THE EFFECTS OF CASTING PARAMETERS ON RESIDUAL STRESSES AND MICROSTRUCTURE VARIATIONS OF AN AL- SI CAST ALLOY

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1 Copyright JCPDS-International Centre for Diffraction Data 29 ISSN THE EFFECTS OF CASTING PARAMETERS ON RESIDUAL STRESSES AND MICROSTRUCTURE VARIATIONS OF AN AL- SI CAST ALLOY S. Mohsen Sadrossadat 1 Sten Johansson 2 1, 2 Division of Engineering Materials, Linköping University, Sweden ABSTRACT Different casting parameters can change the microstructure and residual stresses of castings. The microstructure of Al-Si cast alloys is influenced by the morphology of silicon particles (shape, size and distribution), aluminum grain size and dendrite parameters. Dimensional changes resulting from casting caused by residual stresses can particularly affect the quality of near net shape castings. In this research, the influence of casting process parameters such as modification, superheat temperature, mould hardness and mould design on residual stresses and microstructure of an Al-Si-Mn alloy have been investigated. Experiments were conducted with different superheat, mould hardness, modification with Al-1Sr and two different casting designs. The micro structural changes associated with these parameters have been studied by optical microscopy, scanning electron microscopy and image analysis. The type and extent of residual stresses of all samples were determined using the sectioning method. The results show that all of the mentioned casting parameters have clear effects on residual stresses. The residual stresses decrease with lowered superheat, temperature and mould hardness. It was found that the residual stress increases both with adding a eutectic modifier and with change of casting design. It was also found that microstructure and mechanical properties are influenced significantly by the mentioned parameter. 1. INTRODUCTION Cast aluminum-silicon alloys are being increasingly used in automotive and aerospace industries for critical structure applications because of their excellent castability, low density, acceptable mechanical properties and low cost. Mechanical properties of this family of cast alloys are strongly affected by the shape, size and distribution of the silicon eutectic and different intermetallics in the microstructure. Chemical composition of the alloy, solidification conditions and modification are the most important casting parameters which are able to change the mentioned features of the silicon eutectics and intermetallics. Among the commercial Al-Si modifiers, Al-1Sr master alloy is the most commonly used one. It can modify the mechanical properties considerably by changing the morphology of silicon eutectic from coarse plate like to a fine fibrous structure. Several research findings have shown that iron rich phases are the most effective intermetallics in altering the mechanical properties of the aluminum silicon base alloys depending on their type, size and shape. Solidification rate and the presence of different trace elements such as manganese and copper are affecting the morphology of the intermetallics. Addition of Mn will change β-al 5 FeSi platelets to α-al 15 (Fe, Mn) 3 Si 2 in Cu-free alloys or α- Al 15 (Fe, Mn, Cu) 3 Si 2 in Cu-bearing alloys in the form of Chinese script or polyhedral morphology[1-9].

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3 Copyright JCPDS-International Centre for Diffraction Data 29 ISSN Residual stresses in a cast component are those stresses which may remain in the casting after it has been removed from the mould [1]. The main causes of residual stresses in cast components are: (a) temperature differences within the castings; (b) hindrance of contraction by the mould; and (c) transformations in the solid metal during cooling [11, 12]. According to the following formulas, there is a clear relationship between the residual stresses (σ) of the major parts of a component and their difference in cooling rate [13]: E= σ / ε (1), ε = ΔL/ L= α ΔT (2), then σ = α EΔT (3). Shape change can decrease the residual stresses produced during cooling. So formula 3 will develop into the following formula: σ = α.e.δt (1- ε¹), where ε¹ shows the ductility at failure [14]. On the other hand, according to Chevorinov s rule, the solidification time of molten metal (t), is related to the constant C (which depends on the thermal properties of the mold and the material), the local volume (V) and surface area (A) of the casting section [14]: t = C (V/A) ² =C M ², M is casting modulus. Among the general methods for residual stress measurement, X-ray diffraction, hole drilling and sectioning are the most common methods applied to aluminum alloys. Although the first two methods are non- destructive, the problem is that they are sensitive to microstructure (and texture) and geometry, respectively [15, 16]. The sectioning as a destructive method seems to be an acceptable method for measuring the macro stress within the aluminum cast components. 2. EXPERIMENTAL DETAILS General practical steps of this research are the following: - Calculation of the different grid s dimensions and mould preparation - Alloys selection, melting of the alloys and casting - Performing thermal analysis, removing the casting from the mould and cleaning - Attaching the strain gauges and doing residual stress measurement after sectioning - Doing micro structural and mechanical testing on the gate and grid s member specimens Mould strength, melt s superheat, alloying elements, casting design and modification of the alloy are the main focused casting parameters in experimental tests Calculation of the pattern size and preparation of the moulds As shown in the Fig. 1, the similarity of rectangular and triangular grids is having three different members. The main aim of the grid size calculation was to achieve the same casting modulus for similar members of different grids in order to get the same cooling rate at their members. M1 M2 Fig. 1. Two casting design showing the similar casting modulus sections

