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Isothermal deformation of spray formed Al–Zn–Mg–Cu alloy
Mechanics of Materials 56 (2013) 95–105
Contents lists available at SciVerse ScienceDirect
Mechanics of Materials journal homepage: www.elsevier.com/locate/mechmat
Isothermal deformation of spray formed Al–Zn–Mg–Cu alloy Wenjun Zhao a,b, Fuyang Cao a, Xiaolong Gu b, Zhiliang Ning a, Ying Han c, Jianfei Sun a,⇑ a b c
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Zhejiang Province Key Laboratory of Soldering & Brazing Materials and Technology, Zhejiang Metallurgical Research Institute Co., Ltd, Hangzhou 310030, China Capital Aerospace Machinery Company, Beijing 100076, China
a r t i c l e
i n f o
Article history: Received 19 May 2011 Received in revised form 1 July 2012 Available online 12 October 2012 Keywords: Isothermal deformation Billet Deform Component
a b s t r a c t The isothermal deformation of a V-shaped component was simulated by the DEFORMTM process simulation software. The material flow, internal stress and temperature of the billet’s interior were investigated during the process of deforming, and the simulation results showed that bar billet is more suitable than sheet billet for producing the V-shaped component. The real isothermal deformation process was carried out with bar billet, and the microstructure of the shaped component was greatly refined. After heat treatment, the ultimate tensile strength (UTS) of the component can reach 817 MPa. The results show that isothermal deformation is a low cost and high efficient compaction technique for produce components from the spray formed Al–Zn–Mg–Cu alloy. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Al–Zn–Mg–Cu series super-high strength aluminum alloys have been widely applied as aircraft structural materials due to their low density and high strength (Grant, 2007). Increasing the content of hardening alloying elements is an effective way to strengthen the alloy (Wegmann et al., 2002). Spray forming process has been applied to increase the content of hardening alloying elements up to 13%. For the super-high strength Al–Zn–Mg– Cu alloy, the mass ratio of Zn has a pronounced effect on the mechanical properties. But the presence of porosity is inevitable in the spray formed material. For instance, porosity has been reported for a variety of spray formed Al alloys ranging from 1 to 10% (Cai and Lavernia, 1997). Therefore, forging, extruding and isothermal hot isostatic pressing etc. were used to compact the material. There have been some previous studies on the extrusion research of the spray formed aluminium alloy (Jeyakumar et al., 2010; Ning et al., 2010; Salamci and Cochrane, 2002, ⇑ Corresponding author at: School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China. Tel.: +86 451 86418317; fax: +86 451 86413904. E-mail address: [email protected] (J. Sun). 0167-6636/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mechmat.2012.09.009
2003; Sharma, 2008; Wang et al., 2009). Sharma et al. reported that the mechanical properties of the extruded aluminium alloy of 12Zn–3.3 Mg–1.3Cu–0.3Mn–0.20Zr are tensile stress of 763.8 MPa, yield stress of 755.6 MPa and elongation 8.5% (Sharma et al., 2006), and the results show that the spray formed zinc-rich aluminium alloy is a suitable structural material with high strength. But extrusion is a technique which can only compact the alloy into some product with simple geometry, such as bar, sheet and pipe, not the complex geometry. If the additional machining is applied to produce the complex one, it will increase the cost. Isothermal deformation is a flexible process which can be used to produce some complex components with net size and shape (Flower, 1990), however, there are few studies about the isothermal deformation of the spray formed Al–Zn–Mg–Cu alloy. So the isothermal deformation of this alloy will be analyzed by numerical simulation and testing methods of deformation in this paper (Li et al., 2003; Yamada et al., 1968). Isothermal deformation is carried out within the closed metal mould and hard to be observed in the real experiment, but the deforming process can be simulated by the software DEFORMTM. The material flow, internal stress and temperature of the billet’s interior and the deforming loads of the mould can be obtained by
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Table 1 Chemical compositions of the spray formed aluminum alloy. Element
Zn
Mg
Cu
Mn
Cr
Al
Composition wt.%
12.82
3.20
1.28
0.17
0.17
Balance
DEFORMTM. This software also can predict which part of the component would be stress concentration, and the cracks easily came up in these region. The cracks could be avoided through redesigning the mould and reselecting the working parameters. And the experiment of isothermal
Fig. 1. (a) A typical 23-kg spray formed Al–Zn–Mg–Cu billet. (b) an as-deposited microstructural map showing noncolumnar/dendritic equiaxed polygonal grains.
Fig. 2. True stress–strain curves at different temperatures (a)370 °C, (b)390 °C, (c)410 °C, (d)430 °C.
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Fig. 3. Sketch of V-shaped component.
Fig. 4. Finite element model (1-upper mould, 2-billet, 3-lower mould) (a) sheet billet, (b) bar billet.
Fig. 5. Mesh and boundary condition (a) sheet billet, (b) bar billet.
deformation is used to evaluate the processing given by simulation.
2. Experimental 2.1. Spray formed billet The experimental material of the component is spray formed Al–Zn–Mg–Cu alloy produced by Harbin Institute of Technology, and its chemical composition is shown in
Table. 1 (in wt.%). In spray forming experiment, melt temperature is 760 °C and liquid metal flow rate is 50 g/s. Double-layer atomizer system is employed and the gas pressures of the primary and secondary nozzles are 1.0 and 1.5 MPa. The spray distance is 400 mm. The spray formed billet and the as-deposited microstructure are shown in Fig. 1, and the microstructure shows the grain size is 20–50 lm. At the grain boundary the concentrated second phases are eutectic precipitated from the liquid phase directly. The dispersed white phases within the grains are mainly the strengthening phase
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Fig. 6. Material flow within the sheet billet (a) initial phase, (b) developing phase, (c) intermediate phase, (d) final phase.
