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Radial friction welding interface between brass and high carbon steel
Journal of Materials Processing Technology 212 (2012) 385–392
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Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Radial friction welding interface between brass and high carbon steel Jian Luo a,c,∗ , Junfeng Xiang a , Dejia Liu b , Fei Li a , Keliang Xue a a
The State Key Laboratory of Mechanical Transmission, Chongqing University, Chongqing, 400044, China College of Materials Science and Engineering, Chongqing University, Chongqing, 400044, China c State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, 150001, China b
a r t i c l e
i n f o
Article history: Received 17 May 2011 Accepted 3 October 2011 Available online 7 October 2011 Keywords: The radial friction welding Interface behavior Large size Brass/high carbon steel
a b s t r a c t CT-130 special inertia friction welding machine is used to finish a large size (156 mm diameter) of H90 brass/D60 steel dissimilar welding. SEM, EDS methods are used to analyze the interfacial characteristics of H90/D60 welding joint. The results show that in the welding interface, some furrow-shaped holes appear at the end of welding joint, and a smooth line occurs in the central welding interface and a good welding seam is formed. Thermoplastic brass is droved and flowed to the edge of welding interface under radial pressure, which becomes a lubricating thin film, and reduces the friction coefficient of welding joint edge, leads to that phenomenon. It is found that Fe and Cu elements diffusion appeared at the welding interface. But the density and distance of Fe element diffusion towards H90 brass are larger than that of Cu element towards D60 steel, which is attributed to the difference of the lattice coordination number of Fe/Cu. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Product structures become more complex, to produce innovative products to meet the needs of the market, the applications of dissimilar metals welding are increasingly widespread. Copper, as well as copper alloys, has a good conductivity (Wang et al., 2010a). With the advantages of high strength (copper) and cheap price (steel), the products with hybrid structures of copper/copper alloys and steel are favored (Weigl and Schmidt, 2010). Because of a high-power heat source, preheat and slow cooling after welding, the conventional fusion welding methods used for welding copper/steel dissimilar metals are much complicated. It is mainly attributed to the large differences of thermophysical properties between copper and steel, such as melting point, thermal conductivity, expansion coefficient, and shrinkage rate (Wu et al., 2006). Those dissimilar metals joints may have some welding defects, such as inclusion, cracking, porosity, coarse grains in HAZ and poor mechanical properties of the joints (Magnabosco et al., 2006). Aiming to remove these welding defects, some special welding methods have been proposed to copper/steel dissimilar metals welding process. For example, Mai and Spowage (2004) used laser welding method to achieve the copper/steel dissimilar metal butt joints, and found that laser welding is useful in some tiny geometry of copper/steel joints. Ahmet et al. (2005) applied explosive welding to copper/stainless steel dissimilar metals and obtained a good
∗ Corresponding author at: Chongqing university, No. 174 Shazheng Street, Chongqing, China. Tel.: +86 2365105721. E-mail address: luojian cn [email protected] (J. Luo). 0924-0136/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2011.10.001
quality of joint, the intermetallics were not formed in the bonding interface. Sabetghadam et al. (2010) adopted diffusion welding to acquire the joint of copper and 410 stainless steel, but the reaction layers of Cu/Ni and Ni/SS are bad for the welding joint. Ahmet et al. (2005) analyzed the friction welding process of copper/steel dissimilar metals, and (Jayabharath et al., 2007) realized the continuous drive friction welding between copper and powder metallurgy steel, they all obtained good quality of welding joints. At the effect of radial pressure, the friction heat generated in friction interface is used to heat the welding zone to a high temperature, and then a large radial forging force is applied to the radial friction welding process, which is particularly suitable for welding rotator structures, such as tubular, annular workpieces (Xu et al., 2007). Nicholas and Lilly (1979a,b) first used the radial friction welding to structural materials welding process. Then the microstructure and mechanical properties of Ti-6Al-4V0.1Ru (wt%) titanium alloys joints by radial friction welding were analyzed by Torster et al. (1998a,b), who also studied the thermal cycle effect in radial friction welding process (Torster et al., 1998c). Pinto et al. (2005) used a finite element method to solve the full coupled thermo-mechanical problem, which made a deep study for the residual stress in radial friction welding joint. And a neuron-fuzzy inference system (ANFIS) model was used to study the pressure of pipe joint by radial friction welding (Singh et al., 2009). (Xu et al., 1995a) have designed CT-25 special friction welding machine, and done many research works on the radial friction welding process (Xu et al., 1995b). The mechanism of frictional heat generation in different types of radial friction welding processes was systematically studied by (Wang et al., 2010b), and a three-dimensional non-linear model was proposed to analyze the
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cartridge clip during radial friction welding process (Wang et al., 2010c). Recently, we studied the structures and properties on the small size of T3/35CrMnSi steel inertia radial friction welding joints (Luo et al., 2010a), and found that the spindle speed was more than 1800 rpm, the forging pressure was more than 190 MPa, the welding joints were bonded successfully and had no local spot defects, but some large size brass/high carbon steel inertia radial friction welding joints had not been studied. Although Yao et al. (2009) carried out a research work about welding interface on brass/steel laser welding joint, there is also a lack of study about welding interface behavior on the large size of brass/high carbon steel radial friction welding joints at present, especially. In this paper, focusing on the characteristics of welding interface, we adopt a large size of H90 brass/D60 steel inertia radial friction welding joint as research subject and analyzed the mechanism of elements diffusion and the formation of furrow-shaped holes at the end of welding joint in depth. This study has laid a foundation for the interface behavior of the large size of brass/high carbon steel dissimilar metals welding joints by inertia radial friction welding, and provided a data and theoretical guidance for the removal of distance of the inertia radial friction welding joints at both ends.
2. Experimental procedure The materials used in this study were H90 brass and D60 high carbon steel. H90 brass chemical composition was 88.0–91.0% Cu, ≤0.03% Pb, ≤0.10% Fe, ≤0.05% Sb, ≤0.002% Bi, ≤0.01% P, Zn balance (wt%); and D60 steel chemical composition was 0.62% C, 0.049% Si, 0.65% Mn, 0.04% S, 0.04% P, Fe balance (wt%). The external diameter of brass ring was 166 mm, wall thickness was 5 mm, width was 50 mm, in cold drawing state. D60 steel was a pipe material with 156 mm external diameter, wall thickness was 7 mm, in quenching and tempering state. Schematic diagram of the assembly structure of H90 brass ring/D60 steel tube was shown in Fig. 1. The mixture of phosphoric acid and nitric acid with a volume ratio of 19:1 was used to scrub the surface of welding zone in H90, and then the workpiece was washed and dried. Acetone was used to wash the surface of D60 steel welding parts. The H90/D60 radial friction welding process was performed with the parameters: inertia 95.277 kg m2 , spindle speed 1000 rpm, friction pressure 7.0 MPa, forging pressure 9.0 MPa, and welding time 0.8 s. CT-130 special friction welding machine was used to achieve the brass/high carbon steel inertia radial friction welding. The schematic diagram of typical parameters during inertia friction welding process was shown in Fig. 2a. Eight pressure heads were uniformly arranged in the clamping instrument to apply preload to H90 brass ring during welding process, when the steel pipe
Fig. 1. Schematic diagram of the assembly structure of H90 brass ring/D60 steel tube.
with a flywheel reached predetermined rotational speed, the flywheel divorced from the drive motor, and pressure heads started to apply radial pressure (friction pressure) to the brass ring (Luo et al., 2010b). Then friction and plastic deformation occurred in friction interface, and a large amount of frictional heat was generated, as the friction interface was heated to welding temperature, two workpieces contact surfaces reached a fully thermoplastic state, and then the second radial pressure (forging pressure) was applied. Surface activation, diffusion and recrystallization happened in the friction interface, and under forging pressure, the radial friction welding joint was formed (Luo et al., 2010c). The metallographic samples were prepared from the center and ends of welding joint by WEDM, after polishing and etching with aqua regia for 3–5 s. KQ-100E ultrasonic cleaning machine was used to wash the samples, and then the microstructure and the welding interface characteristics of the H90 brass/D60 steel joint were examined by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). 3. Results 3.1. Welding interface characteristics Fig. 3 shows that the large size of brass/high carbon steel dissimilar metals welding by inertia radial friction welding, successfully. The welding flash is formed naturally at the joint’s end, which is of benefit to good quality of friction welding joint. Because the oxide film in the frictional interface is broke and extruded, and a clean welding joint is obtained, the flash formation process produces a self-cleaning effect on the friction welding joints (Fu et al., 2007). Fig. 4 presents some furrow-shaped holes appeared at the end of welding joint, which implies an un-tightly joint is formed. Fig. 4a
Fig. 2. Schematic diagram of typical parameters and radial force.
