Influence of Microstructure on the Quasistatic and Low ... - Science Direct

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Procedia Engineering 10 (2011) 1339–1347

ICM11

Influence of Microstructure on the Quasistatic and Low Cycle Fatigue Behaviour of an AA2618 Aluminium Alloy Osama Khalila and Karl-Heinz Langb* b

a KIT, IAM-WK, now BP Gelsenkirchen GmbH, Pawiker Straße 30, D-45896 Gelsenkirchen, Germany Karlsruher Institut für Technologie (KIT), Institut für Angewandte Werkstoffe – Werkstoffkunde (IAM-WK), Kaiserstrasse 12, D-76131 Karlsruhe, Germany

Abstract

The high-strength aluminium alloy AA2618 is a typical material to manufacture centrifugal compressor wheels of exhaust turbochargers. In general these components are manufactured from heat treated forged slugs. Depending on the size of the slugs during the heat treatment different microstructure may arise. Especially for large slugs the emerging microstructure will not be uniform within the manufactured component. For this reason, the mechanical properties of the material will depend on the location within the component. To investigate this effect a T6 heat treated forged slug measuring 530 mm in diameter and 200 mm in high was investigated. From the metallographic investigations and from measurements of the hardness three different regions could be defined. Specimens were taken from different locations and their behaviour in tensile tests and low cycle fatigue tests were determined. It turns out that the yield strength depends on the grain size. This behaviour can be described using a Hall-Petch relation. The low cycle fatigue behaviour, the lifetime and the development of fatigue damage are influenced by the local microstructure. The reduction in the lifetime correlates with the number of subgrains. © 2011 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and peer-review under responsibility of ICM11 Keywords: wrougth aluminium alloy; tensile strength; low cycle fatigue; size effect

* Corresponding author. Tel.: +49 721 608 42605; fax: +49 721 608 48044. E-mail address: [email protected].

1877-7058 © 2011 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and peer-review under responsibility of ICM11 doi:10.1016/j.proeng.2011.04.223

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Osama Khalil and Karl-Heinz Lang / Procedia Engineering 10 (2011) 1339–1347

1. Introduction The high temperature resistant aluminium alloy of the type AA2618 is currently used as standard material for compressor wheels of exhaust turbochargers. Since future demands go along with higher circumferential velocities and compressor temperatures, the material will be exposed to higher loadings. Thus, simulation tools are needed, which help to optimize the component during the initial design stage with regard to lifetime, material usage and safety. The development of such simulation tools was the objective of the FVV-research project “Improved methods for lifetime prediction of high temperature aluminium alloy compressor wheels for turbochargers” [1]. One part of this project concentrates on the determination of the properties of the material. Exhaust turbochargers are produced for different sizes of combustion engines. The spectrum varies from car engines up to big ship engines. To satisfy the demand differently big turbochargers are required for differently big engines. Therefore, differently big compressor wheels must be fabricated. To manufacture compressor wheels with different geometries and diameters differently big forging slugs are required. With regard to the influence of the component size on the mechanical properties two effects have to be distinguished. On the one hand, with increasing size of a component the probability for inhomogeneities and defects increases. The reduction of strength which is connected with that is described as statistical size effect [2]. This effect will not be considered here subsequently. On the other hand, increasing component dimensions change the conditions e.g. for hot forming or heat treatment procedures. Therefore, in big components locally different microstructures may appear influencing the local strength significantly. This publication deals with this effect. During the production of the forging slugs local differences of the microstructure may appear due to hot forming as well as due to the heat treatment processes. Particularly within big forging slugs the development of the local microstructure can be very different. In the context of this work a forging slug which has a diameter of 530 mm and a height of 200 mm was examined. The investigations of the microstructure have shown that there are significant differences in the grain size, the grain type and the composition of the microstructure in different places. The aim of this investigation was to determine the consequence of the differences in the microstructure on the mechanical properties of the material. Therefore, specimens were taken from different locations. With these specimens the quasistatic strength as well as the isotherm low cycle fatigue behaviour was identified at 80 and 180 °C. The results are compared to the one received from specimens taken from a smaller forging slug measuring about 190 mm in diameter. 2. Material and Experimental Details The forging slug provided by Atlas Copco Energas GmbH, Cologne, Germany, had a cylindrical form with a diameter of 530 mm and a height of 200 mm. The slug was manufactured of the material AA2618 and delivered in the T6 heat treated condition. The chemical composition is listed in table 1. Table 1. Chemical composition of the A2618 material investigated in wt-%.

