Influence of Mg Content on the Microstructure and Solid Solution Chemistry of Al-7%Si-Mg Casting Alloys During Solution Treatment

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1 Citation & Copyright (to be inserted by the publisher ) Influence of Mg Content on the Microstructure and Solid Solution Chemistry of Al-7%Si-Mg Casting Alloys During Solution Treatment J.A. Taylor 1, D.H. St John 1, J. Barresi 2, M.J. Couper 2 1 Cooperative Research Centre for Cast Metals Manufacturing (CAST), The University of Queensland, Brisbane, QLD 4072, Australia 2 Comalco Research & Technical Support (CRTS), Comalco Aluminium Ltd., Thomastown, VIC 3074, Australia. Keywords: Aluminium foundry alloys, iron intermetallics, T6 heat treatment, phase transformation, magnesium content, iron content, tensile properties Abstract The solution treatment stage of the T6 heat-treatment of Al-7%Si-Mg foundry alloys influences microstructural features such as Mg 2 Si dissolution, and eutectic silicon spheroidisation and coarsening. Microstructural and microanalytical studies have been conducted across a range of Sr-modified Al-7%Si alloys, with an content of 0.12% and Mg contents ranging from wt%. Qualitative and quantitative metallography have shown that, in addition to the above changes, solution treatment also results in changes to the relative proportions of iron-containing intermetallic particles and that these changes are composition-dependent. While solution treatment causes a substantial transformation of π phase to β phase in low Mg alloys ( %), this change is not readily apparent at higher Mg levels ( %). The π to β transformation is accompanied by a release of Mg into the aluminum matrix over and above that which arises from the rapid dissolution of Mg 2 Si. Since the level of matrix Mg retained after quenching controls an alloy s subsequent precipitation hardening response, a proper understanding of this phase transformation is crucial if tensile properties are to be maximised. INTRODUCTION Al-7%Si-Mg foundry alloys are extensively used in the T6 heat-treated condition for high integrity castings used in load-bearing applications. The 356 alloy typically contains 0.3% Mg, while 357 contains approximately 0.6% Mg. The grade of these alloys is determined by the amount of iron impurity present. Iron levels generally vary from 0.04% (best primary metal) to 0.50% (secondary metal), with typical primary levels in the 0.1 to 0.2% range. The microstructure of 356 and 357 alloys in the as-cast state consists of primary grains of α-al solid solution with interdendritic regions of Al-Si eutectic, in which various intermetallic phases such as Mg 2 Si, β-al 5 Si and π-al 8 Mg 3 Si 6 are present. The solution treatment stage of the T6 heat-treatment performs several important functions: dissolution of Mg 2 Si phase; homogenisation of the solid solution; and fragmentation, spheroidisation and coarsening of the eutectic silicon [1]. Dissolution of Mg 2 Si and homogenisation of the matrix occurs within 15 minutes in 356 alloy at 540 C, and within 50 minutes in 357 alloy [2]. The changes to eutectic silicon morphology are generally slower, taking up to several hours, and depend on parameters such as solution temperature and original particle size/shape which in turn are determined by solidification conditions, grain size and eutectic modification. The effect of solution treatment on the iron-containing intermetallic phases is not well understood, but it has been reported that β is stable in alloy 356 [3], and that π is stable in 357 but dissolves in 356 [3-5]. This paper seeks to examine these changes more closely. The tensile properties of Al-7%Si-Mg alloys are governed by various factors, including the level of Mg retained in the matrix after solution treatment and quenching and which is subsequently