(MgZn2) which precipitated from the solid basal body. It’s the characteristic of the deposited material that there are some tiny pores within the microstructure (Grant, 1995), which reduces the mechanical property. Therefore, the as-deposited material is need to be compacted and the microstructure should be refined to increase the mechanical propertied, which can be achieved by the isothermal deformation. 2.2. Hot compression simulation The simulation of the isothermal deformation demands that the necessary deforming property parameter should be imported into the database of the software DEFORMTM, which are obtained by isothermal compression tests. The isothermal compression tests of the spray formed Al–Zn– Mg–Cu alloy under different temperatures or strain rates were performed on a Gleeble-1500 Thermal Simulator. The specimen size used for this test is U 8 12 mm, and true stress–strain curves given by this equipment are shown in Fig. 2, and imported into the material model of DEFORMTM. The results by isothermal compression tests show that the stress didn’t increase sharply with increasing the deformation under the strain rates of 0.01/s, 0.1/s,
1/s, especially when the strain rate is 0.01/s, the stress almost keeps constant during the compressing process. But when the strain rate is 10/s, the materials destabilize due to the fast strain rate. 2.3. Isothermal deformation experiment V-shaped component is the key connecting components, as shown in Fig. 3. The schematic of the component shows the geometry includes the thin wall, high rib, convex platform. Those V-shaped components manufactured with traditional machining production technique cost much time and money. Furthermore, cutting process would cut off the metal fiber structure within the component and decrease the mechanical properties especially the fatigue properties. The component, manufactured with ordinary stamping forming technique, would experience three working procedures, blanking, bending and forming. The problems of dimension variation, shape distortion and surface cracking would come with the components because of the poor plastic forming property. Therefore, to avoid these problems isothermal deformation is adopted to manufacture the net shape components in a single process. This technical needs less operations and raw
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Fig. 7. Material flow within the bar billet (a) initial phase, (b) developing phase, (c) intermediate phase, (d) final phase.
materials, and reduces the cost. The weights of the raw billet and the finished product are 100 and 90 g respectively, and the process yield is up to 90%. To produce this component, two kinds of raw billets, sheet and bar, are prepared. Sheet billet is 100 50 20 mm and bar billet is U 40 100 mm in size. The mould material is H13 steel. The experiment was performed on a 6300KN press. The relative speed between upper and lower moulds is 2 mm/s. Before isothermal deformation, the billets and die moulds were preheated to 430 °C for 30 min. According to hot compression experiment, the friction factor between the row material and the die moulds is 0.3. 2.4. Build the finite element model DEFORMTM (Design Environment for Forming) software system is developed by American Scientific Forming Technologies Corporation (SFTC) company based on S. Kobayashi et al research (Kobayashi et al., 1989) and ‘Analysis of Large Plastic Incremental Deformation’ (ALPID) software compiled by S.I. Oh et al. (Oh et al., 1991) and widely applied by American, Japan, German. The geometry model used to simulate the forming process consists of three parts: upper mould, billet and lower mould, as shown in Fig. 4. As already noted, the sheet billet and the bar billet have been adopted during the simulating process of the V-shaped component. The mesh type should be decided before meshing, and the tetrahedral isoperimetric element was applied. The sheet billet includes
7397 nodes and 31815 cells; the bar billet includes 6641 nodes and 28962 cells, as shown in Fig. 5. To improve the precision, the local mesh refine had been employed at the place where is close to the end of the upper mould or the place near the step of the component. 3. Modeling results and discussions The metal of the billets undergoes the axial, radial and toroidal flows within the mould during the forming process. Comparing with the radial and toroidal flow, the axial flow can be neglected because the restrain of the mould. The flow, internal stress, and temperature of the billet’s interior are most concerned in this paper. In Fig. 6 to Fig. 11, the figures a, b, c and d correspond directly to specific displacements 10, 20, 30 and 35 mm of the upper mould, and show the initial phase, developing phase, intermediate phase and the final phase of the deforming process, respectively. In different phases the flow of the material, the equivalent stress and the temperature field of the billet are studied in the following sections. 3.1. The flow of the material The material flows of the billets’ interior are shown in Figs. 6 and 7 during the deforming process, before that, the sheet was placed in the lower mould, and only two ends touched the lower mould. In the experiment, the upper mould pressed on the middle of the billet as shown
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Fig. 8. Equivalent stress within the sheet billet (a) initial phase, (b) developing phase, (c) intermediate phase, (d) final phase.
in Fig. 4a, and made the sheet billet bend as shown in Fig. 6a. The initial deformation of the sheet billet is visible in the middle where the upper mould is pressing on, meanwhile the material at both side flows towards the middle. The material flow speed in the middle is faster than that at both sides, which causes the bending deformation of the sheet billet in the initial phase. In Fig. 6b, the sheet billet has deformed much more than that in the initial phase. The material still moves faster in the middle than that near two ends of the billet, the flow directions in the developing phase are same to that in the initial phase. In both initial and developing phases, the flow speed difference makes major part of the sheet billet be under the tensile stresses, which would easily lead to cracking. In Fig. 6c the billet surface has completely touched on the inner surface of the moulds. Most part of the cavity between upper mould and lower mould has been filled with the material of the sheet billet at this moment. The material in the middle of the sheet billet still flows in the loading direction of the upper mould, but the flow direction of the material at the two ends of the billet has reversed. At the last period of the forming process, the billet is almost totally transformed into the V-shaped component in geometry, as shown in Fig. 6d. The most part flows slowly except at the two ends where the material is filling the cavity at final stage, and flowing faster.