Si Ti Fe Ni Cu Mg Al 0.18 0.05 1.00 1.00 2.00 1.40 bal. At first a stripe of 16 mm of thickness was sawed out centrically from the forging slug. The respective blanks for the specimens were then taken from the stripe as shown in Fig. 1. From these blanks solid round specimen with a cylindrical gauge length measuring 7 mm in diameter and 17 mm in length were manufactured by turning. In this work only axial oriented specimens were examined. Three different areas were defined as “outside”, “transition” and “core”. The individual specimens were marked with numbers and letters according to the sampling point. The outside zone extends from 0 to 96 mm, the


transition zone from 96 to 192 mm and the core from 192 to 256 mm. The details of the position of the specimens always refer to the distance of the edge. Metallographic examinations show that the three areas are considerably different in the microstructure. The outside area only consists of approximately equally big grains with an ASTM grain size of 2.45. However, the transition and the core area consist of grains and subgrains with different sizes. The ASTM grain size of the bigger grains is 0 and those of the subgrains between 9.2 and 8.2. The grains in the outside area which exhibit an average area of approx. 27,884 m2 or a mean diameter of about 167 m are relatively small. The biggest grains are found in the transition zone. There an average grain area of approx. 170,136 m2 or 378 m of mean diameter were estimated. The average size of the grains in the core Fig. 1. Position for taking the specimens. area amounts to approx. 77,016 m2 or a mean diameter of about 201 m which lies between the values of the outside und the transition zone. The mean grain area of the subgrains in the core zone is with approx. 500 m2 twice as big as those of the subgrains in the transition zone which measure approx. 263 m2. From these areas mean diameters of the subgrains of about 16 m in the transition zone and 22 m in the core zone arise. In the transition zone the proportion of grains and subgrains is about 85% and 15%, respectively. In the core zone about 50% grains and subgrains are found. The grains in the outside zone show no preferential arrangement. However, in the transition and in the core zone the grains show a pronounced elongation along the longitudinal axis of the forging slug. The tensile and the low cycle fatigue experiments were carried out using a Zwick electromechanical testing system with a nominal force of 100 kN. An inductive heating system was used to realize the test temperature. 3. Results 3.1. Tensile Tests Some characteristic values of material properties from tensile tests at T = 80 °C with specimens from the different zones are represented in table 2. For every zone 3 tensile tests were carried out. The given values are the average values from the three tests. The stress strain curves for specimens from all three zones start with approximately the same slope. Hence, in all three zones the material exhibits the same Young's modulus. There is a steady transition from the elastic to the elastic-plastic deformation. The 0.2 % proof stress RP,0.2 as well as the tensile strength Rm of the specimens taken from the outside zone are considerably higher than the characteristic values of the specimens taken from the transition and the core zone core. Specimens from the transition zone show tendentiously little higher strengths than specimens from the core zone. Also with regard to the ductility specimens from the outside zone show a little better characteristic values. An elongation at fracture A of 13 % was determined at specimens from the outside zone. On the other hand, the elongation at fracture for specimens from the transition zone and the core zone amounts to 12 and 11 %, respectively.

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Table 2. Characteristic values of material properties from tensile tests at T = 80 °C.

Zone Outside Transition Core

E [GPa] 77 77 77

Rp,0.2 [MPa] 351 331 330

Rm [MPa] 411 385 382

A [%] 13 12 11

The results from tensile tests at T = 180 °C for specimens from the different areas are assorted in table 3. The 0.2 % proof stress as well as the tensile strength decrease with an increasing distance from the edge. The values of the elongation at fracture are almost identical. Also at 180 °C there is a steady transition from elastic to elastic-plastic deformation. The 0.2% proof stress of specimens from the outside zone is considerably higher than those of the specimens from the transition and the core zone. The same applies to the ultimate tensile strength. Table 3. Characteristic values of material properties from tensile tests at T = 180 °C.