2 2 Title of Publication (to be inserted by the publisher) available for precipitation during artificial ageing. This, together with actual ageing parameters controls the alloy s yield strength and strain hardening characteristics. High matrix Mg results in higher yield strength and increased rates of strain hardening. However, the actual point of tensile fracture (expressed by ductility, s f, and tensile strength, UTS) is controlled by the population of defects (porosity, oxides) [6] and second-phase particles (eutectic silicon, intermetallics) [7, 8]. The Quality Index, QI, which is used to compare the quality of castings made with alloys 356 and 357, is an empirical parameter that is derived from UTS and s f. Castings with low defect numbers and small defect sizes, together with low secondary dendrite arm spacing (SDAS, giving small particle size) and low content (giving less iron-intermetallics) display the highest QI. For a given heat treatment cycle, SDAS and content, alloys with 0.4 to 0.5% Mg display the best QI values [9]. It has recently been demonstrated that the QI can be given physical significance by equating it with the term relative ductility, q, which is the proportion of necking strain achieved at fracture [10]. This work explores the effects of Mg and content on the relative amounts of iron-containing intermetallic particles and the associated matrix chemistry that results from solution treatment and seeks to apply these findings to the observed tensile properties of the alloys. EXPERIMENTAL Cast plates of various alloy compositions were prepared using the Improved Low Pressure (ILP) casting process developed by Comalco Aluminium Ltd.[9, 11]. In this work, all alloys were Srmodified and grain refined with Al-5Ti-1B. The nominal composition of the alloys was Al-7%Six%Mg-0.12%, where x = 0.3, 0.4, 0.5, 0.6 or 0.7%. Samples were cut from each of the cast plates at positions with SDAS of 40 µm. Some as-cast samples were retained while others were given a standard T6 heat-treatment (6 hours at 540 C, followed by a water quench 80 C and then 8 hours at 165 C). Samples of each alloy (as-cast and T6 treated) were prepared to a high polish for metallographic analysis. The alloy microstructures, in particular, the intermetallic particles, were examined qualitatively by conventional optical microscopy and quantitatively using a dedicated quantitative metallography facility, supplemented by phase identification using SEM/EDX and solid solution matrix analysis by WDS electron probe micro-analysis (EPMA). Quantitative metallography measurements on each sample were taken using 40 fields of approx. 81 x 81 µm 2 each, at a magnification of 500 times. EPMA results (Si, Mg and contents) are based on measurements of at least 10 dendrite arm centres for each alloy sample. RESULTS Phase identification. The iron-containing intermetallics observed in the as-cast alloys are β phase (Al 5 Si) occurring as randomly distributed platelets and π phase (Al 8 Mg 3 Si 6 ) occurring in Chinese-script and blocky morphologies, both of which sometimes growg directly upon β platelets. These two phases occur in the interdendritic/intergranular eutectic regions. The same phases are observed in heat-treated samples 1, although there is a reduction in the amount of π phase, especially in the lower Mg alloys (0.3 and 0.4% Mg). In place of π phase, clusters of fine needles/platelets are observed (Fig. 1). The emergence of the clusters and the replacement of π phase is observed as a gradual process, and particles at various stages of 1 Chemical and morphological changes to intermetallic particles are understood to take place during the solution treatment stage of the T6 heat treatment. Subsequent ageing of the samples does not alter the nature of these phases in any macroscopic way. Therefore observations made by optical microscopy techniques on peak-aged samples reflect the changes that occurred during solution treatment only.

3 Journal Title and Volume Number (to be inserted by the publisher) 3 transformation may be observed in any one sample. EDS analysis reveals that the needles have spectra that are typical of β phase (Fig. 1). Al π Al β Mg Si Si 10 µm Fig. 1 Typical micrograph and EDS spectra of Chinese-script π phase in 0.5% Mg alloy that has undergone partial transformation to clusters of fine β needles during solution treatment. Quantitative metallography. Measurements were made of various particle parameters for β, π and Mg 2 Si phases, including number, area and aspect ratio. The original as-cast β platelets and the β needle clusters that form during solution treatment were not distinguished. The volume fraction of the phases present in each alloy, before and after heat treatment, has been equated to the respective phase area fractions using the assumption that each phase is randomly distributed throughout the microstructure. The values of phase volume fraction are plotted against Mg content in Fig 2. Fig. 2 shows that in the lower Mg alloys, solution treatment results in a substantial reduction in the volume fraction of the π phase and an associated increase in the amount of β phase. It can