Page 28 - Demo

Issue 143 August 2024 PCMI Journal 28
PCM of Difficult-to-Etch Metals and Alloys: Nickel- and Cobalt-Based Superalloys
Emeritus Professor David Allen, Cranfield University
Table 12. Electrolytic photoetching of superalloys as an alternative, HSE-friendly
manufacturing technique to spray-etching [9 and 25*]
Superalloy Electrolytic system
yielding fastest etch rate
R
a
(μm)
Etch rate
(μm/minute)
Nickel-based
Hastelloy B-2 10% HCl @ 0.7 A/cm
2 0.68 3.98
Hastelloy C-22 15% NaCl @ 0.7 A/cm
2 0.16 25.4
Hastelloy C-276 15% NaCl @ 0.7 A/cm
2 0.26 5.91
Inconel 617 10% HCl @ 1 A/cm
2 < 1 17
Inconel 617 H
2
SO
4 /H3
PO
4 /H2O (35/45/20 v/v) @ 2 A/cm
2 0.1 10*
Inconel 625 Sat. NH
4Cl/methanol @ 0.7A/cm
2 0.84 3.68
Cobalt-based
Havar 20% H
2
SO
4 @ 0.7 A/cm
2 0.11 6.65
Haynes Alloy 25
(L-605) 1M LiCl/methanol @ 0.7A/cm
2 0.30 5.16
Haynes Alloy 188 1M LiCl/methanol @ 0.7A/cm
2 0.19 6.23
MP35N 15% NaCl @ 0.7 A/cm
2 0.18 5.11
Conclusions
• Inconel alloys have a wide range of etchabilities depending on their
chemical compositions.
• Some nickel- and cobalt-based superalloys can be etched with standard
ferric chloride solution.
• However, some superalloys require very aggressive etchants for PCM to
be successful. These may be ferric chloride formulations with aqua regia
and/or HF additions.
• Such etchants may require durable epoxy photoresists for imaging.
• The HSE-friendly technique of electrolytic photoetching can be used to
manufacture small parts from nickel- and cobalt-based superalloys.
• Electrolytic photoetching reduces HSE impact of aggressive chemical
machining and gives superior edge profiles in comparison to both PCM
and laser cutting. However, tab width needs to be minimised for ease of
separating the etched parts from the superalloy sheet.
• The range of superalloys is still expanding but corrosion resistance of
superalloys makes production of parts by PCM an interesting technical
challenge.