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Fig. 5. Schematic diagram of the characteristics of welding interface. Fig. 3. Macroscopic picture of the H90 brass/D60 steel dissimilar joint in radial friction welding.
and d shows that the region with furrow-shaped holes is approximately 4.5 mm far from the end edge of welding joint, and there exists the furrow-shaped holes at both of the welding joint ends. And the farther away from the end edge of welding joint, the smaller the furrow-shaped holes are. As the distance is about 4.5 mm from the end edge of welding joint, the furrow-shaped holes disappeared, and the mutual occlusive welding interface appeared (Fig. 4e). But the mutual occlusive welding interface has not existed all along the H90/D60 radial friction welding interface. Followed as the furrow-shaped holes disappeared, the mutual occlusive welding interface disappeared too. And the central welding interface
becomes a smooth line and forms a good welding seam (Fig. 4f). But as closer to the other end of welding joint, the furrow-shaped holes emerged again, and the size of the furrow-shaped holes becomes larger while closer to the end edge of welding joint. The schematic diagram of the welding interface characteristics is shown in Fig. 5. 3.2. Elements diffusion Fig. 6 implies that the diffusion of Fe, Cu elements results in the welding interface of H90/D60 joint, that is, Fe and Cu elements diffused each other in the welding interface, but the density and distance of Fe element diffuse in H90 brass are larger that of Cu element in D60 steel.
Fig. 4. The welding interface characteristics of H90 brass/D60 steel radial friction welding joint (a) the end of welding joint (small multiple); (b) near the end of welding joint (small multiple); (c) far from the end of welding joint (small multiple); (d) the end of welding joint (large multiple); (e) far from the end of welding joint (large multiple) and (f) the central of the joint (large multiple).
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Fig. 6. Energy spectrum line scanning of the inertia radial friction welding joint (a) line scanning region of joint and (b) the overall distribution of elements.
According to the diffusion law of Fick, the diffusion fluxes to get across the unit area of perpendicular direction cross-sectional relate with the concentration gradient and diffusion coefficient in unit time (Bydash et al., 2010). The diffusion coefficient is seriously influenced by the diffusion constant, and that the diffusion constant depends on the atomic migration rate. So the larger the atomic migration rate is, the larger the diffusion constant becomes, and then the larger element diffusion quantities will get (Sabetghadam et al., 2010). At the temperature of absolute zero, it is obvious that the lattice coordination number is larger, so there are more neighbor atom places for the atoms to diffuse to, which makes the diffusion constant being larger. Therefore, the lattice coordination number plays an important role in the elements diffusion (Elrefaey and Tillmann, 2009). In this study, H90 brass is fcc lattice, and the lattice coordination number is 12; while D60 steel is bcc lattice, and the lattice coordination number is 8. It is clear that the diffusion constant of Cu element is larger than the Fe element one. Therefore, the density and distance of Fe element diffusion in H90 brass are larger than that of Cu element in D60 steel, and this phenomenon is mainly attributed to the difference of the lattice coordination number of the Fe/Cu two elements. 4. Discussion 4.1. Welding interface characteristics As its content in H90 brass is relatively low, Zn melts into copper and forms typical single phase (␣-Cu), phase transition has not occurred after plastic deformation in H90 brass. But quenching hardened structure in thermal mechanical affected zone (TMAZ) in D60 side could be easily formed because of high carbon content and rapid cooling (Yao et al., 2009). According to the materials properties of H90 brass and D60 steel as shown in Table 1, there are large differences between H90 brass and D60 steel in yield strength, thermal conductivity and melting point. Friction pressure on both sides of the frictional interface is the same, but H90 brass/D60 steel
is different in mechanical properties, and the plastic deformation ability of brass is better than that of steel, so plastic deformation has totally occurred in brass side, almost no plastic deformation happened in D60 steel side (Fig. 4). The refined grains in brass side adjacent to the welding interface can be observed in Fig. 4d–f. Metallographic analysis software was used to analyze the grains size in the base metal of H90 brass and the refined grains zone, and it is found that the average grains size in base metal is 84.6 m, while at the fine grains zone adjacent to the welding interface, the grains were broken, and the grains boundaries were disordered, the metallographic analysis software could not measure the grain size. But in Fig. 4d, the fine grain size was not larger than the grain 10 m. Since the occurrence of plastic deformation makes the grains in H90 brass side adjacent to the friction interface refined, it is noteworthy to estimate the plastic deformation thickness in H90 brass side according to the width of refined grains. Fig. 4d shows the plastic deformation thickness is 50 m at the end of welding joint (position C shown in Fig. 7c). There has about 15 m plastic deformation layer (shown in Fig. 4e) near the end of welding joint (position B shown in Fig. 7c, it is about 4.5 mm distance from point C to point B). The thickness of plastic deformation is 80 m in the central one (position F shown in Fig. 7c). From the differences of the plastic deformation thicknesses at the three places of the welding joint, it is found that the plastic deformation thickness at the end of welding joint (place of C in Fig. 7c) is 35 m thicker than that one near the end of welding joint (place of B in Fig. 7c). And it implies that the most intensive plastic deformation happened at the central welding interface (place of F in Fig. 7c), which makes the most width of refined grains zone approximately 80 m in there. 4.2. Formation mechanism of the welding interface characteristics Fig. 2a presents three stages that can be divided in an inertia radial friction welding process, including the pre-stage and unstable friction stage (t1 period), the stable friction stage (t2 period),
Table 1 Properties of the H90 brass and D60 steel in this study. Materials
b (MPa)
Density (g/cm3 )
Linear expansivity (K−1 )
Heat conduction [W/(m K)]
Melting point (◦ C)
Linear contraction percentage (%)
H90 brass D60 steel
206 620
8.85 7.82
18.1 11.5
244.93 15
1056 1470
– 12
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Fig. 7. Schematic diagram of the contact friction in the H90 brass/D60 steel interface (a) contact region microstructure; (b) force and deformation analysis and (c) heat analysis.
and the forging stage (t3 period). As shown in Fig. 7a, the first radial pressure is imposed to the brass ring at t1 period. At this period, the friction between D60 and H90 is just contact friction in the asperity surface, and the contact area is very small. It can be found that when the radial friction welding process was finished, it was still asperity contact friction at the end edge of welding joint (at the place of C in Fig. 7c) shown in Fig. 4d. And some furrow-shaped holes appeared there, without forming a tight joint interface. A large amount of plastic deformation happened in the brass side, and much friction heat was produced, so sufficient thermoplastic state brass appeared at the welding interface in the condition of high temperature and high strain rate. The friction action of H90/D60 dissimilar metal was stopped at the t3 period (shown in Fig. 2), the temperature of H90/D60 welding joint decreased rapidly at the forging process, so it was difficult to take place recrystallization phenomenon for the thermoplastic brass. As a result, the zone of refined grains was formed at the brass side adjacent to the welding interface. And a smooth line of welding interface was shaped at the central of the joint, and a good welding seam was formed (Fig. 4f). As the same condition of friction pressure and friction time, why the furrow-shaped holes appear at the end of the welding joint, but have not appeared at the central of welding joint. It is noteworthy to comprehensively analyze its formation mechanism from the two important factors of friction heat and plastic deformation in radial friction welding process. (1) Based on the radial friction welding procedure, it can be known that during the effect of the radial pressure, the width of H90 brass ring will become wider, and the height will become more dwarfed. Because the radial pressure is changed from an axial pressure by a special assembly technology clamp, ensuring the radial pressure being applied to the contact surface equably, there are restrictions for the largest contact area and the best contact area in the special assembly technology clamp. Therefore, when the plastic deformation in the width direction exceeds the largest contact area under the frictional pressure applied in the H90 brass ring, the plastic deformation at the two ends and lateral side of H90 brass ring becomes freedom extrusion deformation, which is very similar to the plastic deformation in free forging of the plastic processing field. Fig. 7b depicts that line AB is the boundary of radial pressure line. The radial pressure was applied to H90 brass ring at the A point, and the plastic deformation happened at the B point. So there was a relative motion and mutual friction between the upper surface (A point) and the lower surface (B point), and friction heat was generated. So there was a friction force fa and an effect of severely stress at the internal of H90 brass (the green arrow line shown in Fig. 7b), which made the plastic deformation happened within the horizontal direction. Base on the principle of free forging in the plastic processing discipline
