1,720,968 research outputs found
Microstructural and Mechanical Properties of As Built, Solution treated and Aged 18Ni (300 grade) Maraging Steel Produced by Selective Laser Melting
No full textMechanical and microstructural properties of 18-Ni (300 grade) maraging steel processed by selective laser melting were investigat-
ed to evaluate the effect of the peculiar processing conditions and of the subsequent heat treatment. No need of solution treatment
prior to aging revealed necessary owing to the rapid cooling rate experienced by the material during selective laser melting. Iso-
thermal aging temperature and time played a main role in promoting austenite reversion. Aging also induced a dramatic increase in
strength with respect to the as-built condition and a decrease in fracture elongation. It is proposed that tensile strength and ductility
are mainly governed by the effects brought by the strengthening precipitates, whereas the concurrent reversion of martensite into
austenite is likely to play a minor role. Analysis of defects induced by selective laser melting process and on fracture behavior of the
tensile specimens allowed to improve the understanding of materials performance and to draw guideline for process improvement
Microstructure and Fracture Behavior of 316L Austenitic Stainless Steel Produced by Selective Laser Melting
No full textSelective laser melting is an additive manufacturing method based on local melting of a metal powder bed by a high power laser beam. Fast laser scans are responsible for severe thermal gradients and high cooling rates which produce complex hydrodynamic fluid flow. These phenomena affect crystal growth and orientation and are believed to be the cause of material spattering and microstructural defects, e.g. pores and incompletely melted particles. In this work, the microstructure and texture of 316L bars built along two different orientations and the effect of different distribution of defects on their mechanical response and failure mechanisms were investigated. Partially molten powder particles are believed to be responsible for the scattering in elongation to failure, reduced strength, and premature failure of vertical samples
Conditions for the early onset of blisters during heat treatments of Al-Si-Cu CAST PART
Full text linkThe evolution of subsurface pressurized pores, originating blisters during heat treatment cycles, on High Pressure Die Cast components made of Al-Si-Cu alloys was studied using a Finite Element Model in order to understand under which conditions blister form even below 400°C. The initial defect geometry, depth, inner pressure and the maximum temperature of the thermal cycle were varied in limited ranges, while the material was modelled as elastoplastic, considering creep effects negligible. Results suggested that blisters develop as a result of high plastic strain accumulation in localized regions lying between the pore and the outer surface and that blisters can form even at temperatures of 350-370°C. The simulations were experimentally validated by two sets of experiments carried out on cast parts, monitoring the surface and volume evolution of subsurface defects coupled by optical microscopy and microtomography analyses
Conditions for blister formation during thermal cycles of Al–Si–Cu–Fe alloys for high pressure die-casting
Full text linkThe mechanism of blister formation during thermal cycle in a High Pressure Die Cast A1-9Si-3Cu-Fe alloy (EN 46000) was investigated by means of FEM simulation of a pressurized subsurface defect, considering the high temperature elastoplastic behavior experimentally derived. The effect of parameters related to initial defect geometry, location and maximum temperature was analyzed. Blister formation was considered to occur as the maximum surface displacement after a thermal cycle exceeded 0.005 mm. It was shown that, for a given reference gas pressure, pore volume and depth, as well as temperature, blisters were developed from pores below a critical aspect ratio, as a result of high plastic strain accumulation in localized regions laying between the pore and the outer surface (ligament). Similarly, critical ligament thickness was identified and correlated to temperature and reference gas pressure. The development of blisters at temperatures lower than 400 degrees C from pores with reference pressure 90 MPa was predicted for a wide range of aspect ratios and ligament thickness. The possible occurrence of blisters in conventional HPDC components during heat treatment cycles at 350 degrees C was modeled for different pore pressures in the case of surface-near defects, mainly from lamina-shaped pores and was experimentally confirmed. (C) 2015 Elsevier Ltd. All rights reserved
Influence Of Heat Treatment Condition On Properties Of 1.2709 Maraging Steel Fabricated By Selective Laser Melting
No full textMaraging steels are used in many tooling applications and for the production of high performance parts as they exhibit high strength and toughness due to a martensitic structure and the precipitation of intermetallic compounds. These steels are promising materials for additive manufacturing processes due to their low C content and fairly good weldability.