Zone Outside Transition Core

E [GPa] 68 68 68

Rp,0.2 [MPa] 299 276 267

Rm [MPa] 304 287 282

A [%] 15 15 16

In Fig. 2 the ultimate tensile strength and the 0.2 % proof stress are plotted versus the distance from the edge of the forging slug. The comparison of the two temperatures investigated shows that at T = 80 °C the ultimate tensile strength as well as the 0.2 % proof stress are considerably higher than at T = 180 °C. At 80 °C the difference between Rm and RP,0.2 is generally higher than at 180 °C and for specimens from the outside zone higher than for specimens from the transition and core zone. At T = 180 °C the difference between the two strengths for specimens from the outside zone is relatively small but increases towards the core zone. 3.2. Low Cycle Fatigue Behaviour The isothermal LCF behaviour was investigated using two specimens each from the outside zone as well as the core zone at T = 80 °C. Two total strain amplitudes, Ha,t = 0,5 % and Ha,t = 0,8 %, were chosen. The tests were performed under total strain control with a frequency of 1 Hz as fully reversed tests (RH = 1). From the transition zone only two specimens were examined at Ha,t = 0,5 %. Fig. 3 shows as cyclic deformation curves the stress amplitude and the mean stress plotted versus the logarithmic number of cycles. The specimens from all zones show a slight cyclic softening to a neutral cyclic Fig. 2. 0.2 % proof stress and ultimate tensile strength versus deformation behaviour at both total strain distance from the edge of the forging slug. amplitudes investigated. For specimens from the outside zone the amount of the induced stress amplitude remains nearly constant at about 330 MPa. At the other specimens the stress amplitude decreases steady up to the appearance of macroscopic crack


initiation. Then, the stress amplitudes decrease strongly. At a total strain amplitude of 0.8% the specimens from the core zone show a steady decrease of the stress amplitude up to 240 cycles. Then Va remains practically constant. The specimens from the transition zone show at a total strain amplitude of 0.5 % the same behaviour as the specimens from the core zone. Only the stress amplitude is increased by about 10 MPa compared the specimens from the core zone. For all specimens the induced mean stresses arise to values between 0 and -25 MPa. In the individual tests the mean stress remains almost constant up to macroscopic crack initiation. A comparison for specimens from the three zones at a total strain amplitude of 0.5 % shows that the induced stress amplitudes decrease from the outside tor the core zone. The same facts have to be watched at a total strain amplitude of 0.8 %. However, the differences are less at Ha,t = 0,5 % and lower number of cycles and increases with increasing numbers of cycles. At Ha,t = 0.8 % the differences in the induced stress amplitudes between specimens from the outside and the core zone is higher and decreases with increasing numbers of cycles. Fig. 4 shows the plastic strain amplitude at Ha,t = 0.5 % and 0.8 % plotted logarithmically over the number of cycles for specimens from the three zones. At 0.8 % total strain amplitude, the plastic strain amplitude is as expected considerably higher than at 0.5% (0.3% to 0.05 – 0.09 %). At Ha,t = 0.8 % the plastic strain amplitude remains relatively constant at about 0.3% up to the macroscopic crack initiation for all specimens origin. Specimens taken from the core zone show slightly increased plastic strain amplitudes in comparison with specimens taken from the outside zone (approximately 0.02%). At Ha,t = 0.5 %, however, Ha,p increases weakly with increasing number of cycles up to macroscopic crack initiation at specimens from all three zones. Specimens from the outside zone show considerably smaller plastic strain amplitudes like specimens from the core zone. The plastic strain amplitudes of the specimens from the transition zone lie between this.

Fig. 3. Stress amplitude and mean stress versus number of

Fig. 4. Plastic strain amplitude versus number of cycles for

cycles for tests at T = 80 °C.

tests at T = 80 °C.

In Fig. 5 the total strain amplitude is plotted versus the cycles to macroscopic crack initiation Ni. First of all, the lifetimes of the specimens taken from the three zones of the big forging slug at Ha,t = 0.5 % and 0.8 % are entered. Furthermore, results from experiments with specimens taken from a smaller forging slug measuring about 190 mm in diameter which was investigates mainly in the above mentioned research project [1] are represented. At the two total strain amplitudes, the lifetime of the specimen taken

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from the smaller forging slug are higher than the lifetime of the specimens from the bigger forging slug. The number of cycles to crack initiation of the specimens from the bigger slug increase from specimens taken from to core zone over specimens taken from the transition zone to specimens taken from the outside zone. Only specimens from the outside zone of the big forging slug come close to the lifetime of specimens from the smaller forging slug. At Ha,t = 0.8 % the difference in the lifetime between specimens taken from the bigger slug and specimen taken from the smaller slug is smaller as in the case of Ha,t = 0.5 %. At the smaller total strain amplitude the obtained lifetime differ stronger between each other for specimens taken from the outside, transition and core zone as well as for specimens from the bigger forging slug and specimens from the smaller forging slug.

