Gian Luca Garagnani

Friction and wear behavior of C4Al2O3Al composites under dry sliding conditions

Abstract 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 Al 2 O 3 Al co-continuous ceramic/metal composite (referred to as C 4 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 Al 2 O 3 and Al phases are continuous and interpenetrating. The aim of the present work was to investigate the tribological behavior of C 4 Al 2 O 3 Al 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 C 4 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 Fe 2 O 3 , 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 C 4 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 C 4 composites. These aspects led to an improvement of wear resistance of the C 4 composites over conventional MMCs, under the same experimental conditions, and probably shifted the transition from mild to severe wear regimes to higher critical loads.

Unalloyed copper inclusions in ancient bronze artefacts

Ancient bronze artifacts, that represent a considerable part of the archeological finds, have been largely studied because of their complex degradation phenomena taking place in the long time span that have not been fully understood. One of the peculiar features of ancient bronzes is the presence of inclusions of copper unalloyed with tin. Unalloyed Copper Inclusions (UCI) have been observed in buried archaeological bronze artefacts by several authors, but each paper reports only on a limited number of cases. In our extensive studies on bronze artefacts, UCI have been observed in many bronze artefacts with very different features and purposes. Both as-cast and wrought artefacts were studied, so that the influence of the manufacturing process and the composition of the artefacts on the formation of UCI might be evaluated. The microstructure and composition of these artefacts were studied and the features of UCI have been related with those of the surrounding phases. The results have been discussed and compared with those obtained by other authors. The presence of UCI in buried archaeological bronze artefacts could indicate some unusual corrosion processes that might need to be accounted for when designing conservation treatments.

Mechanical and impact behaviour of (Al2O3)p/2014 and (Al2O3)p/6061 Al metal matrix composites in the 25–200°C range

The present work is aimed at studying the impact behaviour of commercially available Aluminium matrix composites, in a temperature interval ranging from 25°C to 200°C. The results of instrumented impact tests and of microstructural and fractographic observations are correlated with the tensile properties of these materials. A description of the phenomena involved (particles cracking, interfacial failure associated to matrix-reinforcement reaction layers, ductile behaviour of the matrix) is given. The effect of testing temperature as well as that of the matrix characteristics are presented and discussed.

Effects of matrix and reinforcement properties on low- and high-cycle fatigue behaviour of particulate-reinforced MMCs

High and low-cycle fatigue tests were performed on AA 6061/10%Al2O3p, AA 6061/20%Al2O3p and AA 2618/20%Al2O3p metal matrix composites. All the materials were tested in a peak-aged condition (T6 temper). The fatigue properties were analysed in terms of reinforcement and particle/matrix interface properties; the effect of reinforcement volume fraction and size was also studied. OM, SEM and EDS analyses were performed on the composites. SEM analyses were also carried out on the fracture surfaces of fatigue-tested samples to assess crack nucleation and growth mechanisms. Differences between low and high-cycle fatigue and fracture behaviour were discussed for the tested composites. In the high-cycle fatigue regime, the materials showed better resistance than the corresponding un-reinforced alloys only at low stresses. The composite resistance to the low-cycle fatigue was less satisfactory. In this case, a coarser reinforcement reduced the fatigue life, due to greater tendency toward particle damage on loading.

A comparative study on the effects of three commercial Ti–B-based grain refiners on the impact properties of A356 cast aluminium alloy

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 β-platelets size. The concurrent effects of SDAS and β-platelets average maximum length on total impact energy can be taken into account by the multiple regression analysis technique.

Investigation of mechanical properties of AlSi3Cr alloy

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.