field, when the plastic deformation region exceeded the largest contact area between assembly clamp and brass, a free-state upward warping deformation was occurred in the H90 brass side (shown as the yellow arrow in Fig. 7b), as a result of the friction force fa existing (shown as the blue arrow in Fig. 7b). Fig. 7b shows the B point at the lower surface of H90 brass ring, the same as upper surface, there is a friction force fb between the lower surface of H90 brass and the upper surface of D60 steel. But it is different from the point A, because a severely radial friction exists at the interface between the lower surface of H90 brass and the D60 steel. And the temperature at the lower surface of H90 brass ring is much higher, and the thermoplastic deformation happened there. Therefore, the friction force fb at the bottom surface (point B) is smaller than fa in the upper surface (point A), as the effect of big stress (shown as green arrow in Fig. 7b) upon deformation, the thermoplastic brass is extruded out of the assembly technology clamp, which makes the bottom end metal (point C) of H90 brass dynamic flow and occur an upward warping deformation (shown as the purple arrow in Fig. 7b). Applied a decreasing radial pressure, rubbed and adhered with the D60 steel interface, the thermoplastic flow dynamic brass (in a Free and viscoplastic State) is extruded out of the friction interface in BC section (Fig. 7b), when it passed C point and separated from the steel interface, the upward warping deformation happened. Therefore, in the forging process, because of the end of H90 brass ring within a Free State, the H90 brass ring in the BC section cannot generate enough thermoplastic brass with a dynamic flow state, and it only obtain the thermoplastic brass from the central welding interface. Therefore, under the conditions of lower temperature, radial pressure inadequate, and the Free State, the capability for deformation is limited, and some stress is released at the end edge of H90 (compared with the central of H90 brass ring), it fails to eliminate the furrowshaped holes. The results show that the radial friction welding workpieces in BC section have not formed a tight welding seam (as the red arrow in Fig. 7b). In addition, according to the parallel mapping from the parts of upward warping deformation in the upper surface in H90 brass side, it can be roughly estimated that the part of DBC is made by the extruded thermoplastic brass from the central of H90 brass ring. (2) Studying the characteristics of welding interface formation from the view point of energy, shown in Fig. 7c, a symmetrical half of the joint is taken as the analysis sample, line FG is the center symmetry line of H90 brass ring. According to the characteristics of radial friction welding process and the analysis (the force and constraint of the joint) above, of BF section is the actual production heat zone (the actual friction surface), and segment of BC can also produce small amounts of friction heat (made by the adhesion of metal and internal stress), the end region of CEDHA is the boundary of radiation and convection
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heat transfer (as the yellow arrow in Fig. 7c), the heat conduction exists inside the H90 brass and D60 steel, and the upper surface of AG (as the green arrow in Fig. 7c). The production heat mechanism of radial friction welding is similar to the axial friction welding one, that is to say, it is produced by friction in the friction interface, and the friction heat is used to heat the workpieces. The difference is that the friction interface in axial friction welding is the axial cross section, and the angular velocity is equal. But the friction interface of radial friction welding is the circumference surface of the cylindrical (tube) workpieces, the line speed is equal. In this study, the total heat production in the H90 brass/D60 steel radial friction welding (BF segment) is that:
T
Qc =
2r l n(t)(t)F(t) dt
(1)
0
where r is the radius of cylindrical pipe D60, l is the axial contact distance of the H90 brass and D60 steel, T is the total friction time, n(t) is the friction speed, (t) is friction coefficient, F(t) is the radial pressure, is the conversion factors. There are two methods for the heat dissipation in the BF section. One is heat conduction, which conducted the heat to the workpieces, which stored in the workpieces, and increased the temperature adjacent to the welding zone, and the portion dissipated heat is indicated as Qw . The other is the radiation and convection heat transfer, indicated as Qd , it is generated with the welding flash appearance at the end of the joint. Qc = Qw + Qd
(2)
According to the heat conduction equation, the radiation and convection heat transfer Qd is:
Qd = 0
T
s(Tm − T0 ) dt h
(3)
where is the heat transfer coefficient, s is the heat transfer area, h is a constant, Tm is the welding temperature, T0 is the ambient temperature. So the heat used for the temperature increasing and the thermoplastic deformation Qw is:
Qw = Qc − Qd =
T
2r l n(t)(t)F(t)dt − 0
0
T
s(Tm − T0 ) dt (4) h
According to the characteristics of radial friction welding process and the formation discipline of radial friction welding joint of H90 brass ring/D60 steel pipe, at the contact friction interface between H90 brass and D60 steel (BF and BC sections in Fig. 7c), in Eq. (4), r is the radius of cylindrical pipe D60, n(t) is the friction speed, (t) is the friction coefficient, and F(t) is the radial pressure. And the parameters of n(t), (t) and F(t) are variables. This is because the radial friction pressure does not have a direct effect on BC section, and the extruded thermoplastic flow brass is formed in D60 steel surface during the effect of metal adhesion and internal stress. A lubricating thin film is formed at the surface of the thermoplastic state brass, which reduces the friction coefficient of the end edge of welding joint in BC section (Dong et al., 2008). And there is heat loss at the end of welding joint, reducing the useful welding heat, according to the Eq. (4), the friction coefficient at the end edges of welding joint is much smaller, which reduces the production of friction heat there. In addition, the formation of welding flash and the radiation and convection heat transfer reduce the welding heat that is used to increase the temperature of workpieces and produce thermoplastic deformation, which makes the amount of thermoplastic state brass decreased (Fu et al., 2007). While at the central of welding joint, friction coefficient is much larger for the extruded thermoplastic brass, and the radial frictional pressure is applied
directly, without forming welding flash for the heat dissipation. Eq. (4) shows that the more useful energy used for plastic deformation of welding joint is, the stronger of plastic deformation phenomenon of welding joint is, so a large amount of thermoplastic brass occurs in the welding joint. Therefore, there exist a temperature gradient, a friction coefficient gradient and a plastic deformation thickness gradient in the axial direction of the H90/D60 interface (in FC section). As there are no enough heat and restrict conditions for the plastic deformation at the end edges of welding joint, which makes insufficient plastic deformation, and some furrow-shaped holes are appeared there. In summary, there are temperature gradient and plastic deformation thickness gradient in H90 brass ring at the welding interface. And there exists the axial horizontal friction force in the lower and upper surface, as well as the horizontal radial friction, coupled with the restrict conditions at the end edges of welding joint, according to the principles of metallurgy and tribology, the characteristics of H90 steel/D60 steel welding interface (Fig. 4) and the welding flash are formed (Fig. 3). 4.3. The thermoplastic state lubricating thin film The main factors affecting friction coefficient are as follows: the property of metals, surface film, friction speed, temperature and loading. In this study, the property of metals, friction speed, temperature and loading on the friction interface of H90 brass/D60 steel dissimilar metals are steady and uniform, however the surface film is a variable and unsteady factor, which led to the difference of friction coefficient on the interface. Just as a slight clarification, this is not an oxide film on the friction interface, the thermoplastic lubricating thin film is formed on friction surface, which is made up of the thermoplastic brass. During radial friction welding process, the plastic deformation capability of brass is much larger than D60 steel, so it is easy for plastic deformation to take place at the H90 side, and the friction heat makes the H90 brass reached thermoplastic state during the friction process. But the parts at the end edges of welding joint are in contact with the air, the friction heat quickly dissipated into the air, in addition, the constraints and stresses at the end edges of welding joint are different from the one at the central; the constraint at the end edges of welding joint is relatively insufficient, which makes the friction heat relatively smaller, leading to production of little amount of state brass thermoplastic there (shown in Fig. 8a). As the radial pressure gradually increased and stabilized (t2 period in Fig. 2a), thermoplastic flow occurred in the thermoplastic brass surface towards the end edges of welding joint under radial pressure, because the high free degree and the constraints were insufficient there, (it can be confirmed that Fig. 4d shows the clear orbit of directional refined-grains zone formed in the thermoplastic flow process). And it finally makes a large amount of thermoplastic state brass appeared at the two end edges of welding joint, and formed a thermoplastic state thin film. But there is little thermoplastic state brass at the central of welding joint shown in Fig. 8b, (it can be confirmed by the thickness of plastic deformation in Fig. 4d and e, because the amount of plastic deformation has a great relationship with applying time of the radial pressure. Before the forging pressure applied, the amount of plastic deformation at the welding central can be seen as being similar with the one near the end of welding (4.5 mm far from the end edge of welding joint) in forging process finished time. The thickness of plastic deformation at the ends of welding joint is 50 m, while the one near the ends of welding (4.5 mm away) is 15 m. It is obvious that the amount of thermoplastic state brass at the end of welding joint is larger than the central welding joint one). Finally, the radial friction welding process is finished under forging force, and the thermoplastic brass at the two end edges of welding joint extruded, and forms the bend
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Fig. 8. Schematic diagram of the formation of the thermoplastic state lubricating thin film (a) more thermoplastic state brass formed at the central of the joint at the beginning stage; (b) the thermoplastic state brass flowed to the end edge of the joint forming a lubricating thin film and (c) achieved the welding process and formed the welding flash after forging pressure.
welding flash. And there are some furrow-shaped holes appeared at the two end edges of welding joint (Fig. 4d). Duo to thermoplastic brass extruded, a tight welding joint cannot be formed there, and the schematic diagram is showed in Fig. 8c. As the thermoplastic material is in a liquid state, the thermoplastic state brass is in a class-liquid state with micro-nano level solid super plasticity. The thermoplastic brass flows to the end edges of welding interface under radial pressure, which increased the amount of thermoplastic state brass there. And the thermoplastic state brass rotates around the outer surface of D60 steel pipe under rotating force, and a thermoplastic state thin film is formed. But there is no thermoplastic state thin film formed at the central of welding joint because the thermoplastic brass flowed to the ends of welding joint, which decreases the amount of thermoplastic state brass there. Dong et al. (2008) reported that the micro-nano level solid super plasticity material has a strongly lubricating effect. Therefore, this thermoplastic thin film has a strongly lubricating effect on the welding interface in radial friction welding process, so there is little friction heat generated at the end edges of welding joint because of the lubricating effect and the friction coefficient deceased (Luo et al., 2005). Finally under forging force, the thermoplastic brass is extruded out of the welding joint, and forms welding flash, moreover, the furrow-shaped holes appear at the end of welding joint.
