Characterization of semi-solid processing of aluminium alloy 7075 with Sc and Zr additions

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Characterization of semi-solid processing of aluminum alloy 7075 with Sc and Zr additions Ł. Rogal, J Dutkiewicz, H.V. Atkinson, L. Lityńska-Dobrzyńska, T. Czeppe, M. Modigell

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S0921-5093(13)00469-3 http://dx.doi.org/10.1016/j.msea.2013.04.078 MSA29856

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Materials Science & Engineering A

Received date: 6 November 2012 Revised date: 15 April 2013 Accepted date: 24 April 2013 Cite this article as: Ł. Rogal, J Dutkiewicz, H.V. Atkinson, L. LityńskaDobrzyńska, T. Czeppe, M. Modigell, Characterization of semi-solid processing of aluminum alloy 7075 with Sc and Zr additions, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2013.04.078 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Characterization of semi-solid processing of aluminum alloy 7075 with Sc and Zr additions . Rogala, J. Dutkiewicza, H.V. Atkinsonb, L. Lityska-Dobrzyskaa, T. Czeppea, M. Modigellc a

Institute of Metallurgy and Materials Science of the Polish Academy of Sciences, 25 Reymonta St., 30-059 Krakow, Poland b The University of Leicester, Department of Engineering University Road, Leicester, LE1 7RH, United Kingdom c RWTH Aachen - Dept. of Mechanical Process Engineering, 55 Templergraben St., Aachen, Germany

Phone: +48 502 740 215 Fax: +48 122952804 E-mail: [email protected];[email protected]

Abstract: For thixoforming (semi-solid processing) it is necessary to have a fine globular microstructure in a semi-solid range. Here this has been obtained for 7075 aluminium alloy by addition of modifying agents: 0.5 weight % of scandium and zirconium. The thixoforming process was carried out at 632°C which gave about 23 volume % of liquid phase. The microstructure of the thixoformed part (a rotor) consisted of globular grains surrounded by precipitates of secondary phase. The average hardness of thixo-formed parts was 105 HV5 and the tensile strength 300 MPa. T6 heat treatments were performed with solutionisation at 450ºC for 30 min. and 10 hours. In both cases the ageing time was set as 18 hours at 120ºC. The heat treatments led to an increase in average tensile strength up to 495 MPa. Transmission Electron Microscopy (TEM) analysis enabled the identification of precipitates of the metastable dispersoids of L12 – Al3 (Zr, Sc) and ’ (MgZn2) phases in the alloy after the thixoforming and T6 treatment.. The measurements of rheological properties of 7075Al alloy with Sc and Zr 1

additions in the semi-solid range indicated an increase of particle size and spheroidization leading to an observable decrease of viscosity during isothermal shearing. A shear rate jump experiment showed that with increasing shear rate the viscosity rapidly falls. Keywords:7075 alloy with Sc and Zr, thixoforming, semi-solid processing, rheology, grain refinement

1.INTRODUCTION There is a strong drive to semi-solid process 7XXX alloys because they have the highest mechanical properties of all aluminium alloys [1-2]. Previous work is given in references [3-10]. The 7075 series aluminium alloys based on Al–Zn–Mg–Cu have been studied by means of thermodynamic modelling in order to determine the most suitable chemical compositions for semi-solid processability [3]. Spencer et al. [11] originally discovered the thixotropic behaviour of alloys in the semi-solid range. Thixotropy occurs when the semi-solid alloy consists of 20% to 80% of the solid phase in the form of globular grains surrounded by the liquid phase. If the semisolid slurry is subjected to a shear force, a sudden fall of viscosity occurs. Two basic routes of semi-solid processing (SSP) – called “rheocasting” and “thixocasting” have been developed. The rheo-route involves the preparation of SSM slurry from liquid alloys after which the semi-solid slurry forming process takes place. The thixo-route is basically a two-step process which consists in the preparation of feedstock material via the liquid state or the solid state and then reheating the solid feedstock material to a semi-solid temperature and shaping semi-solid slurry into components [12]. The method of obtaining the globular structure in SSM slurry is one of the important technological parameters. Depending on the method employed, a different grain size and a different fraction of the liquid phase are obtained [12]. The most common methods used to obtain a globular structure for Al-Zn-Mg-Cu alloys are: electromagnetic stirring system (EMS) [13], low superheat pouring with shear field (LSPSF) [14, 15], induction stirring with 2

simultaneous forced air cooling (CSIR-RCS) [16], gas induced semi-solid (GISS) [17], warm plastic deformation [18-21]. The grain size and shape factor in all these methods significantly depend on the temperature and time of soaking in the semi-solid state [21-22]. Another method of grain refinement of aluminum alloys is related with the application of agents, such as Al-8B. In the case of Al-Zn-Mg-Cu alloy, 0.5% of B can be used to obtain heterogeneous nucleation [23]. All these methods enable a grain size of between 54 and 140 m to be obtained. Significant grain size decrease can also be obtained by modification with scandium and zirconium in the feedstock. Bunck et al. [24] studied this effect in AlLi2.1Mg5.5 (AA1420) alloy and achieved a thixotropic structure consisting of spherical grains, 30 m in size. Relatively small grains give improved mechanical properties and hence investigating the effect of grain size refinement is the motivation behind this study. Previous work on the addition of Sc and Zr to the 7XXX alloy has shown these decrease the spheroid size and decrease the eutectic mixture content [25-27]. Moreover, additions of Zr and Sc to 7XXX series alloys improved their mechanical properties through the precipitation of metastable dispersoids of L12 – Al3 (Zr, Sc) during saturation. The mechanical properties were also improved by the presence of K’ phases (sequence GP Æ ’ Æ equilibrium  - MgZn2 with hexagonal lattice) characteristic of Al-Mg-Zn-Cu alloys during a complex heat treatment process [2, 6, 28]. In addition, the precipitation of Al3 (Zr, Sc) inhibited coarsening of the microstructure [29]. The viscosity of a semi-solid metal alloy is dependent on a number of parameters: solid fraction, particle shape and size, shear rate, duration of shear, preparation of the alloy and the alloy composition [30-32]. Additionally, according to Modigell et al. [33] viscosity is influenced by particle shape and size. In order to assess the impact of the fine grains resulting from the

