The effect of processing and Mn content on the T5 and T6 properties of AA6082 profiles

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Abstract

Among several alloys within the Al–Mg–Si system, AA6082 alloy is regarded as a high strength alloy and is used for sections requiring tensile strength exceeding 300 MPa in the automotive industries. This alloy needs to be processed in an optimum way in order to meet the everincreasing market demand for improved performance particularly with regard to its strength. Alloy chemistry and processing parameters have a big impact on the strength of the final product and thus need to be fine-tuned to ensure a uniform structure and uniform properties for high strength extruded profiles. The present work was carried out to investigate the effect of T5 and T6 processing as well as the effect of Mn content on the final properties of AA6082 hollow cylinder profiles used in the manufacture of bushings for vibration control in the automotive industry where high strength and dimensional stability is critical.

Introduction

Among the extruded products within the Al–Mg–Si system, AA6061 and AA6082 alloys are regarded as high strength alloys which are widely used for structural applications [1]. While the former is the traditional 6XXX alloy for such applications in North America, AA6082 is preferred in Europe when similar strength levels are required and is receiving a great deal of attention recently for use in automotive extrusions. AA6082 uses an excess amount of silicon to increase age hardening response and an addition of manganese to control grain size [2]. The market increasingly demands improved performance in this alloy, particularly with regard to its strength, which is largely controlled by alloy chemistry and process parameters [3]. For high strength profiles, processing must be optimized to give either fine recrystallized grains or a uniform unrecrystallized structure. Extruded products of this alloy, however, are frequently found to contain mixed grain structures [4], [5].

A popular application for AA6082 is the bushings used in vibration control in the automotive industry. Extruded tubes of the AA6082 alloy are first sectioned into hollow cylinders which are then machined and finally bonded to rubber rings to manufacture antivibration elements. The sectioned aluminum hollow cylinders are tested in uniaxial compression and are allowed to undergo a maximum permanent reduction in height of 0.05 mm after a loading cycle to 95 kN. After repeated testing, the hardness of those parts that have passed the compression test were found to be higher than 100 HB whereas those that have failed were invariably softer than 100 HB (Fig. 1). A hardness level of 100 HB was thus identified as the boundary between the pass and fail situations. The present work was carried out to investigate the effect of Mn content, the quench rate, the solution temperature and the storage before artificial ageing on the T5 and T6 hardness of AA6082 tube profiles for automotive antivibrational elements and to identify the chemistry and the processing conditions that yield a minimum hardness of 100 HB.

Section snippets

Experimental

Three AA6082 alloys, with different Mn contents were cast industrially with a state-of-the art hot top air-slip vertical billet caster in the form of 7400 mm long, 178 mm diameter billets. The chemical compositions of these billets are given in Table 1. These billets were homogenized at 570 °C for 7 h and subsequently cooled to room temperature at a rate greater than 4 °C/min. The billets thus produced were pre-heated to 500 °C and were extruded into tube profiles with internal and external diameters

Results and discussion

A surface layer of the profile extruded from the 0.55% Mn alloy, approximately 1 mm deep, was recrystallized as-received (T4 temper) (Fig. 2). There were only fibrous grains across the transverse and longitudinal cross-sections of the profiles of the 0.70 and 0.90% Mn alloys which were recrystallized only on their outermost skins (Fig. 2). Recrystallized bands are often seen on the periphery of extruded products and are associated with the high levels of stored strain energy near the product

Conclusions

T6 processing has produced a wide range of hardness values between 82 and 134 HB, depending on the quench rate, Mn level of the alloy and the ageing practice. T6 temper hardness levels that were obtained with a proper selection of processing parameters were higher than those offered by the T5 temper. The 24-h room temperature storage before the ageing treatment in the case of T5 processing is believed to be partly responsible for the rather large hardness difference, in favor of the T6 temper.

Acknowledgements

It is a pleasure to thank Osman Çakır and Fahri Alageyik for their help in the experimental part of the work and to ASAS Aluminium Co., for providing the 6082 profiles.

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