Magnesium alloy as a lightweight structural material has recently kindled considerable interest in the automotive and aerospace industries, since lightweighting is considered as one of the salient strategies in reducing fuel consumption and anthropogenic greenhouse gas emissions. The structural applications of magnesium alloys inevitably involve fatigue resistance. This study was aimed at evaluating cyclic deformation behavior and fatigue life of a super-vacuum die cast (SVDC) AM60B alloy using strain-controlled low cycle fatigue tests at two strain ratios of R s  = −1 and R s  = 0.1. The SVDC AM60B alloy exhibited a superior fatigue resistance to the conventional die cast AM60 alloy especially in the high-cycle fatigue region. Fatigue life was longer at R s  = −1 than at R s  = 0.1 at lower strain amplitudes. With increasing total strain amplitude, cyclic stress amplitudes increased, hysteresis loops exhibited a clockwise rotation despite the symmetry in tension and compression at R s  = −1, and fatigue life and psuedoelastic modulus decreased at both strain ratios. Cyclic hardening increased with increasing strain amplitude and strain ratio due to the formation of more twins and their interaction with dislocations. Two types of twins (wider lenticular extension twins and narrower banded contraction twins) were observed in some favorably oriented large α-Mg cells near the fracture surface. Mean stress relaxation occurred mainly within the initial 10–30% of fatigue life. Cyclic hardening exponent was higher than monotonic hardening exponent. Fatigue crack initiation occurred from the specimen surface or near-surface defects, and crack propagation was mainly characterized by fatigue striations coupled with some secondary cracks and tear ridges.

Cyclic deformation behavior of a super-vacuum die cast magnesium alloy

Isothermal and thermal fatigue is one of the most important factors controlling lifetime and damage of tools for hot metal forming processes. Rapid temperature alterations and the resulting inhomogeneous thermal stresses lead to cyclic plastic deformation, crack initiation and to crack propagation until final fracture of the components. In addition, characteristic residual stress states are formed due to inhomogeneous plastic deformations. In this paper results of isothermal and out-of-phase thermal fatigue tests of tool steel AISI H11 (German grade X38CrMoV5-1) in the temperature range 100 °C ≤ T ≤ 650 °C are presented. Stress amplitudes, plastic strain amplitudes and mean stresses were recorded as a function of the number of loading cycles. This allows a comprehensive assessment of the influence of loading parameters on the development of stresses, the formation of cracks and the lifetime of the material.

Cyclic stress–strain behavior and damage of tool steel AISI H11 under isothermal and thermal fatigue conditions

Effect of heat treatment on microstructures and mechanical properties of high vacuum die casting Mg–8Gd–3Y–0.4Zr magnesium alloy

In thermo-mechanically coupled forming processes for industrial mass production residual stresses are an unavoidable consequence of the alternating inhomogeneous fields of temperature and mechanical stress developing in tools and components dependent on the process parameters applied. Hence, a considerable interest exists to get reliable information about origin and distribution of the relevant residual stress fields and to understand the basic principles of their formation. By way of example a metal forming process based on a predefined locally and temporally differential temperature profile is described, which leads to a characteristic materials property profile and geometrical shape. The development of residual stress in tools (steel AISI H11) used for the thermo-mechanical forming operation of cylindrical flange shafts (steel SAE 6150) is outlined. To this end residual stress analyses by X-ray as well as by neutron diffraction were carried out. The loading situation of the tool was simulated by isothermal, thermal and contact fatigue tests, providing information about cyclic stress and plastic deformation during the manufacturing process.

Residual Stress State in Tools Used for Thermo-mechanical Metal Forming Processes