4 Copyright JCPDS-International Centre for Diffraction Data 29 ISSN Oil-based red sand was used for molding. In order to get the compaction criteria, the hardness of the prepared moulds has measured by means of the Disa equipment Materials, melting and casting Melting was carried out in a resistant furnace. The chemical compositions of the alloys used in the experiments are shown in Table 1. The base metal is a casting aluminum alloy EN44, according to European standard. In order to investigate on the effects of alloying elements, manganese, copper was added to that alloy. The alloying step was followed by decreasing the amount of manganese aiming at a composition close to the B 356 alloy. Finally, an Al-1Sr master alloy was added to that alloy to keep the strontium level at.35% in the alloy by slight stirring. The base pouring temperature for most of the castings was 73 C. Two more melts of alloy 1 were prepared at 68 C and 75 C in order to investigate the effect of superheating. Table 1. Chemical composition of different tested alloys Composition (Wt. %) Si Mg Mn Fe Cu Al Modification Alloy Standard No. 1 EN Bal Bal Bal Bal B Bal B 356 Bal. Al- Sr 2-3. Thermal analysis Thermal analysis technique can be used to register the relationship between the temperature and time during cooling. The temperature was registered at six different positions in the mould during casting as is shown in Figs. 2. The acquisition of data was performed by an Almemo data logger. K type (Chromel- Alumel) thermocouples were used (a) (b) Fig. 2. Mold assembly for monitoring the temperature changes versus time: (a) Different thermocouple places in the mould; (b) assembled mould connected to the data logger

5 Copyright JCPDS-International Centre for Diffraction Data 29 ISSN Installation of strain gauges and residual stress measurement The procedure followed in this work was first to measure the stresses in the different grids by using the sectioning method. This was done by polishing the surface of the central member of each grid using down to 5 standard papers; the surface was then cleaned with a degreasing solvent before placing and adhering the strain gauge. The strain gauges were placed at 1/3 of the total length of the central member of each grid. The resistance of the gauge at the mentioned point was measured with the aid of a Wheatstone bridge. The side members of each grid were then cut to release the stresses making it possible to measure the residual strain Tensile and microstructural study Tensile testing was performed on the specimens taken from the gate of each grid using Instron 558 equipment. The tests were carried out according to the ASTM-B5573 standard method in order to measure the tensile properties. Secondary dendrite arm spacing (SDAS) of the grid s members was measured using an optical microscope with an image analysis software (Microgop 2). The microstructural changes were studied by optical and scanning electron microscopy (Hitachi SU-7) which has been equipped with an energy dispersive system (INCA, Oxford). The average compositions of the various phases were measured using EDS and the results reported are based on the average of approximately ten spot analyses. 3. RESULTS AND DISCUSSION 3.1. The effects of sand mixture strength and melt s superheat The influence of superheating the melt on the casting residual strain is shown in Fig. 3. As the superheat increases, the residual stresses increase on both types of casting designs. The reason can be related to the heat affected zone in the surrounding sand (especially core regions) which will begin to grow fast immediately after casting. The result of that effect is higher penetration of the heat to other zones of the casting that can intensify non-uniformity of the heat within the component. The higher temperature difference in the casting may lead to higher residual strains. On the other hand, Fig. 4 illustrates that the residual strains increase with hardness of the mould. After pouring of the melt into the mould, the side members with lower casting modulus begin to solidify and freely contract in solid state. Then the central member reaches to the solidus temperature and experiences solid contraction. The latter contraction opposed by the side members that are already solid and rigid. Consequently, the central member is placed under tensile strain stress. The higher value of the mould hardness leads to higher constraint of the mould mixture (especially cores) that can oppose the shape change of the grid due to contraction of the central member. So, the result is an increased residual stresses. The following effects are consistent with the findings in the literature [14].

