1. P redo
The stainless steel wagon body not only has the characteristics of high strength, good durability, high environmental protection and high regeneration, but also has the advantages of good body firmness, exquisite appearance, long service life and high welding efficiency.
Therefore, little by little he is sought after by people in the sector.
Laser welding is a method that focuses high-energy laser beams on the surface of the connected part to melt the materials to be connected to form a weld.
There are many long, straight welds in the manufacture of stainless steel bodies for passenger railway cars.
Vehicle manufacturing companies in Japan, Germany and China have the capability of laser welding stainless steel bodies.
However, the disadvantages of special laser welding equipment, such as high cost, long cycle, poor accessibility and poor flexibility, hinder the development of laser welding technology on stainless steel bodywork.
In recent years, with the emergence of portable laser welding equipment, it is possible to further apply laser welding technology to the manufacture of stainless steel car bodies.
two . D development of portable laser welding
From the input of electrical energy to the actual formation of the laser, it needs to go through three stages: pump source, gain medium and resonator.
Energy is pumped from the pump source (lamp, diode, radio frequency device, etc.) to the laser medium to stimulate the laser medium to send out laser radiation.
The irradiated laser is amplified in the resonator, and finally the high-power laser is emitted through the optical path.
According to different gain media, lasers can be divided into gas lasers, solid lasers and liquid lasers (not used in industry).
Among them, the widely used solid-state laser medium is composed of single-crystal synthetic yttrium aluminum garnet (YAG) or glass doped with optically activated ions such as neodymium (Nd) or ytterbium (Yb).
The function of the solid-state laser gain medium is to promote maximum absorption of the laser luminescence and ensure that heat is consumed as effectively as possible.
Due to the different structures of gain media, solid-state lasers are divided into disk lasers, fiber lasers, semiconductor lasers and so on.
The gain medium of the traditional solid-state laser is the rod, the gain medium of the disk laser is a thin and wide disc, and the gain material of the fiber laser is a thin fiber.
The emergence of optical fiber transmission equipment solves the problem that the optical path changes according to the direction of usage demand.
The first generation of portable laser welding equipment appears.
Although the first generation of portable laser welding equipment can perform welding within a certain range, it has low photoelectric conversion efficiency, large equipment volume (4~6 times the volume of ordinary argon arc welding equipment), small welding penetration and small weldable range.
The cost of the equipment is high (more than 3 times that of argon arc welding).
Compared with argon arc welding, it is mainly used in billboards and decorative parts with aesthetic requirements.
There is no alternative to argon arc welding for structural connectors.
In recent years, fiber laser has made great progress.
Its advantages mainly include high photoelectric conversion rate, fast heat dissipation, good flexibility, strong anti-interference ability, low cost, long service life, no adjustment, no maintenance, high stability and small volume.
Handheld laser welding equipment using fiber lasers has also been gradually developed.
Laser welding requires high part assembly precision, and welding defects are easy to occur.
To solve this problem, the designer developed a portable laser welding equipment with light spot oscillation with reference to the special laser welding equipment.
The laser oscillates in the “8” or “O” format, which can reduce the assembly accuracy of the part and increase welding penetration.
After a series of optimization and improvements, the current common portable laser welding equipment has a power of 0.5 ~ 1.5KW.
The size and weight of the equipment are equivalent to that of an argon arc welding machine, and can weld metal plates measuring 3 mm or less.
In order to solve the problem of insufficient welding strength of laser welding structure, in recent years, equipment manufacturers have integrated automatic wire feeding devices based on laser welding and developed portable wire filling welding equipment to laser with automatic wire feeding, which basically serves the welding of thin metal sheets below 4mm.
It can basically replace and exceed argon arc welding and realize high-speed, low heat input, small deformation, low cost and environmental protection welding.
Under annual conditions, the manufacturing cost is lower than that of argon arc welding.
3 . C characteristics of portable laser welding
3.1 The advantages of portable laser welding
1) Portable laser welding has the characteristics of high efficiency, low heat input, low cost and environmental protection.
At the same time, because the front end of the welding gun is generally designed with a long guide nozzle, the tip of the guide nozzle can adhere to the workpiece and move along the shape of the workpiece during the welding process. The operator does not need to expend effort as in manual arc welding to overcome the gravity of the welding gun and can operate with one hand.
Requirements for the skill level of welders are low.
Generally, it only takes 5 to 7 days for a welder to learn and use it skillfully, while in the same case, it takes 30 days or more for a qualified manual arc welder.
2) For more complex parts, its efficiency is qualitatively improved compared with traditional manual arc welding.
In terms of flexibility, the welding gun can be customized, which can adapt to welding in various narrow spaces, that is, the position where manual arc welding can be welded, and portable laser welding can also be completed.
3) The laser welding seam is smooth and beautiful, and the weld metal volume is small, which is suitable for parts with high appearance requirements.
The common shape of laser welding joint is shown in Fig.
