Page 26 - Demo

Issue 143 August 2024 PCMI Journal 26
PCM of Difficult-to-Etch Metals and Alloys: Nickel- and Cobalt-Based Superalloys
Emeritus Professor David Allen, Cranfield University
Table 10. Etchability of cobalt-based superalloys in ferric chloride
Cobalt superalloy [FeCl3]
(M)
Etch rate
(μm/minute)
R
a
(μm) Smut Reference
Havar 2.91 7.33 0.22 Slight [9]
Alloy 188 2.91 0 - - [9]
Alloy 25 (L-605) 2.91 0 - - [9]
MP35N 2.91 0 - - [9]
Duratherm 600 2.91 0 - - [20]
acid, all proportions by volume, and sufficient iodine to at least substantially
maintain surface smoothness.” This etchant was used to etch Stellite 21
(R30021), S-816 (R30816), X-40 (R30031) and Alloy 25 (R30605).
Table 11. Surface microetching of L-605 (Alloy 25) in ferric chloride, hydrochloric
acid and methanol
Cobalt superalloy [FeCl3]
(M)
Etch rate
(μm/minute)
R
a
(μm) Temp °C Reference
Alloy 25 (L-605) 0.123 ≈ 0.03 1.42 40 [21]
Electrolytic photoetching of superalloys
Electrolytic photoetching has been established as a viable option to PCM
since the 1950s. In correspondence, Eastman Kodak suggested that etching of
Elgiloy should be carried out electrolytically using aqua regia at room
temperature using current densities less than 3 amp/in2 (0.47 amp/cm2) [23].
Using electrolytic photoetching as a method for fabricating small parts can
produce a polished edge profile that gives electrolytic photoetching a significant
advantage over laser cutting (Figure 2).
A miniature H-spring for a prototype medical device was required to be made
from 41 μm thick Duratherm 600 (Figure 3). This is an age-hardenable, corrosion-
resistant Co-Ni-Cr superalloy developed by Vaccumschmelze GmbH & Co. KG,
Germany. It is a material of choice for spring applications and has excellent
ductility and formability in the soft temper state.
Conventional PCM could not be used to make this part. However, electrolytic
photoetching in a methanol-sulphuric acid electrolyte produced the desired part
[20]. In correspondence, the client company stated that it considered the edge
definition of the manufactured parts to be “quite remarkable”. Table 12 shows