In the present paper, microstructural and mechanical properties of an innovative AlSi3Mg alloy we... more In the present paper, microstructural and mechanical properties of an innovative AlSi3Mg alloy were studied. Particularly, the effect of the addition of Cr and Mn on tensile strength and impact toughness was evaluated. In fact, the presence of these elements leads to the formation of an intermetallic phase with a globular or polyhedral morphology. It was therefore investigated the role played by Cr-Mn containing particles in the failure mechanism and the influence of the heat treatment parameters. Moreover, tensile and impact tests were performed on A356 samples in T6 condition, whose results were compared with the performance of the innovative alloy. Considering the static properties, the innovative alloy showed remarkable values of tensile strength, while ductility was improved only after heat treatment optimization. Poor impact toughness values were measured and the microstructural analysis confirmed the presence of coarse intermetallics, acting as crack initiation and propagation particles, on the fracture surfaces.
La prova di resilienza consente di analizzare il comportamento dei materiali in presenza di un ca... more La prova di resilienza consente di analizzare il comportamento dei materiali in presenza di un carico impulsivo, attraverso la determinazione del valore dell’energia assorbita dal provino durante la rottura. Mediante la cosiddetta prova di resilienza strumentata è possibile inoltre ottenere informazioni relative alle modalità di innesco e propagazione della frattura. In tale contesto e nel caso specifico delle leghe di alluminio questo tipo di prova permette quindi di valutare l’effetto della microstruttura, dei difetti e dei parametri di processo sulle proprietà ad impatto
In aluminium foundry, grain refinement is a fundamental treatment for obtaining good quality cast... more In aluminium foundry, grain refinement is a fundamental treatment for obtaining good quality cast components, increasing static, dynamic and fatigue properties, reducing hot-tearing and improving the distribution of microporosity and intermetallic compounds. Nowadays, in foundry practise the refining of metal alloys is substantially carried out by means of three methods: control of heat transfer, stirring of the melt in the semisolid state and by adding Ti-B based refining agents. Nucleant particles act as heterogeneous nucleating centres in obtaining a great number of dendrites and creating the conditions for a fine and homogeneous microstructure. The present work shows the results of an experimental study designed to evaluate the effect of three different refining agents on the microstructural properties and on the parameters of the cooling curve of the A356 foundry aluminium alloy. Thermal analyses have been correlated with the microstructural data, including SDAS measurements, t...
Dissimilar friction stir welding joints are widely employed in the industrial field due to the ex... more Dissimilar friction stir welding joints are widely employed in the industrial field due to the excellent microstructural and mechanical properties of the resulting joints. Nevertheless, to further enhance the weld properties, the addition of reinforcement particles on the joint-line during the process has been proven effective for increasing its mechanical performance. In the present investigation, the microstructure and the impact behaviour of FSWed joints between AA2024-T351 and AA7075-T651 aluminium plates were investigated, considering the effect of different process parameters selected through a full factorial 2k design of experiments: both the rotational and translational speed of the tool, as well as the addition of Al2O3-SiC microparticles, were considered as input parameters. Unnotched 10 x 5 x 55 mm impact specimens were tested through an instrumented 50 J Charpy pendulum: total impact energy, the two complementary initiation and propagation energies as well as the peak fo...
In this research, an often-used high-pressure die-casting (HPDC) AlSi9Cu3(Fe) alloy was chosen as... more In this research, an often-used high-pressure die-casting (HPDC) AlSi9Cu3(Fe) alloy was chosen as the matrix for an Al-alloy/MWCNT nanocomposite material that could be, in the case of targeted mechanical properties, used for the production of lighter car parts. In the industrial experiment, nanocomposite samples were prepared by a HPDC machine from a base AlSi9Cu3(Fe) alloy with three different mass ratios of MWCNTs placed on two different positions inside the casting machine. The structure and thermal stability of the used MWCNTs were visually confirmed by scanning electron microscopy (SEM). The microstructural and metallographic properties were studied using an optical microscope. The mechanical properties (tensile strength, elongation at break and hardness) were investigated using a universal testing machine and a Vickers hardness tester. The experimental results of this research showed that the HPDC-prepared Al-alloy matrix nanocomposite material had significantly changed mechanical properties even with small MWCNT contents. The elongation at break and the tensile strength of the Al-alloy/MWCNT nanocomposite were increased in comparison with the base alloy. The finest microstructure was shown by samples with 0.05 