Research on friction stir welding process between T2 copper and H62 brass dissimilar materials
Abstract: Friction stir welding process research was conducted on dissimilar materials of T2 copper and H62 brass. The weld formation, joint microstructure and joint mechanical properties of copper and brass with different plate thicknesses under various process parameters were experimentally analyzed. The distribution of the two materials in the joint and the phase composition at the junction were analyzed from a microscopic perspective. Experiments show that appropriate welding process parameters can obtain a copper-brass joint with excellent structure and performance, and there is a transition zone at the junction of the joint. , a transition material with a width of about 1 to 10 μm. The study also found that the microhardness and average tensile strength of the joint are between brass and copper.
CLC classification number: TG453 Document identification code: A
Friction stir welding (FSW) is a welding method that uses friction heat as a heat source, also known as solid phase welding technology. Since its inception, this method has attracted widespread attention from domestic and foreign researchers and has been successfully used. It is mainly used for the welding of aluminum alloys. It has gradually expanded to the welding of magnesium alloys, copper alloys, titanium alloys, stainless steel and other materials [1~6]. There are currently few reports on the research on friction stir welding of dissimilar metals [7]. This article The FSW process test was mainly conducted on T2 copper and H62 brass dissimilar metals. The process parameters that affect the quality of the copper-brass joint and the phase components formed during the welding process were studied. The microstructure of the weld and the joint properties were also studied. The mechanical properties were analyzed.
1. Experimental methods
The test uses 2 mm thick T2 copper and H62 brass and 4 mm thick T2 copper and H62 brass as experimental materials respectively. On the SW-3LM-015 special friction stir welding machine, the FSW experiment was conducted on the copper-brass plate. Experiment When welding copper alloys, use a friction head suitable for welding copper alloys. The length of the stirring needle is 0.2 mm to 0.3 mm shorter than the thickness of the welded plate. The angle between the direction and the vertical line of the workpiece surface is 2 degree . By changing the process parameters, we can obtain the best results. Optimum joint shape and quality. After welding is completed, cut the required specimen in the direction perpendicular to the weld. The prepared metallographic specimen is corroded with ferric chloride hydrochloric acid alcohol solution (10 g FeCl3, 6 ml HCl, 40 ml of H2O, 60 ml of C2H5OH). When etching, the copper side is etched first, and then the brass side is etched. After etching, large-scale optical microscopes MEF3 and ADVANCE 8D X-ray diffraction were used to analyze the structure and junction zone objects. The phase composition was analyzed, and the microhardness and mechanical properties of the joints were tested.
2. Experimental results and analysis
2.1 Effect of process parameters on weld surface formation
During the friction stir welding process, since the temperature of the advancing side is lower than that of the return side, and the thermal conductivity and melting point temperature of red copper are higher than those of brass, brass is mostly placed on the advancing side and red copper is placed on the return side during welding. Experiment Some of the welding process parameters used are shown in Table 1. Table 1 Friction stir welding process parameters of copper and brass dissimilar materials.
Figure 1 shows the welding surface formation of 2 mm thick T2 copper and H62 brass during friction stir welding under different process conditions. The upper side of the picture is the forward side - brass, and the lower side is the return side - copper. From Figure 1c, It can be seen from d that when the workpiece is thin, the rotation speed of the friction head has a greater impact on the surface forming. When the rotation speed is 700 r/min, the welding speed selection range is relatively large, so although the welding speed is increased, the welding speed The seam surface formation begins to deteriorate. This is because the increase in rotation speed greatly increases the heat input per unit length of the weld, making the material flow properties worse. Another way to increase heat is to increase the shoulder pressure of the friction head. Since the plate is thin, the heat generated by shoulder friction plays a major role. At the same rotation speed, the contribution of shoulder pressure to heat is different. Figure 1a and Figure 1b are when the rotation speed remains unchanged and the welding speed is changed. In the obtained surface forming diagram, due to the large amount of pressure and heat generated by the friction head in Figure 1b, the surface forming ring appears uneven in size.
