Application Research of Electrophoretic Coating on Fasteners

1 With the rapid development of China's automobile industry, the quality of automotive products is also getting higher and higher requirements. The high safety, high environmental friendliness and diversified personality of the car are also reflected in the surface protection technology of the fastener. At present, the surface treatment processes of fasteners on automobiles mainly include oxidation (blackening), phosphating, electroplating (galvanizing, zinc-nickel alloy, decorative chrome), and Dacromet coating. It is well known that oxidation (blackening) treatment has the disadvantages of high energy consumption, unstable process, and weak film layer; the corrosion resistance of phosphating treatment is poor; the passivation treatment solution and the Dacromet coating of the plating contain Cr'. Environmentally friendly, the corrosion resistance of ordinary plating layer is about 144 hours, and the corrosion resistance of Dacromet coating can reach 500~1000 hours, but the cost is relatively low, the film layer has poor moisture resistance (240 hours of heat resistance), and it is not wear-resistant.

In order to improve the surface treatment quality of fasteners and meet the requirements of environmental protection regulations, in addition to the above surface treatment processes, foreign automobile companies have used cathodic electrophoretic coating processes to surface fasteners such as bolts. According to the different quality requirements of fasteners, the salt spray resistance can reach 240~1000 hours or more, and the friction coefficient of the paint film is suitable, which does not affect the product assembly. It can replace the passivation process of electroplating. Environmental protection. GM and Ford Motor Company have used the electrophoretic coating process to surface fasteners such as bolts on automobiles.

Cathodic electrophoresis coating is a mature process due to its high degree of mechanization, environmental protection, and excellent corrosion resistance of paint film. It has been applied to automotive body and parts for several years. However, the application of fasteners such as bolts is still a blank in China. It is indeed a new technology and a new field of electrophoretic coating.

Based on the original mature electrophoretic coating process, the research team conducted a large number of experimental studies in the laboratory to verify the feasibility of the application of the electrophoretic coating process on fasteners, and the current application of electroplating and Dacromet coating. The overlay process performs performance and cost comparisons.

2 Test Section 2.1 Test Samples, Materials and Equipment 2.1.1 Types of Electrophoretic Coatings for Tests Type P Black Cathodic Electrodeposition Coatings (hereinafter referred to as P), Electrophoretic Coatings for Fastener Coating; H Type Black Cathodic Electrodeposition Coatings (H ), the formula has been adjusted, suitable for electrophoretic coatings for fastener coating; Y-type black cathodic electrophoretic coating (hereinafter referred to as Y), electrophoretic coating for ordinary parts coating.

2.1.2 Types of Fasteners for Testing Vehicles According to the types and performance requirements of fasteners on automobiles, the following seven fasteners were selected for testing, which are listed in Table 1. All fasteners are supplied by FAW Fuao Standard Parts Branch.

Table 1 Types of Test Fasteners Part No. Use Material Size Remarks Used on the chassis, engine, and transmission. Hardness 10.9 is used on the chassis, engine, and transmission. The hardness is 10.9. It is used on the chassis, engine, and gearbox. Bolts, used on the chassis, engine, gearbox for chassis, engine, gearbox M8 (fine teeth, long) combination bolts, used in the chassis, engine, gearbox M8 (fine teeth, short) hardness 8.8 2.1 .3 Test Small-scale drum electrophoresis device test chamber can be applied by single-piece electrophoresis (refers to the coating method of one or several samples per electrophoresis in the small electrophoresis tank). According to foreign companies, there are three types of fastener electrophoresis coating production lines: drum type, conveyor belt type and basket type. According to the principle of electrophoretic coating, we simulated the large-scale production line equipment. We designed the small-scale drum equipment for the laboratory and used the equipment to test the swimming parts to provide technical basis for the future production line.

2.2 Coating process and preparation of coating film Three coating process schemes are determined through experiments: and compared with the current domestic and international fastener surface treatment application process: no chromium-zinc-aluminum coating, recorded as process scheme F. The three coating processes were determined, and seven kinds of fasteners were respectively subjected to electrophoretic coating using different electrophoretic coatings.

