Role of Direct Aging and Solution Treatment on Hardness, Microstructure and Residual Stress of the A357 (AlSi7Mg0.6) Alloy Produced by Powder Bed Fusion

Applying additive manufacturing (AM) technologies to the fabrication of aluminum automotive components, with an optimized design, may result in improved vehicle light weighting. However, the post-process heat treatment of such alloys has to be customized for the particular AM microstructure. The present study is aimed at investigating the effect of different heat treatments on the microstructure, hardness and residual stress of the A357 (AlSi7Mg0.6) heat-treatable alloy produced by laser-based powder bed fusion (LPBF, also known as selective laser melting). There are two major issues to be addressed: (1) relieving the internal residual stress resulting from the process and (2) strengthening the alloy with a customized heat treatment. Therefore, stress-relief annealing treatment, direct aging of the as-built alloy and a redesigned T6 treatment (consisting of a shortened high-temperature solution treatment followed by artificial aging) were examined. Comparable hardness values were reached in the LPBF alloy with optimized direct aging and T6 treatments, but complete relief of the residual stress was obtained only with T6. Microstructural analyses also suggested that, because of the supersaturated solid solution, different phenomena were involved in direct aging and T6 treatment

A novel heat treatment of the additively manufactured Co28Cr6Mo biomedical alloy and its effects on hardness, microstructure and sliding wear behavior

Co28Cr6Mo alloy (ASTM F75 and F1537) is one of the standard biomaterials for permanent orthopedic implants, utilized especially in case of joint replacement, such as knee and ankle prostheses. At the present, innovative Additive Manufacturing (AM) technologies, such as laser-based powder bed fusion (LPBF), also known as selective laser melting (SLM), enable the production of customized medical devices with improved mechanical properties. When dealing with implants for joint replacement, wear resistance is critical and, unlike compressive and tensile properties, the knowledge on wear behavior of the LPBF Co28Cr6Mo alloy is currently limited. Furthermore, the effect of post-process heat treatment on tribological properties, that have to be customized on the peculiar microstructure induced by LPBF, needs to be assessed. In this view, the present work first focuses on a novel direct aging treatment of the LPBF Co28Cr6Mo alloy, performed in the range 600-900 degrees C up to 180 min, and investigates the effects on hardness and microstructural features, with the optimized heat-treated condition found in case of 850 degrees C for 180 min aging treatment. Then, the attention is driven to the dry sliding wear behavior of as-built and heat-treated LPBF Co28Cr6Mo alloy, considering the conventional wrought alloy as benchmark. For testing conditions closer to the in-service ones, the as-built LPBF alloy showed a wear resistance higher than the conventional wrought alloy. The optimized aging treatment significantly modified the as-built LPBF microstructure, it improved the alloy hardness and, in general, it positively affected its friction and wear behavior

Effect of heat treatment and defects on the tensile behavior of a hot work tool steel manufactured by laser powder bed fusion

Microstructure and tensile properties of a hot work tool steel manufactured via laser powder bed fusion (LPBF) were investigated. Specimens were built under two different orientations and subjected to two quenching and tempering heat treatments, featuring different austenitizing and tempering temperatures and the eventual presence of a sub-zero step. Microstructural analyses revealed a homogeneous tempered martensite structure after both heat treatments, with the only distinction of a higher alloying segregation at a sub micrometric scale length in samples subjected to the highest tempering temperatures. Hardness and tensile tests indicated a negligible effect of building orientation on mechanical properties, but a significant influence of heat treatment parameters. The treatment featuring the lower tempering temperatures and the sub-zero step resulted in higher hardness, tensile strength, and elongation, attributed to a lower martensite tempering and alloying segregation. Tensile fracture occurred via crack initiation and unstable propagation from large LPBF defects in all the investigated conditions

Caratterizzazione microstrutturale e prove di resilienza su giunti Friction Stir Welding e Linear Friction Welding di compositi a matrice metallica

