外文翻译原文.pdf

Journal of Materials Processing Technology 224 (2015) 156168 Contents lists available at ScienceDirect Journal of Materials Processing Technology jo ur nal home p ag e: Die design method for thin plates by indirect rheo-casting process and effect of die cavity friction and punch speed on microstructures and mechanical properties Chul Kyu Jina, Chang Hyun Janga, Chung Gil Kangb, aPrecision Manufacturing System Division, Graduate School, Pusan National University, San 30 Chang Jun-dong, Geum Jung-Gu, Busan 609-735, South Korea bEngineering Research Center for Net Shape and Die Manufacturing, School of Mechanical Engineering, Pusan National University, San 30 Chang Jun-dong, Geum Jung-Gu, Busan 609-735, South Korea a r t i c l e i n f o Article history: Received 12 November 2014 Received in revised form 28 April 2015 Accepted 1 May 2015 Available online 12 May 2015 Keywords: Semi-solid slurry Indirect rheo-casting Electromagnetic stirring Thin plate Filling simulation A356 alloy a b s t r a c t Thin plates with a thickness of 1.2 mm are fabricated from semi-solid A356 alloy through an indirect rheo-casting process both with and without an electromagnetic stirrer (EMS). The thin die cavity for forming is designed with the fl uid analysis software MAGMA. A semi-solid slurry with a solid fraction of 40% is prepared and then injected into the die of a 200 t hydraulic press. Forming tests are performed on the thin plates at two punch speeds (30 and 300 mm/s) and two cavity friction conditions (mf= 0.4 and mf= 0.9). The formability, mechanical properties, and microstructure are then evaluated. The semi-solid slurry obtained with an EMS contains fi ne and globular solid particles; the semi-solid slurry produced without an EMS reveals rosette particles and coarser globular solid particles. At high friction (mf= 0.9), the cavity is mainly fi lled with the liquid phase. At a higher punch rate, the thin plates show better formability and a microstructure with fi ne and even solid particles. The tensile strength and elongation of the thin plate formed with a punch speed of 300 mm/s in the cavity with graphite lubrication (mf= 0.9) are 216 MPa and 10%, respectively. These values are 57 MPa and 5.5% higher, respectively, than those of the thin plate formed at a punch speed of 30 mm/s. 2015 Elsevier B.V. All rights reserved. 1. Introduction The die casting process for aluminium involves the high-speed injection of molten metal, which leads to internal defects because of remaining gas or air in the molten metal, which in turn deteri- orates the mechanical properties. Niu et al. (2000) found that the volume of gas porosity and the pore sizes in the castings are signif- icantly reduced through the use of a vacuum during die casting. This markedly improves the density and mechanical properties, particularly the tensile strength and ductility. The forging pro- cess has limited formability of near-net shapes and reduces the after-treatment productivity and die life, which makes eco-friendly production impossible. Squeeze casting is a metal forming process where molten metal is solidifi ed under a relatively high pressure to reduce gas or shrinkage porosity. However, this process produces a rosette and dendrite structure (Yue and Chadwick, 1996) and has Corresponding author. Tel.: +82 51 510 1455; fax: +82 51 518 1456. E-mail address: cgkangpusan.ac.kr (C.G. Kang). the disadvantages of a shortened die life, limited shape complexity, diffi culty with producing thin parts, and limited maximum size and weight (Ghomashchi and Vikhrov, 2000). Flemings et al. (1976) developed a rheological (semi-solid) material and the rheocasting process as an alternative to metal forming processes such as die casting and forging. Their process produces a highly fl uid slurry of solid spheroids dispersed in liquid. Joly and Mehrabian (1976) showed that the viscosity of the slurry at a given volume fraction of solids decreases with a decreasing cooling rate and increasing shear rate. The rheo-forming method, which is performed on a material in the semi-solid state (i.e., the temperature is above the solid line but below the liquid one), is a solution to the problems of casting and forming processes. In the rheo-forming process, molten aluminium is stirred as the tem- perature is decreased in order to create semi-solid slurry with a controlled grain size, which is then injected into a die and formed with a press. Kapranos