The effects of conventional heat treatments (quenching and tempering) and thermochemical treatment (gas carburizing) on the mechanical characteristics of non standard hot work tool steel, close to either AISI-H11/H13 are investigated. The major elemental composition differences are in carbon, silicon and vanadium. The objective of the carried heat treatments is to obtain an efficient tool performance in terms of hardness, wear resistance and mechanical strength. For conventional heat treatments, experimental results allow an explanation of the surface properties depending mainly on both chemical composition and optimised preheating parameters. After austenitizing at 1050 °C for 15 min, the as-quenched steel in oil bath exhibited the fully martensitic structure connected to a small fraction of retained austenite and complex carbides mainly of M23C6 type. Twice tempering at 500 °C and 600 °C resulted in the secondary hardness effect. As a result, carbide content amounted to 3% while the retained austenite content decreased to 0%. Accordingly, the required mechanical properties in terms of hardness and wear are fulfilled for the expected tool life. For carburizing process, global results are related to (i) formed layers which are characterized by their basic properties such as thickness, depth, formed phases, hardness and wear resistance, (ii) microstructures and diffusion mechanisms which show a close dependence of both time-temperature process and chemical composition of material. Treatment results for various processing times indicate that retained austenite content decreases drastically as processing time increases. Besides, test results provide a significant improvement of wear resistance for specimens presenting higher retained austenite content despite their relatively lower harnesses. Furthermore, productivity of the suggested new material can be extensively enhanced when the above treatment is used. Metallurgical evaluations are carried out using metallographic techniques, optical (MO), scanning electron microscopy (SEM) equipped with an energy-dispersive X-Ray spectrometer (EDS) and X-ray diffraction techniques. Mechanical properties are achieved mainly by standard hardness and wear tests. Résumé: Les effets des traitements thermiques classiques (trempe et revenu) et du traitement thermochimique (cémentation à gaz par exemple) sur les caractéristiques mécaniques d’un acier à outil industriel de travail à chaud, proche de AISI-H11/H13 sont étudiées. La différence dans la composition chimique réside dans les éléments carbone, silicium et vanadium. L'objectif des traitements thermiques est d'obtenir un outil efficace et performant en termes de dureté, de résistance à l'usure et de résistance au choc. Les résultats expérimentaux des traitements thermiques préconisés fournissent une compréhension explicite des propriétés de surface principalement par rapport à la composition chimique et les paramètres optimaux de traitements. Après austénitisation à 1050 °C pendant 15 min, l'acier trempé à l’huile révèle une structure entièrement martensitique liée à une petite fraction d'austénite résiduelle et des carbures complexes de type M23C6. Un double revenu à 500 et 600 °C contribue au phénomène de durcissement secondaire. Le pourcentage de carbures augmente de 2 à 3% tandis que la fraction volumique de l’austénite résiduelle diminue jusqu’à 0%. En conséquence, les propriétés mécaniques requises en termes de dureté et de résistance à l’usure sont atteintes pour une durée de vie définie. Pour le processus de cémentation, les résultats globaux sont liés aux (i) couches formées qui sont caractérisées par leurs propriétés de base telles que l'épaisseur, la profondeur, les phases formées, la dureté et la résistance à l'usure, (ii) microstructures et aux mécanismes de diffusion qui montrent une dépendance étroite entre le temps de traitement et la composition chimique du matériau. Les résultats relatifs aux différents temps de traitement indiquent que la fraction volumique de l’austénite résiduelle diminue de façon drastique avec l’augmentation du temps de traitement. Par ailleurs, les résultats relatifs aux essais montrent une amélioration significative de la résistance à l'usure pour les échantillons qui présentent la fraction volumique d’austénite résiduelle la plus élevée malgré leur bas niveau de dureté. En outre, la productivité des pièces se voit largement améliorée lorsque le traitement suggéré est utilisé. La caractérisation métallurgique est effectuée par le biais de techniques métallographiques, de la microscopie optique (MO) et électronique à balayage (MEB) équipé d'un spectromètre à dispersion d'énergie X-Ray (EDS) et les techniques de diffraction des rayons X. Les propriétés mécaniques sont déterminées principalement par les essais de dureté et d’usure abrasive. صخلم : (ة"نبركلا) ةيرارحويميكلا و (عاجرا + ةياقس) ةيديلقتلا ةيرارحلا تاجلاعملا نع ةبترتملا راثلاا ةسارد تمت ذلاو"فل ة"يكيناكيملا صئا"صخلا ى"لع ةغيص نم بيرق ةيلاع ةرارح تاجرد يف لمعلل صصخم AISI H11/H13 رصانعلا يف فلاتخا دوجو عم : ا .مويدنافلا و مويسيلسلا و نوبركلا فهل ىلا تانيعلا ةرارح ةجرد عفر دعب .تامدصلا و فلتلا ةمةاقم و ةبلاصلا ةيحان نم ةلاعف ةادا داجيا وه تاجلاعملا هذه نم 1050 ° ةد"مل م 15 ند"عملا د د"يدحلا ن"م ة"يمك يو"حت تي"سنترملا ن"م ة"بيرق ة"ينب ب"ستكيتيزلا ى"ف ةعرسب دربملا γ عو"نلا ن"م تا"بكرم و 6 23C M نا ة"جرد د"نع افعا"ضم ا"عاجرا ةرارح 500 ° و م 600 ° نم تاديبركلا ةبسن عافترا ظحلان .ندعملل ةيوناث ةيسقت هنع جتني م 2 % ىلا 3 % د"يدحلا ةبسن يف ضافخنا عم γ ة"فلختملا .رفصلا ىلا ب قلعتت جئاتنلا لمجم ةنبركلل ةبسنلاب : 1 ا راوطلاا و قمعلاب زيمتت يتلا و ةلكشتملا تاقبطلا ( .ةنوكتمل 2 دو"جو ن"ع نهر"بت ي"تلا را"شتنلاا تايلآو ةينبلا ( د"يدحلل ة"يمجحلا ةب"سنلا نا ني"بت ة"جلاعملا تا"قوا ف"لتخمل ةيب"سنلا جئا"تنلا . ند"عملا ة"ينب و ة"جلاعملا ت"قو ني"ب ةقلاع γ يرذ"ج لك"شب ضفخن"ت ة"فلختملا لا ةمواقم يف عافترا ظحلان ىرخا ةهج نم .ةجلاعملا تقو عافترا عم انمازت د"يدحلا ن"م ةر"بتعم ة"يمك ى"لع يو"حت ي"تلا تا"نيعلا د"نع ف"لت γ . ة"فلختملا دقف ةيكيناكيملا صئاصخلا اما ,ةينبلا ديدحتل ةينيسلا ةعشلااب ليلحتلا و حساملا ينورتكلاا و يئوضلا رهجملاب ةناعتسلاا تمت تقو اذه لامعت"ساب ت"سرد .فلتلا ةمواقم و ةبلاصلا سايق ةزهجا