Fig. 7 is the material flow of the bar billet, and Fig. 7a shows the material at the top of the bar billet flows from the middle to the left and right due to the pressure of the upper mould, and the deformation starts from the top. As the upper mould moves down, the material flow speed is increasing as shown in Fig. 7b and larger deformation has performed in the billet than former. In the intermediate and the final phases, the material at two ends flows faster to fill the cavity as shown in Fig. 7 c and d. During the whole deforming process of the bar billet most part of the material is under compressive stress to limit cracking effectively. As the loading continues, change of billet geometry is very small, and parts of metal overflows the cavity along the mould surface until the dimension is up to standard. In the late phase, the material flows of both the sheet and the bar billet are similar with each other. 3.2. The stress analysis Fig. 8 is the equivalent stress of the sheet billet respectively. The equivalent stress distribution isn’t uniform as shown in Fig. 8a due to the uniform deformation. The equivalent stress in the middle is higher than at the two sides because the deformation mainly takes place in the middle, as shown in Fig. 8a and b. With the development of the deformation, the plastic deformation occurs in most
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Fig. 9. Equivalent stress within the bar billet (a) initial phase, (b) developing phase, (c) intermediate phase, (d) final phase.
parts of the sheet billet, and the billet touches on the mould surface gradually. In the intermediate phase, the equivalent stress maintains in equilibrium within the billet, as shown in Fig. 8c. In the final stage the metal fills the whole cavity of the mould and plastic deformation of all parts is uniform and the equivalent stress is greater than former but still uniform, as shown in Fig. 8d. Fig. 9 shows the equivalent stress of bar billet. In the initial phase, the equivalent stress is uniform in most parts of the bar billet. As the upper mould loading continues, the equivalent stress increases a little in most part of the billet, but there is one area with lower equivalent stress at the bottom of the billet, as shown in Fig. 9b, which is due to the restrain of the base angle of the cavity and means that the plastic deformation is hard to take place here. In the developing phase, the equivalent stress reaches the maximum value but is still uniform in the billet as shown in Fig. 9c. Plastic deformation takes place all over the billet. In the last phase, the equivalent stress decrease as shown in Fig. 9d. In the whole process, the equivalent stress is uniform consistently without stress concentration which can limit cracking. During the deforming process of the bar billet, the higher equivalent stress of the developing phase contributes to the compaction of the material, and the lower equivalent stress of the last phase favours the control the final geometry of the component. 3.3. Temperature field Fig. 10 shows the temperature field of the sheet billet during the forming process. At the beginning of the
forming process, the temperature increases in the middle of the billet, the reason for that is larger deformation in this reason, and remains lower temperature at the two ends of billet, as shown in Fig. 10a. In the developing phase, the temperature distribution has nearly changed, except a little temperature rising at the step region of right side, as shown in Fig. 10b. In the intermediate phase, the highest temperature is no longer at the center position, and appears at the step region of right side, as shown in Fig. 10c. In the last phase, the temperature is lower at the center and higher at the two ends, because the deformation almost stops in the middle and the material is filling the last corner of the cavity. The combination of the billet deformation and the frictional interaction between the billet and the surfaces of the moulds makes the maximum temperature 441 °C appear at the left end near side, as shown in Fig. 7d. At the same time, the high temperature may cause the cracks near the edge. Fig. 11 shows the temperature fields of the bar billet. In the initial phase, the temperature increases at the top of the billet where the upper mould is pressing on. Within the billet, the closer from the upper mould, the higher the temperature of the material and at the bottom of the billet temperature keeps low, as shown in Fig. 11 a. The rising of the temperature is due to the accumulation of the deformation energy. The temperature distribution in the developing phase is familiar with that in the initial phase, but increases a little. The temperature is increasing with the moving down of the upper mould. In the intermediate phase, the temperature rises in most parts of the billet
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Fig. 10. Temperature field within the sheet billet (a) initial phase, (b) developing phase, (c) intermediate phase, (d) final phase.
Fig. 11. Temperature field within the bar billet (a) initial phase, (b) developing phase, (c) intermediate phase, (d) final phase.
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Fig. 12. (a) Bar billet placing in the mould. (b) components just after Isothermal deformation.
Fig. 13. (a) Microstructure of the deformed component without heat treatment (SEM). (b) microstructure of the component after solution (TEM). (c) grains after the whole heat treatment. (d) locations of the specimens.
Table 2 Mechanical properties of Al–12.82Zn–3.20 Mg–1.28Cu–0.17Mn–0.17Cr. Experiment
Solution
Aging
YS (MPa)
UTS (MPa)
Elongation (%)
I II
470 °C/1 h + 490 °C/1 h 470 °C/1 h + 485 °C/1 h
120 °C/16 h 120 °C/16 h + 160 °C/5 h
807 748
817 756
1.2 4.1
except a local area round the corner of the billet, as shown in Fig. 11c. In the last phase, the temperature increases in all parts, and the maximum value is 443 °C. The temperature of the last phase is similar to that of the intermediate phase. The highest temperature doesn’t appear at the edge of billet, so as to limit the cracking here.