et al. (2000) described the process of pro- ducing and assessing a high-quality thixoformed component using an aluminium alloy and showed that thixoforming clearly has near net-shape capability. Ji et al. (2001) developed a twin-screw /10.1016/j.jmatprotec.2015.05.002 0924-0136/ 2015 Elsevier B.V. All rights reserved. C.K. Jin et al. / Journal of Materials Processing Technology 224 (2015) 156168 157 rheo-moulding process, and Fan et al. (2005) presented a rheo- diecasting (RDC) process which directly uses liquid Al alloys. Their results indicated that the RDC samples had close to zero porosity and a fi ne and uniform microstructure throughout the entire sample under the as-cast condition. Atkinson (2005) summ- arised routes to spheroidal microstructures, types of semi-solid processing, the advantages and disadvantages of these routes, the background rheology, mathematical theories of thixotropy, the transient behaviour of semi-solid alloy slurries, and computational modelling. The one major drawback of rheo-forming or thixoforming for processing semi-solid metals is controlling the liquid or solid seg- regation (i.e., separation of the solid and liquid phases or uneven distribution of the solid phase). When a semi-solid slurry fi lls the die, the material comes in contact with the cavitys wall. This causes uneven fl ows leading to segregation of the solid phase (primary ?-Al particles) from the liquid phase. Chen and Tsao (1997) proposed semi-solid deformation mechanisms and pre- dicted the segregation phenomenon based on deformation of a phenomenological model. Kang et al. (2007) investigated the effect of changing the injection velocity on the globular microstructure and mechanical properties of a product from semi-solid die cast- ing. They found that the difference in the solid fraction between samples with and without liquid segregation was approximately 1520%. For thin plates, segregation of the solid and liquid phases can be more severe, which makes their mechanical properties uneven at different locations. Because of the problems associated with segregation and the low initial forming temperatures for semi-solid metal processing, no research has been carried out so far on the fabrication of thin plates with the rheo-forming pro- cess. In this study, an indirect rheo-casting process was applied to compensate for the disadvantages of indirect squeeze casting and produce aluminium thin plates for electric and automobile parts and fuel cell bipolar plates. The indirect rheo-casting process for thin plates involves a low pouring temperature in a thin die cav- ity. This makes it highly likely that the material will fail to fi ll the cavity entirely and solidify starting from the centre, which will result in incomplete forming. Therefore, the aim of this study was to fi nd ways to design a die for indirect rheo-casting which are appropriate for the thin plates shape. A gate shape and overfl ow adequate for rheological behaviour were designed with the soft- ware MAGMA to allow the semi-solid slurry to fi ll the cavity. The A356 alloy with a wide solid-liquid coexistent region was used as the semi-solid slurry. A semi-solid slurry with fi ne and globular solid particles was fabricated through the use of an electromag- netic stirrer (EMS) to control the grain size of the A356 alloy. The semi-solid slurry was injected into a die installed in a 200- t hydraulic press to form a thin plate. Experiments for forming thin plates were performed at two punch speeds and two cavity friction conditions, and the effect of the punch speed and fric- tion on the formability, microstructure, and mechanical properties was analysed. The microstructure and mechanical properties of the formed thin plate samples were measured at different loca- tions. 