Essential Criteria for Brazing: Item 2 – Proper cleaning—role of grit blasting in surface-prep (Part C)

As shown in my last three articles, there are a number of very important steps that must be followed in order to ensure good brazing, as shown here once again in Table 1. We’ve looked at some proper design criteria for brazing, and the last two articles have covered some important aspects of cleanliness in brazing. In this current article, we will explore grit blasting and tumble-deburring as common methods many companies use to prepare surfaces prior to their being assembled for subsequent brazing.

Grit blasting

Grit blasting surfaces involves placing parts in a special chamber, as shown in Fig. 1, and then using a high-pressure stream of hard particles being blasted at the surface of the part in order to remove certain surface contaminants, such as surface oxides, that would otherwise interfere with good brazing, i.e., would prevent the molten brazing filler metal (BFM) from being able to effectively alloy with, and bond to, the base metals onto which it is being applied.

IMPORTANT: Never use a grit blasting operation for removal of greases, lubricants, or oils. They are completely ineffective for that.

Grit blasting does NOT replace degreasing operations since grit blasting cannot remove such oily surface contaminants. Instead, the grit blasting would drive those ingredients into the surface of the metals, making them even more difficult to braze! Additionally, such use of a grit blaster will quickly contaminate the grit in the blast-cabinet, rendering it useless for effective removal of surface oxides. It might also so contaminate the blasting cabinet itself, that the cabinet must be shut down and thoroughly cleaned to remove the oils/lubricants, etc., that have now contaminated all surfaces in the blast cabinet, including its pumping and circulating systems.

There are a number of gritty blasting media (grit) in common use today, as shown in Fig. 2. In my experience, the most commonly used grits in the brazing industry that I have observed are those composed of:

1. aluminum oxide
2. silicon carbide
3. sand
4. glass beads
5. stainless steel
6. chilled cast iron
7. nickel-alloy grit
8. dry ice (see the end of this article)

As has been discussed in previous articles, brazing filler metal (BFM) does not like to bond to or flow over, oils, dirt, greases, lubricants, or oxides. If any of those contaminants are present on either of the joint surfaces to be brazed, it can render that joint non-brazeable.

Any of these surface contaminants, as described above, could greatly weaken the integrity (strength) of any braze, and could also greatly increase the number of voids between the joint surfaces.

Thus, it should be the goal of all brazers to keep any such contaminants off the surfaces to be brazed!

REMEMBER — Whatever you blast with WILL be on the surface of the parts!

So, looking at the list of potential blasting media available to most brazing shops, are any of them potentially bad for brazing? Yes.

Any NON-METALLICS used for grit blasting can leave a residue that will not alloy with the metals in the joint area (i.e., with the base metals and the liquid BFM). Such non-metallic residues can remain as hard inclusions in a joint or might outgas at braze temp,etc., thus, forming voids in the brazed joint. Items 1 through 4 in the list above are non-metallics and can cause such negative effects inside a brazed joint. Thus, aluminum oxide grit, silicon-carbide grit, sand, and glass beads are all non-preferred grits for preparing surfaces for brazing, since they can, and will, leave surface residues that will negatively affect brazing.

RECOMMENDATION: Always use a metallic grit, if possible, to blast any surfaces for brazing, since any metallic grit residue will dissolve into, and alloy with, the molten filler metals, thus minimizing any negative effect from any such residue.

NOTE: Grit blasting is generally not done on any aluminum components to be brazed, so it should not be an issue. But, if grit blasting were desired, then consideration might be given to dry ice grit blasting (as discussed further down in this article).

The metallic grits (stainless steel, chilled cast iron, nickel-alloy grit) are very effective for use in blasting cabinets used to prepare many industrial and aerospace components that are made from a wide range of steel alloys and other hard aerospace metals.

PRESSURE. Be careful that the pressures used in any grit blasting operation will not damage the metal being blasted. This is especially true for thin metal structures. Blasting thin surfaces will warp those surfaces, potentially damaging them to the point that they cannot be used in production.

