Fracture–Mechanical Assessment of the Effect of Defects on the Fatigue Lifetime and Limit in Cast and Additively Manufactured Aluminum–Silicon Alloys from HCF to VHCF Regime

Jochen Tenkamp,Peter Starke,Shafaqat Siddique,Mustafa Awd,Frank Walther

doi:10.3390/met10070943

Jochen Tenkamp, Peter Starke + Show 3 more

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https://doi.org/10.3390/met10070943

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Abstract

Aluminum–silicon alloys are commonly used in die-cast and additively manufactured (AM) light-weight components due to their good processability and high strength-to-weight ratio. As both processing routes lead to the formation of defects such as gas and shrinkage porosity, a defect-sensitive design of components is necessary for safe application. This study deals with the fatigue and crack propagation behavior of die-cast alloy AlSi7Mg0.3 and additively manufactured alloy AlSi12 and its relation to process-induced defects. The different porosities result in significant changes in the fatigue stress-lifetime (S–N) curves. Therefore, the local stress intensity factors of crack-initiating defects were determined in the high and very high cycle fatigue regime according to the fracture mechanics approach of Murakami. Through correlation with fatigue lifetime, the relationship of stress intensity factor (SIF) and fatigue lifetime (N) could be described by one power law (SIF–N curve) for all porosities. The relationship between fatigue limit and defect size was further investigated by Kitagawa–Takahashi (KT) diagrams. By using El Haddad’s intrinsic crack length, reliable differentiation between fracture and run out of the cast and AM aluminum alloys could be realized. SIF–N curves and KT diagrams enable a reliable fatigue design of cast and AM aluminum alloys for a finite and infinite lifetime.

Highlights

Aluminum alloys are promising candidates for light-weight applications in highly loaded components like in the automotive or railway industries
The relationship between fatigue limit and defect size was further investigated by Kitagawa–Takahashi (KT) diagrams
Selected samples of batch B were hot isostatically pressed (HIPed) by Bodycote DensalTM process (Bodycote, Haag-Winden, Germany) to significantly reduce the casting porosity compared to as-cast batch DC-1 and DC-2, named batch DC-3

Summary

Introduction

Aluminum alloys are promising candidates for light-weight applications in highly loaded components like in the automotive or railway industries. The low density of 2.66 kg/dm and the excellent strength-to-weight ratio enables a weight reduction of up to 35% in practical applications [1]. Hereby, casting and additive manufacturing (e.g., by laser powder bed fusion (L-PBF)) allow near-net-shape manufacturing from large- to small-scale components. While cast aluminum alloys are used for low-cost solutions with medium quantities, L-PBF aluminum alloys are high-cost solutions for custom or individual components (e.g., internal cooling or heating channels, bionic structures) with high-complexity and small-quantities down to single-item production. Aluminum cast alloys are suitable grouped for according the major element, the Si alloying element 3xxx).

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