Vacuum Brazing Process of Aluminum Alloy Millimeter Wave Components

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1.  Vacuum Brazing Welding process design

Selection of solder and active elements

When brazing aluminum and aluminum alloys, the available brazing materials are shown in Table 1. They are mainly based on aluminum-silicon eutectic, and sometimes some other elements are added to adjust the performance.

During vacuum brazing, magnesium is generally added to the aluminum-silicon solder to improve the fluidity and wettability of the solder. Studies have shown that adding a small amount of bismuth while adding magnesium can reduce the surface tension of the solder, improve the wettability, and reduce the requirement for vacuum. So choose BAI89SiMgBi solder complete the welding test.

In order to remove the oxide film and prevent aluminum oxidation when welding aluminum alloys, it is necessary to use the action of some metal activators at the same time. At present, the commonly used activator is magnesium, because magnesium has a high vapor pressure and is easy to volatilize in a vacuum, which is beneficial to remove the oxide film. In the test, magnesium blocks were selected for the test.In order to prevent the premature volatilization of the magnesium block, the magnesium block was placed in the process box during the test, and the process box was placed near the brazing seam.

Name	Grade	Melting temperature(℃)	Brazing temperature(℃)	Joint strength ab (MPa)
Aluminum silicon solder	BAI88Si	577 – 582	595 ~605	147 ~151
	BA190Si	577	590~ 610	98
	BAI92Si	570 ~ 590	595 -615	–
	BAI85SiMg	559 ~579	582 ~ 604
Aluminum silicon magnesium solder	BAI87SiMg	559 ~ 591	598~604,	130
	BAI90SiMg	559 ~ 607	599 ~ 621
Al-Si-Mg-Bi solder	BAI89SiMgBi	559 ~ 602	600 ~615	130

Table 1 Brazing filler metal for vacuum brazing aluminum and aluminum alloys

2. Vacuum Brazing Welding process parameters

In order to control the deformation well and obtain high-quality welds, the welding process parameters must be selected reasonably. The main process parameters of vacuum brazing include welding temperature, heating rate, holding time and vacuum degree. In general, the welding temperature should be 20C~60C higher than the melting point of the solder. The vacuum furnace adopts the radiation heating method, so the temperature of the furnace body and the temperature of the workpiece will be inconsistent during the heating process, so it is very important to control the heating rate. The holding time determines the quality of the weld. If the holding time is too short, the solder is not completely melted, and if the holding time is too long, the base metal will react excessively with the solder, resulting in erosion. High vacuum degree is an important factor for the broken aluminum alloy oxide film and the realization of vacuum brazing coal. The higher the vacuum degree, the better the flowability and wettability of the brazing material.

The specific parameters tested are as follows:

Vacuum: 1.5×10-‘Pa;

Soldering temperature: 6059C +1C;

Holding time: 1 Min;

Cooling method: cooling with the furnace.

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