3. Because of the small friction coefficient at the two end edges of welding joint, the thermoplastic brass flowed to the edge of welding interface at the effect of radial pressure, which increased the amount of thermoplastic brass there. And the thermoplastic state brass rotated around the outer surface of D60 steel pipe, and a thermoplastic state thin film was formed. It had a lubricating effect, so the friction coefficient at the edge of welding interface was reduced and the thermoplastic state brass was insufficient, so the furrow-shaped holes were formed there. 4. The elements diffusion results showed that there were both the Fe and Cu elements that diffused each other in the welding interface. The density and distance of Fe element diffusion towards H90 brass were larger that of Cu element towards D60 steel. This phenomenon was mainly attributed to the difference of the lattice coordination number of the Fe/Cu two elements.
5. Conclusions
References
1. The CT-130 type special inertia friction welding machine was used to finish a large size (156 mm diameter) of H90 brass/D60 steel dissimilar metals radial friction welding process. The results showed that there were some furrow-shaped holes in the welding interface at the ends of welding joint (about 4.5 mm distance away from the end edges of welding joint), and the farther away from the welding joint end was, the smaller the furrow-shaped holes were. And there was a smooth line in the central welding interface and a good welding seam was formed. 2. The friction at the ends of D60/H90 welding joint (about 4.5 mm) was also contact friction in asperity surface, and there were some furrow-shaped holes at the ends of welding joint. This was because of the insufficient constraints relatively there, as a result, the friction coefficient was smaller and the friction heat was insufficient. In addition, the friction heat was dissipated in heat conduction and at the formation of welding flash. As a result of the lack of friction heat used for plastic deformation, the generated thermoplastic brass is not enough to make up for the gap on the friction interface, so the furrow-shaped holes appear at the ends of welding joint.
Ahmet, D., Behc¸et, G., Fehim, F., 2005. Examination of copper/stainless steel joints formed by explosive welding. Mater. Des. 26 (6), 497–507. Bydash, J., Kasmani, R., Naraharisetty, K., 2010. Metal fume-induced diffuse alveolar damage. J. Thorac. Imaging 25 (2), 27–29. Dong, Y.-k., Zhang, X.-j., Liu, Y., Wen, S.-z., 2008. An elastic ratchet model and scale effect of surface topography on the micro-nano friction. Tribology 28 (4), 333–338. Elrefaey, A., Tillmann, W., 2009. Solid state diffusion bonding of titanium to steel using a copper base alloy as interlayer. J. Mater. Process. Technol. 209 (5), 2746–2752. Fu, L., Du, S.G., Bai, J.-h., 2007. Microstructures and properties of induction friction welded joint of TC4 Ti alloy and LD10 Al alloy. Chin. J. Nonferrous Met. 17 (2), 54–58. Jayabharath, K., Ashfaq, M., Venugopal, P., Achar, D.R.G., 2007. Investigations on the continuous drive friction welding of sintered powder metallurgical (P/M) steel and wrought copper parts. Mater. Sci. Eng. A 454–455 (5), 114–123. Luo, J., Sun Yu, Liu, D.-j., Wu, W., Xu, X.-l., 2010a. Characteristics of inertia radial friction welding joints of small size T3/35CrMnSi dissimilar metal materials. Chin. J. Nonferrous Met. 20 (7), 1309–1315. Luo, J., Zhao, G.-j., Luo, Q., Wang, X.-j., Xu, X.-l., 2010b. Element diffusion on interface of 35CrMnSi/T3 inertial radial friction welding. J. Xi’an Jiaotong Univ. 44 (3), 64–67. Luo, J., Zhao, G.-j., Wang, X.-j., Wu, W., Xu, X.-l., 2010c. Effects of flywheel rotation speed on properties of 35CrMnSi/T3 inertial radial friction welding. J. Chongqing Univ. 33 (9), 24–28. Luo, J.-b., He, Y., Wen, S.-z., Zhong, J., 2005. Challenges to tribology arisen from the development of micro- and nano-manufacturing technology. Tribology 25 (3), 283–288.
Acknowledgements The authors appreciate the financial support from the National Natural Science Foundation of China (no. 51075413), New Century Excellent Talents in University (NCET-08-0607), the Fundamental Research Funds for the Central University (CDJRC11280002), Natural Science Foundation Project of CQ CSTC (no. 2009BA3026) and Excellent Talents Project in Universities of Chongqing Municipal Education Commission (2010024), PR China.
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Magnabosco, I., Ferro, P., Bonollo, F., Arnberg, L., 2006. An investigation of fusion zone microstructures in electron beam welding of copper–stainless steel? Mater. Sci. Eng. A 424 (1–2), 163–173. Mai, T.A., Spowage, A.C., 2004. Characterisation of dissimilar joints in laser welding of steel–kovar, copper–steel and copper–aluminium. Mater. Sci. Eng. A 74 (1–2), 224–233. Nicholas, E.D., Lilly, R.H., 1979a. Radial friction welding. J. Phys., 50–56. Nicholas, E.D., Lilly, R.H., 1979b. Joining of hollow steel components by radial friction welding. Schweisstechnik (Vienna) 33 (5), 69–79. Pinto, Rosa Irene Terra, Strohaecker, Telmo Roberta, 2005. Residual stresses evaluation on radial friction welded joints, in: Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering-OMAE, 24th International Conference on Offshore Mechanics and Arctic Engineering, vol. 3, pp. 43–48. Singh, J., Gill, S.S., 2009. Fuzzy modelling for burst pressure strength in radial friction welding of GI pipes. Int. J. Modell. Ident. Control 7 (3), 305–312. Sabetghadam, H., Hanzaki, A.Z., Araee, A., 2010. Diffusion bonding of 410 stainless steel to copper using a nickel interlayer. Mater. Charact. 61 (6), 626–634. Torster, F., dos Santos, J.F., Hutt, G., Kocak, M., 1998a. Mechanical properties of Ti-6Al-4V-0.1Ru riser pipes joined by radial friction welding. Non-aerospace applications of titanium. In: Proceedings of the 1998 TMS Annual Meeting, pp. 181–188. Torster, F., Hutt, G., dos Santos, J.F., Kocak, M., 1998b. Metallurgical and mechanical properties of radial friction welded Ti-6Al-4V-0.1Ru risers. In: Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering-OMAE. 17th International Conference on Offshore Mechanics and Arctic Engineering, pp. 6–8.
Torster, F., Hutt, G., dos Santos, J.F., Kocak, M., 1998c. A study on thermal cycles in radial friction welding of a Ti-6Al-4V-0.1Ru alloy. Trends in welding research. In: Proceedings of the 5th International Conference, pp. 43–48. Weigl, M., Schmidt, M., 2010. Influence of the feed rate and the lateral beam displacement on the joining quality of laser-welded copper-stainless steel connections. Phys. Procedia 5 (2), 53–59. Wang, J., Cao, J., Feng, J.-C., 2010a. Microstructure and mechanical performance of depositing CuSi3 Cu alloy onto 30CrMnSi steel plate by the novel consumable and non-consumable electrodes indirect arc welding. Mater. Des. 31 (4), 2253–2258. Wang, G.-j., Ling, Z., Kang, D., 2010b. Investigation on heat input numerical models of radial friction welding. Hot Work Technol. 39 (11), 158–159. Wang, G.-j., Ling, Z.-m., Tian, X.-h., Qi, X.-c., Kang, D.-d., Yue, H., 2010c. Threedimensional finite element analysis of clamp-model for radial friction welding. Hot Work Technol. 39 (11), 158–159. Wu, W., Xu, X.-l., Xu, Y.-z., 2006. Study on radial friction welding of typical copperalloy T3, B5, H96 with steel 35CrMnSi. Ordnance Mater. Sci. Eng. 29 (6), 55–58. Xu, X.-l., Xu, Y.-z., Wu, W., 2007. Study on radial friction welding band of small shell. Acta Armamentarii 28 (3), 346–348. Xu, X.-l., Zhu, L.-y., Shen, J., 1995a. Radial friction welding with the axial FRM machine. Electric Weld. Mach. 5 (5), 29–31. Xu, X.-l., Zhu, L.-y., Shen, J., 1995b. Weldabilty of friction welding between copper and high carbon alloy steel. Weld. Joining 6, 17–18. Yao, C.-w., Xu, B.-s., Zhang, X.-c., Huang, J., Fu, J., Wu, Y.-x., 2009. Interface microstructure and mechanical properties of laser welding copper–steel dissimilar joint? Opt. Lasers Eng. 47 (7-8), 807–814.
