Microstructural Features and Mechanical Properties Induced by the Spray Forming and Cold Rolling of the Cu-13.5 wt pct Sn Alloy

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1 J. Mater. Sci. Technol., Vol.24 No.5, Microstructural Features and Mechanical Properties Induced by the Spray Forming and Cold Rolling of the Cu-13.5 wt pct Sn Alloy Xiaofeng WANG, Jiuzhou ZHAO, Jie HE and Jiangtao WANG Institute of Metal Research, Chinese Academy of Sciences, Shenyang , China [Manuscript received December 8, 2006, in revised form June 13, 2007] Copper alloys with high strength and high conductivity are an important functional material with full of potential applications. In the present investigation, a bronze with higher tin content (Cu-13.5 wt pct Sn) was prepared successfully by spray forming, the feasibility of cold rolling this alloy was investigated, and the cold rolling characteristics of this alloy have also been discussed. The results indicate that the spray-formed Cu-13.5 wt pct Sn alloy, compared with the as-cast ingot, shows a quite fine and homogeneous single-phase structure, and, therefore shows an excellent workability. It can be cold-rolled with nearly 15% reduction in the thickness per pass and the total reduction can reach %. The classical border between the wrought and cast alloys is shifted to considerably higher tin contents by spray forming. After proper thermo-mechanical treatment, spray-formed Cu-13.5 wt pct Sn alloy exhibits excellent comprehensive mechanical properties. Particularly, it shows a low elastic modulus ( 88 GPa) and a high flow stress (over 0 MPa) after cold forming. This combination of properties is unique in the domain of metallic materials and could open new possibilities in spring technology field. KEY WORDS: Spray forming; Cu-Sn alloys; Cold rolling; Microstructure; Mechanical properties 1. Introduction Cu-Sn alloys with higher Sn content are appreciated in many applications because they combine high strength with good corrosion resistance and fairly good electrical conductivity [1 3]. Generally, the Sn content is limited to 8 wt pct for the product formed by warm and/or cold processing, such as drawing due to the macrosegregation formed during casting of the alloys [4]. Spray forming is a well-known advanced technique of material processing. Its major advantage is that bulk products with very fine equiaxed grains and low macrosegregation can be fabricated in a single operation through the creation of a spray of droplets by the atomizing gas and the subsequent rapid solidification and consolidation of these droplets [5 14]. It is very suitable for the manufacturing of the bronze with higher tin content. In the present work, Cu wt pct Sn has been prepared by spray forming. The feasibility of cold rolling the spray formed alloy was investigated. 2. Experimental A schematic of the spray atomization and deposition process is shown in Fig.1. In the process, a mixture of pure copper ( 99.9 wt pct) and pure Sn ( 99.7 wt pct) was melt in the vacuum induction furnace. The melt was delivered through a nozzle into a gas atomizer, where the melt was atomized with inert gas into fine droplets. The spray cone was then directed and deposited on a substrate, on which a solid billet was formed. The detail of the set-up is described elsewhere [15]. In the present investigation, the limited atomizer was adopted. Spray forming parameters are shown in Table 1. The size of the spray Prof., Ph.D., to whom correspondence should be addressed, jzzhao@imr.ac.cn. Fig.1 Schematic of spray atomization and deposition process Table 1 Parameters of the spray forming Cu-13.5 wt pct Sn alloy Variables Magnitude Atomizing gas N 2 Atomization pressure/mpa Melt temperature/ C 1150 Deposition distance/mm 450 Inter diameter of delivery tube/mm 4 Spray angle/deg 30 Withdrawal rate/(mm/s) Initial eccentric distance/mm Angle velocity of the substrate/(deg/s) 720 formed billet was 1 mm in height and 150 mm in diameter. The billet was machined and pieces of

2 4 J. Mater. Sci. Technol., Vol.24 No.5, 2008 Cumulative weight / % X-ray Intensity, counts phase phase Powder size / m deg. Fig.2 Size distribution of the powder Fig.3 X-ray diffraction pattern of the Cu-13.5 wt pct Sn powders Fig.4 SEM morphology of microstructure of the powders of (a) 10 µm and (b) 50 µm in diameter 100 mm in diameter were removed from the central region of the deposit for further investigation. Archimedes method was used to determine the density of the spray-formed billet. The alloy blocks, with a thickness of 20 mm, were cut from the asdeposited Cu-13.5 wt pct Sn alloy for cold rolling. The initial total cross-sectional reduction was about 50% with 10% reduction in thickness per pass. Then the cold-rolled specimens were annealed at 0 C for 30 