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Effect of Iron Content on 319 Semi-Solid Alloys using the SEED Method
Samuel, E.; Garat, M.; Major, J. F.
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Effect of Iron Content on 319 Semi-Solid Alloys using the SEED Method
E. Samuel
National Research Council of Canada, Aluminum Technology Centre, Chicoutimi, Canada
M. Garat
1, J.F. Major
21Rio Tinto Alcan, Centre de recherche de Voreppe, Voreppe, France 2Rio Tinto Alcan, Arvida Research and Development Center, Jonquiere, Canada
Copyright 2011 American Foundry Society
ABSTRACT
Aluminum-silicon alloys represent a majority of cast alloys used in the automotive industry, as these alloys can be molded into complex shapes with relative ease. Cast aluminum-silicon-copper alloys, or 319 aluminum alloys, for example, are a popular choice in the fabrication of engine blocks. The addition of copper to Al-Si aids in maintaining the casting integrity when in service (with temperatures exceeding 200°C (392F)). Although numerous publications have explored the use of cast 319 alloys, there is still very little in terms of literature with respect to semi-solid cast 319 alloys. Semi-solid casting, where an alloy is prepared as a slurry, is fast becoming a popular casting method due to the dual liquid-solid nature of the resulting billet. The billet remains still, as if solid, yet flows under the action of an applied shear force. A semi-solid rheocast 319 aluminum alloy was prepared for this study using the SEED (Swirled Enthalpy Equilibration Device) casting method, as developed by Rio Tinto Alcan in collaboration with the Aluminium Technology Centre of NRC Canada. This process uses heat extraction of the liquid aluminum alloy via agitation (swirling) in a confined cylinder to form the semi-solid billet. The resulting microstructure consists of alpha aluminum globules surrounded by the eutectic phase. In this work, the iron content of the 319 semi-solid alloy was altered, and the changes in microstructure and mechanical properties were observed. The chief objective of this ongoing study is to use recycled aluminum (with iron traces) for future casting, as opposed to higher purity aluminum. The SEED process itself has proven successful in obtaining high integrity 356/357 castings, among others; therefore, it is also the authors' intent to develop SEED as a fully industrial process.
Key Words: aluminum casting, semi-solid, rheocasting,
SEED process
INTRODUCTION
Aluminum-silicon alloys are a widely used category of cast aluminum alloys that have been put into practice as the successor to gray cast iron in the realm of shape casting in the automotive industry, by virtue of being low
in density, while maintaining adequate strength [1]. The aluminum-silicon-copper, or '319' (nominal composition (ASM, 1990): 3.0-4.0%Cu, 0.1%Mg max, 0.5%Mn max, 5.5-6.5%Si, 1.0%Fe max, 1.0%Zn max, 0.25%Ti max, 0.35%Ni max, 0.5% others (total) max, Al balance), subset is particularly used in the fabrication of engine blocks. The addition of copper helps to maintain a favourable level of strength in the cast part in service, where temperatures can surpass 200°C (392F) [2]. Several investigators [3-11] have tried to optimize the mechanical properties of 319 aluminum alloys by using T5 and T6 heat treatments. In doing so, the yield and tensile strengths were seen to increase, with an adequate (although sometimes low) value of elongation. The presence of iron in 319 aluminum alloys often results from the use of recycled material, in an attempt to reduce processing costs. However, the presence of Fe can lead to detrimental effects on the elongation of 319 tensile samples due to the formation of β-Al5FeSi ('β-Fe') needles in the microstructure [2, 9, 12-16]. The size of these needles is a function of the iron content in the alloy, with further reductions observed in mechanical properties for increased iron additions. Although the use of primary aluminum stock to cast any aluminum alloy ensures a high level of purity, it is the authors' eventual aim to use recycled aluminum, i.e. containing Fe, in the casting of future alloys.
