Annalisa Pola

Influence of Material Microstructures in Micromilling of Ti6Al4V Alloy

In the most recent decades the introduction of unconventional machining processes allowed the development of micromachining techniques. In this work, the influence of material microstructures on the micromilling process was investigated. Ti6Al4V alloy was selected as workpiece material since it is a very common material for micro applications and because its duplex microstructure can be easily changed by proper thermal treatments. Four different microstructures (namely bimodal, fully equiaxed, fully lamellar and mill annealed) were obtained through recrystallization annealing treatments carried out at different times and temperatures. The mechanical properties of the samples were assessed by microhardness measurements. Nano-indentations were also performed on single grains to understand how the different hardness of phases and structures present in the Ti6Al4V alloy can affect the micromilling process. Microchannels using two flute flat end mills with a diameter equal to 200 µm were realized on the treated samples. Two different feed-per-tooth values were used during the tests. Cutting force, channel shape and burr dimension were investigated. Morphological and energy dispersive spectroscopy (EDS) analyses were performed on tools by means of a scanning electron microscope (SEM): in this way the phenomena mainly influencing the tool status were also identified. Lower cutting forces and reduced tool wear were observed when working fully lamellar microstructures compared to the other ones.

Analysis and design of a low-noise railway wheel

Abstract The design and construction of a new ‘constrained layer’ damping treatment for railway wheels is presented. A numerical procedure was used for the loss factor calculation. This procedure, first verified on a plate by means of experimental modal analyses, allowed the best treatment thickness and arrangement to be chosen among those commercially available and technologically feasible. The work ended with the construction of a prototype and subsequent tests in the laboratory and in the field. The success of these tests justified the start of production of low-noise wheels treated in this way.

Thixo-Extrusion of 5182 Aluminium Alloy

Extrusion is a well established technology for the production of complex sections of aluminium alloys. Thixo-extrusion in comparison to traditional hot-extrusion offers several advantages such as lower extrusion and friction forces, higher material fluidity, longer tool life etc. Aluminium alloy 5182 is an important commercial alloy characterized by high strength and ductility, high corrosion resistance and good formability; it is commonly used for the production of wrought automotive components and it is also suitable for semi-solid applications thanks to its wide solidification range. The aim of this paper is to attempt the shaping of 5182 Al-Mg alloy through the thixo-extrusion process using a ceramic tool and evaluating the effect of different routes of making the feedstock on the semisolid microstructure. Particularly, two different methods were investigated: Near-solidus casting and Roll-casting using a cooling slope. All the samples produced were characterized by metallographic analysis in order to measure globule size and shape factor, as the main criteria used for assessing thixo-formability.

Micromilling of Lamellar Ti6Al4V: Cutting Force Analysis

The aim of this article is to study the influence of a Ti6Al4V microstructure on cutting forces during the micromilling process. Samples were annealed above the β-transus at three different temperatures—1020, 1050, 1080°C—and then cooled in a furnace, air, and water, in order to produce different Widmastätten microstructures. Micromilling tests were carried out on heat-treated samples, and the cutting forces were measured by means of a load cell. The results were correlated to the sample microstructures, which were thoroughly investigated by means of an optical microscope, X-ray diffraction, and microhardness measurements. The highest cutting forces were observed for soft and ductile furnace-cooled samples, suggesting that the most important factor affecting workability is the material ductility, while hardness is a less relevant parameter.

Rheological Characterization of a New Alloy for Thixoforming

A new aluminum alloy (AlSi5Mg0.5Cu0.3Ag) for semisolid die-casting applications was designed, starting from computational thermodynamics calculations by Computherm Database. The goal was to obtain a combination of good castability and proper concentration of hardening elements for strengthening precipitation treatment. The predicted thixotropic properties were verified by measuring the microstructural conventional parameters, such as globule size and shape factor, and the solidification range, by means of differential scanning calorimetry. To complete the characterization of this new alloy and to evaluate its applicability in industrial production, the shear rate and time-dependent flow behavior of the alloy in the semisolid state was studied in a Searle-type rheometer. A future aim of the present research is to try to use rheology testing as the tool to optimize the chemical composition, in order to design an alloy characterized by good mechanical performances and easy processability. Considering the strong influence of the solid fraction content on semisolid alloy viscosity, the rheology tests were interrupted after a certain time and the alloy was deeply freezed using vaporized liquid nitrogen, in order to fix the microstructure and verify the correctness of the thermodynamic simulation.