min. After that, cold rolling was carried out to improve the mechanical properties of the alloy. The total cross-sectional reductions are 50%, %, 70% and %, respectively and the reduction in thickness per pass is 15%. The powders were collected and the values of the mean powder size d 50 and the standard deviation σ were measured. The microstructures of the alloy in various states were characterized by optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). Linear intercept method was used to measure the average grain size. The tensile properties were determined by using an Instron-testing machine AG-5000A. The tensile direction was parallel to the rolling direction. The tensile velocity is 1 mm/min. 3. Results and Discussion Figure 2 shows the powder size distribution, which for a given alloy and atomizer mainly depends on the gas pressure and the metal flow rate. The average powder size d 50 is µm and the standard deviation σ is 0.35 µm indicating that the atomized droplets are very fine. These small-sized droplets assure the fine microstructure of the final deposit. XRD result indicates that the matrix consists of α-phase and δ-phase, see Fig.3. Figure 4 shows the SEM microstructures of the powders. The surface of the powder is regular. A fine dendritic structure was observed. The black part is the primary α-solid solution and the other part is the eutectoid reaction product (ERP), which consists of the α-phase and the δ-phase. The amount of the eutectoid reaction product varies with the powder size. The bigger the powder, the higher the volume fraction of the ERP. For example, the volume fractions of the ERP are about 24% and 38%, respectively, in the powder of 10 and 100 µm in diameter, as shown in Fig.5. Figure 6 shows the microstructure of spray formed and permanent mold cast Cu-13.5 wt pct Sn alloy. The permanent mold casting has a dendritic structure with tin-rich δ phase between the dendrites. The high fraction of this brittle and low melting constituent prevents the hot and cold working. In the spray formed microstructure, it shows clearly fine equiaxed grain morphology. The average grain size is 32 µm. The similar microstructure is invariably presented in the samples from different regions of the spray formed deposit. XRD result shows that single α-phase is present in the microstructure, which is quite different from both permanent mold casting and the powders (Fig.7). The possible reason is as follows: during the atomization stage, the high cooling rate

3 5 J. Mater. Sci. Technol., Vol.24 No.5, measured simulated f ERP The size of the powder / m Fig.5 Relationship between the volume fraction of the eutectoid reaction product (ferp ) and the powder sizes ( K s 1 ) drives the alloy away from the equilibrium condition and most tin elements were solutioned in the matrix. The limit of solid solubility of alloying elements increases compared with the conventional cast alloy[16 19]. However, due to the solute partition, there was a small amount of tin segregated between the dendrites and δ-phases formed in the powders. At the cooling stage, the relatively slow cooling rate of the billet enabled the homogenization of tin alloying element. Therefore, δ-phases disappeared. While for permanent mold casting Cu13.5 wt pct Sn alloy, it seemed to be impossible because the number of the tin-rich δ phase was so much. In addition, there are a few spherical and angular pores in the as-sprayed billet. The relative density of the spray-formed Cu-13.5 wt pct Sn is over 97%. In the middle of the billet, the relative density is as high as 98.1%. In order to eliminate the pores in the billet and enhance the strength of the alloy, deformation is often needed. In general, the deformation ability of as-cast Cu-Sn alloys with tin content more than 8 wt pct is poor due to the presence of a larger number of brittle tin-rich δ-phase[20,21]. A long-time homogenization annealing is often needed to make the tin element distribute uniformly. However, spray formed Cu-13.5 wt pct Sn alloy shows excellent deformation ability. There were no difficulties in cold rolling asdeposited Cu-13.5 wt pct Sn alloy without a prior annealing homogenization treatment. It can be coldrolled with 10% reduction in thickness per pass and the total reduction of the as-deposited alloy can reach 50% prior to the recrystallization annealing. After recrystallization, the total reduction of this alloy can reach more than % with 15% reduction in thickness per pass. Thus, the classical border ( 8 wt pct Sn) between wrought and cast alloys is shifted to considerably higher tin contents ( 13.5 wt pct Sn) by spray forming. Figure 8 shows the microstructure of the asdeposited alloy after cold rolling with a % reduction. It was found that the single phase α-solid solutions