Langsames Ermüdungsrisswachstum in Aluminium‐ und Magnesiumgusslegierungen in Raumluft und in Vakuum

Slow fatigue crack growth in aluminium and magnesium cast alloys in ambient air and in a vacuum The influence of ambient air on near threshold fatigue crack growth in the magnesium cast alloys AZ91 hp, AM60 hp and AS21 hp and in the aluminium cast alloy AlSi9Cu3 has been investigated. Fatigue crack growth properties at a cycling frequency of 20 kHz in ambient air and in a vacuum are significantly different. In a vacuum, the threshold stress intensity amplitude of the aluminium alloy is 30% higher than in ambient air, and the threshold values of the magnesium alloys in a vacuum are up to 85% higher than in ambient air. Moisture of ambient air is responsible for accelerated crack growth at growth rates below 1–3 × 10⁻⁹ m/cycle (AlSi9Cu3) and 2–5 × 10⁻⁸ m/cycle (magnesium alloys), respectively. In ambient air a minimum crack growth rate of 5 × 10⁻¹¹ − 2 × 10⁻¹⁰ m/cycle was observed, whereas far lower minimum growth rates were found in a vacuum.     

Ermüdungsverhalten und Dauerfestigkeit von Graphit und Aluminium – infiltriertem Graphit

Fatigue behaviour and endurance limit of graphite and of aluminium‐infiltrated graphite Fatigue properties of polycrystalline, isotropic graphite FU2590 and of FU2590 infiltrated with AlSi7Mg (FU2590/AlSi7Mg) were investigated in reversed bending tests at 25 Hz at numbers of cycles below 10⁷ and in tension‐compression tests at 20 kHz below 10⁹ cycles. The open porosity of Graphite (10‐11 Vol.‐%) was infiltrated with the aluminium alloy using the squeeze casting infiltration method, which led to an increase of the bending strength by 50 %, increase of tensile strength by 30 % and increase of stiffness by 15 %. Fully reversed tension‐compression loading of FU2590 delivers a mean endurance limit at 10⁹ cycles at the normalized maximum stresses (i.e. maximum tension stress of a cycle divided by the static strength) of 0,65±0,03. Mean numbers of cycles to failure of 10⁴ were found in fully reversed bending tests at the normalized maximum stress of 0,78. The infiltrated material shows approximately 30 % higher cyclic strength in reversed bending tests, and the mean endurance limit under tension compression loading increases by 15 %. The increased endurance limit of the infiltrated material is caused by the increased stiffness. The increased toughness of graphite due to the infiltration with aluminium is of additional beneficial influence at the higher cyclic stresses investigated in reversed bending tests and in static tests.     

High cycle fatigue mechanisms in a cast AM60B magnesium alloy

ABSTRACT We examine micromechanisms of fatigue crack initiation and growth in a cast AM60B magnesium alloy by relating dendrite cell size and porosity under different strain amplitudes in high cycle fatigue conditions. Fatigue cracks formed at casting pores within the specimen and near the surface, depending on the relative pore sizes. When the pore that initiated the fatigue crack decreased from approximately 110 µm to 80 µm, the fatigue life increased two times. After initiation, the fatigue cracks grew through two distinct stages before final overload specimen failure. At low maximum crack tip driving forces (K_max < 2.3 MPa√m), the fatigue crack propagated preferentially through the α‐Mg dendrite cells. At high maximum crack tip driving forces (K_max > 2.3 MPa√m), the fatigue crack propagated primarily through the β‐Al₁₂Mg₁₇ particle laden interdendritic regions. Based on these observations, any proposed mechanism‐based fatigue model for cast Mg alloys must incorporate the change in growth mechanisms for different applied maximum stress intensity factors, in addition to the effect of pore size on the propensity to form a fatigue crack.     

Endurance limit of die-cast magnesium alloys AM50hp and AZ91hp depending on type and size of internal cavities

The aim of this work was to investigate the influence of internal cavities on the fatigue properties of two of the technical most common die-cast magnesium alloys, AM50hp and AZ91hp. For this purpose the endurance limits of altogether three batches of S–N specimens, two conventional cast and one vacural cast, with varying internal defects have been measured. After fatigue failure the fracture surface of each sample has been analysed with respect to the site of crack initiation and, where appropriate, the size of the crack initiating cavity or pore. Moreover, on both alloys crack growth tests have been carried out and the thresholds ΔK_th of the stress intensity factor have been measured.Finally, the experimental data from both, the S–N tests and the crack propagation measurements, were depicted in a modified Kitagawa–Takahashi diagram. Using El Haddad’s and Topper’s approach the distribution function of the endurance limit has been proposed, whose parameters could be determined by fitting them to the experimental results. The knowledge of these parameters allows the calculation of the fracture probability as a function of an equivalent crack length and the stress amplitude.     

