Einfluss von Gussfehlern auf die Dauerfestigkeit von Aluminium‐ und Magnesiumgusslegierungen

Influence of casting defects on the endurance limit of aluminium and magnesium cast alloys The influence of porosity (voids and shrinkage) on the fatigue properties at very high numbers of cycles is shown for the alloys AZ91 hp, AM60 hp, AE42 hp, AS21 hp and AlSi9Cu3 produced by high pressure die casting. Fatigue tests performed with ultrasonic equipment up to 10⁹ cycles show that these alloys exhibit a fatigue limit. The mean endurance limits (50% failure probability) of the magnesium alloys are 8–50 MPa and of the aluminium alloy 75 MPa. Fatigue cracks initiate at porosity, and whether a specimen fractures or not depends on the stress amplitude and the area and the site of the defect. Regarding the cast defect as an initial crack, a critical stress intensity value (K_cr) may be found to propagate a crack until final failure. K_cr of the magnesium alloys is 0,80–1,05 MPa√m, and 1,80 MPa√m was found for AlSi9Cu3. Using K_cr it is possible to correlate the probability of different defect sizes and the failure probability at different stress amplitudes. Additionally, predictions of the influence of rare large casting defects on the endurance limit are possible.     

Einfluss von Temperatur und Vorverformung auf das plastische Werkstoffverhalten von modernen Karosseriestählen

Influence of temperature and prestraining on the plastic material behaviour of modern sheet steels for autobody applications Within the scope of a common research project of the automotive and steel industry, characteristic values describing the plastical behaviour of 20 sheet steels have been determined. In detail, quasistatic tensile tests at the testing temperatures ‐40 °C, 23 °C and 100 °C were carried out to obtain flow curves for the as delivered materials as well as for steels after a defined prestraining or heat treatment. Additionally, sheet metal testing led to forming limit diagrams and limiting drawing ratios including the working ranges for deep drawing. The results of the tensile tests showed significant differences between steel groups with regard to their strain hardening behaviour, which can be described by the ratio of yield and tensile strength R_p0,2/R_m or the Θ_IV‐value, and their temperature sensitivity. Within one steel group, consisting of steels with similar strain hardening behaviour, it might be possible to determine flow curves of one steel in a defined condition in order to calculate the flow curves of other steels with different strength. An advantage would be a lesser number of experimental tests which have to be performed in order to supply reliable input data for numerical material and component modelling.     

Zum Einfluss des Schweißens auf das Kriechverhalten moderner Stähle für die thermische Energieerzeugung

The influence of welding on creep behaviour of modern steels for thermal power generation Un‐ and low alloyed ferritic/bainitic Chromium steels as well as high alloyed ferritic/martensitic 9–12 % Chromium steels are widely used for high temperature components in thermal power generation. Welding in all its variety is the major repair and joining technology for such components. The weld thermal cycle has significant influence on the base material microstructure and its properties. The Heat Affected Zone is often regarded as the weakest link during high temperature service. While weldments of un‐ and low alloyed ferritic Chromium steels can show significant susceptibility to Reheat Cracking in the coarse grained heat affected zone, weldments of high alloyed ferritic Chromium steels generally fail by Type IV Cracking in the fine grained heat affected zone during long term service. In this paper the influence of the weld thermal cycle on the base material microstructure is described. Long‐term creep behaviour of weldments is directly related to the main failure mechanisms in creep exposed ferritic weldments and implications for industries using heat resistant ferritic steels are shown.     

Zum Einfluß von Si und P auf das Verzinkungsverhalten Von Baustählen

The influence of Si and P contents in steels on the galvanizing behavior The galvanizing behavior of 40 structural steels in current use was investigated in relation to the complex influence of the Si/P content under conditions that are usual in the hot dip galvanizing industry (440/450/460 °C,5/10/15 min immersion time). The effect of P on the increase of coating thickness on steels with 0,01 to 0,40 % Si begins to be perceptible at > 0,020 % P. The influence of P increases with decreasing Si content of the steels and decreasing temperature of the melt. In steels with ≤ 0,12 % Si, an increased P content causes a shift of the thickness maxima in the temperature range from 440 to 450 °C, such as is otherwise typical of steels of 0,12 % to 0,28 % Si with < 0,020 % P. Zinc coatings on steels with the critical P/Si content (0 to 0,20 % Si/> 0,020 % P) as a rule are more unstable. With increased immersion time at the temperature stated above, floating away of the ξ phase into the zinc melt may occur. This could also be observed with Sandelin steels in the most critical Si range (0,07 to 0,10 %) with < 0,020 % P, with an immersion time of 15 min. It is possible to reduce the thickness of zinc coatings by adding small amounts of Al to the zinc melt (< 0,03 %). This effect of Al at a concentration that is below what is required for the known inhibitory action by the formation of an thin Fe₂Al₅-or Al-containing δ₁ film on the steel surface is attributed to the instability of the ξ phase, a proportion of which floats away into the zinc melt.     