In this work, a 1.2709 grade steel was fabricated by selective laser melting (SLM) and investigated with focus on evaluation of alloy properties as a function of thermal treatment, by microstructure analyses, tensile test, hardness measurement and differential scanning calorimetry.
The results show that the as built samples feature full aging response, as confirmed by comparisons between as built and solution treated samples. Tensile strength after peak aging reached remarkably high values at the expenses of a significant drop in ductility whereas, in the as built samples a fairly high fracture elongation could be achieved
Interrelation between macroscopic, microscopic and chemical dilution in hardfacing alloys
No full textA common way to extend service life of steel tools under heavy duty service conditions is the use of hardfacing coatings. Coating an economically feasible basematerial like carbon steelwith a hardfacing alloy by any processes based on welding, casting or cladding improves significantly its wear resistance. All these processes involve high heat inputs in order to partially melt the substrate material to create a sound bonding between the substrate and the wear resistant alloy. The intermixing and the elemental diffusion from the hardfacing material into the substrate and vice versa cause a change in microstructure, hardness and wear properties of the diluted alloy. It is not trivial to categorize the amount of dilution and its effects on material properties since composition and microstructure change discontinuously, especially in casting processes, due to different diffusion rates of elements and due to phase transitions. This paper presents a simplified model to correlate the amount of dilution in hardfacing alloys after a casting process considering several reference parameters. Three coefficients are derived to describe the degree of dilution based on crucible casting experiments and are numerically correlated by linear regressions. They specify distinct issues about macroscopic and microscopic dilution, aswell as changes in chemical composition. Themodel is then applied to a spin casting case studywhich can be taken as a reference example for industrial hardfacing processes where dilution effects are clearly observable
Aging Behavior of High-Strength Al Alloy 2618 Produced by Selective Laser Melting
No full textHigh Si-bearing Al alloys are commonly used in additive
manufacturing, but they have moderate mechanical
properties. New high-strength compositions are necessary
to spread the use of additively manufactured Al
parts for heavy-duty structural applications. This work
focuses on the microstructure, mechanical behavior, and
aging response of an Al alloy 2618 processed by selective
laser melting. Calorimetric analysis, electron microscopy,
and compression tests were performed in order to
correlate the mechanical properties with the peculiar
microstructure induced by laser melting and thermal
treatment
Aging behaviour and mechanical performance of 18-Ni 300 steel processed by selective laser melting
Full text linkAn 18-Ni 300 grade maraging steel was processed by selective laser melting and an investigation was carried out on microstructural and mechanical behaviour as a function of aging condition. Owing to the rapid cooling rate, the as-built alloy featured a full potential for precipitate strengthening, without the need of a solution treatment prior to aging. The amount of reversed austenite found in the microstructure increased after aging and revealed to depend on aging temperature and time. Similarly to the corresponding wrought counterpart, also in the selective laser-melted 18-Ni 300 alloy, aging promoted a dramatic increase in strength with respect to the as-built condition and a drop in tensile ductility. No systematic changes were found in tensile properties as a function of measured amount of austenite. It is proposed that the submicrometric structure and the phase distribution inherited by the rapid solidification condition brought by selective laser melting are such that changes in tensile strength and ductility are mainly governed by the effects brought by the strengthening precipitates, whereas the concurrent reversion of the γFe phase in different amounts seems to play a minor role
Effects of V Addition on Microstructure and Hardness of Fe-C-B-Ni-V Hardfacing Alloys Cast on Steel Substrates
Full text linkDilution effects in spin cast Fe-based thick coatings
No full textLAUREA MAGISTRALEIl presente lavoro di tesi si inserisce nel progetto di ricerca Europeo DEBACOAT (Development of High-Performance Barrels with Innovative Gradient
Coatings), come studio sui fenomeni di diluizione in rivestimenti spessi a base Ferro, depositati su barrels di acciaio di estrusori monovite tramite
spin casting.
Nei processi di lavorazione delle materie plastiche, i macchinari utilizzati vanno in contro a degradazione, sia di tipo meccanico sia di tipo chimico.
Per questo motivo, la superficie in contatto con i materiali processati deve essere rivestita con opportuni coatings. In quest’ottica, le leghe hardfacing si presentano come lo stato dell’arte nella protezione dei cilindri per estrusori.