2.3 Electrophoretic coating performance test The main performances of the fastener coating (plating) layer were tested: (1) corrosion resistance (salt spray test) performance; (2) moisture and heat resistance; (3) assembly performance (paint film) Friction coefficient, general tester test); (4) Simulated loading pneumatic wrench test. At present, domestic high-strength fasteners for automobiles rely on imports. An important bottleneck for localization of fasteners is the control of friction coefficient. Under the premise of ensuring the assembly performance can pass, the salt spray resistance and the humidity resistance performance time are as long as possible; in addition, the paint film does not fall off when loaded with a pneumatic wrench, these are the fasteners using the electrophoretic coating process. The key technology is also the difference between electrophoretic coatings for fastener coating and common electrophoretic coatings.

2.3.1 Corrosion resistance (salt spray test) performance and heat and humidity resistance test according to GBT1771-1991 and GB/T13893-1992 test methods. Evaluation method: the electrophoretic coated fasteners are directly placed in the salt spray test chamber and the damp heat test chamber. For the coating of the process plan A, the time for the red rust on the plane and the edge of the nut is mainly evaluated. time. For the coating of process schemes B and C, the time of white rust appeared on the plane and edge of the nut until the time when red rust appeared was the end of the test.

2.3.2 Assembly performance (pass tester test, paint film friction coefficient) According to GBA16823.3 threaded fastener tightening test method, the friction coefficient of fastener electrophoretic paint film was tested, and with zinc electroplating fastener The coefficient of friction was compared.

In addition, the passability of the fastener electrophoretic paint film was tested by a general gauge tester.

2.3.3 Simulated loading pneumatic wrench test In order to check the adhesion of the electrophoretic paint film in the assembly, the simulation production line assembly test was carried out in the laboratory with a simple device.

3 test results and discussion 3.1 salt spray and damp heat test table 2 coating salt spray results sample type / salt spray test results process program electrophoretic coating P electrophoretic coating H electrophoretic coating Y process plan A 480h red rust appeared 288h red rust appeared 168h Red rust process scheme B 144h white rust, 804h red rust 144h white rust, 720h red rust 144h white rust, 360h red rust process plan C 480h white rust, 1000h red rust 360h white rust, 504h Red rust other samples/process plan A 504h red rust appeared 360h red rust appeared 192h red rust other samples / process plan B 144h appeared white rust, 1000h appeared red rust 144h appeared white rust, 804h appeared red rust 144h appeared white Rust, 360h red rust other samples / process plan C 480h white rust, 1200h red rust table 3 coating damp heat test results electrophoretic coating P electrophoretic coating H electrophoretic coating Y test method process plan A 500h, no change in paint film surface Process scheme B 500h, the surface of the paint film has a slight white rust for 500h, the surface of the paint film has a slight white rust for 500h, the surface of the paint film has a slight white rust process plan C 500h, the surface of the paint film has no change for 500h, and the surface of the paint film has no change process. Scheme E 240h, Dacromet coating foaming No matter which process scheme is adopted, the salt spray test result of the electrophoretic coating P coating is better than the salt spray test of the electrophoretic coating H coating, and the salt spray test result of the electrophoretic coating Y coating Worst.

Adopting process scheme A: The salt spray test result of electrophoretic coating P coating is better than the current application scheme D, the salt spray test is raised from 200 hours to 480 hours; the salt spray test result of electrophoretic coating H coating is slightly better than the current one. The applied process scheme D, the salt spray test was increased from 200 hours to 288 hours; the salt spray test result of the electrophoretic coating Y coating was slightly lower than the current application scheme D. The process scheme B: the salt spray of the three coating coatings The test results are superior to the current application scheme D, and the salt spray test is 804 hours. However, it was found in the test that the salt spray test of the electrophoretic coating Y coating showed white rust for a period of time, the paint film began to foam, and the paint film fell off when it was severe, indicating that the electrophoretic paint film and the electrogalvanized layer had poor adhesion (with the previous results). Consistent). The salt spray test results of the electrophoretic coating P and the electrophoretic coating H coating are equivalent to the currently applied process scheme E (about 800 hours).

Adopting process scheme C: The white rust of the sample is later, and the salt spray resistance is the most obvious. It is far from the current application scheme D, and can reach the salt spray test result equivalent to the current application scheme E. .

Regardless of the process scheme used, the salt spray test results for the 1 and 2 samples are inferior to those of the other samples.

