- Email: [email protected]
Thixoforming of an automotive part in A390 hypereutectic Al–Si alloy
Journal of Materials Processing Technology 135 (2003) 271–277
Thixoforming of an automotive part in A390 hypereutectic Al–Si alloy P. Kapranosa,*, D.H. Kirkwooda, H.V. Atkinsonb, J.T. Rheinlanderb, J.J. Bentzenb, P.T. Toftb, C.P. Debelb, G. Laslazc, L. Maennerc, S. Blaisc, J.M. Rodriguez-Ibabed, L. Lasad, P. Giordanoe, G. Chiarmettae, A. Giesef a
Department of Engineering Materials, Thixoforming Group, University of Sheffield, Sheffield S1 3JD, UK b Risoe National Laboratory, Post Box 49, DK 4000 Roskilde, Denmark c Pechiney CRV, BP 27, 38340, Voreppe Cedex, France d CEIT, Paseo de Manuel Lardizabal, P.O. Box 15, 1555-20009 San Sebastian, Spain e Stampal S.p.A., Via Albenga 78, 10090 Cascine Vica, Rivoli, Torino, Italy f Honeywell Bremsbelag GmbH, Glinder Weg 1, D-21509 Glinde, Germany
Abstract Hypereutectic aluminium–silicon alloys offer the possibility of an in situ natural composite (the silicon acting as the reinforcing phase) with properties that make them attractive for a number of automotive applications. However, conventional casting techniques result in excessive growth of the silicon particles in the melt, which adversely affect the mechanical properties. Thixoforming allows hypereutectic Al–Si alloys containing 40–50% fraction liquid to be shaped into complex near net shape components, whilst keeping the silicon particle size quite fine. This paper describes the development of a series of hypereutectic alloys based on the A390 composition (17%Si, 5%Cu, 0.5%Mg), their thixoforming, their resulting microstructures and mechanical properties. Finally the thixoforming of an automotive component using an A390 alloy is also described. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Thixoforming; Hypereutectic; Mechanical properties; Automotive applications
1. Development of the alloys Hypereutectic alloys such as A390 (Al17Si4CuMg) and Al18SiCuNiMg exhibit several very specific and interesting properties, such as high wear resistance, high strength and hardness, and low thermal expansion coefficients. As a result, they are used in heavy wear applications, often at elevated or medium temperatures, such as in pistons, cylinder blocks and AC compressors. However, their use has always been handicapped by several difficulties: in particular, their high latent heat and consequent long solidification time resulting in die wear, difficulty in avoiding segregation of massive primary silicon particles, and generally their unfavourable shrinkage behaviour. Shaping hypereutectic alloys by the semi-solid route (thixoforming) should overcome most of these difficulties [1]. Through thixoforming, the casting temperature and heat
* Corresponding author. E-mail address: [email protected] (P. Kapranos).
content are very much reduced and homogeneously distributed primary silicon is to be expected. Moreover, the shrinkage of a semi-solid material is significantly reduced as compared with that of its liquid counterpart. As soon as the thixoforming route became a proven industrial process with hypoeutectic alloys of the Al7SiMg type, work on a thixotropic version of A390 was undertaken at Pechiney CRV, and the initial results being published in 1999 [2]. Recently, complementary research has been carried out in a European consortium program [3] in order to develop the A 390 composition or variations around it, such as nickel alloying, for the development of a thixoformed automotive brake drum part. This paper describes the initial main results of this research (Table 1). 1.1. Basic metallurgical principles Solidification microstructures of hypereutectic alloys contain primary silicon particles, which provide cast parts with a natural Al–Si composite structure, this feature being of interest in wear applications. During semi-solid
0924-0136/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 8 5 7 - 9
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P. Kapranos et al. / Journal of Materials Processing Technology 135 (2003) 271–277
Table 1 Experimental alloy composition Alloy
Si (%)
Cu (%)
Mg (%)
Ni (%)
Fe (%)
A B C
15.3 15.1 15.7
4.37 4.54 4.56
0.55 0.575 0.544
<0.42 0.2 0.890 0.25 4.10 0.25
Mn (%)
Zn (%)
Ti (%)
<0.005 <0.042 <0.042
<0.018 <0.048 <0.048
0.17 0.17 0.17
