ABSTRACT: The friction stir welding process of t 2 red copper and h 62 brass was studied. The weld formation, microstructure and mechanical properties of the joints of red copper and brass with different thickness were analyzed by experiments under various process parameters, the distribution of the two materials in the joint and the phase composition at the junction were analyzed. The results show that the copper-brass joint with good microstructure and properties can be obtained by suitable welding parameters. There is a transition zone at the junction of the joint and the transition material is about 1 ~ 10 M. It was also found that the microhardness and average tensile strength of the joints were between brass and copper.

Keywords: Friction Stir Welding; dissimilar metals; Red Copper; brass; welding parameters; 

figure classification: TG453 Reference Identification Code: A

In modern industry, it is often necessary to weld materials with different properties into composite parts in order to meet the requirements of various properties, save valuable materials and reduce costs. However, it is usually more difficult to weld dissimilar metals than the same metal due to the great difference in properties between them, the variety of combinations and the different requirements for their joints.

Friction stir welding, or FSW, is a method of welding in which friction heat is used as a heat source, also known as friction stir welding welding. This method has been applied to the welding of aluminum alloy since its appearance. And gradually to welding magnesium alloy, copper alloy, titanium alloy and stainless steel and other materials. Few studies have been reported on friction stir welding welding of dissimilar metals. In this paper, the FSW process test is carried out for t 2 copper and H62 brass dissimilar metals, the process parameters affecting the quality of copper-brass joint and the phase composition formed in the welding process are studied, the microstructure of the Weld and the mechanical properties of the joint were analyzed.

1. Experimental methods

T 2 copper and h 62 brass with 2 mm thickness and t 2 copper and h 62 brass with 4 mm thickness were selected as experimental materials. On the sw-3 lm-015 special friction stir welding machine, the copper-brass plate was FSW tested. In the experiment, the friction head suitable for welding copper alloy was used, the length of stirring pin was 0.2 mm ~ 0.3 mm short relative to the thickness of the welded plate, the direction relative to the vertical line of the workpiece surface was 2 ° , and the technological parameters were changed, to obtain the best joint shape and quality. After welding, cut the specimen perpendicular to the weld line. The prepared metallographic specimen was corroded by hydrochloric acid alcohol solution (10g FECL3,6ML HCl, 40ml H2o, 60ml C2H5OH) . The copper side was corroded first, then the brass side was corroded. After corrosion, the microstructure and phase composition of the joint were analyzed by MEF3 and ADVANCE 8d X-ray diffraction, and the microhardness and mechanical properties of the joint were tested.

2. Experimental Results and analysis of the effect of

 2.1 process parameters on weld surface forming

In the friction stir welding process, because the forward side temperature is lower than the return side, and the thermal conductivity and melting point temperature of red copper are higher than that of brass, the brass is usually placed on the forward side and the red copper on the return side during welding. Some of the welding parameters used in the experiment are shown in Table 1. Table 1 friction stir welding welding process parameters of copper and brass dissimilar materials.

Figure 1 shows the formation of the welded surface of 2 mm thick t2 red copper and H62 brass during friction stir welding welding under different process conditions. The upper side is the forward side-brass, the lower side is the return side-copper. It can be seen from figures 1 C and D that when the workpiece is thin, the rotation speed of the friction head has a great influence on the surface forming. When the rotating speed is 700r/min, the welding speed selection range is relatively large, so the welding speed is increased, but the weld surface formation begins to deteriorate. This is because the increase of rotating speed greatly increases the heat input per unit length of the weld, make the material flow performance become bad. Another way to increase the heat is to increase the friction head shoulder pressure, due to thin plate, shoulder friction heat plays a major role. At the same rotating speed, the contribution of the shaft shoulder pressure to the heat is different. Figures 1A and 1B are the surface forming diagrams obtained when the rotating speed is kept constant and the welding speed is changed, as a result, the size of the surface forming ring is uneven.

2.2 phase analysis of microstructure and interface of joint

Fig. 2 is a cross-sectional view of a 4 mm thick copper-H62 brass FSW joint, with the forward side-brass on the right and the return side-red copper on the left. As can be seen from the diagram, the mixing of copper and brass occurs mainly in the nugget zone. There is an onion ring-like structure in the core zone as shown in figure a [8] . The main component in this zone is brass with only a small amount of red copper doped in it. Mixing Zone is large

Parts of both are connected by large blocky projections. The transfer material of the right side brass (forward side) is mainly in the diameter range of the shaft shoulder and the middle of the stirring pin. The plastic metal of the forward side driven by the shaft shoulder covers the surface of the metal of the return side, while the left Red Copper (back side) moves across the center of the weld to the forward side driven by the rotation of the shaft shoulder and the stirring pin, the materials near the shaft shoulder can reach the thermo-mechanical influence zone of the forward side. Due to the strong plastic shear deformation and flow of the metal in the weld nugget zone, the metal flow in this zone actually moves around the stirring pin in a certain regularity, finally, the onion ring structure at a is formed. From the analysis of the flow of red copper in the diagram, it can be seen that the flow of the material on the forward side can be divided into three types: the metal near the tip of the stirring pin flows from bottom to top, and the onion circulates in the middle of the stirring pin, but on the forward side, the flow direction is consistent with that of the tip flow. The Red Copper B present at the forward side flows upward from the tip of the stirring pin rather than from the same height around the back of the stirring pin. A similar situation occurs in the thin plate t2/H62 joint, where the two materials are connected by inclined plane due to their small thickness, and a part of red copper is completely mixed in brass in the weld nugget zone.

