Effect on the Wear Resistance of Copper Alloy Surface Modification Layer by FSSP Implanting W Particles
2019 Volume 60 Issue 5 Pages 765-769
Details
2019 Volume 60 Issue 5 Pages 765-769
This paper aims to study the surface modification of H62 copper alloy with W nanoparticles implanted on its surface by friction stir surface processing (FSSP). Three implantation methods were studied, with the total implantation depth of 0.2 mm in each method. The first process method is that W powder is extruded to the surface of the copper alloy at a depth of 0.2 mm directly and one-timely through the tool. The second process method is to extrude the W powder into the surface of the copper alloy at a depth of 0.15 mm by a tool firstly, then return the tool to the starting end, and rotate the tool again on the copper alloy surface at a depth of 0.05 mm. The third process method is also to extrude the W powder into the surface of the copper alloy at a depth of 0.15 mm by a tool firstly, then move the tool backward from the stirring end to the starting end on the copper alloy surface at a depth of 0.05 mm. The microstructure, hardness and wear properties of the modified layers of the samples obtained from the three FSSP modified copper alloy surface techniques were tested and analyzed. The results of the three process methods show that W particles can be used to modify the surface properties of copper alloys, but the second process method has the best effect. The second process method can well achieve the uniform distribution of W particles on the copper alloy surface. The hardness of the modified layer was improved compared with the base metal. Among them, the hardness of the modified layer obtained by the second process was increased by 41.4%, the third process method was increased by 35.1%, and the first process method was increased by 16.1%. The friction coefficient of the modified layer obtained by the three technological methods is smaller than that of the base metal, and the second method produces the smallest friction coefficient.
Friction stir surface processing (FSSP)1,2) is a new surface modification technology for metals based on friction stir processing (FSP).3–6) FSP cannot be applied on some key occasions7–10) (Such as pressure vessel internal surface, valve internal surface, etc.) because the stirring needle extrusion into the metal base material, causing large changes in the properties of the metal energy. The FSSP well avoids this shortcoming because it mainly obtains the modified layer by high-speed rotation and extrusion on a metal surface through a needleless tool. Copper alloy is widely used in electronic products, propellers and other key components of large ships because of its good electrical conductivity and wear resistance. The main methods for improving the surface properties of copper alloys are electroless plating, heat treatment, laser cladding and so on.11–15) However, these methods do not guarantee the copper alloy surface properties for a long time, and its safety is low even at critical times. FSP can improve the properties of copper alloys, but it destroys the copper alloys microstructure and reduces the internal properties (Such as fatigue property, tensile property, bending property, etc.), so it cannot be used normally. The FSSP technology is a good method to overcome this problem. It not only maintains the internal structure of the copper alloy, but also improves the surface related properties.
In this study, a method for improving the surface properties of copper alloys is proposed. Firstly, some small holes are drilled on the surface of the copper alloy. Secondly, the W nanoparticles are filled into the holes and compacted. Lastly, surface compaction and extrusion processing of compacted W particles is carried out by FSSP technology. The experiment verifies that surface modification of Wp/H62 copper alloy (W powder was implanted on the surface of H62 copper alloy) can improve the copper alloy surface properties. Therefore, this method is helpful for practical engineering applications.
Hot rolled H62 copper alloy was used in the experiment, and the size of the plate is 200 × 200 × 10 mm. The main chemical constituents are shown in Table 1.
The material injected with copper alloy is W and the particle size is 40 nm.
2.2 Experimental equipmentThe equipment used for preparing the copper alloy surface modification layer is FSW-LS-A10 Friction Stir Welding Equipment developed by China Aviation Industry Beijing Saifost Technology Co., Ltd., as shown in Fig. 1.
The photo of friction stir jointing.
The stirring tool used in the preparation of copper alloy surface modification layer is a special stirring tool without stirring pin. The shoulder with scroll groove. Its specific structure is shown in Fig. 2.
The photo of the pin.
The copper alloy was cut into a uniform specimen block in the experiment by wire cutting machine (200 × 200 × 10 mm). Oil stains and oxides on the surface of copper alloys are treated before FSSP processing. In order to obtain the effective surface modification of Wp/H62 copper alloy, the surface of the treated copper alloy was drilled some holes. The depth of the hole is 0.2 mm, the diameter is 1.5 mm, and the distance between two holes is 3 mm, as shown in Fig. 3.