3

modification with Sc and Zr, the rheological properties of the semi-solid slurries (near to the temperature of the thixoforming process) have been measured here using a rotational viscosimeter, similarly to the studies for SnPb [30, 32, 36], or Al-alloys [33-36]. A knowledge of viscosity is important for modeling semi-solid processing. The aim of the work is to investigate the influence of the method of preparation of 7075 aluminium alloys feedstock (for thixoforming) by modification of Sc and Zr on the microstructure and mechanical properties of thixo-parts. 2 EXPERIMENTAL PROCEDURE 2.1 Starting material The 7075 aluminium with additions of Sc (0.25 wt.%) and Zr (0.25 wt.%) was cast in the form of rods of 30 mm diameter and 100 mm length into a copper die. This material will be termed 7075ScZr. Using the emission optical spectrometer JY 10 000 RF JOBIN YVON, the chemical composition of the alloy was determined to be: Mg- 2.83%, Cu-1.72%, Zn-5.86%, Zr, Sc – 0.5%, Al-88.73% (all in wt.%). A calculated section of the phase diagram for increasing Mg content near the composition of the investigated aluminum alloy is shown in Fig 1, after Rokhlin et al. [26]. The dotted line marks the content of magnesium corresponding to 7075 aluminum alloy with addition of 0.5% (Sc, Zr). The addition of 0.5 % of scandium and zirconium to AlMg-Cu-Zn alloys should cause the formation of ZrAl3 and ScAl3 phases. A significant solubility of zirconium in ScAl3 and a small solubility of scandium in ZrAl3 were observed [26, 27, 29]. The ScAl3 phase can dissolve up to 36%wt. (14 at.%) Zr and the phase ZrAl3 can dissolve up to 5%wt. (‫׽‬5 at.%) Sc [26, 29]. Partial replacement of Sc atoms by Zr atoms leads to the decrease of the parameter of phase lattice L12 and to the formation of Al3(Sc, Zr) phase and hence to a decrease in the mismatching of the lattice to aluminum [27]. 4

2.2 Analysis of microstructure and mechanical properties The samples used for optical microscopy were polished and etched in 5% solution of hydrofluoric acid and observation carried out using a Leica microscope. The quantitative metallographic analysis was performed using a Leica QUIN image analysis system. In order to obtain reasonable statistical results about 1000 particles were evaluated in each specimen. Computer programs were developed for calculating the size distribution and characteristic values. X-ray investigations of the phase composition were performed using CoK filtered radiation with a Philips PW 1710 diffractometer. The analysis of microstructure and chemical composition was made using a Scanning Electron Microscope, FEI SEM XL30, equipped with an energy dispersive X-ray spectrometer (EDAX GEMINI 4000). Chemical content analysis was carried out in three consecutive tests and the arithmetic average was calculated.

Measuring error was

determined depending on the weight in weight concentration of a given element. Microstructure and electron diffraction studies were performed with a Transmission Electron Microscope (TEM) Philips CM20. Samples for TEM were electropolished using the jet method in electrolyte consisting of 10 % HClO4 and C2H5OH at subzero temperatures. Hardness measurements (by the Vickers method) were carried out using a Zwick/ZHU 250 (HV5) in accordance with (HV) ASTM E 92. Tensile strength tests were performed using an INSTRON 6025 machine. The samples of thixo-formed 7075Al with Sc and Zr were investigated in accordance with the standard PN-EN 10002-1:2004 (which corresponds to USA norm - ASTM E8). Thixoformed cuboid shape samples (50 mm x 25 mm x 10 mm) were produced for the tensile test samples to be machined from material perpendicular to the direction of the flow of semi-solid suspension. The tensile test samples were rectangular cross-section with a gauge section 3mm by 4 mm in a gauge length of 25 mm and an overall length 50 mm. 2.3 Rheological and calorimetric measurements 5

The measurement of rheological properties in the semi-solid range of the alloy 7075ScZr was performed using a Searle-rheometer device CDT 501 produced by Anton Paar . The measuring system was made of graphite (cup and rod). The cup diameter was 26 mm and the gap between the cup and the rod was 3 mm wide. In order to avoid wall slip, the rod had 0.5 mm deep grooves. The measuring system was located inside an oven in the argon atmosphere and heated with radiation and convection to temperatures up to the liquidus temperature 680°C. The alloy was cooled at the rate of 4°C/min to temperatures 630°C and 625°C. The applied cooling rate was optimal for the full control of temperature during cooling in viscosimeter, given the thermal capacity of the volume of material. The measurements were performed at shear rates from 10 to 200 s-1. For the analysis of the microstructure during isothermal shearing, the tests were stopped at two points, and the samples were fast cooled with liquid nitrogen vapours. The microstructures presented come from the gap between the rod and the crucible. A DuPont 910 calorimeter was used to measure the thermal effects during heating and cooling at rates of 4 and 15°C/min in a argon atmosphere. 4°C/min cooling rate was applied in order to enable a precise control of liquid phase change as a function of temperature during the viscosimeter analysis, while 15°C/min was applied in order to approach the heating parameters during semi-solid processing. The samples of 1.5 mm thickness and the 3.5 mm diameter were heated and cooled in a silver crucible. The amount of liquid phase as a function of temperature was determined according to the procedure described by L. Rogal [37]. It is expressed in volume percentage. 2.4 Thixoforming procedure The heating conditions were determined before the thixoforming process. Figure 2 illustrates the time-temperature dependence curve of 7075ScZr alloy recorded during heating up to the semi-solid state. A cylindrical billet (diameter – 30 mm, height – 30 mm) of the alloy was placed in the coil of an Inductive Heating Machine. Two thermocouples (type S) were mounted 6

