Solidification behavior of the remnant liquid in the sheared semisolid slurry of Sn 15 wt.%pb alloy

Transcription

1 Scripta Materialia 46 (2002) Solidification behavior of the remnant liquid in the sheared semisolid slurry of Sn 15 wt.%pb alloy S. Ji, A. Das, Z. Fan * Department of Mechanical Engineering, Wolfson Center of Materials Processing, Brunel University, Uxbridge, Middlesex UB8 3PH, UK Received 27 September 2001; received in revised form 25 October 2001; accepted 30 October 2001 Abstract Solidification characteristics of the remnant liquid in a sheared semisolid slurry of Sn 15 wt.%pb alloy is reported. A high shear rate and shear duration combination may promote fine spherical morphology of the secondary solidification product. Resting prior to solidification following shearing appears to wear off the effects of shearing. Ó 2002 Published by Elsevier Science Ltd. on behalf of Acta Materialia Inc. Keywords: Semisolid processing; Alloys; Microstructure; Nucleation; Growth 1. Introduction Following the pioneering discovery by Prof. Flemings and his co-workers at MIT [1], semisolid metal (SSM) processing has received considerable attention as a near-net shape forming technology during the last 30 years. One important discovery related to SSM processing has been the observation of unique rheological properties of thixotropy and pseudoplasticity in alloy slurries with a solid fraction less than 0.6 sheared between their liquidus and solidus temperatures [2 4]. All of the present SSM processing techniques, predominantly thixo- and rheo-casting/moulding routes, * Corresponding author. Tel.: ; fax: address: zhongyun.fan@brunel.ac.uk (Z. Fan). rely upon these properties. It has now been conclusively established that such unique rheological properties originate from the non-dendritic microstructure of the semisolid slurries during SSM processing. Consequently, microstructural evolution in the semisolid slurry under shear has been extensively investigated in the last three decades. It has been observed that shearing results in a rosette type or partially/completely spherical particle morphologies with or without liquid entrapment [2 4]. Although the theoretical understanding of solidification behavior under forced convection is still very rudimentary, different theories have been proposed to explain experimentally observed microstructure in SSM processed alloys. Doherty and co-workers proposed dendrite fragmentation and subsequent early impingement of diffusion fields due to imposed shear to explain the non-dendritic structure in SSM processed alloys based on indirect observations in Al-alloys /02/$ - see front matter Ó 2002 Published by Elsevier Science Ltd. on behalf of Acta Materialia Inc. PII: S (01)

2 206 S. Ji et al. / Scripta Materialia 46 (2002) [5]. Having analyzed the strength of alloys close to their melting point, Hellawell [6] suggested that remelting of dendritic arms at their roots, rather than breaking off by a mechanical force, might achieve grain multiplication. Molenaar et al. [7,8], on the other hand, investigated the structural evolution of Al Cu alloys under simple shear flow and attributed the formation of rosettes to a cellular growth under forced convection. Smith [9] found that the refinement of primary particles in the sheared slurry is due to the increased nucleation rate in stirred Al 19%Si alloy. Mullis [10] recently showed by numerical simulation that dendritic arms bend into the flow due to thermal/ solutal convection, resulting in rosette morphology. The present authors have explained morphological evolution based on the effect of the nature of fluid flow on the diffusion boundary geometry around the growing solid both from experimental observation and Monte-Carlo simulations [11]. All of these proposed theories have definitely helped understanding the solidification behavior in the semisolid slurry and its fascinating rheological properties. However, it should be noted that apart from the processing benefits obtained from the non-dendritic microstructure in sheared semisolid slurries, a non-dendritic microstructure is also envisaged in SSM-processed finished products in order to enhance mechanical properties and restrict segregation. Apart from the initial solidification behavior under shear, the final microstructure is equally influenced by the solidification pattern of the remaining liquid from the SSM processing temperature (which would be termed as secondary solidification throughout the paper). This is especially important for SSM processing close to the liquidus temperature as the solid fraction is low at the shearing temperature and most of the liquid actually solidifies during the secondary solidification process. Unfortunately, in spite of the importance of secondary solidification in determining the final microstructure and properties, no attention has been paid to it so far. The present work attempts to report the solidification characteristics of the remnant liquid following SSM processing. The effects of shear rate and shearing time along with the resting period prior to cooling on the solidified microstructures are investigated. 2. Experimental procedure A modified twin-screw rheomoulding device was used for the experiments [12]. The core of the twin-screw extruder is a pair of closely intermeshing, self-wiping and co-rotating specially deigned screws offering high shear rate and high intensity turbulence unattainable in other standard SSM processing techniques. A series of heating and cooling zones dispersed along the barrel ensure an accurate temperature control within 1 C. The processing temperature is measured directly from the melt between the barrel and the screws. The shear rate between the screw flight tip and the inner surface of the barrel, _c, is calculated as _c ¼ pnððd=dþ 2Þ, where N and D are the rotational speed and the outer diameter of the screw, respectively, and d is the gap between the screw flight and the inner surface of the barrel. In the experiments, predetermined amount of liquid Sn 15 wt.%pb alloy made from industrially pure tin and lead (>99.8% purity) is poured into the steadily rotating twin-screw extruder. The liquid alloy is then rapidly cooled to and isothermally sheared at 204 C (between the liquidus and solidus) converting it into semisolid slurry. Following shearing for predetermined intervals of time, the slurry was discharged into a aluminum mould preheated to 100 C for secondary solidification of the remnant liquid. In the resting experiments, the semisolid slurry was held isothermally at 206 C for different time intervals in a heated crucible following shearing at the same temperature and, subsequently, injected into a sand mould for solidification. Representative specimen were cut from the solidified alloys, ground, polished and etched using standard metallographic technique and subjected to microstructural examination using a LEICA optical microscope. The particle size, shape and the volume fraction of the primary solid phase were measured with an automated image analyzer.

