Luigi Alberto Ciro De Filippis

Prediction of the Vickers Microhardness and Ultimate Tensile Strength of AA5754 H111 Friction Stir Welding Butt Joints Using Artificial Neural Network

A simulation model was developed for the monitoring, controlling and optimization of the Friction Stir Welding (FSW) process. This approach, using the FSW technique, allows identifying the correlation between the process parameters (input variable) and the mechanical properties (output responses) of the welded AA5754 H111 aluminum plates. The optimization of technological parameters is a basic requirement for increasing the seam quality, since it promotes a stable and defect-free process. Both the tool rotation and the travel speed, the position of the samples extracted from the weld bead and the thermal data, detected with thermographic techniques for on-line control of the joints, were varied to build the experimental plans. The quality of joints was evaluated through destructive and non-destructive tests (visual tests, macro graphic analysis, tensile tests, indentation Vickers hardness tests and t thermographic controls). The simulation model was based on the adoption of the Artificial Neural Networks (ANNs) characterized by back-propagation learning algorithm with different types of architecture, which were able to predict with good reliability the FSW process parameters for the welding of the AA5754 H111 aluminum plates in Butt-Joint configuration.

Effect of Friction Stir Process Parameters on the Mechanical and Thermal Behavior of 5754-H111 Aluminum Plates

A study of the Friction Stir Welding (FSW) process was carried out in order to evaluate the influence of process parameters on the mechanical properties of aluminum plates (AA5754-H111). The process was monitored during each test by means of infrared cameras in order to correlate temperature information with eventual changes of the mechanical properties of joints. In particular, two process parameters were considered for tests: the welding tool rotation speed and the welding tool traverse speed. The quality of joints was evaluated by means of destructive and non-destructive tests. In this regard, the presence of defects and the ultimate tensile strength (UTS) were investigated for each combination of the process parameters. A statistical analysis was carried out to assess the correlation between the thermal behavior of joints and the process parameters, also proving the capability of Infrared Thermography for on-line monitoring of the quality of joints.

Optimization and Characterization of the Friction Stir Welded Sheets of AA 5754-H111: Monitoring of the Quality of Joints with Thermographic Techniques

Friction Stir Welding (FSW) is a solid-state welding process, based on frictional and stirring phenomena, that offers many advantages with respect to the traditional welding methods. However, several parameters can affect the quality of the produced joints. In this work, an experimental approach has been used for studying and optimizing the FSW process, applied on 5754-H111 aluminum plates. In particular, the thermal behavior of the material during the process has been investigated and two thermal indexes, the maximum temperature and the heating rate of the material, correlated to the frictional power input, were investigated for different process parameters (the travel and rotation tool speeds) configurations. Moreover, other techniques (micrographs, macrographs and destructive tensile tests) were carried out for supporting in a quantitative way the analysis of the quality of welded joints. The potential of thermographic technique has been demonstrated both for monitoring the FSW process and for predicting the quality of joints in terms of tensile strength.

Investigation and Comparison of Aluminium Foams Manufactured by Different Techniques

Cellular metals offer a large potential for industrial application. Nevertheless, besides the costs, there are a lot of technical improvements necessary in order to gain more widespread use. The reasons for the lack of applications, since now, are multiple and they depend from the physical properties of foams that are still not good enough and not completely experienced, and from the insufficient spreading of research results to designers. To fulfill parts of these requirements, this work investigates in detail three foam production processes, studies the effect of modifications to standard manufacturing methods, clarifies the influence of process parameters on the structure of foams, characterises relevant properties, and finally discusses the difference between powder or melt routes. During experimentation hundreds of samples were realised to reach these objectives. Moreover mechanical, physical, and microstructural properties of the produced foams have been studied using various techniques including compression testing, scanning electron microscopy, visual inspection, and software measurement tool. The three methods for manufacturing aluminium foams, applied in this investigation, are named TiH2, SDP and MGI. Two of them (TiH2 and SDP) start from powdered aluminium as row material, while MGI starts from melt aluminium. Moreover TiH2 and MGI methods let the production of closed cells foam while SDP of open cells foam named, more correctly, metal sponge. The term ‘‘foam’’ was firstly reserved for a dispersion of gas bubbles in a liquid. The morphology of this type of foams can be preserved by letting the liquid solidify, thus obtaining what is called a ‘‘solid foam’’. When speaking of ‘‘metallic foams’’ one generally means a solid foam. The liquid metallic foam is a stage that occurs during the fabrication of the material. For metallic systems it is possible to define the following classification according with Babcsan et al. (2003): • cellular metals are materials with a high volume fraction of voids made up of an interconnected network of walls and membranes; • porous metals have isolated spherical pores and a porosity level of usually less than about 70%; • metal foams are a soubgroup of cellular metals usually having a polyhedral cells with closed or open cells (even if there are no membranes across the faces and the voids are inteconnected, the better definition is metal sponges).