w/% MWCNTs, which was in correlation with the most significant change in the mechanical properties. Keywords: metal-matrix nanocomposites, AlSi9Cu3(Fe) alloy, multi-walled carbon nanotubes (MWCNT), high-pressure die casting (HPDC) Avtorji opisujejo raziskavo izdelave nanokompozitnega materiala s kovinsko osnovo iz zlitine AlSi9Cu3(Fe). Za oja~itveno fazo so uporabili ve~stenske ogljikove nanocev~ice (MWCNT). Za izdelavo kompozita so uporabili visokotla~no brizganje taline v kovinsko orodje oz. deljivi model (HPDC). Ta postopek se najbolj pogosto uporablja za izdelavo lahkih avtomobilskih delov s specifi~nimi mehanskimi lastnostmi. Izvajali so industrijske preizkuse na HPDC stroju in pri tem uporabili kovinsko talino AlSi9Cu3(Fe) s tremi razli~nimi dodatki MWCNT, ki so jih postavili na dve razli~ni mesti v votlini orodja. Strukturno in termi~no stabilnost uporabljenih MWCNT so potrdili s preiskavo ulitkov pod vrsti~nim elektronskim mikroskopom (SEM). Nadalje so izvedli {e metalografske preiskave mikrostrukture pod opti~nim mikroskopom. Mehanske lastnosti izdelanih materialov, natezno trdnost in raztezek po prelomu, so dolo~ili na univerzalnem trgalnem stroju, medtem ko so trdoto dolo~ili na standardnem Vickersovem merilniku. Eksperimentalni rezultati te preiskave so pokazali, da so se mehanske lastnosti s postopkom HPDC izdelanega nanokompozita na osnovi Al zlitine znatno spremenile `e pri dodatku dokaj malih koli~in MWCNT. Natezna trdnost in raztezek po prelomu nanokompozita MWCNT/Al-zlitina sta v primerjavi z osnovno Al zlitino narasla. Najbolj drobnozrnato mikrostrukturo so imeli vzorci z dodatkom 0,05 masnih dele`ev MWCNT, kar se je ujemalo z najve~jo spremembo mehanskih lastnosti. Klju~ne besede: nanokompoziti s kovinsko osnovo, zlitina AlSi9Cu3(Fe), ve~stenske ogljikove nanocev~ice (MWCNT), visokotla~no litje v kovinsko orodje (HPDC)
This paper discusses the effect of different surface treatments on shape memory alloy wires embed... more This paper discusses the effect of different surface treatments on shape memory alloy wires embedded in PolyEster (PE) and VinylEster (VE) polymeric matrices. In particular, two types of chemical etching and a chemical bonding with a silane coupling agent have been performed on the surfaces of the wires. Pull-out tests have been carried out on samples made from a specifically designed Teflon mould. Considering the best results of the pull-out tests obtained with PE resin, the debonding induced by strain recovery of 4%, 5% and 6% pre-strained NiTi wires has been evaluated with the wires being subjected to different surface treatment conditions and then being embedded in the PE matrix. The results prove that the wires functionalised and embedded in the PE resin show the maximum pull-out forces and the highest interfacial adhesion. Finally, it has been found that debonding induced by strain recovery is strongly related to the propagation towards the radial direction of sharp cracks at ...
In foundry practice the occurrence of solidification defects limits the production and the perfor... more In foundry practice the occurrence of solidification defects limits the production and the performance of cast components. In high-pressure die casting (HPDC) the most common defects are shrinkage or gas porosities and bifilms. Thin sections, typical of HPDC components, are the most susceptible to casting defects, and these microstructural imperfections can be detrimental to both the mechanical and aesthetic properties. Even though laminations are not as harmful as internal defects, die cast components with this kind of superficial defects are always rejected by customers, thus requiring further machining operations. The aim of this study is to analyse the lamination defects that occur on the surface of some Al-Si-Cu cast covers produced by means of HPDC. In order to evaluate the influence of the different injection parameters on the final quality of the die casting products, melt temperature, plunger speed and upset pressure have been varied within a range of suitable values. The o...
ABSTRACT In this study, the tensile properties of unmodified A356 alloys with trace additions of ... more ABSTRACT In this study, the tensile properties of unmodified A356 alloys with trace additions of Ni or V were investigated. The nominal concentrations of these elements were set at 600 and 1000 ppm, respectively. Specimens obtained from sand and permanent mould casting in as-cast and T6 heat treated conditions were examined. It was found that the precipitation of Ni-rich intermetallics strongly influences the tensile properties of the sand cast A356 alloy in as-cast condition, leading to a reduction of both the yield strength (Rp0.2) and ultimate tensile strength (UTS) by 87% and 37%, respectively. Conversely, the V addition increased Rp0.2 and UTS by 42% and 25% due to a solid solution strengthening effect. After T6 heat treatment, the sand cast A356 alloys containing V exhibited slightly higher mechanical properties as compared to the A356 reference alloy (≈18%), whereas the detrimental effect of Ni was eliminated. The cooling rate became the main parameter influencing the mechanical properties of permanent mould cast alloys: no significant differences in mechanical properties were observed between the reference alloy and Ni- or V-containing alloys in both as-cast and T6 conditions.