were deformed seriously and squeezed into lamella. The pores present in the deposit were considerably reduced. Due to the serious deformation, the elongated substructures appeared, as shown in Fig.9(a). The cold rolling results in an increase in the number of dislocations. The multiplication of the dislocation leads to the formation of the dislocation cells, see Fig.9(b). The dislocation density in the grain can reach 1012 /cm2[22]. As a result, the hardness and strength increase significantly according to[23] : 1 σ = σ0 + αgbρ 2 (1) where σ is the flow stress, σ0 the friction stress, α a constant, G the shear modulus and ρ the dislocation density. Such a density of dislocation in the present material may lead to a strength increase up to 250 MPa. It is well known that twinning is one of the prominent modes of deformation for a variety of engineering metals and alloys. Formation of the multiple twins is quite common in the deformation process of those materials with a medium or low stacking fault energy, especially at high strain rates and/or at low deformation temperatures when dislocation activities are effectively suppressed. In the present investigation, deformation twins are clearly revealed in the dark field image of the cold rolled as-deposited Cu-13.5 wt pct Sn alloy. The twins form bundles of a few micrometers thick and several micrometers long (as shown in Fig.10). It has preciously been shown in the low stacking fault energy fcc alloys that when a deformation occurs by slip and twinning, the contribution of the twin component to the observed strength can be calculated via a modified Hall-Petch relationship[24] : 1/2 σtwin = (σ0 + kdtwinned )ftwinned (2) where σtwin is the twin component, i.e. strengthening contribution of the twins due to their grain-refinement Fig.6 Microstructure of Cu-13.5 wt pct Sn alloy: (a) as-deposited and (b) permanent mold cast

4 6 J. Mater. Sci. Technol., Vol.24 No.5, 2008 X-ray Intensity, counts phase deg. Fig.7 X-ray diffraction pattern of the as-deposited Cu13.5 wt pct Sn alloy Fig.8 Optical microstructure of the cold rolled asdeposited Cu-13.5 wt pct Sn alloy Fig.9 TEM morphology of (a) the lath microstructure and (b) dislocation cells of the spray formed Cu-13.5 wt pct Sn alloy after a cold rolling of % reduction effect; ftwinned is the fraction of the grains with deformation twins; dtwinned is the inter-twin spacing; σ0 is the friction stress and k is the Hall-Petch slope. In the cold-rolled alloy with % reduction, the average volume fraction of twins was as high as 0.25 with an average twin lamellar thickness about 100 nm. The high density of twins as well as numerous dislocations leads to a very high tensile yield strength. Since there is no possibility for aging this alloy, the strength can be adjusted only by cold rolling. The cross-sectional reduction is selected according to the performance requirements. Figure 11 shows the mechanical properties of the spray-formed Cu13.5 wt pct Sn alloys vs the cross-sectional reduction. It shows that the alloy has a high strength together with a relatively good ductility. With the increase of the cross-sectional reduction, the ultimate tensile strength and the yield strength of the alloys increase, respectively, from 870 MPa and 790 MPa at a 50% cross-sectional reduction to 970 MPa and 920 MPa at a % cross-sectional reduction. While the elongation drops a little. At a 50% cross-sectional reduction the elongation is 9% and at a % cross-sectional reduction the elongation is 7%. The corresponding relationships between the strength, the elongation and the cross-section reduction x can be approximated as the following equations: σb (MPa) = exp( x/474.31) (3) σs (MPa) = exp( x/187.47) (4) δ% = 58.52exp( x/834.96) (5) In addition, this alloy has the particularity of a low elastic modulus ( 88 GPa) and a high flow stress (over 0 MPa). This combination of properties is unique in the domain of metallic materials and could open new possibilities in the spring technology field. Figure 12 shows the morphology of the tensile fracture surfaces of the alloys suffered different crosssection reductions. It is clear that the fracture surfaces are ductile. The size of dimples in Fig.12(b) is smaller than that in Fig.12(a), indicating that the rolling of the alloy can lead to the alteration in the fracture morphology. Inside the grains the density of the dislocation is low and near the grain boundary the density of the dislocation is high. A plenty of dislocations tangle together. Such tangled dislocations are the nucleation sites for the microvoids. The process of fracture can be described as follows: the plastic deformation of the matrix initiates the microvoid. The microvoid then extends when the plastic strain incr-

