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In recent years, due to growing concern over environmental issues, automobile manufacturers are under increasing pressure to improve fuel efficiency. More stringent and restrictive regulations have posed technical challenges to industrial production and material processing. These trends include reducing exhaust emissions, making the vehicle bodies lighter, and extending the service life of components. Advances in material processing have brought unique opportunities to the field of stainless steel tube production. Specifically, manufacturers are asked to produce parts that are lighter in weight, yet still possess corrosion resistance and meet strength requirements. Furthermore, the spatial constraints of the vehicle body further emphasize the importance of formability. Typical applications include exhaust pipes, fuel pipes, fuel injectors, and other components. In the production of stainless steel pipes, a flat steel strip is first shaped, and then its shape is transformed into that of a circular tube. Once formed, the seams of the pipe must be welded together. This weld significantly affects the formability of the part. Therefore, to obtain a weld profile that can meet the stringent testing requirements in the manufacturing industry, it is crucial to choose the appropriate welding technique. Undoubtedly, gas tungsten arc welding (GTAW), high-frequency (HF) welding, and laser welding have all been applied in the manufacturing of stainless steel pipes. High-frequency induction welding: In high-frequency contact welding and high-frequency induction welding, the device that supplies current and the device that applies pressure are separate from each other. Furthermore, both methods can utilize magnetic rods, which are soft magnetic components placed inside the tube body; they help to concentrate the welding flow at the edges of the steel strip. In both cases, after the steel strip is cut and cleaned, it is rolled up and then sent to the welding point. Additionally, a coolant was used to cool the induction coil employed during the heating process. Finally, some coolant will be used in the extrusion process. Here, a large force is applied to the extrusion pulley in order to prevent porosity from forming in the welding area ； However, using a greater pressing force will result in an increase in burrs (or solder beads). Therefore, specially designed cutting tools are used to remove burrs inside and outside the pipes. One of the main advantages of the high-frequency welding process is its ability to process steel pipes at high speeds. However, a typical situation in most solid-state forging is that the joints formed by high-frequency welding are not easily subject to reliable testing using conventional non-destructive testing (NDT) methods. Welding cracks may occur in the flat and thin areas of low-strength joints; such cracks cannot be detected using conventional methods, which may result in a lack of reliability in some high-demand automotive applications. Tungsten inert gas welding (GTAW) is traditionally the method chosen by steel pipe manufacturers for carrying out the welding process. GTAW generates a welding arc between two non-consumable tungsten electrodes. At the same time, an inert shielding gas is introduced from the spray gun to shield the electrodes, generate an ionized plasma stream, and protect the molten weld pool. This is a well-established and understood process that enables the repeated execution of high-quality welding. The advantage of this process is its repeatability, the absence of spatter during welding, and the elimination of porosity. GTAW is considered an electrically conductive process, so relatively speaking, the process is slow. High-frequency arc pulses: In recent years, GTAW welding power sources, also known as high-speed switches, have enabled the arc pulses to reach frequencies of over 10,000 Hz. Customers of steel pipe processing plants are the first to benefit from this new technology, as high-frequency arc pulses result in a downward force on the arc that is five times greater than that in conventional GTAW. Other representative improvement features include an increased blast strength, a faster welding speed, and reduced waste. Customers of steel pipe manufacturers soon found that the weld shape obtained with this welding process needed to be reduced. Furthermore, the welding speed is also relatively slow. Laser welding: In all steel pipe welding applications, the edges of the steel strip are melted, and these edges solidify once the pipe edges are pressed together using clamping brackets. However, for laser welding, its unique characteristic is its high energy beam density. The laser beam not only melts the surface layer of the material but also creates a keyhole, resulting in a very narrow weld shape. If the power density is below 1 MW/cm2, as in GTAW technology, insufficient energy density is generated to create a keyhole. In this way, the keyhole-free process results in a weld profile that is wide and shallow. The high precision of laser welding enables more efficient penetration, which in turn reduces grain growth and results in better microstructural quality ； On the other hand, the higher heat input and slower cooling process of GTAW result in a rough weld structure. Generally, it is believed that the laser welding process is faster than GTAW; they have the same scrap rate, but the former yields better microstructural properties, resulting in higher burst strength and better formability. Compared to high-frequency welding, laser processing of materials does not involve oxidation, which results in a lower defect rate and better formability. Effect of spot size: In welding at stainless steel pipe factories, the welding depth is determined by the thickness of the steel pipe. Thus, the production goal is to improve formability by reducing the welding width, while achieving higher speeds. When selecting the most suitable laser, one cannot consider only beam quality; the accuracy of the tube rolling machine must also be taken into account. Furthermore, before the dimensional errors of the tube rolling machine come into play, it is necessary to first consider the limitations imposed on reducing the spot size. There are many size-related issues specific to steel pipe welding; however, the main factor affecting welding is the seams on the welding coil (more specifically, on the welded strip). Once the steel strip has been shaped and is ready for welding, the characteristics of the weld include: the gap between the steel strips, severe/mild welding misalignment, and variations in the weld centerline. The gap determines how much material is needed to form the weld pool. Excessive pressure will result in excess material at the top of the steel pipe or on its inner diameter. On the other hand, severe or minor welding misalignment can result in an unsatisfactory weld shape. Furthermore, after passing through the welding box, the steel pipe will be further trimmed. This includes adjustments to size and shape (appearance). On the other hand, additional work can remove some severe/mild welding defects, but it may not be possible to eliminate them all. Of course, we aim for zero defects. Generally, as a rule of thumb, welding defects should not exceed 5 percent of the material thickness. Exceeding this value will affect the strength of the welded product. Finally, the presence of a welding center line is important for the production of high-quality stainless steel pipes. As the automotive industry places increasing emphasis on formability, this directly leads to the need for a smaller heat-affected zone (HAZ) as well as a reduced weld profile. Conversely, this promotes the development of laser technology, namely by improving beam quality to reduce the spot size. As the spot size continues to decrease, we need to pay more attention to the accuracy when scanning along the seam line. Generally speaking, steel pipe manufacturers will try their best to minimize this deviation, but in practice, it is very difficult to achieve a deviation of 0.2 mm (0.008 inches). This leads to the need to use a weld tracking system. The two most common tracking techniques are mechanical scanning and laser scanning. On the one hand, mechanical systems use probes to make contact with the area upstream of the weld pool; these probes become covered in dust, suffer from wear and vibration. The accuracy of these systems is 0.25 mm (0.01 inch), which is not precise enough for laser welding that requires a high beam quality. On the other hand, laser weld tracking can achieve the required precision. Generally, a laser beam or laser spot is projected onto the weld surface, and the resulting image is fed back to a CMOS camera, which uses algorithms to determine the location of welds, poor joints, and gaps. Although imaging speed is important, in order to provide the necessary closed-loop control to move the laser focus head directly over the seam, the laser weld tracker must have a fast enough controller to accurately determine the position of the weld. Therefore, the accuracy of weld tracking is very important, and the response time is equally important. Overall, weld tracking technology has been fully developed, allowing steel pipe manufacturers to use higher-quality laser beams to produce stainless steel pipes with better formability. Therefore, laser welding has found its application; it is used to reduce porosity in the weld, minimize the weld profile, while maintaining or even increasing the welding speed. Laser systems, such as diffused cooling strip lasers, have improved beam quality and further enhanced formability by reducing the welding width. This development has led to stricter dimension control and the necessity of laser weld tracking in steel pipe factories. Thus, the success of the welding process in stainless steel pipe factories depends on the integration of all individual techniques, so it must be treated as a complete system. Thank you all for your continued support of our forum~