Versuchs- und Rechendaten zur Dauerschwingfestigkeit von metallischen Werkstoffen unter mehrachsiger Beanspruchung

Experimental Data and Calculated Results about the Fatigue Endurance Limit of Metals under Multiaxial Alternating Load An extensive catalogue of present available experimental data about the fatigue endurance limit of metallic materials under multiaxial loading conditions and thereupon determined deviation ratios between experimental results (long life fatigue tests) and calculated values by five newer computation methods is demonstrated (a further showed statistical analysis of these devations indicates, that in relation to the other failure criterions the ?Quadratische Versagenshypothese”? QVH is preferably recommended for a reliable application). The tabulated data-catalogue totally includes 530 referenced loading cases (limited to various biaxial states of combined normal and torsional alternating stress with sinusoidal synchronous or out-of-phase amplitudes and superimposed mean stresses) with experimental results (probability of survival of P_S = 50%) on metallic materials (unalloyed and alloy steels, non-ferrous metals and wrought aluminium alloys, cast irons and sintered metals).     

The thickness of defect bands in high-pressure die castings

Defect bands following the casting contour are often observed in high-pressure die cast (HPDC) components. In this article, suitable methods for measuring the thickness (w) and grain size in the bands (d_sb) in HPDCs have been developed. The w/d_sb relationship of defect bands has been investigated in HPDC specimens from a range of alloys, casting geometries and band locations within castings. Samples include aluminum alloy, AlSi4MgMn and AlMg5Si2Mn, tensile test bars cast by cold-chamber (cc) HPDC and a magnesium alloy, AM50, hot-chamber (hc) HPDC steering-wheel component. The band thicknesses were measured to be in the range 7–18 mean grains wide. This is substantial evidence that defect bands form due to strain localization in partially solidified alloys during cc and hc HPDC. Additionally, within this range, the w/d_sb ratio decreases with increasing distance from the casting center in a cross-section containing multiple bands.     

The influence of porosity on the fatigue life for sand and permanent mould cast aluminium

The automotive industry always strives to achieve light weight components to reduce fuel consumption and to meet environmental requirements. One way to obtain weight reduction is to replace steel components with components made of aluminium or other light weight materials. Aluminium has good corrosion properties and a high strength to weight ratio which makes it favourable in many applications. The increased use of aluminium castings in the automotive industry does also imply that the need for design data for aluminium increases. Especially for castings, the influence of casting defects are always an issue. For this reason fatigue properties for as-cast sand and permanent mould specimens with different contents of porosity have been studied.

Sand cast and permanent mould cast aluminium specimens of two different geometries were fatigue tested in cyclic bending at R = −1. Prior to fatigue test specimens were examined by X-ray and sorted into three quality groups depending on the porosity level. The aim of this work was to investigate the fatigue life for sand cast and permanent mould cast AlSi10Mg with different amounts of porosity. An additional aim was to predict the largest defect contained in a specified volume of a component, by using a statistical analysis of extreme values, and relate it to the fatigue life.

The results showed that fatigue strength for a smooth specimen geometry decreases by up to 15% with increased porosity. For specimens with a notched geometry, no influence of porosity on the fatigue strength was found. This is believed to be due to a much smaller volume subject to high stress than for specimens with low stress concentration.     

Fatigue failure of suspension arm: experimental analysis and multiaxial criterion

An experimental device have been developed to study fatigue phenomena for nodular cast iron automotive suspension arms. On the base of a detailed fracture analysis, it is shown that the major parameter influencing fatigue failure of casting components are casting defects: the High Cycle Fatigue behaviour is controlled mainly by surface defects such as dross defects and oxides while the Low Cycle Fatigue is governed by multiple cracks initiated independently from casting defects. A methodology is proposed to define the maximum defect size allowable in a casting component. It correlates the empirical method proposed by Murakami to determine the evolution of the fatigue limit with defect size and a multiaxial endurance criterion based on the Dang Van model. The junction between the two approaches gives a concurrent tool for the fatigue design of casting components. Validation of the proposed approach gives encouraging results for surface defects and constant amplitude proportional loading.     