on the axis and at the edge of the sample. The heating curve was plotted using a PC LAB system to register temperature as a function of time. The heating time lasted 168 seconds, after which the sample reached the temperature of 632°C (Fig 2, marked as 1), which corresponded to about 23% of liquid phase in accordance with the DSC curve (for heating rate of 15°C/min) shown later. The thixo-formed rotor parts were produced using a specially built prototype device. A piston velocity of 1 m/s was applied. A billet (diameter – 30 mm, height – 30 mm) of 7075 aluminium alloy with additions of Sc and Zr was placed in the coil of an inductive heating furnace at 632°C. The temperature of the feedstock was measured with a S type thermocouple. The billet was then moved to the shot sleeve of a high-pressure die-casting machine and forced by a piston into the die with a pressing force of 35 kN. The feedstock was pressed into a die in the axial direction. The die was made of M2 steel, pre-heated to 100ºC and 180°C and sprayed with BN. A dozen rotor shape samples (55 mm in diameter, 17 mm high and 2 mm thick in the thinnest wall ) were thixoformed. In addition thixoformed cuboid shape samples (50 mm x 25 mm x 10 mm) were produced for the tensile test. 3 RESULTS 3.1 Characterization of 7075ScZr feedstock Crystallization nuclei based on Al3 (Zr, Sc) phase play a significant role in refining of grains [26, 27]. No dendrites were observed and solid solution globular grains of an average size of 23 μm were observed. Fig 3 shows an optical microstructure of a thixo-formed specimen with an insert of a histogram of the grain size calculated from the microstructure. The grain size was in the range from 5 μm to 45 μm. The largest fraction belonged to grains in the range of 22 μm to 30 μm, indicating the average size of globules. The amount of the secondary precipitates between the spherical grains was estimated at 3 %. At higher magnifications small square precipitates of Al3(Zr, Sc) could be seen in the optical microscope. The phase was visible in the microstructure 7

before etching as distinctly outlined inclusions of grey color, which did not actually change after etching. X-ray diffraction of the 7075ScZr confirmed the dominant presence of (Al) with addition of the intermetallic MgZn2 hexagonal phase [7]. The SEM micrograph in Figure 4 shows aluminum solid solution grains surrounded by precipitates. The EDS quantative microanalysis from the (Al) area, marked 1 in Fig. 4 was presented in the first row in Table 1. Among (Al) grains eutectic mixture is present. EDS analysis was carried out in the areas 2 and 3 in Fig. 4 which correspond to the chemical analysis presented in rows 2 and 3 in Table 1. The analysis confirmed the presence of Zn, Mg, Cu and Al in the eutectic mixture. According to [26, 28], MgZn2, Al2Cu and Al32(Mg, Zn) phases can exist in which atoms of elements may replace each other, which can lead them to have nonstoichiometric composition . The most probable phases which exist in the 7075ScZr feedstock microstructure are MgZn2 and Al2Cu. Additionally, small square-shaped precipitates can be observed within grains. The chemical composition (area 4 in Fig. 4) is presented in the fourth row in Table 1, which indicates the presence of primary precipitates of Al3 (Zr, Sc) phase. 3.2 Calorimetric analysis The curve in Figure 5 show the dependence of the amount of liquid phase as a function of temperature for the 7075ScZr calculated from the DSC measurements for heating. It shows that Al() melting started at 565°C for heating rate 15°C/min. The end of melting took place at 657°C for heating from the solid state. For the determination of the solidus – liquidus range and calculation of the amount of liquid phase as a function of temperature, a high heating rate of 15°C/min was used as this is closer to the heating rate used in thixoforming process. The semisolid processing was carried out at 632°C, which corresponds to 23% of liquid phase in accordance with the DSC analysis for heating rate 15°C (point A marked in Fig 5). The amount of liquid phase was also determined as 23±1.5% on the basis of the content of eutectic and 8

secondary grain phases from the total area of the thixo-formed rotor microstructure but this is after relatively slow cooling in the die and these figures will not be fully representative of the amount of liquid present at the semi-solid processing temperature. 3.3 Rheological properties of 7075ScZr in semi-solid state In order to establish conditions for the thixoforming, measurements of the rheological properties of the 7075ScZr alloy were performed. Figure 6a shows the change of viscosity with shear time (thick curve) and temperature as a function of time (dotted curve) at a constant shear rate of 250 1/s. In the first stage (I), the alloy was melted at 680°C. In the second stage (II), the sample was cooled to the semi-solid range, corresponding to a fraction solid fs = 70% (temperature 625°C, according to DSC analysis for cooling rate 4°C/min, Fig 6b). The initial increase of viscosity indicates the emergence of solid particles. The additional figure 6b presents the DSC curve which corresponds to the temperature curve in Fig 6a obtained during measuring the rheological properties (the cooling rate of 4°C/min was the same for both curves). It can be seen in the curve that crystallization starts at 642.2°C (point d figure 6b) and is close to the temperature - 642.5°C (point e figure 6a), where the increase of viscosity was observed (point e’ figure 6a). Further cooling of the alloy to 625°C corresponds to 70% solid phase in accordance with the DSC curve (point f figure 6b). In the final stage (III) of the experiment, the semi-solid slurry was held under isothermal conditions for 60 min, at a temperature of 625°C, with constant shear rate 250 s-1. When analyzing the viscosity curve in the third stage (III), the viscosity decreases with time of shearing. The complex flow behaviour of the suspension is directly related to the internal structure [32]. The influence of particle diameter and shape on the viscosity curve (discussed in [33]) is clearly visible. In order to determine the relation between the shear time and changes in microstructure in the semi-solid state, a series of experiments were carried out, consisting of fast cooling (using liquid nitrogen) from places in the viscosity curve, marked as points 1 and 2 in Fig 9

