Table of Contents
220.127.116.11.2 Abrasive blasting
Abrasive blasting usually uses non-metallic (ceramic) hard particles. In engine manufacture, these primarily consist of particles of Al2O3 (aluminum oxide) or SiC, which are carried to the surface by an air flow (dry blasting) and/or a water jet (wet blasting, vapor blasting). Wet blasting methods are used when gentle material removal is desired. They are also necessary when there is a danger of explosive metal dusts (e.g. light metals such as Ti, Al, or Mg alloys). The material removal mechanism is based on erosive wear (Volume 1, Ill. 5.3.1-3) that primarily occurs through the cutting action of the blasting particles.
Abrasive blasting processes are used for various purposes and rely on special characteristics of the surface being blasted:
- Removing fouling and reactive layers (usually highly chemically stable oxides) before etching and/or penetrant testing (17.3.1-7).
- Removing coatings during reworking and recoating
- Deburring, rounding edges (Fig. "Safety of parts by deburring").
- Roughening surfaces for better bonding with coatings, especially thermally sprayed coatings (Fig. "Finishing proess affecting bondstrength") and adhesive bonds (Fig. "Design of adhesive bonds").
- Activating the surface for diffusion welds and soldering. In this case, the loading effect (Fig. "Loading effect by blasting processes") can be utilized for certain reactions. This application includes improving the wettability of high-temperature solders (Fig. "Inferring from solder seam contour") on Ni alloys in combination with remaining SiC particles.
- Activating surfaces ahead of diffusion processes.
Naturally, abrasive welding also has certain risks (see Note below). These can be avoided by taking appropriate safety measures.
Figure "Loading effect by blasting processes": During erosive processes, there is a loading effect (Volume 1, Ill. 5.3.1-7). This refers to the apparently paradoxical phenomenon whereby an increase in mass is observed at the start of an erosive process, rather than a decrease in mass as would be expected from the material removal. This is due to the fact that the weight of blasting particles lodged in the surface (detail) is greater than that of the removed material. Only as material removal continues does the loading effect cease (diagram). The loading effect can influence the subsequent finishing steps, as well as the engine.
Positive influences: The improved wettability of Ni-based alloys through high-temperature solders made from the same material is well-known (Ref. 18.104.22.168-2). The intense reactions between SiC and Ni-solders at high temperatures should make themselves known.
Negative influences: Subsequent finishing steps such as diffusion welding or diffusion coating can be compromised by the remaining blasting particles (Fig. "Oxidation protecting Al diffusion coating problems"). This can result in flaws in anti-diffusion coatings, which can promote local oxidation.
The ability to build up vapor deposition layers, such as PVD thermal barrier coatings, can be impaired.
Particles may break loose in the engine and enter the oil circulation system, where they can cause fatigue damage on the bearing tracks.
Figure "Abrasive blasting problems": Abrasive blasting processes can be problematic. They are often a prerequisite for subsequent finishing steps and influence the latter.
Especially important factors are the specified process parameters and shot material. If the shot carries wear products and fouling, these can have several different damaging effects (Chapter 22.214.171.124). Increased temperatures can cause and/or exacerbate damage. Brittle ceramic shot shatters upon impact, altering the grain size distribution. Therefore, the specifications must be adhered to through constant sorting and monitoring of the shot.
Roughing of the surface can complicate penetrant testing through background fluorescence. Related problems occur when flaws such as cracks and cavities are closed or plugged (Fig. "Opening of cracks before penetrant testing").
The dynamic fatigue strength can be compromised by dangerous groove-like notches caused by “glancing shots” (Fig. "Damages by cleaning blasting").
Shot material that has become stuck in the work surface (loading effect, Fig. "Loading effect by blasting processes") can also have a negative influence.
If shot material plugs up flow ducts (bottom left diagram), operating damages such as overheating of hot parts or bearing damage (bottom right diagram) are typical results.
Shot material, such as SiC, that reacts with titanium and nickel alloys can have a damaging effect during heat treatments. This effect is especially problematic if shot residue remains inside turbine rotor blades.
Figure "Damages by cleaning blasting": Similar to shot peening (Ills. 126.96.36.199-11) or extreme erosion in an engine (Volume 1, Ill. 5.3.2-5), abrasive blasting can cause sharp, thin-walled blade edges to roll over (top right diagram). This damage can be prevented through the use of an angled jet direction (left diagram). An axial jet position parallel to the blade profile increases the probability of dangerous axial notches through glancing shots (bottom detail). These can cause fatigue cracks in case of flexural vibrations of the blade. On the other hand, an axial orientation of the roughness has aerodynamic benefits (Volume 3, Ill. 188.8.131.52-9.1)
= References =
184.108.40.206-1 P.Adam, “Fertigungsverfahren von Turboflugtriebwerken”,Birkhäuser Verlag, 1998, ISBN 3-7643-5971-4, pages 105-112, 245.
220.127.116.11-2 ASM “Metals Handbook”, “Volume 5 - Surface engineering”, ISBN 0-87170-377-7, 1999, “Shot Peening” pages 126-135, “Abrasive Cleaning” pages 55-66, 781.
18.104.22.168-3 J.Horowitz, “Das `Shot-peening'-Verfahren”, periodical “Metalloberfläche” 32 (1978) 7, pages 285-292.
22.214.171.124-4 Ch.W.Fabry, “Kugelstrahlen, Theorie-Versuche-Praxis”, periodical “Konstruktion” 17 (1965) Volume 4, pages 141-153.
126.96.36.199-5 publication of the Metals Improvement Company, “Shot Peening”, pages 3-47
188.8.131.52-6 ATSB, Aviation Safety Investigation Report 200205780, “In-flight uncontained engine failure and air turn-back…”, December 8, 2002, ISBN 1877071 83 8, pages 1-43.
184.108.40.206-7 L.Engel, H.Klingele, “Rasterelektronenmikroskopische Untersuchungen von Metallschäden”, Carl Hanser Verlag, 1982, ISBN 3-446-13416-6, pages 179, 180.