a comparative study: zinc phosphate-soap coating

MANİSA CELAL BAYAR UNIVERSITY II. INTERNATIONAL UNIVERSITY INDUSTRY COOPERATION, R&D AND INNOVATION CONGRESS, 14-15 NOVEMBER 2018

A COMPARATIVE STUDY: ZINC PHOSPHATE-SOAP COATING VERSUS POLYMER BASED COATING IN COLD FORWARD EXTRUSION Suna Güneş, Barış Tanrıkulu, Deniz C. Özuğurlu, Cenk Kılıçaslan* and Umut İnce Norm Cıvata San. ve Tic. A.Ş., R&D Centre, Izmir, Turkey Corresponding author: cenk.kilicaslan@norm-fasteners.com.tr

*

Abstract Cold forging process provides to produce high quality components with complex geometries and near net shapes. However, tools in cold forging operations are exposed to high mechanical stresses and extreme tribological conditions due to high normal stresses. The tribological condition of tool and material is so effective on the tool life and surface quality of the product. One of the parameters that affects the tribological conditions in cold forging is coating of the work-piece. The coating which is applied to raw material surface in a separate operation before forging leads to decrease friction. The zinc phosphate-soap coating is mainly used in the cold forging industry however that coating includes heavy metals and leads to generation of hazardous waste. Polymer based coatings are designed to avoid of the above-mentioned disadvantages of zinc phosphate-soap coating. In this paper, 23MnB4 forging steel was coated with zinc phosphate-soap and polymer and flow behavior were compared by conducting cold forward extrusion tests. Numerical models of extrusion tests were also prepared in simufact.forming finite element software to determine the friction coefficient between the work-piece and tool. The results showed that the tribological performance of polymer coating is very close to traditional zinc phosphate-soap coating. Keywords: Cold forging, Lubrication, Tribology, Friction, Friction Coefficient. Introduction Cold forging parts provide high mechanical properties like high fatigue strength and deformation resistance. The cold forged parts are near-net or net shape parts with high surface quality. Work-piece material in cold forging operations is subjected up to nearly 3000% expansion, 2500 MPa pressure and nearly 200 °C tool temperature (may be 600 °C locally) [1]. High expansion level, pressure and the temperature lead to extreme tribological condition. High friction increases wear on tools, forming force, energy consumption and decrease the tool life and surface quality of final products [2, 3]. In general, zinc phosphate-soap/oil coating is used to reduce friction to the range changing between 0.02-0.07 in cold forging process [4]. However, zinc phosphate-soap/oil lubrication has disadvantages huge water consumption and generation of hazardous wastes including heavy metals. Also, raw material properties can be affected by the zinc phosphate-soap/oil coating. Residual zinc phosphate may diffuse into the surface of the workpiece during heat treatment process and may cause surface embrittlement [5]. Several types of coatings like polymer based, wax based and molybdenum disulphide based are developed to avoid these kinds of disadvantages. The coil surface has to be clean and free of contaminants before the any coating processes to avoid uncoated areas. Chemical cleaning process is widely used in the industry. The cleaning process affects the coating and the tribological features of the coil in cold forging process. Also, cleaning process are affected by the initial surface slugs, contaminants of the coil and the chemical condition of the bath. In general, sulphuric acid is used for pickling process at 40-60 °C. Some chemicals are added to the bath to protect the surface overpickling and the Manisa Celal Bayar University, (ÜSİTEM)

MANİSA CELAL BAYAR UNIVERSITY II. INTERNATIONAL UNIVERSITY INDUSTRY COOPERATION, R&D AND INNOVATION CONGRESS, 14-15 NOVEMBER 2018 slug or contaminants deposition. Water soluble salts like silicates, borates, soda are added to the bath to protect the coil any deposition. Pickling inhibitors are added to suppress the clean coil surface and protect the surface any chemical attack. In some cases, hydrochloric acid is used to clean the coil surface and reduce the risk of overpickling thanks to the lower temperature. After cleaning the surface properly, coating become effective. Traditionally soap and oil are used as lubricant. However, this kind of lubricants need lubricant carriers like zinc phosphate/zinc iron phosphate chemicals. These lubricant carriers react with the coil surface and prepare the coil surface for lubricants. The chemical reaction takes place at the surface of the coil. In the lubricant carrier bath, iron is oxidized and H+ ions are reduced into hydrogen gas. Then, crystalline zinc phosphate Zn3(PO4)2 was deposited on the coil surface. After phosphating, coils are dipping into the soap or oil as lubricant. Generally, alkaline soaps are used and the main component of the soap is sodium stearate. The chemical reaction takes place between the sodium soap and the zinc phosphate crystalline deposition create a layer chemically bonded and water insoluble. Figure 1 shows the zinc phosphate-soap deposited layers on the coil surface.

