Development of Experimental and Finite Element Models to Show Size Effects in the Forming of Thin Sheet Metals

Abstract An experimental method was developed that demonstrated the size effects in forming thin sheet metals, and a finite element model was developed to predict the effects demonstrated by the experiment. A universal testing machine (UTM) was used to form aluminum and copper of varying thicknesses (less than 1mm) into a hemispherical dome. A stereolithography additive manufacturing technology was used to fabricate the punch and die from a UV curing resin. There was agreement between the experimental and numerical models. The results showed that geometric size effects were significant for both materials, and these effects increased as the thickness of the sheets decreased. The demonstration presents an inexpensive method of testing small-scale size effects in forming processes, which can be altered easily to produce different shapes and clearances

Teräksen S355N hitsausliitoksien materiaalimallinnustulosten vertailu mikrorakenteen karakterisoinnin ja jäännösjännitysten avulla

The objective of this Master thesis was to answer the following questions. Do the characterized microstructures of VTT S355N samples with their mechanical properties and the experimental results of TIG welds of the Voss‘s doctoral thesis correspond to the modelled material results? Is the mixing of the investigated weld samples of LAHW and MAG welds even? Do X-ray diffraction measured residual stresses of S355N butt weld samples correspond with the measured residual stresses of Contour method? The purpose of these comparisons is to support validation of simulation processes which will be carried out in future. Additionally this thesis discussed if the maximum grain size of coarse-grained zone can be determined by measuring the grain size distribution with micrographs and Matlab. Charasterisation of microstructures of bead-on-plate VTT samples was focused on coarse-grained zones near fusion lines because there are the weakest parts of the welds. Hardness distributions and measuring of dimensions of HAZ (Heat Affected Zone) and weld metal were measured for all bead-on-plate welds of VTT. Recognition of phases of microstructures was carried out with the help of micrographs, hardnesses and recognition micrographs of IIW (International Institute of Welding). Cooling times were measured for all bead-on-plate welds with thermoelements. Values of t8/5 were determined for cooling times of measured, simulated, and of Voss’s doctoral thesis if possible. Determined t8/5 values, measured dimensions of HAZ and weld metals were compared between measured values of VTT samples, simulated values of FLOW-3D and measured values of Voss’s doctoral thesis. Residual stresses were measured for base material, two MAG and LAHW butt welds with Contour method and X-ray diffraction and the measured residual stresses were compared. Mixing of MAG and LAHW S355N samples of VTT were clarified with mixing calculations and with chromium, nickel and iron distributions of stainless weld metal. Distributions of chromium, nickel and iron were measured with EDS (Energy-dispersive X-ray Spectroscopy). Contents of chromium, nickel and iron were measured for one MAG sample with OES (Optical Emission Spectrometry). Mixing of one dissimilar MAG sample was clarified with etching, Schaeffler and WRC-92 diagrams. Reason of high variation of hardness near fusion line was investigated with one dissimilar MAG sample. Maximum grain sizes of coarse-grained zones were measured with optical microscopy and grain size distributions from base material to weld metal were determined with the help of micrographs, image processing and Matlab for all bead-on-plate welds of VTT. Most characterized microstructures of VTT samples with their mechanical properties and the experimental results of TIG welds of the Voss‘s doctoral thesis were near the modelling results. X-ray diffraction measured residual stresses of weld samples correlate with the measured residual stresses of Contour method. Mixing of investigated LAHW and MAG welds are even. Measuring of maximum grain size of prior austenite from coarse-grained zone with the grain size distribution was not possible because Matlab software measures dimensions of martensite blocks inside prior austenite.Tämän diplomityön tavoitteena oli vastata seuraaviin kysymyksiin. Vastaako VTT S355N näytteiden karakterisoidut mikrorakenteet ja niiden mekaaniset ominaisuudet ja kokeellisesti määritetyt Vossin tohtorityön TIG hitsien tulokset materiaalimallinnuksen tuloksia? Onko tutkittujen LAHW ja MAG hitsien sekoittuminen tasaista? Vastaako röntgendiffraktiolla mitatut S355N päittäisliitosten jäännösjännitykset Contour menetelmällä mitattuja jäännösjännityksiä? Tämän vertailun tavoite on tukea tulevaisuudessa tehtäviä simulointiprosessien validointeja. Lisäksi tässä päättötyössä mietittiin voidaanko karkearakeisen vyöhykkeen maksimiraekoko määrittää mittaamalla raekokojakauma mikrorakennekuvien ja Matlabin avulla. VTT:n päällehitsien mikrorakenteiden karakterisoinnit kohdistettiin karkearakeisille vyöhykkeille lähelle sularajoja, koska ne ovat hitsien heikoin kohta. Kaikille VTT:n päällehitseille mitattiin kovuusjakaumat sekä HAZ:ien (Heat Affected Zone) ja hitsien mitat. Mikrorakenteiden faasien tunnistus tehtiin kaikille VTT:n päällehitseille mikrorakennekuvien, kovuuksien ja IIW:n (International Institute of Welding) tunnistuskuvien avulla. Jäähtymisajat mitattiin kaikille VTT:n päällehitseille lämpöelementtien avulla. Simuloiduille, mitatuille ja Vossin työn jäähtymisajoille määritettiin t8/5 arvot, jos mahdollista. Määritettyjä t8/5 arvoja sekä mitattuja HAZ:ien ja hitsien mittoja verrattiin keskenään VTT:n mitattujen arvojen, VTT:n FLOW-3D simulaatioiden ja Vossin tohtorityön mitattujen arvojen kesken. Perusmateriaalin, kahden MAG näytteen ja LAHW näytteen päittäisliitoksien jäännösjännityksiä mitattiin Contour menetelmän ja röntgendiffraktion avulla sekä mitattuja jäännösjännityksiä verrattiin keskenään. VTT:n MAG ja LAHW hitsattujen S355N näytteiden sekoittumista selvitettiin sekoittumislaskelmilla ja ruostumattoman hitsiaineen kromi-, nikkeli- ja rautajakaumien avulla. Kromi-, nikkeli- ja rautajakaumat mitattiin EDS:n (Energy-dispersive X-ray Spectroscopy) avulla. Rauta, nikkeli ja kromipitoisuudet mitattiin yhdelle MAG näytteelle OES:n (Optical Emission Spectrometry) avulla. MAG “eripari” näytteen sekoittumista selvitettiin syövytyksen, Schaeffler ja WRC-92 diagrammien avulla. Suuren kovuusvaihtelun syytä lähellä sularajaa selvitettiin yhdellä MAG eripari hitsillä. Karkearakeisten vyöhykkeiden maksimiraekoko mitattiin valomikroskoopilla ja raekokojakaumat määritettiin perusmateriaalista hitsiin mikrorakennekuvien, kuvan käsittelyn ja Matlabin avulla kaikille VTT:n päällehitseille. Suurin osa VTT:n näytteiden karakterisoiduista mikrorakenteista ja niiden mekaanisista ominaisuuksista ja Vossin tohtorityön mitatuista tuloksista ovat lähellä FLOW-3D simulaatioiden tuloksia. Röntgendiffraktiolla mitatut hitsien jäännösjännitykset korreloivat Contour menetelmällä mitattujen jäännösjännitysten kanssa. LAHW ja MAG hitsien sekoittuminen on tasaista. Karkearakeisen vyöhykkeen maksimiraekoon määrittäminen raekokojakauman avulla ei ole mahdollista, koska Matlab ohjelma mittasi martensiittipakettien mittoja perinnäisen austeniitin rakeiden sisällä