Contents lists available at SciVerse ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Radial friction welding interface between brass and high carbon steel Jian Luo a,c,∗ , Junfeng Xiang a , Dejia Liu b , Fei Li a , Keliang Xue a a
The State Key Laboratory of Mechanical Transmission, Chongqing University, Chongqing, 400044, China College of Materials Science and Engineering, Chongqing University, Chongqing, 400044, China c State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, 150001, China b
a r t i c l e
i n f o
Article history: Received 17 May 2011 Accepted 3 October 2011 Available online 7 October 2011 Keywords: The radial friction welding Interface behavior Large size Brass/high carbon steel
a b s t r a c t CT-130 special inertia friction welding machine is used to finish a large size (156 mm diameter) of H90 brass/D60 steel dissimilar welding. SEM, EDS methods are used to analyze the interfacial characteristics of H90/D60 welding joint. The results show that in the welding interface, some furrow-shaped holes appear at the end of welding joint, and a smooth line occurs in the central welding interface and a good welding seam is formed. Thermoplastic brass is droved and flowed to the edge of welding interface under radial pressure, which becomes a lubricating thin film, and reduces the friction coefficient of welding joint edge, leads to that phenomenon. It is found that Fe and Cu elements diffusion appeared at the welding interface. But the density and distance of Fe element diffusion towards H90 brass are larger than that of Cu element towards D60 steel, which is attributed to the difference of the lattice coordination number of Fe/Cu. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Product structures become more complex, to produce innovative products to meet the needs of the market, the applications of dissimilar metals welding are increasingly widespread. Copper, as well as copper alloys, has a good conductivity (Wang et al., 2010a). With the advantages of high strength (copper) and cheap price (steel), the products with hybrid structures of copper/copper alloys and steel are favored (Weigl and Schmidt, 2010). Because of a high-power heat source, preheat and slow cooling after welding, the conventional fusion welding methods used for welding copper/steel dissimilar metals are much complicated. It is mainly attributed to the large differences of thermophysical properties between copper and steel, such as melting point, thermal conductivity, expansion coefficient, and shrinkage rate (Wu et al., 2006). Those dissimilar metals joints may have some welding defects, such as inclusion, cracking, porosity, coarse grains in HAZ and poor mechanical properties of the joints (Magnabosco et al., 2006). Aiming to remove these welding defects, some special welding methods have been proposed to copper/steel dissimilar metals welding process. For example, Mai and Spowage (2004) used laser welding method to achieve the copper/steel dissimilar metal butt joints, and found that laser welding is useful in some tiny geometry of copper/steel joints. Ahmet et al. (2005) applied explosive welding to copper/stainless steel dissimilar metals and obtained a good
∗ Corresponding author at: Chongqing university, No. 174 Shazheng Street, Chongqing, China. Tel.: +86 2365105721. E-mail address: luojian cn [email protected] (J. Luo). 0924-0136/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2011.10.001
quality of joint, the intermetallics were not formed in the bonding interface. Sabetghadam et al. (2010) adopted diffusion welding to acquire the joint of copper and 410 stainless steel, but the reaction layers of Cu/Ni and Ni/SS are bad for the welding joint. Ahmet et al. (2005) analyzed the friction welding process of copper/steel dissimilar metals, and (Jayabharath et al., 2007) realized the continuous drive friction welding between copper and powder metallurgy steel, they all obtained good quality of welding joints. At the effect of radial pressure, the friction heat generated in friction interface is used to heat the welding zone to a high temperature, and then a large radial forging force is applied to the radial friction welding process, which is particularly suitable for welding rotator structures, such as tubular, annular workpieces (Xu et al., 2007). Nicholas and Lilly (1979a,b) first used the radial friction welding to structural materials welding process. Then the microstructure and mechanical properties of Ti-6Al-4V0.1Ru (wt%) titanium alloys joints by radial friction welding were analyzed by Torster et al. (1998a,b), who also studied the thermal cycle effect in radial friction welding process (Torster et al., 1998c). Pinto et al. (2005) used a finite element method to solve the full coupled thermo-mechanical problem, which made a deep study for the residual stress in radial friction welding joint. And a neuron-fuzzy inference system (ANFIS) model was used to study the pressure of pipe joint by radial friction welding (Singh et al., 2009). (Xu et al., 1995a) have designed CT-25 special friction welding machine, and done many research works on the radial friction welding process (Xu et al., 1995b). The mechanism of frictional heat generation in different types of radial friction welding processes was systematically studied by (Wang et al., 2010b), and a three-dimensional non-linear model was proposed to analyze the
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cartridge clip during radial friction welding process (Wang et al., 2010c). Recently, we studied the structures and properties on the small size of T3/35CrMnSi steel inertia radial friction welding joints (Luo et al., 2010a), and found that the spindle speed was more than 1800 rpm, the forging pressure was more than 190 MPa, the welding joints were bonded successfully and had no local spot defects, but some large size brass/high carbon steel inertia radial friction welding joints had not been studied. Although Yao et al. (2009) carried out a research work about welding interface on brass/steel laser welding joint, there is also a lack of study about welding interface behavior on the large size of brass/high carbon steel radial friction welding joints at present, especially. In this paper, focusing on the characteristics of welding interface, we adopt a large size of H90 brass/D60 steel inertia radial friction welding joint as research subject and analyzed the mechanism of elements diffusion and the formation of furrow-shaped holes at the end of welding joint in depth. This study has laid a foundation for the interface behavior of the large size of brass/high carbon steel dissimilar metals welding joints by inertia radial friction welding, and provided a data and theoretical guidance for the removal of distance of the inertia radial friction welding joints at both ends.
2. Experimental procedure The materials used in this study were H90 brass and D60 high carbon steel. H90 brass chemical composition was 88.0–91.0% Cu, ≤0.03% Pb, ≤0.10% Fe, ≤0.05% Sb, ≤0.002% Bi, ≤0.01% P, Zn balance (wt%); and D60 steel chemical composition was 0.62% C, 0.049% Si, 0.65% Mn, 0.04% S, 0.04% P, Fe balance (wt%). The external diameter of brass ring was 166 mm, wall thickness was 5 mm, width was 50 mm, in cold drawing state. D60 steel was a pipe material with 156 mm external diameter, wall thickness was 7 mm, in quenching and tempering state. Schematic diagram of the assembly structure of H90 brass ring/D60 steel tube was shown in Fig. 1. The mixture of phosphoric acid and nitric acid with a volume ratio of 19:1 was used to scrub the surface of welding zone in H90, and then the workpiece was washed and dried. Acetone was used to wash the surface of D60 steel welding parts. The H90/D60 radial friction welding process was performed with the parameters: inertia 95.277 kg m2 , spindle speed 1000 rpm, friction pressure 7.0 MPa, forging pressure 9.0 MPa, and welding time 0.8 s. CT-130 special friction welding machine was used to achieve the brass/high carbon steel inertia radial friction welding. The schematic diagram of typical parameters during inertia friction welding process was shown in Fig. 2a. Eight pressure heads were uniformly arranged in the clamping instrument to apply preload to H90 brass ring during welding process, when the steel pipe
Fig. 1. Schematic diagram of the assembly structure of H90 brass ring/D60 steel tube.
with a flywheel reached predetermined rotational speed, the flywheel divorced from the drive motor, and pressure heads started to apply radial pressure (friction pressure) to the brass ring (Luo et al., 2010b). Then friction and plastic deformation occurred in friction interface, and a large amount of frictional heat was generated, as the friction interface was heated to welding temperature, two workpieces contact surfaces reached a fully thermoplastic state, and then the second radial pressure (forging pressure) was applied. Surface activation, diffusion and recrystallization happened in the friction interface, and under forging pressure, the radial friction welding joint was formed (Luo et al., 2010c). The metallographic samples were prepared from the center and ends of welding joint by WEDM, after polishing and etching with aqua regia for 3–5 s. KQ-100E ultrasonic cleaning machine was used to wash the samples, and then the microstructure and the welding interface characteristics of the H90 brass/D60 steel joint were examined by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). 3. Results 3.1. Welding interface characteristics Fig. 3 shows that the large size of brass/high carbon steel dissimilar metals welding by inertia radial friction welding, successfully. The welding flash is formed naturally at the joint’s end, which is of benefit to good quality of friction welding joint. Because the oxide film in the frictional interface is broke and extruded, and a clean welding joint is obtained, the flash formation process produces a self-cleaning effect on the friction welding joints (Fu et al., 2007). Fig. 4 presents some furrow-shaped holes appeared at the end of welding joint, which implies an un-tightly joint is formed. Fig. 4a
Fig. 2. Schematic diagram of typical parameters and radial force.