Über den Einfluss von Werkstoffkennwerten auf das Spanbruchverhalten von bleiarmem Messing

Low‐leaded brass materials are gaining in industrial significance due to multiple national and international legislative regulations. Reducing the mass fraction of lead in brass material leads to unfavorable chip forms in machining and an increased chip volume ratio, which is a measure of their bulk volume. Cutting tests show that a certain amount of reduction in lead content can be compensated by adjusting the machining parameters feed and cutting depth. A phenomenological model for the calculation of the chip volume ratio as a function of the lead content and the machining parameters is developed. This work shows that there is no significant correlation between chip volume ratio and standardized material testing procedures such as tensile test, hardness testing or quantitative microstructural analysis.     

Zum Einfluß wechselnder Temperaturen auf das Zeitstandverhalten von Nickellegierungen

On the Effect of Alternating Temperatures on the Creep Strength of Nickel Alloys. The design of parts for duty under creep stresses, such as turbine blades, had conventionally been rested on the assumption of linearly accumulating damage in the presence of alternating temperatures. Experimental work on nickel alloys has now shown that the damage will not generally accumulate linearly. Temperature alternations, when superimposed upon creep stresses, will cause multiplicative damage by impairing the creep strength and on occasion substantially promoting creep.     

Einfluss von Randschichtzustand und Haltezeit auf die thermische Ermüdung der Magnesiumbasislegierung AZ31

In this paper thermal fatigue of magnesium base alloy AZ31 in the temperature range between +50°C and +290°C is investigated. Experiments were carried out under constant total deformation (out‐of‐phase loading) and the resulting stress amplitudes as well as the plastic strain amplitudes were recorded as a function of the number of thermal loading cycles. In particular the consequences of mechanical surface treatments (deep rolling) and of hold‐times were investigated. In both cases no particular influence compared with untreated specimens loaded without hold‐times was observed, which is due to the interaction of deformation and recrystallization processes during thermal fatigue.     

Spannungsfaktoren eines Risses in der Umgebung eines Kreisloches und ihr Einfluß auf das Bruchverhalten

Stress Intensity Factors in the Neighbourhood of a Circular Hole and their Influence on Crack Behavior . A photoelastic method was developed to determine the stress intensity factors K_I and K_II for cracks subjected to mixed-mode loading. The constants in the near field expansion about the crack tip were computed using a non-linear optimization program to give a best fit to the observed isochromatics. Copying the latter onto an equal density film increased their sharpness and, thus, the accuracy of the determination. The method was applied to cracks lying perpendicular to the external stress near a circular hole in a plate under uniaxial tension and the results used to describe the paths of cracks in the neighbourhood of a hole.     

Einfluss der Oberflächenbehandlung auf das Korrosionsverhalten von Magnesiumlegierungen

For the coating of magnesium alloys, especially those that should be used in medical technologies, fluoride coatings are tested and used. These coatings should enable a predictable degradation behavior for future implant materials. Presently the pre‐ and post‐treatment processes vary, so an optimum to achieve defined thicknesses of the fluoride layers is still not established. The present investigations are based on the process parameters known from the literature, they are discussed and compared. Different pre‐treating times of the base material (pure magnesium and a magnesium‐aluminum‐calcium alloy as a comparison) in 200 g/l sodium hydroxide combined with 96 hours of immersion in 40 % fluoride acid are examined.     

Einfluss der PVD‐Prozessparameter auf die Schichtnukleation und Struktur von CrN‐Schichten

Störstellen in Bauteiloberflächen wirken sich negativ auf die Schichtnukleation bei PVD‐Schichtsystemen aus. Sie können im anschließenden Schichtwachstum zu Defekten in der Schichtstruktur führen. Die Konsequenz ist eine Minderung der lokalen Haftfestigkeit, höhere Reibwerte und Fehlstellen in der Schicht. Diese begünstigen in korrosiven Medien die Entstehung von Korrosion. CrN‐Schichten sind für ihre hohe Härte und ihre guten Verschleißeigenschaften bekannt. Zudem weisen sie eine bessere Korrosionsbeständigkeit als Titan basierte Nitride auf. Die strukturellen und mechanischen Eigenschaften dieser Schichtsysteme können unter anderem durch die Veränderung der Biasspannung und des Stickstoffgehalts während des Schichtwachstums beeinflusst werden. Durch gezielte Variation dieser Beschichtungsparameter wurden CrN‐Schichten mit den bevorzugten Kristallgitterstrukturen (111) und (200) abgeschieden, mit dem Ziel deren Abdeckungsfähigkeit von Störstellen zu untersuchen. Für die gezielte Untersuchung und Vergleichbarkeit der Ergebnisse wurden Vickerseindrücke HV 0,5 als bewusst gestaltete Störstellen auf der Grundwerkstoffoberfläche aufgebracht. Als Grundwerkstoff diente niedriglegierter Stahl (100Cr6). Die Kristallgitterorientierungen der abgeschiedenen Schichten wurden zunächst anhand von röntgenographischen Messungen (XRD) identifiziert. Anhand weiterer rasterelektronenmikroskopischer Aufnahmen (REM) wurden die bewusst erzeugten Störstellen in Form von Vickerseindrücken und Ritzspuren sichtbar gemacht, mit dem Ziel die Schichtqualität und die Mikrostruktur um Fehlstellen zu beschreiben. Imperfections on substrate surfaces can cause growth defects in hard coatings prepared by magnetron sputtering, leading to local loss of adhesion, higher friction, voids and corrosion at coating defects. CrN‐coatings are known for their high hardness and good wear resistance. In addition CrN‐coatings are better corrosion resistant than Ti based nitrides. The structure and the mechanical properties of those coatings can be influenced by varying bias voltage and gas flow during film growth. Due to variation of those parameters CrN‐coatings were deposited with preferred crystallized lattice orientations (111) and (200). The main objective of investigation is the potential to cover imperfections. Vickers indentations (HV 0.5) were applied as designed imperfections. As base material low alloyed steel was used (100Cr6). At first crystallized lattice orientations were identified by X‐ray diffraction (XRD). Further scanning electron microscope analysis (SEM) was applied to identify and study crystallized lattice orientations near those imperfections.     