Copper alloy sheet surface drilling.
W powder was filled into small holes and compacted before carrying out the surface of FSSP modified copper alloy. The specific compaction process is as follows: the W powder is now filled into the hole, and then the cylinder bar slightly smaller than the diameter of the hole is clamped on the small drilling machine, the end of the circular bar is aligned with the hole filled with W powder, and the cylinder bar is squeezed into the hole by applying downward pressure through the handle of the small drilling machine to implement the powder compaction process. Three process methods of preparing FSSP alloy surface modification layer were proposed for making the description of process method more clear, first process method: the direct downward pressure of the pin was 0.2 mm; second process method: for the first time, the downward pressure of the pin was 0.15 mm. The second time, according to the path of the first time modification, the modification was carried out again, and the downward pressure of the pin was 0.05 mm; third process method: for the first time, the downward pressure of the pin was 0.15 mm. The second time, according to the opposite path of the first time modification, the modification was carried out again, and the downward pressure of the pin was 0.05 mm. See Table 2 for detail.
The metallographic structure of the modified layer was observed using a dual inverted 4XB type metallographic microscope produced by Shanghai Cai Kang Optical Instrument Co., Ltd. The wear morphology of the modified layer was observed using scanning electron microscope (SEM) of S-3400N tungsten filament produced by Hitachi Co., Ltd.
The hardness test of the modified layer was made using the Rockwell hardness tester HRS-150 produced by Ji’nan Hengxu test machine Co., Ltd. The ball pressure head with a diameter of 1.5875 mm was selected for the experiment. The initial test force was 10 kg, the main test force was 90 kg, and the total test force was 100 kg. The loading and unloading time was 10 s. Each test point was tested 3 times, and the average value is the hardness value.
The wear test of the modified layer was made using the high temperature HT-1000 type friction produced by Lanzhou Zhongkai Hua Technology Co., Ltd. In the friction and wear test, the diameter of the steel ball is 6 mm, the rotation radius of the steel ball is 2 mm, and the force of the steel ball is 600N. Before the experiment, the surface of the sample was polished into a mirror with sandpaper, and then cleaned with anhydrous ethanol and dried.
Figure 4 shows the photos of Wp/H62 copper alloy modified surface under different process methods. Figure 5 shows the metallographic photographs of Wp/H62 copper alloy modified surface under different process methods and base metal. Figure 5(a) shows that the black dots (Wp) are less distributed, but more evenly distributed in Fig. 5(c). However, the overall number of distributions in Fig. 5(c) is relatively small. Figure 5(b) shows that the black dots are distributed most homogeneous, and the tissue size is relatively uniform. This phenomenon indicates that the surface modification layer of Wp/H62 copper alloy prepared by the first method has less distribution of Wp, and most of the Wp disappeared. This may be during the FSSP process when the tool shoulder and pin are extruded to form toe flash. The surface modification layer of Wp/H62 copper alloy prepared by the third method may not be uniform due to insufficient heat generation (Because for the third process method, the rotation speed and the feed speed of the tool are the same, but in the opposite direction, stirring resistance is larger and the loss of heat is more. Therefore, there’s not enough heat being generated), although Wp is also distributed according to diffuse distribution, it is scarce and not evenly distributed. The Wp distribution is more uniform on the modified surface of Wp/H62 copper alloy prepared by the second method, because it could effectively produce the metal plasticizing heat (Because for the second process method, the rotation speed and the feed speed of the tool are the same, and in the same directions, less work is done against resistance and thus less heat is lost. Therefore, more heat is generated by plasticization), so that the Wp flowed better on the surface of copper alloy substrate. Figure 5(b) shows the small and uniform organization of Wp. The grain in Fig. 5(a)–(c) is obviously better than the base metal in Fig. 5(d).
The photos of the modified layer of Wp/H62 copper alloy.
The microstructures of the modified layer of Wp/H62 copper alloy and Base metal.
The grains in the center of the pin are refined and equiaxed. The boundary area between the material outside the center of the pin and the base metal is refined but elongated. This is mainly the result of the extrusion between the pin and the base metal. And then out material have the same flat, long grains as the base metal, because the base material is rolled.
3.2 Hardness analysis of Wp/H62 copper alloy modified surfaceFigure 6 shows the hardness values of Wp/H62 copper alloy modified surface prepared by different methods. The hardness test position in Fig. 6(c) is in the white box area of each photo in Fig. 4.
