Hot isostatic pressing and heat treatments of LPBFed CoCuFeMnNiTi0.13 high-entropy alloy: microstructure and mechanical properties

The present work explores the possibility of processing a CoCuFeMnNiTi0.13 high-entropy alloy by laser powder bed fusion (LPBF). The alloy, produced under optimised processing conditions, presents good densification but also hot cracks, caused by the liquation of an inter-dendritic Cu-rich phase. Microstructure of the as-built alloy is characterised by face centred cubic (FCC) columnar grains, containing Cu-poor dendrites and Cu-rich inter-dendritic areas. The alloy, which was designed to be strengthened by spinodal decomposition and precipitation, was subjected to different thermo-mechanical treatments to try and improve its properties. Direct ageing and solution treatment and ageing produced a strong but brittle material (tensile strength of 683 MPa and elongation to failure of 1.3%), whereas hot isostatic pressing followed by controlled cooling was able to heal pores and cracks while triggering the desired microstructural transformations (spinodal decomposition and precipitation). This resulted into a balanced set of mechanical properties (tensile strength of 473 MPa and elongation to failure of 7.6%). This work shows that proper post-processing can mitigate the issues typically affecting LPBF fabricated HEAs, producing tailored microstructures with satisfactory mechanical performances

On the preparation and characterization of thin NiTi shape memory alloy wires for MEMS

Shape memory alloy (SMA) wires are employed as actuators in small devices for consumer electronics, valves and automotive applications. Because of the continued miniaturization of all the industrial products, nowadays the tendency is to produce MEMS (micro electromechanical systems). Among the most promising functional MEMS materials, the thin SMA wires that are offering a rapid actuating response with high power/weigh ratio of the material, are attracting a world wide interest. This paper is aimed at showing the production process and the characterizations of thin NiTi shape memory wires. The activity was focused on drawing procedure and on functional and TEM characterizations of the final products. In particular, it was evaluated the performance of the SMA wires for actuators in terms of functional fatigue and thermo-mechanical properties by means of an experimental apparatus design ad hoc for these specific test

VHCF response of heat-treated SLM Ti6Al4V Gaussian specimens with large loaded volume

Abstract Among the materials used for the production of components through Additive Manufacturing (AM) processes, the Selective-Laser-Melting (SLM) Ti6Al4V alloy is widely employed in aerospace applications for its high specific strength and in biomedical applications for its good biocompatibility. Actual structural applications are generally limited to static loading conditions where the large defects originating during the SLM process do not play a significant role for the static failure. On the contrary, the same defects strongly affect the fatigue response of the parts since they act as crack initiation sites that rapidly lead to fatigue failure. In the literature, a lot of research has been carried out to investigate the quasi-static and the High-Cycle Fatigue properties of the SLM Ti6Al4V alloy but there are still few studies on its Very-High-Cycle Fatigue (VHCF) response. In the paper, the VHCF response of Ti6Al4V specimens, which are vertically orientated during the SLM building and then subjected to a conventional heat treatment (2 hours heating in vacuum at 850°C), is experimentally assessed. Ultrasonic VHCF tests are carried out on Gaussian specimens with a large risk-volume (2300 mm3). Fracture surfaces are investigated for revealing the defect originating the fatigue failure. The Stress Intensity Factor Threshold associated to the experimental failures is finally estimated

Investigation of TiCr Hydrogen Storage Alloy

A new reversible hydrogen storage material, based on TiCr metal alloy, is proposed. Cr and Ti were mixed and melted in a final atomic ratio of 1,78. Chemical-physical characterisations, in terms of XRD and SEM-EDX, were performed. The quantification of Laves phases was performed through Rietveld refinements. The atomic Cr/Ti ratio was determined by EDX analysis and 1,71 was obtained. The H2 sorption/desorption measurements by Sievert apparatus were carried out. After different tests varying temperature and pressure, a protocol measurement was established; and a H2 sorption value of 0,4 wt% at 200 °C/10 bar with a fast kinetic at 5 bar (Dwt% of about 0,3 wt%) were obtained. Hydrogen desorption measurements performed in the same conditions of T confirmed a totally reversible trend. A confirm of metal hydride formation was recorded by XRD, in fact, comparing X-Ray patterns before and after volumetric tests a notable difference was recorded

On the preparation and characterization of thin NiTi shape memory alloy wires for MEMS

