Antonello Astarita

On the Critical Technological Issues of Friction Stir Welding T-Joints of Dissimilar Aluminum Alloys

In this article, friction stir welded T-joints of innovative dissimilar aluminum alloys have been produced and tested with the aim to investigate the feasibility of using this joining technique, in this configuration, in the aerospace field with the final aim to save weight. The introduction of both this new welding technique and innovative alloys, such as AA 2198 and AA 6056, could allow making lighter and stronger structures. Some experiments, carried out previously, have shown that the fixturing device, the tool geometry, and the tilt angle play a significant role in the joint soundness. A wide experimental characterization has been carried out on FSW T-joints of AA 6056 T4 extrudes to AA 2198 T3 rolled plates. The results attained allow to put in evidence some critical issues on the investigated configuration and can be considered as a further acquired knowledge in the understanding and the design of friction stir processes.

Experimental Study of the Forces Acting on the Tool in the Friction-Stir Welding of AA 2024 T3 Sheets

In this paper, AA 2024 T3-rolled sheets were joined in butt joint configuration through the friction stir welding process. Different joints were carried out varying the principal process parameters (i.e., tool welding speed and tool rotational speed). The aim of this work was the study and the experimental characterization of the influence of the process parameters on the forces acting on the tool during the FSW process. Furthermore, it was studied the correlation between the forces and the grain size, in particular with the extension of the heat-affected zone. Forces acting along the axis parallel to the tool are actually greater than those acting along welding direction. All the recorded forces are strictly dependant on the process parameters adopted. No correlation has been found between the grain dimension within the weld bead and the recorded forces, while the greater the forces, the narrower the extension of the heat-affected zone.

Elevation of tribological properties of alloy Ti – 6% Al – 4% V upon formation of a rutile layer on the surface

The surface morphology, the adhesion of the oxide layer to the substrate, and their effect on the tribological characteristics of rolled sheets from alloy Ti – 6% Al – 4% V are studied at different temperatures and durations of oxidation of the surface. The methods of the study are measuring of microhardness, x-ray diffraction analysis and scanning electron microscopy. The composition of the oxide layer exhibiting good adhesion to titanium, low friction factor and high wear resistance is determined.

Influence of welding parameters and post-weld aging on tensile properties and fracture location of AA2139-T351 friction-stir-welded joints

Tensile properties and fracture location of AA2139-T351 friction stir welded joints are studied in the as-welded and post-weld aged condition. The experimental results show that when the joints are free of welding defects, they fail on the advancing side of the HAZ exhibiting a large amount of plastic deformation. When the revolutionary pitch exceeds a threshold value, some micro-defects are formed in the weld nugget due to insufficient heat input. In this case, the joints fail near the weld center, and the fracture occurs in a mixed mode, both ductile and brittle. However, being less ductile, post-weld aged joints are less defect-tolerant and, then, they fracture closer to the weld center, showing a reduced elongation at fracture and an UTS within the order of magnitude of the as-welded joints.

Selective Laser Post-Treatment on Titanium Cold Spray Coatings

The aim of the present work is to investigate the feasibility and effects of a selective postdeposition laser treatment on titanium coatings. Commercially pure titanium grade 2 powders were deposited by means of a cold spray process on aluminum alloy AA2024-T3 sheets. The surface treatment of the coating was realized using a 220 W diode laser. The influence of heat input and dimensional features of coating layer and substrate was assessed by an experimental campaign conducted following a design of experiments approach. Optical and scanning electron microscopy analysis of the microstructure of the deposited and treated material as well as microhardness measurements showed the formation of a compact layer of titanium oxide on the coating surface and the preservation of the temper state of the aluminum substrate.

Ti-6Al-4V Cutting by 100W Fibre Laser in Both CW and Modulated Regime

Titanium and its alloys are nowadays widely used in many sectors: in the medical field (orthopedic and dental ones), in the architectural field, in the chemical plants field and in aeronautic [1]. In this last field it is more and more used both for its contribution to make lightweight and time durable structures and for its compatibility with new materials, first of all Carbon Fiber Reinforced Plastics (CFRP). Cutting of titanium sheets is one of the primary requirements in the fabrication of most of the components. Laser cutting offers several advantages over conventional cutting methods. It includes narrow kerf width (minimum material lost), straight cut edges, low roughness of cut surfaces, minimum metallurgical and surface distortions, easy integration with computer numerically controlled (CNC) machines for cutting complex profiles and importantly non-contact nature of the process (suitable for cutting in hostile environments and in areas with limited access) [2]. However, due to very limited literature available on laser cutting of titanium, it is very difficult to predict the cut surface quality and optimum process parameters for laser cutting, especially when dross-free cuts are required. Laser cutting of titanium and titanium alloys needs to be carried out with an inert gas, this due to the high reactivity of the titanium with the oxygen at high temperatures [3]. However when the available power is limited, as in the present case, the use of a reactive gas (air) can help to achieve cutting speed value reasonable for industrial applications. The aim of this work is to study the cutting of Ti-6Al-4V rolled sheets 1 mm in thickness, by means of a 100 W fibre laser, (SPI-Red Power) working at wavelength  = 1090 nm. The maximum cutting speed were measured in both CW and pulsed regime at different mean power and different duration. Furthermore, the kerf geometry and the heat affected zone (HAZ) were studied decreasing the cutting speed from the maximum to the 80 % of this values. The results obtained showed that both the power and the cutting speed influence the cutting kerf geometry and HAZ. In particular the synergy of power and speed, resulting roughly into the heat input, seems to rule the whole cutting process.

