"Brazing" refers to a process that joins metals together. By using a braze alloy or filler metal to unite individual pieces, a strong metallurgical bond is created without melting or changing the essential structure of the original materials. With proper technique, the connection between the metals is often stronger than the actual metals themselves, which ensures the longevity of the bond.

Although there are different techniques for brazing, furnace brazing specifically allows for high rates of commercial-scale production. Furnace brazing has many specific advantages. The metals being joined do not must be the same; dissimilar metals with varying thicknesses can also be united. This allows for complex arrangements of different parts, without distorting the original metals. Multiple joints can also be brazed at the same time. Moreover, superior products can be achieved by brazing in a furnace, as furnaces allow for premium reproduction. Furnace atmosphere can be precisely controlled, and this automation is significant. The consistency and quality that brazing in a belt furnace produces cannot be replaced.

Parameters of Brazing Process

Because brazing has certain requirements, brazing furnaces must be able to support these parameters. Components get heated, joined, and cooled inside of a furnace, and this entire process is heavily dependent on temperature. The determining factor for the brazing temperature is based on the melting point of the filler metal. For the CUSIL braze alloy (72% silver and 28% copper, a common copper brazing applications), the temperature should fall around 780 C. The temperature control has a strong influence on success. Usually, there are five steps to a typical brazing process cycle: (1) an optional preheat, including holding, (2) ramp to temperature, (3) brazing, (4) cool down, and (5) exit, as illustrated in Fig. 1. During the thermal brazing cycle, the temperature needs to be precisely controlled by the furnace.

Figure 1. Schematic of a typical brazing process cycle, from "Furnace Materials #4: Brazing of Metals"

Timing is also very important to the brazing process. Brazing requires a minimum duration to ensure that the filler metal can flow through the whole joint to make a strong bond. Generally, 1 or 2 min is enough for this. Too much processing time will yield undesired effects. Typically, the duration of the contact between liquid filler metal and the base metals must be minimized to prevent excess flow of the filler metal. However, in some cases, greater processing time can produce a modified braze alloy that has a higher melting temperature, which could be advantageous in situations where a second brazing may take place.

Atmosphere Control in a Belt Furnace

The atmosphere must be neutral for both the base metal and the braze alloy to avoid oxidation carburizing, decarburization, and nitriding. This ensures that no reactions can occur between the metal, alloy, and the atmospheric elements. In addition to oxygen and carbon, it is possible for nitrogen and hydrogen to also trigger reactions in some circumstances. Likewise, many metals can oxidize rapidly when heated. To avoid this, the brazing atmosphere inside of a furnace must be controlled by utilizing a reducing gas such as hydrogen, forming gas, or cracked ammonia. Controlling the furnace atmosphere is the best way to offset the risk of oxidation and other unwanted issues.

The amount of oxygen and moisture levels in a furnace have a direct influence on the behavior of the filler metal as it melts. For example, when there are low levels of oxygen and moisture in a furnace, the filler metal has a greater ability to melt and navigate into tight joints and create a stronger bond. High levels of these elements can produce the opposite effect, which is averse to brazing. Maintaining strict control of the furnace atmosphere will significantly improve the quality of the brazing process and prevent physical defects. Furnace atmospheres contain both neutral and active gases, and these gases can encounter a multitude of reactions. If parts have not been cleaned before entering the furnace, surface contaminants can interact with the gases and cause unexpected results. Additionally, gases can also react with each other, as well as with vapors from the filler metal. Brazing furnaces allow for precise adjustments within the atmosphere composition to produce a balance between the two types of gases. This equilibrium helps to control oxidizing and reducing, as well as carburizing and decarburizing. The atmosphere also has an inevitable effect on the rate of heat transfer within a furnace's heating and cooling phases.

It is important for atmosphere control systems in brazing furnaces to include both a system to analyze gas and a flow and gas mixture control system to control the balance of gases and maintain atmospheric precision. Continuous brazing furnaces are specifically advantageous for accurate control, in that different furnace zones can accommodate diverse atmosphere composition set points, as shown in Fig. 2.

Fig. 2 Atmosphere control in belt furnace, from "Furnace Materials #4: Brazing of Metals"

Selecting a Furnace for Brazing

A high-quality furnace is essential for to produce top-quality results from brazing. The type of products being brazed, characteristics of the materials, production volumes, and schedule can all influence the furnace that is best suited for the task. A continuous brazing furnace is best for production if a perpetual flow of parts is desired, regardless of whether the parts need to be placed in a tray or basket. A brazing furnace has a metallic muffle, which ensures precise control of the atmosphere while minimizing contamination. This guarantees reliable quality.

The HSA series belt furnace seen below in Fig. 3 is designed to excel at the brazing process. This furnace uses ceramic heater boards to achieve elevated temperatures. The HSA series furnace comes with a refractory heating chamber that is equipped with ceramic fiber FEC (fully enclosed coil) heating board. The heating works to give fast thermal response. The furnace is equipped with a temperature profiling system and a computer monitoring system. Moreover, it can achieve precise atmosphere control for hydrogen, nitrogen, and oxygen by using a dew point and oxygen monitoring system.

Forced air or water cooling is used in the cooling section of the furnace. The muffle is located within the furnace and helps to control atmospheric conditions as well as to maintain a clean environment inside the furnace. As a standard feature, this furnace is equipped with a steel brush for cleaning the conveyor belt, and ultrasonic belt cleaning is available as an extra option.

The HSA series furnace has a microprocessor based PID controller to control the furnace. Type K thermo-couples are used in determining the zone temperatures and the controls are located on the right-hand side of the furnace which can be viewed from the entrance. The central processing unit (CPU) is located at the exit table and is primed with a Windows operating system for ease of use. The computer system is pre-installed with a program for controlling the furnace parameters, including the belt speed, zone temperatures, and atmospheric conditions. Temperature profiles can be stored and retrieved as well for future purposes. The furnace already has programs in the software for capturing/storing, displaying, and printing out the furnace profile. Thermocouple ports are located at the entrance table for connecting the profiling thermocouple directly into the microprocessor. This feature allows for the monitoring and recording of actual temperatures experienced by the part. Additionally, the furnace is equipped with a redundant overheat safety protection system which incorporates an additional type "K" thermocouple in the center of each controlled zone and the multi-loop alarm. Two sample HAS furnaces are listed below.