6a. The quantitative estimation of the eutectic in this micrograph gave a value of 28%-32% of the eutectic. The microstructure in Fig 6.1 (which corresponds to viscosity after 3 min of isothermal shearing, point 1 in Fig 6a) shows that small primary globular grains began to grow via coalescence (average size of 134 μm). The shape of connected grains was irregular and the boundaries of grains were not spherical. In addition, the agglomerated particles also entrapped liquid phases. The structure in Fig 6.2 (which corresponds to viscosity from point 2 in Fig 6a) shows that after 60 minutes of shearing the diffusion process caused the agglomerates of small grains to transform into larger grains. The spherical shape of (Al) of average size 322 μm can be seen. The rate of structure evolution toward more spherical (or ellipsoidal) shape and increase of the particle size leads to a decrease of the viscosity. Figure 7a shows the 7075ScZr alloy flow behavior of metal suspension in isothermal condition. The alloy was cooled to the semi-solid range (54% and 70% of solid fraction, which corresponds to temperature 630°C and 625°C respectively) and held for 60 minutes at a constant shear rate 250 s-1. The starting viscosity was 4.3 Pa·s and 8.3 Pa·s for 54% and 70% of solid fraction, respectively. Isothermal shearing contributed to the decrease of viscosity and after 60 minutes of shearing a steady state was reached. Figure 7b shows the influence of shear rate on the viscosity as a function of shearing time at a constant temperature of 625°C. It can be seen that the starting viscosity (after reaching 70% of solid fraction) strongly depended on the shear rate: 8.9 Pa·s and 25.8 Pa·s for 100 s-1 and 250 s-1, respectively. The viscosity level decreased at higher shear rates because the material is shear thinning. The steady state viscosity was reached after 60 minutes: 2.6 Pa·s and 1.8 Pa·s for 100 s-1 and 250 s1, respectively. The described changes of viscosity were similar to those observed earlier for 7XXX alloys [38], however it seems that the Sc and Zr additions caused an observable increase of viscosity, due to particle size decrease as suggested by Koke and Modigell [39] and also spheroidization of (Al) grains. 10

Figure 8 shows that steady state flow behavior is shear thinning (viscosity decreases with increasing shear rate). Experiments were carried out after reaching a steady state condition during isothermal shearing for shear rates between 10 s-1 to 150 s-1 for two different fractions of solid. The influence of solid fraction on the flow is a very important factor. For a shear rate of 10 s-1 the viscosity was 25 Pa·s for 70% solid fraction and 16 Pa·s, for 54%. With increasing shear rate the viscosity rapidly falls as a result of the decrease of agglomerates of solid particles by the rupture of solid bridges [40].

3.4 Analysis of thixo-formed rotor parts The rotor thixo-formed parts made of 7075 aluminum alloy with the addition of Sc and Zr are presented in Figure 9. The thixo-formed part on the left side (Fig 9a) was produced at a die temperature of 100°C. It can be seen that the blades of the rotor have defects resulting from misrun thixoforming (Fig 9a). A precise reproduction of the die shape without visible defects thixo-formed parts was obtained for a die temperature of 180°C (Fig 9b). The average hardness of the thixo-formed part (un-heat treated condition) was near 106 HV5. The optical microstructure of a thixo-formed part cross-section is shown in Figure 10. Directions of the semi-solid slurry flow have been marked on the microstructures with big arrows. In the central part of the rotor (marked 1 in Fig 10a), where the flow of the semi-solid slurry was easy and the wall thickness was constant, the microstructure was homogeneous, and consisted of globular grains surrounded by the eutectic mixture (Fig 10b). The amount of eutectic mixture was 9%. Figure 10c shows the microstructure from the thermal centre of the rotor (point 2 marked in Fig 10a), more eutectic precipitates could be observed near the thermal centre than in point 1, where the highest fraction of globular grains solidified. It can be seen that the grains are elongated in the direction of the blade. The microstructure of the blade Fig 10d (marked as 3 in 11

Fig 10a), consisted of larger grains of primary (Al), of average size 26 μm, surrounded by smaller secondary grains. The secondary grains crystallized in a second stage from residual liquid during cooling in the die. The precipitates of M ((Cu, Zn, Al)2Mg)) and T ((Cu, Zn, Al)49Mg32)) phases are present among the grains [7, 26, 28]. The amount of the secondary grains and precipitates of M and T phases was estimated as between 15 to 32% (determined on the basis of a metallographic analysis of microstructures by Leica QUIN software). Figure 11a shows a Scanning Electron Microscopy Back Scattered Electron (SEM-BSE) microstructure at the centre of the thixoformed rotor. Visible (Al) grains are surrounded by the precipitates of secondary phases. The EDS analysis of the matrix (point 1, Figure 11a) was presented in the first row in Table 2. The results are similar to the chemical composition of aluminium solid solution in 7075ScZr feedstock . Only partial diffusion of elements from liquid phase to (Al) took place, probably as a result of the relatively fast heating process and short holding time in semi-solid range. The chemical composition analysis of the eutectic mixture present among solid solution (point 2) in Figure 11a, and of small square-shaped precipitates (point 3) in Figure 11 is presented in rows 2 and 3 in Table 2. EDS analysis from the area between globular grains confirms the presence of Mg, Cu and Zn elements, which suggests the formation of intermetallic phases in the eutectic. Primary Al3(Sc, Zr) phase was still present in the microstructure of thixo-elements. Figure 11b shows a TEM microstructure and diffraction from the central part (SAEDP does not include the square shape precipitates). The bright field micrograph presents globular grains of (Al) with the precipitate of Al3 (Sc, Zr) (of size near 3 μm) within the (Al) grain surrounded by secondary phases. The electron diffraction pattern and its solution, shown as an insert in Fig 11b, from the areas near the grain boundaries allowed the identification of these precipitates. On the basis of the distances and angles between reflections, the hexagonal MgZn2 12

phase was identified at zone axis [21 3 1], similarly to the results of works [7, 26, 28], in which the intermetallic  phase in the form of non-stoichiometric [Mg(Zn, Al, Cu)2] phase was reported in 7XXX alloys.

