| Cold plastic deformation is used very commonly in metal processing, e.g., cold extrusion, wire drawing, etc. Nowadays, phosphating has been proved to be a quite necessary pre-treatment process in the above metal processing. The reason is that a ductile phosphate coating is formed on the surfaces of steel workpieces, which implies that the friction coefficient between workpiece and die can be reduced effectively after saponification and thus the speed of clod plastic deformation and the qualities of final products are improved substantially. However, non-ECO-friendly accelerators such as sodium nitrite (NaNO2) are applied very widely and commonly in the modern phosphating industry. Unfortunately, the participants in this field have to face a series of consequences, that is, the issues of carcinogenic characteristics of sodium nitrite and the troubles of the effective processing of phosphate residues. Therefore, it has been shown to be very stringent to find a new environmentally friendly accelerator and improve phosphating treatment . process, which have actually become the key issues in the phosphating industry.In this paper, the author investigated the electrochemical reaction mechanism of phosphating reaction in detail by utilizing the electrochemical theories and diffusion theories. Then, the influences of traditional accelerator (sodium nitrite) and environmentally friendly accelerator (hydroxylamine sulfate) on the formation and frictional properties of phosphating coating were studied. The formation and phase evolution of phosphating coating were investigated by X-ray diffraction method. It is found that the phosphating coatings are mainly composed of hopeite Zn3(PO4)2·4H2O and phosphophyllite ZmFe(PO4)2·4H2O. The microstructural changes of the phosphating coating, as a function of phosphating time, were evaluated by scanning electron microscopy (SEM). Four-ball friction experiments reveal that hydroxylamine suifate can reduce the friction coefficient more effectively in comparison with sodium nitrite. Therefore, hydroxylamine sulfate can be expected to be used as a fast and Eco-friendly accelerator to replace the conventional environmentally unfriendly sodium nitrite.Subsequently, the author studied the Zn-Ca phosphating liquid in order to realize the management without the presence of accelerators. The XRD results reveal that the main components of the phosphate coating are Zn3(PO4)2·4H2O, Zn2Fe(PO4)2·4H2O and CaZn2(PO4)2·4H2O. The microstructural characterizations reveal that a more compact coating is obtained with the increase of the ratio of Zn and Ca. Furthermore, a flake-like and granular crystallizations shift to fine dendritic with increasing the ratio of Zn and Ca. At the same time, the real cold extrusion results indicate that the process precision of steel workpiece is higher and higher when the ratio of Zn and Ca increases, and a highest precision can be obtained when the molecule ratio of Zn and Ca is 0.375. What is more, the reference indexes such as total acid (TA) and float acid (FA) decline exponentially, and whereas acid ratio (AR) and the concentration of Fe2+ ions increase exponentially with the throughputs of steel worpieces.In addition, the phosphate residues produced in the Zn-Ca phosphating process (no accelerators present in the phosphating liquid) are approximately 25% of those produced in the conventional phospahting (i.e., the accelerators such as sodium nitrite are included). Consequently, Zn-Ca phosphating process reduces the phosphate residues substantially and minimizes the field detection indexes. That is to say, production cost is lowered effectively and the efficiency of phosphate processing is markedly improved.In the end, the development tendencies of phosphate liquid applied in the cold plastic working were pointed out and the applications of Internet of Things on the phosphating industry were also preliminarily discussed in this dissertation. |
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