2.2 Joint microstructure and interface phase analysis
Figure 2 is a cross-sectional morphology of a 4 mm thick copper and H62 brass FSW joint. The right side of the picture is the forward side - brass, and the left side is the return side - copper. As can be seen from the picture, the difference between red copper and brass Mixing mainly occurs in the weld nugget area, and the two flow to each other. There is an onion ring structure in the weld nugget area, represented by A in the figure [8]. The main component in this area is brass, and red copper is only doped in a small amount. In the meantime, the mixed area is large
In some areas, both are large-area block-shaped protrusions connected together. The material transferred during the mixing process of the brass on the right (forward side) is mainly within the diameter range of the shoulder and the middle of the stirring needle, driven by the shoulder The plastic metal on the advancing side covers the metal surface on the return side, while the left copper (return side) moves across the center of the weld nugget to the advancing side driven by the rotation of the shaft shoulder and stirring needle. The material close to the shoulder can reach the hot side of the advancing side. Mechanically affected zone. The metal in the weld nugget zone is stirred and mixed with each other due to strong plastic shear deformation and flow [9,10]. The flow of metal in this zone actually moves around the stirring needle according to certain rules. Finally, the onion ring structure at A in the figure is formed. From the analysis of the copper flow situation in the figure, it can be seen that the flow of material on the forward side is divided into three situations: one is that the metal near the end of the stirring needle flows forward from bottom to top; the other is Onion ring flow appears in the middle of the stirring needle, but on the forward side, this flow is consistent with the flow direction at the end; third, a plastic vortex phenomenon appears on the onion ring flow pattern. The copper B that appears on the forward side is upward from the end of the stirring needle. It flows forward instead of moving from the same height of the copper around the back of the stirring needle. A similar situation also occurs in the thin plate T2/H62 joint. Due to the small thickness, the two materials are connected together in an inclined plane, and in the weld nugget The area where some red copper appears is completely mixed in the brass.
Figure 3 is a microstructure diagram of different parts compared to Figure 2. It can be seen from the figure that the size and shape of the grains in each area of the joint are different, and due to the existence of mixed zones, the situation in the joint is more complicated. Figure 3a shows the copper mother Material area, different from when welding the same type of copper, the copper grains near the brass area increase significantly, as shown in Figure 3b, 3d. This is because the heat conduction coefficients on both sides of the friction head are different. Because the temperature of red copper is high, the thermal conductivity is good. , a large amount of heat is transferred from the copper side, and the copper close to the brass side is in the weld nugget area, and the heat conduction on both sides is slow, resulting in a long high temperature residence time in this area, thus causing the copper grains in this area to grow. On the brass side, because the input heat is too high, the grains grow into coarse equiaxed grains. In the weld nugget area, because the two materials are not uniformly mixed, the grain shape increases significantly in the area where the copper is slightly mixed, while in the single The copper grains in the area are significantly larger than those of brass, and the brass grains are fine and evenly distributed, as shown in Figure 3c. Figure 3f shows the microstructure of the brass thermomechanically affected zone on the forward side. Compared with the return side, this area has an obvious boundary, with two dividing lines. The two sides are composed of grains with obvious differences in size. Analyzing the macroscopic picture, it is found that the dividing lines on both sides of the weld nugget area are basically symmetrical with respect to the weld nugget area. This is caused by the low melting point temperature of brass. In the case of red copper and brass, it is Interpenetration occurs at the junction of block connections, but the penetration area is extremely narrow. In the T2/H62 joint, although the grains on the copper side undergo dynamic recrystallization and dynamic recovery during the welding process, compared with the brass side, the grains are smaller. The change in particle size is not obvious. On the other hand, in dissimilar metal welded joints, the connection condition at the interface of the two materials plays an important role in the mechanical properties of the joint. According to the macroscopic view of the joint, it can be seen that most of the connection forms of the two materials are composed of It consists of areas with obvious dividing lines, with only a few mixed zones. From the joints in Figure 4, it can be seen that there are phases at the junction that are different from the two materials, with a width of about 10 μm, and a band-like distribution along the junction line. Figure The black phase infiltrates into the white phase at the junction in 3a, which indicates that the two materials are mainly connected together through metal bonds. ADVANCE 8D X-ray diffraction was used to conduct phase analysis at the junction, as shown in Figure 5. Through the analysis It was found that in addition to the base materials copper and brass, a metal compound Cu5Zn8 also appeared in the joint.