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Fig. 5. Schematic diagram of the characteristics of welding interface. Fig. 3. Macroscopic picture of the H90 brass/D60 steel dissimilar joint in radial friction welding.
and d shows that the region with furrow-shaped holes is approximately 4.5 mm far from the end edge of welding joint, and there exists the furrow-shaped holes at both of the welding joint ends. And the farther away from the end edge of welding joint, the smaller the furrow-shaped holes are. As the distance is about 4.5 mm from the end edge of welding joint, the furrow-shaped holes disappeared, and the mutual occlusive welding interface appeared (Fig. 4e). But the mutual occlusive welding interface has not existed all along the H90/D60 radial friction welding interface. Followed as the furrow-shaped holes disappeared, the mutual occlusive welding interface disappeared too. And the central welding interface
becomes a smooth line and forms a good welding seam (Fig. 4f). But as closer to the other end of welding joint, the furrow-shaped holes emerged again, and the size of the furrow-shaped holes becomes larger while closer to the end edge of welding joint. The schematic diagram of the welding interface characteristics is shown in Fig. 5. 3.2. Elements diffusion Fig. 6 implies that the diffusion of Fe, Cu elements results in the welding interface of H90/D60 joint, that is, Fe and Cu elements diffused each other in the welding interface, but the density and distance of Fe element diffuse in H90 brass are larger that of Cu element in D60 steel.
Fig. 4. The welding interface characteristics of H90 brass/D60 steel radial friction welding joint (a) the end of welding joint (small multiple); (b) near the end of welding joint (small multiple); (c) far from the end of welding joint (small multiple); (d) the end of welding joint (large multiple); (e) far from the end of welding joint (large multiple) and (f) the central of the joint (large multiple).
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Fig. 6. Energy spectrum line scanning of the inertia radial friction welding joint (a) line scanning region of joint and (b) the overall distribution of elements.
According to the diffusion law of Fick, the diffusion fluxes to get across the unit area of perpendicular direction cross-sectional relate with the concentration gradient and diffusion coefficient in unit time (Bydash et al., 2010). The diffusion coefficient is seriously influenced by the diffusion constant, and that the diffusion constant depends on the atomic migration rate. So the larger the atomic migration rate is, the larger the diffusion constant becomes, and then the larger element diffusion quantities will get (Sabetghadam et al., 2010). At the temperature of absolute zero, it is obvious that the lattice coordination number is larger, so there are more neighbor atom places for the atoms to diffuse to, which makes the diffusion constant being larger. Therefore, the lattice coordination number plays an important role in the elements diffusion (Elrefaey and Tillmann, 2009). In this study, H90 brass is fcc lattice, and the lattice coordination number is 12; while D60 steel is bcc lattice, and the lattice coordination number is 8. It is clear that the diffusion constant of Cu element is larger than the Fe element one. Therefore, the density and distance of Fe element diffusion in H90 brass are larger than that of Cu element in D60 steel, and this phenomenon is mainly attributed to the difference of the lattice coordination number of the Fe/Cu two elements. 4. Discussion 4.1. Welding interface characteristics As its content in H90 brass is relatively low, Zn melts into copper and forms typical single phase (␣-Cu), phase transition has not occurred after plastic deformation in H90 brass. But quenching hardened structure in thermal mechanical affected zone (TMAZ) in D60 side could be easily formed because of high carbon content and rapid cooling (Yao et al., 2009). According to the materials properties of H90 brass and D60 steel as shown in Table 1, there are large differences between H90 brass and D60 steel in yield strength, thermal conductivity and melting point. Friction pressure on both sides of the frictional interface is the same, but H90 brass/D60 steel
is different in mechanical properties, and the plastic deformation ability of brass is better than that of steel, so plastic deformation has totally occurred in brass side, almost no plastic deformation happened in D60 steel side (Fig. 4). The refined grains in brass side adjacent to the welding interface can be observed in Fig. 4d–f. Metallographic analysis software was used to analyze the grains size in the base metal of H90 brass and the refined grains zone, and it is found that the average grains size in base metal is 84.6 m, while at the fine grains zone adjacent to the welding interface, the grains were broken, and the grains boundaries were disordered, the metallographic analysis software could not measure the grain size. But in Fig. 4d, the fine grain size was not larger than the grain 10 m. Since the occurrence of plastic deformation makes the grains in H90 brass side adjacent to the friction interface refined, it is noteworthy to estimate the plastic deformation thickness in H90 brass side according to the width of refined grains. Fig. 4d shows the plastic deformation thickness is 50 m at the end of welding joint (position C shown in Fig. 7c). There has about 15 m plastic deformation layer (shown in Fig. 4e) near the end of welding joint (position B shown in Fig. 7c, it is about 4.5 mm distance from point C to point B). The thickness of plastic deformation is 80 m in the central one (position F shown in Fig. 7c). From the differences of the plastic deformation thicknesses at the three places of the welding joint, it is found that the plastic deformation thickness at the end of welding joint (place of C in Fig. 7c) is 35 m thicker than that one near the end of welding joint (place of B in Fig. 7c). And it implies that the most intensive plastic deformation happened at the central welding interface (place of F in Fig. 7c), which makes the most width of refined grains zone approximately 80 m in there. 4.2. Formation mechanism of the welding interface characteristics Fig. 2a presents three stages that can be divided in an inertia radial friction welding process, including the pre-stage and unstable friction stage (t1 period), the stable friction stage (t2 period),
Table 1 Properties of the H90 brass and D60 steel in this study. Materials
b (MPa)
Density (g/cm3 )
Linear expansivity (K−1 )
Heat conduction [W/(m K)]
Melting point (◦ C)
Linear contraction percentage (%)
H90 brass D60 steel
206 620
8.85 7.82
18.1 11.5
244.93 15
1056 1470
– 12
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Fig. 7. Schematic diagram of the contact friction in the H90 brass/D60 steel interface (a) contact region microstructure; (b) force and deformation analysis and (c) heat analysis.
and the forging stage (t3 period). As shown in Fig. 7a, the first radial pressure is imposed to the brass ring at t1 period. At this period, the friction between D60 and H90 is just contact friction in the asperity surface, and the contact area is very small. It can be found that when the radial friction welding process was finished, it was still asperity contact friction at the end edge of welding joint (at the place of C in Fig. 7c) shown in Fig. 4d. And some furrow-shaped holes appeared there, without forming a tight joint interface. A large amount of plastic deformation happened in the brass side, and much friction heat was produced, so sufficient thermoplastic state brass appeared at the welding interface in the condition of high temperature and high strain rate. The friction action of H90/D60 dissimilar metal was stopped at the t3 period (shown in Fig. 2), the temperature of H90/D60 welding joint decreased rapidly at the forging process, so it was difficult to take place recrystallization phenomenon for the thermoplastic brass. As a result, the zone of refined grains was formed at the brass side adjacent to the welding interface. And a smooth line of welding interface was shaped at the central of the joint, and a good welding seam was formed (Fig. 4f). As the same condition of friction pressure and friction time, why the furrow-shaped holes appear at the end of the welding joint, but have not appeared at the central of welding joint. It is noteworthy to comprehensively analyze its formation mechanism from the two important factors of friction heat and plastic deformation in radial friction welding process. (1) Based on the radial friction welding procedure, it can be known that during the effect of the radial pressure, the width of H90 brass ring will become wider, and the height will become more dwarfed. Because the radial pressure is changed from an axial pressure by a special assembly technology clamp, ensuring the radial pressure being applied to the contact surface equably, there are restrictions for the largest contact area and the best contact area in the special assembly technology clamp. Therefore, when the plastic deformation in the width direction exceeds the largest contact area under the frictional pressure applied in the H90 brass ring, the plastic deformation at the two ends and lateral side of H90 brass ring becomes freedom extrusion deformation, which is very similar to the plastic deformation in free forging of the plastic processing field. Fig. 7b depicts that line AB is the boundary of radial pressure line. The radial pressure was applied to H90 brass ring at the A point, and the plastic deformation happened at the B point. So there was a relative motion and mutual friction between the upper surface (A point) and the lower surface (B point), and friction heat was generated. So there was a friction force fa and an effect of severely stress at the internal of H90 brass (the green arrow line shown in Fig. 7b), which made the plastic deformation happened within the horizontal direction. Base on the principle of free forging in the plastic processing discipline
