Selective chemical milling of recast surfaces

摘要

Disclosed is a process for machining nickel-base superalloys wherein a thermal-effect process, such as laser or electric discharge machining, is first used to remove material but leaves a recast layer. Next a chemical milling process is used wherein the etchant only attacks and removes the recast layer. The etchant is comprised by volume percent of 40-60 HNO3, 5-20 HCl, and balance H2O, with which is included 0.008-0.025 moles/liter FeCl3 and at least 0.016 moles/liter CuSO4. The FeCl3 improves removal rate but tends to cause unwanted pitting and intergranular attack. These tendencies are inhibited by the addition of CuSO4; preferably the molar ratio of CuSO4 to FeCl3 is 2:1. The beneficial combination of FeCl3 and CuSO4 is usable in other etchants.