EXTRUSION MODELING OF A356 ALLOY AND COMPOSITE E.V. Konopleva and H.J. McQueen Mech. Eng., Concordia University, Montreal, Canada H3G 1M8 EXTRUSION MODELING OF A356 ALLOY AND COMPOSITE. E.V.Konopleva and H.J.McQueen Mech. Eng., Concordia University Montreal, Quebec, Canada H3G 1M8 t.: (514) 848-3145; Fax: (514) 848-3175 ABSTRACT The extrusion modeling of A356 alloy and A356/15%SiCp composite was carried out using the finite element analysis software DEFORM. The constitutive laws determined by the torsion tests have been used in the model to calculate the flow stresses. Models were developed for the initial billet temperatures 350-500°C, extrusion ratio R=31 and ram speeds VR = 2.6 and 5 mm/s. For each condition the distribution of velocity, strain rate, strain, mean stress and maximum temperature were determined. The load and the increase in maximum temperature decreased as billet temperature was raised and ram speed reduced. The maxima in temperature, effective and mean stresses, strain and strain rate were found at the die exit. In comparison with the alloy, A356/15%SiCp composite exhibited higher load and stress at 350-400°C with almost no difference in maximum temperatures, strain and strain rate. For extrusion at 350-450°C, the exit temperatures were below the incipient melting point. Comparisons are made to the extrusion of 6061 alloy and 6061/15%SiCp. Keywords: Al356/15%SiCp, metal matrix composite, extrusion, finite element modeling, microstructures. INTRODUCTION Aluminum alloys reinforced with particles of Al2O3 or SiC possess higher strength and stiffness as well as greater wear resistance and improved high temperature properties. [1-4].Metal-matrix composites (MMC) produced by liquid metal mixing are secondarily fabricated by traditional mechanical forming (extrusion, forging or rolling) [5-8]. Because of reduced ductility and fracture toughness, the workability of MMC is expected to be somewhat lower than that of the matrix alloy; hot shaping is expected to reduce cracking and plastic degradation. A356 is a hypoeutectic Al - Si casting alloy (7.0% Si, 0.35% Mg, 0.2% Cu, 0.2% Fe and 0.1% Mn [9]) containing 6.5 vol% Si eutectic particles (1-12 m) which cause considerable strengthening but also reduce the ductility. There is some additional strengthening from fine Si and Mg2Si precipitates in the solid that dissolve at high temperatures. The composite with the matrix of A356 combined with 15 vol% SiC particles has much finer (0.5 - 3 m) eutectic Si phase as a result of strontium modification of the melt [9,10]. Hot working behavior of A356 alloy and A356/15%SiCp composite has been studied previously by torsion testing [9-14]. Materials were deformed over the temperature range 300 to 540°C and strain rates 0.1 to 5 s-1. The composite was found to be generally stronger by 25-50% than the matrix alloy but the difference reduced at higher temperatures (>400°C). Although the ductility of A356 is not high and the composite exhibited even lower fracture strain, about 25% below the alloy, the ductility of the MMC increased intensively between 400 and 500°C and became higher than that of many wrought alloy composites [9,15-23].The limited ductility of A356 resulted from linking up of cracks nucleated at large Si particles whereas in the composite with much refined eutectic phase, the decohesion voids initiated at the SiC particles [14].Predominant softening mechanism for both materials is dynamic recovery. However, in the MMC, dynamically recrystalized nuclei were observed in the vicinity of SiC particles but they did not grow over substantial regions of the matrix [10-14]. In the present work the extrudability of A356 and A356/15%SiCp composite was compared by finite element analysis. Modeling was performed to determine the temperature, strain rate, velocity and stress distribution in the billet during this forming process. Varying the extrusion parameters provided information on how they affect the process and the extreme values of load and of temperature, strain and stress in the distributions. EXTRUSION MODELING The extrusion of A356 and A356/15%SiCp was modeled using the finite element software DEFORM. This program uses a flow formulation approach and an updated Lagrangian procedure; it possesses an automatic remeshing scheme to allow the modeling of large or localized deformations [24]. Owing to axisymmetry a radial section of the extrusion press was analyzed using a two-dimensional model. The four objects (billet, ram, container and die) were meshed with the spacing