treatment, the primary silicon is not remelted. In the thixoforming route, the size and homogeneity of primary silicon depends mainly on the feedstock structure and only slightly on the further semi-solid treatment and thixoforming operations [1]. This slight dependence of cast parts on feedstock microstructure is a potential advantage to be exploited, insofar as fine and evenly distributed structure should be obtained more easily in a high solidification rate DCC billet than in any other foundry process. When reheating the feedstock to the semi-solid state, the rheology will depend on the morphology of the remaining solid phases. The remaining solid is composed partly of aluminium spheroids, primary silicon particles and mainly of a fraction of the aluminium eutectic phase not remelted during reheating. Preliminary work by Pechiney using A390 alloy has shown that favourable spheroidisation of the aluminium phase occurs as a result of natural separation and coalescence of eutectic constituents [2]. 1.2. Compositional aspects of the A390 thixo alloys Efficient primary silicon refinement in feedstock has been achieved with the conventional phosphorous addition taking advantage of the high solidification rates during DCC billet casting. Favourable rheological microstructures can be obtained in A390 alloy. Since the alloy contains Cu and Mg alloying elements, during the reheating and holding in the semi-solid state, the Cu and Mg enriched fraction of the eutectic (quaternary eutectic areas at the end of solidification) remelts while the rest of the eutectic remains solid but undergoes a major structural evolution. The former secondary a-Al dendrites become globular and the balance silicon is deposited on the pre-existing Si crystals, which grow in the process. The resulting microstructure resembles that of a particulate MMC, with Si instead of SiC particles.
Initial characterisation through optical microscopy, thermal analysis, and soundness control has shown the following: 1. leak-out problems during the continuous casting of feedstock make the manufacturing with high silicon content (20%) difficult, therefore a silicon content of 14–18% has been considered as a technological limit; 2. nickel additions of around 1% do not adversely affect the feasibility of feedstock production; 3. for a nickel content of 4%, however, soundness problems (cracks) and inappropriate structures (coarse intermetallic phases, coarse aluminium phase) have been clearly observed. Complementary experiments in small laboratory scale have shown that using low copper alloys cannot solve the cracking problem associated with the high nickel content.
2. Thixoforming at laboratory scale Non-dendritic feedstock produced by Pechiney was thixoformed at the University of Sheffield in graphite dies. A number of test specimens were produced in the three compositions: standard A390, A390 þ 1%Ni, and A390 þ 4%Ni. The process conditions of thixoforming these alloys were varied, i.e. injection speed, fraction liquid, and soaking time, in order to assess the effect of these process parameters on the quality of the final product. Components such as that shown in Fig. 1 were thixoformed and used for generating tensile specimens for mechanical testing.
3. Non-destructive testing of thixoformed products The thixoformed specimens, to be used for mechanical properties assessment, were examined by using X-ray techniques and ultrasonics at Risoe National laboratories, in order to assess their internal soundness. This lead to an initial assessment of internal soundness related to process
1.3. Feasibility test of A390 feedstock having different silicon contents and Ni additions In this part of the work, Pechiney CRV experimented with the feasibility of producing several hypereutectic compositions based on or around the standard A390 (Al–Si 17%) and produced them using a continuous casting pilot plant.
Fig. 1. Thixoformed component for tensile specimens.
P. Kapranos et al. / Journal of Materials Processing Technology 135 (2003) 271–277
273
Fig. 2. X-ray radiograph of thixoformed fingers, the one on the left at low fraction liquid and the one on the right under optimised conditions [2].
parameters, allowing the prediction of a process window. In addition it allowed the selection of sound specimens for the further mechanical testing. Although internal defects were found in some thixoformed specimens [2], the majority of tensile specimens generated were of sound internal structure as represented by the defect-free finger specimen shown in Fig. 2.
4. Heat treatment In order to define the T6 heat treatment conditions required for the three thixoformed alloys, first the onset of melting was calculated with a DSC 2920 (Universal V2.3C TA Instruments) equipment. Samples were analysed
under a microscope equipped with EDAX and Thermocalc calculations were made to identify and then study the stability and solubility of the intermetallic phases present in each alloy. The intermetallic phases involved are not the same in the three thixoformed alloys, but the onset of melting occurred at similar temperatures. Fig. 3 corresponds to the DSC thermogram of alloy A390 þ 1%Ni, showing that the onset of melting takes place at 513 8C. Thermocalc calculations suggest that it corresponds to the eutectic Al–CuAl2–Si– Cu2Mg8Si6Al5. In the case of A390 thixoformed alloy, the onset of melting was identified at 511 8C and for alloy A390 þ 4%Ni at 519 8C (in this later case it corresponds to Al7Cu4Ni intermetallic phase). From these results, in order to avoid partial melting, a temperature of 500 8C
Fig. 3. DSC Thermogram of alloy A390 þ 1%Ni, showing the onset of melting at 513 8C.