3 S. Ji et al. / Scripta Materialia 46 (2002) Fig. 1. Optical micrographs of SSM processed Sn 15 wt.%pb alloy under different shearing conditions. Refer to Table 1 for a description of parameters pertaining to individual figures. 3. Results 3.1. Effect of rate, duration and temperature of shearing Fig. 1 shows representative microstructures of Sn 15 wt.%pb alloy sheared at different shear rate, shearing time and shearing temperature combinations. Table 1 summarizes the parametric values pertaining to the experimental conditions. Before commenting on the secondary solidification structure it is worthwhile to mention that Table 1 Processing parameters for the Sn 15 wt.%pb alloy in the present study Fig. 1 (a) (b) (c) (d) Pouring temperature ( C) Shearing temperature ( C) Shear rate (s 1 ) Shearing time (s) Vol. fr. solid at the shearing temperature previous investigations with semisolid slurry structure under shear have established the dependence of microstructure of primary solidification products on the shear rates. It is now clear that the solidification structure is rosette type or granular with liquid entrapment under low to moderate intensity shear flow and spherical with nearly monosize distribution of particles under high intensity turbulence produced in a twin-screw rheomoulding machine. Only the secondary solidification structure formed during cooling to room temperature from the shearing temperature will be discussed here. At a very high shear rate (5200 s 1 ) both the primary (coarse particles) and secondary (fine particles) solidification products are spherical in nature as shown in Fig. 1(a). At a lower shear rate (80 s 1 ) the secondary solidification products became coarser compared to that at high shear rate (Fig. 1(a)) and the morphology of the particles became rosette type (Fig. 1(b)). Primary solidification products in Fig. 1(b) are still imperfect spheres. On reducing the duration of shearing from Fig. 1(b) (180 s) to Fig. 1(c) (20 s) the rosette type morphology of secondary solidification product

4 208 S. Ji et al. / Scripta Materialia 46 (2002) transformed to a dendritic morphology (Fig. 1(c)). The primary particles in Fig. 1(c) resemble coarse rosettes. If the shearing is performed near the solidus temperature of the alloy (185 C) most of the solid forms during the isothermal shearing with hardly any new particles forming during the secondary solidification process except for the eutectic solidification of the matrix (Fig. 1(d)). Primary solidification products in Fig. 1(d) have globular morphologies. It is therefore evident from Fig. 1 that the shear rate and duration not only have profound influence on the solidification morphology during the processing but also have significant influence on the solidification morphology of the remnant liquid during cooling to room temperature. It is, indeed, interesting to observe that a prior shearing history can modify the secondary solidification structure in spite of no shearing being exerted during the cooling process. The present observation is significant in the sense that a high shear rate and duration combination not only appears to produce spherical particle morphology during the SSM processing but also during the subsequent solidification stages ensuring a complete non-dendritic and spherical particle dispersed in the final product. Therefore, apart from ensuring the useful rheological properties during processing, the benefit of non-dendritic structure may be assured in the final component with a proper selection of shearing conditions, especially when the SSM processing temperature is close to the liquidus (which is normally the case) Effect of resting time Fig. 2 illustrates the effect of resting time on the secondary solidification microstructure of Sn 15 wt.%pb alloy allowed to rest isothermally after shearing prior to secondary solidification. The morphology of the solid formed during secondary solidification changed significantly from fine rosettes in the absence of resting (Fig. 2(a)) to coarse rosettes after 60 s resting (Fig. 2(b)), finally to moderate and coarse dendrites for 180 and 300 s of resting, respectively, in Fig. 2(c) and (d). It is evident from Fig. 2(c) and (d) that apart from secondary solidification of coarse dendrites, an Fig. 2. Optical micrographs of SSM processed Sn 15 wt.%pb alloy sheared at 2014 s 1 for 180 s at 206 C followed by resting at 206 C for (a) 0, (b) 60, (c) 180, and (d) 300 s prior to solidification in a sand mould.