also be seen that, besides changes to phase ratios, there is a dramatic reduction in the overall volume fraction of iron-containing intermetallics after solution treatment. Other quantitative measures clarify these changes. Table 1 shows that the number of π particles drops and the number of higher aspect ratio, smaller area β particles increases markedly. In the high Mg alloys, solution treatment results in very little change from the as-cast situation in terms of volume fractions and particle numbers. Alloys with intermediate Mg level appear to show a transitional behaviour. TABLE 1 Numbers of intermetallic particles (40 fields) Mg content (nom. wt%) β phase (as-cast) β phase (heat-treated) π phase (as-cast) π phase (heat-treated)

4 4 Title of Publication (to be inserted by the publisher) Phase volume fraction (%) (a) As-cast alloys β π Mg2Si All Phase volume fraction (%) (b) Heat-treated alloys β π Mg2Si All Magnesium content (wt%) Magnesium content (wt%) Fig. 2 Volume fraction of β, π and Mg 2 Si phases in (a) as-cast, and (b) heat-treated alloys with varying Mg content (0.3 to 0.7%) and fixed content (0.12%). Solid solution microanalysis. WDS analysis of the chemical constitution of the α-aluminium solid solution dendrites in heat-treated alloys 2 indicates that the Si and Mg contents vary with alloy composition (Fig. 3). In Figure 3, the measured matrix compositions are shown as individual Mg/Si data points rather than average values. The level in the matrix of the alloys was typically low at wt%. Two lines, with differing Mg:Si ratio, are constructed in Fig. 3 to highlight that a change in heat-treated matrix chemistry seems to occur at a mid-range Mg content similar to that at which the change in the balance of iron-containing intermetallics occurs (Fig. 2b). The average values of matrix Mg content in the alloys are shown in Fig. 4, together with the predicted maximum levels of matrix Mg (based on nominal alloy content and a 0.62 % maximum solid solubility of Mg in an Al-7%Si alloy [5]) and the values of matrix Mg that have been estimated on the basis of phase volume fractions as determined by quantitative metallography. Matrix Si content (wt%) Mg 0.4Mg 0.5Mg 0.6Mg 0.7Mg Matrix Mg content (wt%) Fig. 3 EPMA-determined matrix compositions (Mg versus Si) for each measured dendrite of each alloy after heat treatment. The constructed lines indicate a possible change in behaviour at mid Mg levels similar to the transition point observed in Fig 2b. 2 Microanalysis of matrix compositions of as-cast samples was not examined in detail because of the complex solute segregation profiles that occur across the dendrite arms.

5 Journal Title and Volume Number (to be inserted by the publisher) 5 Matrix Mg content (wt% ) As-cast (est.) HT (est.) HT (meas'd) Predicted Nominal alloy Mg content (wt%) Fig. 4 Measured matrix Mg content versus nominal alloy Mg content, together with predicted maximum Mg levels and estimated levels based on measured phase fractions. DISCUSSION The results show that a phase transformation takes place during solution treatment of certain Al-Si- Mg alloys. The transformation can be best described as a process by which some particles of Mgcontaining π phase gradually break up, either partially or fully, into clusters of fine needles of another iron intermetallic phase that does not contain Mg (probably β phase, or one of similar constitution). The transformation goes almost to completion in alloys with 0.3 or 0.4% Mg, but becomes increasingly less likely to occur as Mg increases to 0.7%. The phase transformation in the low Mg alloys is accompanied by an overall reduction in total volume fraction of intermetallics. This is most likely due to a combination of the change in the atomic proportion of in the β and π phases (1 atom in 7 c.f. 1 in 18) and the increased density of the β phase compared to π (3.35 c.f [12]). The microanalytical data presented in Figs. 3 and 4 show that during solution treatment there is a release of Mg into the matrix and that the extent to which this occurs depends on the alloy composition. Some of the Mg comes from the readily dissolved Mg 2 Si phase but a proportion is also liberated as a consequence of the π to β transformation. In the low Mg alloys (0.3, 0.4%), it can be seen that the final matrix Mg concentration reaches the predicted limit for those alloys (Fig. 4, dotted line), meaning that all the as-cast Mg 2 Si has dissolved and the π phase has transformed to the non-mg