being refined in regions with high localized deformations and maximum temperature. Heat transfer between the billet and tools was analyzed but interface heat transfer between the different tools was ignored. Thermal conductivity, heat capacity and emissivity for the alloy and composite were correspondingly 160 W m-1 K-1, 2.59 x 103 kJ K-1 m-2 and 0.19. Thermal properties of billet and tools (steel H13) were assumed not to be temperature dependent within the range studied. Since non-lubricated forward extrusion was modeled, sticking friction was assumed between the billet and the surrounding tools. Extrusion was modeled for a billet with diameter 178 mm and height 305 mm, an extrusion ratio R = 31 and ram speed VR = 2.6 or 5 mm/s in similarity to previous modeling [25-30]. All tools were considered as completely rigid materials while the billet was assumed to be rigid-plastic with a flow-stress dependent on both temperature and strain rate . The constitutive laws determined by the torsion tests have been used in the model to calculate flow stresses [9-14]. Models were developed for the initial billet temperatures TB from 350 to 500°C. As the billet temperature was increased, the initial tooling temperatures were adjusted ( about 30°C less for die and chamber, always 175°C for the ram block). The most important parameters of the extrusion modeled for A356 alloy are summarized in Table I. Extrusion simulation starts with movement of the ram to upset the billet. The metal flows towards the die-chamber corner and when the chamber is filled completely it starts to flow through the die aperture. At this step (the end of the upsetting stage) pressure reaches a maximum value (Fig. 1a). As extrusion proceeds and the billet shortens, pressure decreases as a result of deformation heating and reduction of friction. Finally, a steady state is achieved as seen in the load-stroke curves. Fig.1a also shows that load decreases as extrusion temperature rises. TABLE I. Results of modeling extrusion of A356 alloy and composite. Alloy VR R TB (°C) Tmax (°C) Lmax m max max MPa A356 “ “ “ “ “ “ 2.6 “ “ “ 5 “ “ 31 “ “ “ 31 “ “ 350 400 450 500 350 400 450 446 481 519 558 462 499 536 15.0 11.3 9.5 7.8 14.8 11.7 9.8 MPa 73.8 66.1 61.3 54.7 79.2 71.5 63.9 A35615% SiCp “ “ “ “ 2.6 “ “ “ 5 “ “ 31 “ “ “ 31 “ “ 350 400 450 500 350 400 450 449 483 517 550 471 500 530 15.6 13.8 9.7 6.8 16.1 13.5 10.1 81.2 68.3 56.6 44.8 86.5 76.1 62.8 max s-1 104.2 90.2 68.4 107.7 94.4 83.1 Effect strain max 3.1 3.1 3.0 3.1 3.2 3.1 3.0 16.5 17.2 16.1 18.5 36.2 30.7 31.2 Vel. max mm/s 91.2 92.9 94.5 95.3 187.0 169.7 184.2 115.5 94.4 62.0 118.1 98.1 79.7 3.0 3.1 3.1 3.1 3.2 3.2 3.1 16.0 17.7 18.0 17.9 34.2 31.5 35.5 84.3 90.9 94.2 87.4 164.3 187.4 181.6 For VR = 2.6 mm/s, Lmax = 15.0 MN at 350°C while at 500°C, Lmax is half as high (7.8 MN) (Table I). Extrusion at higher ram speed 5 mm/s requires a higher pressures equivalent to a decrease of about 10°C in TB. Distribution of the parameters of the process such as temperature T, effective stress and mean stress m, strain , strain rate and velocity V (Fig.2) indicates that the die corner is the area with the largest localized deformation. Maximum values of these parameters in the billet are observed near the die aperture. The most critical stage of extrusion is the end of upsetting; almost all parameters (, m, , and V) achieve their maximum value at this moment near the die corner; this is similar to other materials modeled by computation [25-30] or by physical simulation [31-33]. As the process develops, they decrease to some extent. Only two parameters T and continue to increase and reach their maximum value at the beginning of the steady stage. As extrusion temperature TB increases from 350 to 500°C, maximum temperature of the billet Tmax rises (see Table I) from 446 to 558°C for VR = 2.6 mm/s (distribution of T at 400°C can be seen in Fig.2a) but the increase in temperature T (Fig.3a) reduces linearly from 96 to 58°C. The higher pressure required for ram speed VR = 5 mm/s (more work per unit time) results in greater increase in T (T is higher by 16-18°C). It should be noted that extrusion at 500°C might cause melting of the eutectic component because Tmax exceeds incipient melting point of A356 alloy which is equal to 555°C. Both effective stress and mean stress reduce with rising TB , following a decreased load and increased T in the deformation zone (Table I, Figs.2c, 3c). These parameters possess the highest value at the end of the upsetting stage, than decrease