2. Experimental procedure 2.1. Semi-solid fl ow model The fl uid model of semi-solid materials shows that the viscos- ity depends on the shear rate. Semi-solid materials lose viscosity drastically as the shear rate increases but have nearly constant vis- cosity at very low shear rates. In order to express the dependence of viscosity on the shear rate in a high shear rate region, an empirical formula called a power law is used (Kim and Kang, 2000; Atkinson, 2005): ? = k ?n1(1) where ? is the shear stress, ? is the shear rate, k is the power law fac- tor, and n is the power law index. When n = 1, the material becomes a Newtonian fl uid whose viscosity ? is the same as k. Kim and Kang (2000) set n = 1 for the Newtonian fl uid model. For the Ostwaldde Waele fl uid model, which is for the semi-solid state, the experimen- tally obtained n value was 0.48 to 0.45 (shear rate = 32500 s1). This was applied in MAGMASOFT for comparison of the fi lling anal- ysis results within the die. Their results demonstrated that the Ostwaldde Waele fl uid model is consistent with the experimen- tal results. The viscous behaviour model of MAGMASOFT uses the Ostwaldde Waele model, which expresses the non-Newtonian aspect of semi-solid materials through a power law: n = ?m ?n1(2) where ? is the apparent dynamic viscosity, m is the Ostwaldde Waele coeffi cient, n is the Ostwaldde Waele exponent, and ? is the density. The governing equations of MAGMASOFT are the control volume fi nite difference method, continuity equation, NavierStokes equa- tion, energy equation, and volume of fl uid (VOF) method. These are the same used for liquids. 2.2. Simulation preparation for die design When a semi-solid slurry is compressed, the liquid phase moves towards the surface of the slurry, which leads to surface cracks. The void content and segregation of the solid and liquid phases become more severe at the side of the compressed specimen. Seo et al. (2002) conducted compression experiments to investigate the deformation behaviour of a semi-solid material with vary- ing processing parameters, such as the test specimen size and strain-rate. They suggested that the rheo-forging die for thin plates must be designed as an indirect type of structure. To form thin plates using a semi-solid slurry, a die for indirect rheo-casting was designed where fi lling is carried out by compressing the semi-solid slurry in closed upper and bottom dies with a punch. Because this structure is similar to the die casting process, the design of the gate system and overfl ow is a critical variable. Analysis of the behaviour of the semi-solid slurry as it goes through the gate and fi lls the cav- ity is a major factor for the design of thin plates. Seo et al. (2007) analysed how the gate shape affects the liquid segregation of a semi-solid slurry and concluded that a wider gate makes fi lling more likely to be done in order and liquid segregation less likely to form. In order to examine the fi lling behaviour of a semi- solid slurry according to the gate shape, a thin plate cavity (150 mm 150 mm 1.2 mm) was simulated with different gate shapes. The simulations were performed by using the A356 thixo- module (Ostwaldde Waele model) of MAGMA. Table 1 lists the Table 1 Simulation parameters. Parameters Values Molten metalMaterial A356 Liquidus temperature (TL) 617C Solidus temperature (TS) 547C Initial temperature (TM) 596C Latent heat (Q) 430 kJ/kg DieMaterial SKD 61 Initial temperature (TD) 300C Heat transfer coeffi cientMaterial and die 7000 W/m2K Die and die 1000 W/m2K 158 C.K. Jin et al. / Journal of Materials Processing Technology 224 (2015) 156168 Table 2 Chemical composition of A356 alloy (wt%). Si Mg Ti Fe Ni Mn Zn Pb Al 7.08 0.35 0.17 0.08 0.07 0.01 0.01 0.01 Bal. conditions and heat transfer coeffi cient values used in the simula- tion. 2.3. Fabrication of semi-solid slurry A356 alloy was used for the semi-solid slurry, and an EMS was used to control the size of the solid particles. Because A356 alloy has great fl uidity at the two-phase mushy zone and can enhance the mechanical strength with heat treatment, it is used in automobile parts such as knuckles, arms, and housing that require reliability. In particular, the mechanical properties of A356 are closely related to the size of the primary particles, secondary dendrite arm spacing (SDAS), and Si particle shape and distribution within the eutectic matrix. Table 2 lists the chemical compositions of A356 alloy. Fig. 1 shows the solid fraction versus temperature of the A356 alloy. The solid phase fraction of A356 alloy at different temperatures was determined by using the obtained differential scanning calorime- try (DSC) curve. The liquidus and solidus temperatures for A356 alloy were 617 and 547C, respectively, and the solid fraction for a temperature of 596C was 40%. Fig. 2(a) shows a photo of the EMS used in this study. The EMS consisted of three phases (P, R, S) and three poles with a coil placed Fig. 1. Solid fraction versus temperature of A356 alloy. Fig. 2. Electromagnetic stirrer: (a) real picture and (b) schematic diagram. Fig. 