Thermo-mechanical Forming of Al–Mg–Si Alloys: Modeling and Experiments

In an ongoing quest to realize lighter vehicles with improved fuel efficiency, deformation characteristics of the material AA 6016 is investigated. In the first part of this study, material behavior of Al–Mg–Si sheet alloy is investigated under different process (temperature and strain rate) and loading (uniaxial and biaxial) conditions experimentally. Later, warm cylindrical cup deep drawing experiments were performed to study the effect of various parameters on warm forming processes, such as the effect of punch velocity, holding time, temper and temperature on force-displacement response. The plastic anisotropy of the material which can be directly reflected by the earing behavior of the drawn cups has also been studied. Finite element simulations can be a powerful tool for the design of warm forming processes and tooling. Their accuracy will depend on the availability of material models that are capable of describing the influence of temperature and strain rate on the flow stresses. The physically based Nes model is used to describe the influence of temperature and strain rate and the Vegter yield criterion is used to describe the plastic anisotropy of the sheet. Experimental drawing test data are used to validate the modeling approaches

Effect of temperature on anisotropy in forming simulations of aluminum alloys

A combined experimental and numerical study of the effect of temperature on anisotropy in warm forming of AA 6016-T4 aluminum was performed. The anisotropy coefficients of the Vegter yield function were calculated from crystal plasticity models with an adequate combination of extra slip systems. Curve fitting was used to fit the anisotropy coefficients calculated at discrete temperatures. This temperature dependent constitutive model was successfully applied to the coupled thermo-mechanical analysis of deep drawing of aluminum sheet and results were compared with experiments

