Temperature dependent properties of heat treated aluminium alloys

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treated aluminium alloys. A. G. Esmeralda. 1,2. , A. Rodrıguez. 2. , J. Talamantes-Silva. 2 and R. Colás*. 1. The mechanical properties of different aluminium ...
Temperature dependent properties of heat treated aluminium alloys A. G. Esmeralda1,2, A. Rodrı´guez2, J. Talamantes-Silva2 and R. Cola´s*1 The mechanical properties of different aluminium alloys used in the manufacture of power train automotive parts were studied. The alloys were cast in wedge shape ingots that promote a onedimensional solidification gradient. Tensile samples were machined from bars that were cast with different degrees of microstructural refining and were heat treated at times and temperatures that depended on their composition. The mechanical properties were measured from samples that had been held for 200 h within the temperature range of 25 to 300uC. It was found that the increase of Cu in alloys of the Si–Mg type alloys enhanced the mechanical properties above room temperature. The best properties were found in the Al–Cu alloy. Keywords: Aluminium alloys, Castings, Heat treating, Mechanical properties

Introduction Improvements in the fuel efficiency of automobiles without impairing their performance are one of the driving forces behind the development of newer products and processes in such a competitive industry. Replacement of cast iron by light metals alloys has become commonplace for parts such as cylinder heads, engine blocks and manifolds.1,2 Aluminium alloys in engine components enhance their power rating as the higher thermal conductivity of these alloys, in comparison with cast irons, allows for higher working temperatures in the combustion chamber that contribute to delivering more power for a given cylinder size while requiring reduced amounts of refrigeration liquids.3 However, these higher temperatures promote a reduction in the strength of the material at a time that increases the possibility of promoting thermal fatigue.1,4 Most commercial automotive castings are based on the 3XX series, as silicon imparts good fluidity and the addition of either magnesium or copper allows for enhancing their strength by heat treating.1,2,5–10 The microstructures in these alloys are made of aluminium dendrites surrounded by the Al–Si eutectic aggregate; the presence of tramp elements from scrap promote the formation of intermetallic phases. Mg is added to some alloys to strengthen by precipitation of Mg2Si particles, but should be avoided in castings hardened by Al2Cu precipitates, as these elements tend to form a low melting point eutectic that forms at temperatures below 500uC. Modifying elements are added to change the form of the Al–Si eutectic by promoting the formation of fibrous silicon branches; titanium and boron are added to refine grain size by promoting the nucleation of new grains in front of the solidification front.5–10 The 1 2

Universidad Auto´noma de Nuevo Leo´n, Me´xico Nemak S.A de C.V., Me´xico

*Corresponding author, email [email protected]

ß 2014 IHTSE Partnership Published by Maney on behalf of the Partnership DOI 10.1179/1749514813Z.00000000092

continuous demand for higher mechanical properties makes 3XX alloys unsuitable for certain uses. Such properties can be fulfilled by Al–Cu alloys, but these alloys exhibit low fluidity and are more difficult to cast than silicon bearing alloys.8–10 The aim of this work is to present the results of work conducted to evaluate the effect that temperature exerts on the mechanical properties of samples held for 200 h at temperatures within the 25 to 300uC range for alloys used in the manufacture of power train components (Table 1).

Experimental procedure The experimental alloys were prepared in a 250 kg gas fired furnace. The melt was degassed with nitrogen before adding master alloys of titanium and boron, to refine the grain size, and strontium to modify the Al–Si eutectic aggregate. The liquid metal was poured at 740uC into moulds designed to impose a one-direction thermal gradient by using a grey iron chill plate at their bottom.11,12 The ingots were cut at three different heights as measured from the bottom to obtain samples with the desired microstructural characteristics. The samples obtained were machined into tensile specimens and heat treated following the cycles shown in Table 2; these specimens were subjected to a preconditioning treatment to simulate the working conditions in an internal combustion engine, such treatment consisted of holding the specimens for 200 h before conducting the tests at the testing temperatures of 25, 150, 200, 250 or 300uC.

Results and discussion Figure 1 shows the microstructural characteristics of Al–Si–Mg (A), Al–Si–Mg–Cu (C), Al–Si–Cu (E) and Al–Cu (F) alloys. These images correspond to the ascast structures of the samples cut from a height of around 100 mm, as measured from the bottom of the

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1 Microstructures of samples from alloys: a A, b C, c E and d F in as cast condition; aluminium dendrites or grains are marked as A, Al–Si eutectic aggregate as B, Fe rich intermetallics of a- and b-types by C and D and Cu rich particles as E

respectively show the variation of the tensile strength and that of the total elongation as a function of temperature. The values that are shown in these graphs correspond to material that was cooled after solution treatment by immersion in hot water. The quality index Q is used in castings to assess them and, in the case of Al–Si–Mg alloys, is given by14

ingot. The images identify the aluminium dendrites, in Al–Si alloys, or grains, in Al–Cu, as A, the Al-Si eutectic aggregate as B, Fe rich intermetallics of the a- and btypes as C and D and Cu rich particles as E. Figures 2 and 3 show the effect that the solidification and cooling rate exerts on the microstructure of Al–Si–Mg (A) and Al–Cu (F) alloys respectively. The solidification and cooling rate not only refine the dendrite arm spacing (DAS), as grain size and the size and amount of pores are also affected (Fig. 4). The mechanical properties of the material were evaluated by means of the yield sy and tensile or ultimate su strength and by the elongation at the ultimate strength Dlu and at fracture or total Dlt. Tensile tests were carried out on specimens that were cooled down in air or immersed in water close to boiling as precipitation can take place during slow cooling from solution treatment temperature.13 Figures 5 and 6

Q~su z150 log(Dlt )

where su and Dlt are the ultimate tensile strength and the total elongation. Figures 7 and 8 compares the values of the parameters used to compute Q for samples tested with the finer (Fig. 7) and coarser (Fig. 8), microstructures, to compare the effect that microstructural refining exerts. These plots can also be used to attest the high quality rating that can be achieved by the Al–Cu alloy. The second best ratings correspond to the Al–Si–Mg alloy with added Cu.