Multi-Scale Modeling of Dynamic Recrystallization in Metals Undergoing Thermo-Mechanical Processing

This study focuses on devising a unified multi-scale numerical framework to predict the grain size evolution by dynamic recrystallization in metals and alloys for an array of severe plastic thermo-mechanical deformation conditions. The model is developed to predict the temporal and spatial grain size evolution of the material subjected to high strain rate and temperature dependent deformation. Dynamic recrystallization evolves by either a continuous grain refinement mechanism around room temperatures or by a discontinuous grain nucleation and growth mechanism at elevated temperatures. The multi-scale model bridges a dislocation density-based constitutive framework with microscale physics-based recrystallization laws to predict both the types of recrystallization phenomena simultaneously. The simulations are conducted within an integrated probabilistic cellular automata-finite element framework to capture the physics of the recrystallization mechanisms. High strain rate loading experiments in conjunction with microstructural characterization tests are conducted for pure copper to characterize the dynamic grain size evolution in the material and evaluated against the model predictions. Synchrotron X-rays are integrated with a modified Kolsky tension bar to conduct in situ temporal characterization of the grain refinement mechanism operating during the dynamic deformation of copper and evaluated against the developed model kinetics. Finally, the model is implemented to predict the grain size evolution developed during the friction stir spot welding of Al 6061-T6 for varying tool rotational speeds. The experiments show that the original microstructure is completely replaced by a recrystallized fine-grained microstructure with the final average grain size and morphology dependent on the process parameters. The model accurately predicts the process temperature rise with increasing tool rotational speeds, which results in a higher rate of grain coarsening during the dynamic recrystallization phenomenon

