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Semi-solid extrusion of aluminum alloy ZL116

*Zhao Dazhi1, Lu Guimin2 and Cui Jianzhong1

(1. Key Laboratory of Electromagnetic Processing of Materials, Ministry of Education; Northeastern University, Shenyang 110004, China;

2. School of Resources and Environmental Engineering, East China University of Science and Technology, Shanghai 200237, China)

Abstract: The semi-solid forward-extruding feasibility of reheated ZL116 alloy cast by the near-liquidus semi-continuous casting process was studied by analyzing the microstructures and properties of forward-extruded bars. The results show that the microstructure of the ZL116 alloy billets cast by near-liquidus semi-continuous casting is mainly made up of homogeneous, fi ne global- or rosette-shaped grains. The microstructure of the billets, reheated and held at 575℃, contains stable and net-spherical grains which are suitable for semi-solid thixoforming. The semi-solid forward-extruded bars of the ZL116 alloy billet are facially smooth, microstructurally fine and homogeneous. Therefore the feasibility of semi-solid forward-extrusion of ZL116 alloy is thus excellent.

Key words: near-liquidus semi-continuous casting; ZL116 alloy; reheated; semi-solid forward-extrusion; microstructure and property

CLC Number: TG249.2/TG146.2+1 Document Code: A Article ID:1672-6421(2008)02-0104-06

The technology of semi-solid metalforming (SSM)[1-4] not only combines the quality characteristics of the liquid

and solid metal deformation, but also effectively produces net-shaped components with complex geometries. The semi-solid metal has significantly lower viscosity than the solid counterpart and flows easily under shearing. Because of the smaller solidifi cation shrinkage than that of fully liquid casting, the forming process of semi-solid metal has the capability for producing high quality products with precise dimensions and more homogeneous microstructure and lower internal defects, such as porosity and segregation, etc. Further, the mechanical properties of semi solid products could be improved due to the plastic-deformation of the solid particles in the slurry during forming. Therefore, the SSM process has started to become a more attractive forming process in recent years.

The improvement of the plasticity of aluminum alloy castings is restricted because of its high strength and forming process. On the other hand, the application of cast aluminum alloys in the traditional extrusion process is limited due to its low capacity for plastic deformation. Thus the question how to improve the properties of aluminum alloy castings by the forming process is becoming more crucial. During deforming of the SSM slurry, its low resistance to deformation due to the favourable rheology and thixotropy reduces markedly the extrusion stress, and allows it to deform easily. The characteristics of the SSM process would make feasible the deformation by extrusion of low ductility and high strength

alloys. At the same time, the properties of the components could also be improved, their microstructures more compact and homogeneous, and the grains fi ner along the section and extruding direction. This is due to the fact that the slurry is solidifi ed and shaped when impacted out the extrusion die [5-7]. In the present study, the semi-solid extrusion process of ZL116 alloy cast by the near-liquidus semi-continuous casting process was conducted. Compared with the solid extrusion on the microstructure and property, the semi-solid forward-extruding feasibility of reheated ZL116 alloy with a high solid fraction was investigated.

1 ExperimentA commercial cast alloy of ZL116 was used in the experiment. The chemical composition of ZL116 alloy is given in Table 1. The solidus and liquidus temperatures of ZL116 alloy are 575℃ and 611℃, respectively, from a differential scanning calorimetry (DSC) measurement.

Table 1 Chemical compositions of ZL116 alloy used for experiment, mass %

Si Mg Be Ti Fe Cu Zn Zr Al

7.26 0.33 0.15 0.12 0.51 0.20 0.10 0.21 Bal.

The alloy was melted in an induction furnace and grain refined above 750℃. After holding at 615℃ for 20 min in the holding furnace, the melt of ZL116 alloy was cast into cylindrical billets of diameter 162 mm and length 1,200 mm by near-liquidus semi-continuous casting process. The holding temperature was precisely controlled to ±2℃. The casting speed varied between 150 and 180 mm/min. The as-cast microstructure of samples selected from the billet was examined and analyzed by using the optical microscope.