Different position distribution of hardness value on the modified layer.
Figure 6(a) shows that the middle position of the modified surface belongs to the second process method, the first process method has the highest hardness, and followed by the third process method. The first process method was used to prepare the modified surface at last. The average hardness (The average hardness value is obtained by dividing the total number of hardness values tested at all hardness test points in the same direction in the same process by the number of test points) of modified surface layer prepared by the second method was 68.4HRB, and at the same time, the average hardness prepared by the third process method was 65.32HRB, the average hardness prepared by first process method was 56.16HRB. However, the hardness of H62 copper alloy base metal is 48.36HRB. It can be verified that the hardness of Wp/H62 copper alloy modified surface layer prepared by FSSP is improved. The hardness of the modified surface produced by the second process method is 41.4% higher than that of the base metal, the third process method is 35.1% higher than that of the base metal, and the first process method is 16.1% higher than that of the base metal. The surface hardness of the modified surface prepared by the second process method is higher than that of the third process method. The hardness test results in Fig. 6(a) are consistent with the microstructure mentioned above. In Fig. 5(b), the grains are fine and uniform, followed by Fig. 5(c) and the worst is Fig. 5(a). Grain size and uniformity determine the hardness value.
Figure 6(b) is the hardness value which measured along the centerline of the modified surface. The average hardness of the modified surface prepared by the second process method is 69.9HRB, the third process method is 64.1HRB, the first process method is 61.7HRB. Their hardness values are higher than those of copper alloys, which is consistent with Fig. 6(a).
3.3 Wear resistance analysis of Wp/H62 copper alloy modified surfaceFigure 7 is the friction coefficient of Wp/H62 modified surface prepared by different FSSP methods at room temperature. Wear test position is shown white box area in Fig. 4. It can be seen from Fig. 7 that the friction coefficient of Wp/H62 modified copper alloy surface modified by second method is the smallest. At the same time, the surface hardness is high and the surface is wear-resistant, which is consistent with the hardness test. The friction coefficient of Wp/H62 copper alloy modified surface prepared by the first process method and the surface modification of Wp/H62 copper alloy prepared by the third process method were studied. In fact, the friction coefficient of Wp/H62 modified copper alloy surface modified by the third process method is small at the beginning, then the first process method was studied. This is because at the beginning, the surface of the modified Wp/H62 copper alloy surface prepared by the third process method has a certain Wp distribution which ensures the high surface hardness. When the friction reaches a certain degree, the Wp is worn out, and the substrate surface, hardness decreases, friction coefficient increases. The surface of Wp/H62 modified copper alloy modified by the first method has less Wp and low hardness initially, however, the friction surface is continuously extruded under long time friction. On the contrary, the hardness of the extruded surface is increased, resulting in the increase of friction coefficient behind. Figure 7 shows that the friction coefficient of the base metal is larger than that of the modified layer obtained by the three process methods. This is consistent with the hardness rules, which indicates that the wear resistance of copper alloy surface could be improved by implantation of W particles through FSSP.
Friction coefficients of W/H62 copper alloy modified layer by different process and Base metal.
Table 3 shows the difference in weight of Wp/H62 copper alloy modified surface prepared by three different methods at room temperatures before and after friction and wear.
Table 3 shows that the wear weight of the modified layer prepared by the second process method is the smallest, the wear weight of the first process method and the third process method are the same, but both less than the base metal.
Figure 8 is the SEM image of friction and wear tests of modified surface by different methods at normal temperature. As can be seen from the Fig. 8, most of the grinding cracks are rolling marks, and only a small part of them are crushed.
Grinding crack SEM photos of the modified layer by different process and Base metal.
Figure 8(a)–(d) shows that the shape of the wear marks is basically the same, except that particle grinding crack in Fig. 8(a) and (c) is more than that in Fig. 8(b). It is mainly because of the rolling and sliding form, so the friction coefficient is low.
This study was supported by the Key Research and Development Project from Anhui Province of China (Grant No. 1704a0902053); The Key Natural Science Foundation of Anhui Higher Education Institutions of China (Grant No. KJ2016A681); The Major Natural Science Foundation of Anhui Higher Education Institutions of China (Grant No. KJ2016SD55); Supported by open research project of Anhui simulation design and modern manufacture engineering technology research center (Huangshan University) (NO. SGCZXZD03); Huangshan University Talent Start Project (2018xkjg006).
