Nicrobraz Technical Articles Library
Grit Blasting vs. Ni-Plating for Superalloy Aerospace Components?
Question:
We are currently nickel plating our Rene base materials prior to brazing. But this is a very expensive and tedious operation. After reading (Nicrobraz News, Summer '92) about the use of NicroBlast Grit as a means of preparing base metals for brazing, we have several questions. What air pressure should we use? Do we need to dust-off or degrease the braze joint before or after blasting? What happens if grit gets caught in the joint and is not removed prior to brazing, and how long can NicroBlast Grit be used before it must be replaced?
Answer:
Grit blasting base metals prior to brazing improves the wet-ability of the surface for the brazing filler metal (BFM). The grit "works" the surface, essentially "peening" it, which helps the BFM wet the surface better (spreads out further) than the original untreated surface. Nickel plating also improves surface wet-ability; one of its main advantages is that it covers, and thus, prevents oxidation of, metallic elements such as aluminum and titanium in the base metal.
Figure 1 shows qualitative comparisons of how a super-alloy base metal surface was wetted by a nickel BFM after the super-alloy surface was prepared by several different techniques. The wetting angle (0) shows how well a surface is wetted by a BFM. The smaller the angle, the better the wetting.
We normally recommend that a 100 psig air pressure be used in your blasting operation. We also suggest that you use a pressure-type blaster instead of a suction-type. In a pressure-type blaster, air pressure powers the gun as well as forces the blasting particles through it; versus the suction-type unit, where the exhaust fan pulls the air through the blaster.
Caution: When blasting thin sheet metal parts, it may be necessary to lower the air pressure to prevent distortion of the parts. In some cases, the thin material should not be blasted at all (such as a material that may distort excessively even under light pressure). For example, when blasting a 1" diameter tube with a 0 032'1 wall thickness, the O.D. of the tube will increase in size as the blasting time is increased. This is due to the compressive stress in the surface which results from continuous blasting (peening). See Figure 2.
We strongly recommend that any parts which are to be blasted be thoroughly degreased before blasting. Be sure the degreasing fluids are clean, and that all oil and grease have been completely removed from the parts prior to blasting.
After the parts have been degreased and blasted, we recommend that they be dusted with a clean cloth prior to BFM application. If the parts have been tack-welded prior to being blasted, there will probably be some particles of grit lodged in the joint area. If the grit particles cannot be easily removed, and the blasting media used was NicroBlast Grit, it is acceptable to leave them in place because the nickel BFMs used in your brazing operation will dissolve (or bond to) any NicroBlast particles. Since they are non-contaminating, they will not interfere with the brazing operation or joint properties. However, if the grit particles are non-metallic (aluminum oxide, glass bead, etc.), voids may result in the area of entrapment.
The blasting cabinet should have an adequate air sweep to ensure that all fine particles are continuously removed from the cabinet during the blasting operation. While blasting is taking place, the NicroBlast Grit will be broken up into finer particles. As long as the parts being blasted are free of all oxides, oil, grease, carbon and other contaminants, then it is not necessary to replace the NicroBlast Grit. It is only necessary to remove the dust created by the continual wearing away of the blasting grit, and to add grit to the cabinet, as required. An interesting experience occurred some years ago, which illustrates the importance of air sweep. Although some parts, which were free of all contaminants, had been blasted, the blasting cabinet did not have adequate air sweep. Consequently, the fine particles, which were being created from the breakdown of the NicroBlast Grit, continued to buildup until they resembled dark talcum-like powder, which felt like pasty carbon to the touch. This film coated the parts being blasted and prevented any further effective blasting operations. Even though we had presumed that the parts were oily and greasy, the contamination was merely due to our failure to remove the fine dust that was being created by continual breakdown of the grit over time.
Other considerations. Some Rene alloys contain large amounts of titanium and aluminum (about 8% total). This raises the following items:
NicroBlast Grit is very effective if the brazing temperature is maintained at 1950° F (1066° C) after blasting. If the parts are brazed at 2150° F (1177° C) instead, the aluminum and titanium diffuse to the surface much faster, forming an oxide layer which may inhibit the wetting action of the BFM. Thus, the brazing temperature will have a large effect on the results of the blasting operation (the higher the brazing temperature, the higher the risk that the blasting operation may not remain effective).
- The atmosphere quality will also effect the results of the blasting operation. When the brazing furnace atmosphere quality is exceedingly good (maximum leak-up rate of 1-to-2 microns per hour, and an extremely clean furnace with no visible surface contamination on any inside surfaces), these base metals can be brazed immediately after the degreasing operation, with no other surface preparation required. However, there are very few furnaces that have this capability.
- Where the furnace atmosphere quality is less than perfect, the presence of any contamination in the furnace and/ or small leak in the furnace can result in oxidation of both the titanium and aluminum content in the base metal. This oxidation may occur so rapidly that the blasting operation could be rendered totally ineffective (particularly with base metals in which titanium and aluminum levels are high).
Hopefully this information provides you with valuable insights into both the strengths and limitations of NicroBlast Grit, and how to maximize its effectiveness in your operations.
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