notably (see and m at Tmax in Table I). An increase in ram speed raises and m by about 2 to 5 MPa (~8% rise). Distribution of m (Fig.2c) shows that although the deformation zone is mainly in compression, tensile stresses are observed in the surface of the extrudate near the die exit. This may cause cracking at the surface. Maximum effective strain depends neither on the temperature conditions of extrusion nor the ram speed (see Table I). Strain rate is only slightly affected by TB but it doubles upon raising ram speed to 5 mm/s. Extrusion of composite A356/15%SiCp requires higher pressure at all stages of the process at lower temperatures 350-400°C (Fig.1b) and results in higher and m than that of the alloy (Table I, Figs.3b,3c). At 450°C, there is almost no difference in the mechanical behavior of the alloy and composite while at 500°C, the composite appears to be much softer than the bulk material. This corresponds to flow stresses in the torsion tests [10-14]. An increase in Tmax is slightly higher for the composite at 350-400°C and lower at temperatures 450-500°C (Fig.3a). The effect of ram speed on the extrusion of composite is the same as for A356. The values and distribution of effective strain and strain rate and the way in which they change as extrusion proceeds are similar in both materials. The billet was highly distorted near the die exit while at the die-container corner there was a dead metal zone. Load and stresses were the highest at the beginning of extrusion when the metal started to emerge through the die aperture. DISCUSSION Modeling results show that there are less differences in the mechanical behavior of A356 and A356/15%SiCp composite compared to what one notices in the deformation during torsion testing. In torsion, the peak stress for the composite is very high at low temperature but decreases rapidly so that the difference is much less above 400°C [10-14]. Strain to fracture of the composite is also very lower but attains quite high values above 400°C (Fig.4a). The stress calculated from the constitutive equation (Fig.4b) show the composite to be much softer above 500°C, although this is somewhat exaggerated compared to the data. In the extrusion modeled for 350-400°C, maximum loads and T were observed higher for the composite while above 450°C both materials exhibit very similar results. For T B between 400 and 450°C both composite and alloy can be expected to have similar good extrudability since they have similar strength and good ductility. However extrusion with TB near 500°C may cause melting of eutectic, consequently, the most reasonable temperature for extrusion of composite seems to be about 450°C. Extrusion of A356 alloy and composite can be compared with other Al materials for which modeling has been performed previously [25-30]. The 7075 alloy and composites have relatively poor extrudability because of high alloying content (5.83%Zn, 1.46% Cu, 2.86%Mg, 0.23% Cr) and low ductility above 450°C because of large grain boundary precipitates forming [9,19,20,29,30]. The mechanical behaviors of 7075 materials during extrusion were similar to the A356 alloy and composite. Maximum load and subsequent increase in temperature of billet T at 400-450°C and VR = 2.6 mm/s were almost the same in both Al alloys (Table II). The billet distribution patterns of temperature, strain, strain rate and mean stress were much alike [29,30]. The 7075/10%Al2O3 and 7075/15%Al2O3 composites possessed considerably higher flow stresses at 350-400°C in torsion tests but the differences reduced above 450°C [19,20]. Under extrusion conditions, the increase in maximum pressure and T were larger only at 350°C; while at 400-450°C, extrusion of composite required the same load as for the matrix alloy. While for A356 alloy, the recommended extrusion TB is about 450°C, for the 7075, 400°C is more favorable for extrusion of its composites because ductility of these materials drops rapidly as T rises above 450°C. The modeled maximum pressures for 7075 alloy agreed with measured loads in experimental extrusions reported by Sheppard and Tunnicliffe [34]. Both A356 and 7075 alloys differ considerably from the 6061 (1.0%Mg, 0.7%Fe, 0.6%Si, 0.3%Cu). The latter is much softer and has very good extrudability [32,33]. It requires 20% lower pressure for extrusion and an increase in temperature is ~20°C less than for stronger A356 or 7075 [25,27,28] (Table II). Nevertheless, the composites of 6061 with 10 and 20% Al2O3 show considerable increased load and temperature compared to the