Dina Palmeri

Material Flow in FSW of AA7075-T6 Butt Joints: Continuous Dynamic Recrystallization Phenomena

In the paper the continuous dynamic recrystallization (CDRX) phenomena occurring in the FSW of AA7075-T6 butt joints is investigated at the varying of the most relevant technological and geometrical parameters. In particular, both experiments and numerical simulations obtained utilizing a 3D Lagrangian implicit, coupled, rigid-viscoplastic model have been developed on FSW butt joints. The resulting microstructure at the core of the weldings is correlated to the material flow occurring during the FSW process.

Friction stir spot welding of AA6082-T6: Influence of the most relevant process parameters and comparison with classic mechanical fastening techniques

Abstract The results of an experimental study on friction stir spot welding (FSSW) of AA6082-T6 are reported. In particular, process mechanics is highlighted and joint strength is considered in relation to varying the most relevant process parameters. Furthermore, the results obtained are compared with those derived from the application of traditional mechanical fastening techniques such as clinching and riveting. In this way the effectiveness of FSSW is highlighted.

FSW of AA2139-T8 Butt Joints For Aeronautical Applications

The effect of process parameters on mechanical and microstructural properties of AA 2139 T8 joints produced by friction stir welding (FSW) was analysed by means of statistical tools. Three different parameters were taken into account: angular speed (ω), welding speed (va), and plunging depth (p); each of them was varied on three levels. Forces and temperatures achieved during the welding were monitored and analysed for all joints. An accurate microstructural analysis was performed: the occurrence of some remarkable defects, typical of FSW, such as tunnel or lack of penetration, was related to actual process parameters adopted. The extension of each FSW characteristic zone was measured and the main grain dimension within each of these zones was measured. The mechanical properties of the joints were evaluated by means of Vickers microhardness measurements and tensile tests at room temperature. It was observed that the joint welded with lowest angular speed, highest welding speed, and greatest plunging depth attained the best performances. The statistical data treatment allowed providing a numerical model capable of predicting the mechanical performances of joints.

Microstructural Changes Determining Joint Strength in Friction Stir Welding of Aluminium Alloys

In the paper the results of a wide experimental activity on friction stir welding (FSW) of aluminum alloys are reported. In particular the butt joints of two different materials, namely AA1050-O and AA6082-T6 were considered. Grains dimensions and precipitates density were investigated both in the parent materials and after the welding processes. Furthermore post-welding heat treatments effects on the joint strength were studied.

Improving of the Productivity and the Quality of a Manufacturing Robotized Cell for MIG/MAG Welding

During the last years, globalization and the necessity to reduce production costs have pushed industry to employ more and more manufacturing robotized cells. This trend has been assimilated by arc welding too. The use of robotic systems allows to increase the performance of productive systems by means of reduction in lead-time, reproducibility of processes and increase in quality. However, process automation needs pre-scheduling of manufacturing cell movements and also a setup of different welding parameters, such as power provided by the welding generator, arc length and robot speed, all of which allows to get a joint without defects. An online scheduling of robotic systems brings certainly a delay in production times, whereas offline scheduling allows to maintain the desired production without any production stop. An interface that allows to easily translate the coordinate points established by a simulation software (such as Robcad) in some complete programs that can be sent to the controller of the manufacturing cell, has been developed. Besides, in order to improve the joint’s quality, an analysis of the joint quality has been carried out by macrographic analysis. The employment of image analysis techniques on the obtained macrographs allowed to translate the morphology of each joint in terms of a matrix. After different morphologies have been distinguished in numerical terms and correlated to the joint’s quality, these data have been introduced as input to the interface. The output, constituted by the program for the controller of the cell and the best welding parameters, under such conditions, has allowed to increase both the cell’s productivity and the quality of the joints. Therefore, during offline scheduling, the initial phase of loop control of the joint morphology allows to evaluate the best welding parameters both in terms of productivity and joint quality. The same interface allows also to plan the best parameters for different welded joints with varying thickness and typology, which makes the system more eflexible and maintains a high level of productivity.

Influence of Process Parameters for Thixotropic Alloys

With reference to a metallic alloy, the attribute thixotropic is utilized to indicate the behaviour of it in the semi-solid state when its microstructure consists of spheroids in a liquid matrix. Such alloys are characterized by very low values of viscosity under shearing stress in the semi-solid state, while after solidification they show relevant mechanical properties. Actually a structural change from a dendritic structure to a globular one, with the globular grains finely dispersed in a liquid matrix, is observed after particular thermo-mechanical treatments. In the present paper the authors present the results of a wide experimental campaign on the AA 7075 aluminium alloy that shows a large semi-solid window temperature range. In particular, a mechanical device was set up in order to develop a mechanical treatment with the aim to produce thixotropic material to be subsequently forged. The influence of the main operative parameters was taken into account and micrographic analysis and mechanical tests were performed highlighting properties of the obtained structures.

ANN Model to predict the bake hardenability of Transformation-Induced Plasticity steels

Neural networks are useful tools for optimizing material properties, considering the material’s microstructure and therefore the thermal treatments it has undergone. In this research an artificial neural network (ANN) with a Bayesian framework able to predict the bake hardening and the mechanical properties of the Transformation-Induced-Plasticity (TRIP) steels was designed. The forecast ability of the ANN model is achieved taking into account the operating parameters involved in the Intercritical Annealing (IA), in the Isothermal Bainite Treatment (IBT) and also considering the different prestrain values and the volume fraction of the retained austenite before the Bake Hardening (BH) treatment. This approach allowed one to overcome the need to know the metallurgical rules that describe all the active phenomena in multiphase steels. The neural network approach allowed one to overcome the lack of prediction capability in the existing numerical models.

Design of Experiments for the optimization the process parameters of thixotropic aluminum alloy

The success of the thixoforming process depends on the possibility to confer to material, when it is found in the semisolid state, a microstructure characterized by globular particles of solid phase surrounded by a continuous film of liquid phase; such microstructure is obtainable through particular thermo-mechanical treatments. In the present research, in order to optimize the influence of process parameters in the step in which the thixotropic properties are conferred to the AA7075 aluminum alloy, the statistic technique of the Design Of Experiments (DOE) has been used. The advantages in the application of such technique are expressible in terms of reduction the times of development of process and more efficient use of resources. A Central Composite Design factorial plan with two levels has been realized to allow the evaluation of experimental error and to check the adequacy of the model. The experimental tests foreseen from the same plan have been therefore performed. Using the method of the response surfaces (RSM), the function of response has been formulated, and the analysis of the experimental data has been realized by the linear regression method. Finally, the analysis of variance (ANOVA) allowed to value the causes of variability of the results. Statistic test has been performed on the significance of single factors and their interactions.