FATIGUE-MICROSTRUCTURE RELATIONSHIPS IN SOME AGED ALUMINIUM ALLOYS

Abstract Aluminium–copper–magnesium alloys show a high response to age hardening but have relatively poor fatigue properties, whereas the reverse is true for aluminium magnesium alloys. Small additions of silver (∼0.1 atomic %) change both the type and dispersion of precipitates in Al–Cu–Mg alloys and promote age hardening in Al–Mg alloys in which it is normally absent. The paper is concerned with the effects of silver on the behaviour of representative Al–Cu–Mg and Al–Mg alloys tested under fatigue conditions.
Despite the fact that silver increases the response to age hardening in Al–Cu–Mg alloys, the fatigue endurance limit is reduced. This effect is even more marked with the alloy Al–5% Mg in which the 0.2% proof stress was increased from 85 to 200 MPa by adding silver, whereas the fatigue endurance limit fell from ±87 to ±48 MPa. In both systems, silver promotes formation of finely dispersed precipitates, and the poor fatigue properties are associated with the concentration of dislocations in intense slip bands. On the other hand, when large precipitate particles are present, dislocations are more uniformly dispersed and the fatigue properties are improved.     

Abnormal da/dN – ΔK curves: New failure mode with Ti – alloys

Fatigue crack propagation (FCP) -rates and -threshold values have been determined on the titanium alloys IMI 834, IMI 685 (turbine disk materials) and Ti-6Al-6V-2Sn (plate material). K_max-constant tests were executed in laboratory air at room temperature and run with 50 Hz on C(T) specimens. It was found that FCP-rates in K_max-constant tests followed the well known FCP behavior up to a certain limiting value K_max, denoted as °K_max. Below °K_max, the FCP-rates da/dN decrease with decreasing ΔK down to the threshold value ΔK_T (ΔK for 10^?7 mm/cycle). For K_max-constant tests with K_max > °K_max, the FCP-rates initially decreased with decreasing ΔK, but reached 10^?7 mm/cycle at smaller ΔK. For K_max ≧ ∧K_max > °K_max, FCP-rates of 10^?7 mm/cycle were never reached as ΔK decreased to and below ΔK_T. Instead, as ΔK approaches or gets smaller than ΔK_T, the FCP-rates stay either constant or increase again. The limit value °K_max for this abnormal FCP-behavior had been determined for IMI 834 to be 22 to 28 MPa√m, for IMI 685 to be 46 MPa√m and for Ti-6A1–6V-2Sn to be 26 MPa√m. The important result from a practical stand-point is the large difference in °K_max for comparable Ti alloys, i.e., IMI 834 and IMI 685.     

Application of ultrasound for fatigue testing of lightweight alloys

The use of aluminium and magnesium alloys offers a great potential for weight reduction in automotive applications. Load-bearing car components are subjected to 10⁸ cycles and more during service, and the high-cycle fatigue properties of construction materials are therefore of great interest.
The time-saving ultrasound fatigue testing method has been used to study the fatigue properties of a high-pressure, die-cast magnesium alloy AZ91  hp and a post-forged, cast-aluminium alloy AlSi7Mg0.3 in ambient air and saltwater (5wt% sodium chloride) spray. In ambient air, fatigue cracks in AZ91  hp emanate from voids, and it is possible to correlate void areas with the numbers of cycles-to-failure. Post-forging of AlSi7Mg0.3 reduces the numbers and size of voids. The remaining small voids (void areas smaller than 9000  μm² ) do not significantly reduce lifetimes. Saltwater deteriorates the fatigue properties of both the lightweight alloys. With increasing numbers of cycles, the influence of the corrosive liquid on fatigue strength becomes more pronounced.     

Fatigue and fatigue crack growth of aluminium alloys at very high numbers of cycles

The application of ultrasonic frequency (20 kHz) loading to test fatigue and fracture mechanical properties of materials is briefly reviewed and recent investigations on high strength aluminium alloys are reported. Very high cycle endurance tests and near threshold crack growth experiments were performed with the 2024-T351 aluminium alloy. Lifetimes are approximately 10–100 times lower, if cycled in distilled water instead of ambient air. Fatigue experiments under randomly varying loads showed that linear damage summation calculations overestimated lifetimes by approximately a factor 2. Fracture mechanics studies in ambient air, dry air and in vacuum served to investigate the role of air humidity on near threshold fatigue crack growth at ultrasonic frequency. The threshold value was 2.1 MPa√m in ambient air and 3.3 MPa√m in vacuum. The aluminium alloy AlZnMgCu1.5-T66 and the aluminiumoxyde particle reinforced alloy 6061-T6 were tested at 100 Hz and 20 kHz to investigate frequency influences on high cycle fatigue properties, and similar lifetimes were found at both frequencies.     