To conclude, within the bar billet the material deforms uniformly, there is no stress concentration near the edge, and the highest temperature at the end of forming does not appear at the edge. Both of these effects result in the absence of any cracking near the edge. For making the V-shaped component the bar billet is superior to the sheet billet.
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4. Isothermal deformation Fig. 12 shows the photos of the bar billet before and after isothermal deformation experiment. There are no cracks in the surface of the component after the isothermal deformation, and the surface is flat and smooth without any raised or sunken place in it. There is some overflow material connected with the component which can ensure the filling of the cavity entirely. Fig. 13a shows the microstructure of the components before solution, and the specimen is cut from zone ‘A’ as shown in Fig. 13d. The porosities as deposited have been eliminated. The coarse precipitated phases on the grain boundary of as-deposited (Fig. 1b) were tore into many little pieces of white phases lining in the flow direction of the material, and some other precipitated phases locates within the grain. Most of the white phases would play the role of solid solution strengthening. While the precipitated phases on the grain boundary were tore into pieces, the grains were great refined, so that the fine-grain strengthening had been achieved. The grain size of the component is no more than 5 lm after deformation as shown in Fig. 13c, and the specimen is cut from zone ‘C’ as shown in Fig. 13d. After isothermal deformation, the components were divided into two groups (I and II) randomly. The technologies for heating processing are shown in Table 2. Before aging, the solution and quenching of the alloy are necessary. Higher solution temperature makes the secondary phases dissolve into the base phase thoroughly, that does a good strengthening effect but also results in overburning of the alloy which should be avoided. As the deformed alloy contains refractory phases i.e. Al2Cu, AlCu or Mg32(AlZn)49, double-stage (low and high temperature) solution is adopted. In the first stage (low temperature stage) the low melting phase MgZn2 dissolves into the matrix; in second stage (high temperature stage) the refractory phases dissolve and there are no overburning low melting phases. The hardening medium is warm water and the quenching transferring time must be less than 10 s. Fig. 13b shows the microstructure of II group component after solution, and the specimen is cut from region ‘B’ as shown in Fig. 13d. Most of the white precipitated phases have been dissolved in the base matrix. Only some of strengthening precipitations are dispersing in the matrix, and solution strengthening has achieved a desired result. The location of tensile test specimens is region ‘c’ as shown in Fig. 13d. The results of tensile test are shown in Table 2. The heat treatment process of two groups I and II are double-stage solution adding T6 peak aging and double-stage solution adding double-stage solution adding T7 over aging, respectively. The peak aging can make the alloy reach the highest strength, and the ultimate tensile strength (UTS) achieves 817 MPa. Over aged alloy shows a little lower strength, but higher elongation.
5. Conclusion The spray formed Al–12.82Zn–3.20 Mg–1.28Cu– 0.17Mn–0.17Cr alloy shows good deforming properties
during isothermal deforming process. The results by both simulation and real experiment show that the V-shaped component can be produced with bar billet rather than sheet billet. In the forming process, the deformation of the material is more uniform without cracking at the edges of the bar billet. The real forged experiment shows that the microstructure has been significantly refined after the isothermal deformation. The heat treated component achieves higher tensile strength. The good deformability and mechanical property of the spray formed alloy demonstrate that the material has great potential to be a engineering materials. Isothermal deformation is an effective process for producing components from spray formed super-high strength aluminium alloys. The process simulation software DEFORM TM also shows its usefulness in guiding the plastic deformation technique.
Acknowledgement This research was supported in part by the National Basic Research Program of China (Contract No. 2010CB631205) and the National Natural Science Foundation of China (Contract No. 51074145).
Reference Cai, W.D., Lavernia, E.J., 1997. Modeling of porosity during spray forming. Materials Science and Engineering A-Structural Materials Properties Microstructure and Processing 226, 8–12. Flower, H.M., 1990. Microstructural development in relation to hotworking of titanium-alloys. Materials Science and Technology 6, 1082–1092. Grant, P.S., 1995. Spray forming. Progress in Materials Science 39, 497– 545. Grant, P.S., 2007. Solidification in spray forming. Metallurgical and Materials Transactions A-Physical Metallurgy and Materials Science 38A, 1520–1529. Jeyakumar, M., Kumar, S., Gupta, G.S., 2010. Microstructure and properties of the spray-formed and extruded 7075 Al alloy. Materials and Manufacturing Processes 25, 777–785. Kobayashi, S., Oh, S.I., Altan, T.Knovel, 1989. Metal Forming and the FiniteElement Method. Oxford University Press, Oxford, UK. Li, L., Zhou, J., Duszczyk, J., 2003. A 3D FEM simulation study on the isothermal extrusion of a 7075 aluminium billet with a predetermined non-linear temperature distribution. Modelling and Simulation in Materials Science and Engineering 11, 401. Ning, Z.L., Guo, S., Cao, F.Y., Wang, G.J., Li, Z.C., Sun, J.F., 2010. Microstructural evolution during extrusion and ECAP of a spraydeposited Al–Zn–Mg–Cu–Sc–Zr alloy. Journal of Materials Science 45, 3023–3029. Oh, S.I., Wu, W.T., Tang, J.P., Vedhanayagam, A., 1991. Capabilities and applications of FEM code deform: the perspective of the developer. Journal of Materials Processing Technology 27, 25–42. Salamci, E., Cochrane, R.F., 2002. Investigation of secondary phase particles in spray deposited 7000 series aluminium alloys. Materials Science and Technology 18, 1445–1452. Salamci, E., Cochrane, R.F., 2003. Microstructure and mechanical properties of spray deposited and extruded 7000 series aluminium alloys. Materials Science and Technology 19, 1130–1136. Sharma, M.M., 2008. Microstructural and mechanical characterization of various modified 7XXX series spray formed alloys. Materials Characterization 59, 91–99. Sharma, M.M., Amateau, M.F., Eden, T.J., 2006. Aging response of Al–Zn– Mg–Cu spray formed alloys and their metal matrix composites. Materials Science and Engineering A-Structural Materials Properties Microstructure and Processing 424, 87–96. Wang, F., Xiong, B.Q., Zhang, Y.G., Liu, H.W., He, X.Q., 2009. Microstructural development of spray-deposited Al–Zn–Mg–Cu alloy during subsequent processing. Journal of Alloys and Compounds 477, 616–621.