containing β phase, together releasing all their Mg to the matrix. As the alloy Mg content increases, it can be seen that it becomes increasingly more difficult for the matrix Mg levels to reach their predicted potential limit. Since very little Mg 2 Si remains after solution treatment in even the highest Mg alloys, this limiting behaviour appears to be due to an increased stability of the π phase. The difference between the measured and estimated Mg matrix levels in heat-treated alloys (Fig. 4, lines with closed and open triangles respectively) probably stems from quantitative metallography experimental errors associated with distinguishing partially-transformed π particles from β particles. It must be noted that the predicted limits of matrix Mg are based on data from the Al-Si-Mg ternary diagram and this does not take into account the effect of the 0.12% in this quaternary system. Preliminary calculations using Thermocalc software [13] for the quaternary alloy compositions have indicated that the equilibrium levels of matrix Mg after a 540 C solution treatment are actually of the order obtained experimentally by microanalysis. It furthermore indicates that the iron-rich intermetallic phases observed in these alloys after heat treatment are

6 6 Title of Publication (to be inserted by the publisher) those predicted by the software. Possible explanations as to why the π to β phase transformation occurs in the low Mg alloys, while the π phase is more stable in the higher Mg alloys are presented elsewhere [14]. It is expected that the solution temperature may play an important role in determining the extent to which the π to β transformation takes place and the rate at which it occurs. However, it remains to be seen whether further Mg can be driven into solution in the higher Mg alloys (as suggested by ternary phase diagram predictions) through heat treatment manipulation, or whether the levels obtained have already attained equilibrium (as suggested by Thermocalc). Related research work on mechanical properties of these alloys [9] has also shown that optimum tensile properties (as expressed by Quality Index values) occur in the same % Mg alloy range in which partial π to β transformation occurs. Given that tensile fracture is governed in part by particle populations, these gains may result from the loss of the large brittle π particles and their substitution by very fine β needles, as well as an overall reduction in the total volume fraction of intermetallic particles. CONCLUSIONS In addition to Mg 2 Si dissolution, homogenisation and morphological changes to eutectic silicon, the solution treatment stage of a T6 heat treatment also affects the nature and balance of the ironcontaining intermetallics in Al-Si-Mg casting alloys. In low Mg alloys (0.3, 0.4%), most of the π phase is transformed to small clustered needles of β phase and there is an accompanying release of Mg into solution that allows maximum predicted Mg matrix levels to be reached. In high Mg alloys, the π phase becomes more stable and transformation to β phase appears to be unfavourable. As a consequence, Mg is retained in the intermetallic particles and cannot be released into solution. Actual matrix Mg levels fall short of the potential levels predicted using ternary phase diagram data. References [1] D. Apelian, S. Shivkumar and G. Sigworth: AFS Trans., Vol 97 (1989), pp. 727ff. [2] P.A. Rometsch, L. Arnberg and D.L. Zhang: Int. J. Cast Metals Res., Vol 12 (1999), pp. 1ff. [3] G. Gustafsson, T. Thorvaldsson and G.L. Dunlop: Met. Trans, Vol 17A-1 (1986), pp. 45ff. [4] B. Closset and J.E. Gruzleski: Met. Trans., Vol 13A-6, (1982), pp. 945ff. [5] D.A. Granger, R.R. Sawell and M.M. Kersker: AFS Trans., Vol 92 (1984), pp. 579ff. [6] N.R. Green and J. Campbell: AFS Trans., Vol 102 (1994), pp. 341ff. [7] C.H. Caceres, C.J. Davidson, J.R. Griffiths and Q.C. Wang: Met. Mater. Trans, Vol 30A-10, (1999), pp. 2611ff. [8] O. Vorren, J.E. Evensen and T.B. Pedersen: AFS Trans., Vol 92 (1984), pp. 459ff. [9] J. Barresi, M. Kerr, H. Wang and M.J. Couper: to be presented, 104 th AFS Casting Congress, Pittsburgh PN, (2000). [10] C.H. Caceres: Inter. J. Cast Metals Res., Vol 10 (1998), pp. 293ff. [11] U.S. Patent , 03/29/94, Casting of metal objects. [12] L.F. Mondolfo: Aluminium alloys: Structure and properties, (Butterworth, London, 1976). [13] N. Saunders: Light Metals 97, TMS (1997), pp. 911ff. [14] J.A. Taylor, D.H. StJohn, M.J. Couper: Aluminum Trans., to be published, 2 parts.