Male, born in 1970, doctoral candidate. Research interest: metal semi-solid technology. E-mail: zhaodz@126.comReceived: 2008-01-06; Accepted: 2008-04-10

*Zhao Dazhi

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large number of grain nuclei are provided. In casting, the melt is undercooled rapidly (the measured temperature at the melt surface was 605℃), and a large number of heterogeneous nuclei are formed throughout the melt. From

the stability condition [8]:

Where C= v/∆Sf ; G, the temperature gradient; V, interface

growth velocity; D, diffusive coefficient; k, equilibrium partition ratio; ∆T0, temperature difference between the solidus and liquidus of initial solute concentration; C, Gibbs-/Thomson coefficient; ∆Sf, fusion entropy; v, solid-/liquid interface energy, it could be expected that in suitable cooling intensity and casting velocity, the nuclei would tend towards the spherical growth. Due to the small temperature gradient and the large quantity of nuclei in the melt, the grains would interfere with each other in dendrite growth in a relatively uniform growing environment. The growth resistance from the surface tension of the ‘grain bud’ could not be eliminated by the small constitutional undercooling in front of the solid-liquid interface of a growing grain. The tendency for further dendrite growth is limited. The secondary dendrite arms would be broken off due to the corrosion of low melting point compositions and thermal fl uctuation in the melt. The grains tended towards the spherical - or rosette-liked growth [9-11]. Therefore, the semi-solid billets with the homogeneous and fine net-global or rosette-shaped microstructures could be obtained by the near-liquidus semi-continuous casting process. The processing route to produce semi-solid slurries need not use complex apparatus for mechanical or electromagnetic stirring, and it is an effective and economic way to produce semi-solid billets for rheo- or thixo-forming.

To examine the microstructure evolution of the prepared ZL116 billets during reheating, samples of 25 mm length, 18 mm width and 35 mm height machined from as-received billets were reheated in a resistance furnace respectively at 575℃ and 585℃, and isothermally held for 5, 10, 15 and 20 min, then quenched in water in order to keep the microstructure of semi-solid slugs for observation and analysis. The reheating temperature of the samples was monitored with K-type thermocouples inserted into the furnace and the samples, respectively. The reheating temperature of samples was precisely controlled to ±1℃.

The operation of semi-solid forward extrusion was carried out with the hydraulic extrusion equipment having the maximum load capacity of 800 t. The cylindrical billets of 150 mm diameter and 350 mm length machined from as-received billets cast by near-liquidus semi-continuous casting process were reheated isothermally at 575℃ for 15 min. K-type thermocouples were fi tted to the surface and 20 mm below the surface of the billet for monitoring the reheating temperature. The profile of the extruded bar is designed as shown in Fig.1. The extrusion ratio is 13. The temperature of the die was controlled above 200℃ during extrusion. The temperature of the container was above 500℃ and controlled by the self-heated device. The thermocouples were withdrawn from the billets just before extruding, and the ram cooling at the exit of the extruding die was used. Respectively, the billets cast by near-liquidus semi-continuous casting process were extruded in solid at 530℃ and 560℃ and in semi-solid reheated at 575℃ and held for 15 min.

To weaken the compositional segregation in the as-cast billets, the billets for solid extrusion were pretreated at 500℃ for 10 hours for homogenization. The billet extruded in semi-solid could not be homogenized for its forming in the two-phase zone of solid and liquid.

The microstructures of special sections in the extruded bars, shown in Fig. 1, were examined by means of Lica optical microscope. The mechanical properties of the extruded bars were also measured.

cast by near-liquidus semi-continuous casting process. It is mainly composed of primary a-Al phases and eutectic a+Si phases, and the primary a-Al phases are mainly distributing as the homogeneous and fine rosette-shaped or net-globular structures across the section of the billets.

Being held and poured near the liquidus temperature of ZL116 alloy, the distribution of the composition and temperature in the melt tends to be uniform. A lot of clusters of atoms form in the melts. The conditions for forming a

Fig.1 Schematic diagram of sample from the extruded bar

Fig.2 Microstructure of as-cast ZL116 alloy

2 Results and discussion2.1 Microstructure of as-cast billetsFig. 2 shows the as-cast microstructure of the ZL116 alloy

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(b)(a)

2.2 Microstructures of reheated billetsReheated to the semi-solid state and held isothermally, grains spheroidizing would be taking place driven by a decrease in the interface energy of the grains. Thus the microstructure of semi-solid slurry for thixo-forming could be realized by reheating the billets.

