TY - BOOK

T1 - Heat treatment optimization and advanced alloy design for enhanced mechanical properties of AlMgSi alloys

AU - Schmid, Florian

N1 - embargoed until 29-04-2022

PY - 2021

Y1 - 2021

N2 - In view of the rising demand for structural materials used in all areas of modern applications, aluminium alloys need to undergo constant improvement to stay competitive against others such as advanced steel grades or carbon fibre reinforced polymers. Due to extreme strength improvements of these adversary materials, the advantage of aluminium in terms of a high strength to weight ratio is fading. In addition, aluminium alloys do not incorporate such a high formability as for example low-carbon steels do, which might limit usage as outer body sheet material. Triggered by these observations the present thesis seeks to improve the mechanical parameters of AlMgSi-alloys – the most used age-hardenable alloys. On the one hand, an enhanced stability during room temperature storage combined with a high artificial aging response is aimed for. Additionally, a novel approach of cluster hardening is intended to increase elongation values at comparably high strength values. The latter measures are both applied to a EN AW-6016 like sheet material, which is industrially produced. On the other hand, advanced alloy design comprising an alteration of the chemical composition as well as an adaption of the heat treatment parameters are applied using a EN AW-6082 as starting point and reference. Advanced electron microscopy as well as atom probe tomography are deployed to interpret results obtained primarily from hardness and tensile testing. The main outcomes are summarized as follows. A combination of micro-alloying of Sn to alter the mobility of quenched-in vacancies and pre-aging necessitates an adaption of the pre-aging parameters themselves, but helps to suppress natural aging for at least six months without deteriorating artificial aging kinetics in a EN AW-6016. An implementation of a short term high temperature spike substantially influences contained vacancies and is shown to contribute to an even larger increase in stability. The concept of influencing the vacancy content by the means of a spike enables a combination of 230 MPa yield strength and 29 % in a EN AW-6016. Atom probe tomography and scanning transmission electron microscopy attribute these results to narrow precipitation free zones and a high volume fraction of clusters. A combination of various measures (increase of the amount of age-hardenable elements, addition of Zirconium in combination with an adapted homogenization route and interrupted quenching) applied to a EN AW-6082 leads to an increase of the yield strength by over 40 % up to 411 MPa. Advanced electron microscopy characterization methods reveal a synergistic effect. First, the artificial aging response is enhanced significantly due to the addition of Mg, Si and Cu. Second, the addition of Zirconium suppresses dislocation and grain boundary movement to a large extent, which in turn results in subgrain strengthening. Third, interrupted quenching effectively leads to the formation of nuclei and strength-increasing precipitates.

AB - In view of the rising demand for structural materials used in all areas of modern applications, aluminium alloys need to undergo constant improvement to stay competitive against others such as advanced steel grades or carbon fibre reinforced polymers. Due to extreme strength improvements of these adversary materials, the advantage of aluminium in terms of a high strength to weight ratio is fading. In addition, aluminium alloys do not incorporate such a high formability as for example low-carbon steels do, which might limit usage as outer body sheet material. Triggered by these observations the present thesis seeks to improve the mechanical parameters of AlMgSi-alloys – the most used age-hardenable alloys. On the one hand, an enhanced stability during room temperature storage combined with a high artificial aging response is aimed for. Additionally, a novel approach of cluster hardening is intended to increase elongation values at comparably high strength values. The latter measures are both applied to a EN AW-6016 like sheet material, which is industrially produced. On the other hand, advanced alloy design comprising an alteration of the chemical composition as well as an adaption of the heat treatment parameters are applied using a EN AW-6082 as starting point and reference. Advanced electron microscopy as well as atom probe tomography are deployed to interpret results obtained primarily from hardness and tensile testing. The main outcomes are summarized as follows. A combination of micro-alloying of Sn to alter the mobility of quenched-in vacancies and pre-aging necessitates an adaption of the pre-aging parameters themselves, but helps to suppress natural aging for at least six months without deteriorating artificial aging kinetics in a EN AW-6016. An implementation of a short term high temperature spike substantially influences contained vacancies and is shown to contribute to an even larger increase in stability. The concept of influencing the vacancy content by the means of a spike enables a combination of 230 MPa yield strength and 29 % in a EN AW-6016. Atom probe tomography and scanning transmission electron microscopy attribute these results to narrow precipitation free zones and a high volume fraction of clusters. A combination of various measures (increase of the amount of age-hardenable elements, addition of Zirconium in combination with an adapted homogenization route and interrupted quenching) applied to a EN AW-6082 leads to an increase of the yield strength by over 40 % up to 411 MPa. Advanced electron microscopy characterization methods reveal a synergistic effect. First, the artificial aging response is enhanced significantly due to the addition of Mg, Si and Cu. Second, the addition of Zirconium suppresses dislocation and grain boundary movement to a large extent, which in turn results in subgrain strengthening. Third, interrupted quenching effectively leads to the formation of nuclei and strength-increasing precipitates.

KW - alloy design

KW - AlMgSi alloys

KW - sheet

KW - plate

KW - Legierungsdesign

KW - AlMgSi-Legierungen

KW - Blech

KW - Platte

M3 - Doctoral Thesis

ER -