Amélioration des propriétés de surface d'un acier a outils de travail à chaud par des traitements thermiques et thermochimiques (Cémentation)

Isothermal Fatigue of Tool Steel AISI H11

Thermal Fatigue of Shot Peened or Hard Turned Hot‐Work Steel AISI H11

Thermal fatigue tests of the magnesium-base alloy AZ91 were carried out under total strain control and out-of-phase-Ioading conditions in a temperature range between 50°C and +190°C Specimen produced by a vacuum die casting process were loaded under constant total strain and uniaxial homogeneous stress To simulate the influence of different mean stresses, experiments were started at different temperature levels, i e the lower, mean or upper temperature of the thermal cycle The thermal fatigue behaviour is described by the resulting stress amplitudes, plastic strain amplitudes, and mean stresses as a function of the number of thermal loading cycles Depending on the maximum temperature, and the number of loading cycles, cyclic softening as well as cyclic hardening behaviour is observed The upper temperatures or the respective temperature amplitudes determine the total fatigue lifetime

Thermal Fatigue of Magnesium Base Alloy AZ91

Thermische Ermüdung des Warmarbeitsstahls X38CrMoV 5‐1 in hartgedrehtem oder kugelgestrahltem Zustand

Primary progressive multiple sclerosis (PPMS) is characterised by gradual worsening of disability from symptom onset. Knowledge about the natural course of PPMS remains limited.PPMS patients from the German NeuroTransData (NTD) MS registry with data from 56 outpatient practices were employed for retrospective cross-sectional and longitudinal analyses. The cross-sectional analysis included a contemporary PPMS cohort with a documented visit within the last 2 years before index date (1 Jan 2021). The longitudinal analysis included a disease modifying therapy (DMT)-naïve population and focused on the evolution of expanded disability status scale (EDSS) from the first available assessment at or after diagnosis within the NTD registry to index date. Outcome measures were estimated median time from first EDSS assessment to first 24-week confirmed EDSS ≥ 4 and ≥ 7. Besides EDSS change, the proportion of patients on disability pension were described over time.The cross-sectional analysis included 481 PPMS patients (59.9% female, mean [standard deviation, SD] age 60.5 [11.5] years, mean [SD] EDSS 4.9 [2.1]). Estimated median time from first EDSS assessment after diagnosis to reach 24-week confirmed EDSS ≥ 4 for DMT-naïve patients was 6.9 years. Median time to EDSS ≥ 7 was 9.7 years for 25% of the population. Over a decade mean (SD) EDSS scores increased from 4.6 (2.1) to 5.7 (2.0); the proportion of patients on disability pension increased from 18.9% to 33.3%.This study provides first insights into the German NTD real-world cohort of PPMS patients. Findings confirm the steadily deteriorating course of PPMS accompanied by increasingly limited quality of life. 

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Martin Krauß

Papers

Isothermal Fatigue of Tool Steel AISI H11

Thermal Fatigue of Shot Peened or Hard Turned Hot‐Work Steel AISI H11

Thermal Fatigue of Magnesium Base Alloy AZ91

Thermische Ermüdung des Warmarbeitsstahls X38CrMoV 5‐1 in hartgedrehtem oder kugelgestrahltem Zustand

The natural history of primary progressive multiple sclerosis: insights from the German NeuroTransData registry