5 S. Ji et al. / Scripta Materialia 46 (2002) Fig. 3. Intercept length as a function of resting time for solid formed during primary and secondary solidification in samples sheared at 2014 s 1 for 180 s at 206 C. increased resting time produces a further dendritic growth from primary solidification products unlike in Fig. 1 where the primary solidified particles do not appear to grow further during the secondary solidification process. Fig. 3 shows the measured particle size of the solid formed during both initial and secondary solidification as a function of resting time. It is evident from Fig. 3 that the grain size of the solid formed during initial solidification (during shearing) undergoes a steady increase, although nominal, with an increase in the resting time during the subsequent cooling. This perhaps reflects a growth of primary solidification products during resting due to Ostwald ripening. On the other hand, the size of the particles formed during the secondary solidification increases very sharply with an increase in the resting time (Fig. 3). It therefore appears that the advantage of shearing the alloy intensely during the semisolid processing wears off during any resting period allowed prior to the cooling down toroom temperature. Similar behavior is expected for very slow cooling following SSM processing. 4. Discussion The present experiments clearly demonstrate that the solidification behavior of Sn 15 wt.%pb alloy after shearing in semisolid state is quite different from that of conventional casting. Instead of conventionally solidified dendritic microstructure, the SSM processed alloy may have spherical particles, rosettes or dendrites depending on the shearing conditions during both solidification at the processing temperature and secondary solidification below the processing temperature. A carefully chosen shearing condition comprising high shear rate and long shearing time with minimum delay in subsequent solidification (low resting period) may ensure a distribution of spherical particles in the final microstructure, thus reducing segregation. However, most of the advantage of a spherical particle morphology formed due to shearing at the processing temperature and during the secondary solidification may be lost if the semisolid alloy is allowed to rest prior to casting. How shearing may influence the microstructural characteristics of secondary solidification product is difficult to interpret. A detailed and systematic investigation into this area is necessary to formulate a universally acceptable explanation. It has been shown by the present authors that turbulence introduced by high intensity shear can reduce the solutal diffusion boundary layer thickness around a growing solid promoting compact spherical growth. Destabilization of diffusion boundary layer also prevents solute build up ahead of the growing solid liquid interface and thus prevents dendritic growth due to constitutional undercooling. Therefore, high intensity shear ensures spherical particle morphology during shearing (primary solidification product). During the secondary solidification following shearing, solid fraction may increase due to further growth from the existing solid particles or separate nucleation of the solid. It is evident from the present investigation that growth from primary particles is restricted and fresh nucleation occurs during the secondary solidification if a high shear rate and shear time combination is employed. A high shear rate promotes intense turbulence in the semisolid slurry and establishes a uniform temperature distribution throughout the melt, especially in case of a high-intensity shearing device like the twin-screw rheomoulder. This condition is ideal for nucleation throughout the melt. In addition,

6 210 S. Ji et al. / Scripta Materialia 46 (2002) a compact spherical morphology of the primary particles and the absence of prominent diffusion boundary layer around them restrict the growth of these particles due to less available kinks at the surface. Perhaps, for this reason, solidification through fresh nucleation throughout the melt is kinetically favored over growth of existing particles if secondary solidification follows immediately after shearing. Resting the semisolid slurry, however, promotes relaxation of the liquid. Local temperature fluctuations and solutal diffusion layer formation reinstate in the melt reducing nucleation events and producing coarse secondary dendrites and growth of primary solidification products (Fig. 2(b) and (c)). After a very long resting time the liquid gets completely relaxed and secondary solidification pertains to solidification conditions in a conventional casting producing dendritic growth of existing solids in the slurry and solidification of very coarse dendrites (Fig. 2(d)). 5. Conclusions The solidification characteristics of the remnant liquid in sheared semisolid slurry of Sn 15 wt.%pb alloy have been investigated. It has been observed that following long shearing time at high shear rate the remnant liquid solidifies as fine spherical particles without any visible growth of the existing solid particles. Reducing the shear rate and shearing time results in a departure to fine rosette and fine dendritic morphology of the secondary solidification product. If the semisolid slurry is allowed to rest prior to solidification following shearing, the remnant liquid solidifies as coarse rosettes or dendrites as a function of resting time. After a long resting period the solidification structure resembles coarse dendritic structure as in conventional solidification. References [1] Spencer DB, Mehrabian R, Flemings MC. Metall Trans 1972;3:1925. [2] Flemings MC. Metall Trans A 1991;22A:269. [3] Kirkwood DH. Inter Mater Rev 1994;39:173. [4] Fan Z. Inter Mater Rev, in press. [5] Doherty RD, Lee H-I, Feest EA. Mater Sci Eng A 1984;65:181. [6] Hellawell A. Proceedings of 4th international conference on semi-solid processing of alloys and composites. Sheffield, UK, 1996, p. 60. [7] Molenaar JMM, Salemans FWHC, Katgerman L. J Mat Sci 1985;20:4335. [8] Molenaar JMM, Katgerman L, Kool WH, Smeulders RJ. J Mat Sci 1986;21:389. [9] Smith DM, Eady JA, Hogan LM, Irwin DW. Metall Trans A 1991;22A:575. [10] Mullis AM. Acta Mater 1999;47:1783. [11] Das A, Ji S, Fan Z. Acta Mater, submitted for publication. [12] Ji S, Fan Z, Bevis MJ. Mater Sci Eng A 2001;299:210.