W. Zhao et al. / Mechanics of Materials 56 (2013) 95–105 Wegmann, G., Gerling, R., Schimansky, F.P., Zhang, J.X., 2002. Spray forming and subsequent forging of gamma-titanium aluminide alloys. Materials Science and Engineering A-Structural Materials Properties Microstructure and Processing 329, 99–105.
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Yamada, Y., Yoshimura, N., Sakurai, T., 1968. Plastic stress-strain matrix and its application for the solution of elastic-plastic problems by the finite element method. International Journal of Mechanical Sciences 10, 343–354.
Contents lists available at SciVerse ScienceDirect
Mechanics of Materials journal homepage: www.elsevier.com/locate/mechmat
Isothermal deformation of spray formed Al–Zn–Mg–Cu alloy Wenjun Zhao a,b, Fuyang Cao a, Xiaolong Gu b, Zhiliang Ning a, Ying Han c, Jianfei Sun a,⇑ a b c
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Zhejiang Province Key Laboratory of Soldering & Brazing Materials and Technology, Zhejiang Metallurgical Research Institute Co., Ltd, Hangzhou 310030, China Capital Aerospace Machinery Company, Beijing 100076, China
a r t i c l e
i n f o
Article history: Received 19 May 2011 Received in revised form 1 July 2012 Available online 12 October 2012 Keywords: Isothermal deformation Billet Deform Component
a b s t r a c t The isothermal deformation of a V-shaped component was simulated by the DEFORMTM process simulation software. The material flow, internal stress and temperature of the billet’s interior were investigated during the process of deforming, and the simulation results showed that bar billet is more suitable than sheet billet for producing the V-shaped component. The real isothermal deformation process was carried out with bar billet, and the microstructure of the shaped component was greatly refined. After heat treatment, the ultimate tensile strength (UTS) of the component can reach 817 MPa. The results show that isothermal deformation is a low cost and high efficient compaction technique for produce components from the spray formed Al–Zn–Mg–Cu alloy. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Al–Zn–Mg–Cu series super-high strength aluminum alloys have been widely applied as aircraft structural materials due to their low density and high strength (Grant, 2007). Increasing the content of hardening alloying elements is an effective way to strengthen the alloy (Wegmann et al., 2002). Spray forming process has been applied to increase the content of hardening alloying elements up to 13%. For the super-high strength Al–Zn–Mg– Cu alloy, the mass ratio of Zn has a pronounced effect on the mechanical properties. But the presence of porosity is inevitable in the spray formed material. For instance, porosity has been reported for a variety of spray formed Al alloys ranging from 1 to 10% (Cai and Lavernia, 1997). Therefore, forging, extruding and isothermal hot isostatic pressing etc. were used to compact the material. There have been some previous studies on the extrusion research of the spray formed aluminium alloy (Jeyakumar et al., 2010; Ning et al., 2010; Salamci and Cochrane, 2002, ⇑ Corresponding author at: School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China. Tel.: +86 451 86418317; fax: +86 451 86413904. E-mail address: [email protected] (J. Sun). 0167-6636/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mechmat.2012.09.009
2003; Sharma, 2008; Wang et al., 2009). Sharma et al. reported that the mechanical properties of the extruded aluminium alloy of 12Zn–3.3 Mg–1.3Cu–0.3Mn–0.20Zr are tensile stress of 763.8 MPa, yield stress of 755.6 MPa and elongation 8.5% (Sharma et al., 2006), and the results show that the spray formed zinc-rich aluminium alloy is a suitable structural material with high strength. But extrusion is a technique which can only compact the alloy into some product with simple geometry, such as bar, sheet and pipe, not the complex geometry. If the additional machining is applied to produce the complex one, it will increase the cost. Isothermal deformation is a flexible process which can be used to produce some complex components with net size and shape (Flower, 1990), however, there are few studies about the isothermal deformation of the spray formed Al–Zn–Mg–Cu alloy. So the isothermal deformation of this alloy will be analyzed by numerical simulation and testing methods of deformation in this paper (Li et al., 2003; Yamada et al., 1968). Isothermal deformation is carried out within the closed metal mould and hard to be observed in the real experiment, but the deforming process can be simulated by the software DEFORMTM. The material flow, internal stress and temperature of the billet’s interior and the deforming loads of the mould can be obtained by
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Table 1 Chemical compositions of the spray formed aluminum alloy. Element
Zn
Mg
Cu
Mn
Cr
Al
Composition wt.%
12.82
3.20
1.28
0.17
0.17
Balance
DEFORMTM. This software also can predict which part of the component would be stress concentration, and the cracks easily came up in these region. The cracks could be avoided through redesigning the mould and reselecting the working parameters. And the experiment of isothermal
Fig. 1. (a) A typical 23-kg spray formed Al–Zn–Mg–Cu billet. (b) an as-deposited microstructural map showing noncolumnar/dendritic equiaxed polygonal grains.
