ou have to get to your customers before trouble starts.” So says Don Paris, president of Parfuse Corporation, Westbury, N.Y. Paris refers to customer engineers who design parts to be brazed. Parfuse, a job-shop brazer, specializes in dip brazing and heat treating of 6000-series-aluminum-alloy assemblies.

The Parfuse shop, a 9,000-square foot single-story building staffed by 35 employees, brazes subassemblies for heat exchangers, cabinets, tube assemblies, and complex components for customers in the aerospace and avionics, electronics, medical-equipment, automotive, and instrumentation industries. Business averages $2.5 million annually.

Convinced that an understanding of brazing operations is key to design of brazed components, Paris hosts customers’ design engineers for one-day hands-on training at his shop. Says Paris, “They come into the shop to see what goes into brazing an assembly, and they get an education so they can design brazements that we can produce.” No customer is too big to learn: engineers from ITT, IBM, and other fortune 500 manufacturers have spent a day at the Parfuse shop.

One goal of this program is to persuade customers’ engineers to design joints that are self-fixturing. Tabbing and slotting of parts minimizes fixturing for brazing and improves tolerances on the final product. Typical tolerances on parts brazed at Parfuse is plus or minus 0.001 to 0.005 inch.

A first-piece prototype for a cabinet for medical equipment. A Parfuse engineer evaluates the assembly design for ease of brazing and purposes of brazing procedure. Note the tabbed fitup, on the right side of the assembly, typical of good design for brazing.

Base materials, fillers

Parfuse brazes 6000-series aluminum, mostly alloy 6061. This is a heat-treatable alloy that gains its stength and hardness from precipitation hardening after fabrication. Composition: 0.4–0.8 Si, 0.7 Fe. 0.15–0.40 Cu, 0.15 Mn, 0.8–1.2 Mn, 0.04–0.35 Cr, 0.25 Zn, 0.15 Ti. Stock comes in T-0, -4, or -6 tempers. After brazing, the parts are in an annealed state and undergo heat treatment to restore hardness and strength.

Fillers are Alumibraze or MarBraze (AWS A5.8 BAlSi-4) from Omni Technologies Corporation, Exeter, N.H., a powder that Parfuse brazers mix with water; clad sheet; or alloy 4047 wire or foil. The clad material is based on alloy 6051 or 4047.

Clad sheet finds use on assemblies where joint areas are wide, requiring broad, uniform distribution of filler for good, uniform bonding. The first choice for filler is wire, Paris told us, since wire-filled joints show little porosity.

One of several cleaning tanks at Parfuse, this one 4-1/2 feet square and 3 feet deep, contains hot water to rinse off materials following immersion in a nitric-acid-bath solution.

Starting a job

New customers typically send a drawing of an assembly and ask for a quote. At this point, Paris often recommends modifications to improve part design and reduce fabrication costs.

Having garnered the job, Parfuse waits for material to arrive per the customer order. Customers normally supply their own material as raw stock for fabrication or as partially fitted assemblies ready for brazing. Incoming material undergoes inspection for documentation, cleanliness, and dimension and chemical test to check alloy composition.

Readying parts for brazing

Because every brazed product is different and heat-flow pattern varies with thickness, material, and part geometry, braze schedules are unique to each job. To prescribe a brazing procedure, Parfuse operators run a test product, starting with a procedure based on experience. Inspecting this first brazement, they adjust cycle times and temperatures to obtain a smooth, well-fused braze fillet. When the part proves satisfactory, both in integrity of the braze joint and dimension of the brazement — distortion and subsequent straightening should be minimal — production proceeds.

The next step is cleaning of the raw stock, done to Parfuse in-house specs that meet MIL requirements and other customer requirements. This step entails degreasing in caustic solution followed by a nitric-acid-bath rinse and hot-water rinse.

Then material undergoes assembly, which may necessitate gas-tungsten-arc (GTA) tack welding, and application of the filler material. Power for welding comes from one of three Miller Synchrowave 250 sources, at 275 A ac.

