Papers by Author: Emanuela Cerri

Abstract: Titanium (Ti) and its alloys are used extensively in aerospace industry where there is a critical need for material with high strength-to–weight ratio and high elevate temperature properties. Friction stir welding (FSW) is a new solid state welding process in which a cylindrical–shouldered tool with an extended pin is rotated and gradually plunged into the joint between the workpieces to be welded. The material is frictionally heated to a temperature at which it becomes more plastic but no melting of the blanks to be welded is reached therefore the presence of defects typically observed in and close to the welding seam is strongly reduced. The final result is the improvement of the mechanical performances of the welded joints even in some materials with poor fusion weldability. In this paper the authors analyze the microstructure of FSW joints of Ti-6Al-4V processed at the same travel speed (50 mm/min) and at different rotation speed (300-500rpm). The microstructure of base material (BM) is not homogenous. It is characterized by distorted α/ β lamellar microstructure together with smashed zone of fragmented β layer and β retained grain boundary phase. The BM has been welded in the as received state, without any previous heat treatment. The microstructure of the transverse section of joints is not homogeneous. Close to the top of weld cross sections a much finer microstructure than the initial condition has been observed while in the center of the joints the microstructure is mixed and less refined.

Abstract: A high pressure die cast (HPDC) magnesium alloy was friction stir processed (FSP) at high rotation rates with different advancing speeds. The AZ91 plate was 3 mm thick and the pin had a truncated cone tip. The friction stir processing induced the disappearance of porosity (typical of the HPDC process) in the nugget zone but some tunnel defects were introduced. The presence of characteristic FSP zones is not obvious due to the low plastic deformation of this alloy. The stirred zone is very narrow around the tool and this makes the FSP difficult to occur in the whole thickness of the plate. Microhardness values and electrical conductivity are sensitive to stirred zone and differences arise according to FSP parameters. The grain size is refined and homogenized by FSP due to partial solubilisation and disappearance of the eutectic phase surrounding Mg-alpha. X-rays diffractometry was performed on FSP samples to index phases and calculate peak shifts of Mg-alpha.

Abstract: Friction Stir Welding process introduces a degree of deformation in the material that is related to process parameters. They directly regulate the heat input of welding process and then the morphology and the microstructure characteristics. In the present work, an investigation was carried out on 6082T6 butt joints obtained by Friction Stir Welding process. The microstructure and mechanical characterizations were performed before and after a post-welding treatment at 535°C for 2 hours and cold deformation. The thermal stability of fine recrystallized grains in the nugget zone depends on process parameters and post-welding heat treatment and it has unusual consequences on mechanical properties. In fact abnormal grain growth, that occurs in the nugget zone, increases ductility of the joints and homogenizes the hardness profile of the whole joint.

Abstract: The influence of severe plastic deformation induced by ECAP on microstructure modification and aging effect was studied in two modified Al-Mg-Si aluminium alloys. The microstructure of both alloys in different heat treated and deformed state was characterised by X-Rays diffraction and polarised light microscopy. The effect of artificial aging was investigated after ECAP performed on samples in the as extruded condition. The aging effect was followed by hardness and electrical conductivity measurements. At higher aging temperature (170°C) the alloys showed an increasing softening with time due to recovery or/and grain coarsening effect. At the lower aging temperature, the hardness remains almost constant due to enhanced precipitation hardening effect.

Abstract: ECAP (Equal Channel Angular Pressing) is a very interesting method for modifying microstructure in producing UFG (Ultra Fine Grained) materials. It consists of pressing test samples through a die containing two channels, equal in cross section and intersecting at an angle Φ. As a result of pressing, the sample theoretically deforms by simple shear and retains the same cross sectional area to repeat the pressing for several cycles. 2-D and 3-D FEM simulations of both one and four ECAP passes of two modified aluminium alloys were performed in order to investigate the deformation state of processed workpiece and, moreover, the effect of the different alloy related Strain Hardening Rate (SHR), die geometry (in terms of variation of channel outer angle) and friction on deformation distribution and magnitude. FEM results showed a lower equivalent plastic strain on the outer side of both cross and longitudinal sections of the billets after one and four passes. Microhardness tests performed on the same sections of ECAP processed billets supported these findings. Moreover, FEM analysis indicated that an higher SHR means a greater strain inhomogeneity on cross section of the processed billet. The same effect was observed by increasing the channel outer angle by computing friction.

Abstract: In this study, the effect of various solution and aging treatments on microstructure and mechanical properties of an as cast Al-5.5Zn-1.2Mg alloy has been investigated by optical microscopy, hardness measurements and room temperature tensile test. The solution heat treatments performed at temperatures between 400 and 490°C have little effects on hardness while electrical conductivity values increased at the lower temperature because of dissolved atoms and vacancy rich clusters. Concerning aging, only T6 improves mechanical properties of the undeformed alloy, while aging performed on tensile tested samples results in a decrease of hardness due to accelerated kinetics and heterogeneous nucleation of equilibrium phase stimulated by dislocation network. Tensile tested samples of the as cast alloy exhibits the fastest recrystallization time during annealing because of the absence of fine precipitates and the high strain hardening.

Abstract: Constitutive equations for hot working are of great importance in optimizing forming processes to balance reductions in preheating and force, to avoid defects and to improve properties. Flow curve shapes and. constitutive parameters are affected by variations in composition, in homogenization, in grain morphology and significantly in texture. Confidence in published analyses is enhanced by existence of many data for the same or similar alloys. In this paper, constitutive equations have been collected for commercial Mg alloys from torsion, compression and tension tests in the range from 10-3 to 10 s-1 and 180 – 500°C. Some data were determined by the authors’ but more came from published reports; in some cases they have been re-calculated in a common manner. The deformation and restoration mechanisms that control the flow curve shape and the material parameters are summarized. Microstructure investigations of strained samples are illustrated. Applications to extrusion or rolling are discussed; comparisons to Al are made as appropriate.

Abstract: Microhardness and hardness maps have been measured in different sections of samples processed by Equal Channel Angular Pressing (ECAP). In order to investigate the homogeneity of the hardening induced by ECAP as a function of deformation, the hardness maps have been based on samples deformed at different strains (after one and four passes). After one pass the specimen’s outer side, both on its cross and longitudinal planes, is less hard than all other zones. Moreover, after 4 passes via route Bc, the hardening induced on the sample cross section is lower at the surface than in the central area of the billet. The reduced hardness regions are compared to the distributions of plastic equivalent strain generated by finite element analysis.