3.3 Analysis of thixo-formed part after heat treatment In the next stage of this study, a heat treatment of rotor samples was performed. The temperatures of saturation and aging were determined based on previous work [1, 2, 9, 28]. The heat saturation time was determined on the basis of the coefficient of diffusion: for zinc in Al DZn 450°C = 4,244 × 10-14 m2s-1 and for copper: DCu

450°C

= 1,63 × 10-14 m2s-1 [41]. The 10-hour saturation time was

determined by taking into account the calculated coefficient of diffusion and the average grain size of about 28 m. Additionally a 30-minute saturation time was applied in the study similarly to conventionally treated AW7075 alloy [42]. Two types of heat treatment were chosen (T61) solutionising at 450°C for 0.5h and ageing for a period of 18 hours, at temperature of 120ºC, (T62) solutionising at 450°C for 10h and ageing for 18 hours at 120ºC. In order to estimate the effect of precipitation during ageing after two different times of supersaturation the samples were investigated using a DSC calorimeter. Figure 12 shows the DSC curves for the 7075 alloy during heating (at rate 20°C per minute) in the range 60°C - 180°C for three different states of 7075Al with Sc and Zr: after annealing (450°C/1h and slow cooling to room temperature), after solutionising at 450°C for 0.5h and after solutionising at 450°C for 10h. It can be seen that after annealing the thermal effect did not appear (i.e. the peak). During heating of the sample supersaturated at 450°C for 0.5 hour a large exothermic effect in the range 62°C to 146°C (the enthalpy of this process was 3.4 J/g) became visible. This effect was connected with the precipitation of ’ phase [1, 2]. The most significant exothermic effect appeared at the temperatures between 63°C and 167°C after the saturation at 450°C for 10h. The enthalpy of this 13

effect was 5.3 J/g and was most probably caused by dissolution of larger amounts of Mg, Zn and Cu elements in (Al) during solutionising for 10h in comparison with the solution heat treatment for 0.5 hour. Figure 13a presents the microstructure of a thixo-formed part in cross-section after the T61 treatment (solution at 450°C for 0.5h and ageing at 120°C for 18h). Significant changes in the microstructure of the blade part of the rotor are visible. The average grain size increased up to 30 μm (Fig 10b).The small secondary and eutectic grains that were present between the large grains have dissolved, resulting in the coarsening of primary grains (Fig 13c). The amount of insoluble secondary phase is higher in the blades than in the centre of the rotor, due to the greater amount of the eutectic phase. In the central part (Fig 13d), a small amount of secondary phases between grains is clearly visible. The average hardness of the thixo-formed parts after the treatment was 184 HV5 (Table 4). Figure 14 shows SEM-BSE microstructure of the thixo-formed part after T61 treatment. Significant changes in the structure morphology, in the form of a smaller amount of the secondary phase, are visible. Chemical analysis was performed on the areas marked by squares in Fig.14 and the results are presented in Table 3. The content of elements in the solid solution (area 1 in Fig. 14, first row, Table 3) is higher than in the part directly after processing, which suggests partial dissolution of elements in globules, although fine

’ precipitates resulting from ageing

could not be seen at that magnification. As a result of T61 heat treatment, MgZn2 phase was generally dissolved, while the Al2Cu phase remained in the microstructure (area 2 in Fig. 14, row 2 in Table 3). In the microstructure primary Al3 (Sc, Zr) phases were also present (area 3 in Fig. 14, row 3 in Table 3).

14

Figure 15a shows a TEM bright field image obtained under Bragg’s conditions at g = 111 and the corresponding diffraction pattern for the thixo-formed part after heat treatment. The characteristic Ashby-Brown’s coffee bean contrast formed around the spherical and coherent particles is connected with stresses in the crystal lattice around precipitates. The line of lack of contrast is perpendicular to the g . The supersaturation at 450°C brought about the precipitation of zirconium-scandium particles in the coffee bean shape. The electron diffraction pattern from this area shows the reflections from Al3(Sc, Zr) (100) and (01 1 ) at zone axis [011]. Further ageing at 120°C led to the formation of very fine precipitates of ’ phase, coherent with the matrix and responsible for the strengthening of Al-Mg-Zn-Cu alloys [1, 2, 28]. Figure 15b shows a high-resolution transmission electron microscopy (HRTEM) image taken of the thixo-formed part after the same heat treatment. A nanocrystalline precipitate  12 nm size can be seen in the central part of the micrograph and ’ planar precipitates are distributed homogeneously in the (Al) matrix. The ’ plates are coherent with the matrix. Two variants of plate like ’ are perpendicular to the foil. The ’ plates nucleate near the Al3(Sc, Zr) particles, due to strains associated with their existence. The spherical particles give a clear lattice crossgrating contrast, identified as metastable dispersoids of L12 - Al3(Sc, Zr) of cubic lattice cP4 structure. From the lattice distances and their angles measured using a fast Fourier transform (FFT) analysis as shown in the insert, the [011] zone axis orientation was identified, which fitted well to the simulation of the reciprocal lattice section at that orientation. The inverse FFT obtained using Digital Micrograph software, applying masks near reflections in the FFT, showed much better contrast as can be seen in the right side insert in Fig 15b. The heat treatment of the thixo-formed part solutionised at 450°C for 10h and aged 18h caused an increase of the average hardness up to 199 HV5. A higher value of hardness was 15