Index to NASA Tech Briefs, January - June 1966

Index to NASA technological innovations for January-June 196

Formability Enhancement of AA5182-O During Electro-Hydraulic Forming

In this research, formability improvement of AA5182-O aluminium sheet during electrohydraulic forming (EHF) was investigated by means of experimental and finite element analysis. Free and conical die formed EHF was carried out on grid sheet blanks and formability improvement was measured by comparing grid analysis results at each EHF condition with different forming limit curves (FLCs). It is found that AA5182-O shows minor improvement in formability when formed freely into EHF. But a significant rise in effective strain in safe grids is observed when EHF into 34, 40 and 45-degree conical dies provided a critical threshold of input energy has been used. In order to understand the mechanical aspects of formability improvement, related factors such as strain rate, stress triaxiality, and compressive through-thickness stress were studied using finite element simulation with an accurate description of the hardening behaviour of AA5182-O. Another advantage of the numerical simulation carried out in this work is that unlike previously published works, the driving force for EHF deformation was not simplified as uniform pressure and it resembles the actual process of EHF. Ignition and growth model was used in conjunction with Coupled Eulerian-Lagrangian (CEL) approach to simulate the EHF pulse formation. Moreover, 3D solid elements were used instead of shell elements and this facilitated measurement of stress in elements located in the bulk of sheet material. The tensile flow behaviour of AA5182-O sheet was investigated in the strain rate range of 0.001 to 1000s−1 and at different material directions (RD, DD, and TD) by means of both phenomenological models and neural networks (NNs). Genetic algorithm (GA) and linear regression analysis methods were used to calculate the constants in Johnson-Cook (JC), Khan-Huang-Liang (KHL) and modified Voce hardening functions, and user-material subroutines were developed and used in FE software. Moreover, in order to predict the rheological behaviour of AA5182-O without the limitations of a mathematical function, two types of feed-forward back-propagation neural networks were trained and used in the FE model. Simulation results were compared with experimental tensile flow curves and the most accurate method is used to predict the mechanical response of AA5182-O in FE simulations of the EHF process. FE results suggested that a combination of EHF process related parameters including compressive through-thickness stress (negative stress triaxiality) generated during the deformation could postpone the failure, when specimens are formed into a die cavity (EHDF). Also, the increased velocity and significant impact pressure at the final stage of deformation not only prevent strain localization but also help in further suppressing the damage. It is found that very high peak strain rates develop in the sheet as it contacts the die surface which further postpone the failure since AA5182-O exhibits positive strain rate sensitivity at such high-strain rates. Moreover, damage mechanisms of AA5182-O sheets were investigated during EHF tests and are compared with those occur during quasi-static (QS) deformation. The results confirm that void nucleation, growth and coalescence are the main damage mechanisms of AA5182-O at both high and low strain rates. It is found that Mg2Si particles do not significantly influence void formation and the main source of void nucleation is cracking of Al3(Fe-Mn) intermetallic particles. More importantly, it is found that specimens deformed under QS conditions contained more voids in areas away from the sub-fracture surface but EHF specimens exhibit higher rate of void growth close to sub-fracture areas. Optical microscopy results confirmed that void formation is suppressed by increasing the applied energy in EHF. And more importantly, the growth of voids is suppressed due to the high-velocity impact of the sheet against the die which plays an important role in increasing formability of AA5182-O aluminium sheet in EHF. Optical microscopy showed that AA5182-O grains experience significant shearing strain during the EHF deformation in the apex area of conical EHDF specimens. The results of transmission electron microscopy showed that dislocation density increases when specimens are formed using EHF process but the magnitude of this increase is not significantly greater than in quasi-static deformations. Finally, it is concluded that the combination of high strain rate deformation and compressive through-thickness stress during the deformation, leads to formability improvement of AA5182-O in EHDF

Low-Cost Prototype to Automate the 3D Digitization of Pieces: An Application Example and Comparison

This work is aimed at describing the design of a mechanical and programmable 3D capturing system to be used by either 3D scanner or DSLR camera through photogrammetry. Both methods are widely used in diverse areas, from engineering, architecture or archaeology, up to the field of medicine; but they also entail certain disadvantages, such as the high costs of certain equipment, such as scanners with some precision, and the need to resort to specialized operatives, among others. The purpose of this design is to create a robust, precise and cost-effective system that improves the limitations of the present equipment on the market, such as robotic arms or rotary tables. For this reason, a preliminary study has been conducted to analyse the needs of improvement, later, we have focused on the 3D design and prototyping. For its construction, there have been used the FDM additive technology and structural components that are easy to find in the market. With regards to electronic components, basic electronics and Arduino-based 3D printers firmware have been selected. For system testing, the capture equipment consists of a Spider Artec 3D Scanner and a Nikon 5100 SLR Camera. Finally, 3D models have been developed by comparing the 3D meshes obtained by the two methods, obtaining satisfactory results

