Fatigue life of additively manufactured Ti–6Al–4V in the very high cycle fatigue regime

The formation mechanism of characteristic region at crack initiation for very-high-cycle fatigue of high-strength steels

Very high cycle fatigue measuring techniques

Preparation and properties

The effects of stress ratio on high-cycle fatigue (HCF) and very-high-cycle fatigue (VHCF) behavior of a Ti-6Al-4V alloy were systematically investigated in this paper. Fatigue tests with ultrasonic frequency (20 kHz) were performed on specimens of a bimodal Ti-6Al-4V alloy with stress ratios of -1, -0.5, -0.1, 0.1 and 0.5. Three types of crack initiation mode were observed on the fracture surfaces of the specimens that failed in the HCF and the VHCF regimes, which were explicitly classified as surface-without-facets, surface-with-facets and interior-with-facets. With the increase of stress ratio from 1 to 0.5, the number of specimens for surface-without-facets decreased, that for surface-with-facets increased, and that for interior-with-facets increased first and then decreased. For the failure types of surface-with-facets and interior-with-facets, the fatigue strength decreased sharply in the VHCF regime, and the S-N curve switched from an asymptote shape to a duplex shape. Then, a new model based on Poisson defect distribution was proposed to describe the effects of stress ratio on the occurrence of different failure types, i.e., the competition of alternative failure types. The observations also showed that there is a rough area at the crack initiation region for interior initiation cases, and the values of the stress intensity factor range for the rough area are within a small range, with the mean value being close to the threshold for the crack starting to grow in vacuum environment of the alloy. The estimated value of plastic zone size at the periphery of rough area is close to the average diameter of the primary cc grains of the alloy. (C) 2014 Elsevier B.V. All rights reserved.

Effects of stress ratio on high-cycle and very-high-cycle fatigue behavior of a Ti-6Al-4V alloy

New method for evaluation of the Manson–Coffin–Basquin and Ramberg–Osgood equations with respect to compatibility

The fatigue properties of the spray formed hypereutectic aluminium silicon alloy DISPAL ® S232 were studied at constant load amplitude and load ratio R  = −1 using the 20 kHz ultrasonic testing technique. S–N curves were measured at 20 °C and 150 °C with peak aged material heat treated for improved ductility, named S232 T6x. Increasing testing temperature shifts the S–N curve to approximately 80% of the stress amplitude measured at 20 °C. Comparison with measurements with conventional frequency equipment shows that a frequency effect does not exist at both temperatures. Correlation of fracture surface features with stress amplitudes and corresponding fatigue lifetimes reveals insight into the crack initiation mechanisms. In the HCF regime partly Al 2 O 3 particles besides several other kinds of defects serve as crack starters at the surface or in the interior of a specimen. With decreasing load amplitude and increasing fatigue lifetimes in the VHCF regime, internal fracturing becomes more probable. Crack initiation at Al 2 O 3 particles is observed less frequently, and initiation at porous areas or at areas with higher Si content is observed more often.

Fatigue properties of spray formed hypereutectic aluminium silicon alloy DISPAL® S232 at high and very high numbers of cycles

Spray-formed hypereutectic aluminium silicon alloy DISPAL® S232–T6x is cycled with variable amplitude at ultrasonic frequency up to the very high cycle fatigue (VHCF) regime under fully reversed tension–compression loading The Powder Metallurgy alloy is tested using a Gaussian cumulative frequency distribution of load cycles, and lifetimes are compared with constant amplitude data Miner calculation delivers mean damage sums between 05 and 09 for mean lifetimes between 8 × 107 and 16 × 1010 cycles, respectively Cracks are initiated at voids, at inclusions or at distributed inhomogeneities (porous areas or oxides) at the surface or in the interior In situ analysis of vibration properties indicates that cracks are formed and start growing from the beginning of fatigue cycling, even if failure occurs in the very high cycle fatigue regime Crack initiation stage is negligible Lifetime prediction calculation is performed using an adapted Paris-law and considering lifetime as cycles necessary to propagate an initial crack to failure Measured and predicted mean lifetimes differ by factor 04–10 Large crack-initiating defects strongly reduce the fatigue lifetimes, which is successfully covered in the crack propagation model

Variable amplitude loading of spray‐formed hypereutectic aluminium silicon alloy DISPAL® S232 in the VHCF regime