Table 1 Chemical composition of alloys* Si

Cu

A B C

7.4 7.3 7.2

0.001 0.41 0.15 0.06 0.18 140 20 0.19 0.40 0.38 0.25 0.18 140 20 0.47 0.32 0.18 0.09 0.14 140 20

D E F

7.5 2.9 8.1 3.1 0.002 4.9

Fe

Mn

Table 2 Heat treating of alloys

Alloy Type A356 356 A356z 0.5Cu 319 319 Al5Cu

Mg

Ti

Sr

B

0.32 0.62 0.41 0.18 140 20 0.32 0.69 0.40 0.18 140 30 0.31 0.15 0.20 0.05 … 30

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Alloy Solution

Aging

A B C D E F

4.5 4.5 4.5 4.5 4.5 4.5

5 5 5 5 5 2

h h h h h h

at at at at at at

530uC 510uC 530uC 495uC 495uC 505uC and 3 h at 525uC

h h h h h h

at at at at at at

Temper 160uC 210uC 190uC 235uC 235uC 190uC

T6 T7 T6 T7 T7 T6

*Cooling from solution was in air or by immersion in hot water (90–95uC).

*Values in mass-% for all elements except Sr and B that are given in ppm.

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2 Microstructures of alloy A in as cast conditions in samples cut at heights of a 30 mm, b 60 mm and c 100 mm: grain size at 100 mm is shown in d

Conclusions The results obtained in this work show how a series of parameters affect the behaviour of different aluminium cast alloys. It is found that the alloys exhibit acceptable strength

up to temperatures of 150uC; alloys of the Al–Si–Cu and Al–Cu can extend these values up to 200uC. The favourable effect that microstructural refining exerts is documented.

3 Microstructures of alloy F in as cast conditions in samples cut at heights of a 30 mm, b 60 mm and c 100 mm: grain size at 100 mm is shown in d

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4 Microstructural changes as function of distance

5 Tensile strength as function of temperature for samples that solidified at different rates and were immersed in water after solution

6 Total elongation as function of temperature for samples that solidified at different rates and were immersed in water after solution treatment

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7 Plot of tensile strength as function of total elongation of samples with finer microstructure that were cooled by immersion in water: dashed lines correspond to values of Q calculated with equation (1)

8 Plot of tensile strength as function of total elongation of samples with coarser microstructure that were cooled by immersion in water: dashed lines correspond to values of Q calculated with equation (1)

Acknowledgements

6. L. Ba¨ckerud, G. Chai and J. Tamminen: ‘Solidification characteristics of aluminum alloys’, Vol. 2, ‘Foundry alloys’; 1990, AFS/ Skanaluminium, Des Plains. 7. L. Arnbert, L. Ba¨ckerud and G. Chai: ‘Solidification characteristics of aluminum alloys’, Vol. 3, ‘Dendritic coherency’; 1996, Des Plains, AFS. 8. A. Keaney and E. L. Rooy: ‘ASM handbook’, Vol. 2, ‘Properties and selection of nonferrous alloys and special-purpose materials’, 123–151; 1990, Materials Park, OH, ASM International. 9. A. L. Keaney: ‘ASM handbook’, Vol. 2, ‘Properties and selection of nonferrous alloys and special-purpose materials’, 152–177; 1990, Materials Park, OH, ASM International. 10. E. L. Rooy: ‘ASM handbook’, Vol. 15, ‘Castings’, 743–770; 1992, Materials Park, OH, ASM International. 11. M. A. Talamantes-Silva, A. Rodrı´guez, J. Talamantes-Silva, S. Valtierra and R. Cola´s: Metall. Mater. Trans. B, 2008, 39B, 911– 919. 12. E. Carrera, J. A. Gonza´lez, J. Talamantes-Silva and R. Cola´s: Metall. Mater. Trans. B, 2011, 42B, 1023–1030. 13. J. L. Cavazos and R. Cola´s: Mater. Char., 2001, 47, 175–179. 14. M. Drouzy, S. Jacob and M. Richard: Rev. Metall., 1978, 75, 51– 59.

This paper is based on a presentation at the 2nd Mediterranean Conference on Heat Treatment and Surface Engineering, held in Cavtat, Croatia on 11–14 June 2013.

References 1. J. Campbell: ‘Castings’, 2nd edn; 2003, Oxford, ButterworthHeinemann. 2. R. Cola´s, E. Velasco and S. Valtierra: ‘Castings’, ‘Handbook of aluminum’, Vol. 1, ‘Physical metallurgy and processes’, (ed. G. E. Totten and D. S. MacKenzie), 591–641; 2003, New York, M. Dekker. 3. J. B. Heywood: ‘Internal combustion engine fundamentals’; 1989, New York, McGraw-Hill Int. Ed. 4. R. B. Gundlach, B. Ross, A. Hetke, S. Valtierra and J. F. Mojica: Trans. AFS, 1994, 104, 205–211. 5. I. J. Polmear: ‘Light alloys’; 1980, Arnold, London, Metallurgy of the Light Metals.

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