The microstructures of ZL116 semi-solid samples, that reheated at 575℃ and 585℃ and held for 5, 10, 15 and 20 min, respectively, are shown in Figs. 3 and 4. Reheating at 575℃, the remelted liquid in the samples is mainly located at the circumference of the globular solid phase, and entrapped liquid in the globular solid phase is low. When reheating at 585℃ for 15 min, the entrapped liquid in the globular solid

Fig.4 Microstructure of as-cast ZL116 alloy reheated at 585℃ for: 5 min (a); 10 min (b); 15 min (c) and 20 min (d)

Fig.3 Microstructure of as-cast ZL116 alloy reheated at 575℃ for: 5 min (a); 10 min (b); 15 min (c) and 20 min (d)

(a) (b)

(c) (d)

(c) (d)

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phase increases with the holding time obviously (shown in Fig. 4c and 4d). When holding time is 5 min at 575℃, there are still some rosette-shaped primary grains being spheroidized (shown in Fig. 3a) because of the short holding time. Being held from 10 min to 15 min, the primary grains become globular obviously, but the primary grains begin to grow up with the increase in holding time. When reheated for 20 min, the primary grains abnormally grow up and coalesce (shown in Fig. 3d), and the quantity of large-sized grains is increased. When reheated and held at 585℃, the spheroidizing rate of the primary grains become faster, and some of the primary grains coalesce when holding time is 5 min (shown in Fig. 4a). With the increase of holding time from 10 min, the spheroidizing effi ciency of primary grains is dropping and some of the small grains begin to remelt. Further, increasing the holding time leads to an increase in the average grain diameter and the difference in grain diameters is obviously due to coarsening and coalescence phenomena.

Measured average size (the equal-area-circle diameter) and shape (roundness) of the primary grains by the image analyzing software indicated that the spheroidizing effi ciency of primary grains reheated at 575℃ from 10 to 15 min was the most favorable. The average diameter of primary grains is about 116.5 µm, and the roundness is about 0.66. Stable, rounded grains surrounded by the liquid phase could be obtained, and the reheated billets are more suitable for the semisolid thixo-forming when reheated at 575℃ for 15 min.

2.3 Forward extruding of ZL116 alloyIn observing the surface of extruded bars of ZL116 alloy, small repeating cracks in the intersecting area of two ends of the solid-extruded bars at 530℃ and 560℃ could be seen, as shown in Fig.5. Extruded in semi-solid state at 575℃, the extrusion integrity and the high formability of the bar without the above defects could be obtained. The surface fi nish of the bar is very smooth, as shown in Fig.6.

The plastic deformability of the experimental alloy in solid is poor. During the solid extrusion, the fl ow of outer section in the billet is restricted from the restriction of the die shape and the friction from the inner wall of the extrusion container so that the fl ow velocities at the inner and outer sections of the die cavity are different. Usually the inner fl ow is faster than the outer, especially at the thick section. The outer metal would be affected by the tensile stress, and the inner metal would be affected by the compressive stress. The basic stress state in the deforming zone has been changed. Cracks on the bar surface, especially in the intersecting section of the two walls, would occur and expand to the inner section when the above stresses exceed the maximum crack-resisted intensity of the metal [12-13]. During the semi-solid extrusion, the tendency to form large stress in the metal, caused by the plastic deformation of solid grains while being shaped, would decrease because of the liquid fi lm surrounding the solid grains in the materials. Surface cracks could form, but the inner structure of semi-solid extruded bars is more compact [14-16]. Obviously, high quality bars of ZL116 alloy with a smooth surface fi nish could be obtained using the semi-solid forward-extruding process.

2.4 Microstructures and propert ies of extruded bar of ZL116 alloy

The reticulation in the reticulate eutectic phase in the as-cast billet was observed to be broken, when semi-solid billets of ZL116 alloy were reheated and held at 575℃. A large quantity of fi ne silicon particles and precipitates was dispersed in the matrix when semi-solid forward-extruding. The microstructures over all of the cross-section of the semi-solid extruded bar are very homogeneous, as shown in Fig.7. The dynamic recovery and recrystallization in the solid grains took place during the plastic deformation, and made the grains fi ner.

