1. Introduction
Electron beam welding is widely used in industry, especially in aeronautics and aerospace engineering. The weld seam connects the welded parts and the base metal after electron beam welding (EBW). The weld seam has a certain excess height, and the surface quality cannot meet the requirements. Grinding is an effective way to improve the aero-engine performance of aero-engine components. Grinding is used to treat the surface of aero-engine parts, grinding the extra height to produce a uniform surface, and improving its surface quality. Grinding has the characteristics of flexibility and precision, which are widely used in the precision processing of aero-engine parts. However, few scholars in the field of belt grinding have explored the formation mechanism of the ribbed surface by belt grinding and its influence on aerodynamic performance. Therefore, it is necessary to study the aerodynamic performance of the ground surface by belt grinding on titanium alloy EBW.
Scholars have done a great deal of research on the properties of titanium alloy welded and EBW. Kar et al. [
1] studied the effect of electron beam scanning on the porosity generation and fatigue performance of welding joints of TC4 titanium alloy, and the results showed that electron beam scanning could reduce the porosity of welding joints and improve the fatigue performance of joints. Garud et al. [
2] elaborated on the relationship between mechanical strength and working life of welded joints as weak links of mechanical equipment. Experiments show that the fatigue life of the welded toe by hemispherical burr tool is 76% higher than that of the welded toe without polishing. Liu et al. [
3] evaluated the influence of the microstructure unevenness of the electron beam weld of TC4 thin plate on the tensile performance and low-cycle fatigue performance and found that the tensile ductility and low-cycle fatigue strength of the joint was lower than that of the base material. Baptista et al. [
4] polished the toes of the welds of two different sheets of steel under different environmental conditions. The results showed that the fatigue life of the welds polished in a 3% sodium chloride environment was longer, and the fatigue strength was higher. Through experiments, Lan et al. [
5] studied the effect of welding pores on the fatigue life of titanium alloy EBW joints. It was shown that the dispersion of pore size and depth was the key factor leading to the widespread dispersion of the fatigue life of welded joints. A model of the influence of a welding pore on the fatigue life of welded joints was established.
In the field of belt grinding, there is also much research, which includes robotic belt grinding technology, prediction of surface roughness of belt grinding, and precision grinding methods of aviation parts belt grinding, to name a few areas of interest. Single factor belt grinding tests of titanium alloy aero-engine blades were carried out by Chai et al. [
6]. The effects of belt speed and belt feed speed on belt blockage during grinding were analyzed. Xiao et al. [
7] developed an adaptive belt grinding method for an integral blade disc. The deformation error of the blade was compensated by measuring and controlling contact pressure. Precision grinding was achieved by controlling the rotating speed. Experiments showed that the method can be used as a precision processing method for the blade disc. Pandiyan [
8] developed a machine learning belt grinding system based on deep learning, which can monitor and predict the new height distribution of welding seams and the endpoint of belt grinding in extra real-time height. Wang and Yan et al. [
9,
10] studied robot belt grinding technology from two aspects—the robot belt grinding model and surface quality—which laid a theoretical foundation for robot belt grinding technology.
Many scholars have conducted research on ground surfaces and their characteristics. Some achievements have been made in reducing noise, improving hydrophobicity, and improving aerodynamic performance. Frohnapfel et al. [
11] studied the surface of turtle skin and simplified the groove structure on the surface of turtle skin; they found that the blade surface had the best drag reduction effect, and the turbulence dissipation rate between the grooves had a significant impact on the final drag reduction effect. Chen et al. [
12] prepared strip textures with different spacing on the surfaces of 3Cr2W8V, H13, and HD(4Cr3Mo2NiVNb) materials, and compared them with smooth surfaces to investigate changes in the wear amount. The study found that, compared with smooth surfaces, the wear amount of strip textures was smaller, and the wear resistance of texture surfaces decreased with the increase of stripe spacing. Ma et al. [
13] studied the ground surface and found that it had excellent drag reduction performance. Xiao and colleagues [
14] established a parameterized mathematical model for grinding a multi-particle abrasive belt and proposed a method for grinding a micro-bionic serrated surface abrasive belt. Finally, with the blade profile of titanium alloy as the object, a primary experimental platform for grinding the micro-bionic serrated surface abrasive belt with titanium alloy as the typical material was built, and the bionic surface was tested and verified, making it possible to process the rib surface.
From the analyses above, many scholars have focused on the welding performance of titanium alloy electron beams and improving the welding quality. Belt grinding, as a standard finishing method for aero-engine blade surfaces, is very useful for blade surface processing, but research on the ground surface is relatively minimal at present. Therefore, it is of considerable significance to explore the belt grinding performance of complex surfaces with bionic structures; thus, belt grinding experiments were carried out on the welds of titanium alloy EBW. The surface quality of the welds and the aerodynamic performance of the ground surfaces were analyzed to judge the property of titanium alloy EBW belt grinding with a bionic structure.
3. Analysis of Experimental Results
3.1. Analysis of Grinding Surface Quality
Grinding is often used for material removal, eliminating excess weld height, or polishing, which also makes the weld surface smoother. The surface of the grinding workpiece is observed by an optical microscope at 50
× magnification. We obtained an observation graph, as shown in
Figure 6A,A’, respectively, which show the micrographs of the weld surface and the weld ridge before grinding. By comparing the micrographs before and after grinding, it can be seen from the figure that the surface texture before grinding is relatively messy with obvious texture fracture, while the surface texture after grinding is uniform and continuous.