Journal of Materials Engineering and Performance, 2014
The paper presents the results of wettability tests by a sessile drop method coupled with residua... more The paper presents the results of wettability tests by a sessile drop method coupled with residual gas analysis for the HASTELLOY Ò X substrates contact heated with brazing filler metal of Palnicro 36M. The contact angles formed between the substrate and the molten brazing filler metal were recorded by high-speed high-resolution CCD camera using two different heating procedures: (1) fast continuous heating to a temperature of 960 °C; (2) isothermal heating at a temperature of 995 °C for 10 min after step heating to the test temperature. In this study, the reference sample was HASTELLOY Ò X in a delivery state (D1). The base material with a different surface finish was also tested: after polishing (P1), after polishing and oxidation at a temperature of 600 °C for 1 h in the air (U1). The tests confirmed very good wetting properties of Palnicro 36M on HASTELLOY Ò X, independently on its surface finish and the heating procedure used in this study, i.e., after 10 min at 995 °C, the final contact angle values were h f = 7.6 ± 0.1°, 6.03 ± 0.07°and 6.2 ± 1.0°for D1, P1 and U1, respectively. However, oxidation of HAS-TELLOY Ò X surface affects the wetting kinetics and spreading of molten Palnicro 36M on BM substrates by: (1) increasing the initial contact angle values (h 0 = 51 ± 4°, 65 ± 13°and 83 ± 2°for D1, P1 and U1, respectively), (2) delaying the wetting process and (3) reducing the spreading area since after wettability tests at 960 °C, the largest area covered with solidified Palnicro 36M drop was noted for D1 (92 mm 2 ) while the smallest one for U1 (52 mm 2 ).
ABSTRACT The effect of three commercial Ti–B-based grain refiners on the impact properties of the... more ABSTRACT The effect of three commercial Ti–B-based grain refiners on the impact properties of the A356 cast aluminium alloy was assessed. The impact tests were performed by means of an instrumented Charpy pendulum. During impact testing, the maximum load and the total impact energy, as well as its complementary contributions, the energy at maximum load and the crack propagation energy, were measured. Impact properties were studied as a function of size and shape of the main microstructural features, which were analysed by means of optical microscopy and scanning electron microscopy. The results show that the influence of grain refinement on microstructure involves beneficial and detrimental concurrent effects which strongly affect impact properties. The total impact energy decreases with the addition of all the grain refiners due to a shift from a mixed transgranular–intergranular fracture mode to a more severe transgranular mode. Crack initiation and propagation occur mainly through the fracture of Fe-intermetallics and brittle Si particles, and the mechanism of void coalescence. No direct correlation between grain size and impact properties is found. Moreover, the aspect ratio of eutectic Si particles does not change with grain refinement, implying that there are no mutual poisoning effects between Sr and B. Total impact energy is found to depend on both SDAS and b-platelets size. The concurrent effects of SDAS and b-platelets average maximum length on total impact energy can be taken into account by the multiple regression analysis technique.
Instrumented impact strength tests have been carried out on KV sub-size Charpy samples drawn from... more Instrumented impact strength tests have been carried out on KV sub-size Charpy samples drawn from A356 aluminium alloy 17-in. wheels, produced by a low-pressure die casting. The wheels show different geometry and thermal treatment. In this paper, the effects of microstructure and defects on the impact properties are studied. The results indicate that the impact energy is lower in as-cast wheel than in T6 heat-treated wheels. A finer microstructure always corresponds to higher impact strength, while a direct correlation between the resistance to crack propagation values and secondary dendrite arm spacing (SDAS) exists. Casting defects, revealed by means of X-ray and density measurements techniques, become critical when concentrated around the V-notch, where they reduce the load bearing area of Charpy specimens. The fracture profile and surface of Charpy specimens have been investigated revealing how the crack crosses the interdendritic eutectic region where a significant fraction of cracked eutectic silicon and intermetallic particles is found. Numerical simulations have been performed to study the filling and solidification behaviour of the alloy of the wheels analysed, in order to predict the final microstructure and shrinkage formation. Solidification times, estimated by means of SDAS measurements and calculated with a numerical simulation approach, show a good correspondence. Critical areas, as concern hot spots and shrinkage porosities, are generally revealed in the zone of the wheels between the spoke and the rim, as well as in the rim area.
This paper focuses on the identification of alloys and the manufacturing techniques used for insc... more This paper focuses on the identification of alloys and the manufacturing techniques used for inscribed bronze vessels from Sumhuram (modern Khor Rori, Sultanate of Oman). A group of 19 fragments (dating back to the first to second century AD), thought to belong to the same inscribed vessel, were investigated with the aim of identifying the manufacturing process. The fragments were analysed by scanning electron microscopy with both energy-dispersive spectroscopy (SEM-EDS) and micro-Raman probe, flame atomic absorption spectroscopy (FAAS), as well as X-ray diffractometry (XRD). The results indicate that these fragments belong to different artefacts that all consist of leaded bronze. However, three different groups were identified on the basis of the composition and microstructure of the fragments. The fragments bearing inscriptions were produced by lost-wax casting with the 'wax-thread moulding' technique (in which each letter is shaped separately with segments of wax lace on the sheet of the wax model).