Esse riescono a garantire vite di servizio maggiori rispetto a quelle ottenibili tramite altri processi di indurimento, quali carburazione e nitrurazione, ponendole come scelta preferenziale in ambiente industriale.
I meccanismi di degrado presenti all'interno di un estrusore variano lungo l’asse dello stesso: nella regione prossima all'alimentazione e in quella prossima allo stampo il meccanismo dominante è quello dell’abrasione. Nella parte centrale, invece, il meccanismo dominante è quello della corrosione, sebbene sia presente anche abrasione.
Sono state studiate due leghe hardfacing: la lega A, ottimizzata contro l’abrasione, e la lega B, resistente ad abrasione ed a corrosione. Entrambe
le leghe sono state depositate tramite tecnologia delle polveri.
Durante lo spin casting, a causa delle elevate temperature, si verificano fenomeni diffusivi che causano la diluizione delle leghe. Gli elementi che
forniscono le proprietà antiusura e anti-corrosione ricercate (specialmente Boro, Carbonio, Cromo, Molibdeno, Nickel e Vanadio) diffondono verso il substrato, mentre il Ferro diffonde in direzione opposta. La diluizione comporta significative variazioni microstrutturali, al punto che, se le leghe non diluite solidificano con microstruttura ipereutettica, a seguito del processo diffusivo il coating può avere microstruttura eutettica o ipoeutettica.
Prima di iniziare il presente lavoro sono state effettuate simulazioni in crogioli di Allumina, nei quali furono inserite, insieme alle polveri delle leghe A e B, diverse percentuali di polvere di Ferro. Questi esperimenti avevano lo scopo di simulare gli effetti della diluizione sulle leghe depositate su substrati di acciaio. La trasformazione ipereutettica-ipoeutettica per la lega A avviene per diluizioni dell’ordine del 10-20%, mentre per la lega B tra il 30 e il 40%.
Durante la prima fase del lavoro sperimentale sono state effettuate diverse prove in crogioli di acciaio, per determinare gli effetti della diluizione sulle leghe in esame quando vengono depositate su acciaio. I primi crogioli ad essere studiati sono stati quelli con camera singola. Successivamente sono stati preparati crogioli con due o tre camere, separate da barriere di acciaio.
Gli esperimenti sui crogioli multipli sono stati eseguiti variando la disposizione delle polveri depositate ed il tempo di casting. Gli scopi di
tali esperimenti sono: studiare la variazione microstrutturale delle leghe in funzione del tempo di casting e misurare il tempo necessario per fondere le barriere separatrici.
I crogioli sono stati analizzati tramite microscopia stereo, ottica ed elettronica e con prove di microdurezza. Le microstrutture ottenute sono state confrontate con quelle derivanti dalle simulazioni in crogioli di Allumina, ottenendo un valore percentuale di diluizione per ogni campione.
La teoria della diffusione di Einstein è stata utilizzata come base per sviluppare un modello che permettesse il calcolo di un coefficiente di diffusione basato sulla diluizione, D* , per le leghe studiate.
Dalle analisi sperimentali sui crogioli è emerso che:
• un tempo di 15 min è sufficiente ad indurre la transizione ipereutettica-ipoeutettica per la lega A, ma non per la lega B. Ciò equivale a dire che la lega A ha subito una diluizione superiore al 10-20%, mentre la lega B inferiore al 40%, • per fondere la barriera che separa le camere in un crogiolo doppio, la quale ha uno spessore di 2mm, è sufficiente un tempo compreso tra 20 e 25 min,
• per fondere le barriere che separano le camere in un crogiolo triplo, le quali hanno uno spessore di 4mm, è sufficiente un tempo intorno ai 35 min,
• si osserva la formazione di un layer all'interfaccia tra la lega B e l’acciaio del crogiolo, che potrebbe essere martensite, arricchita da
elementi di lega,
• si osserva la presenza di una fase depositata ai bordi dei grani all'interfaccia tra la lega A e l’acciaio del crogiolo. Analisi EBSD hanno dimostrato che si tratta di cementite e
• il modello proposto per il calcolo del coefficiente D* fornisce risultati comparabili a quelli trovati in letteratura.