In questo studio sono stati caratterizzati giunti Friction Stir Welding e Linear Friction Welding su compositi a matrice in lega di alluminio e rinforzo particellare ceramico. Il processo FSW è stato applicato a due compositi ottenuti con processo fusorio, quindi estrusi e trattati termicamente T6: AA6061/20%vol.Al2O3p e AA7005/10%vol.Al2O3p. I giunti LFW sono stati invece realizzati su un composito con matrice in lega di alluminio e rinforzo particellare in carburo di silicio, ottenuto mediante metallurgia delle polveri, quindi forgiato e trattato termicamente T4: AA2124/25%vol.SiCp. Sono stati esaminati gli effetti della saldatura sullecaratteristiche microstrutturali dei giunti, avvalendosi di tecniche di microscopia ottica con analisi di immagine e di microscopia elettronica in scansione (SEM) con microsonda a dispersione di energia (EDS). Sono state quindi condotte prove di resilienza con pendolo strumentato Charpy. Lo studio dei meccanismi di danneggiamento è stato effettuato mediante analisi al SEM delle superfici di frattura. Entrambi i processi di saldatura hanno portato a giunti sostanzialmente esenti da difetti. La microstruttura dei cordoni è risultata dipendente sia dalle caratteristiche microstrutturali iniziali dei compositi considerati, sia dalla tipologia di processo di saldatura. Nel caso dei compositi AA6061/20%Al2O3p e AA7005/10%Al2O3p saldati FSW si è osservato un sostanziale incremento di resilienza, rispetto al materiale base, in conseguenza dell’affinamento dei grani della matrice, della riduzione della dimensione media delle particelle di rinforzo e della loro spigolosità, indotte dal processo di saldatura. Il composito AA2124/25%SiCp saldato LFW ha presentato valori di resilienza confrontabili con quelli del materiale base, in conseguenza, soprattutto, dei limitati effetti della saldatura su dimensione e distribuzione delle particelle di rinforzo

Effect of the Austempering Process on the Microstructure and Mechanical Properties of 27MnCrB5-2 Steel

AbstractThe effect of austempering parameters on the microstructure and mechanical properties of 27MnCrB5-2 steel has been investigated by means of: dilatometric, microstructural and fractographic analyses; tensile and Charpy V-notch (CVN) impact tests at room temperature and a low temperature.Microstructural analyses showed that upper bainite developed at a higher austempering temperature, while a mixed bainitic-martensitic microstructure formed at lower temperatures, with a different amount of bainite and martensite and a different size of bainite sheaf depending on the temperature. Tensile tests highlighted superior yield and tensile strengths (≈30%) for the mixed microstructure, with respect to both fully bainitic and Q&T microstructures, with only a low reduction in elongation to failure (≈10%). Impact tests confirmed that mixed microstructures have higher impact properties, at both room temperature and a low temperature

Mechanical and microstructural features of wire-and-arc additively manufactured carbon steel thick plates

Metal additive manufacturing (AM), in particular wire-and-arc additive manufacturing (WAAM), has become over the last few years the breakthrough technology to reduce the environmental impact and increase the efficiency of steel structures. Although intense research effort has been paid toward the mechanical characterization of WAAM-produced thin walls, little attention has been devoted to the investigation of multi-layered thick parts. These latter would indeed expand the application of WAAM for large-scale constructions requiring thicker cross-sections to withstand high loading conditions. The present work provides a comprehensive experimental investigation of mild steel WAAM thick plates from the fabrication to the mechanical and microstructural characterization. First, the fabrication process is presented in full details. From that, microstructural and mechanical characterization is described and discussed, showing a homogeneous microstructure with little influence on the mechanical response along the wall plate thickness, also considering different specimen orientations with respect to the printing directions. The results confirm good mechanical properties of the printed outcomes, in line with those of structural mild steels manufactured with conventional technologies. Little influence on the response along the thickness is reported, thus proving the required quality of WAAM thick parts for applications in the construction sector

Friction Stir Welding of aluminium based composites reinforced with a L2O3 particles: effects on microstructure and charpy impact energy

The aim of the present research was to study the effect of the Friction Stir Welding process on the microstructure and impact toughness of the composites W6A20A (AA6061 reinforced with 20vol.% of Al2O3 particles) and W7A10A (AA7005 reinforced with 10vol.% of Al2O3 particles). FSW, because of the concurrent effect of severe plastic deformation and frictional heating during welding, had effects both on the reinforcing particles and the aluminium matrix. It induced a significant reduction in the reinforcement particles size and their better distribution in the welded zone as well as a grain refinement of the aluminium alloy matrix in the nugget due to dynamic recrystalizzation. The frictional heating, moreover, had effects on the growth, dissolution and re-precipitation of hardening precipitates. The impact tests showed that the total impact energies increased in the FSW composites, respect to the corresponding base materials

Heat treatment response and influence of overaging on mechanical properties of C355 cast aluminum alloy