Kurzfassung Auf dem Gebiet des Plasmanitrierens von Al-Knet-bzw. Gusslegierungen wurden in den letzten Jahren beachtliche Fortschritte erzielt. Das Nitrierverhalten wird in entscheidendem Mase von den Legierungselementen bestimmt. Fur Al-Knet-bzw. Gusslegierungen ist bekannt, dass Mg-Anteile das Nitridschichtwachstum befordern, wogegen es durch hohere Si-Konzentrationen gehemmt wird. Spruhkompaktierte Al-Si-Legierungen weisen, bedingt durch den speziellen Herstellungsprozess, typischerweise hohe Si-Gehalte (15…35%) zur Verbesserung der Verschleisbestandigkeit sowie fur eine gute Warmfestigkeit hohe Anteile an Fe, Ni, Co u. a. auf. Erstmalig durchgefuhrte Untersuchungen zum Plasmanitrieren derartiger Werkstoffe ergaben in Abhangigkeit von den Nitrierparametern rissfreie AlN-Schichten mit einer Dicke von bis zu 6 μm. Diese ausgezeichnete Nitrierbarkeit, trotz hoher Si-Gehalte, ist offensichtlich auf die werkstofftypischen ausschlieslich primar erstarrten, fein verteilten, kleinen (2…5 μm) Si-Ausscheidungen ...

Plasmanitrieren von sprühkompaktierten Al-Legierungen*

Due to high cooling rates spray forming is an appropriate process to produce aluminum alloys with a high content of Mg2Si. Compared to common casting processes, a fine microstructure can be achieved yielding in improved mechanical properties. In this work, billets were spray formed from the two alloys AlMg15Si8Cu2 (22 mass-% Mg2Si) and AlMg20.5Si11Cu2 (30 mass-% Mg2Si) under different spraying conditions. The analysis of the microstructure showed that the size of Mg2Si dispersoids is very sensitive to process parameters. Besides the well known thermal effects of melt superheat (carried out from –40 K to +170 K) and GMR (varied from 2.0 to 6.3) a strong influence of the scanning frequency of the atomizer nozzle (7 Hz and 15 Hz) could be observed. Similar effects could be found for the occurrence of porosity. A new parameter, the enthalpy flow to gas flow ratio (EGR), was defined from these two parameters of which correlations of Mg2Si dispersoid size and amount of porosity were found.



Auf Grund der hohen auftretenden Abkuhlraten ist das Spruhkompaktieren ein geeigneter Prozess zur Herstellung von Aluminium-Legierungen mit hohen Gehalten an Mg2Si. Im Vergleich zu konventionellen Giesverfahren ergibt sich eine deutlich feinere Mikrostruktur mit entsprechend verbesserten mechanischen Eigenschaften. In den durchgefuhrten Untersuchungen wurden Bolzen aus den Legierungen AlMg15Si8Cu2 (22 Massen-% Mg2Si) und AlMg20,5Si11Cu2 (30 Massen-% Mg2Si) mit verschiedenen Prozessparametern spruhkompaktiert. Eine Analyse der Mikrostruktur zeigte, dass die Grose der Mg2Si-Dispersoide stark von den Prozessparametern abhangt. Neben bekannten thermischen Effekten der Schmelzeuberhitzung (variiert zwischen –40 K und 170 K) und des GMR (variiert von 2,0 bis 6,3) konnte ein starker Einfluss der Scanfrequenz (7 und 15 Hz) festgestellt werden. Ahnliche Effekte traten hinsichtlich der Porositat im gespruhten Material auf. Ein neuer Parameter, das Enthalpie- zu Gasmassenstrom-Verhaltnis (EGR) wurde auf der Basis der Parameter Schmelzeuberhitzung und GMR gebildet. Es zeigten sich Abhangigkeiten der Dispersoidgrose und der Porositat von diesem neuen Parameter.

P. Krug

Papers

New method for evaluation of the Manson–Coffin–Basquin and Ramberg–Osgood equations with respect to compatibility

Fatigue properties of spray formed hypereutectic aluminium silicon alloy DISPAL® S232 at high and very high numbers of cycles

Variable amplitude loading of spray‐formed hypereutectic aluminium silicon alloy DISPAL® S232 in the VHCF regime

Plasmanitrieren von sprühkompaktierten Al-Legierungen*

Influence of spray forming process parameters on the microstructure and porosity of Mg2Si rich aluminum alloys