The effect of porosity on the fatigue life of cast aluminium‐silicon alloys*

Fatigue strength optimization of cast aluminium alloys requires an understanding of the role of micropores resulting from the casting process. High cycle fatigue tests conducted on cast A356‐T6 show that the pore size and proximity to the specimen surface significantly influence fatigue crack initiation. This is supported by finite element analyses (both elastic and elastic–plastic) which demonstrate that high stress/strain concentration is induced by pores which are both large and near to the specimen surface. A new pore‐sensitive model based on a modified stress‐life approach has been developed which correlates fatigue life with the size of the failure‐dominant pore. The model prediction is in good agreement with experimental data.     

The influence of porosity on the fatigue strength of high-pressure die cast aluminium

Aluminium is a lightweight material with high strength and good corrosion resistance among other beneficial properties. Thanks to these properties, aluminium is more extensively used in the vehicle industry. High‐pressure die casting of aluminium is a manufacturing process that makes it possible to attain complex, multi‐functional components with near‐net shape. However, there is one disadvantage of such castings, that is, the presence of various defects such as porosity and its effect on mechanical properties. The aim of this work was to investigate the influence of porosity on the fatigue strength of high‐pressure die cast aluminium. The objective was to derive the influence of defect size with respect to the fatigue load, and to generate a model for fatigue life in terms of a Kitagawa diagram. The aluminium alloy used in this study is comparable to AlSi9Cu3. Specimens were examined in X‐ray prior to fatigue loading and classified with respect to porosity level and eventually fatigue tested in bending at the load ratio, R, equal to ?1. Two different specimen types with a stress concentration factor of 1.05 and 2.25 have been tested. It has been shown that the fatigue strength decreases by up to 25% as the amount of porosity of the specimen is increased. The results further showed that the influence of defects was less for the specimen type with the higher stress concentration. This is believed to be an effect of a smaller volume being exposed to the maximum stress for this specimen type. A Kitagawa diagram was constructed on the basis of the test results and fracture mechanics calculations. A value of 1.4 Mpa m^1/2 was used for the so‐called stress intensity threshold range. This analysis predicts that defects larger than 0.06 mm² will reduce the fatigue strength at 5 × 10⁶ cycles for the aluminium AlSi9Cu3 material tested.     

Schwingfeste Auslegung von Schweiß‐ verbindungen aus der Magnesiumknetlegierung AZ31(ISO‐MgAl3Zn1) mit dem Konzept der fiktiven Ersatzradien von rf = 1,0 mm und 0,05 mm

Fatigue design of welded joints from the wrought magnesium alloy AZ31 (ISO‐MgAl3Zn1) by the local stress concept with the fictitious notch radius of r_f = 1.0 mm and 0.05 mm The investigations were carried out with three different types of MIG‐ and TIG‐welded magnesium joints of the alloy AZ31. The evaluation of the results showed that the local stress concept using the fictitious notch radius of r_f = 1.0 mm can be applied to magnesium welded joints from plates with thicknesses t ≥ 5 mm independently of the weld geometries (fully or partially penetrated butt welds, transversal stiffeners). Design curves are proposed for different stress ratios, i.e. R = ‐1 as well as 0 and 0.5, which allow the consideration of residual stresses as well as load induced mean stresses. The results permit also the suggestion of Δσ = 28 MPa as FAT‐value for the IIW‐Fatigue Design Recommendations. Further, the FAT‐value Δσ = 73 MPa for the fictitious radius of r_f = 0.05 mm to be applied to welded thin magnesium joints is derived, too. These FAT‐values are compared with already known data for steel and aluminium joints. A linear relationship between the FAT‐values and the Young’s modulus is determined.     

