Research on Influencing Factors of Coating Indentation of Aluminum Alloy Wheels

Author: Du Yunpeng, Zhang Sheng Chao, Zhang Lin, Li Guangcheng, Ray round

Source of this article: Issue 3 of "Coating Industry" in 2021

The wheel is an important part of the automobile.
It not only has the safety requirements of bearing the quality and driving load of the entire vehicle, but also has the aesthetic requirements of balancing the appearance of the entire vehicle.
The material of the wheel has gradually developed from steel rim to aluminum alloy wheel.

1 Experimental part

1.
1 Experimental materials

ZL101A aluminum wheels, 6061 aluminum plates, acrylic transparent powder coatings, pearl cotton, paper-plastic partitions: commercially available.

1.
2 Experimental equipment

Pen hardness tester: 318, ERICHSEN; Differential Scanning Calorimeter (DSC): DSC3, METTLER TOLEDO; Micro Fourier Transform Infrared Spectrometer (Miro-FTIR): spotlight200, PerkinElmer; Stereo microscope : M205C, Leica; Scanning Electron Microscope (SEM): EVOMA25/LS25, Zeiss.

1.
3 Experimental method

The aluminum wheel simulation indentation test program was used to verify the indentation of the wheel coating surface, as shown in Figure 1.

Figure 1 The simulated indentation test scheme for aluminum wheels

As shown in Figure 1, the wheels are in accordance with the packaging process requirements.
Paper-plastic partitions are placed on the bottom support, the wheel hub is stacked, and 4 are stacked in a single layer.

1.
4 Performance testing

A stereo microscope and a scanning electron microscope were used to detect the morphology of the coating film; a differential scanning calorimeter was used to detect the glass transition temperature of the coating film at a heating rate of 10℃/min; a micro Fourier transform infrared spectrometer was used to detect the coating film Measure the composition of organic matter; use a pen-type hardness tester to test the scratch pressure.
The indenter diameter is 0.

2 Results and discussion

2.
1 Analysis of coating film performance

2.
1.
1 Analysis of indentation morphology

The surface morphology of the indentation area is shown in Figure 2.

Figure 2 Surface morphology of the indentation area

As shown in Figure 2, the result of the stereomicroscope photo shows that the indentation area is obviously wrinkled, and the coating film in the non-indentation area is flat and smooth; the SEM photo shows that the coating film has a fibrous structure, similar to the surface fiber shape of the packaging material (pearl cotton).

2.
1.
2 DSC analysis

Scrape the front surface coating film of the spokes from the indented wheel hub, and conduct a DSC test to determine the degree of coating film curing.
The experimental results are shown in Figure 3.

Figure 3 DSC curve of coating film

Glass transition temperature (Tg) is an important parameter of amorphous polymer.
Before and after the cross-linking and curing reaction of thermosetting coatings, the Tg will change accordingly.
If the reaction is complete, the corresponding Tg will no longer change.
As shown in Figure 3, Tg1 is the glass transition temperature of the indentation sample to be tested, and then continue to heat up until the coating film is fully cured (>150℃, according to the transparent powder chemical technical data sheet); Tg2 is the transparent powder after the sample is fully cured Glass transition temperature.
In engineering, ΔTg=Tg2-Tg1<2℃ is the criterion for judging that the coating film is fully cured.
The sample Tg1=71.
35℃, Tg2=72.
79℃, ΔTg=1.
44℃, the DSC result proves that the acrylic transparent powder coating has been completely cured.
The indentation has nothing to do with the degree of curing.

2.
1.
3 Miro-FTIR analysis

Scrape a small amount of the coating film under the microscope in the indentation area and non-indentation area of ​​the hub respectively, and conduct the Miro-FTIR test to confirm the change of the coating film composition at the indentation area.
The test results are shown in Figure 4.

Figure 4 Miro-FTIR comparison of coating film between indentation area and non-indentation area

It can be seen from Figure 4 that the composition at the indentation is exactly the same as the composition on the surface of the normal coating film, and no other foreign matter remains.
Comprehensive DSC results show that the indentation on the surface of the wheel hub is a wrinkle on the surface of the coating film, which may be caused by the deformation of the coating film caused by external force after the complete coating film is formed.

2.
1.
4 Analysis of the relationship between scratches and temperature

A pen-type hardness tester was used to measure the relationship between the scratch pressure on the surface of the acrylic transparent powder coating film and the surface temperature, and the results are shown in Figure 5.

Figure 5 The relationship between the scratch pressure on the surface of the coating and the surface temperature

It can be seen from Figure 5 that when the surface temperature is less than 45°C, the scratch pressure on the polished surface of the finished car remains unchanged with the increase in temperature, and when the surface temperature is greater than 45°C, the scratch pressure decreases with the increase in temperature.
It shows that when the wheel surface temperature reaches 45°C, the coating film begins to soften, and the hardness decreases with the increase of temperature.

2.
2 Analysis of the impact of coating process

2.
2.

Figure 6 shows the test results of the hardness of the wheel hub acrylic powder coating film after rolling off the assembly line and the storage time after baking.

Figure 6 The relationship between film hardness and offline time

It can be seen from Figure 6 that the hardness of the coating film is HB after the hub is off the assembly line, and it becomes F after 2 hours and remains F for a long time.

2.
2.
2 Baking temperature

The wheels were baked at high, medium, and low temperatures in the transparent powder baking interval, and then the indentation simulation test was performed.
The results are shown in Table 1.

Table 1 Indentation on the surface of the wheel coating film at different baking temperatures

It can be seen from Table 1 that the coating film of the wheel surface cured at 173℃, 177℃, 181℃ will produce indentation after the test, and the proportion of indentation products is all >40%, indicating the way to promote the complete curing of the coating film by increasing the baking temperature Will not eliminate the indentation of the coating film.

2.
2.
3 Cooling method

The different surface temperatures of the wheels are controlled by adjusting the amount of cold air, and the wheels are packaged after reaching this temperature.
The influence of the packaging temperature on the indentation of the coated wheel hub is shown in Table 2.

Table 2 The influence of packaging temperature on film indentation

It can be seen from Table 2 that under the condition of not higher than 50℃, there is no obvious difference in the proportion of indentation products of different packaging temperature test wheels, indicating that the packaging temperature has no obvious influence on the indentation.

2.
3 Analysis of the impact of packaging methods

2.
3.

The influence of different packaging materials on the indentation of the coating film is shown in Table 3.

Table 3 Comparison results of indentation of different packaging methods

2.

Store the test wheels indoors (below 30°C), outdoor storage (up to 60°C), and outdoor shading storage (below 40°C).

Table 4 The influence of storage conditions on the indentation of the coating film

It can be seen from Table 4 that the indoor and outdoor shading storage test wheels have almost no indentation.

Combining the analysis of the results of Figures 3 and 5, although the Tg of this acrylic transparent powder coating is 72.

2.

Prepare 14 pallets of wheels (20 pcs/pallet), a set of 3 pallets, and store them outdoors at the same time after packaging.

Table 5 The influence of outdoor storage time on the impression of wheel coating film

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3

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