Fig. 2. True stress–strain curves at different temperatures (a)370 °C, (b)390 °C, (c)410 °C, (d)430 °C.
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Fig. 3. Sketch of V-shaped component.
Fig. 4. Finite element model (1-upper mould, 2-billet, 3-lower mould) (a) sheet billet, (b) bar billet.
Fig. 5. Mesh and boundary condition (a) sheet billet, (b) bar billet.
deformation is used to evaluate the processing given by simulation.
2. Experimental 2.1. Spray formed billet The experimental material of the component is spray formed Al–Zn–Mg–Cu alloy produced by Harbin Institute of Technology, and its chemical composition is shown in
Table. 1 (in wt.%). In spray forming experiment, melt temperature is 760 °C and liquid metal flow rate is 50 g/s. Double-layer atomizer system is employed and the gas pressures of the primary and secondary nozzles are 1.0 and 1.5 MPa. The spray distance is 400 mm. The spray formed billet and the as-deposited microstructure are shown in Fig. 1, and the microstructure shows the grain size is 20–50 lm. At the grain boundary the concentrated second phases are eutectic precipitated from the liquid phase directly. The dispersed white phases within the grains are mainly the strengthening phase
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Fig. 6. Material flow within the sheet billet (a) initial phase, (b) developing phase, (c) intermediate phase, (d) final phase.
(MgZn2) which precipitated from the solid basal body. It’s the characteristic of the deposited material that there are some tiny pores within the microstructure (Grant, 1995), which reduces the mechanical property. Therefore, the as-deposited material is need to be compacted and the microstructure should be refined to increase the mechanical propertied, which can be achieved by the isothermal deformation. 2.2. Hot compression simulation The simulation of the isothermal deformation demands that the necessary deforming property parameter should be imported into the database of the software DEFORMTM, which are obtained by isothermal compression tests. The isothermal compression tests of the spray formed Al–Zn– Mg–Cu alloy under different temperatures or strain rates were performed on a Gleeble-1500 Thermal Simulator. The specimen size used for this test is U 8 12 mm, and true stress–strain curves given by this equipment are shown in Fig. 2, and imported into the material model of DEFORMTM. The results by isothermal compression tests show that the stress didn’t increase sharply with increasing the deformation under the strain rates of 0.01/s, 0.1/s,
1/s, especially when the strain rate is 0.01/s, the stress almost keeps constant during the compressing process. But when the strain rate is 10/s, the materials destabilize due to the fast strain rate. 2.3. Isothermal deformation experiment V-shaped component is the key connecting components, as shown in Fig. 3. The schematic of the component shows the geometry includes the thin wall, high rib, convex platform. Those V-shaped components manufactured with traditional machining production technique cost much time and money. Furthermore, cutting process would cut off the metal fiber structure within the component and decrease the mechanical properties especially the fatigue properties. The component, manufactured with ordinary stamping forming technique, would experience three working procedures, blanking, bending and forming. The problems of dimension variation, shape distortion and surface cracking would come with the components because of the poor plastic forming property. Therefore, to avoid these problems isothermal deformation is adopted to manufacture the net shape components in a single process. This technical needs less operations and raw
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Fig. 7. Material flow within the bar billet (a) initial phase, (b) developing phase, (c) intermediate phase, (d) final phase.
materials, and reduces the cost. The weights of the raw billet and the finished product are 100 and 90 g respectively, and the process yield is up to 90%. To produce this component, two kinds of raw billets, sheet and bar, are prepared. Sheet billet is 100 50 20 mm and bar billet is U 40 100 mm in size. The mould material is H13 steel. The experiment was performed on a 6300KN press. The relative speed between upper and lower moulds is 2 mm/s. Before isothermal deformation, the billets and die moulds were preheated to 430 °C for 30 min. According to hot compression experiment, the friction factor between the row material and the die moulds is 0.3. 2.4. Build the finite element model DEFORMTM (Design Environment for Forming) software system is developed by American Scientific Forming Technologies Corporation (SFTC) company based on S. Kobayashi et al research (Kobayashi et al., 1989) and ‘Analysis of Large Plastic Incremental Deformation’ (ALPID) software compiled by S.I. Oh et al. (Oh et al., 1991) and widely applied by American, Japan, German. The geometry model used to simulate the forming process consists of three parts: upper mould, billet and lower mould, as shown in Fig. 4. As already noted, the sheet billet and the bar billet have been adopted during the simulating process of the V-shaped component. The mesh type should be decided before meshing, and the tetrahedral isoperimetric element was applied. The sheet billet includes
7397 nodes and 31815 cells; the bar billet includes 6641 nodes and 28962 cells, as shown in Fig. 5. To improve the precision, the local mesh refine had been employed at the place where is close to the end of the upper mould or the place near the step of the component. 3. Modeling results and discussions The metal of the billets undergoes the axial, radial and toroidal flows within the mould during the forming process. Comparing with the radial and toroidal flow, the axial flow can be neglected because the restrain of the mould. The flow, internal stress, and temperature of the billet’s interior are most concerned in this paper. In Fig. 6 to Fig. 11, the figures a, b, c and d correspond directly to specific displacements 10, 20, 30 and 35 mm of the upper mould, and show the initial phase, developing phase, intermediate phase and the final phase of the deforming process, respectively. In different phases the flow of the material, the equivalent stress and the temperature field of the billet are studied in the following sections. 3.1. The flow of the material The material flows of the billets’ interior are shown in Figs. 6 and 7 during the deforming process, before that, the sheet was placed in the lower mould, and only two ends touched the lower mould. In the experiment, the upper mould pressed on the middle of the billet as shown
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Fig. 8. Equivalent stress within the sheet billet (a) initial phase, (b) developing phase, (c) intermediate phase, (d) final phase.