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P. Kapranos et al. / Journal of Materials Processing Technology 135 (2003) 271–277
Fig. 4. Variation of hardness as a function of ageing time: A390 at 180 8C and with Ni additions at 160 8C.
was adopted for the solution heat treatment of the three alloys. For the determination of the hold time, samples were solution heat treated at 500 8C in a salt bath for different times (up to 10 h) and quenched within 5 s, performing hardness measurements (at least 8) for each condition. After 3 h no further softening is measured in A390 thixoformed alloy, while in the case of Ni containing alloys a minimum time of 5 h is required. These two different soaking times were defined as adequate for the solution treatments at 500 8C. Concerning artificial ageing treatments, two temperatures were studied: 160 and 180 8C. To obtain the optimum time for artificial ageing, samples were kept for different times (up to 25 h) at the temperatures chosen and hardness measurements were done for each condition. The curves
obtained are shown in Fig. 4. The maximum hardness values were very similar for both temperatures, but at 160 8C the maximum was achieved for longer maintenance periods. From the figure, it can be observed that beyond the peak a very small reduction in hardness is measured. In view of the above results, a heat treatment at 180 8C for 4 h was selected for the three alloys.
5. Microstructural and mechanical testing Microstructural analysis of the thixoformed products has been carried out and the results are related to the process parameters and product properties. Fig. 5 shows the micrographs of the A390 alloy, illustrating the microstructural changes during processing.
Fig. 5. Microstructure of alloy A390: (a) as-received; and (b) as-thixoformed (100).
P. Kapranos et al. / Journal of Materials Processing Technology 135 (2003) 271–277
275
Fig. 6. Microstructures after reheating at 570 8C (100): (a) A390 þ 1%Ni; (b) A390 þ 4%Ni; and (c) A390.
A number of characteristic features can be deduced: (i) During thixoforming, major spherodisation of the Al matrix takes place. (ii) The globules are oblong in the direction of flow and have average sizes of 60–70 and 90–100 mm in the direction transverse to and parallel to the direction of flow, respectively. (iii) The globules contain small, 1–2 mm, silicon particles. (iv) The polyhedral primary silicon particles are homogeneously distributed in the matrix and there is no
change in their particle shape and size (30 mm) as compared with the raw alloy. (v) Eutectic silicon and intermetallic phases are seen between the globules. (vi) The different processing temperatures gave no notable differences in the microstructures. (vii) The crystalline phases as identified by X-ray diffraction analysis (XRD) were aluminium, silicon,
Table 2 Mechanical properties of thixoformed A390 alloys Alloy
E (GPa)
Y.S., R0.2 (MPa)
UTS, Rm (MPa)
Elongation, A5 (%)
Hardness (HRB)
As-thixoformed A 77.7 B 76.3 C 90.6
215 225 252
271 277 261
1.04 0.72 0.3
67 71 76
T6 A B C
NA NA 348
429 411 354
0.16 0.10 0.25
84 84 83
78.4 81.7 93.8
Fig. 7. Thixoformed brake drum.
276
P. Kapranos et al. / Journal of Materials Processing Technology 135 (2003) 271–277
Fig. 8. Brake drum: weight comparison for different materials/processes.
Al8Si6Mg3Fe, Al1.9CuMg4.1Si3.3, CuAl2, and traces of Mg2Si. The identified phases in alloy A before and after thixoforming were identical. The properties have been related to the process parameters and resulting microstructures. Table 2 gives an
example of values obtained from thixoformed specimens in the as-thixoformed and T6 conditions. If one considers the microstructures of the three different compositions employed in the project (Table 1), it can be seen that the increasing content of Ni appears to adversely affect the morphology of the microstructure (Fig. 6). The
Fig. 9. Brake drum: cost comparison for different materials/processes.
P. Kapranos et al. / Journal of Materials Processing Technology 135 (2003) 271–277
effect manifests itself as a general decrease in the spheroidicity of the microstructure, a factor that is crucial in thixoformability.