Effect of the Addition of Nickel Powder and Post Weld Heat Treatment on the Metallurgical and Mechanical Properties of the Welded UNS S32304 Duplex Stainless Steel

No presente estudo, foi investigado o efeito da adicao de po de niquel e a aplicacao de um tratamento termico pos-soldagem (PWHT) nas propriedades de soldagem do aco inoxidavel lean duplex UNS S32304, a fim de melhorar a microestrutura e as propriedades mecânicas. Po de niquel foi diretamente vertido na abertura da junta e misturado com o metal de adicao durante o processo de soldagem a arco com eletrodo de tungstenio e protecao gasosa (GTAW). Alem disso, o tratamento termico foi estabelecido em 1100 °C por 10 min. As juntas foram caracterizadas por microscopia optica (MO) e a evolucao das percentagens de fase nas diferentes zonas foi estudada por meio da tecnica de analise de imagem. Os ensaios de tracao e de microdureza foram realizados, a fim de avaliar a melhoria das propriedades mecânicas das juntas. Os resultados mostraram que tanto a adicao de po de niquel durante o processo de soldagem e o tratamento termico pos-soldagem permitiram melhorar as propriedades mecânicas da junta soldada. PWHT mostrou o melhor efeito em restaurar a percentagem igual de ferrita e austenita comparado com a adicao de po de niquel.

Correlation between Aging Effects and High Temperature Mechanical Properties of the Unmodified A356 Foundry Aluminium Alloy

In this study, the effect of aging on the mechanical properties of unmodified A356 aluminium casting alloy with trace additions of Ni or V was investigated. Trace elements were added in concentrations of 600 and 1000 ppm of Ni and V, respectively. Samples from sand and permanent mould castings in as cast and T6 heat-treated conditions were tested. Tensile tests were performed at both room and high temperature (235 °C). Taking into account the results from both testing conditions, Vickers hardness was measured in order to endorse the hypothesis of artificial aging occurring during high temperature tensile tests. In order to study this effect, a series of specimens was aged at 235 °C for different aging times, and aging curves were plotted. The occurrence of static and dynamic aging was evaluated by comparing hardness values of tensile specimens and aged samples, particularly in the range of 5-20 min, as this range corresponds to the time necessary for pre-heating and testing of the tensile samples. A basic correlation between tensile strength and hardness is also given.

Tribological Behavior of a Cr2O3 Ceramic Coating/Steel Couple Under Dry Sliding and Heavy Loading Conditions

This work evaluates the effects of heavy loadings on a Cr2O3 ceramic coating/steel couple under dry sliding and in conditions similar to those occurring in heavy-duty diesel engines. Two types of wear tests were performed by a standard pin-on-disc tribometer under high constant and variable loads. Before wear tests, microstructure and mechanical properties of coating and steel were determined by optical emission spectrometry, optical microscopy, roughness, microhardness and x-ray diffractometry (XRD). The friction coefficient was directly calculated by the tribometer. The wear rate of Cr2O3 ceramic coating was evaluated by the wear volume and that of pins by weighing them before and after tests. The wear tracks on pins and coating surfaces were analyzed by XRD and by scanning electron microscopy with energy-dispersive spectroscopy. For all loads except the lowest constant one, the results indicated high friction and wear involving material removal from the coating surface, through a severe-oxidational wear associated with extensive cracking of the surface of the coating, softening of the carburizing steel and removal of the carburized layer of pins. The wear mechanism was mild-oxidational under the lowest constant load, which was therefore suggested as the limit for acceptable performance of the sliding couple under study.

Metallurgical Characterization of a Weld Bead Coating Applied by the PTA Process on the D2 Tool Steel

In this investigation, a nickel-base powder mixed with tungsten carbide particles was applied by Plasma Transferred Arc welding (PTA) on the surface of the D2 cold work tool steel to improve surface quality and to extend its lifetime during applications. The Design of Experiment (DoE) method was applied to obtain the appropriate combination of hardfacing parameters and to run the minimum number of tests. Current, travel speed and preheat were considered as variable parameters. These parameters are important to reach a final layer with an appropriate bead geometry accompanied with good metallurgical properties. All samples were prepared for metallurgical investigations and the effect of process parameters on the weld bead geometry was considered. For each experiment run, weld bead geometry parameters were measured including dilution, penetration and reinforcement. Microstructures and the distribution of tungsten carbide particles after welding were analyzed by Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) equipped with an EDS microprobe. In addition, hardness tests were performed to evaluate the mechanical properties of the weld bead layers. Finally, among all the experiments, the best sample with appropriate bead geometry and microstructure was selected.