Although much research has been devoted to the use of cast 319 alloys in terms of casting, microstructure and mechanical properties, there are fewer publications regarding the use of semi-solid cast 319 alloys [5-7]. Semi-solids, exhibiting flow properties resembling both solid and liquid, have found an increased level of interest in the automotive industry owing to their ability to be formed into high integrity castings with a much lower potential for major casting defects and flaws such as gas porosity, shrinkage, etc. The 319 alloys (AlSi6Cu3Mg) used in this study were prepared using SEED. The SEED process [17, 18] is a novel semi-solid rheocasting procedure which allows the slurry to be formed on site. In SEED, the liquid molten aluminum is essentially stirred via mechanical agitation in a metallic cylinder, thus allowing the material to cool to slurry. The resulting semi-solid billet is then ready to be cast into shape. This
flexibility enables one to avoid the extra processing steps and costs involved in thixocasting such as transportation, storage and reheating of the semi-solid billets. Moreover, there is a reduced exposure to the air (i.e. during transportation or storage of the thixocast billets), resulting in a more sound casting, free of oxides.
Although still not extensively used in the mass production of cast parts, the SEED method has proven successful in the case of 356/357 and 6061 alloys, among others, with sound castings and good mechanical properties being reported [19-24]. From the work of Lemieux et al. [23], for example, the SEED processed aluminum alloy billets were found to exhibit very good die filling capabilities, owing to the uniform globular microstructure.
EXPERIMENTAL PROCEDURE
The 319 alloys (AlSi6Cu3Mg, with or without Fe) were cast using the SEED process and a high pressure die-cast (HPDC) press at the Aluminium Technology Centre (ATC-NRC) facility in Chicoutimi, Quebec, Canada. A general schematic of the steps involved in the SEED process is given in Figure 1. Any recycled material can be reused in the cycle shown in Figure 1.
Fig. 1. SEED rheocasting process (a-d) with an HPDC press (e) [23].
In the present study, wedge shape plates were cast; from these, round bar tensile test samples were machined along the width of the plates, according to the ASTM B557M standard, with a gage length of 25 mm (2 inches) and a gage diameter of 6.35 mm (¼ inches)). It was also the authors' goal to obtain a minimum tensile yield strength and tensile strength of 240 and 300 MPa, respectively, with the highest possible amount of elongation. Although the wedge mold varies in thickness from end to end, the tensile samples were machined with the same dimensions. A general schematic of the tensile sample locations along the wedge is shown in Figure 2.
Fig. 2. Tensile sample locations along wedge mold.
The chemical compositions of the three cast alloys (0.09, 0.4, 0.6%Fe) are given in Table 1.
Table 1. Chemical composition (wt%) of the 319 alloys used in this study (Al = balance)
Si Mg Cu
6.46±0.05 0.19±0.02 2.80±0.10
Fe Ti Sr
0.09, 0.40, 0.60 ≤ 0.10 0.004±0.0005 Casting was carried out using a 750-kg capacity electrical resistance furnace, and the mold was kept heated at 240-300°C (464-572F). A filling speed and injection pressure of 0.1 m/s and 900 bars, respectively, were also used. The cast plates were subjected to a T7 heat treatment comprising of a 5-hour solution heat treatment at 495°C (923F), followed by a room temperature water quench and a 4-hour artificial aging step at 210°C (410F). Tensile tests were carried out using a 50-kN capacity MTS servohydraulic tensile tester. Ten to fifteen samples were tested for each alloy, and the average yield and tensile strengths, with error, were calculated at each 0.5% interval of strain, starting at 1% (it was found that the yield point occurred at a strain of ~0.6%). The elongation itself was recorded using an extensometer attached to the testing apparatus. Scanning electron microscopy was carried out, in order to image the blocky CuAl2 and β-Al5FeSi needle phases, using a Hitachi SU-70 field emission scanning electron microscope coupled with an Inca 300 series EDX detector. Samples for microscopy were taken from a point just below the tensile fracture area, ground and polished using diamond suspension.