Fig. 5. Total strain amplitude versus number of cycles to macroscopic crack initiation at T = 80 °C.

Fig. 6 shows the courses of the induced stress amplitudes and mean stresses into dependence of the number of cycles at a temperature of 180 °C. All examined specimens exhibit a weak cyclic softening deformation behaviour at both total strain amplitudes investigated. The induced stress amplitudes decrease relatively slowly before the appearance of a macroscopic macroscopic crack initiation and after this relatively strongly. Specimens from the outside zone at Ha,t = 0.5 % form an exception. At about 200 cycles, these start to soften more strongly than all other examined specimens. The induced stress amplitudes get even smaller than those of the specimen from the core zone. The induced mean stresses stay at first almost constant during the tests and amount to values between 5 and -5 MPa. Only with the initiation and the growth of a macroscopic crack strongly increasing compression mean stresses are build up. A comparison between the specimens taken from the outside and the core zone at a total strain amplitude of 0.5 % shows that the induced stress amplitudes decrease from the outside to the core zone. The same facts have to be watched also at a total strain amplitude of 0.8 %. The difference of the induced stress amplitude between Ha,t = 0.5 % and 0.8 % amounts to about 25 MPa. In Fig. 7 the plastic strain amplitude is plotted versus the the number of cycles at 0.5 % and 0.8 % total strain amplitude and T = 180 °C for specimens taken from the outside and the core zone. As expected, at 0.8 % total strain amplitude, the plastic strain amplitude is with Ha,p = 0.35% considerably higher than at Ha,t = 0.5 % with values between Ha,p = 0.04 and 0.2 %. At Ha,t = 0.8 % the plastic strain amplitude stays relatively constant up to macroscopic crack initiation. Specimens from the core and from the outside zone show approximately the same plastic strain amplitudes. However, at Ha,t = 0.5 % specimens taken from the outside zone exhibit a little lower plastic strain amplitude as specimens taken from the core


zone. The plastic strain amplitudes increase respectively until failure. Specimens from the outside zone show more cyclic softening than specimens from the core zone.

Fig. 6. Stress amplitude and mean stress versus number of

Fig. 7. Plastic strain amplitude versus number of cycles for

cycles for tests at T = 180 °C.

tests at T = 180 °C.

Fig. 8 illustrates the lifetime behaviour at T = 180 °C in a Wöhler diagram where the total strain amplitude is plotted versus the number of cycles to macroscopic crack initiation. Included are results from LCF tests with specimens taken from the three zones of the bigger forging slug in comparison to results from specimens taken from smaller slugs. The number of cycles to macroscopic crack initiation is considerably higher for specimens taken from the smaller slug than for specimens from the bigger slug. Regarding specimens taken from the bigger slug the lifetime increase for specimens taken from the core zone over specimens taken from the transition zone to specimens taken from the outside zone. However, even the specimen from the outside zone did not reach the lifetime of the specimens taken from the smaller slug. At Ha,t = 0.8 % the difference of the lifetime between specimens from the bigger and the smaller slug is lower the at Ha,t = 0.5 %. At a given total strain amplitude the difference in lifetime between the specimens taken from different zones of the bigger slug is higher than the difference in lifetime between specimens taken from the bigger and the smaller slug.

Fig. 8. Total strain amplitude versus number of cycles to macroscopic crack initiation at T = 180 °C.

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4. Discussion The investigations have shown that the microstructure of the bigger forging slug depends on the position within the slug. The grain size increases with increasing distances from the surface. With respect to the flow stress the well known Hall-Petch relation Vy = V0 + k d-1/2

(1)

is often used to describe the influence of the grain size d on the flow stress Vy. V0 is the initial stress which is mainly determined by the inner friction (Peierls stress). The coefficient k represents the degree of the dependence of the flow stress on the grain size. In comparison with steel aluminum and its alloys have a considerably lower dependence of the yield stress on the grain size. In this lower grain size dependence the distinct cross slip behaviour caused by the low stacking fault energy of aluminium manifests itself [3]. In Fig. 9 the 0.2 % proof stress RP,0.2 is plotted versus the reciprocal value of the square root of the grain diameter. The given values of the grain size refer to the diameters of normal grains and not to the dimensions of the subgrains. The respective values for T = 80 and 180 °C are approximately on straights which go parallel to each other. With decreasing grain size the value of RP,0.2 increases as expected. Due to the parallelism of the straights for the two temperatures the relationship of the increase of RP,0.2 with the grain size are identical. From this one can conclude that the for the investigates wrought aluminium the influence of the grain size on the quasistatic strength could be described very well with the Hall-Petch relation for a practice relevant application field. For the appearing flow stress only the normal grains have to be considered. Obviously, the subgrains have no significant influence.