Cleaning surfaces after blasting.

a. Ultrasonic cleaning

The use of ultrasonics can be highly effective at removing any trapped blasting media from the tiny “nooks and cranies” on the faying surfaces of the parts to be brazed. Ultrasonics can thus be also effective at removing most of the residues that may remain after blasting with alumina or silicon carbide. Sand blasting and glass-bead blasting leave glassy, smeared-on residues that may not be effectively removed even by ultrasonics.

NOTE: Ultrasonic cleaning has been discussed and reviewed recently in one of my previous articles on this website.

b. Rinsing

If you do not have any ultrasonic equipment available to you, then you may find it wise after grit blasting to wash parts in a good alkaline-bath, or in a high pressure water stream, in an effort to remove any grit blasting residue from the surfaces to be brazed.

WATER TYPE — It is STRONGLY recommended that only deionized (DI) water or Reverse Osmosis (RO) water be used for cleaning parts, since such water can be carefully controlled in your plant from day to day, to ensure that good quality, clean water is used for rinsing operations. Too many problems can occur with city-water (it has sometimes become so contaminated that cities have had to warn their residents to boil the water prior to use. Fortunately this is rare, but it can happen.) Similarly, well-water or “softened-water” contains too many minerals such as calcium, salt, etc., some of which are actually great “brazing stop-off” ingredients, which can then hurt brazing significantly. It is not necessary to go to the extent of using distilled water, since making it can be a very slow process, and buying it in bottles can be quite expensive.

Dry-Ice grit blasting

Dry-ice grit-blasting is a unique grit-blasting media that can be used for smaller, or more delicate metallic parts. Dry-ice grit blasting uses crushed dry-ice (frozen carbon dioxide), which is much softer than metallic grits. The dry-ice particles are accelerated toward the surface at near supersonic speeds, and when these tiny particles hit the surface they can literally cause tiny explosions at the surface, which “blasts” away the undesirable surface contaminants, such as light surface oxides. The medical industry has found this method to work well for some of their needs. This can also be readily used on aluminum parts, since it will clean off the surface, and leave it smooth and shiny, with no “matte” finish.

A big benefit of this process is that the media itself instantly vaporizes (sublimates), leaving no blasting-grit residue. The only residue is that of what was physically removed from the surface, such as oxide, or paint, etc.

An example of dry-ice grit-blasting equipment is diagrammed in Fig. 3, where a dry-ice grit blasting unit is being used to remove paint from a concrete wall.

Dry ice grit-blasting equipment is available in a variety of sizes from table-top units used in laboratories, all the way up to large units on wheels (such as shown in Fig. 3) for use out in the field.

There are many videos online to see how this process works. It’s fun to watch. Merely go online and type in “dry-ice grit blasting” or something similar, and you’ll be surprised at the amount of information you will see regarding this fascinating, growing process.

CONCLUSION

Grit-blasting can be a very effective method by which metallic surfaces can be prepared for brazing by removing surface oxides. Because of the softness of aluminum parts, and many copper parts, these procedures may not be common for either of those two soft metals. But for steel parts, stainless, super-alloys, etc., grit-blasting surfaces with a metallic grit is commonly used.

As discussed in this article, be careful that any non-metallic blasting grit is NOT used, since they can leave a non-brazeable residue on the surface of the parts. Instead, use a stainless or nickel-alloy metallic grit when grit-blasting.

The exception to some of this is when dry-ice is considered for the process since it leaves no residue of its own. However, being a soft blasting media may render it useful for aluminum and copper products, but it’s effectiveness at blasting away oxides needs to be evaluated on a case-by-case basis.

Finally, remember again that the purpose of grit-blasting is NOT to remove oils and lubricants, which needs to be done with effective degreasing solutions before proceeding with any grit-blasting operations.

In the next article in this series; “Essential Criteria for Brazing: Item 2 – Proper cleaning—role of tumble deburring in surface-prep (Part D)” we will complete this review of cleaning procedures for braze-prep, looking at tumble-deburring operations and some of the precautions that need to be observed for that method of surface-prep prior to brazing.