2.3 Analysis of joint mechanical properties
Figure 6 is the distribution of microhardness values measured at certain distances along the direction from copper to brass on the cross section of the T2/H62 joint. Figure 6a is the joint obtained when the plate thickness is 4 mm and the rotation speed is 600 r/min. Hardness value distribution. In the experiment, the average hardness value of the copper base material is 95HV, and the average hardness value of the brass base material is 160HV. The entire curve distribution is low (copper) - fluctuating in a lower range, increasing (brass) decreasing. There is an increasing trend. Since the hardness of the brass base material is higher than that of red copper, the microhardness increases significantly in the transition zone from red copper to brass. There is a softening phenomenon in the entire joint. Compared with red copper, the hardness value of brass decreases more. Large, dropped by 40 ~ 60HV, while the hardness of red copper only dropped by 10 ~ 20HV. Figure 6b shows the impact of the welding process on the microhardness value of the joint. It can be seen from the figure that the rotation speed is 450 r/min and the welding speed is The microhardness value of the joint at 80 mm/min is higher than that of the joint at higher rotation speed and higher welding speed. This phenomenon is especially obvious on the brass side, while the microhardness difference on the copper side is not significant. This It is related to the melting point and thermal conductivity of the two. Since brass has a low melting point, it is easier to soften than copper at higher temperatures, so the hardness of brass decreases more than that of copper. Since the plate to be welded is relatively thin, the rotation speed of the friction head The contribution to the heat of the weld is relatively large, so the high rotation speed generates more heat, which has a great impact on the joint and the softening phenomenon is serious. The hardness value in the weld nugget area increases, which is related to the large number of uniform and fine grains in this area. Because The T2/H62 boundary line is very narrow, and the hardness peak of the metal compound Cu5Zn8 in this area is basically not measured in the figure. Although the metal compound was found in the phase analysis, due to its small content, it has a greater impact on the mechanical properties of the joint. Small. From the fracture surface of the sample, it can be seen that the fracture did not simply break from the junction of the two materials, but broke toward the copper side in the weld nugget area. A mixed interlayer of brass and copper appeared in the fracture surface, and the joint appeared obvious before the fracture. The necking is a ductile fracture. In the tensile test of a joint with a plate thickness of 2 mm, most fractures occurred on the copper side, not at its junction.
Figure 7 is a comparison of the elongation and tensile strength of the welds obtained by welding copper and brass with a plate thickness of 2 mm under different process parameters. From the figure, it can be seen that the average tensile strength of the joints is basically the same as that of the copper joints. The tensile strength is equal. When the friction head rotation speed is 600 r/min and the welding speed is 55 mm/min, the joint elongation is maximum, and the tensile strength can also reach maximum under different combinations of rotation speed and welding speed. Maximum value. But overall, qualified joints can be obtained by keeping the rotation speed between 450 and 600r/min, and the obtained joint elongation and tensile strength values are relatively ideal. When the rotation speed is increased to 700r/min , Since the increase in rotation speed increases the heat input of the joint, the range of welding speed selection narrows. When the welding speed is improperly selected, the joint elongation and tensile strength decrease significantly, thereby increasing the difficulty of controlling the welding quality.
3.Conclusion
1) By selecting appropriate welding process parameters, the friction stir welding connection of copper-brass dissimilar metals can be realized, and the joint structure and performance are excellent.
2) Due to the different physical properties of copper and brass, there is a large difference in the size of the copper and brass grains in the copper-brass joint after welding. The brass grains in the welding nugget area are refined, while the copper grains appear to a certain extent. There is a transition material between copper and brass in the joint. X-ray diffraction analysis shows that it is Cu5Zn8, and the width of the transition zone is about 1 to 10 μm.
3) After welding, the microhardness of the joint softens to varying degrees, and the softening amplitude on the brass side is greater than that on the copper side. The fracture of the joint occurs on the copper side, and the average tensile strength of the joint is between the tensile strength of brass and copper. .
