Deoxidation of copper alloys is the process of reducing Cuprous oxide in copper solution. The addition of an element with a greater affinity for oxygen than copper has for oxygen reduces the copper in the Cuprous oxide, resulting in a deoxidation product that rises to the surface and is removed, a process called deoxidation. A substance added to a liquid copper alloy to reduce its Cuprous oxide is called a deoxidizer.

The principle of phosphorus copper deoxidation: when phosphorus copper is added to the copper solution, the deoxidation reaction is carried out in the whole copper water. The deoxidation reaction is as follows: The first stage: Phosphorus Vapor (the boiling point of phosphorus is 280 °C) reacts immediately with CU2O IN THE COPPER SOLUTION:

The P2O5 formed by the reaction has a boiling point of 347 °C and is in a gaseous state in the copper solution. As a result, a part of P2O5 bubbles escape to the liquid surface and another part of P2O5 bubbles continue to react with Cu2O in the copper solution during the rising process. Phase II:

Copper metaphosphate (Cupo3) can also be formed directly by the reaction of phosphorus with Cu2o when the content of Cu2o is high and the escape of Phosphorus Vapor is slow:

The CuPO3 produced has low melting point, small specific gravity, and is a ball-like liquid in copper liquid, which is easy to aggregate and float. The amount of phosphorus and copper is controlled: If the amount of phosphorus is too much, the residual phosphorus content in the copper alloy liquid is too high, which will cause the copper alloy liquid to react with the water vapor in the casting mould, and the hydrogen produced by the reaction will dissolve in the copper alloy liquid very quickly, as a result, the pinhole degree of the casting is increased. This phenomenon is also called “Mold reaction”. The addition of 0.03% ー0.04% phosphorus in sand mold casting and 0.05% ー0.06% phosphorus in metal mold casting can not only deoxidize completely, but also the mold reaction is very weak. The addition of phosphorus in tin bronze smelted by oxidation method is higher than that of other copper alloys, when the tin bronze is cast in permanent mold, 0.07-0.1% phosphorus can be added.

The addition mode of phosphor copper is that phosphor copper is often added twice. The first time is to add 2/3 of the total amount of phosphor copper when the copper melt temperature reaches 1150 ~ 1200 °c. the main purpose is to reduce Cu2O in pure copper and then add other alloying elements to avoid these elements being oxidized by CU2O in pure copper, thus, the oxidation and burning loss of elements and the content of oxide inclusions are reduced. The second time, the remaining 1/3 phosphor copper is added before pouring to assist deoxidation and refining. Effect of addition amount of P-Cu alloy on microstructure of up-drawing Copper Rod

It can be seen that the metallographic structure of unadded and added P-cu is obviously different. With the addition of P-cu and the increase of the residual P content in the billets, the grains of the billets become coarser and the precipitates appear. When the addition of P-Cu reaches 0.05% , the flaky precipitates begin to appear in the billets, as shown in fig. B. When the content of p-Cu is 0.15% , the casting structure will be changed obviously. The number of lamellar precipitates increases and most grains become coarse, as shown in fig. C. It is found that the precipitates are intermetallic compounds of P and Cu, which are often distributed in the form of EUTECTIC (Cu3P + α) lamellae on the grain boundary. The higher the residual P content is in continuous casting, the more the precipitated phase is, the more obvious the influence on the microstructure uniformity is. As can be seen from FIG. D, The change of casting structure is especially prominent when the mass of P-Cu is 0.25% . The original regular columnar crystal and equiaxed crystal at the edge of Slab Cross section will be banded and flaky. The grains with the same size changed into the grains with different sizes and shapes. As can be seen from the diagram, when the residual P content of the slab increases to a certain value, the DENDRITE and equiaxed grains which are relatively uniform in the slab cross section become polygonal heteromorphous crystals with different sizes. Due to the increase of the amount of P-cu, CU3P compounds were precipitated from P during solidification. The compound is hard and brittle, and tends to be distributed unevenly on the grain boundaries of different positions, which hinders the growth and movement of the grain boundaries of different positions, as a result, the direction of grain growth is obviously different, which leads to the crystal anisotropy, and makes the regular grains tend to sheet and bar shape. This phenomenon is more obvious when the actual residual P content in Cu solution is above 0.03% . 

Source: Copper Alloy Casting