Umberto Prisco

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.

Influence of filler material on micro- and macro-mechanical behaviour of laser-beam-welded T-joint for aerospace applications

3.2mm thick laminated skins of AA7475 alloy and 2.5mm thick extruded stringers of the alloy PA765 were T-jointed by laser beam welding. Two different joints were fabricated using identical welding parameters but two different alloys as welding wires, AA5356 and AA4047. The microstructure, the tensile, and the failure behaviours of the two welded joints are studied by means of optical and electronic microscope, micro-indentation, and tensile testing. Weld beads extension using AA5356 are larger than those using AA4047. Microstructure of the welded joints is quite similar: at the centre of the fusion zone, cellular dendrites are present; near the fusion line in the fusion region, a chill zone and parallel dendrite arms are found; and an evident heat-affected zone is observed between the fusion line and the parent material. Microhardness tests reveal that the partially melted zone is a weak band in the welded specimens. Indeed, for both joints, failure took place at the partially melted zone upon tensile loading. Hoop testing demonstrates that AA4047 joints present a lower yield stress than AA5356 joints although both joints show similar tensile strength and elongation at break. AA4047 joints show more strength than AA5356 joints upon T-pull testing.

Experimental Evaluation of Properties of Cross-Linked Polyethylene

This article describes several aspects of the production of cross-linked polyethylene. Advantages and disadvantages of chemical, or electron beam irradiation, cross-linking processes are discussed. The material under test is thermoshrinking low density polyethylene, produced at the Megarad s.r.l. (Cassino, Italy) firm. It is mostly used as blanket insulation for low and medium electrical voltage cables and as anticorrosion protection for gas pipes. The performance of end products is verified through destructive tests. In particular, we evaluated the gel fraction, the modulus of elasticity at 150°C and the elongation percent on commercial pipes and on specimens manufactured by varying certain parameters, such as the irradiation dose.

Thermal conductivity of flat-pressed wood plastic composites at different temperatures and filler content

Abstract The thermal conductivity of wood flour (WF) filled high-density polyethylene composites (wood plastic composite, WPC) is investigated experimentally as a function of filler content and temperature. Samples are prepared by compression molding process of previously blended and extruded WPC pellets, up to 50% weight content of WF. The thermal conductivity is measured by the heat flow meter technique in a temperature range from -15°C to 80°C. Experimental results show that the WPC thermal conductivity decreases with temperature and WF content, with the last effect due to the increase in porosity with the filler content, as confirmed by density measurements. Using the thermal conductivity of bare WF, the thermal conductivity of the wood material in WPC is estimated. This value successfully predicts the upper and lower bounds of the WPC thermal conductivity by means of the parallel and series conduction model of a multiphase composite material.

Optimization of friction stir welds of aluminium alloys

Publisher Summary This chapter illustrates an experimental study that investigated the effect of rotating and welding speed on the mechanical properties of AA6056 joints made by friction stir welding. Different welds with rotating speed of 1000 and 1600 RPM and travel speeds of 230, 325, and 460 mm/min were produced. An ultrasonic (UT) non-destructive testing (NDT) procedure was applied to characterize the presence of possible weld defects prior to mechanical destructive testing. The joint mechanical properties were evaluated by means of static tensile tests and fatigue tests. The correlation between process parameters and weldment static properties was carried out through an analysis of variance (ANOVA) of the data. The results prove that both static and fatigue properties show an appreciable improvement as both travel and rotating speed increase. However, in the case of maximum travel speed and maximum rotating speed, the welded specimens may display weldment defects that reduce the evaluated fatigue limit for such processing conditions.

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.

NONDESTRUCTIVE CONTROL OF POLYETHYLENE BLANKET INSULATION BY MEANS OF LOCK-IN THERMOGRAPHY

It is well known that cross-linking of polyethylene molecules into three-dimensional networks improves material properties; in particular, it enhances impact strength, thermal performance and chemical resistance. A product of good characteristics can be obtained if the manufacturing process is adequate and all involved machines are working properly. In this context, infrared thermography as a remote imaging technique may be a valuable tool to monitor the polyethylene cross-linking processes. Experimental tests were carried out on specimens without treatment and on specimens treated with either chemical processes or electron beam irradiation. Our results prove that lock-in thermography can detect local non-uniformity of material characteristics due to either the extrusion or cross-linking processes, and material differences linked to the different compound.

Characterization of NiTinol under torsional loads through a numerical implementation of the Boyd?Lagoudas constitutive model and comparison of the results with experimental data

Shape memory alloys (SMA) are a particular family of materials, discovered during the 1930s and only now used in technological applications, with the property of returning to an imposed shape after a deformation and heating process. The study of the mechanical behaviour of SMA, through a proper constitutive model, and the possible ensuing applications form the core of an interesting research field, developed in the last few years and still now subject to studies driven by the aim of understanding and characterizing the peculiar properties of these materials. The aim of this work is to study the behaviour of SMA under torsional loads. To obtain a forecast of the mechanical response of the SMA, we utilized a numerical algorithm based on the Boyd?Lagoudas model and then we compared the results with those from some experimental tests. The experiments were conducted by subjecting helicoidal springs with a constant cross section to a traction load. It is well known, in fact, that in such springs the main stress under traction loads is almost completely a pure torsional stress field. The interest in these studies is due to the absence of data on such tests in the literature for SMA, and because there are an increasing number of industrial applications where SMA are subjected to torsional load, in particular in medicine, and especially in orthodontic drills which usually work under torsional loads.

Microstructure of a Hot Forged Ti 5-5-5-3 Aeronautical Component

The changes in microstructure during the forging of a large Ti-5-5-5-3 component are studied. The effect of the different plastic strain and cooling rates experienced by the various zones of the component is analyzed and compared in terms of the microstructures that they induce. The results show that the microstructures vary in the different zones of the forging with the α phase spheroidization increasing with the amount of plastic strain sustained by the material. A correlation between the level of α phase spheroidization and the plastic softening of the forging has been found.

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.