Giovina Marina La Vecchia

Influence of ultrasound treatment on cavitation erosion resistance of AlSi7 alloy

Ultrasound treatment of liquid aluminum alloys is known to improve mechanical properties of castings. Aluminum foundry alloys are frequently used for production of parts that undergo severe cavitation erosion phenomena during service. In this paper, the effect of the ultrasound treatment on cavitation erosion resistance of AlSi7 alloy was assessed and compared to that of conventionally cast samples. Cavitation erosion tests were performed according to ASTM G32 standard on as-cast and heat treated castings. The response of the alloy in each condition was investigated by measuring the mass loss as a function of cavitation time and by analyzing the damaged surfaces by means of optical and scanning electron microscope. It was pointed out that the ultrasound treatment increases the cavitation erosion resistance of the alloy, as a consequence of the higher chemical and microstructural homogeneity, the finer grains and primary particles and the refined structure of the eutectic induced by the treatment itself.

Effect of Globular Microstructure on Cavitation Erosion Resistance of Aluminium Alloys

In this paper the effect of globular microstructure on the cavitation erosion resistance was assessed and compared to that of conventional dendritic one. Three different wrought aluminum alloys in as-cast conditions were investigated. The samples were completely characterized by metallographic analyses and microhardness measurements. Cavitation erosion tests were performed according to ASTM G 32 standard. The volume loss was evaluated during the test by periodical interruptions. It was identified the damaging mechanism in case of both dendritic and semisolid microstructure. It was also found that the globular microstructure increases the cavitation erosion resistance only for one of the studied alloys.

Aluminum Segregation in ZA27 Rheocast Alloy

Aluminum segregation in zinc alloys is a well-known problem during melting and casting. In molten Zn alloys, in fact, Al tends to float on the surface of the bath because of its lower density than zinc. The same problem also occurs during casting solidification, causing chemical gradients along part thickness and soundness problems. As a consequence, the use of high aluminum content zinc alloys, such as ZA27, can be limited.In this paper the effectiveness of rheocasting method on decreasing Al-segregation phenomenon in ZA27 was investigated. The slurry was obtained by using the ultrasound technique, starting from the fully liquid down to different solid fraction percentages. The metal was then cast into a permanent steel mold. The produced samples were completely characterized by metallographic analysis (optical and scanning electron microscope), to assess the microstructure modification induced by the treatment. Hardness tests were also performed to evaluate mechanical properties. For comparison, the same tests were performed on conventionally cast samples.The analyses show that the use of a semisolid alloy increases the homogeneity of the part in terms of hardness, chemical composition and microstructure; furthermore casting’s shrinkage porosity is significantly reduced.

Investigation on Microblasting Applied to CrN Coatings

A microblasting treatment carried out on CrN coated samples was studied to investigate the induced effect on corrosion and wear resistance. CrN coating was deposited through Cathodic Arc Evaporation technique on quenched and tempered steel. The properties of the coating were studied by hardness measurements, scratch, potentiodynamic, and pin-on-disk tests. The results show that microblasting reduces the corrosion resistance while improving the wear behavior.

Nd:YOV4 laser surface texturing on DLC coating: Effect on morphology, adhesion, and dry wear behavior

The surface of structural components is usually subjected to higher stresses, greater wear or fatigue damage, and more direct environmental exposure than the inner parts. For this reason, the interest to improve superficial properties of items is constantly increasing in different fields as automotive, electronic, biomedical, etc. Different approaches can be used to achieve this goal: case hardening by means of superficial heat treatments like carburizing or nitriding, deposition of thin or thick coatings, roughness modification, etc. Between the available technologies to modify components surface, Laser Surface Texturing (LST) has already been recognized in the last decade as a process, which improves the tribological properties of various parts. Based on these considerations the aim of the present research work was to realize a controlled laser texture on a Diamondlike Carbon (DLC) thin coating (about 3 m thick) without damaging both the coating itself and the substrate. In particular, the effect of laser process parameters as marking speed and loop cycle were investigated in terms of texture features modifications. Both qualitative and quantitative analyses of the texture were executed by using a scanning electron microscope and a laser probe system to select the proper laser parameters. Moreover, the effect of the selected texture on the DLC nanohardness, adhesion and wear behavior was pointed out.