3. Variations in magnetic induction density at three positions as function of stirring current. vertical to the core. The core for fi xing the coil position was fabri- cated by piling up several 0.35 mm thick plates. The core consisted of 240 unit laminations of SiZn alloy plates, and the coil was wound around the core. Each phase was placed in a cylindrical direction, as shown in Fig. 2(b), to let the current move alongside the coil, and an electromagnetic force was generated in the cylindrical direc- tion to stir the molten metal. The electromagnetic force of the EMS was measured with a gaussmeter at three positions. Fig. 3 shows the variations in the magnetic induction density measured at three positions inside the EMS as a function of the current. The measured magnetic induction density was proportional to the increase in the current at each position. At a current of 60 A, the magnetic induc- tion densities at the upper, middle, and lower positions were 640, 680 and 1120 G, respectively. The stirring force induced shear stress in the molten aluminium, which controlled the growth of dendrite arms which form during solidifi cation. Thus, it controlled the grain size of the solid phase and made the grains globular. Fig. 4 shows the process to make a semi-solid slurry. First, the cup is inserted into the EMS, and a ladle is used to scoop the molten metal from the furnace. Then, stirring starts as the electric current is applied while the molten metal is poured into the EMS cup. (The molten metal is at a temperature of 680C in the furnace, 635C in the ladle, and 620C in the cup.) Stirring is performed until the temperature of the molten metal in the cup cools to 596C, i.e., the solid fraction (fs) is 40%. It takes about 78 s of stirring to reach this level. The variables for the stirring experiment were the molten metal temperature at the start of stirring (TS), the stirring current (A), and the stirring time (t). Bae et al. (2007) suggested that fi ne and globular solid particles can form when the molten metal tem- perature at the start of stirring is below 655C, the stirring current is 60 A, and the stirring time is 60 s based on an electromagnetic stirring experiment using A356. Therefore, in the present experi- ment, the molten metal temperature was 620C, and the stirring current was 60 A, as given in Table 3. Stainless steel 304 was used for the EMS cup. Stainless steel is nonmagnetic because it is austenitic, so it is not affected by elec- tromagnetic forces. In addition, stainless steel 304 does not deform even at temperatures over 700C. Seo et al. (2002) performed a compression experiment on a semi-solid slurry and examined how changes in the height and diameter of the billet affect the liquid seg- regation. They concluded that a larger billet diameter makes it less likely for pores and liquid segregation to occur. In addition, a greater billet length increases the void content gets. Thus, after the amount of material required for thin plate forming was considered, the EMS cup was designed to have a similar diameter to the inner diameter of the die sleeve (60 mm) and a much lower height. Fig. 5(a) and (b) illustrates the shape dimensions of the EMS cup and actual cup, C.K. Jin et al. / Journal of Materials Processing Technology 224 (2015) 156168 159 Fig. 4. EMS process for fabricating semi-solid slurry with fi ne and globular solid particles. Table 3 Experimental conditions for semi-solid slurry fabrication. Parameters Values Stirring method Electromagnetic stirring Stirring current (A) 60 A Temperature at the start of stirring (TS) 620C Temperature at the fi nish of stirring (TF)/solid fraction (fs) 596C/40% Stirring time (t) 60 s Fig. 5. Geometries of stirring cup and slurry: (a) section of cup, (b) photo of cup, and (c) photo of semi-solid slurry. respectively. Fig. 5(c) shows the semi-solid slurry. The cup is 2 mm thick and 100 mm long. Because the molten metal rotates during stirring because of the stirring force, the molten metal will fl ow over the cup if the cup is fi lled to the inlet. Therefore, the cup was fi lled to a height of 90 mm with the molten metal. The volume of the fabricated semi-solid slurry was 165,597 mm3, and the volume of the thin plate model calculated using the space fi nder function of the software UG NX6 was 161,304 mm3. The amount of semi-solid slurry was adequate to fi ll the thin cavity. 2.4. Indirect rheo-casting process The semi-solid slurr