Regardless of which process scheme is adopted, the damp heat test results are superior to the current process plan E. 3.2 Assembly performance (pass tester test, paint film friction coefficient) 3.2.1 General tester test results Test results are listed in Table 4 in.

Table 4 General Tester Test Results Firmware Type Paint Process Plan A Process Plan B Process Plan c Process Plan A Process Plan B Process Plan C Process Plan A Process Plan B Process Plan C Note: X is not passed the pass test; In order to pass the general test.

With process scheme A, both electrophoretic coating P and electrophoretic coating H can pass the general test, and the electrophoretic coating Y fails; with process scheme B, fine fasteners (1, 2, anti-loose bolts) are difficult to pass the general test. The coarse tooth fasteners (3, 4) and other samples can pass the general test; with the process plan C, the fine tooth fasteners have not passed the general test.

If the fasteners with torque requirements are surface treated with Process Plan B and Process C, we consider whether the tolerance dimensions of the fastener white parts can be enlarged. In addition, the galvanized samples before electrophoretic coating were directly tested, and it was found that individual samples could not pass, indicating that the thickness of the galvanized layer itself was not uniform, which also directly affected the assembly after electrophoretic coating. performance.

3.2.2 Paint film friction coefficient Table 5 Fastener coating (plating) layer friction coefficient sample galvanized total friction coefficient (average value) total friction coefficient (arithmetic value) tightening torque (average value) (Nm) tightening torque ( Arithmetic value) It can be seen from the results in Table 5 that the total friction coefficient and the dispersion coefficient (average value) of the fastener film of the electrophoretic coating P and the electrophoretic coating H electrophoresis are superior to those of the electrogalvanized parts.

Excessive friction coefficient causes the energy (torque) consumed by tightening the fastener to be too large; and the difference in friction coefficient causes the fastener to be tightened under the same torque, and the final axial force is too large, which is not conducive to the use of fasteners. .

It can be seen from the test results that the electroplated zinc parts did not meet the requirements, which may be caused by poor manufacturing quality control of the sample pieces and uneven thickness of the galvanized layer (same result as the general test).

3.3 Comparison of performance with different protective layers According to the above test results, the main performance comparison of each process plan and different protective layers is shown in Table 6.

Table 6 Performance Comparison of Different Protective Layers Electrophoretic Coating Electroplated Zinc Layer Dacromet Coating Chromium-Free Aluminum Coating Thickness (im) Salt Spray Test (h) Damp Heat Test (h) Friction Coefficient 3.4 Cost Analysis Based on Experience and Production Application In the actual situation, the comprehensive cost of the surface treatment process schemes D, E, and F of the process schemes A, B, and C and the current fasteners are analyzed, and are listed in Table 7, Table 8.

Table 7 Cost analysis of each process plan No. Process plan Comprehensive cost (yuan/m2) Process plan A Process plan B Process plan C Process plan D 'Process plan E Process plan F Table 8 Cost comparison Process plan Comprehensive cost comparison and process plan D comparison Compared with process plan E and process plan F, process plan A is reduced by about 5 yuan/m. Process plan B is about 10å…€/m. The process plan C is increased by about 12å…€/m (5~8) yuan/m. 4 Conclusion The experimental study on the application process of electrophoretic coating on fasteners concluded that the process of surface treatment of fasteners by electrophoretic coating is feasible.

Fastener-specific electrophoretic coatings must be used to achieve fastener surface finish quality requirements.

The fastener adopts the electrophoretic coating process: in terms of quality and cost, compared with the electro-galvanizing process, the surface corrosion resistance is improved, the salt spray test reaches 240 hours or more; the overall cost of the process plan A is reduced, and the process plan B and the process plan C are integrated. The cost is improved; compared with the Dacromet coating process and the chromium-free aluminum-aluminum coating, the surface corrosion resistance of the process scheme B and the process scheme C is equivalent, the salt spray test reaches about 1000 hours, and the moist heat resistance is improved (the moist heat reaches 500 hours). ), the overall cost is reduced. From the environmental protection, the coating does not contain Cr' is conducive to environmental protection and has good social benefits.

Adopting process scheme A: the friction coefficient of the paint film is suitable, and does not affect the product assembly; the process scheme B: the fasteners are not affected by the fasteners with strict torque requirements; the process scheme C is applicable to the fasteners without the torque requirement. .

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