The Effect of Surface Roughness on the Performance of Electrostatic Powder Spray Coating of Transmission Shaft

Abstract: According to different sandblasting conditions, the surface of the workpiece for automobile transmission shaft is sandblasted, so that the surface of the workpiece has different surface roughness

Keywords: automobile drive shaft; electrostatic powder coating; surface roughness; freezing impact test; cyclic salt spray corrosion

0 Preface

In the coating process, the initial state of the workpiece surface will directly affect the final coating performance

With the development of the automobile industry, the requirements for the surface treatment and protection performance of chassis parts such as drive shafts are becoming higher and higher, and there is an urgent need to determine a suitable mechanical pretreatment process to treat the workpiece before electrostatic powder spraying.

1 test

1.

1.

The material of the drive shaft is similar to No.

1.

The sample is sandblasted with SH-4 4-station automatic sandblasting machine produced by Shanghai Jichuan Machinery Equipment Co.

1.

After the sandblasting, the Zeiss AxioComHRc stereo microscope was used to observe the surface morphology of the sample after mechanically removing the oxide scale; the German Marr roughness tester was used to detect the surface roughness (Ra) of the sample

1.

After the sample is sandblasted, it is treated with electrostatic powder spraying

After phosphating, VEGA3 TESCAN scanning electron microscope was used to observe the crystalline state of the phosphating film on the surface of the sample
.

1.
3 Performance evaluation of coated samples

1.
3.
1 Cold shock test

The coated samples were aged for 24h at a low temperature of (-40+3)°C, and then, in accordance with DINISO4532-1995 "Determination of the impact resistance of glaze and enamel layers" According to the regulations in, the coating sample in the frozen state is placed on a harder backing plate, the steel ball is bounced onto the surface of the coating sample (90N), and the impact test is carried out
.
The surface of the coating sample has cracks due to impact.
Observe the diameter of the cracked area of ​​the paint film to judge the low-temperature impact resistance of the coating sample
.

1.
3.
2 Cyclic corrosion test

Before the test, use a special tool to make a straight line with a length of 50mm, a width of 1mm, and a depth to the metal substrate of the coated sample on the surface of the coated sample in the axial direction, as shown in Figure 2
.
Afterwards, the cyclic corrosion test was carried out according to the requirements of the DBL7381-2012 standard
.
Every 7d is a cycle, among which, the first day is a 24h neutral salt spray test [NaCl mass concentration (50±10)g/L, temperature (35±2)°C]; the second to 5th days are 8h hot and humid per day Environment static [(40±2)°C, 100%RH], 16h normal temperature static [(23±2)°C, (50±20)%RH]; 24h normal temperature static on the 6th and 7th day Set [(23±2)°C, (50±20)%RH]
.
After the 6-week cyclic corrosion test, observe the corrosion of the paint film at the marking position on the surface of the workpiece
.

2 Results and discussion

2.
1 The morphology of the sandblasted sample

It can be seen from Figure 3 that the surface of the unblasted sample (Sample A) is black.
This is because more oxide scale is generated during the heat treatment process; the oxide scale is unevenly distributed on the surface of the A sample and accumulates more.
Area, the oxide scale can be easily scraped off
.
After sandblasting, the entire surface of the sample showed a uniform color, and the oxide scale was completely removed
.
It can be seen from Figure 4 that after sandblasting, many fine wave peaks and troughs appear on the surface of the sample, which is conducive to the formation of phosphating crystals and the combination of the electrostatic powder spray paint film and the sample (4)
.

It can be seen from Table 3 that the surface roughness of A sample is relatively low; after sandblasting, the surface roughness of the sample increases significantly, and as the sandblasting pressure and sand diameter increase, the surface roughness of the sample increases
.

The sandblasted sand is made of brown corundum with a hardness greater than 9.
0 (Mohs hardness), and the surface hardness of the heat-treated sample is 54-60HRC.
The hardness of the sand is much greater than the hardness of the sample surface, so the sand can remove the oxidation attached to the sample surface Skin, and impact the metal matrix into uniform pits.
Increase the specific surface area of ​​the sample
.
The larger the diameter of the sand grain, the greater the roughness of the sample surface after sandblasting
.
Too small surface roughness is not conducive to the improvement of coating adhesion, and too large roughness will cause uneven coating distribution, insufficient film thickness at the wave peak, causing "spot rust", and will also affect the quality of the coating.
If the roughness is too large, it will cause the "loss" of the film thickness and increase the cost of the coating [1, 4]
.