Einfluss der Mikrostruktur auf das Verschleiß‐ verhalten der hartanodisierten Aluminium‐legierungen EN AW‐6082 und EN AW‐7075

Microstructural effect on the wear behaviour of the hard‐anodised aluminium alloys EN AW‐6082 and EN AW‐7075 The suitability of hard‐anodising of high‐strength Al alloys (EN AW‐7075‐T651) for the fabrication of protective coatings which are also applicable on screws was investigated. A medium‐strength AlSi1MgMn alloy (AA60682‐T6), generally rated as applicable for anodising, was used as reference material. After possible setting phenomena of a screw joint, the load‐bearing surface of the screw can be subjected to an oscillating relative movement. The damaging tribological load was simulated in an oscillation wear test. The resulting wear appearances have revealed that the untreated oxide coatings on the EN AW‐6082 substrate are not capable of providing protection against tribological load. Since hot‐water sealing increases the hardness of the coating but also contains the technology‐induced risk of softening the substrate material, other tribological protection methods have been looked for. The analysis of the tribological tests (characterisation of the structure and the resulting properties of the material, measurement of the wear amount and analysis of the wear appearance) have shown that the films sealed with wax emulsion on both substrate materials are the most promising candidates for the application of devices under oscillation wear. The obtained roughness, friction coefficients and hardness values confirm the positive behaviour of the anodically oxidised EN AW‐7075‐T651 alloy under the chosen tribological load.     

Haftfestigkeit von Metallschichten auf Stählen bei statischer und wechselnder Beanspruchung

Influences of Static and Dynamic Load on the Adhesive Strength of Thermally Sprayed Metal Coatings on Steels It were the aims of this work to produce metal-sprayed specimens and workpieces with high resistance to static and dynamic load, to prove these properties by appropriate measurements and to give advices for practical application. For this purpose, two different substrate materials were metal-coated with five materials by use of three spraying techniques. As a result of the investigations, it can be summarized that specimens, which were shot-peened prior to spray-coating reached the same fatigue limit as only shot-peened specimens, independent on the spraying technique applied.     

Einfluß von Spannungsfrei-Glühung und Abkühlungsgeschwindigkeit auf Mikrostruktur und Festigkeit von TiAl6V4

Influence of stress relieving of TiA16V4 The stress relieving of Ti6A14 V has been investigated by electron microscopy and by tensile tests. Partial recrystallization could be detected at 740 °C. The cooling rate from the stress relieving temperature exerts a marked influence on strength and elastic modulus. Slow cooling increases tensile strength, probably because of a higher volume fraction of alpha and because of an oxygen ordering reaction. It is concluded that after stress relieving, slow cooling down to a temperature of 300 °C is of advantage.     

Einfluss der Porosität auf die Schwingfestigkeit von Proben und Bauteilen aus Aluminiumdruckguss

Influence of Porosity on Fatigue Strength of Aluminium Die‐Cast Specimens and Components Aluminium die‐casting is widely used in the automotive industry. Brackets, engine blocks, wheels and suspension arms etc. are typical applications. Due to the gases contained in the melt, more or less spherical pores arise in the cast components during the cooling procedure, usually with a higher concentration in the middle. The distribution of the pores depends both on the casting condition and on the form of the components and is stochastic. In the past, the simple way is often chosen for the fatigue life estimation of aluminium die‐casting: The fatigue strengths of the material, e.g. in the form of S‐N curves, are determined by an unnotched specimen which are then used in the fatigue calculation. However, the application of these data to real components with notches leads in many cases to fatigue life estimations which are by far too conservative. In the present paper a simple geometrical model of pores is presented. Using this model, the notch stress caused by a pore can be estimated. The behaviour of pores in a real component with notches will be treated. It will be shown that the mentioned conservative nature of the fatigue life prediction and the apparent notch insensitivity of aluminium die‐casting can be explained by the facts that the stress distribution in a real component, contrary to an unnotched specimen, is inhomogeneous and that the porosity density on the surface is often low.