6. Automotive part An automotive part (brake drum) was thixoformed by Stampal S.p.A. using the A390 standard alloy. The part shown in Fig. 7 is in the T6 heat treated condition, weighs 1.7 kg, and is substituting for a cast iron part. The part offers lower weight and good wear resistance. The part has been bench tested at room temperatures, 200 and 350 8C.
277
8. Conclusions There are clear weight benefits to be gained from thixoforming a brake drum out of A390 as opposed to cast iron, however, there are higher costs involved and of course there are still the results of bench testing to be considered. Nevertheless, there is scope for cost reductions at all fronts as thixoforming develops further and makes inroads into new markets where no further cost reductions are expected, as with cast iron technology. This work is only a starting point, but nevertheless an exciting starting point in using a new near net shaping technology to produce lighter automotive parts in the continuous drive to reduce weight, save energy, and therefore manufacture with an environmentally friendly process.
7. Discussion Hypereutectic alloys, based around the standard A 390 (Al–17%Si) alloy, have been produced in the required spheroidal microstructure. However, the addition of Ni does affect the rheological behaviour of these alloys and as a result their behaviour when thixoformed. Clearly further development is necessary in the feedstock production route in order to optimise the microstructures, especially for the added Ni content alloys. Optimisation of heat treatment could possibly offer some improvement in the elongations obtained thus far. The thermal stability and wear properties of the thixoformed alloys have compared quite favourably with cast and squeeze-cast products tested as part of this work. Appropriate processing windows for thixoforming have been established for the various compositions tested, and the resulting mechanical properties are encouraging. Finally, a weight and cost comparison has been made for the automotive part produced, the results being shown graphically in Figs. 8 and 9, respectively.
Acknowledgements This study has been conducted within the EU BRITEEURAM project BE96-3652 (HAforAC). The fruitful discussions with partners of the consortium and the financial support of the European Commission are gratefully acknowledged.
References [1] P.J. Ward, et al., Semi-solid processing of novel MMCs based on hypereutectic aluminium–silicon alloys, Acta. Mater. 44 (5) (1996) 1717–1727. [2] M. Garat, S. Blais, L. Maenner, G. Laslaz, The thixotropic version of the A390 hypereutectic alloy, Hommes et Fonderie (298) (1999) 14–21. [3] P. Kapranos, J. Rheinlander, Quantitative NDE by X-radiography for optimisation of the thixocasting process, INSIGHT—J. Br. Inst. NDT 41 (1) (1999) 25–30.
Thixoforming of an automotive part in A390 hypereutectic Al–Si alloy P. Kapranosa,*, D.H. Kirkwooda, H.V. Atkinsonb, J.T. Rheinlanderb, J.J. Bentzenb, P.T. Toftb, C.P. Debelb, G. Laslazc, L. Maennerc, S. Blaisc, J.M. Rodriguez-Ibabed, L. Lasad, P. Giordanoe, G. Chiarmettae, A. Giesef a
Department of Engineering Materials, Thixoforming Group, University of Sheffield, Sheffield S1 3JD, UK b Risoe National Laboratory, Post Box 49, DK 4000 Roskilde, Denmark c Pechiney CRV, BP 27, 38340, Voreppe Cedex, France d CEIT, Paseo de Manuel Lardizabal, P.O. Box 15, 1555-20009 San Sebastian, Spain e Stampal S.p.A., Via Albenga 78, 10090 Cascine Vica, Rivoli, Torino, Italy f Honeywell Bremsbelag GmbH, Glinder Weg 1, D-21509 Glinde, Germany
Abstract Hypereutectic aluminium–silicon alloys offer the possibility of an in situ natural composite (the silicon acting as the reinforcing phase) with properties that make them attractive for a number of automotive applications. However, conventional casting techniques result in excessive growth of the silicon particles in the melt, which adversely affect the mechanical properties. Thixoforming allows hypereutectic Al–Si alloys containing 40–50% fraction liquid to be shaped into complex near net shape components, whilst keeping the silicon particle size quite fine. This paper describes the development of a series of hypereutectic alloys based on the A390 composition (17%Si, 5%Cu, 0.5%Mg), their thixoforming, their resulting microstructures and mechanical properties. Finally the thixoforming of an automotive component using an A390 alloy is also described. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Thixoforming; Hypereutectic; Mechanical properties; Automotive applications
1. Development of the alloys Hypereutectic alloys such as A390 (Al17Si4CuMg) and Al18SiCuNiMg exhibit several very specific and interesting properties, such as high wear resistance, high strength and hardness, and low thermal expansion coefficients. As a result, they are used in heavy wear applications, often at elevated or medium temperatures, such as in pistons, cylinder blocks and AC compressors. However, their use has always been handicapped by several difficulties: in particular, their high latent heat and consequent long solidification time resulting in die wear, difficulty in avoiding segregation of massive primary silicon particles, and generally their unfavourable shrinkage behaviour. Shaping hypereutectic alloys by the semi-solid route (thixoforming) should overcome most of these difficulties [1]. Through thixoforming, the casting temperature and heat
* Corresponding author. E-mail address: [email protected] (P. Kapranos).