2.
2 Morphology characterization of phosphating film

It can be seen from Figure 5 that after phosphating the original sample with oxide scale on the surface, the phosphating film crystals are cubic, but the crystal size is different, and the coverage of the entire phosphating film is less than 70%; the oxide scale is removed by sandblasting After that, the phosphating crystals generated on the surface of the workpiece are needle-like, with a size of about 5-20 µm; the surface of the C sample (roughness of 3.
0-4.
0 µm) has the densest phosphating crystals, with a coverage rate of over 93%; continue to increase the sample The surface roughness of the phosphating crystal increases but the coverage rate decreases
.

It can also be seen from Figure 5 that the phosphating crystals are mainly needle-shaped, leaf-shaped, granular and so on
.
Due to the anchoring effect of the phosphating layer, the phosphating crystals are closely combined with the paint film, which can improve the adhesion of the paint film [5-7]
.
Therefore, the phosphating crystals with small size, uniform distribution and high coverage have good bonding performance with the paint film
.

2.
3 Performance characterization of the coating

2.
3.
1 Freezing impact performance

Under the condition of low temperature of -40°C, the brittleness of the phosphating film on the workpiece surface and the epoxy resin film formed by electrostatic powder spraying increases.
Under the action, it is easier to separate from the phosphate layer and form peeling [8]
.

It can be seen from Figure 6 that the paint film formed by electrostatic powder spraying for the A sample without sandblasting treatment has poor adhesion.
After the cold impact test, the maximum diameter of the paint film peeling off is about 6.
3mm; and after sandblasting After treatment, the adhesion of the coated sample is greatly improved
.
This is because after the sandblasting process is used to remove the oxide scale on the surface of the sample, the crystal size of the phosphating film is uniform and compact, which can well improve the adhesion of the paint film to the workpiece
.
When the surface roughness of the substrate sample is 3.
0~4.
0μm (C sample), after the electrostatic powder spraying, the paint film peeling diameter after the cold shock test is the smallest (2.
0-3.
5mm), that is, the C coating sample The freezing impact performance of the coating sample is the best.
Continue to increase the surface roughness of the substrate sample, the coverage of the phosphating film will decrease, the crystal size will become larger, and the freezing impact performance of the coating sample will become worse
.

2.
3.
2 Cyclic corrosion salt spray test

The cyclic corrosion test can objectively and quickly evaluate the performance of automotive coatings, and can well simulate the destructive factors such as the alternation of cold, heat, water, and salt spray [8-9]
.
By measuring the unilateral expansion of the coating sample after the cyclic corrosion salt spray test, the performance of the coating can be inferred intuitively [10]
.

It can be seen from Figure 7 that after 6 cycles of cyclic salt spray test of A sample, the unilateral expansion is about 4.
2mm; and after sandblasting, the resistance of the coated sample to cyclic salt spray corrosion is greatly improved
.
Among them, the single-sided corrosion of the C-coated sample is the smallest, about 1.
0mm
.
Phosphating crystals are densest when the surface roughness of the substrate is 3.
0-4.
0μm, and has the highest coverage rate to the metal substrate, which can effectively improve the corrosion resistance of the coating
.

3 Conclusion

Using sandblasting to remove the oxide scale on the surface of the automobile drive shaft workpiece to improve the surface roughness of the workpiece can effectively improve the adhesion of the paint film and the salt spray corrosion resistance
.
Using No.
80 brown corundum grit, the workpiece is sandblasted for 3 minutes under a pressure of 0.
35 MPa, so that the surface roughness of the workpiece is Ra3.
0-4.
0 µm.
After phosphating, the crystal of the phosphate film is the densest, and the coverage rate exceeds 93.
%, the peeling diameter of the coated sample after electrostatic powder spraying is less than 3mm after the cold shock test, and the unilateral expansion after 6 weeks of cyclic salt spray corrosion is less than 2mm
.