Spray formed and rolled aluminium‐magnesium‐scandium alloys with high scandium content

Aluminium‐magnesium‐scandium alloys offer good weldability, high corrosion resistance, high thermal stability and the potential for high strength by precipitation hardening. A problem of aluminium‐scandium alloys is the low solubility of about 0.3 mass‐% scandium when using conventional casting methods. The solution of scandium can be raised by higher cooling rates during solidification. This was realised by spray forming of Al‐4.5Mg‐0.7Sc alloys as flat deposits. Further cooling rates after solidification should also be high to prevent coarse precipitation of secondary Al₃Sc. Therefore a cooling device was designed for the spray formed flat deposits. The flat deposits were rolled at elevated temperatures to close the porosity from spray forming. Microstructures, aging behaviour and tensile properties of the rolled sheets were investigated. Strength enhancements of about 100 MPa compared to conventional Al‐Mg‐Sc alloys were achieved.     

Micromechanisms of fatigue crack growth in cast aluminium piston alloys

The fatigue crack growth behaviour in as-cast and hot isostatically pressed (HIP) model cast aluminium piston alloys with hypoeutectic Si compositions of 6.9 wt% and 0.67 wt% has been investigated. The HIP alloys showed slightly improved fatigue crack growth resistance. Analysis of the crack path profiles and fracture surfaces showed that the crack tends to avoid Si and intermetallic particles at low ΔK levels up to a mid-ΔK of ∼7 MPa√m. However, some particles do fail ahead of the crack tip to facilitate crack advance due to the interconnected microstructure of these alloys. At higher levels of ΔK, the crack increasingly seeks out Si and intermetallic particles up to a ΔK of ∼9 MPa√m after which the crack preferentially propagates through intermetallic particles in the 0.67 wt%Si alloy or Si and intermetallics in the 6.9 wt%Si alloys. It was also observed that crack interaction with intermetallics caused crack deflections that led to roughness-induced crack closure and possibly oxide-induced crack closure at low to mid-ΔK. However, crack closure appears unimportant at high ΔK due to the large crack openings and evidenced by the fast crack growth rates observed.     

The influence of post‐heat treatments on the tensile strength and surface hardness of selectively laser‐melted AlSi10Mg

To improve the mechanical properties of cast aluminium alloys several post‐heat treatments are known. However, these treatments cannot directly be transposed to additively via selective laser melting manufactured aluminium alloys, e. g., aluminium‐silicon‐magnesium (AlSi10Mg). Therefore, this study aims to determine suitable post‐heat treatments to optimise the mechanical properties of SLM‐built AlSi10Mg specimen. The influence of various post‐heat treatment conditions on the material characteristics was examined through hardness and tensile tests. The findings indicate that the Vickers hardness and ultimate tensile strength could not be improved via secondary precipitation hardening, whereas the fracture elongation shows a value which is distinctly higher than the values of a comparable cast alloy. Solution annealing at 525 °C reduces the hardness and the ultimate tensile strength by about 40 % and increases the fracture elongation three times. A subsequent precipitation hardening allows recovery of 80 % of the as‐built hardness, and 90 % of the previous ultimate tensile strength combined with maintaining an improved fracture elongation of about 35 % compared to the respective as‐fabricated condition.     

Influence of transverse load on the high cycle fatigue behaviour of low pressure cast AZ91 magnesium alloy

A new testing procedure, employing transverse load was adopted to investigate the high cycle fatigue behaviour of low pressure cast AZ91 magnesium alloy. The tests were conducted with an electro dynamic shaker system by employing specimens fabricated as per ASTM standard. S–N plot was generated from the test results and compared with that of gravity cast AZ91 alloy tested in identical ambience. The influence of transverse load on the fatigue behaviour of these alloys is discussed. As fatigue cracks were found to have initiated in pores in most of the tested samples, pores were assumed as initial cracks as per linear fracture mechanics and the critical stress intensity amplitude (K_cr) was estimated. Structure–fatigue property correlations are discussed using fractographs. Mean stress effect on the fatigue properties and effects of alloying constituents are also discussed.     

Comparison of retardation behaviour of 2024-T3 and 7075-T6 Al alloys

Retardation in fatigue crack growth rate following the application of single and periodic tensile overloads was studied for 2024‐T3 and 7075‐T6 aluminium alloys. Tests were performed at constant stress and at constant stress intensity factor ranges, at a load ratio of R= 0.1, at a baseline ΔK in the 10–20 MPa√m range which corresponds to the Paris regime. Overload ratios of 1.3–1.65 were studied with overload spacing, n, varying from 20 to 10 000 cycles. 2024‐T3 displayed an order of magnitude higher retardation, N_d, due to single tensile overloads compared to 7075‐T6. Periodic overloads induced maximum retardation when n/N_d≈ 0.5 for both alloys, the magnitude being only 15% higher for 2024‐T3.