in Fig. 4a, and made the sheet billet bend as shown in Fig. 6a. The initial deformation of the sheet billet is visible in the middle where the upper mould is pressing on, meanwhile the material at both side flows towards the middle. The material flow speed in the middle is faster than that at both sides, which causes the bending deformation of the sheet billet in the initial phase. In Fig. 6b, the sheet billet has deformed much more than that in the initial phase. The material still moves faster in the middle than that near two ends of the billet, the flow directions in the developing phase are same to that in the initial phase. In both initial and developing phases, the flow speed difference makes major part of the sheet billet be under the tensile stresses, which would easily lead to cracking. In Fig. 6c the billet surface has completely touched on the inner surface of the moulds. Most part of the cavity between upper mould and lower mould has been filled with the material of the sheet billet at this moment. The material in the middle of the sheet billet still flows in the loading direction of the upper mould, but the flow direction of the material at the two ends of the billet has reversed. At the last period of the forming process, the billet is almost totally transformed into the V-shaped component in geometry, as shown in Fig. 6d. The most part flows slowly except at the two ends where the material is filling the cavity at final stage, and flowing faster.
Fig. 7 is the material flow of the bar billet, and Fig. 7a shows the material at the top of the bar billet flows from the middle to the left and right due to the pressure of the upper mould, and the deformation starts from the top. As the upper mould moves down, the material flow speed is increasing as shown in Fig. 7b and larger deformation has performed in the billet than former. In the intermediate and the final phases, the material at two ends flows faster to fill the cavity as shown in Fig. 7 c and d. During the whole deforming process of the bar billet most part of the material is under compressive stress to limit cracking effectively. As the loading continues, change of billet geometry is very small, and parts of metal overflows the cavity along the mould surface until the dimension is up to standard. In the late phase, the material flows of both the sheet and the bar billet are similar with each other. 3.2. The stress analysis Fig. 8 is the equivalent stress of the sheet billet respectively. The equivalent stress distribution isn’t uniform as shown in Fig. 8a due to the uniform deformation. The equivalent stress in the middle is higher than at the two sides because the deformation mainly takes place in the middle, as shown in Fig. 8a and b. With the development of the deformation, the plastic deformation occurs in most
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Fig. 9. Equivalent stress within the bar billet (a) initial phase, (b) developing phase, (c) intermediate phase, (d) final phase.
parts of the sheet billet, and the billet touches on the mould surface gradually. In the intermediate phase, the equivalent stress maintains in equilibrium within the billet, as shown in Fig. 8c. In the final stage the metal fills the whole cavity of the mould and plastic deformation of all parts is uniform and the equivalent stress is greater than former but still uniform, as shown in Fig. 8d. Fig. 9 shows the equivalent stress of bar billet. In the initial phase, the equivalent stress is uniform in most parts of the bar billet. As the upper mould loading continues, the equivalent stress increases a little in most part of the billet, but there is one area with lower equivalent stress at the bottom of the billet, as shown in Fig. 9b, which is due to the restrain of the base angle of the cavity and means that the plastic deformation is hard to take place here. In the developing phase, the equivalent stress reaches the maximum value but is still uniform in the billet as shown in Fig. 9c. Plastic deformation takes place all over the billet. In the last phase, the equivalent stress decrease as shown in Fig. 9d. In the whole process, the equivalent stress is uniform consistently without stress concentration which can limit cracking. During the deforming process of the bar billet, the higher equivalent stress of the developing phase contributes to the compaction of the material, and the lower equivalent stress of the last phase favours the control the final geometry of the component. 3.3. Temperature field Fig. 10 shows the temperature field of the sheet billet during the forming process. At the beginning of the
forming process, the temperature increases in the middle of the billet, the reason for that is larger deformation in this reason, and remains lower temperature at the two ends of billet, as shown in Fig. 10a. In the developing phase, the temperature distribution has nearly changed, except a little temperature rising at the step region of right side, as shown in Fig. 10b. In the intermediate phase, the highest temperature is no longer at the center position, and appears at the step region of right side, as shown in Fig. 10c. In the last phase, the temperature is lower at the center and higher at the two ends, because the deformation almost stops in the middle and the material is filling the last corner of the cavity. The combination of the billet deformation and the frictional interaction between the billet and the surfaces of the moulds makes the maximum temperature 441 °C appear at the left end near side, as shown in Fig. 7d. At the same time, the high temperature may cause the cracks near the edge. Fig. 11 shows the temperature fields of the bar billet. In the initial phase, the temperature increases at the top of the billet where the upper mould is pressing on. Within the billet, the closer from the upper mould, the higher the temperature of the material and at the bottom of the billet temperature keeps low, as shown in Fig. 11 a. The rising of the temperature is due to the accumulation of the deformation energy. The temperature distribution in the developing phase is familiar with that in the initial phase, but increases a little. The temperature is increasing with the moving down of the upper mould. In the intermediate phase, the temperature rises in most parts of the billet
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Fig. 10. Temperature field within the sheet billet (a) initial phase, (b) developing phase, (c) intermediate phase, (d) final phase.