Experiência de 6 Anos de Cirurgia Micrográfica de Mohs num Hospital Português

Introduction: Mohs micrographic surgery (MMS) is a surgical method of treating skin tumors that consists of histological control of the tumor margins using horizontal slices in thin, freshly frozen layers. This technique is a preferred indication for cutaneous facial tumors (basal and squamous cell carcinoma, among others) associated with higher risk of recurrence, recurrent tumors, tumors with undefined clinical margins, when there is perivascular and perineural involvement or with aggressive histological subtypes. Methods: The authors carried out a retrospective study of a 6-year period (from July 2012 until June 2018) at a Portuguese hospital – Egas Moniz hospital, to define the characteristics of patients undergoing Mohs micrographic surgery and to evaluate the advantages of this technique. The following factors were analyzed: age, gender, provenience, diagnosis, tumor location, number of stages of MMS, type of reconstruction of the surgical defect, follow-up and recurrences. Results: A total of 835 tumors were excised, 459 in male patients and 376 in female patients. The mean age at surgery was 71 years old (range 23- 95 years). Most of the lesions submitted to MMS were basal cell carcinomas (87%) and the most common location was the nasal pyramid (43%). In 44% of the cases, it was necessary to perform more than one micrographic stage. Half of the surgical defect closures were performed by graft or skin flap. Considering patients with a follow up equal or superior to 3 years, the recurrence rate was 4.9%. Discussion: The most frequent diagnosis was basal cell carcinoma, reflecting the importance of MMS in this type of skin malignancy, in both primary and persistent/recurrent lesions. Compared to other European surgery reviews, our recurrence rate is within normal range. This fact is particularly relevant, if we consider that our institution receives patients sent from hospitals and dermatologists from all over the country, specially selected for tumor aggressiveness or previous relapse. More than half of these patients were referred from other centers and consisted of persistent/recurrent basal cell carcinomas. The follow-up at recurrence for MMS was 27.9 months, supporting that a long follow-up of these patients is necessary. Conclusion: MMS allows lower rates of recurrence compared to simple surgical excision, so its practice should be encouraged in selected tumors. The surgical control of the margins instead of the “blind margins” is an advantage in saving healthy tissue and allows the complete excision of the tumor in the same surgical time.Introdução: A cirurgia micrográfica de Mohs é um método cirúrgico de tratamento dos tumores cutâneos que consiste no controlo histológico das margens do tumor usando cortes horizontais em camadas finas congeladas a fresco. Esta técnica é uma indicação preferencial para tumores cutâneos da face (carcinoma basocelular e espinocelular, entre outros) associados a maior risco de recidiva, tumores recidivados, com margens clínicas indefinidas, quando existe envolvimento perivascular e perineural ou com subtipos histológicos agressivos. Métodos: Os autores realizaram um estudo retrospetivo de um período de 6 anos (de julho 2012 até junho 2018) num hospital Português – Hospital de Egas Moniz, para definir as características dos doentes submetidos a cirurgia micrográfica de Mohs e avaliar as vantagens desta técnica. Foram analisados os fatores: idade, género, proveniência, diagnóstico, localização do tumor, número de estádios da cirurgia micrográfica de Mohs, tipo de reconstrução do defeito cirúrgico, seguimento e recidiva. Resultados: Foram operados 835 tumores no total, 459 em doentes do sexo masculino e 376 em doentes do sexo feminino. A média de idades foi de 71 anos (intervalo entre 23- 95 anos). A maioria dos tumores submetidos a cirurgia micrográfica de Mohs foram carcinomas basocelulares (87%) localizados na pirâmide nasal (43%). Em 44% dos casos, houve necessidade de realizar mais de um estadio micrográfico. Metade dos encerramentos do defeito cirúrgico foram realizados por enxerto ou retalho cutâneo. Em doentes com seguimento igual ou superior a 3 anos, a taxa de recidiva foi de 4,9%. Discussão: O diagnóstico mais frequente foi o de carcinoma basocelular, o que reflete a importância da cirurgia micrográfica de Mohs, neste tipo de neoplasia cutânea, quer em lesões primárias como persistente/recorrentes. Salientamos que comparativamente a outras revisões europeias de cirurgia de Mohs, a taxa de recidiva dos tumores se encontra dentro dos parâmetros normais. Este dado é particularmente relevante, se considerarmos que o nosso serviço recebe doentes enviados de hospitais e dermatologistas de todo o país, especialmente selecionados quanto à agressividade tumoral ou já recidivados. O intervalo médio de recorrência para MMS foi de 27,9 meses, o que demonstra a necessidade de um seguimento a longo prazo destes doentes. Conclusão: A cirurgia micrográfica de Mohs permite menores taxas de recidiva comparativamente á excisão cirúrgica simples pelo que a sua prática deve ser encorajada em tumores devidamente selecionados. O controlo cirúrgico das margens em vez das “margens cegas” é uma mais-valia tanto na poupança de tecido são como na garantia da excisão completa do tumor num mesmo tempo cirúrgico

Tensile mechanical properties, constitutive parameters and fracture characteristics of an as-cast AA7050 alloy in the near-solidus temperature regime

The knowledge on constitutive mechanical behavior at the temperatures close to the solidus is essential for predicting high-temperature deformation and fracture, e.g. cold and hot cracking of aluminum alloys. In this work we studied the tensile mechanical properties of an as-cast AA7050 alloy in a near-solidus temperature regime. Tensile tests were carried out using Gleeble-3800TM system at temperatures from 400 to 465 °C and at strain rates from 0.0005 to 0.05 s-1. The results show that the strength decreases with increasing temperature and decreasing strain rate. Meanwhile, ductility decreases with the increase of temperature and strain rate. The constitutive parameters were extracted by fitting the test data to the extended-Ludwik and creep-law equations. The parameters for the extended-Ludwik equation are continuous with the values from a lower temperature regime obtained earlier, while the parameters for the creep-law equation are comparable with those obtained on other 7XXX aluminum alloys. We observed a change in fracture mode at 450 °C; from ductile transgranular to intergranular. This temperature coincides with the discontinuity point of the temperature-ductility slope. On the fracture surface of a sample that was deformed at 465 °C with a strain rate of 0.0005 s-1, we observed features characteristic of micro-superplasticity. Considering the test conditions, viscous flows of incipient melt or liquid-like substances are suggested to be responsible for the formation of this feature