Influence of Cr and Mn Addition and Heat Treatment on the Corrosion Behaviour of an AlSi3Mg Alloy

In the present work, the effect of Cr and Mn addition on corrosion resistance was investigated on AlSi3Mg alloy. Potentiondynamic corrosion tests in a 3.5 wt. % NaCl solution were performed on samples in different heat-treated conditions, and corrosion current density and potential were determined by Tafel method. Brinnel hardness measurements were also carried out in order to couple corrosion resistance with mechanical properties. It was interestingly found that Cr presence enhanced mechanical properties and corrosion resistance in comparison with the base alloy.

Comprehensive Numerical Simulation of Filling and Solidification of Steel Ingots

In this paper, a complete three-dimensional numerical model of mold filling and solidification of steel ingots is presented. The risk of powder entrapment and defects formation during filling is analyzed in detail, demonstrating the importance of using a comprehensive geometry, with trumpet and runner, compared to conventional simplified models. By using a case study, it was shown that the simplified model significantly underestimates the defects sources, reducing the utility of simulations in supporting mold and process design. An experimental test was also performed on an instrumented mold and the measurements were compared to the calculation results. The good agreement between calculation and trial allowed validating the simulation.

Computational Model For Spray Quenching Of A Heavy Forging

Considering heavy forged parts presented that water spray is an appropriate technique as quenching operation. In general, the rapid cooling causes residual stress field due to the dis-homogeneity of the distribution of temperature in the part and the different microstructural phases. In comparison with other conventional quenching methods, spray-quenching method could have a capability to control the temperature distribution through the surface. In order to gain entry the appropriate mechanical properties, the spray control valves need to adjust to the foundation of heavy forgings cross sections. In this situation, the modelling of the spray quenching operation necessarily deserves for more elaboration. The present paper is aimed at developing and simulating a spray quenching process of a heavy forged shaft produced in a hardening and tempering steel.

Thixoforging of Ultrasound Treated 6060 Aluminum Alloy

In the last years researches on thixotropic materials have been developed in order to introduce this new technology in manufacturing processes. For instance, when considering high pressure die-casting, several applications are present in literature mainly related to low melting point alloys (Al and Mg) because of the limited die life experienced when casting higher melting materials. In this case, semi-solid metal forming allows to work at lower temperature with subsequent increase in die life and reduction in production costs, combined with lower porosity level in the casting. On the other hand, in the case of conventional forging, semi-solid processing needs higher performance materials and/or coatings for the mould because of the working temperatures; however, the advantages of obtaining near net shape part in a single step, with reduced machining and finishing costs, make the semi-solid technology competitive. The present paper deals with the thixoforging of aluminum 6061 alloy, whose semi-solid feedstock material was obtained by ultrasound treatment. The application of ultrasonic waves to liquid or solidifying alloys has been already demonstrated to be an effective technique for the obtainment of globular microstructure. Along with a refining effect, ultrasound can also produce a series of beneficial effects, such as hydrogen degassing or oxide and non-metallic inclusion removal, which all improve mechanical properties of the component. The aim of this research was to investigate the influence of process parameters on final forged part quality. The solid fraction percentage as a function of temperature was measured by differential scanning calorimetric analysis. The geometry of the die was properly designed and optimized by FEM simulation in order to be suitable for forging semi-solid material, allowing a comparison with conventional forging process. 14 K-type thermocouples were used for monitoring the temperature of top and bottom dies; an instrumented 100 ton press was also equipped with load cells to acquire the forging force. A deep metallurgical analysis of the forged parts was performed in order to evaluate their mechanical properties and quality.