Page 24 - Demo

Issue 143 August 2024 PCMI Journal 24
PCM of Difficult-to-Etch Metals and Alloys: Nickel- and Cobalt-Based Superalloys
Emeritus Professor David Allen, Cranfield University
ADEX, a TDFS, 5-75 μm thick, applied by hot lamination; TOK TMMF S2000 DFR
and DuPont PerMX DFR.
Spraying the same etchant onto the metal at 50°C using an “all-plastic” etching
machine resistant to HF will give a fast etch rate comparable to that achieved in
chemical milling by immersion at 60-71°C. The epoxy photoresist can be stripped
as recommended by the manufacturer.
Cobalt-based superalloys
In developing and producing the turbocharger early in World War II, a discovery
of major significance to the cobalt industry was made. The investment-cast,
cobalt-base dental alloy, known as Vitallium, was observed to possess
mechanical properties and microstructural stability that made it suitable for
supercharger buckets operating at temperatures > 1000°F. Shortly thereafter, a
modification of Vitallium, designated Haynes Stellite 21, was developed
specifically for such high-performance, high-temperature applications. The use
of this alloy expanded to buckets and guide vanes for aircraft turbojet engines
operating at temperatures as high as 1500°F, a performance considered
remarkable at the time – hence the term “superalloy” was coined [19].
Among the cast alloys, X-40 soon displaced the original Stellite 21 because of its
substantially greater elevated-temperature strength. Of the wrought alloys, S-
816 and L-605 (Haynes Alloy 25) were highly successful materials that emerged
early in the history of wrought superalloy development. Haynes Alloy 188 and
CM-7 are modifications of L-605. The strength of Haynes Alloy 188 is essentially
the same as that of L-605, but it has much better oxidation resistance, making it
a strong candidate for numerous applications where Hastelloy X is employed.
Some cobalt-based superalloys originated and were developed within the US
watch industry, namely Elgiloy (Elgin National Watch Co., Elgin, IL, USA) and
Havar (Hamilton Watch Co., Lancaster, PA, USA). This development was
necessitated by the requirement to prevent corrosion of metal mainsprings
when operating in saline and humid environments in wartime when accurate
timekeeping is critical.
A selection of cobalt-based superalloys is shown in Table 8 and the various
chemical compositions are shown in Table 9.