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 microblasting on cathodic arc evaporation CrN coatings

Abstract Cathodic arc evaporation CrN double layer coatings were deposited onto tool steels in two steps by industrial apparatus. Microblasting was performed on CrN first layer in order to remove metallic droplets and surface defects and to promote the adhesion of the second layer. Surface morphology effects were assessed by a scanning electron microscope coupled with Image analyser software. Nanoindentation and scratch tests were performed to evaluate the changes of coating adhesion and mechanical properties. Structure, microstructure and residual stress were evaluated by X-ray diffraction. In particular, by means of Cu and Cr wavelengths, the residual stresse of both steel substrates and coatings was calculated. The results show that microblasting has no effect on coating hardness and slightly affects the adhesion of the coating to the substrate. On the other hand, microblasting increases coating resistance to cohesive failure and the substrate compressive stress at the coating interface.

Modeling of shear induced coarsening effects in semi-solid alloys

Abstract In long-term rheological shear experiments with semi-solid alloys, coarsening of the particles will falsify the interpretation of the experimental results. The coarsening is intensified by the shear induced convection and the mean size of the particles is changed significantly during the experiments. A simple model has been set up which takes the influence of the convection into account. The resulting growth law has been simplified for diffusion and convection dominated growth. The growth law was verified with shear experiments in a Searl-rheometer with A356 and tin-lead alloys. The experiments demonstrated that under convection the growth follows a linear time law and that the rate constant depends on the root of the shear rate. The correction of experimental results to gain the true viscosity function is demonstrated for a shear jump experiment with A356.

Design and Production of New Aluminum Thixotropic Alloys for the Manufacture of Structural Components by Semisolid Die Casting

Semi-solid processing is nowadays a powerful technology for the realization of structural components; in addition to the increase in their mechanical properties, due to the globular structure instead of the dendritic one, further developments are most likely to be expected from alloy chemical composition adjustments in order to achieve higher performances compared with the industrially used A356 and A357. Aim of this research is to try to set up new aluminium alloys for semisolid foundry applications, starting from the standard Al-Si system, at the base of all known casting processes. Different chemical compositions, based on either foundry or wrought Al alloys, have been investigated by means of computational thermodynamics (Pandat®), producing quantitative data about solidus-liquidus interval, solid fraction as a function of temperature, phase diagrams i.e. potential for age hardening, etc.. Some selected alloys, fitting the needs of good castability, proper concentration of hardening elements in the alpha phase and, obviously, easy production of feedstock material have been mechanically stirred by means of an experimental apparatus designed and self-constructed in the foundry laboratory of the university; the effect of different stirring tool configurations on the semi-solid state obtainment has also been assessed. Subsequently, the manufactured alloys have been reheated and cast into a simple die, properly designed, for the production of small samples. Microstructural investigations have been done on the stirred alloy (before and after re-heating), on the as cast and the heat treated samples to evaluate the efficiency of the designed system and of the defined alloys. Experimental tests on the processed alloys have been carried out by means of an instrumented crucible in order to verify the predicted thermodynamic properties supplied by simulation study (i.e. fs-temperature curve).

Advanced Casting Methodologies: Investment Casting, Centrifugal Casting, Squeeze Casting, Metal Spinning, and Batch Casting

This chapter explores various advanced casting techniques. One such technique is investment casting, a process that came about as a result of the demand for complex geometry components in high-temperature capability materials in the first half of the twentieth century for use in aircraft engines. Such components could not be formed or would be too expensive to produce using the more common processes, such as forging or machining. The industry has developed rapidly since then, encompassing everything from the jewelry market to the aerospace field. Castings from a few grams to several hundred kilograms in weight can be produced using investment casting. Centrifugal casting is a technique in which liquid metal is poured into a rotating mold. The centrifugal force induced by the spinning causes the melt to be thrown against the inner mold wall, and rotation is maintained until the metal solidifies from the outer to the inner diameter of the casting.