Figure 6 (a)–(d), (a’)–(d’) represents the surface machined under the grinding parameters in
Table 3. Under different parameter conditions, the surface after grinding is different, but compared with the surface without grinding, it is relatively smooth, and the consistency of surface texture is very good. It can be seen that the diamond belt grinding can eliminate well the welding margin of titanium alloy electron beam welds so that the welding margin meets the requirements of parts and achieves a smooth transition between the machined surface and the substrate.
In this experiment, the roughness instrument TR200 was used to measure the roughness parameter Ra. The surface roughness of the welded surface under the grinding parameters in
Table 3 is measured, asshown in
Figure 7.
From the results of the roughness measurement, it can be seen that the roughness decreased after grinding with the decrease in feed speed. With the increase of belt speed, the roughness decreased after grinding. The surface roughness of the welded face was generally lower than that of the welded ridge, which shows better grinding machinability.
Due to the larger width of the welding surface, grinding can reduce the surface roughness of the workpiece and improve the surface finish. Therefore, the face of EBW should be the main working face of the workpiece, while the ridge of welding should be the secondary working face or non-working face of the workpiece. Similarly, the belt grinding of titanium alloy EBW seam follows the basic rule of material removal. With an increase in cutting speed and decrease in feed speed, the surface quality and roughness improve. Therefore, when choosing the grinding speed of the belt, we should try to choose a faster belt speed and lower feed speed when considering the factor of roughness.
3.2. Analysis of Formation Law of Ground Surface
In order to better observe the surface, microscopic observation on the processed surface was conducted by an ultra depth of field electron microscope. The test results are shown in
Figure 8, where (a)–(d) and (a’)-(d’) represent the surface under the grinding parameters in
Table 3.
From the detected results of ultra depth of field electron microscopy, it can be seen that the belt rows out the ravines on the surface of the welded seam, and the ravines distribute on the surface of the weld. The ground ribbed surfaces range in width from 20 to 40 μm and in height from 3 to 10 μm. The angles of the ribs are about 50 to 70 degrees. In the case of lower belt speed and grinding speed, the microstructure of the grinding surface is more prominent. The low feed speed is beneficial in prolonging the processing time between the strip and the workpiece surface and the overall surface treatment. The lower linear velocity of the friction band can avoid the damage of friction particles to the surface of the rib.
In order to facilitate the study of the surface characteristics of the stiffened ribs, some stiffened surfaces were selected to simplify the geometric surface models analyzed and make the stiffened rib surface more scientific and reasonable. The fifth group and the third group of experiments have visible ground surface characteristics, so this paper extracted the geometric features of the two groups of surfaces. Two kinds of 3D models were modeled according to the geometric elements, referred to as type I and type II. The smooth surface was also modeled to contrast, called type II.
Figure 9 and
Figure 10 are the geometric feature extraction models and simulation analysis models of the ground surface.
Figure 9 shows the type I model.
Figure 9a represents the microscopic view of the weld, while
Figure 9b represents the profile of the weld section, and a standard model was used to fit the monitoring results. By analyzing the detection results, the average depth of the valley floor was equal to 10 μm, and the average width between the two valley peaks was 40 μm; thus, the model in
Figure 9c was obtained. It can be seen from the figure that the reinforcing rib presents a gully shape. The width of the gully is about 40 μm, the height is 10 μm, and the angle is about 60–70 degrees. For this reason, we simplified the model, as shown in
Figure 10.
Figure 10 shows the type II model.
Figure 10a represents the microscopic view of the weld, while
Figure 10b represents the profile of the weld section. A standard model was used to fit the monitoring results. By analyzing the detection results, the average depth of the valley floor is equal to 3 μm, and the average width between the two valley peaks was 20 μm; thus, the model in
Figure 10c was obtained.
3.3. Analysis of Ground Parameters
According to the above orthogonal test results, the influence of grinding parameters on the grinding effect of welds was analyzed. The results are shown in
Figure 11.
As shown in
Figure 11, the influence of grinding parameters such as grinding pressure, feed speed, and grinding line speed on Ra of the face and ridge surfaces of the weld is shown. Under the conditions of grinding pressure of 15 N and 25 N, the face Ra of the weld was 1.72 μm and 0.98 μm, and the ridge Ra was 1.77 μm and 1.03 μm, respectively. Under the feed rate of 0.01 m/s and 0.015 m/s, the Ra of the face of the weld was 1.48 μm and 1.22 μm, and the Ra of the ridge of the weld was 1.57 μm and 1.28 μm, respectively. When the grinding speed was 10 m/s and 15 m/s, the Ra of the face of the weld was 1.4 m/s and 1.3 m/s, and Ra of the ridge of the weld was 1.5 m/s and 1.35 m/s, respectively. The range indicates the difference between different grinding parameters under the same grinding factor.
From the above experimental results, it can be seen that the surface roughness of titanium alloy EBW can be reduced to less than 0.15 μm after grinding. Ra will decrease with the increase of belt speed, the decrease of feed speed, and the improvement of grinding positive pressure. At the same time, it can be seen that the range of grinding positive force is more significant than the other two factors. We can thus say that the effect of Fn on Ra is followed by vp then vs. This conforms to the fundamental technological rules of cutting. In addition, the surface roughness on the weld face is generally lower than weld ridge. This is mainly because the width of the weld face is more comprehensive, and the grinding smoothness is better.