RESULTS AND DISCUSSION
As the SEED process has established itself in the casting of semi-solid aluminum alloys such as 356/357 and 6061 [19-24], it was only natural to extend the existing work on SEED to other materials such as 319 which already has an established reputation in the automotive industry. By implementing such a cost-effective casting method, the resulting aluminum castings can then be heat treated and mechanically deformed.
TENSILE PROPERTIES
The mechanical property behavior of the SEED processed 319 semi-solid alloys was ascertained using room temperature tensile tests. Results were obtained in the form of force-displacement data, which were then converted into stress-strain curves. It was observed that the samples strain hardened, without ever necking. Therefore, during the course of this work, the term 'tensile strength' will be used in place of 'ultimate tensile stress' to denote the maximum tensile strength achieved, at a given strain. Stress-strain curves were extrapolated at various points along the strain axis, at intervals of 0.5% strain. The yield strength was recorded for all tests carried out, per alloy. It was found that the yield strain occurs at
~0.6%. Moreover, the average yield strength (± error), as a function of the iron content of a given alloy, was found to be comparable from one alloy to the next. This data is shown in Figure 3, where the squares represent the average of 10-15 tests.
Fig. 3. Tensile yield strength vs. iron content for 319 alloys used in this study.
The tensile strength vs. strain behavior for the 319 alloys is shown in Figure 4, where, again, the squares represent the average of 10-15 tests. As can be seen, the curves generated for all three alloys overlap each other. Therefore, at a given strain, the values of tensile strength from one alloy to another are comparable. The primary difference is that there is a reduction in ductility from 319+0.09%Fe to 319+0.4%Fe to 319+0.6%Fe. Although only one point was obtained for the 319+0.09%Fe at 7.5% strain as well as for the 319+0.6%Fe at 2% strain, the authors felt these were worth showing, in terms of the alloys' potential ductility. With respect to the other data points shown, the error in each point is approximately ±10-12 MPa. The best combination of properties was found in the 319+0.09%Fe alloy, having a maximum
repeatable ductility of 7% strain with an associated tensile strength of ~360±6 MPa. This is remarkable since normally, an T7/overaged alloy can result in a marked decrease in stress, compared to T6/peak aged alloys (Table 2).
Considering the brittle nature of this alloy, the 319+0.4%Fe alloy still maintained a favourable level of strain at 4.5% with an associated tensile strength of ~353±12 MPa. For the same alloy with 0.6%Fe, however, there is a marked decrease in tensile stress owing to the increased brittleness of the alloy and decreased ductility. Table 2 illustrates how the mechanical properties obtained in this work compare to those found in the literature.
Table 2. Comparison of 319 alloys mechanical properties
Alloy YS
(MPa) (MPa)UTS %El. (%)
Semi-solid 319(+0.09%Fe)-T7a 283±9 360±6 7 Semi-solid 319(+0.4%Fe)-T7a 282±9 353±12 4.5 Semi-solid 319(+0.6%Fe)-T7a 282±8 321±6 1.5 Semi-solid 319-Fb (modified) 128 307 18.6 Semi-solid 319-T4b (modified) 234 370 13.4 Semi-solid 319-T5b 224 327 5.2 Semi-solid 319-T6b (modified) 299-351 387-409 4.8-10.2 Semi-solid 319-T5c (modified) 237 292 2.8 Semi-solid 319-T6c (modified) 334 379 1.6 Semi-solid 319-T6d (modified) 320 405 5.0 Sand cast 319-Fe 125 185 2.0 Sand cast 319-T5e 179 207 1.5 Sand cast 319-T6e 165 250 2.0 Permanent mold 319-Fe 130 235 2.5 Permanent mold 319-T6e 185 280 3.0
a: present work, b: Bergsma et al. (2001), c: Bergsma & Kassner (1999), d: Garat (1998, 2000), e: ASM vol 2 (1990)