caused by a higher amount of the ’ precipitates. In addition, solute atoms of scandium and zirconium in the (Al) solid solution precipitated in large amount in the form of Al3(Sc, Zr) phases, during 10 hour-supersaturation. 3.4 Analysis of mechanical properties The feedstock Vickers hardness was 129 HV5 (Table 4). The relatively high value was due to the rapid cooling in the copper die, which could have resulted in saturation of (Al) and natural ageing. After the thixoforming process the average hardness was 101HV5 (Table 4). In this case the smaller hardness (in relation to the feedstock) could have been caused by lower saturation of the (Al) as a result of cooling from the semi-solid state to the temperature of preheated die. The highest values of hardness were achieved during the measurements of the samples after heat treatments (solutionised at 450°C for 0.5h and aged at 120°C for 18h) and (solutionised at 450°C for 10h and aged at 120°C for 18h). They were respectively 184 HV5 and 198 HV5 (Table 4). This difference was due to the different lengths of saturation time of (Al), which was shorter for T61 and could have been responsible for the solution of a smaller amount of elements in (Al). Figure 16 shows the tensile strength of the thixo-formed part 7075ScZr directly after SSM and of the samples heat treated at 450°C for 0.5 hour and 10 hours, followed by ageing at 120°C for 18 hours. The lowest tensile strength of 300.3 MPa was for the sample directly after thixoforming. At that stage, the thixo-formed part microstructure of the (Al) solid solution was slightly supersaturated due to the conditions of the thixoforming process (large rate of cooling from 632°C to the temperature of the metal die). A few days delay between the thixoforming process and the mechanical properties test caused precipitation-hardening in a natural way. The thixo-formed parts after T61 heat treatment: solutionising at 450°C for 0.5 h and ageing at 120°C for 18 h, had significantly higher tensile strength of 482 MPa, due to the presence of ’ and 16

Al3(Sc, Zr) phases responsible for hardening of the alloy. The heat treatment (T62) of thixoformed part at 450°C for 10h with ageing for 18h caused the increase of the tensile strength up to 498 MPa. When comparing the tensile strength of 7075ScZr thixo-formed part with the properties of AA7075 with Sc after the extrusion and T6 treatment [43], it should be noted that the average values of tensile strength were significantly higher i.e. 619 MPa. The AA7075 with Sc after the plastic deformation had tensile elongation 7 %, while the alloy after thixoforming showed plastic strain about 3.5%. Figure 17a shows the image of the fracture surface, and its cross-section (Fig 17b). Cracks started from the grain boundaries, where the eutectic precipitation of MgZnCu phase had occurred. The failure occurred around the spheroids of the brittle MgZn2 phase, where a liquid phase was present during thixoforming process. This could account for the lower elongation. 4 DISCUSSION The developed method of preparing the globular microstructure in the 7075 aluminum alloy through scandium and zirconium additions was successful. A globular microstructure with the (Al) grain size of 23 m was obtained in the feedstock for the thixoforming process. Similar research was carried out by Bunck et al. [24] in aluminum-lithium alloys (AlLi2.1Mg5.5). After modification with Sc (0.25%) and Zr (0.25%), they obtained globular grains of 30 m size in the feedstock, and used such billets for semi-solid processing. The difference in the grain size was probably caused by a different level of liquid alloy superheating above the solidus temperature (T) during casting: for the Al-Li alloy TAl-Li  80°C [12, 24], while for 7075ScZr T7075ScZr  40°C. The globular microstructure in the 7075 aluminum alloy was reported in numerous studies [46, 9, 13-21] as a result of other technological processes such as electromagnetic stirring system (EMS), cooling slope casting system (CS), low superheat pouring with shear field (LSPSF), 17

induction stirring with simultaneous forced air cooling (CSIR-RCS), gas induced semi-solid (GISS), liquid semi-continuous casting (LSC), cold plastic deformation – RAP, hot plastic deformation – SIMA and Severe Plastic Deformation (SPD) by Equal Channel Angular Pressing (ECAP). The average globular grain size obtained after the applied methods was 54-140 m. Strong microstructure refining in 7075 ScZr was caused by the Al3(Sc, Zr) phase precipitation identified in Fig. 3 as responsible for the heterogeneous nucleation of aluminum (Al) solid solution [26, 27, 29]. The addition of Sc and Zr reduced the amount of eutectic mixture on the boundaries of (Al) compared with the 7075 alloy without Sc and Zr. This was probably caused by the nucleation of intermetallic phases with a higher melting temperature than that of the eutectic mixture. It led to the reduction or almost a complete elimination of porosity [27, 29]. The measurement of rheological properties of alloys in the semi-solid state is usually carried out using a rotational viscosimeter [32-36, 38, 39]. The increase in viscosity during cooling of alloy from the liquid to the semi-solid phase was caused by the increase of solid fraction [11, 40, 44, 45], a volume of about 20% led to the non-Newtonian flow of slurry. The viscosity of 7075ScZr for a fraction of solid fS 54% at temperature 630° C and fS  70% at 625°C was lower compared with the unmodified 7075 alloy with the same amount of solid fraction investigated by Kim et al. [38]. This probably resulted from the lower average grain size and more globular shape, a finding consistent with, AlSi7Mg, AlCu4.5Mg1.5, Sn15Pb alloys [33, 36, 44, 46]. Rheological research conducted under conditions in which the semi-solid slurry was characterized by a homogenous distribution of grain size and shape (after the material preparation procedure [32, 35]), enabled the measurements of viscosity resulting from the interaction between the solid and liquid phases [12, 32, 47]. The analysis of the results (Fig. 7a) implied that the viscosity of 7075ScZr aluminum alloy decreased with the shear rate, which was in accordance with other studies [32] which claimed that it depended on the number of agglomerated grains 18

