Vacuum Aluminum Brazing – What Matters Most
Vacuum aluminum brazing is a careful balance of time, temperature and vacuum level. These parameters are controlled to maintain the fundamental brazing success parameters – load the parts, heat the parts, get the braze joints clean, melt the braze filler and get the parts out.
Vacuum aluminum brazing is done in a specific work environment utilizing sophisticated controls to ensure fast pumping, low parts per million (PPM) of oxygen and exceptional temperature uniformity combined in one synergistically designed vacuum furnace system.
Types of Aluminum Brazing
Aluminum brazing can be done with or without flux and includes many different methods for creating the bond.
In flux brazing, the flux flows into the joint and is displaced by the liquidus filler metal entering the joint in order to remove oxides on the part to create a strong, solid braze. Flux comes in several different forms: paste, liquid or powder. Some brazing rods are coated with flux or have a flux core in order to apply necessary flux during the brazing process. Flux brazing processes include torch brazing (manual and automatic), induction, salt bath (dip brazing) and controlled atmosphere (CAB).
Brazing performed in a vacuum furnace is considered fluxless brazing because it does not use flux to create the joint. Fluxless brazing processes can be performed using inert-gas atmospheres or in vacuum furnaces. Such processes include but are not limited to semiconductor manufacturing, ceramic-to-copper brazing and so on. Due to the cleanliness of the vacuum environment, flux is not needed. Magnesium is used as an additive, or getter, in the vacuum aluminum brazing process.
Vacuum Aluminum Brazing
Benefits of Vacuum Aluminum Brazing
Brazing has many advantages when compared to other metal-joining processes. Given that brazing does not melt the base metal of the joint, it allows for more precise control of tolerances and provides a clean joint with no need for additional finishing. The meniscus (crescent-shaped) formed by the filler metal in the brazed joint is ideally shaped for reducing stress concentrations and improving fatigue properties.
Ideal situations for brazing include:
- Joining parts of very thin or thick cross sections
- Compact components containing many junctions to be sealed (e.g., heat exchangers) or deep joints with restricted access
- Joining dissimilar metals such as copper and stainless steel
- Assemblies with a large number of joints
Specifically, vacuum aluminum brazing minimizes distortion of the part due to uniform heating and cooling as compared to a localized joining process. This type of brazing creates a continuous hermetically sealed bond. Components with large surface areas and numerous joints can be successfully brazed.
Hardening can also be accomplished in the same furnace cycle if hardenable alloys are utilized, and the furnace system is integrated with a forced-cooling system, thus reducing cycle time.
Vacuum furnace brazing offers extremely repeatable results due to the critical furnace parameters that are attained with every load (i.e., vacuum levels and temperature uniformities). Capillary joint paths (even long paths) are effectively purged of entrapped gas during the initial evacuation of the furnace chamber, resulting in more-complete wetting of the joint.
Vacuum aluminum brazing is ideal for oxidation-sensitive materials. Vacuum brazing is considered a flux-free process that eliminates corrosive flux residue. Post-brazed parts are clean with a matte-grey finish. The process is relatively non-polluting, and no post-braze cleaning is necessary.
Examples of Vacuum Aluminum Brazed Parts
Examples of vacuum aluminum brazed parts often include heat exchangers, condensers and evaporators used in automotive, aerospace, nuclear and energy industries. Some of these parts are shown in Figures 1 and 2.
Vacuum Aluminum Brazing Process
The vacuum aluminum brazing process is usually a relatively short cycle due to the fast pumping and heating characteristics of the furnace, the excellent temperature uniformity at soak temperatures and the high thermal conductivity of the aluminum parts being brazed. Figure 3 shows a typical vacuum aluminum brazing cycle.
The vacuum pumping capacity must be adequately sized in order to minimize the pump-down time of a new load to a deep vacuum level; to initiate the heating cycle; and to have adequate throughput to keep up with the significant outgassing that takes place during the heating cycle due to magnesium vaporization. A deep vacuum level is an important process parameter because it ensures a relatively pure environment for brazing (less PPM of oxygen). Table 1 illustrates the change in purity levels in relation to the various vacuum levels.
A key component of vacuum aluminum brazing is the use of magnesium as an additive to the filler metal and/or the base metal of the parts to be brazed. It is necessary in this fluxless brazing environment for the following reasons:
- When the magnesium vaporizes starting at around 1058°F (570°C), it acts as a “getter” for oxygen and water vapor, thus improving the purity of the brazing vacuum.
- Magnesium will also reduce the aluminum oxide (alumina) that exists on the surface of the aluminum to promote uniform accelerated wetting of the joint surfaces.