The wear resistance of MMCs can be improved by increasing the volume fraction of the reinforcing ... more The wear resistance of MMCs can be improved by increasing the volume fraction of the reinforcing ceramic phase (HRC) by as much as 70%. Among the various types of HRC composites, a new Al2O3Al co-continuous ceramic/metal composite (referred to as C4 composite) has been recently produced at The Ohio State University. A new method of preparation, based on a displacement reaction between a silica precursor (whose frame is faithfully reproduced in the composite product) and molten aluminum, allows the production of a MMC where both the Al2O3 and Al phases are continuous and interpenetrating. The aim of the present work was to investigate the tribological behavior of C4Al2O3Al composites under dry sliding conditions using a computer-controlled slider-on-cylinder tribometer. The tests were carried out at applied loads in the range 5–30 N and sliding speeds in the range 0.3–1.8 m/s. for sliding distances up to 10 km. Wear scars and debris were characterized by means of scanning electron microscopy (SEM), electron probe microanalysis (EPMA), with an EDS analyzer and X-ray diffraction (XRD) analysis. Under the adopted testing conditions, the C4 composites underwent only mild wear, mainly as a consequence of the high load-bearing capacity of the ceramic phase which enhances wear resistance. The high wear resistance is also related to the nature of the third-body, mainly constituted by Fe2O3, which is produced by the abrasive action carried out by the hard ceramic phase on the counterfacing steel. Under the adopted testing conditions, the transition from mild to severe wear, observed at a critical load in conventional MMCs, was never observed in the C4 materials. Both the continuity between aluminum and alumina, and the increase in interfacial bonding, played an important role in influencing the wear behavior of the C4 composites. These aspects led to an improvement of wear resistance of the C4 composites over conventional MMCs, under the same experimental conditions, and probably shifted the transition from mild to severe wear regimes to higher critical loads.
Aluminium based metal matrix composites (MMCs) offer higher specific stiffness and strength, bett... more Aluminium based metal matrix composites (MMCs) offer higher specific stiffness and strength, better wear resistance and greater thermal stability when compared to the corresponding unreinforced alloys [1]. One of the outstanding challenges in the use of these materials concerns their joining, since traditional fusion welding processes generally lead to microstructural defects, also due to the presence of the ceramic reinforcement and, consequently, to a general decrease in their mechanical properties [2]. These problems can be significantly reduced by the use of solid state joining techniques, such as Friction Stir Welding (FSW) [3]. A main limitation of this process, however, could be the severe wear of the pin [4]. This problem could be overcome by using Linear Friction Welding (LFW), in which the bonding of two flat-edged components is achieved through frictional heating, caused by their relative reciprocating motion, under an axial compressive force [5], without using any consum...
Nella memoria e presentata una attivita di ricerca riguardante la caratterizzazione microstruttur... more Nella memoria e presentata una attivita di ricerca riguardante la caratterizzazione microstrutturale e la valutazione del comportamento ad usura di quattro diverse tipologie di rivestimento, Al2O3-13TiO2, Cr2O3, WC-Co e Cr3C2-37WC-18Me, che sono stati depositati su dischi in acciaio da cementazione. I primi due rivestimenti sono stati applicati mediante tecnica APS, i restanti mediante tecnica HVOF. Le prove di usura sono state effettuate utilizzando un tribometro DUCOM in configurazione pin-on-disk, secondo normativa ASTM G99-05, con pin costituito da un agglomerato di allumina. Le prove sono state eseguite in condizioni variabili di umidita relativa, mantenendo costanti tutti i rimanenti parametri quali carico applicato, durata della prova, velocita di rotazione e temperatura. L’analisi delle superfici usurate mediante tecniche di microscopia ottica ed elettronica ha evidenziato l’instaurarsi di differenti meccanismi di danneggiamento, a conferma dell’importanza dell’umidita in re...
The possibility of realising adaptive structures is of great interest in the control/automation f... more The possibility of realising adaptive structures is of great interest in the control/automation fi elds, owing to the benefi ts related to enhanced performance. To accomplish this, a challenging approach is the employment of Shape Memory Alloys (SMAs) as active elements, which can recover seemingly permanent strains by temperature-induced phase transformations whereby the so-called Shape Memory Effect (SME) takes place. This paper deals with an experimental investigation of the bending recovery performance of a functional structure. The active material was a near-equiatomic NiTi alloy in the form of strips, which were embedded into a custommade polymeric matrix. To study the infl uence of heating/cooling rates on the characteristic transformation temperatures of the NiTi material, several analyses were carried out by means of Differential Scanning Calorimetry (DSC). Prior to the insertion, the strips were thermo-mechanically treated to memorise a bent shape through experimentally ev...
The use of full cores, for producing automotive components by means of the permanent mould castin... more The use of full cores, for producing automotive components by means of the permanent mould casting technique, allows good castings with a low level of defects to be obtained. Nevertheless, an extra phase of the production cycle is necessary to have an optimal emptying of the internal cavities of the castings, with an increase in costs. This scenario motivated the present research, in particular to study the effect of replacing full cores with empty cores. The component analysed is one of the three parts of a motorcycle frame, realised in aluminum alloy A356 by permanent mould casting and T6 heat-treated. Several castings have been produced with both full and empty cores; tensile strength tests have been performed on samples drawn from four different zones, in order to compare the mechanical and microstructural properties of the castings realised with the two different kinds of core. A slight decrease in elongation at fracture has been verified in the samples drawn from the castings ...
