In recent years, due to increasing attention to environmental issues, automakers are increasingly under pressure to improve fuel efficiency. More stringent and more restrictive regulations have brought technical challenges to industrial production and material processing. Among these trends are reduced exhaust emissions, lighter bodywork, and extended component life. Advances in material processing have brought unique opportunities for stainless steel tube production. Specifically, people are required to produce such parts. They must have lighter weight, but they must still have anti-corrosion characteristics and meet the strength requirements. In addition, the space limitations of the car body emphasize the importance of formability. Typical applications include exhaust pipes, fuel pipes, injectors, and other components. In general, it is believed that the laser welding process is faster than GTAW. They have the same scrap rate, while the former brings better metallurgical properties, which leads to higher burst strength and higher formability. When compared with high-frequency welding, the process of laser processing materials does not oxidize, which results in a lower reject rate and higher formability. Mini Fiber Laser Marking Machine Small Fiber Laser Marking Machine, laptop Fiber Laser Marking Machine Feiquan Laser Technology Wuxi Co., Ltd. , https://www.fq-lasers.com
In the production of stainless steel pipes, a flat strip of steel is first formed, which is then made into a round tube shape. Once formed, the seams of the tubes must be welded together. This weld greatly affects the formability of the part. Therefore, to obtain a welding profile that meets the rigorous testing requirements in the manufacturing industry, choosing the right welding technology is extremely important. Undoubtedly, tungsten gas shielded arc welding (GTAW), high frequency (HF) welding, and laser welding have been applied to the manufacture of stainless steel pipes.
High-frequency induction welding
In high-frequency contact welding and high-frequency induction welding, the device for supplying current and the device for providing pressing force are independent from each other. In addition, both methods can use magnetic rods, which are soft magnetic elements that are placed inside the tube, which helps to concentrate the welding flow at the edge of the strip.
In both cases, the strip is cut and cleaned, rolled up, and then sent to the weld. In addition, coolant is used for cooling the induction coil used during heating. Finally, some coolant will be used in the extrusion process. Here, a large force is exerted on the compression pulley to avoid creating porosity in the weld zone; however, the use of a larger extrusion force will result in an increase in burrs (or weld beads). Therefore, specially designed tools are used to remove the burrs inside and outside the pipe.
One of the main advantages of the high-frequency welding process is that it enables high-speed machining of steel tubes. However, in most cases of solid-phase forging, it is typical that high-frequency welded joints cannot easily be reliably tested using conventional non-destructive techniques (NDT). Welding cracks may occur in flat, thin areas at low-strength joints. Such cracks cannot be detected using conventional methods and may lack reliability in some demanding automotive applications.
Tungsten gas shielded arc welding (GTAW)
Traditionally, steel pipe manufacturers have selected tungsten gas shielded arc welding (GTAW) to complete the welding process. GTAW creates a welding arc between two non-consumable tungsten electrodes. At the same time, an inert protective gas is introduced from the spray gun to shield the electrodes, generate an ionized plasma stream, and protect the molten weld pool. This is an already established and understood process that will repeat the high quality welding process.
The advantages of this process are its repeatability, the absence of splatter during the welding process, and the elimination of porosity. GTAW is considered to be a process of electrical conduction, so, relatively speaking, the process is relatively slow.
High-frequency arc pulse
In recent years, GTAW welding power supply, also known as high-speed switching, has caused arc pulses to exceed 10,000 Hz. Customers of steel pipe processing plants benefited the most from this new technology, and high-frequency arc pulses caused the arc down pressure to be five times larger than that of the conventional GTAW. The representative improvements brought about include: increased blasting strength, faster welding line speed, and reduced scrap.
Customers of the steel pipe production plant soon discovered that the welding profile obtained by this welding process needs to be reduced. In addition, the welding speed is still relatively slow.
Laser welding
In all steel tube welding applications, the edge of the strip is melted and the edges solidify when the edges of the steel tube are pressed together using a clamp bracket. However, the unique property for laser welding is that it has a high energy beam density. The laser beam not only melts the surface layer of the material, but also creates a keyhole so that the weld profile is very narrow.
If the power density is less than 1 MW/cm2, such as GTAW technology, it will not generate enough energy density to generate keyholes. In this way, the keyless hole process results in a wide and shallow solder profile. The high precision of laser welding leads to a more efficient penetration, which in turn reduces grain growth and leads to better metallurgical quality. On the other hand, the higher heat input and slower cooling process of the GTAW result in Rough welded structure.
Effect of spot size
In the welding of stainless steel pipe factory, the welding depth is determined by the thickness of the steel pipe. In this way, the production goal is to improve formability by reducing the weld width while achieving higher speeds. When selecting the most suitable laser, one cannot only consider the beam quality, but must also consider the accuracy of the tube mill. In addition, before the error in the size of the rolling mill is effected, it must be considered before the reduction of the light spot.
There are many problems in the peculiar size of the steel pipe welding, however, the main factor influencing the welding is the joint on the welding box (more specifically, the welding coil). Once the strip is ready for welding in the forming process, the characteristics of the weld include: strip clearance, severe/slight misalignment of welds, and changes in the weld midline. The gap determines how much material is used to form the weld pool. Too much pressure will result in excess material on the top or inner diameter of the steel tube. On the other hand, severe or minor misalignment of the weld will result in a poor weld appearance.
In addition, after passing through the welding box, the steel tube will be further trimmed. This includes adjustments in size and shape (outline). On the other hand, extra work can remove some serious/slight weld defects but may not be able to remove them all. Of course, we want to achieve zero defects. In general, the rule of thumb is that weld defects do not exceed 5% of the material thickness. Exceeding this value will affect the strength of the welded product.
Finally, the presence of a welded midline is important for the production of high quality stainless steel tubes. With the growing emphasis on formability in the automotive market, it is directly related to the need for a smaller heat affected zone (HAZ) and a reduced solder profile. This, in turn, promotes the development of laser technology that improves beam quality to reduce spot size. As the spot size keeps getting smaller, we need to pay more attention to the accuracy when scanning the seam centerline. In general, steel pipe manufacturers will reduce this deviation as much as possible, but in practice, it is difficult to achieve a deviation of 0.2 mm (0.008 inch).
This brings with it the need to use a weld seam tracking system. The two most common tracking techniques are mechanical and laser scanning. On the one hand, mechanical systems use probes to contact the joints upstream of the weld pool. They can become ashed, worn, and vibrated. The accuracy of these systems is 0.25 mm (0.01 inch), which is not accurate enough for high beam quality laser welding.
On the other hand, laser seam tracking can achieve the required accuracy. In general, the laser light or laser spot is projected on the surface of the weld, and the resulting image is fed back to a CMOS camera that uses algorithms to determine the position of the weld, mis-joint, and gap.
Although the imaging speed is important, the laser seam tracker must have a sufficiently fast controller to accurately compile the position of the weld while providing the necessary closed-loop control to move the laser focus head directly over the seam. Therefore, the accuracy of weld tracking is very important, and the response time is equally important.
In general, weld seam tracking technology has been fully developed and can also allow steel pipe manufacturers to use higher quality laser beams to produce more formable stainless steel pipes.
Therefore, laser welding finds a place where it is used to reduce the porosity of the weld and reduce the weld appearance while maintaining or increasing the weld speed. Laser systems, such as diffusion-cooled slab lasers, have improved beam quality and further improve formability by reducing weld width. This development has led to more stringent dimensional control and the need for laser weld tracking in steel tube plants.
In this way, the success of the stainless steel pipe welding process depends on the integration of all individual technologies, so it must be treated as a complete system.