As aluminum brazing continues to grow, some folks are asking: “Can I use available furnace time in my aerospace vacuum furnace to perhaps do some aluminum brazing?” There are a number of reasons why that is a VERY bad idea, but I will briefly review just two of the more obvious ones below.

Temperature control – Shown below is a photograph of a typical hi-temp aerospace vacuum furnace metal hot zone. Notice that there are six (6) heating elements connected around the OD of the hot-zone, as well as heating elements on the back wall of the furnace. By Dan Kay

Picric acid (2,4,6-trinitrophenol, [(NO2)3C6H2OH]) is widely used in metallography labs for the common steel etchants known as picral, a 4% solution in ethanol, Vilella’s reagent, 1 g picric acid and 5 mL HCl and 100 mL ethanol,  and alkaline sodium picrate (2 g picric acid, 20 g NaOH, 100 mL water) for coloring M3C and M6C carbides, as well as several other formulations.

Picric acid was formulated by Peter Woulfe, a British chemist, in 1771, although Glauber is claimed to have written about it in 1742. The name comes from the Greek word pikros which means bitter, as picric acid has a bitter taste (it is toxic). Initially it was used to dye fabrics yellow. In the early 20th century, workers producing picric acid were sometimes called canaries, because their skin also became stained yellow.  By George Vander Voort

Numerous etchants have been used to selectively reveal matrix phases and second-phase constituents in ferritic, martensitic, ferritic-martensitic, austenitic, ferritic-austenitic (duplex) and precipitation hardenable stainless steels. Procedures for identification of second-phases, such as carbides, sigma and chi, and delta ferrite in austenitic or precipitation hardenable stainless steels using selective etchants are described.

The microstructural constitution of stainless steel is quite complex and exposure to high operating temperatures adds to the complexity as a variety of phases can be observed. In addition to the matrix phases of ferrite, austenite and martensite, and duplex austenite-ferrite and (less commonly) ferrite-martensite, there are numerous possible minor constituents.  In the carbide family, M₂₃C₆ (face-centered cubic) and M₇C₃ (hexagonal) carbides are the most common, but M₆C (face-centered cubic) and MC (face-centered cubic) carbides, are also observed in certain alloys. Certain nitrides may be observed and the intermetallic phases, sigma, σ (tetragonal), chi, χ (body-centered cubic) and Laves, η (hexagonal). By George Vander Voort

As the name of this product-type indicates, a brazing “stop-off” is supposed to be something that “stops” molten brazing filler metal (BFM) from flowing into areas where it is not supposed to be, thus keeping it “off” surfaces that are supposed to remain clean and free from the presence of any BFM.

First of all, brazing filler metals (BFMs) do not like to bond to (or flow over) dirts, greases, or oxides.  The presence of any of these contaminants on the surface of parts to be brazed can literally prevent the BFM from flowing over surfaces on which any of these contaminants are located. By Dan Kay

Next month:  As aluminum-brazing continues to grow, some folks are asking: “Can I use available furnace time in my aerospace vacuum furnace to perhaps do some aluminum brazing?”  Let’s look at why that is a VERY bad idea!

The paper describes the approach taken in the author’s first metallographical study of plasma sprayed coating. After consideration of the preparation problems known to be associated with porous materials, the coating materials and the substrate material, and coatings in general, literature on metallographic studies of plasma sprayed coatings was identified, obtained and reviewed.

A coating section of each type was carefully removed from the substrate, refrigerated in liquid nitrogen, broken and examined with the SEM to reveal the nature and degree of porosity and bonding defects. Specimens were prepared using two different approaches and two variations of one of the approaches. Polishing was conducted using DP-Plan cloths and two different diamond sizes followed by two stages of diamond polishing with conventional cloths and finally using colloidal silica on a vibratory polisher. Vibratory polishing time must be critically controlled for two of the three coatings. Conventional etchants were tried to enhance microstructural detail but the Pepperhoff interference layer method was found to be more useful. By George Vander Voort

Color has historically seen limited use in metallography, mainly due to the cost of film and prints and the difficulty and cost of reproducing images in publications. However, with the growth of digital imaging, capturing color images is much simpler and cheaper. Also, printing images in color is inexpensive for in-house reports, and can be distributed cheaply on CDs, although reproduction in journals is still expensive. Color does have many advantages over black and white. First, the human eye is sensitive to only about forty shades of gray from white to black, but is sensitive to a vast number of colors. Tint etchants reveal features in the microstructure that often cannot be revealed using standard black and white etchants. Color etchants are sensitive to crystallographic orientation and can reveal if the grains have a random or a preferred crystallographic texture. They are also very sensitive to variations in composition and residual deformation. Further, they are usually selective to certain phases and this is valuable in quantitative microscopy. By George Vander Voort

(Courtesy of the Microscopy Society of America – www.microscopy.org) – Original Published by Microscopy Today, November 2005 – Volume 13, Number 6 – www.microscopy-today.com

This is Part 2 of the article Shelf-Life and Expiration-Dates for Brazing Paste

Brazing paste is not difficult to make yourself. All you need is some brazing filler metal powder, a gel-binder, and a paint-shaker. Sound easy enough? Let’s see…

First, procure the desired brazing filler metal (BFM) in powder form from one of the BFM manufacturers.· I show a listing of such manufacturers on my website at http://www.kaybrazing.com/sources.htm, and each company’s name is a “hotlink” to that company’s webpage. By Dan Kay

Next Month: In next month’s article, we’ll consider brazing stop-offs, and how they are correctly and incorrectly used in many brazing shops today!