Process parameters of friction stir welding, types of defects, and detection methods

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Modern aircraft are evolving toward higher performance, lower weight, longer lifespan, greater reliability, improved comfort, and **lower manufacturing costs. As a new welding technique, friction stir welding has been widely used in aerospace structures. Friction stir welding has not only improved aviation structures but also contributed to the advancement and development of techniques for assessing the integrity of such structures. 1 Process characteristics and applications of friction stir welding 1.1 Process characteristics of friction stir welding Friction stir welding is one of the technologies that has seen rapid development internationally in recent years. It features minimal damage to the materials being welded, low welding deformation, high weld strength, and sustainable manufacturing properties, and is regarded as \"the most revolutionary welding technology of our time\". Friction stir processing is primarily achieved through the high-speed rotation and movement of a stir tool, which consists of a shoulder and a stir pin. During the processing, the mixing head rotates at high speed and is slowly pushed into the area of the workpiece that is to be processed, until the shoulder of the shaft makes tight contact with the surface of the workpiece. The stirring needle penetrates into the material to cause friction and mixing; the shear friction heat generated by its rotation softens the metal surrounding the needle, thereby enabling thermoplasticization and inducing plastic flow in the material at the processing site. While the stirring head rotates at high speed, it moves relative to the workpiece in the processing direction. The thermoplasticized material is transferred from the front to the back of the mixing head, and undergoes strong plastic deformation due to the forging action at the shaft shoulder of the mixing head. Compared to the oxy-acetylene welding process, friction stir welding has many advantages, including the ability to eliminate various welding materials such as gases, electrodes, and welding rods. Since the joining process is carried out under the effect of frictional heat generated by mechanical action, friction stir welding requires control of only three main welding parameters: impact force, rotational speed, and welding speed. 1.2 Applications of friction stir welding Friction stir welding holds broad application prospects and has already been widely used in the welding of aviation aluminum alloys. The 2XXX series of aluminum alloys, with their high strength and low weight, have long been the primary materials for aircraft structures. New materials such as the A12195 aluminum-lithium alloy represent significant improvements in terms of the base material compared to the previous generation of material, A12219. At room temperature and low temperatures, the strength of the new alloy improved significantly; however, its weldability sometimes presented problems, which prompted efforts to improve the welding processes, leading ultimately to the development and implementation of friction stir welding. The A12195 alloy can be effectively used in the friction stir welding process, overcoming the difficulties that arise during welding of A12195 using conventional oxy-acetylene welding methods. Friction stir welding can also be used to weld various aluminum alloy materials, such as high-strength alloys like Al, Cu, AlL-Mg, Al-Mg-Si, Al-Zn, and Al-Li, producing excellent welded joints in the process. The applications of friction stir welding in aviation structures mainly include wings, fuselages, tails, fuel tanks, and auxiliary fuel tanks. Boeing has applied friction stir welding technology to the manufacturing of the floors for the C-17 Globemaster III military transport aircraft, achieving a production efficiency 10 times higher than that of traditional riveting methods. Airbus has begun to apply friction stir welding in the manufacture of large civil aircraft. On August 26, 2005, according to Speed News, Airbus has been working to introduce friction stir welding technology into the production of the A340 aircraft, and to use it on a large scale in the manufacturing of the A350, as well as for joining the longitudinal seams of the A340-500 and A340-600 models, thereby replacing traditional riveting techniques. Airbus claims that using friction stir welding instead of riveting to manufacture aircraft fuselages can reduce the weight by 09 kg per meter of weld. 2 Types of Defects in Friction Stir Welding The friction stir welding process has a tendency to generate harmful defects, and the type of these defects depends on the parameter settings and control used in the process. 2.1 Holes: The formation of hole defects is mainly due to insufficient heat input during welding; as a result, not enough of the material reaches a plastic state, leading to inadequate flow of the material and thus the creation of unfilled areas within the weld. When welding is performed using a welding head with straight or conical stir rods that lack threads, voids are likely to form in the joint. Such defects are usually located in the middle-lower part on the advancing side of the joint and near the weld surface. Holes located near the surface of the weld run in the same direction as the welding direction; when they extend over a long distance along the length of the weld, they are also referred to as tunnel-type defects. 