content are very much reduced and homogeneously distributed primary silicon is to be expected. Moreover, the shrinkage of a semi-solid material is significantly reduced as compared with that of its liquid counterpart. As soon as the thixoforming route became a proven industrial process with hypoeutectic alloys of the Al7SiMg type, work on a thixotropic version of A390 was undertaken at Pechiney CRV, and the initial results being published in 1999 [2]. Recently, complementary research has been carried out in a European consortium program [3] in order to develop the A 390 composition or variations around it, such as nickel alloying, for the development of a thixoformed automotive brake drum part. This paper describes the initial main results of this research (Table 1). 1.1. Basic metallurgical principles Solidification microstructures of hypereutectic alloys contain primary silicon particles, which provide cast parts with a natural Al–Si composite structure, this feature being of interest in wear applications. During semi-solid
0924-0136/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 8 5 7 - 9
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P. Kapranos et al. / Journal of Materials Processing Technology 135 (2003) 271–277
Table 1 Experimental alloy composition Alloy
Si (%)
Cu (%)
Mg (%)
Ni (%)
Fe (%)
A B C
15.3 15.1 15.7
4.37 4.54 4.56
0.55 0.575 0.544
<0.42 0.2 0.890 0.25 4.10 0.25
Mn (%)
Zn (%)
Ti (%)
<0.005 <0.042 <0.042
<0.018 <0.048 <0.048
0.17 0.17 0.17
treatment, the primary silicon is not remelted. In the thixoforming route, the size and homogeneity of primary silicon depends mainly on the feedstock structure and only slightly on the further semi-solid treatment and thixoforming operations [1]. This slight dependence of cast parts on feedstock microstructure is a potential advantage to be exploited, insofar as fine and evenly distributed structure should be obtained more easily in a high solidification rate DCC billet than in any other foundry process. When reheating the feedstock to the semi-solid state, the rheology will depend on the morphology of the remaining solid phases. The remaining solid is composed partly of aluminium spheroids, primary silicon particles and mainly of a fraction of the aluminium eutectic phase not remelted during reheating. Preliminary work by Pechiney using A390 alloy has shown that favourable spheroidisation of the aluminium phase occurs as a result of natural separation and coalescence of eutectic constituents [2]. 1.2. Compositional aspects of the A390 thixo alloys Efficient primary silicon refinement in feedstock has been achieved with the conventional phosphorous addition taking advantage of the high solidification rates during DCC billet casting. Favourable rheological microstructures can be obtained in A390 alloy. Since the alloy contains Cu and Mg alloying elements, during the reheating and holding in the semi-solid state, the Cu and Mg enriched fraction of the eutectic (quaternary eutectic areas at the end of solidification) remelts while the rest of the eutectic remains solid but undergoes a major structural evolution. The former secondary a-Al dendrites become globular and the balance silicon is deposited on the pre-existing Si crystals, which grow in the process. The resulting microstructure resembles that of a particulate MMC, with Si instead of SiC particles.
Initial characterisation through optical microscopy, thermal analysis, and soundness control has shown the following: 1. leak-out problems during the continuous casting of feedstock make the manufacturing with high silicon content (20%) difficult, therefore a silicon content of 14–18% has been considered as a technological limit; 2. nickel additions of around 1% do not adversely affect the feasibility of feedstock production; 3. for a nickel content of 4%, however, soundness problems (cracks) and inappropriate structures (coarse intermetallic phases, coarse aluminium phase) have been clearly observed. Complementary experiments in small laboratory scale have shown that using low copper alloys cannot solve the cracking problem associated with the high nickel content.