As can be seen from Table 2, the values obtained in this work demonstrate very favourable combinations of strength and ductility, especially at Fe levels of 0.09% and 0.4%. Moreover, although semi-solid 319-T6 alloys demonstrate greater values of tensile yield and tensile strength, it is usually at the expense of ductility. At peak aging (Figure 5), elongation is at its minimum. However, in keeping with our objective of attaining tensile yield strength and tensile strength values of 240 and 300 MPa, respectively, the authors are very satisfied with the results obtained for the SEED-processed 319 alloy samples. The 319 alloy has been reported to exhibit issues with thermal growth and dimensional stability [2, 3] when a given cast part is put into service; these cast parts were noted to have undergone changes in volume of up to 0.1%. In parts requiring strict dimensional tolerances, this is enough to severely affect the performance of the parts in question. It is thought that these changes in dimension are due to a supersaturation of Cu after solution heat treatment. Due to the various elements found in 319, the complex precipitation reactions are often treated as co-precipitation [25], using the binary Al-Cu system:
αss→ GP zones → θ'' → θ' → θ
Under peak aging conditions, the thermodynamically unstable θ' phase forms, yet with further aging time (i.e. overaging), this phase disappears and is replaced by the stable θ (CuAl2) phase. Given this finding, a T7 heat treatment was adopted, where the overaging temperatures allowed the cast part to maintain its shape [2, 3]. From the work of Mancha-Molinar et al. [2], based on HRTEM observations, it was found that the precipitation of secondary phases was accompanied by corresponding changes in sample dimensions. These 'thermal growth' changes were distinct from reversible dimensional changes caused by thermal expansion. However, little research exists to elaborate on the mechanis responsible for thermal growth in aluminum alloy systems.
In the case of the present work, dimensional stability effects were not taken into consideration, given that mechanical tests were carried out on machined samples from cast plates, rather than on cast parts. Moreover, a T7 heat treatment was used to allow for a compromise between strength, usually achieved through peak aging, and ductility, which increases during underaging or overaging, and is lowest at peak aging (however, overaging at too long a time results in a drastic decrease in strength (Figure 5)).
METALLOGRAPHY
In the 319+0.09%Fe alloy, evidence of the blocky CuAl2 phase was observed. This phase can exist in either a fine, eutectic form or a coarser, blocky form, where the latter has been observed in cast 319 alloys that have been modified with strontium [9]. Strontium is added to many Al-Si cast alloys in order to alter the morphology of the
Fig. 5. Hardness vs. temperature schematic outlining the three aging regimes.
acicular Si phase, so as to improve the mechanical properties of the alloy [2, 26]. However, it has been reported [9] that the presence of Sr promotes the segregation of Cu, often resulting in the formation of a blockier form of the CuAl2phase. Therefore, although the addition of Sr is used to improve mechanical properties by modifying the acicular Si into a finer form, the presence of a blocky CuAl2 phase restricts this potential improvement in strength. As can be seen from Figure 6, the presence of a blocky CuAl2 phase (white) is noted. Moreover, the CuAl2particles are seen to be within the vicinity of each other. This may be due to the reported segregation of Cu due to the addition of strontium [9, 26].
Fig. 6. Typical microstructure (600X) of the 319+0.09%Fe alloy. The micrograph reveals the presence of the blocky CuAl2 phase with no strong
evidence for the formation of β-Fe needles.
As mentioned before, the presence of Fe can lead to detrimental effects on the elongation and peak stress due to the formation of β-Al5FeSi ('β-Fe') needles [2, 9,
12-16]. The size of these needles is a function of the iron
content in the alloy. According to the work of Li et al. [9], a base 319 alloy with up to 0.4%Fe will demonstrate adequate values of stress and ductility, as the β-Fe needles which form are uniformly distributed throughout the alloy. This supports the observations made in Figure 4
where the ductility of the alloys decreases with the addition of Fe.
