I. Brazing Alloy
Copper and phosphorus are the two main elements in copper-phosphorus brazing alloys. It comes mostly in rod, flat rod and ring shapes. This type of brazing alloy is widely applied in the brazing of copper and copper alloys, especially in the HVAC-R industries. They have good fluidity and an economical price, which contribute to their popularity.
We are going to focus on AWS BCuP-2, which is one of the most typical copper-phosphorus brazing products. Its phosphorus content is relatively high, in the 7.0 - 7.5% range. Because of this high phosphorus content, their melting point is low, in the 710 - 793 ºC (1310 - 1459.4 ºF)range. And their fluidity is quite good.
As you may know, this brazing alloy has self-fluxing property, which means that we don’t need to add any flux during brazing. At Hua Guang we produce BCuP-2 in many different shapes, such as rod, flat rod and ring. The one you see here is a flat rod with the diameters 1.3*3.2*451 mm.
II. Brazing Steps
We choose flame brazing, as it is straightforward in terms of equipment selection, and it also gives us a high level of flexibility. The flame we use here is acetylene and oxygen flame. We can classify the flame into neutral flame, carbonized flame and oxidized flame based on the ratio of oxygen and acetylene in the mixture. We don’t recommend carbonized flame and oxidized flame, because if we choose carbonized flame, soot will be released during the burning process, which will negatively affect brazing performance. Whereas if we choose oxidized flame, oxidation of the base metals will speed up, which will in term obstruct solder’s flow and wetting process. Therefore, we use neutral flame, and we use the outer flame to preheat and heat the designated area.
BCuP-2 is a self-fluxing brazing product. The phosphorus content in the alloy fulfills the role of a deoxidizer, so we don’t need to use any flux. We repeatedly point our brazing torch from side to side on the heating area, so that we won’t overheat a particular spot. Flame is to be kept at a 45° position with the length of the copper part for most kind of joints, with the exception of socket joint, where flame is to be kept level with bell mouth of the tubing.
When we see that the copper base metal is turning light red, it is an indicator that preheating is sufficient, and we can add our BCuP-2 brazing alloy on the opposite side of the flame. Something to be noted, when we braze tubings, sometimes we need to switch the heating position of the flame symmetrically to the opposite side of the tube, so that brazing filler metal can fill every spot of the gap.
In the brazing process, flame is used to melt BCuP-2 brazing alloy, and the molten alloy then wet the copper base metals. Under capillary action, molten brazing alloy fill the gap at the braze joint, and it dissolves and diffuses with the copper. Afterwards, allow it to be air cooled and solidify for a while,
and joining of the base metals is now completed.
We put it into the water bath, after taking it out, we use a towel to wipe any slag on the joint, then we use an air blower to blow it using cool air, so that it won’t be corroded afterwards.
For these brazed parts, we need to run some relevant testing to ensure that they meet our specific application requirements. Some applicable tests for the joints include appearance inspection and metallographic observation for microstructure analysis. A good joint should not have defects such as slag-inclusion, pin hole, crack and insufficiency in brazing. And metallurgical bonding with the base metal should be optimal. For socket joint, we will do penetration inspection, and if needed, will also test its tightness.
To recap, we used the common BCuP-2 brazing alloy and flame brazing to braze copper and copper using T joint and socket joint. The most important things to remember are the brazing steps. First, clean the copper parts. Secondly, assemble and fix the parts in place. Thirdly, preheat and heat the copper with flame. Fourthly, molten BCuP-2 then wet the base metals and fill the joint. Lastly, clean the brazed parts and run the required testing.