2.2 Flash: The flash defect appears on the surface of the weld; it is usually caused by excessive welding pressure, which leads to more plastic material being forced out from both sides of the shoulder, and this results in a defect after cooling. The friction stir welding process is one in which the volume of the weld material remains constant. During actual welding, the shoulder and tip of the stir tool, along with the unmelted base metal, form a \"compression die,\" through which the plastically deformed material flows. If the welding pressure is too high, that is, if the mixing head penetrates too deeply, the volume of the \"extrusion die\" will be smaller than that during normal welding, causing some material to be forced out on both sides of the shaft shoulder; this results in burr defects after cooling. 2.3 Lack of weld fusion: Lack of weld fusion refers to the “crack-like” defect that occurs when no connection or an incomplete connection is formed at the bottom of the weld; insufficient weld pressure can easily lead to lack of weld fusion at the root. The occurrence of unwelded areas is essentially due to an insufficient length of the stirring pin. During friction stir welding, if the length of the stirring pin is appropriate, the oxides on the joint surface between the two plates being joined are broken apart as the stirring pin rotates and moves, resulting in a dense joint at the back of the stir head, with the oxides dispersed throughout that joint. However, if the length of the stirring pin is shorter than the normal size, it is unable to fully stir the material in the weld thickness during welding, especially the material at the lower part of the weld. Coupled with the presence of oxides on the joint surface of the plates, crack-like unwelded defects will appear at the root of the joint after welding. 2.4 Grooves: Groove defects are a significant type of defect that occurs when the mixing head fails to form a proper connection after mechanically stirring the surface of the butt joint plate; they are usually found on the surface of the forward weld seam. The formation of groove defects is mainly due to insufficient pressure during welding, which results in severely inadequate heat input and a significant reduction in the amount of material that undergoes plastic deformation ； Furthermore, the fluidity of the material decreases, causing the plasticized material on the forward side of the weld to flow around from the rear side and fail to return to the forward side, thereby forming voids near the surface of the weld on the forward side. As the flowability of the material further decreases, the area where voids form expands, eventually resulting in groove defects on the upper surface of the weld. 2.5 Other defects: Due to the presence of an oxide film on the welding surface, an oxide layer that differs from that inside the weld may form on the surface of the weld after welding ； Since the oxide film on the joint surfaces may not be completely disrupted during welding, it remains in the weld in a continuous distribution, and is referred to as an “S-line” or “Z-line”” ； In lap or T-joints, a defect known as a residual interface line tends to form; this also falls under the category of lack-of-fusion defects. 3 Inspection methods for friction stir welding: The inspection of welded structures primarily relies on visual inspection and non-destructive testing. The emergence of new friction stir welding processes has imposed new requirements on these inspection techniques. 3.1 Visual inspection. Visual inspection is the most direct and simplest inspection technique; it is also the best method for assessing surface conditions. It allows for a visual detection of any burrs, scratches, edge holes, or uneven welds on the surface of parts. These defects are caused by incorrect welding parameters, such as too fast movement speed, too fast rotation speed, insufficient impact force, and an inappropriate weld shape. The most unacceptable defect at the root of a weld is lack of fusion; it is the most important type of defect, and most non-destructive testing is carried out to detect this type of defect. After penetrant testing of welded structures, unwelded defects can be detected visually. Erosion is a chemical treatment carried out after welding; it usually requires mechanical processing of the surface before penetrant testing is performed. In this case, the etching process clearly shows the dynamically recrystallized zone (DXZ) at the weld site and its surrounding heat-affected zone (HAZ), enabling professionals to detect welding defects through visual inspection. Visual inspection is a reliable technique for identifying suspected weld defects. 3.2 Penetrant Testing Penetrant testing using P135E and P6F4 can be carried out on friction stir welding test plates in the as-welded state, as well as under conditions of single or double erosion. Additionally, the penetrant testing can be performed with and without developer, under various penetrant exposure times. Penetrant testing performed during welding in its original state is considered an unacceptable method due to poor detection capability and high background noise. Under erosion conditions, inspecting friction stir welded structures using P135E and P6F4 enables the successful detection of unwelded defects at the root. Due to the different detection sensitivities of each penetrant, the test results also vary. Using P135E, unseam defects with a depth of 1.626 m or more can be successfully detected, while using P6F4, unseam defects with a depth of 1.270 m or more can be successfully detected. Compared to single etching, double etching using a corrosive etchant prior to the application of a penetrant can improve the detection rate of unfused defects. The difference between single erosion and double erosion is that single erosion can remove 0.005–0.010 mm of metal, while double erosion can remove 0.010–0.015 mm of metal. Test results show that, prior to the application of a penetrant, at least 0.010 m of metal can be removed by erosion, thereby improving the detection rate of unfused defects. Research shows that penetrant testing should include removing a metal layer of 0.010–0.015 mm using an etching solution before applying the penetrant solution. Extending the penetration time of the penetrant and using a developer do not improve the detection rate of unwelded defects. 