In Figures 7 and 8, the β-Fe needles, distributed throughout the microstructure, are only evident when an adequate amount of Fe is present. Figure 7 shows the presence of needles in the 319+0.4%Fe and 319+0.6%Fe alloys, respectively. The micrographs of Figure 8 reveal a stronger presence of β-Fe needles than in Figure 7.
Fig. 7. Typical microstructure (600X) of the 319+0.4%Fe alloy. The presence of β-Fe needles (19±6 μm) is evident in this alloy.
Fig. 8. Typical microstructure (600X) of the 319+0.6%Fe alloy. The presence of β-Fe needles (37±9 μm) is evident in this alloy.
A large portion of the β-Fe needles observed for the 319+0.6%Fe alloy are much longer than for the 319+0.4%Fe alloy. This is due to the increased iron content, which influences the size of the needles. This adds to the overall reduction in mechanical properties of the alloy with increased iron content. In terms of needle length, the values obtained were 19±6 μm and 37±9 μm for the 319+0.4%Fe and 319+0.6%Fe alloy, respectively. These values are based on 250 measurements taken for the 319+0.4%Fe, as well as the 319+0.6%Fe alloy. Moreover, it was noted that the concentration of β-Fe
needles was greater in the 319+0.6%Fe alloy than in the 319+0.4%Fe, thus further contributing to the overall brittleness and reduced mechanical properties of the 319+0.6%Fe alloy.
CONCLUSIONS
The use of Al-Si-Cu alloys in the automotive field has sparked a lot of interest in terms of its optimization. Many investigators have looked into improving the alloy's performance with respect to strength and ductility. Although the presence of iron introduces some difficulty in achieving a suitable combination of properties, especially at 0.6%Fe, it was found that the alloy at 0.09% and 0.4%Fe resulted in a favourable combination of strength and ductility. This implies that one can substitute a 319 aluminum alloy with little (0.09%) to no iron with one containing up to 0.4%Fe, for example, and still maintain a favourable level of tensile strength and strain. In summary, after having carried out this study, it was observed that:
1. The tensile yield strength behavior with iron addition shows no noticeable difference from one alloy to another. The yield strain was observed to occur at ~0.6%.
2. The increase in iron content shows no noticeable change in tensile strength from the 319+0.09%Fe to the 319+0.4%Fe alloy (and to a lesser extent, the 319+0.6%Fe alloy). However, the ductility reduces from one alloy to the next as the iron content is increased.
3. In spite of the reduction in strain with Fe, a tensile strength of ~360±6 MPa and strain of 7% was attained in the 319+0.09%Fe alloy. With an increase in Fe content to 0.4%, a tensile strength of ~353±12 MPa and strain of 4.5% was observed.
4. For an alloy containing 0.6%Fe, however, the strain is drastically reduced from 7-7.5% to 1.5-2%.
5. The CuAl2 phase, primarily observed in the 319+0.09%Fe alloy, was seen to form as a blocky phase and cluster together with other CuAl2phase particles.
6. The presence of β-Al5FeSi needles was observed primarily in the 319 alloys with 0.4% and 0.6% Fe. From a count of 250 needles for each of these two alloys, these needles were found to measure 19±6μm and 37±9μm in length for the 319+0.4%Fe and 319+0.6%Fe alloy, respectively. With an increase in Fe (0.6%), these β-Fe needles were seen to be greater in length and concentration in the 319+0.6%Fe alloy, thus contributing to the increased
brittleness and decreased mechanical properties observed for the 319+0.6%Fe alloy.
ACKNOWLEDGMENTS
The authors would like to gratefully acknowledge support from the National Research Council Canada (NRC), Rio Tinto Alcan and STAS. The authors would also like to thank Chang-Qing Zheng, Dany Drolet, Marie-Eve Larouche, Helene Gregoire and Genevieve Simard and Alain Simard of the Aluminium Technology Centre (ATC-NRC, Chicoutimi (QC)) for their input, support and technical expertise and assistance.
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