Formation of Peripheral Coarse Grain in Thin-Walled Al–Mg–Si Extrusion Profiles

ORIGINAL RESEARCH ARTICLE Formation of Peripheral Coarse Grain in Thin-Walled Al–Mg–Si Extrusion Profiles P. GOIK, A. SCHIFFL, and H.W. HÖPPEL Investigations on microstructure and texture development during processing of Al–Mg–Si sheet and extrusion profiles focused on describing and modelling the changes in the bulk. However, understanding the evolution of microstructure in the sub-surface region is highly relevant, as this governs material performance under corrosion and bending deformation, which are crucial applications in mobility and transportation. The aim of this study was to correlate the effect of alloy composition and extrusion parameters to the formation of texture and peripheral coarse grain (PCG) layer. For this, five alloys of the AA6xxx class with varying content of dispersoid forming elements were extruded at increasing extrusion speeds into thin-walled hollow profiles. The microstructure at a plane section was investigated by means of EBSD and characterized in terms of distinct texture component distribution. It was shown that with increasing grain boundary mobility, through adapting dispersoid content and extrusion speed, the PCG layer grows into a fine-grained bulk. This growth of the PCG layer is only counter-acted upon, when the grain boundary mobility in the bulk is high enough, so that extensive growth of Cube oriented grains occurs. The PCG layer shows distinct orientations related to h101ijjTD, so that a possible mechanism for the orientation selection during grain growth is proposed. https://doi.org/10.1007/s11661-023-07144-3 Ó The Author(s) 2023 I. INTRODUCTION THE application profile on structural safety components in transportation such as automobiles, busses and trains demand for high strength and ductility of the material. This is necessary to absorb maximum crash energy in case of an accident, while retaining lightweight potential for construction. Extrusion profiles of age-hardening Al–Mg–Si alloys fulfill these requirements including a high degree of hot workability due to a low strength at high temperature and a sufficient strength in the final state after age hardening.[1] While the age hardening process affects both strength and ductility,[2,3] the grain microstructure and texture developing during the hot working process primarily influence the ductility and formability of the final material state.[4–6] Development of grain structure and texture of Al–Mg–Si alloys depend on the alloy chemistry and the initial microstructure after casting and homogenization heat treatment prior to the hot working step. Most of the microstructure and texture evolution of wrought aluminum alloys during working has been performed on sheet alloys of different classes of wrought alloys, such as pure Al,[7,8] Al–Mn,[9–11] Al–Mg[12,13] and Al–Mg–Si.[4,14–16] Although the chemistry of these alloy classes is different, the mechanisms of microstructure and texture formation are related and depend on the following factors: I. II. III. IV. P. GOIK and H. W. HÖPPEL are with the Department of Materials Science & Engineering, Institute I: General Materials Properties, Friedrich-Alexander-Universität Erlangen-Nürnberg, Martensstraße 5, 91058 Erlangen, Germany. Contact e-mail: A. SCHIFFL is with the Hammerer Aluminium Industries Extrusion GmbH, Lamprechtshausener Straße 69, 5282 Ranshofen, Austria. Manuscript submitted January 27, 2023; accepted July 13, 2023. Article published online August 1, 2023 3940—VOLUME 54A, OCTOBER 2023 plastic deformation mechanism governed by the matrix’ crystal system effects of phases on the plastic deformation mechanism applied strain state and temperature during deformation driving forces on recrystallization and grain growth Regarding the first factor, Hirsch and Lücke[7] identified that texture formation is linked to the plastic deformation mechanism of the fcc crystal structure of Al with a medium to high stacking fault energy, depending on the alloy composition and the working temperature, which in turn affect the number of elements in solid solution during forming. Metals with a high stacking fault energy show a small tendency for twinning under METALLURGICAL AND MATERIALS TRANSACTIONS A deformation. Cold rolling of such metals leads to the formation of the Copper f112gh111i texture component. Introduction of an increasing number of soluble elements in the solid solution reduces the stacking fault energy and shifts the resulting texture after cold rolling to the Brass f011gh211i texture component. The formation of these distinct texture components is explained by the Taylor Model,[17] taking into account the lattice rotation that is affected either by slip or twinning deformation. Deformation textures of alloys are then characterized by the spread between those texture components along the b orientation fiber. The second factor describes the role of particles that are formed prior to the extrusion process, and are not soluble in the solid solution. In Al alloys Fe plays an important role for the formation of such particles. Fe is present as an impurity in technical alloys with a very low solubility in fcc a-Al.[18] Depending on the exact chemical composition and heat treatment during casting and homogenization, this leads to the formation of Al3Fe, a- and b-AlFeSi and, in presence of Mn and/or Cr, a-Al(FeMnCr)Si.[19–22] In technological relevant Al–Mg–Si alloys the Si-containing phases are dominant. During casting, these particles form in the solidification interval and accumulate as interdendritic particles with a size of 5–50 lm. These particles cannot be resolved upon heat-treatment and are named constituent particles (CP). The following homogenization heat treatment leads to a spheroidization of the CP. This is of special importance if large, needle-shaped b-AlFeSi are formed. During prolonged aging they transform into spheroidized a-AlFeSi[19] which reduce the high temperature flow stress during hot working. After casting, a small fraction of Fe and larger amounts of alloying elements such as Mn, Cr, Zr, V or Mo can sustain in the super-saturated grains of a-Al.[23–34] Throughout homogenization, the super-saturation leads to the precipitation of 20–400 nm sized particles, named dispersoids due to their incoherent interface and stability up to the solidus temperature.[23–25] Therefore, these elements are further referred to as dispersoid forming elemtens (DFE). In case of Mn and Cr, the dispersoids are linked to the a-Al(FeMnCr)Si-phase showing cubic or hexagonal crystallographic structure of complex and variable composition. Although incoherent to the surrounding a-Al matrix,[26] their precipitation sequence is described as a heterogeneous nucleation mechanism at a transition phase, the U-phase, that forms during heating up between 100 °C to 350 °C on b0 -Mg9Si5 precipitates.[24] In Zr containing alloys the coherent L12 Al3Zr-phase is formed via homogeneous nucleation, with the peak transformation rate between 400 °C and 500 °C.[ (...truncated)