Oxygen Film on Precipitation of Color Alloy Forging
Forging is a process which involves the shaping of a metal in a certain pressure, ensuring the surface of the metal is compressed and the internal strain is eliminated. It is an important method for manufacturing of mechanical parts and production of precision parts such as wristwatch cases and engine blocks. It is a very ancient technology and foundry industry has been developed in China for many centuries. As for the forging technique of color alloy, it is more mature nowadays, which involves various steps, with the final forging accomplished in a certain temperature and pressure.
Precipitation of colored alloy forging is a kind of advanced forging technology. Compared with other die processes, it has the characteristic of generating far fewer internal stresses and a finer grain structure of the part. But precipitation of colored alloy forging can cause some defects, and one of them is the oxygen film.
Oxygen film refers to a thin and unstable layer that adheres to the surface of a colored alloy forging part. It is caused by the presence of oxygen molecules in the non-metallic inclusions and holes, which results in the formation of oxides inclusions on the surface. In this case, a uniform and thin layer will be formed on the surface of parts, and the oxygen film will increase the friction and wear resistance of alloy parts.
The causes of oxygen film formation in the precipitation of colored alloy forgings are mainly determined by the process parameters. First, temperature control affects the formation of oxides in the alloy. If the temperature fluctuates during the process, oxides will be easily generated and form an oxygen film on the surface. Second, the aluminum content in the alloy also determines the formation of oxides, previously adding aluminum can increase the oxidation speed of the liberated oxygen, and increase the oxidation surface, resulting in an increased thickness of the oxygen film. At last, controlling the pressure is important. Too high pressure will increase the internal strain during the forging, generating tiny scratches on the surface and making it easier to generate oxygen films.
To avoid oxygen films on the parts, two processes should be adopted. The first is called surface flux. This method adopts a chemical reaction to remove the oxide layer on the surface, and according to forging material, the right flux should be selected to achieve good results. The second is called surface mechanical grinding. If the surface of the part has a thick and uneven layer of oxides, mechanical grinding can be adopted. This method helps to eliminate the surface layer and oxides on the surface, which form the oxygen film.
In conclusion, oxygen film is a common defect in the practice of colored alloy forging. Its formation is determined by the process parameters of temperature, pressure and aluminum content in alloy. To prevent and eliminate the defect, two methods – surface flux and mechanical grinding – can be adopted. With these techniques, the surface of colored alloy forging parts will have better performance in wear and friction resistance.