In the present paper, microstructural and mechanical properties of an innovative AlSi3Mg alloy we... more In the present paper, microstructural and mechanical properties of an innovative AlSi3Mg alloy were studied. Particularly, the effect of the addition of Cr and Mn on tensile strength and impact toughness was evaluated. In fact, the presence of these elements leads to the formation of an intermetallic phase with a globular or polyhedral morphology. It was therefore investigated the role played by Cr-Mn containing particles in the failure mechanism and the influence of the heat treatment parameters. Moreover, tensile and impact tests were performed on A356 samples in T6 condition, whose results were compared with the performance of the innovative alloy. Considering the static properties, the innovative alloy showed remarkable values of tensile strength, while ductility was improved only after heat treatment optimization. Poor impact toughness values were measured and the microstructural analysis confirmed the presence of coarse intermetallics, acting as crack initiation and propagation particles, on the fracture surfaces.
La prova di resilienza consente di analizzare il comportamento dei materiali in presenza di un ca... more La prova di resilienza consente di analizzare il comportamento dei materiali in presenza di un carico impulsivo, attraverso la determinazione del valore dell’energia assorbita dal provino durante la rottura. Mediante la cosiddetta prova di resilienza strumentata è possibile inoltre ottenere informazioni relative alle modalità di innesco e propagazione della frattura. In tale contesto e nel caso specifico delle leghe di alluminio questo tipo di prova permette quindi di valutare l’effetto della microstruttura, dei difetti e dei parametri di processo sulle proprietà ad impatto
In aluminium foundry, grain refinement is a fundamental treatment for obtaining good quality cast... more In aluminium foundry, grain refinement is a fundamental treatment for obtaining good quality cast components, increasing static, dynamic and fatigue properties, reducing hot-tearing and improving the distribution of microporosity and intermetallic compounds. Nowadays, in foundry practise the refining of metal alloys is substantially carried out by means of three methods: control of heat transfer, stirring of the melt in the semisolid state and by adding Ti-B based refining agents. Nucleant particles act as heterogeneous nucleating centres in obtaining a great number of dendrites and creating the conditions for a fine and homogeneous microstructure. The present work shows the results of an experimental study designed to evaluate the effect of three different refining agents on the microstructural properties and on the parameters of the cooling curve of the A356 foundry aluminium alloy. Thermal analyses have been correlated with the microstructural data, including SDAS measurements, t...
Dissimilar friction stir welding joints are widely employed in the industrial field due to the ex... more Dissimilar friction stir welding joints are widely employed in the industrial field due to the excellent microstructural and mechanical properties of the resulting joints. Nevertheless, to further enhance the weld properties, the addition of reinforcement particles on the joint-line during the process has been proven effective for increasing its mechanical performance. In the present investigation, the microstructure and the impact behaviour of FSWed joints between AA2024-T351 and AA7075-T651 aluminium plates were investigated, considering the effect of different process parameters selected through a full factorial 2k design of experiments: both the rotational and translational speed of the tool, as well as the addition of Al2O3-SiC microparticles, were considered as input parameters. Unnotched 10 x 5 x 55 mm impact specimens were tested through an instrumented 50 J Charpy pendulum: total impact energy, the two complementary initiation and propagation energies as well as the peak fo...
In this research, an often-used high-pressure die-casting (HPDC) AlSi9Cu3(Fe) alloy was chosen as... more In this research, an often-used high-pressure die-casting (HPDC) AlSi9Cu3(Fe) alloy was chosen as the matrix for an Al-alloy/MWCNT nanocomposite material that could be, in the case of targeted mechanical properties, used for the production of lighter car parts. In the industrial experiment, nanocomposite samples were prepared by a HPDC machine from a base AlSi9Cu3(Fe) alloy with three different mass ratios of MWCNTs placed on two different positions inside the casting machine. The structure and thermal stability of the used MWCNTs were visually confirmed by scanning electron microscopy (SEM). The microstructural and metallographic properties were studied using an optical microscope. The mechanical properties (tensile strength, elongation at break and hardness) were investigated using a universal testing machine and a Vickers hardness tester. The experimental results of this research showed that the HPDC-prepared Al-alloy matrix nanocomposite material had significantly changed mechanical properties even with small MWCNT contents. The elongation at break and the tensile strength of the Al-alloy/MWCNT nanocomposite were increased in comparison with the base alloy. The finest microstructure was shown by samples with 0.05 