alloy within the TB range of 300 to 500°C. This is consistent with the torsion flow stresses [9,16-21] but differs from behavior of the composites of both A356 and 7075 which require higher pressures than the matrix alloys only at the low temperatures (<400°C). TABLE II. Comparison of modeling extrusion of A356, 7075 and 6061 alloys and composites (R = 31 and VR = 2.6 mm/s) Material A356 alloy A356/15%SiCp 7075 alloy 7075/10%Al2O3 7075/15%Al2O3 7075 alloy 7075/10%Al2O3 7075/15%Al2O3 6061 alloy 6061/10%Al2O3 6061/20%Al2O3 TB (°C) 450 450 400 400 400 450 450 450 450 450 450 L max (MN) 9.5 9.7 12.8 12.9 13.9 9.4 9.0 9.2 7.0 7.6 10.0 T max (°C) 519 517 482 482 484 514 512 510 495 500 509 The constitutive strengths of the alloy and composite decrease less rapidly as temperature rises; however, the fractional decline actually rises: at 400°C it is about -2.8 %/10° for the alloy and -4.0 %/10° for the composite and at 500°C these have risen to -3.2 %/10° and -5.5 %/10° respectively. The increased fraction follows from the Arrhenius dependence on T and is perhaps exaggerated in the composite because of the higher activation energy derived from the larger drop in composite strength at low T. The ductility markedly rises in the range 350 to 500°C reflecting the reduction in crack-inducing stress concentration associated with the above decline in strength. Both the flow stress and ductility changes arise from the enhanced softening mechanisms due to thermal activation and the coalescence of Si or Mg2Si particles. The distribution of Si particles in the alloy and SiC particles in the composite are shown in Fig.5a,b. In the alloy the softening is reflected entirely in the improved dynamic recovery as observed in the increase in spacing of the subgrain walls and of the dislocations in both the walls and the subgrains (Fig.5c) [10,14]. This behavior is similar to most other Al alloys; the presence of the Si particles reduces all these parameters to a small degree raising the strength above that of Al (QHW 156 kJ/mol). However, the composite softening depends on more complicated changes in substructure which is far more heterogeneous than the alloy as a result of the turbulent and intense plastic flow surrounding the rigid particles. About a third of the matrix volume exhibits subgrains but these are considerably less recovered than in the bulk alloy for the same conditions (Fig.5d) [6,10,14]as observed in other composites [16-19,21]. The remaining two thirds exhibit high dislocation densities, about half having little apparent cellularity compared to the subgrains (Fig.5e). However in the other half, there are scattered fine cells, highly misoriented, which could be classed as dynamic recrystallization (DRX) nuclei and are well high-lighted by dark field TEM (Fig.5f). Such cells clearly provide softening compared to the dense substructures; however they have not been observed to grow to produce classical discontinuous DRX which would give a marked downturn in the flow curves. Such fine grains could give rise to grain boundary sliding and localized superplastic deformation. The low constant flow stress over large strains in 6061 composite was mechanically analyzed to give a strain rate sensitivity of about 0.3, supporting this idea [22]. The relatively high composite ductility near 500°C and 1 s-1 indicates a good potential for avoiding surface cracking at the die corner and hence satisfactory extrudability in combination with the modest rises in peak load. Furthermore, the composites are likely to retain the heterogeneous substructure because of pinning by both Si and SiC particles so that the extrusions would have high strength in comparison to those of the alloy. CONCLUSIONS From the constitutive constants determined by torsion for the A356 alloy and composite with 15% SiCp, the distribution in the billet and extrusion of distortion, temperature, stress, strain and strain rate were modeled for the initial temperatures 350-500°C, ram speeds 2.6, 5 mm/s and extrusion ratio 31. At the end of upsetting, almost all parameters (, m , and V) achieve their maxima near the die corner. Maximum temperature of the billet rises considerably as initial temperature is raised but this increase reduces linearly with TB. For extrusion at 350-450°C, the exit temperatures were below the incipient melting point. The load stroke curves have a sharp peak followed by decline to a steady state. The pressure decreases as TB is raised and ram speed reduced. 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