Page 16 - Demo

Issue 143 August 2024 PCMI Journal 16
PCM of Difficult-to-Etch Metals and Alloys: Nickel- and Cobalt-Based Superalloys
Emeritus Professor David Allen, Cranfield University
PCM of difficult-to-etch metals and alloys: nickel- and cobalt-based superalloys
David M. Allen
Emeritus Professor of Microengineering, Cranfield University, UK
(Presented at the PCMI Conference, Frankfurt, Germany on 13th and 15th May 2024)
Abstract
A superalloy is a metal specifically designed for applications requiring a
high level of resistance to elevated temperatures, high tensile strength, and
resistance to corrosion. Superalloys are therefore often used in chemical
conversion plants, industrial and aerospace turbines, medical implants and
devices containing precision springs and diaphragms.
These materials are extremely difficult to machine by conventional cutting
methods as the materials work-harden when being machined. Fabrication by
chemical machining is therefore a preferred technique. However, as these
materials are also corrosion-resistant, they can also be difficult to etch.
This presentation reviews how a wide range of nickel- and cobalt-based
superalloys can be etched by aggressive chemical etchants or, alternatively, by
electrolytic photoetching techniques with reduced Health, Safety and
Environmental (HSE) impacts.
Background
A superalloy is a metal specially designed for applications requiring a high
level of resistance to elevated temperatures, high tensile strength, and
resistance to corrosion [1,2]. These alloys are usually categorized under three
possible types:
• a cobalt base (30-60% Co),
• a nickel base (≈50% Ni) and
• an iron-nickel base (15-60% Fe and 25-45% Ni) also known as a super
stainless steel.
Over the past few years, I have received several enquiries concerning how to
etch certain superalloys (a term first used in the mid-1940s), especially Inconel
625 which is rated as a “difficult-to-etch” material. Apparently, the interest is
worldwide as I have received enquiries from UK, Continental Europe and USA .