Typically, the entire section of sheet is masked and then a template is used to scribe a pattern in the area to be chemically milled.
Regardless of chemical milling method, there are numerous process variables in a state of flux requiring the utmost in process controls and technician experience. Masking, racking, chemical control and technician experience are key attributes in controlling this one-shot, get-it-right process.
Hydrogen embrittlement is a critical concern in milling ferrous materials as is intergranular attack and end grain pitting. Hydrogen absorption is a key control attribute in chemical milling of titanium alloys in addition to IGA and EGP.