The research activity was focused on the optimization of heat treatment parameters for C355 (Al-Si-Cu-Mg)cast aluminum alloy and on its microstructural and mechanical characterization in T6 condition, also evaluatingthe effect of subsequent high temperature exposure. Differential thermal analyses were carried out to identifythe solution heat treatment optimal temperature. After solution heat treatment and quenching, samples weresubjected to artificial aging, at different times and temperatures, as to obtain the corresponding hardnesscurves. Samples for successive hardness and tensile tests were subjected to hot isostatic pressing (HIP) and T6heat treatment, according to the parameters optimized in the foregoing research phase. Some of the T6 heattreated samples were also characterized after overaging, induced by holding at 210 °C for 41 h. Aiming to carryout a comparative study, tensile properties of C355 alloy, both in T6 and overaged conditions, were comparedto those of A356 alloy (results from a previous study), which is currently more widely employed than C355.Experimental results showed how C355-T6 alloy is characterized by superior mechanical properties as comparedto A356-T6, especially in the overaged condition, due to the higher thermal stability induced by Cu-basedstrengthening precipitates

COMPORTAMENTO SUPERPLASTICO DELLA LEGA DI MAGNESIO AZ31 PRODOTTA MEDIANTE “TWIN ROLL CASTING”

Le leghe di Magnesio hanno grandi potenzialità come materiali strutturali, soprattutto per applicazioni nel settore dei trasporti, per il loro basso peso specifico. La scarsa deformabilità a temperatura ambiente ne limita, tuttavia, l’impiego a componenti ottenuti prevalentemente per fusione. Studi recenti hanno evidenziato la possibilità di avere superplasticità in alcune leghe di magnesio, che sarebbero quindi utilizzabili in processi di formatura, per la produzione di componenti near-net-shape di forma complessa. È noto, tuttavia, che il comportamento superplastico è tipicamente rilevabile solo in materiali a microstruttura controllata (grano fine e stabile nel corso della deformazione), quando deformati a temperature e con velocità di deformazione opportune. In questo lavoro sono presentati i risultati di prove di trazione in regime di superplasticità, effettuate sulla lega di magnesio AZ31, prodotta mediante Twin Roll Casting (un processo di colata continua diretta all’interno di due cilindri controrotanti e raffreddati ad acqua) e successivamente laminata a caldo. Il materiale così trattato ha presentato un comportamento superplastico a temperature comprese tra 400 e 420 °C, con un valore massimo dell’allungamento a rottura del 370% a 400 °C e con velocità di deformazione di 1·10-3 s-1. Tali valori sono correlabili alla favorevole microstruttura a grani fini ed equiassici, che favorisce la deformazione controllata dal meccanismo di scorrimento a bordo grano (GBS), come confermato da analisi metallografiche e frattografiche condotte sui campioni deformati

Caratterizzazione microstrutturale e prove di resilienza su giunti Friction Stir Welding e Linear Friction Welding di compositi a matrice metallica

In questo studio sono stati caratterizzati giunti Friction Stir Welding e Linear Friction Welding sucompositi a matrice in lega di alluminio e rinforzo particellare ceramico. Il processo FSW è stato applicato a duecompositi ottenuti con processo fusorio, quindi estrusi e trattati termicamente T6: AA6061/20%vol.Al2O3p eAA7005/10%vol.Al2O3p. I giunti LFW sono stati invece realizzati su un composito con matrice in lega dialluminio e rinforzo particellare in carburo di silicio, ottenuto mediante metallurgia delle polveri, quindi forgiatoe trattato termicamente T4: AA2124/25%vol.SiCp. Sono stati esaminati gli effetti della saldatura sullecaratteristiche microstrutturali dei giunti, avvalendosi di tecniche di microscopia ottica con analisi di immagine edi microscopia elettronica in scansione (SEM) con microsonda a dispersione di energia (EDS). Sono statequindi condotte prove di resilienza con pendolo strumentato Charpy. Lo studio dei meccanismi didanneggiamento è stato effettuato mediante analisi al SEM delle superfici di frattura. Entrambi i processi disaldatura hanno portato a giunti sostanzialmente esenti da difetti. La microstruttura dei cordoni è risultatadipendente sia dalle caratteristiche microstrutturali iniziali dei compositi considerati, sia dalla tipologia diprocesso di saldatura. Nel caso dei compositi AA6061/20%Al2O3p e AA7005/10%Al2O3p saldati FSW si èosservato un sostanziale incremento di resilienza, rispetto al materiale base, in conseguenza dell’affinamento deigrani della matrice, della riduzione della dimensione media delle particelle di rinforzo e della loro spigolosità,indotte dal processo di saldatura. Il composito AA2124/25%SiCp saldato LFW ha presentato valori di resilienzaconfrontabili con quelli del materiale base, in conseguenza, soprattutto, dei limitati effetti della saldatura sudimensione e distribuzione delle particelle di rinforzo