Fig. 3 is a micrograph of different parts relative to Fig. 2. The grain size and shape are different in each zone, and the mixed zone makes the joint more complex. Fig. 3A is the base metal of red copper. The grain size of red copper near the brass region is obviously larger than that of the same copper during welding. Fig. 3B, 3d. This is because of the difference in the heat transfer Coefficient between the two sides of the friction head. Due to the high temperature and good heat transfer coefficient of red copper, a large amount of heat is transferred from the side of red copper, this results in a long high temperature residence time in the region, which results in the growth of copper grains in the region. Because of the high heat input, the grains of brass grow into coarse equiaxed grains. In the nugget zone, because the two materials are not uniformly mixed, the grain shape of copper in the slightly uniform zone increases obviously, but in the single zone, the grain size of copper is obviously larger than that of brass, and the grain size of brass is fine and uniformly distributed as shown in Fig. 3C. Fig. 3F shows the microstructure of the forward-side thermo-mechanical influence zone (TMAZ) of the brass. The zone is clearly demarcated from the return side. The two sides of the TMAZ are composed of grains of different sizes. It is found that the boundary between the two sides of the nugget zone is basically symmetrical, which is caused by the low melting temperature of brass. Mutual penetration occurs at the junction of copper and brass, but the penetration area is very narrow. In the T 2/h 62 joint, although dynamic recrystallization and dynamic recovery occurred in the copper side during the welding process, the grain size of the copper side did not change significantly compared with that of the brass side. On the other hand, in dissimilar metal welded joints, the connection between two materials plays an important role in the mechanical properties. According to the macro-diagram of the joint, most of the two materials are composed of regions with distinct boundaries, and only a few of the mixed regions exist. From the joint in Fig. 4, it can be seen that there are different phases at the junction, the width of which is about 10 m, and the shape of the Strip along the junction is similar to that of the two materials. In Fig. 3 A, the black phase penetrates the white phase at the junction, which indicates that the two materials are mainly connected by metal bonds. Advance 8 D X-ray diffraction was used to analyze the phase at its junction, as shown in Fig. 5. In addition to copper and brass, a metal compound cu 5 zn 8 was found in the joint.

2.3Analysis of mechanical properties

Fig. 6 is the microhardness distribution of the t2/H62 joint measured at intervals along the brass direction on the cross section. Fig. 6A is the microhardness distribution of the t2/H62 joint when the thickness of the plate is 4 mm and the rotation speed is 600 r/min. In the experiment, the average hardness of Copper Matrix is 95HV, and that of Brass Matrix is 160HV. The whole curve distribution shows a trend of low (purple copper) in a lower range. Because the hardness of Brass Matrix is higher than that of copper, the microhardness of transition zone from copper to brass increases obviously. Compared with red copper, the hardness of brass decreased more than 40 ~ 60HV, but the hardness of red copper decreased only 10 ~ 20hv. . Fig. 6B shows the effect of the welding process on the microhardness of the joint, the microhardness of the joints with a rotating speed of 450 r/min and a welding speed of 80 mm/min is higher than that of the joints with a higher rotating speed and a higher welding speed, the microhardness on the copper side was not different. This is related to the melting point and thermal conductivity of the two metals. The hardness of brass decreases more than that of copper because of its low melting point, which is easier to be softened than that of copper at higher temperatures. Due to the thin sheet, the rotation speed of the friction head contributes a lot to the heat of the weld, so the high rotation speed produces a lot of heat, which has a great influence on the joint and causes serious softening. The hardness increases in the nugget zone, which is related to a large number of uniform fine grains. Because the t 2/h 62 boundary is very narrow, the hardness peak of the metal compound cu 5 zn 8 in this region has not been measured. Although metal compounds were found in phase analysis, their contents had little effect on mechanical properties. From the fracture of the specimen, it can be seen that the fracture is not only from the junction of the two materials, but from the welding core zone to the copper side. There is a brass and copper mixed interlayer in fracture, and the joint has obvious necking before fracture, it belongs to Ductile fracture. In the tensile test of the joints with 2 mm thickness, the fracture mostly occurred on the copper side, not at its junction.

Fig. 7 is a comparison of the elongation and tensile strength of the weld obtained by welding copper and brass with a plate thickness of 2mm under different process parameters, as can be seen from the diagram, the average tensile strength of the joint is basically equal to that of the copper joint. When the rotating speed of the friction head is 600 r/min and the welding speed is 55 mm/min, the elongation of the joint is maximum, and the tensile strength can also reach the maximum value under the combination of different rotating speed and welding speed. But on the whole, if the rotation speed is kept between 450 ~ 600 R/min, the qualified joints can be obtained. The elongation and tensile strength of the joints obtained are all ideal. When the rotating speed is increased to 700r/min, the heat input of the joint is increased due to the increase of the rotating speed, resulting in the narrow selection range of welding speed. When the welding speed is not properly selected, the elongation and tensile strength of the joint decrease greatly, therefore, the difficulty of controlling the welding quality is increased.

3. Conclusion

1) with proper welding parameters, the copper-brass dissimilar metal friction stir welding can be realized, and the microstructure and properties of the joints are excellent. 

2) because of the difference in physical properties between copper and brass, the grain size of copper and brass in the joint after welding is quite different. The grain size of brass in the nugget zone is refined, while the grain size of copper appears to grow up to a certain extent. There is a transition material between copper and brass in the joint. The x-ray diffraction analysis shows that Cu5Zn8 and the width of transition band is about 1 ~ 10 M. 

3)The microhardness of the joint is softened in different degrees after welding, and the softening extent of brass side is larger than that of copper side. The average tensile strength of the joint is between that of brass and that of copper. 

Source: Chinanews.com, by Wang Xijing

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