Fig. 9. 0.2 % proof stress versus reciprocal value of the

Fig.10. Number of cycles to macroscopic crack initiation

square root of the grain diameter.

versus distance from the edge of the forging slug at a total strain amplitude of 0.5 %.

The influence of the microstructure or the micro structural conditions on the fatigue behaviour is fundamentally more complicated. The comparison of the tests at 80 and 180 °C shows different dependences of the number of cycles to macroscopic crack initiation on the distance from the edge as Fig. 10 demonstrates for a total strain amplitude of 0.5 %. At T = 180 °C and specimens taken from the


outside zone Ni is considerably higher than at 80 °C but decreases much more strongly with increasing distance from the edge. At T = 80 °C the lifetime decreases only slightly within about 120 mm distance from the edge. In contrast, at 180 °C at about 56 mm distance from the edge Ni drops to approximately 1,100 cycles which is about less than the half of the lifetime of the specimens taken from the outside zone. The different lifetimes of specimens taken from different positions are result of the local microstructure of the material on hand. The differences in the grain size and the precipitation structure are caused by the technologically hot deformation and heat treatment procedures. At the development of fatigue damage a complex interaction between the microstructure and the loading occurs. Both, at T = 80 °C as well as at T = 180 °C fatigue cracks form preferably at or nearby grain boundaries. Obviously, grain boundaries and the direct surroundings of the grain boundaries are weakened areas where also traces of dislocation movements are to be watched. Therefore, the subgrains play now an important role in the development of fatigue damage due to the higher offer of grain boundaries. Because fatigue cracks initiate preferably at grain boundaries the presence of subgrains reduce the lifetime. This effect is more pronounced at higher temperature. Therefore, the decrease in lifetime for specimens taken from higher distances from the surface of the bigger forging slug is at T = 180 °C more pronounced than at T = 80 °C. More detailed examinations are represented in [1]. Altogether, the lifetimes found out from specimens taken from the bigger forging slug are smaller than the one of specimens from the smaller forging slug. At T = 80 °C the lifetime of specimens taken from the outside zone of the bigger forging slug hit approximately the lifetime of specimens taken from the smaller slug. However, specimens taken from the transition and the core achieve only distinct lower lifetimes. The results indicate that the differences in the lifetime get greater with decreasing fatigue loading. At T = 180 °C also the specimens taken from the outside zone of the bigger forging area show a lower lifetime than specimens taken from the smaller forging slug. Again is indicated, that the differences in the lifetime increase with decreasing fatigue loading. To understand the low cycle fatigue behaviour completely further investigations will be carried out. 5. Acknowledgements The authors thank the Bundesministerium für Wirtschaft und Technologie (BMWT) and the Arbeitsgemeinschaft industrieller Forschungsvereinigungen e. V. (AiF) for funding the project. The Forschungsvereinigung Verbrennungskraftmaschinen e.V (FVV), Frankfurt a.M., Germany, is thanked for the coordination of the project. The FVV working group under the guidance of Dr. Böschen, MTU Friedrichshafen GmbH, Germany, is thanked for the guidance of the project with inspiring discussions and valuable advice. Thanks also to Atlas Copco Energas, Cologne, Germany, for supplying the material. 6. References [1] Verbesserte Methoden zur Lebensdauerberechnung von Abgasturbolader-Radialverdichterrädern aus hochwarmfesten Aluminiumlegierungen. Final report FVV-research project No. 897, Vol. 911 (in german), Forschungsvereingung Verbrennungskraftmaschinen e.V., Frankfurt (Main), Germany, 2010. [2] Y. Murakami, M. Endo: Effects of defects, inclusions and inhomogeneities on fatigue strength. International Journal of Fatigue, Volume 16, Issue 3, 1994, 163-182. [3] F. Ostermann: Anwendungstechnologie Aluminium. Springer Verlag Berlin - Heidelberg - New York, 2007.

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