Optimization the machinability and mechanical properties of PM steel components by development a new machining additive

L'usinabilité des composants en acier PM est nettement inférieure à celle des aciers corroyés en raison de la présence de porosité résiduelle et de l'hétérogénéité de leur microstructure. Les problèmes liés à l'usinabilité constituent une part importante des coûts de production globaux des pièces en acier PM. La stratégie la plus populaire pour améliorer leur usinabilité consiste à mélanger un composé chimique, tel que MnS, MoS2 ou BN-h, à la poudre de base. Ces améliorateurs d'usinabilité améliorent le comportement d'usinage des aciers PM en diminuant les forces de coupe impliquées dans la formation de copeaux et en lubrifiant la surface de l'outil de coupe, ce qui en retour réduit à la fois l'usure en flanc et l'usure en cratère. Cette étude met en évidence une nouvelle approche pour développer des activateurs d'usinabilité conçus pour maximiser l'usinabilité des composants d'acier PM sans affecter les propriétés mécaniques ni la résistance à la corrosion. Ainsi, il a été décidé de revêtir les particules de MnS d'une couche de nickel pouvant agir comme une barrière pour neutraliser leur nature hygroscopique et augmenter la résistance à la corrosion des pièces en acier PM. De plus, il a été prévu que les propriétés mécaniques peuvent être améliorées en raison de la formation de liaisons métallurgiques entre le revêtement de nickel des particules de MnS et la matrice d'acier, tandis que le MnS améliore simultanément l'usinabilité de la pièce en acier PM. Une comparaison avec des additifs d'usinage commerciaux a été effectuée en termes d'usinabilité et de propriétés mécaniques. Il a été constaté que la résistance à la corrosion des échantillons contenant du MnS recouvert de nickel était excellent et identique à celle des échantillons sans additifs. De plus, les propriétés mécaniques ne sont pas affectées par la présence de l'additif nouvellement développé par rapport à ce qui a été mesuré lorsque le MnS a été utilisé. Enfin, la caractérisation de l'usinabilité a montré que l'ajout du MnS revêtu de nickel comme additif d'usinage pouvait améliorer l'usinabilité aussi bien que le MnS.Machinability of PM steel components is significantly lower than that of wrought steels due to the presence of residual porosity and the heterogeneity of their microstructure. Machinability-related issues constitute a significant portion of the overall production costs of PM steel parts. The most popular strategy for improving their machinability involves admixing a chemical compound, such as MnS, MoS₂ or BN-h, to the base powder. These machinability enhancers improve the machining behavior of PM steels by decreasing the cutting forces involved with chip formation and by lubricating the surface of the cutting tool, which in return, reduces both flank wear and crater wear. This study highlights a novel approach for developing machinability enhancers engineered to maximize the machinability of PM components without affecting their mechanical properties nor corrosion resistance. Thus, it was decided to coat MnS particles with a nickel layer that can act as a barrier to neutralize their hygroscopic nature and increase the corrosion resistance of PM steel parts. Moreover, it was anticipated that mechanical properties could be improved due to the formation of metallurgical bonds between the nickel coating of the MnS particles and the steel matrix, while the MnS core of the additive would improve machinability of the PM steel component simultaneously. A comparison with commercial machining additives was performed in terms of both machinability and mechanical properties. It was found that the corrosion resistance of the samples containing nickel-coated MnS was excellent and identical to that of samples without additives. Moreover, mechanical properties are not affected by the presence of the newly developed additive compared to what was measured when MnS was used. Finally, machinability characterization showed that the addition of the nickel-coated MnS as a machining additive could improve machinability as well as MnS does

A Complete Electron Microscopy Volume of the Brain of Adult Drosophila melanogaster.

Drosophila melanogaster has a rich repertoire of innate and learned behaviors. Its 100,000-neuron brain is a large but tractable target for comprehensive neural circuit mapping. Only electron microscopy (EM) enables complete, unbiased mapping of synaptic connectivity; however, the fly brain is too large for conventional EM. We developed a custom high-throughput EM platform and imaged the entire brain of an adult female fly at synaptic resolution. To validate the dataset, we traced brain-spanning circuitry involving the mushroom body (MB), which has been extensively studied for its role in learning. All inputs to Kenyon cells (KCs), the intrinsic neurons of the MB, were mapped, revealing a previously unknown cell type, postsynaptic partners of KC dendrites, and unexpected clustering of olfactory projection neurons. These reconstructions show that this freely available EM volume supports mapping of brain-spanning circuits, which will significantly accelerate Drosophila neuroscience. VIDEO ABSTRACT