Carla Velotti

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.

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.

Beta Forging of Ti-6Al-4V: Microstructure Evolution and Mechanical Properties

Titanium alloys are finding an increasing use in the aeronautical field, due to their characteristics of high mechanical properties, lightness and corrosion resistance. Moreover these alloys are compatible with the carbon fibre reinforced plastics that are also finding a wide use in the aeronautical field. On the other hand the use of these alloys implies some drawbacks, for example titanium alloys are often considered more difficult to form and generally have less predictable forming characteristics than other metallic alloys such as steel and aluminum. In this paper was studied both the microstructure evolution and the mechanical properties of a Ti-6Al-4V rolled bar after hot forging. The thermo-mechanical response of a Ti-6Al-4V alloy was studied in elevated temperature compression tests (CT). Furthermore numerical simulations were carried out in order to do a comparison between numerical data and experimental results. The simulations were carried out using an implicit commercial code able to conduct coupled thermo-mechanical-microstructural analysis of hot forming processes of metal alloys.

FSW of AA 2139 Plates: Influence of the Temper State on the Mechanical Properties

Nowadays the fiber reinforced materials are finding more and more widespread use in aeronautic field due to their features of lightness, high strength and flexibility of manufacturing systems. The only way for metals to remain competitive for the aerospace applications is to improve new technologies and alloys in order to realize lighter and more resistant structures. The development of new alloys (lighter and stronger) and technologies will allow to use metals also in the future for aerospace applications. In this scenario the research activity has a fundamental importance, and the key point is to work simultaneously on both innovative materials and new technologies that allow to obtain the best performances with the innovative alloys. Welding is nowadays playing a fundamental role in transport industry thanks to the important advantages it allows. Friction Stir Welding (FSW) [1] is one of the most promising welding techniques, particularly suitable for applying to light alloys. FSW in butt joint configuration allows to achieve very high mechanical performances, often absolutely superior to those achievable with all other joining techniques, and lots of researches and results are now available [2]. The AA 2139 is an innovative Al-Cu-Ag alloy that has higher mechanical performances than the conventional 2xxx series aluminum alloys. The AA 2139 is designed to work in service in T8 temper condition, but is simplest to work in T3 temper condition. The aim of this work is to compare the performances of AA 2139 butt joints welded in T8 temper conditions, presented in a previous work [3], with the ones of joints welded in T3 condition and heat treated post welding in order to achieve the T8 temper condition.

Numerical Optimization of Selective Superplastic Forming of Friction Stir Processed AZ31 Mg Alloy

Superplastic forming is a near net shape process used to produce various items with complex geometry. However in many cases, only some portions of the workpiece undergo superplastic deformation. In these cases, instead of choosing expensive starting sheet material with superplastic properties, a low-cost conventional material can be chosen and a grain refinement process can be performed in the selected regions to enhance superplastic properties locally [1]. This process is known as “selective superplastic forming” [R.S. Mishra, M.W. Mahoney, US Patent 6,712,916, 2002]. In some previous works the use of Friction Stir Processing (FSP) was used to obtain locally a microstructure with ultrafine grains in the AZ31 magnesium alloys [2, 3]. In this study a modeling approach was adopted thanks to a commercial FE code and different simulations were conducted in order to correlate the experimental and numerical results for the model optimization [4, 5]. Free bulge forming tests of friction stir processed AZ31 sheets, in conjunction with numerical simulations, were used to evaluate the proposed optimization approach, with the aim to reduce the time and costs in the design of components with complex geometry.

Characterization of Ti-6Al-4V Tribopairs: Effect of Thermal Oxidation Treatment

This paper deals with the study of the influence of the thermal oxidation (TO) treatment on the tribological properties of Ti-6Al-4V tribopairs. A detailed experimental campaign, including tribological tests, microgeometrical measurements, microhardness tests and phase composition analyses, was carried out on both treated and untreated components. The tribological behavior was studied through the pin-on-disk tests in four different contact conditions: treated disk coupled with untreated pin, untreated disk coupled with treated pin, both treated and both untreated. The effectiveness of the treatment in enhancing the tribological properties of the Ti-6Al-4V alloy sheets was found. In particular, the thermal oxidation treatment, promoting hardness enhancement and the formation of a superficial rutile layer, changed the wear mechanism of the titanium alloy, passing from adhesive wear type, for the untreated case, to abrasive wear, in the treated one.

Mechanical characterization by DOE analysis of AA6156-T4 friction stir welded joints in as-welded and post-weld aged condition

Abstract Tensile properties of AA6156-T4 friction stir welded joints were studied in the as-welded and T62 post-weld heat treated condition. To analyze the influence of rotating and welding speed on the 0.2 % proof stress and tensile strength of the two classes of joints, a complete full factorial design with three levels for each studied parameter was performed. Statistical analyses were carried out to establish empirical models of the tensile properties of the joints as a function of the studied welding parameters. The obtained models were validated by statistical tools such as Mallows CP, S, R2 and R2(adj). The developed regression models can be effectively used to predict the mechanical properties of the joints at 95 % confidence level.

Confocal microscopy and imaging profilometry: A new tool aimed to evaluate aesthetic procedures

According to the American Academy of Aesthetic Plastic Surgeons, more than 11 million cosmetic surgical and nonsurgical procedures were performed by board-certified plastic surgeons, dermatologists and otolaryngologists in the United States, totaling more than 12 billion dollars. We performed a retrospective observational multi-centric study on patients treated with a non-animal origin cross-linked hyaluronic acid with different molecular weights for nasolabial folds, evaluating through a new imaging system, profilometric techniques with the confocal microscopy, the durability, the efficacy and the safety of this product. From 25 patients, 150 silicone casts were obtained: 75 casts of the right nasolabial fold and 75 casts of the left nasolabial fold. Roughness arithmetical average of the right fold at T2 decreased by 50% versus T0 and by 40% compared to T1; at T2, it decreased by the 45% versus T0 and by 35% compared to T1. No side effects were reported. Results proved that the analysis of the skin microreliefs through confocal microscopy is a new imaging system that allows to evaluate with precision and safety the results of aesthetic treatments such as fillers objectively.

Study of the solid state joining of additive manufactured components

The additive manufacturing technologies are finding an increasing use due to their peculiarities. A limit that still hampers their massive application is the maximum dimension of the components than can be produced. This paper deals with the study of the joining process of additive manufactured parts. In particular, two different cases have been studied: i) Ti6Al4V parts made through electron beam melting joined through linear friction welding; ii) AlSi10Mg parts made through direct metal laser sintering joined through friction stir welding. The results obtained showed the feasibility of both the processes.

On the Investigation of the Portevin-Le Chatelier Effect in the Rolling of AA 5083

The aim of this paper is to investigate about the Portevin-Le Chatelier Effect (PLC) on a AA 5083 sheet. In order to study the minimizing of the PLC effect, three different rolling cyles have been carried out and an experimental campaign on the three different AA 5083 rolled has been carried out. In particular, the experimental campaign, to better understand the evolution of the phenomena during the rolling process, is based on: microstructural analysis, tensile tests and fractographic observations. Finally it has been found that the greater grain size the smaller the PLC effect, even if this effect cannot be totally removed, furthermore the PLC effect occurs only in the rolling direction. The best rolling cycle is the one that provide a hot rolling until 4mm, then a cold rolling up to 2mm and a final heat treatment of annealing.