Influence of melt feeding scheme and casting parameters during direct-chill casting on microstructure of an AA7050 billet

© The Minerals, Metals & Materials Society and ASM International 2012Direct-chill (DC) casting billets of an AA7050 alloy produced with different melt feeding schemes and casting speeds were examined in order to reveal the effect of these factors on the evolution of microstructure. Experimental results show that grain size is strongly influenced by the casting speed. In addition, the distribution of grain sizes across the billet diameter is mostly determined by melt feeding scheme. Grains tend to coarsen towards the center of a billet cast with the semi-horizontal melt feeding, while upon vertical melt feeding the minimum grain size was observed in the center of the billet. Computer simulations were preformed to reveal sump profiles and flow patterns during casting under different melt feeding schemes and casting speeds. The results show that solidification front and velocity distribution of the melt in the liquid and slurry zones are very different under different melt feeding scheme. The final grain structure and the grain size distribution in a DC casting billet is a result of a combination of fragmentation effects in the slurry zone and the cooling rate in the solidification range

Semi-solid constitutive parameters and failure behavior of a cast AA7050 alloy

Materials innovation institute (www.m2i.nl) research framework, project number M42.5.09340; Norsk Hydro Fond for SINTEF; Modelling assisted INnovation for Aluminum DC Casting process (MINAC

Microtubules in Bacteria: Ancient Tubulins Build a Five-Protofilament Homolog of the Eukaryotic Cytoskeleton

Microtubules play crucial roles in cytokinesis, transport, and motility, and are therefore superb targets for anti-cancer drugs. All tubulins evolved from a common ancestor they share with the distantly related bacterial cell division protein FtsZ, but while eukaryotic tubulins evolved into highly conserved microtubule-forming heterodimers, bacterial FtsZ presumably continued to function as single homopolymeric protofilaments as it does today. Microtubules have not previously been found in bacteria, and we lack insight into their evolution from the tubulin/FtsZ ancestor. Using electron cryomicroscopy, here we show that the tubulin homologs BtubA and BtubB form microtubules in bacteria and suggest these be referred to as “bacterial microtubules” (bMTs). bMTs share important features with their eukaryotic counterparts, such as straight protofilaments and similar protofilament interactions. bMTs are composed of only five protofilaments, however, instead of the 13 typical in eukaryotes. These and other results suggest that rather than being derived from modern eukaryotic tubulin, BtubA and BtubB arose from early tubulin intermediates that formed small microtubules. Since we show that bacterial microtubules can be produced in abundance in vitro without chaperones, they should be useful tools for tubulin research and drug screening

Use of limited proteolysis and mutagenesis to identify folding domains and sequence motifs critical for wax ester synthase/acyl coenzyme A:Diacylglycerol acyltransferase activity

Triacylglycerols and wax esters are synthesized as energy storage molecules by some proteobacteria and actinobacteria under stress. The enzyme responsible for neutral lipid accumulation is the bifunctional wax ester synthase/acyl-coenzyme A (CoA): diacylglycerol acyltransferase (WS/DGAT). Structural modeling of WS/DGAT suggests that it can adopt an acyl-CoA-dependent acyltransferase fold with the N-terminal and C-terminal domains connected by a helical linker, an architecture demonstrated experimentally by limited proteolysis. Moreover, we found that both domains form an active complex when coexpressed as independent polypeptides. The structural prediction and sequence alignment of different WS/DGAT proteins indicated catalytically important motifs in the enzyme. Their role was probed by measuring the activities of a series of alanine scanning mutants. Our study underscores the structural understanding of this protein family and paves the way for their modification to improve the production of neutral lipids