3.3 Ultrasonic testing: NDT engineers and technicians from Lockheed Martin conducted ultrasonic testing on the friction stir welding specimens, using conventional ultrasonic probes as well as multi-probe systems, along with L-wave, shear-wave, and multi-angle sensors. The research results show that these techniques can detect unwelded defects at 15%–20% of the material thickness. Changes in the friction stir welding tool directly affect the metallographic characteristics of unfilled defects, making them closer together and more difficult to detect. While improving the friction stir welding process, it is necessary to research and improve the corresponding inspection methods. Through improvements to phased array ultrasonic testing technology, welding defects at 25%–30% of the material thickness can be detected. Phased array ultrasonic testing technology can provide echoes from multiple directions, offering information about the location of defects as well as information regarding defects throughout the entire thickness. This phased array ultrasonic testing technique uses 32 ultrasonic probes; as these probes scan automatically along the weld, the ultrasonic beam is adjusted via an electronic grating to generate ultrasonic images. 3.4 Radiographic testing Radiographic testing involves taking film or digital photographs of friction stir welding specimens. The research results show that this method can reliably detect unwelded defects that are greater than or equal to 30% of the material thickness, with a probability of 90% and a 95% confidence level. However, it is difficult to use X-ray film photography to inspect the welded joints of different alloys; in such cases, it is hard to distinguish between lack-of-fusion defects. There are two reasons for this: First, when different aluminum alloys are welded together, a mixture of the two alloys is formed at the joint, and the concentrations of copper and lithium in this mixture differ by a few percentage points. Different copper contents have a significant impact on the transmission of X-rays; therefore, those involved need to \"train\" their eyesight in order to be able to accurately interpret the resulting film images. Secondly, after applying friction stir welding to different alloys, in the alloy mixing zone (such as the mixture of A12219 and A12195), unwelded defects become more closely packed and are difficult to detect. 3.5 Eddy Current and Conductivity Testing Lockheed Martin conducted traditional eddy current testing on the friction stir welding specimens using a 1MHz pen-type probe and a 300kHz differential rotating probe. Initial eddy current testing results indicate that, using technologies from the Marshall Space Flight Center and Lockheed Martin, it is possible to reliably detect friction stir welds of the same alloy (A12195/A12195) that have unfilled defects at least 1.651 mm deep or more. The response of eddy currents as they pass through welds of different alloys is completely different; all specimens exhibited an eddy current response, making it impossible to distinguish between those with unseamed defects and those without such defects. Using automatic non-destructive testing techniques for reliable inspection is key to ensuring the structural integrity of aviation components. To evaluate the latest eddy current testing technologies, Lockheed Martin contacted Jentek Sensors to develop testing techniques for friction stir welding. This new eddy current testing method is based on conductivity. From 1998 to 2001, Jentek Sensor Company utilized its own testing systems to carry out extensive work in welding process inspection and weld inspection. Later, this conductivity-based detection technology was improved, and sensors specifically designed for friction stir welding were developed. This research complements the inspection methods currently widely used in friction stir welding, such as penetrant testing, ultrasonic testing, and X-ray radiography, thereby further reducing the inspection risks associated with friction stir welding. 4 Conclusion In the manufacturing process of new aviation equipment in our country, new manufacturing processes and technologies are being applied on a wider scale. The application of new processes and technologies can improve the structural and manufacturing efficiency of aviation components, while also posing new challenges to the integrity testing of aviation structures. With the growth and development of China’s aviation industry, it is necessary not only to improve manufacturing technologies and enhance production standards, but also to develop comprehensive inspection techniques suited to new technologies through continuous innovation, in order to ensure the reliability of aviation structures.