field, when the plastic deformation region exceeded the largest contact area between assembly clamp and brass, a free-state upward warping deformation was occurred in the H90 brass side (shown as the yellow arrow in Fig. 7b), as a result of the friction force fa existing (shown as the blue arrow in Fig. 7b). Fig. 7b shows the B point at the lower surface of H90 brass ring, the same as upper surface, there is a friction force fb between the lower surface of H90 brass and the upper surface of D60 steel. But it is different from the point A, because a severely radial friction exists at the interface between the lower surface of H90 brass and the D60 steel. And the temperature at the lower surface of H90 brass ring is much higher, and the thermoplastic deformation happened there. Therefore, the friction force fb at the bottom surface (point B) is smaller than fa in the upper surface (point A), as the effect of big stress (shown as green arrow in Fig. 7b) upon deformation, the thermoplastic brass is extruded out of the assembly technology clamp, which makes the bottom end metal (point C) of H90 brass dynamic flow and occur an upward warping deformation (shown as the purple arrow in Fig. 7b). Applied a decreasing radial pressure, rubbed and adhered with the D60 steel interface, the thermoplastic flow dynamic brass (in a Free and viscoplastic State) is extruded out of the friction interface in BC section (Fig. 7b), when it passed C point and separated from the steel interface, the upward warping deformation happened. Therefore, in the forging process, because of the end of H90 brass ring within a Free State, the H90 brass ring in the BC section cannot generate enough thermoplastic brass with a dynamic flow state, and it only obtain the thermoplastic brass from the central welding interface. Therefore, under the conditions of lower temperature, radial pressure inadequate, and the Free State, the capability for deformation is limited, and some stress is released at the end edge of H90 (compared with the central of H90 brass ring), it fails to eliminate the furrowshaped holes. The results show that the radial friction welding workpieces in BC section have not formed a tight welding seam (as the red arrow in Fig. 7b). In addition, according to the parallel mapping from the parts of upward warping deformation in the upper surface in H90 brass side, it can be roughly estimated that the part of DBC is made by the extruded thermoplastic brass from the central of H90 brass ring. (2) Studying the characteristics of welding interface formation from the view point of energy, shown in Fig. 7c, a symmetrical half of the joint is taken as the analysis sample, line FG is the center symmetry line of H90 brass ring. According to the characteristics of radial friction welding process and the analysis (the force and constraint of the joint) above, of BF section is the actual production heat zone (the actual friction surface), and segment of BC can also produce small amounts of friction heat (made by the adhesion of metal and internal stress), the end region of CEDHA is the boundary of radiation and convection
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heat transfer (as the yellow arrow in Fig. 7c), the heat conduction exists inside the H90 brass and D60 steel, and the upper surface of AG (as the green arrow in Fig. 7c). The production heat mechanism of radial friction welding is similar to the axial friction welding one, that is to say, it is produced by friction in the friction interface, and the friction heat is used to heat the workpieces. The difference is that the friction interface in axial friction welding is the axial cross section, and the angular velocity is equal. But the friction interface of radial friction welding is the circumference surface of the cylindrical (tube) workpieces, the line speed is equal. In this study, the total heat production in the H90 brass/D60 steel radial friction welding (BF segment) is that:
T
Qc =
2r l n(t)(t)F(t) dt
(1)
0
where r is the radius of cylindrical pipe D60, l is the axial contact distance of the H90 brass and D60 steel, T is the total friction time, n(t) is the friction speed, (t) is friction coefficient, F(t) is the radial pressure, is the conversion factors. There are two methods for the heat dissipation in the BF section. One is heat conduction, which conducted the heat to the workpieces, which stored in the workpieces, and increased the temperature adjacent to the welding zone, and the portion dissipated heat is indicated as Qw . The other is the radiation and convection heat transfer, indicated as Qd , it is generated with the welding flash appearance at the end of the joint. Qc = Qw + Qd
(2)
According to the heat conduction equation, the radiation and convection heat transfer Qd is:
Qd = 0
T
s(Tm − T0 ) dt h
(3)
where is the heat transfer coefficient, s is the heat transfer area, h is a constant, Tm is the welding temperature, T0 is the ambient temperature. So the heat used for the temperature increasing and the thermoplastic deformation Qw is:
Qw = Qc − Qd =
T
2r l n(t)(t)F(t)dt − 0
0
T
s(Tm − T0 ) dt (4) h
According to the characteristics of radial friction welding process and the formation discipline of radial friction welding joint of H90 brass ring/D60 steel pipe, at the contact friction interface between H90 brass and D60 steel (BF and BC sections in Fig. 7c), in Eq. (4), r is the radius of cylindrical pipe D60, n(t) is the friction speed, (t) is the friction coefficient, and F(t) is the radial pressure. And the parameters of n(t), (t) and F(t) are variables. This is because the radial friction pressure does not have a direct effect on BC section, and the extruded thermoplastic flow brass is formed in D60 steel surface during the effect of metal adhesion and internal stress. A lubricating thin film is formed at the surface of the thermoplastic state brass, which reduces the friction coefficient of the end edge of welding joint in BC section (Dong et al., 2008). And there is heat loss at the end of welding joint, reducing the useful welding heat, according to the Eq. (4), the friction coefficient at the end edges of welding joint is much smaller, which reduces the production of friction heat there. In addition, the formation of welding flash and the radiation and convection heat transfer reduce the welding heat that is used to increase the temperature of workpieces and produce thermoplastic deformation, which makes the amount of thermoplastic state brass decreased (Fu et al., 2007). While at the central of welding joint, friction coefficient is much larger for the extruded thermoplastic brass, and the radial frictional pressure is applied
directly, without forming welding flash for the heat dissipation. Eq. (4) shows that the more useful energy used for plastic deformation of welding joint is, the stronger of plastic deformation phenomenon of welding joint is, so a large amount of thermoplastic brass occurs in the welding joint. Therefore, there exist a temperature gradient, a friction coefficient gradient and a plastic deformation thickness gradient in the axial direction of the H90/D60 interface (in FC section). As there are no enough heat and restrict conditions for the plastic deformation at the end edges of welding joint, which makes insufficient plastic deformation, and some furrow-shaped holes are appeared there. In summary, there are temperature gradient and plastic deformation thickness gradient in H90 brass ring at the welding interface. And there exists the axial horizontal friction force in the lower and upper surface, as well as the horizontal radial friction, coupled with the restrict conditions at the end edges of welding joint, according to the principles of metallurgy and tribology, the characteristics of H90 steel/D60 steel welding interface (Fig. 4) and the welding flash are formed (Fig. 3). 4.3. The thermoplastic state lubricating thin film The main factors affecting friction coefficient are as follows: the property of metals, surface film, friction speed, temperature and loading. In this study, the property of metals, friction speed, temperature and loading on the friction interface of H90 brass/D60 steel dissimilar metals are steady and uniform, however the surface film is a variable and unsteady factor, which led to the difference of friction coefficient on the interface. Just as a slight clarification, this is not an oxide film on the friction interface, the thermoplastic lubricating thin film is formed on friction surface, which is made up of the thermoplastic brass. During radial friction welding process, the plastic deformation capability of brass is much larger than D60 steel, so it is easy for plastic deformation to take place at the H90 side, and the friction heat makes the H90 brass reached thermoplastic state during the friction process. But the parts at the end edges of welding joint are in contact with the air, the friction heat quickly dissipated into the air, in addition, the constraints and stresses at the end edges of welding joint are different from the one at the central; the constraint at the end edges of welding joint is relatively insufficient, which makes the friction heat relatively smaller, leading to production of little amount of state brass thermoplastic there (shown in Fig. 8a). As the radial pressure gradually increased and stabilized (t2 period in Fig. 2a), thermoplastic flow occurred in the thermoplastic brass surface towards the end edges of welding joint under radial pressure, because the high free degree and the constraints were insufficient there, (it can be confirmed that Fig. 4d shows the clear orbit of directional refined-grains zone formed in the thermoplastic flow process). And it finally makes a large amount of thermoplastic state brass appeared at the two end edges of welding joint, and formed a thermoplastic state thin film. But there is little thermoplastic state brass at the central of welding joint shown in Fig. 8b, (it can be confirmed by the thickness of plastic deformation in Fig. 4d and e, because the amount of plastic deformation has a great relationship with applying time of the radial pressure. Before the forging pressure applied, the amount of plastic deformation at the welding central can be seen as being similar with the one near the end of welding (4.5 mm far from the end edge of welding joint) in forging process finished time. The thickness of plastic deformation at the ends of welding joint is 50 m, while the one near the ends of welding (4.5 mm away) is 15 m. It is obvious that the amount of thermoplastic state brass at the end of welding joint is larger than the central welding joint one). Finally, the radial friction welding process is finished under forging force, and the thermoplastic brass at the two end edges of welding joint extruded, and forms the bend
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Fig. 8. Schematic diagram of the formation of the thermoplastic state lubricating thin film (a) more thermoplastic state brass formed at the central of the joint at the beginning stage; (b) the thermoplastic state brass flowed to the end edge of the joint forming a lubricating thin film and (c) achieved the welding process and formed the welding flash after forging pressure.
welding flash. And there are some furrow-shaped holes appeared at the two end edges of welding joint (Fig. 4d). Duo to thermoplastic brass extruded, a tight welding joint cannot be formed there, and the schematic diagram is showed in Fig. 8c. As the thermoplastic material is in a liquid state, the thermoplastic state brass is in a class-liquid state with micro-nano level solid super plasticity. The thermoplastic brass flows to the end edges of welding interface under radial pressure, which increased the amount of thermoplastic state brass there. And the thermoplastic state brass rotates around the outer surface of D60 steel pipe under rotating force, and a thermoplastic state thin film is formed. But there is no thermoplastic state thin film formed at the central of welding joint because the thermoplastic brass flowed to the ends of welding joint, which decreases the amount of thermoplastic state brass there. Dong et al. (2008) reported that the micro-nano level solid super plasticity material has a strongly lubricating effect. Therefore, this thermoplastic thin film has a strongly lubricating effect on the welding interface in radial friction welding process, so there is little friction heat generated at the end edges of welding joint because of the lubricating effect and the friction coefficient deceased (Luo et al., 2005). Finally under forging force, the thermoplastic brass is extruded out of the welding joint, and forms welding flash, moreover, the furrow-shaped holes appear at the end of welding joint.
