Case Study On-Site Brazing Audit Improves Client s Joint Quality and Optimizes Brazing Process Overview of Process

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1 Case Study On-Site Brazing Audit Improves Client s Joint Quality and Optimizes Brazing Process The Lucas-Milhaupt Technical Services Group recently conducted an on-site audit of a client s torch brazing operation. This equipment manufacturer uses a brazing process to join steel framing members. The client requested this audit because of an issue with braze assembly rejections. They saw voids along the exterior of the braze joint that appeared after coating, which meant the assemblies had to be reworked or scrapped. The client s objective was to confirm proper brazing procedures and then optimize the process for improved outcome. The audit report consists of six main sections following the Six Fundamentals of Brazing. These categories are proper fit and clearance, clean metals, proper fixturing, proper flux, proper heating and final cleaning. Additional topics are included, detailing discussions with the client during the audit. Overview of Process The client employs flame braze equipment to braze assemblies. Several operators utilize this equipment, depending upon schedules and product demand. Lucas-Milhaupt Easy-Flo 45 is the alloy used for brazing, primarily in ring form, along with Lucas- Milhaupt Handy Flux in paste form. Easy-Flo 45 and Handy Flux are a common alloyand-flux combination for open-air steel brazing. I. Proper Fit and Clearance Proper joint clearance for torch brazing falls within the range of ( mm). Typically, clearances between components brazed in the facility are held in the ( mm) per side range, depending on specific internal drawing requirements. During the audit, examination of several parts confirmed that clearances appeared to be within this range. The exception was the area in which the braze alloy ring was placed. This ring groove was in excess of (1.27mm) and often retained a large amount of alloy, even after heating. A cross section of a braze joint with a large ring groove exhibited a large void, and some retained braze alloy. By definition, clearance is the distance or gap between the two base materials being joined. This joint gap is the critical area of a brazed joint that is filled with the brazing alloy. In brazing, clearance has a direct effect on the capillary action that pulls the braze alloy through the joint area. Capillary action disappears when the joint gap exceeds

2 0.012 (0.305mm). When the assembly is up to brazing temperature, the braze alloy should flow through the joint interface completely. Clearances also have a great impact on the mechanical performance of a brazed joint, so maintaining consistent clearance is very important. Figure 1 shows the relation between clearances and overall joint strength. Figure 1: Joint Strength vs. Joint Clearance The joint interface of the brazed joint is the key determinant in a successfully brazed joint, not the fillet. The fillet that is formed on the exterior of the brazed joint accounts for very little braze joint strength and joint integrity. If the filler metal flows completely through the joint interface, the joint integrity and strength will be the highest. The client s current joint fit seemed acceptable and was properly controlled by component drawings, but we recommended that the joint clearance be periodically checked to confirm that the proper clearance was consistently maintained. In addition, we recommended that standardizing joint clearances be considered. Although all the assemblies Lucas-Milhaupt observed were within recommended clearances, the clearances did range from part to part. One part exhibited a maximum clearance of (0.076mm) per side while another exhibited a maximum clearance of (0.1143mm) per side. Standardizing clearances should make the manufacture of these assemblies easier and may allow some standardization of the ring size and wire diameter. We also recommended that the groove be redesigned or removed completely. II. Clean Metal The client primarily brazes steel in its facility, and the majority of the components are machined prior to brazing, using a lubricant. It is essential that these components are free of any oils or lubricants that may be present from a previous manufacturing process. During the audit, the base metal surfaces were wiped down with no residue appearing on the tissue, as shown in Figure 2, indicating that the components had been thoroughly cleaned prior to brazing. Immediately before brazing, the braze operator observed if the components were free from residue and oxidation. From a brazing perspective, clean base metals are critical. Braze alloy only flows properly when the surfaces of the base metals are clean and free from oxides. Any oil, dirt, grease or cutting fluid residue inhibits the flow of the braze alloy by increasing the surface tension on the surface of the base metal. We recommended that occasional white glove/tissue tests be performed on incoming parts to ensure consistent cleanliness of the components. Mechanical cleaning could be employed to remove