Shape memory alloy (SMA) wires are employed as actuators in small devices for consumerelectronics, valves and automotive applications. Because of the continued miniaturization of all the industrialproducts, nowadays the tendency is to produce MEMS (micro electromechanical systems). Among the mostpromising functional MEMS materials, the thin SMA wires that are offering a rapid actuating response withhigh power/weigh ratio of the material, are attracting a world wide interest. This paper is aimed at showing theproduction process and the characterizations of thin NiTi shape memory wires. The activity was focused ondrawing procedure and on functional and TEM characterizations of the final products. In particular, it wasevaluated the performance of the SMA wires for actuators in terms of functional fatigue and thermo-mechanicalproperties by means of an experimental apparatus design ad hoc for these specific test

Microstructure and preliminary fatigue analysis on AlSi10Mg samples manufactured by SLM

Nowadays, selective laser melting (SLM) is considered as the most challenging technology for manufacturing complex components in different industrial fields, such as biomedical, aerospace and racing. It is well-known that SLM may yield to microstructures significantly different from those obtained by conventional casting, thus affecting the mechanical properties of the component. In the present paper, microstructural and mechanical tests were carried out on AlSi10Mg samples manufactured by SLM technique in the XY building configuration. Homogeneous composition and typical microstructures were achieved for all the investigated samples. The mechanical properties were assessed through a tensile test and through the Impulse Excitation Technique (IET). The feasibility of ultrasonic Very High Cycle Fatigue (VHCF) tests with Gaussian specimens characterized by large loaded volumes (risk-volumes) was also experimentally verified in the paper. A Gaussian specimen was designed and manufactured. A preliminary ultrasonic test was then carried out on the manufactured specimen and the fracture surface was finally investigated

Metalli a struttura ultrafine ottenuti mediante deformazione plastica severa

Nella memoria viene presentata una breve rassegna sull’evoluzione microstrutturale, le proprietà e letecniche di produzione di metalli massivi a struttura ultrafine o nanostrutturati. Alcune caratteristichevengono esposte con il supporto di dati sperimentali ottenuti su leghe di alluminio e su acciai deformatimediante tecnica “Equal Channel Angular Pressing” o mediante laminazione a temperature pocosuperiori a quella ambiente. Si mette in evidenza come l’evoluzione della microstruttura con ladeformazione plastica severa a basse temperature segua dei principi comuni per i diversi metalli,schematizzabili secondo la teoria della ricristallizzazione continua. Esistono alcuni processi adottabili inlaboratorio ormai maturi per la sintesi di metalli ultrafini o nanostrutturati in forma massiva mentre sonoancora in fase di valutazione e sviluppo analoghe tecnologie che si possano inserire nelle linee diproduzione industriale dei metalli. Infine, i metalli ultrafini si caratterizzano per una struttura a granisubmicroscopici in uno stato molto lontano dall’equilibrio, specialmente per quanto concerne lacondizione dei bordi grano. La conseguente elevata reattività modifica profondamente le cinetiche didiffusione, di ingrossamento del grano e trasformazione di fase

Design for the Damping of a Railway Collector Based on the Application of Shape Memory Alloys

A new design of a Cu based SMA/GFRP lateral horn of a railway collector is proposed. Synergistic contribution of the performance parameters associated with the SMA, including specific damping, specific stiffness, and volume fraction, as well as those associated with the host composite such as flexural rigidity, SMA through-the-thickness location, and SMA-host interfacial strength, is taken into account. The aim is to increase the structural damping of the first flexural mode of the horn without significantly changing its flexural stiffness and weight. The focus of this work also applies to manufacturability and the cost effectiveness of the component for future industrial production

High performance shape memory effect (hp-sme): un innovativo percorso termo-meccanico per lo sviluppo di attuatori sma ad elevate prestazioni

Recentemente è stato proposto un percorso termo-meccanico innovativo per l’utilizzo di leghe a memoria diforma basato sul ciclaggio termico della martensite indotta da sforzo. Questo fenomeno viene chiamato HighPerformance Shape Memory Effect (HP-SME). Questa soluzione consente di utilizzare elementi austenitici conun notevole incremento del carico necessario al funzionamento dell’attuatore a memoria di forma e di mantenereuna elevata capacità del materiale di recupero della deformazione. Attuatori basati su questo principio mostranoproprietà funzionali migliorate rispetto agli attuatori convenzionali basati sull’effetto a memoria di forma (Shapememory effect, SME). In questo lavoro vengono riportati i risultati dei test di recupero della deformazione a caricocostante di fili sottili austenitici e confrontati con quelli di fili martensitici dello stesso diametro