6 Copyright JCPDS-International Centre for Diffraction Data 29 ISSN Residual strain (Microstrain) Residual strain (Microstrain) Melt's superheat (C) Sand mixture strength (N/Cm2) Fig. 3. The effect of melt s superheat on Fig. 4. The effect of mould hardness on the residual strain in alloy number 1 the residual strain in alloy number The effects of alloying elements and modification on the microstructure Fig. 5 shows the different microstructures of the tested alloys. The β-al 5 FeSi with needle-like morphology was found in almost all of the alloys in different quantities, size and shape. It was also found that the β particle is the most common intermetallic type in alloys no. 5 and 6 in comparison with the others samples. The reason for that can be the Mn/ Fe ratio which is equal to.5 in these alloys that is much lower than the corresponding ratio of 4 in the other samples. The EDS microanalyses also proved that the chemical composition of the mentioned phase in alloy 1, 2, 3 and 4 was nearly the same: 8-11% Si, %Mn, 5-6% Fe, -.5% Cu, Al (rest). The results showed completely different composition in alloy no. 5 and 6 with no Mn but higher Fe and Si [1, 4]. The amount of Chinese script in sample 4 was higher than in the other samples. This can be due the optimal conditions for growth of this phase. The same Mn/Fe as in sample 1, 2 and 3 but smaller amount of copper can lead to less porosity, higher local heat conduction and consequently higher degree of conversion of β-al 5 FeSi to α-al 15 (Fe, Mn) 3 Si 2 [3, 6, 4]. Fig 5. SEM microstructures of the side members of cast alloys with rectangular design: (a) alloy no 1, (b) alloy no. 2, (c) alloy no. 3, (d) alloy no. 4, (e) alloy 5, (f) alloy 6 A comparison between the microstructures in Fig. 5, (e, f) shows that due to the effect of modification, the size of silicon eutectic is quite smaller and the shape seems to be more fibrous and rounded. Also the length of the needle-like precipitate after modification is much higher [2, 8, 9].

7 Copyright JCPDS-International Centre for Diffraction Data 29 ISSN Effects of alloying elements on the mechanical properties and residual stresses The effects of different alloying elements on the residual strain height and ultimate tensile strength of the tested grids (which are taken from ingates) are shown in Fig. 6 and table 2. Highest UTS value of the modified alloy can be due to the modification effect (alloy no. 6). Lowest UTS value for the alloy 1 with 91 Mpa, is probably due to the low amount of solid solution and intermetallics hardeners (Cu and Mg). As the value of Mn goes from.5 in no. 5 to.4 in no. 3 and 4, the UTS value increases [1, 3]. Table2.The UTS results for the different tested Composition (Wt. %) UTS (Mpa) Alloy No. Standard 1 EN B B Compresive residual strain (Microstrain) Ultimate tensile strength (Mpa) 3.4. Effect of casting design on the microstructure Fig. 6.The effect of superheat of the melt on the residual strain in alloy number 1 The figure 7 shows the cooling curves of the different members of the specimen (central member) for alloy 1. The 1, 3, 3, 1, 2 and 2 members have the lower cooling rate, respectively. Analysis of the mentioned results shows that for the rectangular grids the effect of cooling rate is more pronounced compared with triangular grids, which will be explained more. The reason for the lowest cooling rate of the neighboring side members in the center of the mould can be due to the reciprocal heat effects on each other. On the other hand, while the ratio of the side member casting modules to the central member is equal on both grid types, the lower cooling rate of the triangular one can be due to heat affects of the central member on the side member in comparison with rectangular design. The presence of the smaller size of sand core between the triangular members results in much higher heat effects on the side members of that design. Cooling curves of the members for the alloy Time (sec) Fig. 7. Cooling curves for the alloy No. 1 Typical cast microstructures of alloy no. 4 with the rectangular design for both side and central members are shown in Figs., 8 and 9. These figures illustrate the effect of higher cooling rate on the side member in comparison with the central member which has the lower cooling rate due to the difference in casting modulus. Temperature (C)

