From: Heribert Gray
Dear Dr. Peener
Spherical Conditioned Steel - SCCW 62 , hardness 60-65 HRC ( 640 HV1 ) cut wire is being used for such kind of Shot Peening - after thermal spraying for instance - to achieve a high penetration of the compressive residual stress affected zone below the surface area due to reason that you are familiar with.
Stainless steel Cut Wire ( even less Stainless Shot) is not recommended due to two reason:
1.) Stainless Steel is more precious than Aluminum, particular to the 7000 series Al- Alloys with regard to galvanic reaction.
Consequently, the galvanic corrosion occurs preferably and intense at the Al- Alloy!
Steel causes some discoloration on the parts in contrary to STEEL CUT WIRE but faces no major damage of the component made of Al-Alloy. The local galvanic corrosion stops as soon as the whitish Al-Oxide does insulate the flow of galvanic current.
2.) Do to the higher resistance of stainless steel alloys to any acids compared to steel, the removal of the contamination causes longer or more intensive chemical treatments.
This fact rises the risk of a hydrogen embrittlement in the Al-Alloy, particular in the case of 7000 series - Al-Alloys when being peak or overaged.
The longer the parts are exposed to acid treatment, the more Al-metal is dissolved as salt into the acid bath.
An approved alternative is the use of Zirconium Beads that are partially stabilized with Cerium Oxide but having no glassy Phase like Si-Oxide : 80% Zr-Dioxide with 20 % Ce-Dioxide, hardness 1100 HV
( 9 Mohs ) at crushing strength of 125 kg on 2 mm diameter beads.
Size: 1.2 - 1.7 mm
Shape : spherical
Appearance: smooth glossy satin finish
Specific weight: 6-6.1 g/cm³ - Apparent Density ~3.7 g/cm³
The beads are inert and leaving no chemically reacting residues on the surface to be exposed to Shot Peening !
These kind of professional peening is applied in the F-1 Racing Sports of TA-Plasma Sprayed High Temperature Components and Titanium Springs, high strength components and jet engine parts in the Aeronautical Industries, for Off-shore Oil Drilling Tools and last but not least for biomedical application ( implants ) .
The higher cost for such kind of media are paying off due to less or no cost for the post treatments like decontamination, at the bottom line!
Additionally, the requested intensity is reached faster due to the high hardness and the coverage can be adjusted to <= 100 % instead of >> 100 %.
This way the Peening Process in general is becoming less time consuming and less costly itself already.
The flip side of the coin is : this kind of media is limited to pressure pot peening or to suction peening with a gravimetric feeding system of the media.
hope we could help