The effect of anodic oxide coating on the fatigue behaviour of AA6082 with an ultrafine‐grained microstructure

Abstract

In this work, the high-cycle fatigue behaviour of the precipitation hardening aluminium wrought alloy AA6082 was investigated for a conventionally-grained and an ultrafine-grained substrate condition, with each a hard and a soft electrolytic anodic oxide (EAO) coating. The investigations were focused on the susceptibility of the coatings to anodising-and fatigueinduced cracking of the coatings and its impact on the fatigue life. For both substrate materials, the fatigue strength of the coated specimens was significantly reduced compared to the uncoated materials. Considering the resistance to crack initiation in the coating, the soft-anodised coatings were found to be superior compared to the hard-anodised ones. Moreover, the coatings on the ultrafine-grained substrate showed an enhanced resistance to anodisinginduced as well as fatigue-induced cracking. However, it is supposed that the lower resistance to crack transition from the coating into the substrate is one of the most important factors which strongly limits the fatigue life of the EAO-coated specimens. This might be the reason why the fatigue strength of the ultrafine-grained substrates is relatively low, despite their higher resistance to crack initiation.Keywords: equal-channel angular pressing (ECAP) / electrolytic anodic oxidation (EAO) / high-cycle fatigue (HCF) / ultrafine-grained / Al-Mg-Si alloy / In dieser Arbeit wird die Ermüdungsfestigkeit der ausscheidungshärtbaren Aluminiumknetlegierung AA6082 an einen normalkörnigen sowie einem ultrafeinkörnigen Zustand mit jeweils einer hart-und einer weichanodisierten Schicht (electrolytic anodic oxiddation, EAO) untersucht. Die Untersuchungen sind darauf ausgerichtet, die Anfälligkeit der Schichten für Rissbildung, sowohl beim Anodisierprozess als auch bei der Ermüdungsbelastung, zu untersuchen und Verbindungen zur Ermüdungsfestigkeit herzustellen. Für beide Substratmaterialien ergibt sich für die beschichteten Zustände eine deutliche Verschlechterung der Ermüdungsfestigkeit. Dabei zeigen sich die hartanodisierten Schichten generell als deutlich rissanfälliger im Vergleich zu den weichanodisierten Schichten. Darüber hinaus kann im Vergleich der Substratzustände festgestellt werden, dass beide Schichttypen auf dem ultrafeinkörnigen Substrat einen erhöhten Widerstand gegen eine anodisierprozessbedingte oder ermüdungsbedingte Anrissbildung besitzen. Die Untersuchungen zeigen jedoch auch, dass vor allem der Widerstand gegen einen Übergang des Risses von der Schicht ins Substrat die Lebensdauer bestimmt. Für die beschichteten ultrafeinkörnigen Substrate könnte hier der Grund für deren relativ niedrige Ermüdungsfestigkeit liegen, die sich trotz des erhöhten Widerstandes gegen Anrissbildung ergibt.Schlüsselwörter: equal-channel angular pressing (ECAP) / elektrolytisch anodische Oxidation (EAO) / ultrafeinkörnig / Ermüdungsfestigkeit / Al-Mg-Si-Legierung /

Influence of anodized depth on fatigue life for bicycle cranks

“…According to previous studies (Hockauf et al, 2011), the crack transition is enhanced for thicker coating layers. Consequently, the coating thickness plays a crucial role for the overall fatigue strength.…”

“…This informs that a greater dispersion of probable values of the fatigue life is expected as the depth of anodizing increases, so that the behavior of the crank is less predictable affecting to the quality and performance of the component. This could be due to the facts observed by Hockauf et al, 2011).…”

"High-cycle fatigue and tensile strength of AZ91 Mg alloy with MAO-epoxy duplex coating "

“…The fatigue strength of the MAO coatings decreases compared to the base material since the crack progresses rapidly and results in damage due to the porosity of the MAO structure. In other words, at the endurance limit, the transition of oxide-coated samples to the infinite life zone starts significantly earlier than the uncoated base material due to the inhibition of the cracking onset by the porous oxide layer [6,31]. When the S-N curves of the MAO-epoxy duplex process in Fig.…”

Effects of Micro-Shot Peening on the Fatigue Strength of Anodized 7075-T6 Alloy

“…The effect of anodization on the fatigue strength/life of Al alloys is counted on several process variables. Moreover, the fatigue strength of a soft anodized coating is superior to that of a hard anodized one [ 12 ]. Chromic acid anodizing is less harmful to fatigue strength than sulfuric acid anodizing [ 13 , 14 ].…”

“…Increasing the thickness of the anodizing coating lowers the fatigue strength/life of AA 7075 alloy [ 10 ]. Moreover, a coating on an ultrafine-grained substrate has shown enhanced resistance to anodizing-induced and fatigue-induced cracking of AA 6682 alloy [ 12 ]. It is reported that the microcracks in the anodized film and irregularities beneath the film are the reasons for the decrease in fatigue strength [ 10 ].…”