3 some of the surface oxide on the base metal surface prior to brazing. The rougher surface produced by this mechanical cleaning also improves wetting and strength. Recommended surface roughness is between 30 and 80 microns (RMS). III. Proper Fixturing Limited fixturing was used during the brazing process. Self-fixturing methods (gravity held) were employed to directly hold the mating tube-to-tube components together during heating. An adjustable fixture held the entire assembly in place and positioned the joint line evenly within the heating profile. There was minimal contact between the joint area and the fixture, which limited the amount of heat sink effect produced and also allowed the torch flame to thoroughly heat the assemblies. The current setup used for holding parts in place during heating appeared to provide proper alignment of the base components during heating. We recommended that current processes be maintained. IV. Proper Flux Easy-Flo 45 rings were placed in a ring groove in the tube-to-tube assembly prior to brazing. Handy Flux was then applied by brush to the steel surface. The consistency/viscosity of the flux applied by the operator was acceptable, but the physical amount of flux applied to the joint surface was minimal. Flux was not in contact with both base materials along the entire joint interface. Employees indicated that this was done intentionally to limit alloy flow and that occasionally, the flux was diluted/thinned down using tap water. During torch heating, gas flux was also applied to the joint region through the flame to improve alloy flow and joint cleanliness on the exterior of the assembly. Note that gas flux does not exhibit much joint penetration and does not necessarily improve internal alloy flow. We recommended that the amount of flux and the coverage area of the flux be increased. Flux helps prevent and dissolve oxidation from a base metal s surface. Areas where the flux was not applied oxidize readily during heating and limit braze alloy flow. If the amount of braze alloy flow is a concern, braze joint designs should be re-evaluated so that only the desired length of braze alloy flow is achieved. Limiting the flux application to the joint creates inconsistency in alloy flow and lowers joint quality. It is common to see flux dry out over time, and reconstitution of the flux is recommended, but this should be done with distilled or de-ionized water to prevent contamination. V. Proper Heating The client uses a flame-brazing machine to heat steel tubes during brazing. Four burners positioned at the joint line, oscillating back and forth, perform heating. This oscillation helps ensure even heating throughout the entire joint length. All assemblies

4 were heated uniformly to an appropriate temperature and allowed proper alloy flow to the outside of the joint. The heating cycle and the temperature of the assembly were controlled by an infrared pyrometer. After heating, the burners were moved away from the assembly, and the braze alloy was allowed to solidify naturally prior to movement of the assemblies. Uniform heating is one of the most critical aspects of brazing. It is essential that the assembly be heated uniformly to the braze temperature. Braze temperature is considered to be above the liquidus temperature of the braze alloy. If the braze joint area is not up to braze temperature, the filler metal does not flow properly into the joint area. The brazing alloy only flows onto surfaces of the base metal that are at temperature. The client s heating profile provided uniform heating and should be continued. VI. Final Cleaning After the heating cycle was completed, the brazed assemblies were allowed to cool for a short time before being placed in a water bath adjacent to the braze machine. The parts were allowed to soak in the water for a time sufficient to remove the majority of flux residue. We observed that the water solution was dirty. The water solution was changed periodically when it appeared to be saturated. When using brazing flux, post-braze cleaning is important because flux residue is corrosive. Cleaning/quenching procedures appeared to leave the material surface clean of flux, but we recommended that the tube assemblies not be quenched until the braze alloy had completely solidified. All red color in the assembly should be gone prior to the water quench. The water bath solution should be changed on a regular basis. If the solution becomes saturated with flux, little flux residue is removed. To improve the removal of the flux residue, the water could be heated to F ( C). Other Considerations Ring Groove and Ring Placement The client uses grooves machined into the assembly to retain the braze alloy ring during heating. While providing proper alloy placement, these grooves inherently cause brazequality issues. The alloy flow out of the groove is often inconsistent, with some alloy remaining inside of the groove. When this occurs, there may not be a sufficient amount of alloy present to completely fill the braze region. These ring grooves can also retain flux and possibly trap gas inside of the joint. This may be causing the voids in the joint region after coating. The ring groove was located very close to the outside edge of the braze joint. Entrapped flux or gas inside of the ring groove could be outgassing during the coating process and cause gaps/voids along the joint line. Figure 3 shows an example of a ring groove similar the client s while Figure 4 shows the preferred joint design(s) that we discussed during the brazing audit.

5 Eliminating the ring groove should help improve overall joint quality and limit the voiding seen after coating. Figure 3: Current Alloy Placement (Ring Groove) Figure 4: Preferred Alloy Placement (No Ring Groove) Conclusions The majority of the braze practices performed were acceptable, but there were improvements to be considered. We suggested the client: 1) Standardize joint clearances across all assemblies. 2) Consider removal of the ring groove, and re-evaluate ring placement and braze joint design. 3) Apply brazing flux across all joint surfaces. 4) Allow the materials to lose all color prior to water quenching to avoid affecting the joint strength and quality. 5) Change the post-braze cleaning bath on a regular basis. Lucas-Milhaupt recommended these improvements to increase quality and consistency of the client s brazed assemblies for more robust joints with reduced failure and scrap. Questions about brazing? Contact us for further assistance. For detailed questions regarding specific applications, please call the Lucas-Milhaupt Technical Services Department at