8 Copyright JCPDS-International Centre for Diffraction Data 29 ISSN Fig. 8. SEM microstructure of the side member of the alloy no.1 Fig. 9. SEM microstructure of the central member of the alloy no.1 The Fig. 1 shows that the rectangular grid could perform a better cooling between the members compared with the triangular grid for all of the tested alloys. The reason can be again due to the lower thermal effect of the central member on the side members in rectangular grid. Fig. 11 shows that the mean value of SDAS for two members of each triangular grid increases compared with the rectangular gird. It means that the general cooling capability of rectangular grid is higher than the triangular design. Difference of the SDAS values of the members (Microstrain) Mean value of SDAS of each specimen (Microstrain) Alloy 1 Alloy 2 Alloy 3 Alloy 4 Alloy 5 Alloy 6 Different tested alloys different casting design Fig. 1. The effect of casting design on the strain the different tested alloys Fig. 11. The effect of casting design on residual residual strain of the different tested alloys 3.5.The effects of casting design and modification on residual stresses Fig. 12 shows that the residual strains in triangular grids are higher than for rectangular grids. Although the rectangular grids are more capable of making the higher difference of cooling rate between their members compared with triangular grids, but this result can be due to the higher rigidity of triangular than rectangular grid. Residual strain (Microstrain) Different cast alloys Fig. 12. A comparison of the residual strains in different grids of the tested alloys According to the literature, modification of an alloy can improve strength and ductility. On the other hand residual stresses increases as ductility decreases and they both increase as strength of an alloy increases.

9 Copyright JCPDS-International Centre for Diffraction Data 29 ISSN Fig. 13 shows the effect of modification on the residual strains in both members of the different grids. It seems that the improving strength effect of modification dominates the ductility effect. Residual strain (Microstrain) Unmodified Modified CONCLUSIONS Fig. 13. The effect of modification on the residual As the sand mixture strength and melt s superheat increase, the residual stresses increase tangular grid can make a better cooling rate difference compared with triangular grid. angular grid specimen produces higher value of residual stress. Modification can lead to higher values of residual stresses. Mechanical properties of the alloys which are affected by microstructure can alter the residual response of the alloys. The optimum ratio of Mn/Fe is one of the parameters which can affect converting the β- Al 5 FeSi platelets to α-al 15 (Fe, Mn) 3 with Chinese script morphology. The other influencing. parameter can be local cooling rate and other alloying elements. Thermal analysis is a powerful technique to predict and evaluate the residual stresses. within the different sections of a casting. Casting modulus plays an important role in residual stress issue. REFERENCES [1] Hwang, J. Y., Doty, H. W., Kaufman, M. J., Mater. Sci. Eng., 27, A 488 (1-2), [2] Moustafa, M. A., J. Mater. Proc. Tech., 28, Article in press [3] Shabestari, S. G., Moemeni, H., J. Mater. Proc. Tech., 24, (1-3), [4] Taghaddos, E., Hejazi. M. M., Taghiabadi, R., Shabestari, S: G., J. All. Comp., 28, Article in press [5] Qian, Z., Liu, X., Zhao, D., Zhang, G., Mater. Lett., 28, 62(14), [6] Du tta, B., Rettenmayer, M., Mater. Sci. Eng., 2, A 283, [7] Zhang, L. Y., Jiang, Y. H., Ma, Z., Shan, S. F., Jia, Y. Z., Fan, C. Z., Wang, W. K., J. Mater. Proc. Tech., 27, Article in press [8] Liao, H., Sun Y., Sun G., Mater. Sci. Eng., 22, A335, [9] Osorio, W. R., Garcia, L. R., Goulart P. R., Garcia, A., Mater. Chem. Phys., 27, 16, [1] Withers, P. J., Bhadeshia, H. K. D. H., Mater. Sci. Tech., 21, 17 (4), [11] Parkins, R. N., Cowan, A., Proc. Inst. Brit. Found., 1953, A99-A19 [12] Report of sub- committee T.S.32, Proc. Inst. Brit. Found., 1952, A179- A189 [13] Campbell, J., Castings, Elsevier: England, 23, [14] Farhangi, H., Norouzi, S., Nili-Ahmadabadi, M., J. Mater. Proc. Tech., 24, , [15] Lu, J., Handbook of measurement of residual stresses, The Fairmont press: USA, 1996 [16] Withers, P. J., Bhadeshia, H. K. D. H., Mater. Sci. Tech., 21, 17 (4),