3. Because of the small friction coefficient at the two end edges of welding joint, the thermoplastic brass flowed to the edge of welding interface at the effect of radial pressure, which increased the amount of thermoplastic brass there. And the thermoplastic state brass rotated around the outer surface of D60 steel pipe, and a thermoplastic state thin film was formed. It had a lubricating effect, so the friction coefficient at the edge of welding interface was reduced and the thermoplastic state brass was insufficient, so the furrow-shaped holes were formed there. 4. The elements diffusion results showed that there were both the Fe and Cu elements that diffused each other in the welding interface. The density and distance of Fe element diffusion towards H90 brass were larger that of Cu element towards D60 steel. This phenomenon was mainly attributed to the difference of the lattice coordination number of the Fe/Cu two elements.
5. Conclusions
References
1. The CT-130 type special inertia friction welding machine was used to finish a large size (156 mm diameter) of H90 brass/D60 steel dissimilar metals radial friction welding process. The results showed that there were some furrow-shaped holes in the welding interface at the ends of welding joint (about 4.5 mm distance away from the end edges of welding joint), and the farther away from the welding joint end was, the smaller the furrow-shaped holes were. And there was a smooth line in the central welding interface and a good welding seam was formed. 2. The friction at the ends of D60/H90 welding joint (about 4.5 mm) was also contact friction in asperity surface, and there were some furrow-shaped holes at the ends of welding joint. This was because of the insufficient constraints relatively there, as a result, the friction coefficient was smaller and the friction heat was insufficient. In addition, the friction heat was dissipated in heat conduction and at the formation of welding flash. As a result of the lack of friction heat used for plastic deformation, the generated thermoplastic brass is not enough to make up for the gap on the friction interface, so the furrow-shaped holes appear at the ends of welding joint.
Ahmet, D., Behc¸et, G., Fehim, F., 2005. Examination of copper/stainless steel joints formed by explosive welding. Mater. Des. 26 (6), 497–507. Bydash, J., Kasmani, R., Naraharisetty, K., 2010. Metal fume-induced diffuse alveolar damage. J. Thorac. Imaging 25 (2), 27–29. Dong, Y.-k., Zhang, X.-j., Liu, Y., Wen, S.-z., 2008. An elastic ratchet model and scale effect of surface topography on the micro-nano friction. Tribology 28 (4), 333–338. Elrefaey, A., Tillmann, W., 2009. Solid state diffusion bonding of titanium to steel using a copper base alloy as interlayer. J. Mater. Process. Technol. 209 (5), 2746–2752. Fu, L., Du, S.G., Bai, J.-h., 2007. Microstructures and properties of induction friction welded joint of TC4 Ti alloy and LD10 Al alloy. Chin. J. Nonferrous Met. 17 (2), 54–58. Jayabharath, K., Ashfaq, M., Venugopal, P., Achar, D.R.G., 2007. Investigations on the continuous drive friction welding of sintered powder metallurgical (P/M) steel and wrought copper parts. Mater. Sci. Eng. A 454–455 (5), 114–123. Luo, J., Sun Yu, Liu, D.-j., Wu, W., Xu, X.-l., 2010a. Characteristics of inertia radial friction welding joints of small size T3/35CrMnSi dissimilar metal materials. Chin. J. Nonferrous Met. 20 (7), 1309–1315. Luo, J., Zhao, G.-j., Luo, Q., Wang, X.-j., Xu, X.-l., 2010b. Element diffusion on interface of 35CrMnSi/T3 inertial radial friction welding. J. Xi’an Jiaotong Univ. 44 (3), 64–67. Luo, J., Zhao, G.-j., Wang, X.-j., Wu, W., Xu, X.-l., 2010c. Effects of flywheel rotation speed on properties of 35CrMnSi/T3 inertial radial friction welding. J. Chongqing Univ. 33 (9), 24–28. Luo, J.-b., He, Y., Wen, S.-z., Zhong, J., 2005. Challenges to tribology arisen from the development of micro- and nano-manufacturing technology. Tribology 25 (3), 283–288.
Acknowledgements The authors appreciate the financial support from the National Natural Science Foundation of China (no. 51075413), New Century Excellent Talents in University (NCET-08-0607), the Fundamental Research Funds for the Central University (CDJRC11280002), Natural Science Foundation Project of CQ CSTC (no. 2009BA3026) and Excellent Talents Project in Universities of Chongqing Municipal Education Commission (2010024), PR China.
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J. Luo et al. / Journal of Materials Processing Technology 212 (2012) 385–392
Magnabosco, I., Ferro, P., Bonollo, F., Arnberg, L., 2006. An investigation of fusion zone microstructures in electron beam welding of copper–stainless steel? Mater. Sci. Eng. A 424 (1–2), 163–173. Mai, T.A., Spowage, A.C., 2004. Characterisation of dissimilar joints in laser welding of steel–kovar, copper–steel and copper–aluminium. Mater. Sci. Eng. A 74 (1–2), 224–233. Nicholas, E.D., Lilly, R.H., 1979a. Radial friction welding. J. Phys., 50–56. Nicholas, E.D., Lilly, R.H., 1979b. Joining of hollow steel components by radial friction welding. Schweisstechnik (Vienna) 33 (5), 69–79. Pinto, Rosa Irene Terra, Strohaecker, Telmo Roberta, 2005. Residual stresses evaluation on radial friction welded joints, in: Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering-OMAE, 24th International Conference on Offshore Mechanics and Arctic Engineering, vol. 3, pp. 43–48. Singh, J., Gill, S.S., 2009. Fuzzy modelling for burst pressure strength in radial friction welding of GI pipes. Int. J. Modell. Ident. Control 7 (3), 305–312. Sabetghadam, H., Hanzaki, A.Z., Araee, A., 2010. Diffusion bonding of 410 stainless steel to copper using a nickel interlayer. Mater. Charact. 61 (6), 626–634. Torster, F., dos Santos, J.F., Hutt, G., Kocak, M., 1998a. Mechanical properties of Ti-6Al-4V-0.1Ru riser pipes joined by radial friction welding. Non-aerospace applications of titanium. In: Proceedings of the 1998 TMS Annual Meeting, pp. 181–188. Torster, F., Hutt, G., dos Santos, J.F., Kocak, M., 1998b. Metallurgical and mechanical properties of radial friction welded Ti-6Al-4V-0.1Ru risers. In: Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering-OMAE. 17th International Conference on Offshore Mechanics and Arctic Engineering, pp. 6–8.
Torster, F., Hutt, G., dos Santos, J.F., Kocak, M., 1998c. A study on thermal cycles in radial friction welding of a Ti-6Al-4V-0.1Ru alloy. Trends in welding research. In: Proceedings of the 5th International Conference, pp. 43–48. Weigl, M., Schmidt, M., 2010. Influence of the feed rate and the lateral beam displacement on the joining quality of laser-welded copper-stainless steel connections. Phys. Procedia 5 (2), 53–59. Wang, J., Cao, J., Feng, J.-C., 2010a. Microstructure and mechanical performance of depositing CuSi3 Cu alloy onto 30CrMnSi steel plate by the novel consumable and non-consumable electrodes indirect arc welding. Mater. Des. 31 (4), 2253–2258. Wang, G.-j., Ling, Z., Kang, D., 2010b. Investigation on heat input numerical models of radial friction welding. Hot Work Technol. 39 (11), 158–159. Wang, G.-j., Ling, Z.-m., Tian, X.-h., Qi, X.-c., Kang, D.-d., Yue, H., 2010c. Threedimensional finite element analysis of clamp-model for radial friction welding. Hot Work Technol. 39 (11), 158–159. Wu, W., Xu, X.-l., Xu, Y.-z., 2006. Study on radial friction welding of typical copperalloy T3, B5, H96 with steel 35CrMnSi. Ordnance Mater. Sci. Eng. 29 (6), 55–58. Xu, X.-l., Xu, Y.-z., Wu, W., 2007. Study on radial friction welding band of small shell. Acta Armamentarii 28 (3), 346–348. Xu, X.-l., Zhu, L.-y., Shen, J., 1995a. Radial friction welding with the axial FRM machine. Electric Weld. Mach. 5 (5), 29–31. Xu, X.-l., Zhu, L.-y., Shen, J., 1995b. Weldabilty of friction welding between copper and high carbon alloy steel. Weld. Joining 6, 17–18. Yao, C.-w., Xu, B.-s., Zhang, X.-c., Huang, J., Fu, J., Wu, Y.-x., 2009. Interface microstructure and mechanical properties of laser welding copper–steel dissimilar joint? Opt. Lasers Eng. 47 (7-8), 807–814.