a b s t r a c tA novel MMM (Multi-frequency, Multimode, Modulated) ultrasonic (US) technology was used to refine the as cast microstructure and improve the mechanical properties of a AlSi9Cu3 alloy. Ultrasonic vibration was isothermally applied to the melt for 120 s at different temperatures slightly above the liquidus temperature of the alloy, using different electric power values, before pouring into a metallic mould. The microstructure of the cast samples was characterized by optical and scanning electron microscopy and energy dispersive spectrometry. Ultrasonic vibration promoted the formation of small ˛-Al globular grains, changed the size and morphology of intermetallic compounds and distributed them uniformly throughout the castings. Ultimate tensile strength and strain were increased to 332 MPa and 2.9%, respectively, which are 50% and 480% higher than the values obtained for castings produced without vibration. The microstructure morphology and the alloy mechanical properties were found to depend on the electric power and the melt temperature, and by using a suitable combination of these parameters it is possible to achieve high refinement efficiency by treating the melts in the liquid state.

The corrosion of five materials for implant suprastructures (cast-titanium, machined-titanium, gold alloy, silver-palladium alloy and chromium-nickel alloy), was investigated in vitro, the materials being galvanically coupled to a titanium implant. Various electrochemical parameters E(CORR), i(CORR) Evans diagrams, polarization resistance and Tafel slopes) were analyzed. The microstructure of the different dental materials was observed before and after corrosion processes by optical and electron microscopy. Besides, the metallic ions released in the saliva environment were quantified during the corrosion process by means of inductively coupled plasma-mass spectrometry technique (ICP-MS). The cast and machined titanium had the most passive current density at a given potential and chromium-nickel alloy had the most active critical current density values. The high gold content alloys have excellent resistance corrosion, although this decreases when the gold content is lower in the alloy. The palladium alloy had a low critical current density due to the presence of gallium in this composition but a selective dissolution of copper-rich phases was observed through energy dispersive X-ray analysis.

a b s t r a c tThe grain refinement effect and the ageing behaviour of Al-0.5 wt.% Sc, Al-0.7 wt.% Sc, and Al-1 wt.% Sc alloys are studied on the basis of optic microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) observations and hardness measurements. In Al-Sc alloys the higher grain refinement is observed for Sc contents greater than 0.5 wt.% accompanied by a notorious morphology modification, from coarse columnar grains to a fine perfect equiaxed structure. The as cast structures are characterized by a rich supersaturated solid solution in Sc, that promotes a great age hardening response at 250°C and 300°C. The age hardening curves also demonstrate a low overageing kinetics for all the alloys. Although the higher Sc content in solid solution for the alloys with 0.7 and 1 wt.% Sc, the age hardening response of all the Al-Sc alloys remains similar. The direct age hardening response of the as cast Al-0.5 wt.% Sc is shown to be greater than the solutionised and age hardened alloy.

a b s t r a c tThe influence of high intensity ultrasound (US) propagating through a steel mold on the microstructure and mechanical properties of die-cast AlSi9Cu3(Fe), was studied for different levels of electric power and at different distances to the waveguide/mold interface. The effect of those parameters on the morphology of a-Al and eutectic Si and on the volume of porosity were investigated and characterized.The morphological characterization revealed that the high intensity vibration not only promoted the formation of small a-Al globular grains but also modify the eutectic silicon, as well as decreased the volume of porosity. Besides microstructure modification, US treatment improved the alloy mechanical properties, namely UTS and strain, which maximum values were 339 MPa and 2.9% respectively by comparison to the values obtained for castings produced without US vibration. A mechanism of eutectic Si modification based on theoretical-experimental analysis is proposed.

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Car exhaust manifolds are critical components subjected to cyclic thermo-mechanical

Purpose In a printed circuit board assembly (PCBA), the coefficient of thermal expansion (CTE) mismatch between the solder joint materials has a detrimental impact on reliability. The mechanical stresses caused by the thermal changes of the assembly lead to fatigue and sometimes the failure of the solder joints. The purpose of this study is to propose a novel pad design to obtain an interrupted solder/substrate interface, to improve the PCBA reliability. Design/methodology/approach An interruption in the continuous intermetallic compound (IMC) layer of a solder joint was implemented, by the deposition of a silicone film in the pad, changing its geometry. That change allows a redistribution of stresses in the most ductile zone of the solder joint, the solder. The stress concentration at the solder/substrate interface is reduced, as well as the general state of stress at the solder joint. Findings A new way was developed to reduce the stress on the solder joints, caused by thermal variations, because of the different components CTEs mismatch. This new method consists of interrupting the IMC layers of the solder joint, strategically, redirecting the usual stresses to a more ductile area of the joint, the solder. This is an innovative method that allows increase the lifetime of PCBAs and the equipments. Originality/value In this study, a new pad design concept for higher solder joint reliability was developed to reduce the shear stress in the solder joints because of the CTE mismatch between all the solder joint components.

The demand for high-strength castings at competitive prices, for applications where the ratio strength/unit mass is a critical factor, such as automotive and aeronautics, has increased exponentially. Among the aluminum casting alloys stand out the Al-Si alloys (with or without Cu and/or Mg additions), which represent 80% of aluminum casting alloys. One of the microstructural parameters that most influence the properties of Al-Si alloys is the morphology (volume fraction, size, shape and distribution) of eutectic silicon. The modification of the eutectic can be obtained by chemical modification by the addition of several elements [1,2]. It was recently reported that some transition metals can promote the modification of the eutectic, among which stands out scandium (Sc) [3]. The addition of scandium to aluminum alloys has been the subject of several studies in recent years, since the addition of small amounts induces a significant increase in the mechanical properties of these alloys. This work focuses on evaluating the effect of the addition of Sc on the grain refinement and the modification of eutectic silicon of an as-cast AlSi9 alloy. The OM micrographs presented in Fig. 1 revealed that the addition of Sc obviously reduced the sizes of the-Al primary phase and eutectic silicon particles as well as secondary dendrite arm spacing value. According to the microstructures shown in Fig. 2, it is evident that the Sc has a high potential to change the morphology of eutectic silicon. In fact, the silicon lamellar/acicular morphology, characteristic of unmodified alloys (Fig. 2a), was transformed into a fibrous structure with rounded edges and compact shaped (Fig. 2b and c), characteristic of modified alloys. The EDS analysis (Fig. 3) showed that Sc precipitates mainly in the interdendritics regions forming Fe and Si rich intermetallic phases, which reduces drastically the amount of Sc on-Al and consequently the hardening effect of the Al-Si (Sc) alloys.