To evaluate the differences in the extruding formability, the same cast alloy in the fully solid state was extruded at 530℃ and 560℃ using the same die. Fig. 8 and 9 show respectively the microstructures of various sections of ZL116 bars extruded in solid at 530℃ and 560℃. The microstructure deformation level in various sections of the bars will obviously vary (Fig. 1). The deformation level in section B is the lowest, and the outline of the as-cast microstructure could still be seen. The deformation level in section A is the highest, and the as-cast outline is no longer evident. The deformed microstructures are fi ber-like along the longitudinal direction of section C. With the increase in the extruding temperature, the difference in the microstructures deformation in the various sections of the extruded bars is reduced as the deformation resistance of the billet is decreased.

When extruded in semi-solid state with the high solid fraction, the globular grains in the slurry are surrounded by the liquid fi lm, and would fl ow as a viscoplastic fl uid under the extruding force. The forming process is mainly composed of sliding between solid grains and the plastic deformation of solid grains. Due to the small yield strength of the material in the semi-solid state, the heat generation during the deformation could be negligible. The process is almost isothermal; the temperature distributes uniformly on the cross section of the

Fig.5 Surface defects of profi led bar by the solid extrusion

Fig.6 ZL116 bars produced by semi-solid extrusion at 575℃

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(b) (c)(a)

Fig.7 Microstructures of the ZL116 bar SSM-extruded at 575℃: position A (a), B (b), C (c) as shown in Fig.1

Fig.9 Microstructures of the ZL116 bar solid-extruded at 560℃: position A (a), B (b), C (c) as shown in Fig.1

Fig.8 Microstructures of the ZL116 bar solid-extruded at 530℃: position A (a), B (b), C (c) as shown in Fig.1

billet during the deformation and no temperature increase is observed. The stress and fl ow velocity distribution in various directions over all of the billet cross section is uniform during the forming, and is affected less by the wall size of the bar. The uniformly deformed microstructures are easy to obtain [17-19].

When extruded in the solid state, the metal is deformed as the elastoplastic fl uid. Due to the large yield strength of the material, noticeable deformation heat is generated but the temperature across the cross section is not uniform. As the fl ow of the metal is affected by the restriction of the die shape and the frictional resistance, the fl ow velocity over the cross section is not uniform [19-21]. Therefore, the deformation level, as well as the microstructure over the bar cross section is not uniform. During the elastoplastic deformation of the metal, which is affected by the shape and the wall size of the bar,

the deformation level is weakened from the edge to the inner section and from the thin wall to the thick wall of the cross section. The deformed microstructures in the center of the cross section, such as section C, are fiber-like as the tensile stress is decreasing from the center of the cross-section to the intersecting area of its two sides [22-23].

Another factor which determines the feasibility of the forward extrusion in the semi-solid state is the mechanical properties of the bar. The properties of ZL116 bar extruded in semi-solid and solid states are shown in Table 2 for comparison. The hardness of each section extruded in semi-solid state is higher than those extruded in solid state. The tensile strengths from SSM are close to the best value in the solid state. The SSM elongations are also higher. The bar extruded in the semi-solid state thus possesses very favorable mechanical properties.

StateExtruded temperature

℃

Hardness, HB Rm, MPa A, %

A B C A B C A B C

Solid 530 44.5 46.9 47.4 177 177 176 19.4 18.0 20.0

560 51.0 54.1 55.3 194 220 220 15.3 17.3 15.3

Semi-solid 575 53.8 57.2 60.6 200 205 205 17.6 16 16

Table 2 Mechanical properties of ZL116 alloy parts extruded in semi-solid and solid state

(a) (b) (c)

(a) (b) (c)

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3 ConclusionsCast by the near-liquidus semi-continuous casting process, the homogeneous, fine net-globular or rosette-shaped microstructures in ZL116 semi-solid billets could be obtained. Reheated and held at 575℃ from 10 to 15 min, the solid grains in the ZL116 semi-solid samples are homogeneous, fi ne and rounded, and have good remelting stability. The reheated billet is suitable for the semi-solid thixo-forming.

Extruding with a high solid fraction in the semi-solid state at 575℃ for 15 min, the extruded bar’s surface is smooth. The deformed microstructures are homogeneous in the cross section. A large volume of fine silicon particles and precipitates was dispersed in the matrix. The grains are also fi ner because of the dynamic recovery and recrystallization,

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The project is supported by the National Key Scientific and Technical Research Program of China (106058), National Doctoral Program Foundation of China (20050145007) and National Natural Science Foundation (50674032).