Technical ReportFriction stir welded T-joints optimization
Introduction
The T-joint is an important geometry when improved inertia and strength of thin skins, without significant weight increase, are needed. This geometry is commonly used in the reinforcement of aircraft fuselages, railway cars and many other applications where the weight of the structure is of particular importance [1], [2], [3], [4], [5].
T-joints of aluminium alloys welded by fusion techniques presents high residual stresses and significant distortion due to high temperatures achieved during welding. These are difficult to avoid without time-consuming and costly additional operations [2], [6]. Friction stir welding is an excellent candidate for the replacement of conventional fusion welding processes to fabricate T-joints. This process is a solid-state joining technology where a special tool in rotation is inserted into the work pieces and transverse along the joint line providing a complex stirring of the material (see Fig. 1). In this work T-joints were created using three components: two skins and a stiffener, as presented in Fig. 2. The probe depth should be high enough to promote the skins and stiffener stirring. When compared with fusion techniques FSW presents improved surface finishing, absence of porosity, absence of segregation or hot cracking and no need for consumables. Nevertheless, the analysis of process parameters and their interactions effect in the joint mechanical strength, is still necessary [1], [2], [3], [6], [7].
Only a small number of studies concerning FS (friction stir) welded T-joints may be found in the literature. Studies on FS welded T-joints, made of aluminium alloys were performed by Fratini et al. [1], [2], [3], [6]. AA6082-T6 FS welded T-joints bending properties were compared with metal inert gas (MIG) welds and extruded T-parts [6], and microstructural and mechanical properties were also evaluated [2]. The tilt angle was reported to play a relevant role in the joint strength and several defects were verified revealing the necessity of an appropriated choice of tool geometries and set of process parameters. Numerical simulations, and experimental tests using a thin copper sheet placed between the skin and stringer, were performed to study the material flow in FS welded T-joints [3]. A similar behaviour to butt joints material flow was verified in the skin area. FS welded AA2024-T4 and AA6082-T6 T-joints plastomechanics were studied in [1]. It was verified that the two aluminium alloys present different behaviour in the plastic region. The AA6082-T6 joints revealed improved tensile strength when compared to the AA2024-T4. Also, it was verified that the shoulder diameter and the tool probe geometry affects the FSW process.
Erbslöh et al. [4], produced 4 mm thick AA6013-T4 FSW T-joints, where defects as oxides lines and cracks in the proximity of the T corner were verified. It was verified that a round fillet radius may increase the propensity to crack formation. Acerra et al. [5] presents an industrial case study of aeronautical AA2024-T4 and AA7075-T6 dissimilar T-joints obtained by FSW. In this work it was concluded that large shoulder diameters are required in order to achieve sufficient heat to plasticize the material, leading to the filling of the joint fillets. Distributions of defects and mechanisms for its formation in AA6061-T4 FS welded T-joints, using three different combinations of skins and stringers, were investigated by Cui et al. [7]. The formation of defects were related to the initial matching modes of the blanks, insufficient heat input and material flow patterns. The use of a large shoulder diameter was also mentioned to provide the best welds. In a work by Castro et al. [8], FS welded T-joints residual stresses were measured using the sectionioning and the hole drilling methods. Also, mechanical joint characterization of FS welded AA6056 and AA7075 dissimilar T-joints was evaluated in [9]. The multimaterial T-joint revealed promising mechanical properties. Probe geometry effect in FS welded AA6082-T6 T-joints was also studied in [10]. Joints welded using two of the five probe geometries studied present good results, achieving an ultimate tensile strength of 226–242 MPa and a yield strength of 148–161 MPa.
The Taguchi method is a quality technique widely used for optimization material processing technologies. This method is based on statistical analysis enabling the reduction of experiments. In order to determine optimum level for parameters and their influence in the process, data may be analysed using an analysis of variance (ANOVA) or signal to noise (S/N) ratio. The method used in this work was the ANOVA analysis.
In the literature there is still a lack of studies concerning a proper determination of effective set of process parameters for FS welded T-joints. Most of the studies that may be found are concerned with the prediction of properties and parameters optimization of FS welded butt joints, e.g. [11], [12], [13], [14], [15], [16], [17].
In this work a step forward is made aiming at the industrial application of FSW. A study concerning parameters optimization of FS welded AA6082-T6 T-joints using the Taguchi method and ANOVA analysis is presented. Also, interaction between the most influent parameters on the joint mechanical strength were analysed, and linear equations for mechanical strength prediction were derived.
Section snippets
Methods and material
The welds were produced using three AA6082-T6 aluminium alloy parts. Plates of 380 × 100 × 3 mm were used for the skins, and 380 × 31 × 3 mm plates for the stringer (see Fig. 2). The welds were performed in the material rolling direction using a modified milling machine.
The parameters selected to be optimized and their levels are presented in Table 1. The probe profile and diameter were maintained constant (shoulder/probe diameters ratio was defined by variation of the shoulder diameter). The probe used
Experimental results
Mechanical properties of the base metal are shown in Table 3. Results of tensile and bending tests from the 27 experiments resultant from the Taguchi method analysis are shown in Table 4.
Results discussion
Analysing the tensile test results, joint efficiencies (weld/base material strengths) of 56% and 51% for tensile and yield strengths respectively, were observed. In the literature, results for this alloy T-joints are still scarce, nevertheless values of the same magnitude were found in [1], [10].
Analyses of variance considering a level of confidence of 95%, were performed, in order to determine which parameters have the highest influence in the different properties evaluated. The ANOVA analyses
Conclusions
It was demonstrated that Taguchi’s robust orthogonal array design method is suitable for the optimization of FS welded T-joints.
The ANOVA approach leaded to the contribution of each parameter and their interaction in the properties analysed. Also, linear equations for a joint mechanical properties prediction were derived.
The joints presented an efficiency of 56% for the tensile and 51% for yield strengths regarding base material properties.
It was observed that the tool rotational speed was the
Acknowledgments
This work was supported by the FCT project PTDC/EME-TME/114703/2009 and FCT project PTDC/EME-TME/117596/2010. Dr. Moreira acknowledges POPH – QREN-Tipologia 4.2 – Promotion of scientific employment funded by the ESF and MCTES. The collaboration of Mr. José Almeida during the welding trials is gratefully acknowledged.
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