which caused the entrapment of liquid phase. The number of agglomerated grains, in turn, increased with the solid fraction, and decreased with the shear rate. However, while comparing the results with other alloys e.g. the Sn15Pb, A356 aluminum, or even X210CrW12 steel investigated under similar conditions [12, 30, 36, 46], higher viscosity was observed in the 7075ScZr alloy, which probably resulted from a finer solid grain size in the modified 7075ScZr alloy. These results are in line with the earlier studies [36, 44, 46], whose authors established that viscosity increased with the decrease of grain size. In accordance with the developed technology a few dozen thixo-elements of rotor were obtained from the 7075 aluminum alloy modified with Sc and Zr (Fig. 9). These construction elements were obtained without any detectible defects. Liu et al. [4] and Chayong et al. [48] also obtained defect-free thixo-cast parts by conducting thixoforming of 7075 alloy (with CS, SIMA and RAP feedstock preparation method) on a vertical machine [49] with a higher liquid fraction (30-50%) and a pressing force of 100 kN. The mechanical properties of 7075ScZr thixo-formed part (directly after the thixoforming process and the T6 heat treatment) which were obtained using the pressing force of 35 kN were similar to those obtained in the 7075 alloy (thixo-cast) after applying the pressing force from 100 kN – 3500 kN [4, 6, 50]. In all the investigated feedstock in which the globular structure was obtained through plastic deformation (RAP, SIMA), entrapment of liquid phase inside the globular grains was observed [4, 6, 8, 10] and led to a 3 10% decrease in the effective amount of liquid phase in the semi-solid slurry. In the 7075 ScZr feedstock the effect was not observed, which enabled the application of lower liquid fraction during the process. This led to obtaining good quality thixo-formed parts with lower segregation and porosity, while applying a relatively smaller pressing force most probably due to a fine grain size of 27 m, with high homogeneity of shape and distribution. In other studies [4, 6, 8, 16, 50] the average grain size of 7075 thixo-casts (without T6) was 19

estimated to be between 80 150 m. This was probably caused by typical mechanisms of structure coarsening in the semi-solid range, i.e. Ostwald ripening and coalescence [40, 52]. The Al3Zr and Al3Sc phases are present in 7075ScZr in accordance with the phase diagram shown in Fig. 1 [26] at a wide range of temperatures in the solidus liquidus; these probably block the coarsening of grains during SSM processing. It may be assumed that the above phenomenon only exists if suitable proportions of liquid phase and Al3(Sc, Zr) precipitates are present in the semi-solid range. However, conducting the thixoforming process in the Al-Li alloy with additions of Sc and Zr at the 40% contribution of liquid phase, Bunck et al. [24] observed almost an twofold increase in the grain size. This might have resulted from the fact that the amount of Al3(Sc, Zr) precipitates present in the liquid phase was too low in relation to the liquid fraction, which enabled structure coarsening. After the heat treatment of the 7075ScZr thixo-elements, a significant increase in hardness and tensile strength was achieved: for T61 - 482 MPa, and for T62 - 498 MPa, as compared with the state directly after thixoforming which was Rm 303 MPa. After the T6 heat treatment of the unmodified 7075 alloy (with saturation at 465 480°C for 4-17 hours, and aging at 120°C for 4 34 hours) some authors [4, 6, 10, 16, 50, 53, 54] obtained a tensile strength of 405-522 MPa. The differences in tensile strength may have resulted from technological parameters such as the amount of liquid fraction, the pressing force, as well as the presence of Al3(Sc, Zr) precipitates. It should be noted that the previously cited authors used a lengthier saturation time in comparison with here [54, 55], and in addition their alloys were not modified with Sc and Zr. Hence large primary precipitates were inhomogeneously distributed in the eutectic mixture in the thixo-casts. In addition the average size of the -aluminum spheroids was larger. The coarse microstructure and large precipitates increased the diffusion time of elements during saturation [10, 11].

20

The analysis of microstructure of thixo-elements after T61 (Fig. 15) shows that the greatest increase of mechanical properties is accompanied by a fine microstructure, consisting of metastable and coherent precipitates: ’ (formed during aging) [1, 2, 28] and Al3(Sc, Zr) dispersoids (which precipitated during saturation) [26, 28]. It should be noted that in the 7075ScZr alloy after the T61 or T62 heat treatment the grain size did not exceed 32 m, which was due to the primary Al3(Sc, Zr) phase present among grains.

5 CONCLUSIONS 1) The modification of molten 7075 aluminum alloy with 0.5% Sc and Zr is an effective route for feedstock preparation for the thixoforming process. The feedstock microstructure consisted of homogeneously distributed globular grains of solid solution of average size 23 μm. Al3(Sc, Zr) primary precipitates were identified inside the (Al) grains, of size near 5μm. 2) The microstructure of the thixo-formed parts of 7075Al with Sc and Zr consisted of primary globular grains surrounded by a fine mixture of secondary grains and intermetallic  - MgZn2 phase. The average hardness of thixoformed samples was 106 HV5, while the tensile strength attained was 300 MPa. 3) The T62 heat treatment (solution at 450°C for 10h and ageing 18h) caused a slight increase of grain size and the precipitation within the (Al) of two kinds of phases responsible for the increase of tensile strength and hardness: zirconium-scandium particles of the coffee bean shape Al3(Sc, Zr) and ’ MgZn2. The average thixo-formed part hardness increased up to 199 HV5. The tensile strength was 498 MPa with an elongation of 3.5%. 4) The measurement of the rheological properties of the 7075 alloy with Sc and Zr additions in the semi-solid state (fs=70%, shear rate 250 s-1) allowed the determination of the influence of the shearing time and particle size of the solid solution on the viscosity during isothermal 21

experiments. The increase of the particle size and spheroidization led to an observable decrease in viscosity during isothermal shearing. The steady state viscosity of the metal suspension depended on the amount of solid fraction and shear rate. The shear rate jump experiment indicated that the viscosity rapidly decreased with increasing shear rate. ACKNOWLEDGMENTS The research was supported by Polish science financial resources „Doctus” by the European Science Foundation (COST Action 541) REFERENCES [1]

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[50] A. Tietmann, T. Bremer, G. Hirt, in: M.C. Flemings, S.B. Brown (Eds.), Proceedings of the Second International Conference on Semi-Solid Processing of Alloys and Composites, Boston, USA, 10–12 June 1992, MIT Press, Cambridge, MA, 1992, 170-179. [51]

