1.Background
In recent years, the surface treatment of titanium steel composite plates has mainly focused on the grinding process of the steel substrate, while there are few reports on the surface treatment process of the titanium composite layer. Due to the unique physical and chemical properties of titanium coating, titanium absorbs hydrogen at 200 ℃, oxygen at 400 ℃, and nitrogen at 600 ℃ . When the surface treatment process is not chosen properly, brittle phases such as oxides are easily generated on the surface of the titanium coating during grinding, resulting in a decrease in surface cleanliness and a significant increase in hardness, which affects the subsequent bonding quality between titanium and steel. At present, mechanical treatment, chemical treatment, laser treatment and other methods are mainly used to remove the surface grease and oxide film of titanium and titanium alloys, in order to obtain a high-purity titanium coating surface. The mechanical processing method mainly uses machine tools, grinders, sandblasting and other methods. Although it can effectively remove surface oxides, it is time-consuming and difficult to process titanium coatings with large processing areas and thin thicknesses. Chemical treatment methods include acid washing, alkali washing, chemical coating, etc., which are suitable for cleaning thin parts but leave adsorption layers composed of corroded liquids and gases. Laser treatment is often used as a surface modification method for titanium materials, and the degree of cleaning is affected by factors such as its power parameters. Currently, it has not been put into large-scale industrial production and further experimental verification is needed. Therefore, it is necessary to conduct in-depth research on the surface cleaning treatment process of GR2 Cladding Surface of Titanium Steel Composite Plate in response to the above key points. This article explores the effects of three processes, sandblasting, acid washing, and laser cleaning, on the surface cleanliness of GR2 industrial pure titanium Combined with the surface microstructure and microhardness, a reasonable titanium coating surface treatment process is proposed, which has important guiding significance for the industrial production of rolled titanium steel composite plates
2. Experimental Materials and Methods
This article uses GR2 industrial pure titanium as the experimental material, and the measured composition is shown in Table 1
Three processes, namely sandblasting, acid washing, and laser cleaning, were used to treat the surface of GR2 industrial pure titanium plate. The macroscopic morphology of the GR2 surface after treatment is shown in Figure 1. During sandblasting, 400 mesh quartz sand was used, and during acid washing, a mixed acid solution of (3mlHF+37mlHNO3+60mlH2O) was used; The HST-100 laser cleaning machine is used for laser cleaning, with a center wavelength of 1064nm, a maximum output average power of 100W, and a maximum laser pulse frequency of 200kHz. Three different surface treated GR2 pure industrial titanium plates were cut into samples with dimensions of 10mm × 10mm × 10mm using wire cutting. The morphology of the surface and longitudinal section of the sample without surface treatment was observed using metallographic microscope (OM) and scanning electron microscope (SEM), and the surface composition was analyzed by energy dispersive spectroscopy (EDS). Use Qness 60 Vickers hardness tester to analyze the surface microhardness of GR2 matrix and samples after different surface treatments.
Fig.1 Macroscopic surface morphology of GR2 Cladding Surface of Titanium Steel Composite Plate
Results and Discussion
2.1 Sandblasting treatment
The surface morphology of GR2 industrial pure titanium plate after sandblasting treatment is shown in Figure 2 (a) and (b). To determine the thickness of the oxide layer, the morphology of the longitudinal section perpendicular to the sample surface was observed as shown in Figure 2 (c), and EDS energy spectrum analysis was performed at different positions as shown in Table 2. Based on the macroscopic morphology analysis in Figure 1 (a), it was found that the surface of GR2 after sandblasting treatment was uneven, with no obvious metallic luster and a dark gray color. Through SEM morphology observation, it was found that the surface of GR2 after sandblasting was grooved, with irregular particles embedded in the matrix, with a particle size of about 30 μ or less. EDS analysis at positions 1-4 on the surface revealed the presence of significant O and Si elements on the GR2 surface after sandblasting treatment. The O element content ranged from 6wt% to 10wt%, while the Si element content was below 7wt%. Among them, the O element was residual surface oxide scale, and the Si element indicated that irregular particles embedded in the matrix were caused by sandblasted quartz sand . Based on the energy spectrum analysis at positions 2 (c) and 5, it can be found that there is a uniformly distributed dense oxide layer rich in Ti and Si on the surface of the sample, with a thickness of about 2 μ m, while there are no obvious O and Si elements at position 6, which is consistent with the composition of the TA2 matrix. This phenomenon indicates that the surface cleanliness of GR2 after sandblasting treatment is poor, with residual oxide scale and quartz sand, which is not conducive to the subsequent rolling and composite of titanium coating and steel substrate.
Fig.2 GR2 Cladding Surface of Titanium Steel Composite Plate
2. 2 Acid washing treatment
The surface and cross-sectional morphology of GR2 industrial pure titanium plate after acid washing treatment are shown in Figure 3. EDS energy spectrum analysis was performed at different positions, as shown in fig. 3
Fig.3 Surface and section morphology of GR2 after pickling treatment
The results showed that the surface cleanliness of GR2 was significantly improved after acid washing treatment compared to sandblasting treatment, and the surface oxide scale was almost completely removed. The O element content was only below 3wt%. The cross-sectional morphology in Figure 3 (c) is in sharp contrast to sandblasting treatment, and there is no obvious oxide layer on the surface of TA2 after acid washing. However, after acid washing treatment, GR2 reacted strongly with the mixed acid solution, resulting in irregularly distributed corrosion pits on the surface of the sample, as shown in Figure 3 (b)
Upon magnification observation at the concave pit, it was found that a large amount of fine and unknown inclusion particles were adsorbed inside the corroded pit, which would be a hidden danger for the subsequent rolling and composite of titanium and steel. To solve this problem, after pickling the titanium coating, it is necessary to clean the surface of the titanium material with cleaning solutions such as acetone or anhydrous ethanol. The surface morphology of GR2 before and after cleaning is shown in Figure 4. The results indicate that organic solvents such as acetone can effectively remove fine inclusions adsorbed in acid washed corrosion pits, and this method is a reasonable surface treatment process for titanium steel composite plates with titanium coating.
3. Conclusion
(1) After sandblasting treatment, the surface of GR2 is grooved with residual oxides and quartz sand. The content of O element is between 6wt% and 10wt%, and the thickness of the oxide layer is about 2 μ m. The surface microhardness is about 290HV.
(2) After acid washing treatment, the oxide scale on the surface of GR2 is completely removed, and fine inclusions are adsorbed in the local corrosion pits. Acetone needs to be used for further cleaning. The surface microhardness is close to the GR2 substrate, which is about 220HV, making it a reasonable titanium coating surface treatment process.
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