2. Thixoforming at laboratory scale Non-dendritic feedstock produced by Pechiney was thixoformed at the University of Sheffield in graphite dies. A number of test specimens were produced in the three compositions: standard A390, A390 þ 1%Ni, and A390 þ 4%Ni. The process conditions of thixoforming these alloys were varied, i.e. injection speed, fraction liquid, and soaking time, in order to assess the effect of these process parameters on the quality of the final product. Components such as that shown in Fig. 1 were thixoformed and used for generating tensile specimens for mechanical testing.
3. Non-destructive testing of thixoformed products The thixoformed specimens, to be used for mechanical properties assessment, were examined by using X-ray techniques and ultrasonics at Risoe National laboratories, in order to assess their internal soundness. This lead to an initial assessment of internal soundness related to process
1.3. Feasibility test of A390 feedstock having different silicon contents and Ni additions In this part of the work, Pechiney CRV experimented with the feasibility of producing several hypereutectic compositions based on or around the standard A390 (Al–Si 17%) and produced them using a continuous casting pilot plant.
Fig. 1. Thixoformed component for tensile specimens.
P. Kapranos et al. / Journal of Materials Processing Technology 135 (2003) 271–277
273
Fig. 2. X-ray radiograph of thixoformed fingers, the one on the left at low fraction liquid and the one on the right under optimised conditions [2].
parameters, allowing the prediction of a process window. In addition it allowed the selection of sound specimens for the further mechanical testing. Although internal defects were found in some thixoformed specimens [2], the majority of tensile specimens generated were of sound internal structure as represented by the defect-free finger specimen shown in Fig. 2.
4. Heat treatment In order to define the T6 heat treatment conditions required for the three thixoformed alloys, first the onset of melting was calculated with a DSC 2920 (Universal V2.3C TA Instruments) equipment. Samples were analysed
under a microscope equipped with EDAX and Thermocalc calculations were made to identify and then study the stability and solubility of the intermetallic phases present in each alloy. The intermetallic phases involved are not the same in the three thixoformed alloys, but the onset of melting occurred at similar temperatures. Fig. 3 corresponds to the DSC thermogram of alloy A390 þ 1%Ni, showing that the onset of melting takes place at 513 8C. Thermocalc calculations suggest that it corresponds to the eutectic Al–CuAl2–Si– Cu2Mg8Si6Al5. In the case of A390 thixoformed alloy, the onset of melting was identified at 511 8C and for alloy A390 þ 4%Ni at 519 8C (in this later case it corresponds to Al7Cu4Ni intermetallic phase). From these results, in order to avoid partial melting, a temperature of 500 8C
Fig. 3. DSC Thermogram of alloy A390 þ 1%Ni, showing the onset of melting at 513 8C.
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P. Kapranos et al. / Journal of Materials Processing Technology 135 (2003) 271–277
Fig. 4. Variation of hardness as a function of ageing time: A390 at 180 8C and with Ni additions at 160 8C.
was adopted for the solution heat treatment of the three alloys. For the determination of the hold time, samples were solution heat treated at 500 8C in a salt bath for different times (up to 10 h) and quenched within 5 s, performing hardness measurements (at least 8) for each condition. After 3 h no further softening is measured in A390 thixoformed alloy, while in the case of Ni containing alloys a minimum time of 5 h is required. These two different soaking times were defined as adequate for the solution treatments at 500 8C. Concerning artificial ageing treatments, two temperatures were studied: 160 and 180 8C. To obtain the optimum time for artificial ageing, samples were kept for different times (up to 25 h) at the temperatures chosen and hardness measurements were done for each condition. The curves
obtained are shown in Fig. 4. The maximum hardness values were very similar for both temperatures, but at 160 8C the maximum was achieved for longer maintenance periods. From the figure, it can be observed that beyond the peak a very small reduction in hardness is measured. In view of the above results, a heat treatment at 180 8C for 4 h was selected for the three alloys.
5. Microstructural and mechanical testing Microstructural analysis of the thixoformed products has been carried out and the results are related to the process parameters and product properties. Fig. 5 shows the micrographs of the A390 alloy, illustrating the microstructural changes during processing.
Fig. 5. Microstructure of alloy A390: (a) as-received; and (b) as-thixoformed (100).