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Figure Captions: Figure 1 Calculated section of the pseudo-binary phase diagram for composition near 7075 aluminium alloy with Sc and Zr additions (total amount 0,5 wt.%) and with Mg content (0-8%) [26]. The dashed line shows content of Mg corresponding to the studied aluminium alloy composition. Figure 2 The heating curve of billets in the thixo-process (two different thermocouple positions are marked) Figure 3 Microstructure of the as cast 7075ScZr feedstock for thixoforming and quantitative analysis of the grain size distribution Figure 4 SEM-BSE microstructure of starting material made of 7075ScZr alloy Figure 5 Liquid fraction vs. temperature curves for the 7075ScZr alloy obtained from heat flow vs temperature data recorded during heating from the solid state Figure 6 a) Temperature and viscosity at constant shear rate 250 s-1 of 7075ScZr alloy for 70% solid fraction with microstructure from point 1 and 2 marked in viscosity curve, b) DSC analysis for cooling rate 4°C/min of 7075ScZr alloy, 6.1 Frozen microstructure after shearing for 3 min, 6.2 Frozen microstructure after shearing for 60 minutes Figure 7 a) Viscosity as a function of shearing time at constant shear rate 250 s-1 and various the temperatures as well as the solid fractions; 54% - 630°C and 70% - 625°C Figure 7 b) Viscosity as a function of shearing time for constant temperature: 625°C for 7075Al with Sc and Zr (fs=70%) partially solidified with shear rates 100 s-1 and 250 s-1. Figure 8 Steady state viscosity curves of 7075ScZr for 54% and 70% of solid fraction . Figure 9 Thixo-formed parts of rotors Figure 10 The rotor thixo-formed part with marked direction of flow in the die a) cross-section of rotor thixo-formed part, b) microstructure of the heat centre c) microstructure of rotor blade, d) microstructure of the rotor cross-section centre . Figure 11 a) SEM-BSE microstructure of the centre of thixo-formed rotor part,

26

Figure 11 b) TEM micrographs thixo-formed part and SAEDP as an insert with indexation of reflections from the same place Figure 12 DSC curves for heating thixo-formed part after annealing, supersaturation at 450°C for 0,5h hour and 450°C for 10h

Figure 13 The rotor thixo-formed part after heat treatment (solutionised at 450°C for 0,5h and aged at 120°C for 18h), a) rotor thixo-formed part cross-section, b) quantitative analysis of grain size distribution, c) microstructure of rotor blade, d) microstructure of the rotor centre cross-section Figure 14 SEM-BSE microstructure of 7075ScZr thixo-formed parts after heat treatment (solutionised at 450°C for 0,5h and aged at 120°C for 18 h) Figure 15 a) TEM micrographs of 7075ScZr thixo-formed part after heat treatment (soluted at 450°C for 0,5h and aged at 120°C for 18h) and SAEDP as an insert with indexed reflections Figure 15 b) HRTEM image of 7075ScZr thixo-formed parts after heat treatment (soluted at 450°C for 0,5h and aged at 120°C for 18h) taken at [011] (Al) zone axis. Figure 16 Block diagram representing tensile strength and plastic deformation of thixoformed 7075Al with Sc and Zr sample in the as thixoformed and after T6 heat treatment at given temperatures. Figure 17 a) Optical micrograph for the fracture profile after static tensile test of 7075ScZr thixo-formed part after T6 treatment (450°C for 10 h and ageing at 120°C for 18h), Figure 17 b) The SEM micrograph of the fracture surface

27

Tables caption Table 1 Results of EDS analysis of 7075ScZr feedstock Table 2 Results of EDS analysis of 7075ScZr thixoformed element Table 3 EDS analysis of 7075ScZr thixoformed element after T61 Table 4 Results of hardness: feedstock, thixoformed element directly after processing and after T61 and T62 heat treatment

28

Table 1 Results of EDS analysis of 7075ScZr feedstock Content [weight %] Area of analysis Mg

Cu

Zn

Sc

Zr

Al

1

1.9 ±0.2

1.0±0.1

5.5±0.2

0.1±0.05

0.2±0.1

91.6±1.8

2

15.2±0.6

0.9±0.45

75.7±1.5

-

-

8.2±0.3

3

1.5±0.1

53.4±1.1

3.1±0.3

-

-

42.0±0.8

4

1.3±0.1

21.6±0.4

4.5±0.4

9.9±0.4

10.6±0.4

52.1±1.1

Table 2 Results of EDS analysis of 7075ScZr thixoformed element Content [weight %] Area of analysis

Mg

Cu

Zn

Sc

Zr

Al

1

2.0±0.1

1.1±0.1

3.7±0.1

0.3±0.05

0.4±0.04

92.5±0.9

2

17.2±0.1

27.8±0.2

34.6±0.2

-

-

20.4±0.3

3

1.8±0.1

0.6±0.04

3.0±0.1

5.5±0.1

6.2±0.1

82.9±0.8

Table 3 EDS analysis of 7075ScZr thixoformed element after T61 Area of analysis

Content [weight %] Mg

Cu

Zn

Sc

Zr

Al

1

3.2±0.3

0.9±0.4

6.9±0.3

-

-

89.0±1.8

2

1.5±0.1

56.9±1.3

5.6±0.2

-

-

36.0±0.6

3

0.7±0.35

19.3±0.4

3.8±0.4

10.8±0.4

13.1±0.5

52.3±1.1

29

Table 4 Results of hardness: feedstock, thixoformed element directly after processing and after T61 and T62 heat treatment Technological state of 7075ScZr

Average hardness [HV5]

Feedstock 7075ScZr

Thixo-formed element

129±5

101±6

Thixo-formed elements after heat treatment: (T61) 450°C/0.5h; 120°C/18h

(T62) 450°C/10h; 120°C/18h

184±3

198±6

30

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7a

Figure 7b

Figure 8

Figure 9

Figure 10

Figure 11a

Figure 11b

Figure 12

Figure 13

Figure 14

Figure 15a

Figure 15b

Figure 16

Figure 17a

Figure 17b