Capability of infrared thermography for studying the friction stir welding process

The Friction Stir Welding (FSW) is an innovative solid-state welding method based on frictional and stirring phenomena, discovered and patented by TWI Ltd in 1991, providing high quality components for aerospace, marine and automotive industrial fields. In this process, a rotating non-consumable tool that plunges into the work piece and moves forward produces the heat necessary to weld the parts together. The much lower temperatures compared with those achieved in traditional welding processes by melting, determine the following main advantages of FSW: minimal mechanical distortion, excellent surface finish, absence of splash, no crack formation and porosity after welding, thanks to the low input of total heat. This work deals with the use of thermographic techniques for monitoring the friction stir welding process applied on AA 5754-H111 plates, in order to evaluate the quality of the produced joints in terms of presence of defects and Mechanical strength. The adopted experimental approach was addressed to study and optimizing the FSW process by analyzing the thermographic sequences and extracting several indexes related to the heating involved in the process. Such the indexes, the maximum temperature, the heating and cooling rate of the material, correlated to the frictional power input and the presence of defects respectively, have been investigated for different process parameters (the travel and rotation tool speeds) configurations. The results of the research have been quantitatively supported and characterized by destructive and non-destructive techniques.

Potentialities of thermal signal analysis approach for a rapid mechanical characterisation of high diffusivity materials

One of the most important advantages of using high-diffusivity alloys like aluminium, in industry, is to reduce the weight without renouncing to high strength components. To accelerate the time of the mechanical characterisation, frequently experimental methods based on temperature measurements are adopted, even if in this case, these methods could involve in wrong estimations. In particular, the study of energy dissipations could produce some assessment errors of fatigue limit due to the fact that the fraction of the detected energy dissipated could be lower if compared to the effective energy intrinsically dissipated in the material due to damage. Furthermore, the fatigue life assessment of Aluminium alloys is problematic due to a non-distinct ‘knee’ in the S-N curve. To take into account these issues and to estimate the fatigue strength in rapid and accurate way, in this work, a method providing a specific thermal signal analysis is presented applied to an aluminium alloy 5754 H-111. Firstly, the well-known methods based on direct temperature measurements for estimating fatigue strength of metals, were applied on an aluminium alloy 5754 H111 in order to demonstrate their problematic application for high-diffusivity materials. Furtherly, a specific thermal signal analysis was adopted for extracting first and second order temperature variations as better parameter for fatigue strength assessment. This work questions the use of direct temperature evaluation in high diffusivity materials and fully replaces it in favor of an approach based on in-depth analysis of thermal signal by using thermoelastic and dissipative temperature variations.

Thermographic signal analysis of friction stir welded AA 5754 H111 joints

Aluminium alloys present some criticalities in terms of fatigue life characterisation due to the absence of a point representing the ‘fatigue limit’, the topic becomes complicated when the material is welded. In this case, the fatigue characterisation lies on design specifications which have to clearly explain the guidelines for the performing the tests and for evaluating the failures, in order to design tailored welded joints. However, the fatigue of welded joints is a difficult subject since the welding process makes the material different, introducing residual tensions, defect, etc. Also, the standard test methods provide only the estimation of the strength at fixed loading cycles but no information on the damage processes occurring in the material. Prompted by these issues researchers deal with the study of other approaches to achieve not only information on fatigue resistance but also damage information. In particular, the thermography can be used for thermal signal analysis of dissipative heat sources involved in fatigue of material undergoing cyclic test. In this paper, this approach is adopted to study the fatigue behavior of friction stir welded joints of AA5754-H111 during specific loading conditions. The component of the temperature related to intrinsic dissipations is assessed and the fatigue strength is evaluated together with a graphical study of the location of damaged areas.

Correlation between Thermal Behaviour of AA5754-H111 during Fatigue Loading and Fatigue Strength at Fixed Number of Cycles

The characterization of the fatigue behaviour of aluminium alloys is still capturing the attention of researchers. As it is well known in literature, for certain alloys, in a specific range of cycles number, the S-N curves do not present any asymptote. So that, problems result in the assessment of the fatigue life. In these conditions, the concept of the fatigue limit has to be replaced by the fatigue strength at a fixed number of loading cycles. Temperature acquisitions during fatigue tests allow for a specific analysis that can support the researchers in understanding the complex processes that are involved in fatigue and their influence on fatigue life, even for aluminium alloys. In fact, the analysis of the surface temperature signal that was detected during a self-heating test provides a curve that is characterized by a distinct slope-change point at a specific stress value. Even though researchers have been investigating fatigue life characterisation and temperature variations for more than a decade, it is not clear what this point represents in terms of fatigue strength. The aim of the present paper is to find out a possible correlation between the thermal behaviour of AA5754-H111 undergoing self-heating testing procedure and fatigue strength at a specific loading cycles.