The following reactions occur during the vacuum brazing process:
Mg + H2O Þ MgO + H2
2Mg + O2 Þ 2MgO
3Mg + Al2O3 Þ 3MgO + 2Al
3Mg + N2 Þ Mg3N2
Also known as a “mag burst,” the vaporization of magnesium produces heavy outgassing for a short period of time. The slower the heating rate, the smaller the magnesium vaporization rate. Due to this gas load, the vacuum pumps must be adequately sized to maintain a good working vacuum (10-4 to 10-5 torr range).
Heating Control and Temperature Uniformity
Second to the deep vacuum level, precise temperature control and uniformity are also important process parameters. Accepted temperature uniformity during a brazing cycle is +/-5°F (3°C) of setpoint.
Aluminum brazing has a very narrow window of acceptable brazing temperatures. The governing rule for aluminum brazing is that the filler metal has to liquidize before the base metal reaches its solidus temperature. This temperature difference may be as small as 10-18°F (5-10°C).
For example, a base metal 6061 alloy will have a solidus temperature of 1099°F (593°C) and a liquidus temperature of 1206°F (652°C). Brazing temperature range would be 1049-1085°F (565-585°C) depending on the filler metal used.
It is necessary to use a heating step at a soak temperature just below the solidus point of the filler metal to ensure all the parts and joints to be brazed reach the correct temperature at approximately the same time. At this time, the ramp to brazing temperature starts, the filler metal begins to melt and the capillary wetting of the braze joints occurs.
Braze-temperature time duration must be kept to a minimum because the melted filler metal is vaporizing in the deep vacuum as it is trying to wet the braze joints. Too much loss of filler metal to vaporization will result in poor joint wetting and subsequent loss of joint strength and sealing ability.
After the brazing temperature soak duration is complete, it is followed by an immediate vacuum cooling cycle, which solidifies the filler metal in the braze joints and stops the vaporization of material.
The type of precise temperature control and uniformity needed for vacuum aluminum brazing is achieved through the use of several heating control zones around the parts while at the same time maintaining the surface temperatures of the heating elements as near to the part temperature as possible. A large delta in temperature between the heating elements and the parts would result in overheating of the parts’ surface, possibly above the solidus temperature for the material as the filler metal begins to melt.
Types of Braze Joints
In general, the difference between the favorable and unfavorable types of joints is the amount of overlapping that results in a good braze joint. A stronger braze joint has a large surface area that is wetted by the filler material. Too much overlapping is detrimental to the joint because the filler material will not cover the entire surface when it flows into the joint.
Braze-joint strength is dependent on two primary mechanical characteristics: joint wetted surface area and the size of the gap into which the filler metal flows. Gaps of between 0.003-0.008 inch (0.08-2.0 mm) work best for vacuum furnace brazing. Joint gaps are controlled by the manufacturing tolerances of the parts to be brazed and by proper clamping (pre-loading) of the part assemblies to be brazed.
Fixturing of Parts
Part assemblies must be fixtured properly for brazing in order to maintain joint gaps, joint alignment, flow passage alignment and overall assembly tolerances. Some examples of fixturing for assemblies are shown in Figures 4 and 5.
Fixturing materials must be chosen carefully due to different coefficients of expansion for varying materials. Fixture designs are also extremely part-dependent, thought out in great detail and proprietary in some cases since they are an integral part of the manufacturing process.
Cleaning of Parts
Along with good joint design and fixturing, brazing requires part assemblies to be cleaned properly prior to assembly and then handled with care so as not to introduce contamination prior to the brazing cycle. All grease, oil and particulates must be cleaned from the parent and filler-metal surfaces. Assemblers must be careful not to transfer oils from their skin to these surfaces when stacking the parts together. Typical types of cleaning methods are vapor degreasing, hydrocarbon wash, aqueous washing, acid etching and vacuum de-oiling.
What matters most in vacuum aluminum brazing? The key process parameters are deep vacuum levels, precise temperature control and excellent temperature uniformity, which are all provided by optimum furnace design and controls. Keys to successful part brazing include proper joint design with regard to joint surface area and joint gaps, cleanliness of the parts and correct fixturing of the part assemblies.
For more information: Contact Ipsen, Inc., 984 Ipsen Road, Cherry Valley, IL 61016; tel: 815-332-4941; fax: 815-332-4995; e-mail: sales@IpsenUSA.com; web: www.IpsenUSA.com. Author Jim Grann is senior technical manager and Craig Moller is chief engineer. For technical assistance, call 1-844-Go-Ipsen. For more maintenance advice, how-to guides and instructional videos, visit our blog at www.IpsenHarold.com.