Figure 1. Layers on coil surface while zinc phosphate-soap coating. Polymer is the one of non-reactive coating. The water dispersed polymer chains that has high affinity the steel mechanically hold the surface of the coil. Then, the water vaporization process is applied to obtain dense layer of polymer coating on the surface. Figure 2 shows the polymer coating generation on the coil surface. While any chemical reaction could not take place between polymer coating and the base material, surface cleanness of the coil is more important. The polymer coating could not deposit up to the contaminant or slug. That cause the breakage of the continuity of the polymer coating.

Dipping

During Drying Figure 2. Polymer coating deposition on coil surface. Manisa Celal Bayar University, (ÜSİTEM)

After Drying

MANİSA CELAL BAYAR UNIVERSITY II. INTERNATIONAL UNIVERSITY INDUSTRY COOPERATION, R&D AND INNOVATION CONGRESS, 14-15 NOVEMBER 2018 In this paper, 23MnB4 forging steel was coated with zinc phosphate-soap and polymer coatings separately, and tribological performance of these coatings were revealed by terms of friction coefficient that occurs between work-piece and tool in cold forward extrusion. The samples were cut from coated work-pieces and extrusion tests were conducted at ram speed of 5 mm/min on a Zwick Z400 Red model test machine. The finite element models of extrusion tests were prepared in simufact.forming finite element software. Materials and methods Coils of 23MnB4 forging steel were chosen to test tribological features of polymer and zinc phosphatesoap coating. The coating procedures starts with the cleaning the surface of steel coils with water. Then the coils were moved to sulphuric acid bath that cleans the surface contaminants. The cleaning part is important to achieve straight results. When cleaning is not proper especially for polymer coating, the test results could not consider as comparable because of the lack of the coating. Then neutralization was conducted in the following tank. After the neutralization, coils of 23MnB4 forging steel were coated with zinc phosphate-soap and polymer coatings separately. Figure 3 shows the coating procedure of coils of 23MnB4 forging steel. After the coating process, the surface of the coils must be clean and fully coated to achieve logically comparable results. Figure 4 shows the responsibly polymer and zinc phosphate-soap coated coils.

Figure 3. Coating procedure of coils of 23MnB4 forging steel.

(a)

(b)

Figure 4. (a) Polymer coated coil. (b) Zinc phosphate-soap coated coil.

Manisa Celal Bayar University, (ÜSİTEM)

MANİSA CELAL BAYAR UNIVERSITY II. INTERNATIONAL UNIVERSITY INDUSTRY COOPERATION, R&D AND INNOVATION CONGRESS, 14-15 NOVEMBER 2018 Tribological features of the cold forging lubricants could be tested various methods like ring compression, ball penetration, double cup backward extrusion, spike etc. In this paper, forward extrusion is selected because of the similarity of the cold forging process. Extrusion test specimens were prepared from these acid cleaned and separately zinc phosphate-soap and polymer coated coils. Figure 5 shows the test specimens. Extrusion tests were conducted at 5 mm/min with a forward extrusion die having 55% area reduction. To unsure reproducibility of the results, at least 3 tests were conducted.

Figure 5. Extrusion test specimens were taken from the coils. The numerical models of die, punch and the sample were prepared in Catia V5R20. Then the extrusion tests were regenerated by simufact.forming finite element software. Cross-section of the numerical model of the extrusion test is shown in Figure 6. The model includes elastic WC-Co (G40) die and stress ring (made of H13 tool steel) and rigid punch. The specimen is modeled as elastic-plastic material. Coulomb friction model was defined as friction model and friction constant was determined with a parametric study for each test. Friction coefficient was varied to obtain the experimental curve in an error range of %5.

Figure 6. Simulation model of extrusion test.


MANİSA CELAL BAYAR UNIVERSITY II. INTERNATIONAL UNIVERSITY INDUSTRY COOPERATION, R&D AND INNOVATION CONGRESS, 14-15 NOVEMBER 2018 Results The force (kN) – stroke (mm) graphs were obtained by the experiments and the numerical simulations. The comparison of experimental/numerical curves of both zinc phosphate-soap and polymer coated work-pieces are given Figure 7. The friction coefficient of the polymer coating under the given conditions was determined 0.05 whereas the zinc phosphate-soap coating was 0.06. There is no remarkable difference between polymer and zinc phosphate-soap coating as the lubricant on tribological features. The gap between the forming forces of polymer coated sample and the zinc phosphate-soap coated sample in extrusion test is ~3 kN, but the error range is %5. So, the coatings could be assume the same lubrication and tribological behavior.

Figure 7. Experimental and numerical stroke-force curves of extrusion of polymer and zinc phosphate soap coatings. Conclusions The new lubrication coatings are developed to substitute traditional zinc phosphate-soap coating to eliminate disadvantages and environmental issues. Polymer coating is the one of these lubrication coatings. In this paper, tribological features are investigated both polymer and zinc phosphate-soap coatings and the results are promising. According to experimental and numerical findings, polymer-based coating offers similar lubrication performance between specimen and die. As seen in Figure 7, the coefficient of friction of both coatings (0.05 for polymer and 0.06 for zinc phosphate-soap) are very close to each other. The study showed that polymer-based coating can be an effective alternative to traditional zinc phosphatesoap coating in cold forging.


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