w/% MWCNTs, which was in correlation with the most significant change in the mechanical properties. Keywords: metal-matrix nanocomposites, AlSi9Cu3(Fe) alloy, multi-walled carbon nanotubes (MWCNT), high-pressure die casting (HPDC) Avtorji opisujejo raziskavo izdelave nanokompozitnega materiala s kovinsko osnovo iz zlitine AlSi9Cu3(Fe). Za oja~itveno fazo so uporabili ve~stenske ogljikove nanocev~ice (MWCNT). Za izdelavo kompozita so uporabili visokotla~no brizganje taline v kovinsko orodje oz. deljivi model (HPDC). Ta postopek se najbolj pogosto uporablja za izdelavo lahkih avtomobilskih delov s specifi~nimi mehanskimi lastnostmi. Izvajali so industrijske preizkuse na HPDC stroju in pri tem uporabili kovinsko talino AlSi9Cu3(Fe) s tremi razli~nimi dodatki MWCNT, ki so jih postavili na dve razli~ni mesti v votlini orodja. Strukturno in termi~no stabilnost uporabljenih MWCNT so potrdili s preiskavo ulitkov pod vrsti~nim elektronskim mikroskopom (SEM). Nadalje so izvedli {e metalografske preiskave mikrostrukture pod opti~nim mikroskopom. Mehanske lastnosti izdelanih materialov, natezno trdnost in raztezek po prelomu, so dolo~ili na univerzalnem trgalnem stroju, medtem ko so trdoto dolo~ili na standardnem Vickersovem merilniku. Eksperimentalni rezultati te preiskave so pokazali, da so se mehanske lastnosti s postopkom HPDC izdelanega nanokompozita na osnovi Al zlitine znatno spremenile `e pri dodatku dokaj malih koli~in MWCNT. Natezna trdnost in raztezek po prelomu nanokompozita MWCNT/Al-zlitina sta v primerjavi z osnovno Al zlitino narasla. Najbolj drobnozrnato mikrostrukturo so imeli vzorci z dodatkom 0,05 masnih dele`ev MWCNT, kar se je ujemalo z najve~jo spremembo mehanskih lastnosti. Klju~ne besede: nanokompoziti s kovinsko osnovo, zlitina AlSi9Cu3(Fe), ve~stenske ogljikove nanocev~ice (MWCNT), visokotla~no litje v kovinsko orodje (HPDC)
This paper discusses the effect of different surface treatments on shape memory alloy wires embed... more This paper discusses the effect of different surface treatments on shape memory alloy wires embedded in PolyEster (PE) and VinylEster (VE) polymeric matrices. In particular, two types of chemical etching and a chemical bonding with a silane coupling agent have been performed on the surfaces of the wires. Pull-out tests have been carried out on samples made from a specifically designed Teflon mould. Considering the best results of the pull-out tests obtained with PE resin, the debonding induced by strain recovery of 4%, 5% and 6% pre-strained NiTi wires has been evaluated with the wires being subjected to different surface treatment conditions and then being embedded in the PE matrix. The results prove that the wires functionalised and embedded in the PE resin show the maximum pull-out forces and the highest interfacial adhesion. Finally, it has been found that debonding induced by strain recovery is strongly related to the propagation towards the radial direction of sharp cracks at ...
In foundry practice the occurrence of solidification defects limits the production and the perfor... more In foundry practice the occurrence of solidification defects limits the production and the performance of cast components. In high-pressure die casting (HPDC) the most common defects are shrinkage or gas porosities and bifilms. Thin sections, typical of HPDC components, are the most susceptible to casting defects, and these microstructural imperfections can be detrimental to both the mechanical and aesthetic properties. Even though laminations are not as harmful as internal defects, die cast components with this kind of superficial defects are always rejected by customers, thus requiring further machining operations. The aim of this study is to analyse the lamination defects that occur on the surface of some Al-Si-Cu cast covers produced by means of HPDC. In order to evaluate the influence of the different injection parameters on the final quality of the die casting products, melt temperature, plunger speed and upset pressure have been varied within a range of suitable values. The o...
ABSTRACT In this study, the tensile properties of unmodified A356 alloys with trace additions of ... more ABSTRACT In this study, the tensile properties of unmodified A356 alloys with trace additions of Ni or V were investigated. The nominal concentrations of these elements were set at 600 and 1000 ppm, respectively. Specimens obtained from sand and permanent mould casting in as-cast and T6 heat treated conditions were examined. It was found that the precipitation of Ni-rich intermetallics strongly influences the tensile properties of the sand cast A356 alloy in as-cast condition, leading to a reduction of both the yield strength (Rp0.2) and ultimate tensile strength (UTS) by 87% and 37%, respectively. Conversely, the V addition increased Rp0.2 and UTS by 42% and 25% due to a solid solution strengthening effect. After T6 heat treatment, the sand cast A356 alloys containing V exhibited slightly higher mechanical properties as compared to the A356 reference alloy (≈18%), whereas the detrimental effect of Ni was eliminated. The cooling rate became the main parameter influencing the mechanical properties of permanent mould cast alloys: no significant differences in mechanical properties were observed between the reference alloy and Ni- or V-containing alloys in both as-cast and T6 conditions.