Fig. 11. Temperature field within the bar billet (a) initial phase, (b) developing phase, (c) intermediate phase, (d) final phase.
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Fig. 12. (a) Bar billet placing in the mould. (b) components just after Isothermal deformation.
Fig. 13. (a) Microstructure of the deformed component without heat treatment (SEM). (b) microstructure of the component after solution (TEM). (c) grains after the whole heat treatment. (d) locations of the specimens.
Table 2 Mechanical properties of Al–12.82Zn–3.20 Mg–1.28Cu–0.17Mn–0.17Cr. Experiment
Solution
Aging
YS (MPa)
UTS (MPa)
Elongation (%)
I II
470 °C/1 h + 490 °C/1 h 470 °C/1 h + 485 °C/1 h
120 °C/16 h 120 °C/16 h + 160 °C/5 h
807 748
817 756
1.2 4.1
except a local area round the corner of the billet, as shown in Fig. 11c. In the last phase, the temperature increases in all parts, and the maximum value is 443 °C. The temperature of the last phase is similar to that of the intermediate phase. The highest temperature doesn’t appear at the edge of billet, so as to limit the cracking here.
To conclude, within the bar billet the material deforms uniformly, there is no stress concentration near the edge, and the highest temperature at the end of forming does not appear at the edge. Both of these effects result in the absence of any cracking near the edge. For making the V-shaped component the bar billet is superior to the sheet billet.
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4. Isothermal deformation Fig. 12 shows the photos of the bar billet before and after isothermal deformation experiment. There are no cracks in the surface of the component after the isothermal deformation, and the surface is flat and smooth without any raised or sunken place in it. There is some overflow material connected with the component which can ensure the filling of the cavity entirely. Fig. 13a shows the microstructure of the components before solution, and the specimen is cut from zone ‘A’ as shown in Fig. 13d. The porosities as deposited have been eliminated. The coarse precipitated phases on the grain boundary of as-deposited (Fig. 1b) were tore into many little pieces of white phases lining in the flow direction of the material, and some other precipitated phases locates within the grain. Most of the white phases would play the role of solid solution strengthening. While the precipitated phases on the grain boundary were tore into pieces, the grains were great refined, so that the fine-grain strengthening had been achieved. The grain size of the component is no more than 5 lm after deformation as shown in Fig. 13c, and the specimen is cut from zone ‘C’ as shown in Fig. 13d. After isothermal deformation, the components were divided into two groups (I and II) randomly. The technologies for heating processing are shown in Table 2. Before aging, the solution and quenching of the alloy are necessary. Higher solution temperature makes the secondary phases dissolve into the base phase thoroughly, that does a good strengthening effect but also results in overburning of the alloy which should be avoided. As the deformed alloy contains refractory phases i.e. Al2Cu, AlCu or Mg32(AlZn)49, double-stage (low and high temperature) solution is adopted. In the first stage (low temperature stage) the low melting phase MgZn2 dissolves into the matrix; in second stage (high temperature stage) the refractory phases dissolve and there are no overburning low melting phases. The hardening medium is warm water and the quenching transferring time must be less than 10 s. Fig. 13b shows the microstructure of II group component after solution, and the specimen is cut from region ‘B’ as shown in Fig. 13d. Most of the white precipitated phases have been dissolved in the base matrix. Only some of strengthening precipitations are dispersing in the matrix, and solution strengthening has achieved a desired result. The location of tensile test specimens is region ‘c’ as shown in Fig. 13d. The results of tensile test are shown in Table 2. The heat treatment process of two groups I and II are double-stage solution adding T6 peak aging and double-stage solution adding double-stage solution adding T7 over aging, respectively. The peak aging can make the alloy reach the highest strength, and the ultimate tensile strength (UTS) achieves 817 MPa. Over aged alloy shows a little lower strength, but higher elongation.
5. Conclusion The spray formed Al–12.82Zn–3.20 Mg–1.28Cu– 0.17Mn–0.17Cr alloy shows good deforming properties
during isothermal deforming process. The results by both simulation and real experiment show that the V-shaped component can be produced with bar billet rather than sheet billet. In the forming process, the deformation of the material is more uniform without cracking at the edges of the bar billet. The real forged experiment shows that the microstructure has been significantly refined after the isothermal deformation. The heat treated component achieves higher tensile strength. The good deformability and mechanical property of the spray formed alloy demonstrate that the material has great potential to be a engineering materials. Isothermal deformation is an effective process for producing components from spray formed super-high strength aluminium alloys. The process simulation software DEFORM TM also shows its usefulness in guiding the plastic deformation technique.
Acknowledgement This research was supported in part by the National Basic Research Program of China (Contract No. 2010CB631205) and the National Natural Science Foundation of China (Contract No. 51074145).
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