P. Kapranos et al. / Journal of Materials Processing Technology 135 (2003) 271–277
275
Fig. 6. Microstructures after reheating at 570 8C (100): (a) A390 þ 1%Ni; (b) A390 þ 4%Ni; and (c) A390.
A number of characteristic features can be deduced: (i) During thixoforming, major spherodisation of the Al matrix takes place. (ii) The globules are oblong in the direction of flow and have average sizes of 60–70 and 90–100 mm in the direction transverse to and parallel to the direction of flow, respectively. (iii) The globules contain small, 1–2 mm, silicon particles. (iv) The polyhedral primary silicon particles are homogeneously distributed in the matrix and there is no
change in their particle shape and size (30 mm) as compared with the raw alloy. (v) Eutectic silicon and intermetallic phases are seen between the globules. (vi) The different processing temperatures gave no notable differences in the microstructures. (vii) The crystalline phases as identified by X-ray diffraction analysis (XRD) were aluminium, silicon,
Table 2 Mechanical properties of thixoformed A390 alloys Alloy
E (GPa)
Y.S., R0.2 (MPa)
UTS, Rm (MPa)
Elongation, A5 (%)
Hardness (HRB)
As-thixoformed A 77.7 B 76.3 C 90.6
215 225 252
271 277 261
1.04 0.72 0.3
67 71 76
T6 A B C
NA NA 348
429 411 354
0.16 0.10 0.25
84 84 83
78.4 81.7 93.8
Fig. 7. Thixoformed brake drum.
276
P. Kapranos et al. / Journal of Materials Processing Technology 135 (2003) 271–277
Fig. 8. Brake drum: weight comparison for different materials/processes.
Al8Si6Mg3Fe, Al1.9CuMg4.1Si3.3, CuAl2, and traces of Mg2Si. The identified phases in alloy A before and after thixoforming were identical. The properties have been related to the process parameters and resulting microstructures. Table 2 gives an
example of values obtained from thixoformed specimens in the as-thixoformed and T6 conditions. If one considers the microstructures of the three different compositions employed in the project (Table 1), it can be seen that the increasing content of Ni appears to adversely affect the morphology of the microstructure (Fig. 6). The
Fig. 9. Brake drum: cost comparison for different materials/processes.
P. Kapranos et al. / Journal of Materials Processing Technology 135 (2003) 271–277
effect manifests itself as a general decrease in the spheroidicity of the microstructure, a factor that is crucial in thixoformability.
6. Automotive part An automotive part (brake drum) was thixoformed by Stampal S.p.A. using the A390 standard alloy. The part shown in Fig. 7 is in the T6 heat treated condition, weighs 1.7 kg, and is substituting for a cast iron part. The part offers lower weight and good wear resistance. The part has been bench tested at room temperatures, 200 and 350 8C.
277
8. Conclusions There are clear weight benefits to be gained from thixoforming a brake drum out of A390 as opposed to cast iron, however, there are higher costs involved and of course there are still the results of bench testing to be considered. Nevertheless, there is scope for cost reductions at all fronts as thixoforming develops further and makes inroads into new markets where no further cost reductions are expected, as with cast iron technology. This work is only a starting point, but nevertheless an exciting starting point in using a new near net shaping technology to produce lighter automotive parts in the continuous drive to reduce weight, save energy, and therefore manufacture with an environmentally friendly process.
7. Discussion Hypereutectic alloys, based around the standard A 390 (Al–17%Si) alloy, have been produced in the required spheroidal microstructure. However, the addition of Ni does affect the rheological behaviour of these alloys and as a result their behaviour when thixoformed. Clearly further development is necessary in the feedstock production route in order to optimise the microstructures, especially for the added Ni content alloys. Optimisation of heat treatment could possibly offer some improvement in the elongations obtained thus far. The thermal stability and wear properties of the thixoformed alloys have compared quite favourably with cast and squeeze-cast products tested as part of this work. Appropriate processing windows for thixoforming have been established for the various compositions tested, and the resulting mechanical properties are encouraging. Finally, a weight and cost comparison has been made for the automotive part produced, the results being shown graphically in Figs. 8 and 9, respectively.
Acknowledgements This study has been conducted within the EU BRITEEURAM project BE96-3652 (HAforAC). The fruitful discussions with partners of the consortium and the financial support of the European Commission are gratefully acknowledged.
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