Journal of Materials Engineering and Performance, 2014
The paper presents the results of wettability tests by a sessile drop method coupled with residua... more The paper presents the results of wettability tests by a sessile drop method coupled with residual gas analysis for the HASTELLOY Ò X substrates contact heated with brazing filler metal of Palnicro 36M. The contact angles formed between the substrate and the molten brazing filler metal were recorded by high-speed high-resolution CCD camera using two different heating procedures: (1) fast continuous heating to a temperature of 960 °C; (2) isothermal heating at a temperature of 995 °C for 10 min after step heating to the test temperature. In this study, the reference sample was HASTELLOY Ò X in a delivery state (D1). The base material with a different surface finish was also tested: after polishing (P1), after polishing and oxidation at a temperature of 600 °C for 1 h in the air (U1). The tests confirmed very good wetting properties of Palnicro 36M on HASTELLOY Ò X, independently on its surface finish and the heating procedure used in this study, i.e., after 10 min at 995 °C, the final contact angle values were h f = 7.6 ± 0.1°, 6.03 ± 0.07°and 6.2 ± 1.0°for D1, P1 and U1, respectively. However, oxidation of HAS-TELLOY Ò X surface affects the wetting kinetics and spreading of molten Palnicro 36M on BM substrates by: (1) increasing the initial contact angle values (h 0 = 51 ± 4°, 65 ± 13°and 83 ± 2°for D1, P1 and U1, respectively), (2) delaying the wetting process and (3) reducing the spreading area since after wettability tests at 960 °C, the largest area covered with solidified Palnicro 36M drop was noted for D1 (92 mm 2 ) while the smallest one for U1 (52 mm 2 ).
ABSTRACT The effect of three commercial Ti–B-based grain refiners on the impact properties of the... more ABSTRACT The effect of three commercial Ti–B-based grain refiners on the impact properties of the A356 cast aluminium alloy was assessed. The impact tests were performed by means of an instrumented Charpy pendulum. During impact testing, the maximum load and the total impact energy, as well as its complementary contributions, the energy at maximum load and the crack propagation energy, were measured. Impact properties were studied as a function of size and shape of the main microstructural features, which were analysed by means of optical microscopy and scanning electron microscopy. The results show that the influence of grain refinement on microstructure involves beneficial and detrimental concurrent effects which strongly affect impact properties. The total impact energy decreases with the addition of all the grain refiners due to a shift from a mixed transgranular–intergranular fracture mode to a more severe transgranular mode. Crack initiation and propagation occur mainly through the fracture of Fe-intermetallics and brittle Si particles, and the mechanism of void coalescence. No direct correlation between grain size and impact properties is found. Moreover, the aspect ratio of eutectic Si particles does not change with grain refinement, implying that there are no mutual poisoning effects between Sr and B. Total impact energy is found to depend on both SDAS and b-platelets size. The concurrent effects of SDAS and b-platelets average maximum length on total impact energy can be taken into account by the multiple regression analysis technique.
Instrumented impact strength tests have been carried out on KV sub-size Charpy samples drawn from... more Instrumented impact strength tests have been carried out on KV sub-size Charpy samples drawn from A356 aluminium alloy 17-in. wheels, produced by a low-pressure die casting. The wheels show different geometry and thermal treatment. In this paper, the effects of microstructure and defects on the impact properties are studied. The results indicate that the impact energy is lower in as-cast wheel than in T6 heat-treated wheels. A finer microstructure always corresponds to higher impact strength, while a direct correlation between the resistance to crack propagation values and secondary dendrite arm spacing (SDAS) exists. Casting defects, revealed by means of X-ray and density measurements techniques, become critical when concentrated around the V-notch, where they reduce the load bearing area of Charpy specimens. The fracture profile and surface of Charpy specimens have been investigated revealing how the crack crosses the interdendritic eutectic region where a significant fraction of cracked eutectic silicon and intermetallic particles is found. Numerical simulations have been performed to study the filling and solidification behaviour of the alloy of the wheels analysed, in order to predict the final microstructure and shrinkage formation. Solidification times, estimated by means of SDAS measurements and calculated with a numerical simulation approach, show a good correspondence. Critical areas, as concern hot spots and shrinkage porosities, are generally revealed in the zone of the wheels between the spoke and the rim, as well as in the rim area.
This paper focuses on the identification of alloys and the manufacturing techniques used for insc... more This paper focuses on the identification of alloys and the manufacturing techniques used for inscribed bronze vessels from Sumhuram (modern Khor Rori, Sultanate of Oman). A group of 19 fragments (dating back to the first to second century AD), thought to belong to the same inscribed vessel, were investigated with the aim of identifying the manufacturing process. The fragments were analysed by scanning electron microscopy with both energy-dispersive spectroscopy (SEM-EDS) and micro-Raman probe, flame atomic absorption spectroscopy (FAAS), as well as X-ray diffractometry (XRD). The results indicate that these fragments belong to different artefacts that all consist of leaded bronze. However, three different groups were identified on the basis of the composition and microstructure of the fragments. The fragments bearing inscriptions were produced by lost-wax casting with the 'wax-thread moulding' technique (in which each letter is shaped separately with segments of wax lace on the sheet of the wax model).