Dopo aver ottenuto informazioni riguardo gli effetti della diluizione in crogioli di acciaio, l’analisi sperimentale si è rivolta a campioni provenienti da barrels fabbricati da una nota azienda, leader nel campo degli estrusori bimetallici. I tre barrels analizzati presentano una disposizione assiale delle leghe A-B-A. Le polveri sono state separate, nel primo e nel secondo barrel, da appositi anelli separatori, mentre nel terzo barrel esse si trovano in diretto contatto.
E' stata preparata solamente una sezione del primo cilindro, in corrispondenza dell'anello separatore, mentre per il secondo ed il terzo cilindro è stato studiato metà barrel. Tutti i campioni estratti sono stati analizzati tramite microscopie stereo, ottiche ed elettroniche e con prove di micro- e macrodurezza, per descrivere i fenomeni diffusivi agenti durante un processo di deposizione spin casting.
Dall’analisi del primo barrel è emerso che l’anello separatore non rimane in posizione durante il processo di deposizione. Per evitare la traslazione della barriera è stato proposto l’utilizzo di C-rings ai lati della stessa.
Dallo studio del secondo cilindro è emerso, però, che la presenza dei C-rings non ha impedito il movimento del separatore.
Analisi chimiche, microscopiche e meccaniche sul secondo e sul terzo barrel hanno dimostrato la presenza di un gradiente di composizione, di microstruttura e di microdurezza lungo l’asse del cilindro. La composizione misurata a seguito del processo di deposizione è stata confrontata con quella delle leghe non diluite, ottenendo un valore percentuale di diluizione.
La presenza dei separatori, nonostante il loro movimento, ha favorito lo sviluppo un gradiente più pronunciato, sia della composizione chimica del coating, sia dei valori di microdurezza. Si è notata una composizione costante nelle sezioni in prossimità della posizione iniziale del separatore, la quale fa supporre che esso vada incontro a movimento solo nella parte finale del processo di casting.
La scelta di quale configurazione adottare dovrà essere basata su un bilancio del rapporto costi-benefici. Se, a seguito di prove in condizioni di processo standard, l’utilizzo di separatori risultasse troppo costoso in rapporto ai benefici forniti, il loro mancato posizionamento non inficerebbe in maniera drastica l’aumento delle caratteristiche protettive del rivestimento e, quindi, della sua vita di servizio rispetto ai coatings ad oggi disponibili.In this thesis work the dilution effects in Iron-based thick coatings, spin cast at the inner surface of extruder barrels, are studied.
During extrusion, the internal surface of the barrel is exposed to severe degradation. To increase its service life a protective coating is required; thick hardfacing coatings represent the state of the art for extruders protection.
The wear mechanisms change along the extruder axis; a single alloy would not counteract efficiently all the mechanisms. This is the reason for the deposition of a gradient coating.
Two alloys have been studied: alloy A, optimized against abrasion, and alloy B, optimized against abrasion and corrosion.
Diffusive movements during casting cause microstructural changes in the alloys: in the undiluted state they solidify hypereutectically, while after dilution their microstructure may be eutectic, or even hypoeutectic.
Tests on steel crucibles were performed to study the microstructural changes of the alloys as a function of the casting time and to measure, in multiple crucibles, the time required to melt the steel wall separating the moulds.
It was observed that 15 min is a sufficient casting time to induce the hypereutectic to hypoeutectic transition for alloy A, while circa 25 min are needed to induce the same transition for alloy B.
A time comprised between 20 and 25 min is sufficient to melt the separating barrier of double crucibles, 2mm thick, while about 35 min are needed to melt the separating barriers of triple crucibles, 4mm thick.
Einstein diffusion theory was chosen to develop a model to calculate a dilution-related coefficient, D , for the interstitial elements of the two alloys.Three barrels, produced by a leader industry in the bimetallic extruder market field, were then analyzed to study the dilution effects after an industrial spin casting process. The powders deposition sequence was A-B-A for all the barrels. In the first and in the second barrel the powders were separated
by separators, while in the third they were in direct contact.
Both in the second and in the third barrel a gradient in the composition and in microhardness was present, but the use of the separators induced steeper gradients
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