Aluminum alloys are utilized in diverse industries due to their good mechanical properties, good manufacturability, and the existence of well-known inspection procedures [1]. Recently, the increasing concerns of energy economy, and environmental preservation have raised the requirement for light-alloy structures. The aluminum alloy Al-7075 has been primarily developed for aircraft structural applications. Its major alloying elements are zinc, magnesium, and cooper. The Al-7075 alloy generates its strength from second phases and precipitates such as of Mg2Zn and Al2CuMg phases.
Friction stir welding (FSW) is a solid-state joining process established to joint materials using a permanent tool that rotates while is plunged into the material generating stirring of the weld zone [2]. The solid state nature of FSW allows the reduction of distortions and residual stresses due to the lower welding temperatures in contrast with fusion welding process [3]. FSW comprises a rotary tool that generates friction heat between the tool shoulder and plate interfaces [4]. The welding parameters (axial load, rotational speed, and feed rate) and weld characteristics (base metal, joint type, and thickness) impact the mechanical behavior of the joint [5].
Diverse studies have been exploring the influence of tools and process parameters in FSW of Al alloys. Mohammadi et al. [6], investigated the impact of welding parameters on the material and tensile strength of the joint. Different pin profiles were investigated and rotational speed of welded joints of Al-7075-T6 alloy. The results revealed that the pin plays an important role in the form and microstructure of the welded zone. A large weld nugget and a fine grain size were obtained using a threaded conical pin. It was observed that the strength grows with increasing welding speed.
Shah et al. [7], developed a FSW process using different tool diameters to explore the thermal effect of Al-7075 alloy. The experimental temperature distribution was recorded during the FSW process showing a higher temperature on the forward side of the weld joint. An appropriate temperature for a defect-free Al-7075 alloy was observed between 375–420˚C and welds with a shoulder diameter of 20 mm provided the highest performance.
FSW exhibits typically four distinct regions: a stir region, a thermo-mechanical affected zone, heat affected zone and base metal. In the stir zone the material changes substantially due to the high temperature gradients and localized strain conditions. Thermo-mechanical affected zone present elongated grains and the heat affected zone only includes thermal gradients with slightly deformation effect. Despite of vast literature reported regarding FSW processing, a complete understanding of friction welding of high strength materials is still limited [8].
There are different types of flaws that occurred during FSW processing, such as internal cavities know as tunnel defect, joint-line flaws where there is a lack of coalescence and hooking defects due to presence of oxides on the weld. The detection of tunnel defect requires a post-welding examination. The inspection of this type of flaws requires a small joint sample, which is effective only for some welding conditions [9].
Typically, the inspection of friction welded aluminum alloys in aerospace manufacturing includes phased array ultrasound. However, this method needs high skills and require complex techniques due to probe adjustments. Recently, non-destructive testing base on X-Ray computed tomography (CT) has been utilized to detect defects during FSW. Hamade and Baydoun [10] evaluated defects by CT method after friction weld Al plates. A voxel size of 0.0223 mm3 was utilized for defined sectioned zones in the welds. CT images were converted to 3D models reveling shapes and places of identified defects.
In other study, CT method was utilized to evaluated Al-foam microstructure and determine characteristics such as density, pores, and phases [11]. The occurrence of discontinuities in welded steel tubes was explore by Moura et al. [12] using CT measurements detecting cutoffs in the range of 0.3 mm to 13 mm. A comparative between X-Rays and ultrasonic waves was performed by Santos [13] to reveal the existence of pores in FSW. It was found that X-Rays and ultrasonic inspection can be utilized to identify defects, nevertheless, an inaccurate evaluation of surface defects can occur for ultrasonic transmission.
Experimental and numerical methods help in the understanding of non-uniform temperature during welding. Several studies have been developed to calculate thermo-mechanical response of friction welding. Discretization using finite element and finite volume methods were developed to establish the thermal and mechanical response of aluminum alloys [14]. The tool-material flow interaction was explored by Fanelli et al. [15], in which investigated the internal structure of the joint by a robust finite element approach to evaluate the mechanical response of welds. The understanding of the FSW process is a difficult task, since it requires the interaction of diverse physical phenomena, dynamic transitions, and a continuous plastic process at elevated temperature.
In the present work, the computed tomography non-destructive method was utilized to detect defects after friction welding using a voxel size of 46 mm. The thermo-mechanical prediction of FSW Al-7075 plates was assess using the finite element procedure. The objective of this research is dedicated to investigating the soundness of friction welded joints using a non-destructive method and evaluate the impact of process variables in the reliability of the joints.