Selective Laser Treatment on Cold-Sprayed Titanium Coatings: Numerical Modeling and Experimental Analysis

In this paper, a selective laser post-deposition on pure grade II titanium coatings, cold-sprayed on AA2024-T3 sheets, was experimentally and numerically investigated. Morphological features, microstructure, and chemical composition of the treated zone were assessed by means of optical microscopy, scanning electron microscopy, and energy dispersive X-ray spectrometry. Microhardness measurements were also carried out to evaluate the mechanical properties of the coating. A numerical model of the laser treatment was implemented and solved to simulate the process and discuss the experimental outcomes. Obtained results highlighted the key role played by heat input and dimensional features on the effectiveness of the treatment.

Beta Forging of a Ti6Al4V Component for Aeronautic Applications: Microstructure Evolution

Ti–6Al–4V is an alloy increasingly used for structural applications in aeronautics due to its characteristics of high mechanical properties, lightness, and corrosion resistance. This alloy is conventionally forged below the beta transus temperature in order to control the microstructure evolution, to obtain a component with the desired properties. In this paper, some experiences of an innovative beta forging process of the Ti–6Al–4V alloy are reported. A preliminary campaign of forging tests in the beta field on cylindrical coupons was carried out in order to study the microstructural evolution in different forging conditions, in terms of both temperature and strain rate. Moreover, in order to study the microstructural evolution due to the beta forging in a complex shaped component, a case study is presented. The forged component showed a microstructure coherent with the forging process experienced; moreover, the hardness values measured were similar to the ones of the Ti–6Al–4V alloy in mill-annealed conditions.

Laser Cutting of Aluminium Sheets with a Superficial Cold Spray Titanium Coating

In the manufacturing of metal components both wear and corrosion have to be considered. In particular, corrosion is a primary problem in the aeronautic field, where the aluminium alloys are affected by several corrosion typologies. Furthermore, nowadays carbon fibre reinforced plastics (CFRP) are finding an increasing use, but they can induce galvanic corrosion phenomena when coupled with aluminium alloys. To overcome this problem, corrosion resistant coatings are used on aluminium components. On these premises, the realization of a titanium coating on aluminium components could allow the coupling of CFRP and aluminium alloys, improving the corrosion resistance. Cold Spray Deposition is a recent technology to realize these coatings. This technology allows the production of near fully dense coatings on metallic surfaces. In many applications the coated aluminium sheets have to be machined (cut or drilled). Machining with conventional cutting methods leads to both tool wear and damages in the coating. Laser cutting represents a promising alternative: it does not involve any mechanical cutting force or tool wear and, thanks to the small laser beam spot, it allows to realize complex shapes. In this paper, laser cutting of an Al alloy sheet (0.6 mm thick) coated with Ti Alloy, was studied. The adopted laser source was a 150 W, lamp pumped Nd:YAG, specifically developed for micro-machining operations on metals. During the tests, the process parameters (cutting speed, pulse duration and entrance side) were changed and the kerf geometry was analysed as a function of the process parameters.

Hot Stretch Forming of a Titanium Alloy Component for Aeronautic: Mechanical and Modeling

The development of Hot Stretch Forming (HSF) by the Cyril Bath Company was in response to airframe designers needing to use Titanium airframe components in new commercial aircraft. Many of the airframe component structures are designed to fit against the inside radius of the fuselage curvature. By combining traditional stretch forming technology with hot titanium forming techniques, the HSF guarantees a saving in material and machining time, which are two serious cost issues for today’s aircraft manufacturers. In addition, the process allows for consistent quality in a productively efficient manner, assuring the sustainable attainment of delivery and build schedules. The HSF is an innovative process on the cutting edge of the technologies, so focused research is needed in order to better understand this technology and develop new applications for this process. in this paper the HSF process is investigated: the machine and the different steps that characterized the process were described and the results of a preliminary experimental campaign was discussed focusing the attention on the metallurgical aspect. Moreover a modeling of the process was executed in order to study the stresses and strains undergone by the material among the deformation.