Einfluss von Kalt- und Warmumformprozessen auf die lokalen Korrosionsmechanismen von Al-Mg-Si-Cu-Legierungen

• Influence of cold and hot rolling processes on the localized corrosion mechanisms of Al-Mg-Si-Cu alloys

Müller-Jena, Roland; Zander, Brita Daniela (Thesis advisor); Springer, Hauke Joachim (Thesis advisor)

Düren : Shaker Verlag (2022)
Book, Dissertation / PhD Thesis

In: Schriftenreihe des Lehrstuhls für Korrosion und Korrosionsschutz 4
Page(s)/Article-Nr.: xx, 217 Seiten : Illustrationen, Diagramme

Dissertation, RWTH Aachen University, 2022

Abstract

Wrought 6xxx series aluminum alloys (AlMgSiCu) are one of the most used alloy systems in the automotive industry due to their favorable combination of medium strength and good formability and corrosion properties. However, they are prone to intergranular corrosion (IGC), which limits their application due to safety concerns. While the impact of the copper content and the heat treatment condition on IGC is well researched, the influence of the degree of deformation has been recently observed and is therefore not well understood. Existing studies are either contradictory, specific for one alloy system or processing route or fail to explain the observed effects, which result in multiple isolated and non correlatable model representations. However, the effect of the degree of deformation on the IGC susceptibility is highly critical as 6xxx series alloys are usually produced in complex, strongly differentiating deformation processes such as forging, rolling or extrusion depending on their end-use application. Components of the same alloy system but with different processing routes can exhibit significantly different corrosion mechanisms and susceptibilities that cannot be explained with the current state of the literature not. The aim of the present thesis is therefore a detailed mechanism analysis of the influence of process-related deformations on the microstructure and local corrosion mechanisms. The work is divided into two main parts. Based on the presented literature, the effect of a stepwise cold forming of 5-60 % of the sheet alloys EN AW 6082, 6010 and 6110A with increasing copper content was analyzed. Secondly, the effect of the hot forming processes forging and extrusion on local corrosion was developed and analyzed using an EN AW 6082 and 6110A alloy with increasing copper content. The deformation extensively influenced the corrosion mechanisms and penetration depths. Cold forming by rolling resulted in a reduction of the size and continuity of the grain boundary phases with simultaneously less pronounced Cu depletion zones. The coarse matrix phases led to a change in mechanism from IGC to a localized matrix dissolution and reduced the penetration depth by up to 80 % for small deformations. As the degree of deformation increased, the penetration depths increased by up to 290 % due to the coarsening of the matrix phases. The effect could be attributed to pinning of the elements to dislocations and matrix phases and altered phase formation at dislocations. In the forged and slightly deformed alloys (5 %), $Al_{v}Fe_{w}(Mn_{x},Cr_{y}),Si_{z}$- and $Mg_{2}Si$-phases were formed as well as depletion zones at the grain boundary, which led to a sensitization of the grain boundaries to local corrosion. With increasing deformation (50 %), these cathodic $Al_{v}Fe_{w}(Mn_{x},Cr_{y})Si_{z}$-phases were aligned in rows in the matrix. This alignment led to increased matrix dissolution and pitting corrosion phenomena. As such, the corrosion penetration depth increased by up to 250 % compared to the undeformed state. The pitting corrosion was probably enhanced by increased grain orientation spread (GOS). The extruded alloys, on the other hand, showed the formation of a fine, densely occupied sub-grain network due to the high degree of deformation of 90 %. At this sub-grain network, a fine selective corrosion was initiated, especially in the vicinity of the cathodic and aligned $Al_{v}Fe_{w}(Mn_{x},Cr_{y})Si_{z}$-phases. In the low-copper alloys, the aligned and cathodic $Al_{v}Fe_{w}(Mn_{x},Cr_{y})Si_{z}$-phases led to the formation of hole-shaped corrosion phenomena oriented in the direction of deformation. Depending on the orientation of these phases (width vs. depth) strong differences in the penetration depth of 255 % occurred. In the high-copper EN AW 6110A alloys, intergranular corrosion was the dominant corrosion mechanism due to the formation of the Q-phase on sub-grains. Those sensitized sub-grains initiated intergranular corrosion and formed a fine dissolution network accompanied by low matrix dissolution. This change in the corrosion mechanism from pitting through the aligned $Al_{v}Fe_{w}(Mn_{x},Cr_{y})Si_{z}$-structures to intergranular corrosion by the formation of Q-phases at sub-grain boundaries with increasing copper content reduced the penetration depth by 35 % compared to the copper-depleted state. The summarized discussion of the displayed corrosion mechanisms enabled the development of a general corrosion mechanism concept, which additionally expands the mechanistic models in the literature by the aspect of sensitization of sub-grain boundaries and depletion of deformation structures and puts the current literature into context of those new findings.

Institutions

• Division of Materials Science and Engineering [520000]
• Chair of Corrosion and Corrosion Protection [522710]