5 J. Mater. Sci. Technol., Vol.24 No.5, Fig.10 TEM morphology of the microstructure of the spray formed Cu-13.5 wt pct Sn alloy after a cold rolling of % reduction showing the deformation twins in the matrix: (a) BF image, (b) DF image, (c) SAD,( d) schematic of SAD of deformation twins 10 9 Tensile strength Yield strength Strength / MPa 8 8 / % Elongation rate Cross-section reduction / % Fig.11 Tensile properties of the specimens with different cross-sectional reductions eases. Cracking takes place after the microvoids grow to a large size and coalesce. The size of the dimples on a fracture surface is governed by the number and distribution of the microvoids nucleated[22]. Apparently microvoid nucleation sites in the alloy with a 50% cross-sectional reduction is less and more widely spaced than that in the alloy with a % cross-sectional reduction. For the former one, the microvoids grow to a large size before coalescing and the result is a fracture surface that contains large size dimples. While for the latter, small dimples are formed when a numerous nucleation sites are activated and the growth of the individual microvoids is limited. Fig.12 Morphology of the tensile fracture surface of the alloy with (a) a 50% cross-sectional reduction and (b) a % cross-sectional reduction

6 8 J. Mater. Sci. Technol., Vol.24 No.5, Conclusion A bronze with a higher tin content (Cu-13.5 wt pct Sn) was prepared by spray forming. The cold rolling characteristics of this alloy were studied. The results indicate that the spray-formed Cu-13.5 wt pct Sn alloy, compared with the as-cast ingot, has a quite fine and homogeneous single-phase structure due to rapid solidification, and, therefore, shows excellent workability. It can be cold-rolled with nearly 15% reduction for one pass and the total reduction can reach %. The classical border between wrought and cast alloys is shifted considerably to the high tin contents by spray forming. After a proper thermo-mechanical treatment, the spray-formed Cu-13.5 wt pct Sn alloy shows excellent comprehensive mechanical properties. The corresponding relationships between the strength, the elongation and the cross-section reduction can be approximated as: σ b (MPa) = exp( x/474.31) σ s (MPa) = exp( x/187.47) δ% = 58.52exp( x/834.96) In addition, this alloy shows a low elastic modulus ( 88 GPa) and a high flow stress (over 0 MPa) after cold forming. This combination of properties is unique in the domain of metallic materials and could open new possibilities in the spring technology field. Acknowledgements The authors gratefully acknowledge the financial support from the Hundred-Talent-Person Project of Chinese Academy of Sciences. REFERENCES [1 ] A.Lawley and A.G.Leatham: Mater. Sci. Forum, 1999, 299(3), 407. [2 ] K.Siegert and S.Huber: Mater. Sci. Eng. A, 2002, 326, 63. [3 ] H.R.Müller, S.Hansmann and K.Ohla: Proc. of the SDMA, Bremen, Germany, 2000, 205. [4 ] J.P.Tardent and P.A.Isler: Proc. of the SDMA, Cardiff, UK, 1996, 211. [5 ] Z.Y.Li, J.Shen, F.Y.Cao and Q.C.Li: J. Mater. Process. Technol., 2003, 137,. [6 ] X.F.Wang, J.Z.Zhao and C.Tian: Acta. Metall. Sin., 2005, 41, (in Chinese) [7 ] X.F.Wang, J.Z.Zhao, J.He and J.T.Wang: Acta. Metall. Sin., 2005, 41, 923. (in Chinese) [8 ] X.F.Wang, J.Z.Zhao, D.M.Liu, J.T.Wang and G.Y.Chen: Special Casting Nonferrous Alloys, 2005, 6, 350. (in Chinese) [9 ] J.H.Zhao, D.M.Liu and H.Q.Ye: J. Mater. Sci. Technol., 2003, 19(5), 398. [10] J.F.Sun, J.Shen and F.Y.Cao: J. Mater. Sci. Technol., 2001, 17(1), 105. [11] F.Y.Cao, C.S.Cui and Q.C.Li: J. Mater. Sci. Technol., 2001, 17(1), 101. [12] X.Liang and E.J.Lavernia, Q.Xu, V.V.Gupta and E.J.Lavernia: Metall. Mater. Trans. B, 1999, 30(3), 527. [13] P.S.Grant: Prog. Mater. Sci., 1995, 39, 497. [14] B.Cantor, K.H.Baik and P.S.Grant: Prog. Mater. Sci., 1997, 42, 373. [15] Lin YANG: Ph.D. Thesis, Institute of Metal Research, CAS, (in Chinese) [16] D.M.Liu, J.Z.Zhao and H.Q.Ye: Mater. Sci. Eng. A, 2004, 372, 229. [17] J.S.Zhang, H.Cui and X.J.Duan: Mater. Sci. Eng. A, 2000, 276(1-2), 257. [18] A.Manonukul and F.P.E.Dunne: Acta Mater., 1999, 47(17), [19] H.Zhang, Z.H.Chen, Y.Sun, P.Y.Huang and F.L.Yang: Rare Metal Mater. Eng., 1998, 27(4), 226. (in Chinese) [20] J.P.Tardent and G.Himstead: Future Diection of Copper-based Alllys for Electrical and Electronic Applications, Fleck Materials Conference, September 17-18; Indian Wells, California, [21] J.P.Tardent and B.Demestral: Draht, 1999, 1, 43. [22] M.Niewczas, Z.S.Basinski and J.D.Embury: Philos. Mag. A, 2001, 81, [23] H.Conrad, S.Fuerstein and L.Rice: Mater. Sci, Eng., 1967, 2, 157. [24] A.Rohatgi, K.S.Vecchio and G.T.Gray: Metall. Mater. Trans. A, 2001, 32A, 135. [25] G.F.Pittinato, V.Kerlins, A.Phillips and M.A.Russo: SEM/TEM Fractography Handbook, Metals and Ceramics Information Center, 1975.