The wear resistance of MMCs can be improved by increasing the volume fraction of the reinforcing ... more The wear resistance of MMCs can be improved by increasing the volume fraction of the reinforcing ceramic phase (HRC) by as much as 70%. Among the various types of HRC composites, a new Al2O3Al co-continuous ceramic/metal composite (referred to as C4 composite) has been recently produced at The Ohio State University. A new method of preparation, based on a displacement reaction between a silica precursor (whose frame is faithfully reproduced in the composite product) and molten aluminum, allows the production of a MMC where both the Al2O3 and Al phases are continuous and interpenetrating. The aim of the present work was to investigate the tribological behavior of C4Al2O3Al composites under dry sliding conditions using a computer-controlled slider-on-cylinder tribometer. The tests were carried out at applied loads in the range 5–30 N and sliding speeds in the range 0.3–1.8 m/s. for sliding distances up to 10 km. Wear scars and debris were characterized by means of scanning electron microscopy (SEM), electron probe microanalysis (EPMA), with an EDS analyzer and X-ray diffraction (XRD) analysis. Under the adopted testing conditions, the C4 composites underwent only mild wear, mainly as a consequence of the high load-bearing capacity of the ceramic phase which enhances wear resistance. The high wear resistance is also related to the nature of the third-body, mainly constituted by Fe2O3, which is produced by the abrasive action carried out by the hard ceramic phase on the counterfacing steel. Under the adopted testing conditions, the transition from mild to severe wear, observed at a critical load in conventional MMCs, was never observed in the C4 materials. Both the continuity between aluminum and alumina, and the increase in interfacial bonding, played an important role in influencing the wear behavior of the C4 composites. These aspects led to an improvement of wear resistance of the C4 composites over conventional MMCs, under the same experimental conditions, and probably shifted the transition from mild to severe wear regimes to higher critical loads.
Aluminium based metal matrix composites (MMCs) offer higher specific stiffness and strength, bett... more Aluminium based metal matrix composites (MMCs) offer higher specific stiffness and strength, better wear resistance and greater thermal stability when compared to the corresponding unreinforced alloys [1]. One of the outstanding challenges in the use of these materials concerns their joining, since traditional fusion welding processes generally lead to microstructural defects, also due to the presence of the ceramic reinforcement and, consequently, to a general decrease in their mechanical properties [2]. These problems can be significantly reduced by the use of solid state joining techniques, such as Friction Stir Welding (FSW) [3]. A main limitation of this process, however, could be the severe wear of the pin [4]. This problem could be overcome by using Linear Friction Welding (LFW), in which the bonding of two flat-edged components is achieved through frictional heating, caused by their relative reciprocating motion, under an axial compressive force [5], without using any consum...
Nella memoria e presentata una attivita di ricerca riguardante la caratterizzazione microstruttur... more Nella memoria e presentata una attivita di ricerca riguardante la caratterizzazione microstrutturale e la valutazione del comportamento ad usura di quattro diverse tipologie di rivestimento, Al2O3-13TiO2, Cr2O3, WC-Co e Cr3C2-37WC-18Me, che sono stati depositati su dischi in acciaio da cementazione. I primi due rivestimenti sono stati applicati mediante tecnica APS, i restanti mediante tecnica HVOF. Le prove di usura sono state effettuate utilizzando un tribometro DUCOM in configurazione pin-on-disk, secondo normativa ASTM G99-05, con pin costituito da un agglomerato di allumina. Le prove sono state eseguite in condizioni variabili di umidita relativa, mantenendo costanti tutti i rimanenti parametri quali carico applicato, durata della prova, velocita di rotazione e temperatura. L’analisi delle superfici usurate mediante tecniche di microscopia ottica ed elettronica ha evidenziato l’instaurarsi di differenti meccanismi di danneggiamento, a conferma dell’importanza dell’umidita in re...
The possibility of realising adaptive structures is of great interest in the control/automation f... more The possibility of realising adaptive structures is of great interest in the control/automation fi elds, owing to the benefi ts related to enhanced performance. To accomplish this, a challenging approach is the employment of Shape Memory Alloys (SMAs) as active elements, which can recover seemingly permanent strains by temperature-induced phase transformations whereby the so-called Shape Memory Effect (SME) takes place. This paper deals with an experimental investigation of the bending recovery performance of a functional structure. The active material was a near-equiatomic NiTi alloy in the form of strips, which were embedded into a custommade polymeric matrix. To study the infl uence of heating/cooling rates on the characteristic transformation temperatures of the NiTi material, several analyses were carried out by means of Differential Scanning Calorimetry (DSC). Prior to the insertion, the strips were thermo-mechanically treated to memorise a bent shape through experimentally ev...
The use of full cores, for producing automotive components by means of the permanent mould castin... more The use of full cores, for producing automotive components by means of the permanent mould casting technique, allows good castings with a low level of defects to be obtained. Nevertheless, an extra phase of the production cycle is necessary to have an optimal emptying of the internal cavities of the castings, with an increase in costs. This scenario motivated the present research, in particular to study the effect of replacing full cores with empty cores. The component analysed is one of the three parts of a motorcycle frame, realised in aluminum alloy A356 by permanent mould casting and T6 heat-treated. Several castings have been produced with both full and empty cores; tensile strength tests have been performed on samples drawn from four different zones, in order to compare the mechanical and microstructural properties of the castings realised with the two different kinds of core. A slight decrease in elongation at fracture has been verified in the samples drawn from the castings ...
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2017 by Gian Garagnani
Papers by Gian Garagnani