For critical brazements, the assembly is cleaned after GTAW to remove oxides that may have formed during welding. At this point the assembly undergoes pre-braze inspection to assure that all parts are in place and aligned and that braze filler is properly applied.

Now assemblies load onto dip baskets. Fixturing, of type 300 stainless steel or Inconel, is costly, so Parfuse strives to keep fixtures simple and multi-purpose. This is where use of twist tabs and other self-locking joints save time and material.

Prior to brazing, parts go into an electric furnace, one of six in the shop supplied by Grieve Corporation. Here they preheat for 3 to 45 minutes, length of time depending on the mass of the part, to just 30 degrees below the melting point of the filler metal, 1,000 F to 1,040 F. Controls hold temperatures to plus or minus 10 degrees. This process readies the assembly for the dip bath, avoiding cooling of the salt bath and minimizing distortion and thermal shock to the assembly.

Into the bath

Dip brazing follows, done at 1,100 F and held to a tight plus or minus 5 degrees Fahrenheit. Operators submerge parts in one of two Ajax brazing pots, 27 by 27 by 39 and 24 by 24 by 36 inches. The bath solution is a mixture of chlorides (sodium, potassium, and lithium) and fluorides (sodium and aluminum), supplied by HeatBath Corporation, a division of Park Metallurgical. Bath heating is internal, by electric-resistance elements.

In the salt bath, the bath medium contacts the work evenly, heating it uniformly by conduction to minimize distortion. Brazing is rapid — salt-bath brazing proceeds four to five times faster than atmosphere-furnace brazing, resulting in less thermal degradation of the brazement at half the cost of furnace brazing.

To meet specifications (ASTM E-960), samples of the bath material go monthly to an outside test lab for analysis. Other than this, shop workers, alert to effects of bath composition on brazement quality, replenish the bath as necessary. This calls for “a trained eye,” Don Paris told us. “Our employees can tell what’s happening in the bath by looking at the brazements that come out of it.”

Following brazing, parts undergo inspection. Inspectors look closely for pinholes. Parts then undergo straightening if necessary, followed by grinding and light finishing.

An operator GTA tack-welds an assembly prior to dip brazing. He uses aluminum alloy 1100 filter rod for tacking. If deformation is unacceptable after welding, straightening follows.

A heat-exchanger part for a guide for an electronic board takes braze paste applied by hand. The paste is a water solution of BAISi-4 powder.

For fixturing and holding devices, of costly stainless steel or Inconel, simpler is better. Above, multi-purpose dip racks hold braze assemblies. Below, three ceramic blocks hold parts in place.

An operator lowers a brazement-laden hanger into the dip-brazing bath.

A typical Parfuse product, this cylindrical heat exchanger uses aluminum brazing foil, just visible here wrapped around the concentric shells, to join the corrugated fin material to the shells.

Nothing surpasses the discerning trained eye. At the end of the brazing line, a trained inspector eyes every joint prior to shipment.

On to heat treating

Following brazing, parts move on to heat treatment. After brazing, heating brings alloying elements — copper, magnesium, zinc, and silicon — out of solid solution to harden and strengthen the assembly. All fitting, straightening, and other mechanical deformation must be done prior to heat treatment, while the material is ductile; following heat treatment (also called artificial aging, precipitation-hardnening), when the alloy reaches maximum hardness and strength, no further mechanical deformation is possible.

Brazements now undergo cleaning again — by grinding or glass-bead blasting. Ultrasonic cleaning removes trapped flux from critical heat exchangers.

Quality wins always

All work in the shop, commercial and military, is done to military specifications: MIL-B-7883 (brazing); MIL-H-6088 (heat treatment); MIL-I-45208 (inspection); and MIL-STD-45662A (calibration). Grade A brazements undergo x-ray inspection, Grade B 100-percent visual inspection.