Generally, conventional joints of 3mm and below can be completed by manual laser welding.
Fig. 1 Common types of laser welded joints
3.2S advantages of portable laser welding
1) Because the laser can be dangerous to eyes and skin, whether direct or reflected, when welding aluminum, aluminum alloy, copper, gold, silver and other high-reflective materials, most of the laser will be reflected by the materials.
Laser belongs to invisible light. Pay attention to the reflection angle during operation. Cross operation should be avoided in the manual laser welding process.
At the same time, in order to avoid injury caused by laser reflection, scattering and diffuse scattering in the laser equipment area, isolation protection facilities need to be installed and operators need to wear anti-laser glasses.
2) For common laser welding welds, the gap of the workpiece after assembly should be ≤ 0.1mm.
In order to reduce assembly accuracy requirements and reduce the manufacturing cost of accessories, manual wire-filled laser welding equipment and portable oscillating laser welding equipment can be used.
Different release tests can be carried out according to the equipment capacity and product conditions to finally meet the usage requirements.
3) Portable laser welding equipment transmits laser through optical fiber.
When the bending radius of the equipment transmission line is too small, the optical fiber will be damaged.
Therefore, very large bending angles must be avoided during the welding process.
Generally, the length of single and continuous welding seam should be controlled within the range of personnel activities.
For long straight welds longer than 2m, it can be considered that they are carried out in sections, and a welding gun cable suspension device can be installed.
4) Portable laser welding is still used in manual welding, and fatigue will occur during prolonged operation.
For long, straight welds with good welding accessibility, special machines or automatic equipment should be considered for welding.
4. An Application Perspective of Stainless Steel Body
The stainless steel body has the characteristics of strong corrosion resistance, good plastic strength and free from coating, which is preferred by customers.
Stainless steel body laser welding has incomparable advantages over traditional welding.
It has high welding resistance, can effectively reduce defects and has a beautiful appearance, so it is highly respected.
The stainless steel body of railway vehicles is generally composed of thin plates.
For the lightweight stainless steel body (see Figure 2), except that the plate thickness of the lower frame supporting part exceeds 4 mm, other parts are made of thin-walled materials with a plate thickness of 0.8 ~ 3 mm.
Among them, the thinnest corrugated plate on the top cover is only 0.8mm, while the steel material of the side wall plate and end wall plate required by the outer surface is generally 2mm, which is within the range plate thickness applicable to portable laser welding.
Fig. 2 A lightweight stainless steel body
4.1 Aesthetic requirements and welding application perspective
The overlap welds around the door frame and sidewall cladding (see Fig. 3) are located at the passenger compartment entrance of the car body, the fillet welds between the endwall drain and the endwall cladding (see Fig. 4), and the overlapping welds between the end edge and the sidewall cladding (see Fig. 5).
They are visible to passengers on the platform. They have high requirements for the aesthetics of the weld.
Manual arc welding is used to form unsightly welds such as full-length welding.
Large welding heat input will cause deformation of the outer surface, but to ensure that the vehicle is not corroded for a long time and meets structural needs, the conventional scheme is argon arc welding + bonding treatment, while arc welding of argon the weld is fish scale, the weld position is uneven, the weld joint is concave or convex, ugly, and the final welding gap bonding treatment operation is complicated.
The thickness of the adhesive joint is small, easy to age and fall off, and the maintenance cost is high.
Fig.3 Overlapping the door frame and side wall
Fig. 4 Corner connection between end wall drainage and skin
Fig. 5 Overlapping of the end edge and skin
For the welds in the above positions, generally, the gap in the weld joint can be controlled within 0.1 mm.
Portable laser welding can completely replace argon arc welding + gluing, significantly reduce manufacturing cost and improve efficiency.
Welds are straight and uniform after welding without grinding, which can improve customer satisfaction.
4.2 Application perspective of large deformation welding
The lap weld between the diaphragm and the bent beam above the top cover and the fillet weld between the diaphragm and the bottom plate (see Figure 6) are about 3m long.
When manual arc welding is used, the heat input is large.
Even if sectional jump welding is used, the deformation after welding is still large.
The deformation on the bottom plate is more than 5mm and the deformation on the diaphragm is more than 10mm, which requires a lot of manpower and material resources for adjustment and repair. Portable laser welding is used.
The welding heat input is greatly reduced and the welding speed can be increased by more than three times, which can greatly reduce the subsequent shape adjustment and correction.
At the same time, the weld formation is beautiful and there are few defects in the joints, which can reduce the thinning of the joints, so as to reduce costs and improve efficiency.
Fig. 6 Diaphragm connection above the top cover
5. Conclusion
At present, advanced laser welding technology is gradually adopted for the stainless steel body of railway cars, and the corresponding technical standards are gradually improved.
The joint fatigue resistance and mechanical strength of laser welding are better than those of traditional arc welding. In the manufacture of lightweight stainless steel car bodies, portable laser welding has greater flexibility and a wider range of applications.
With the continued progress of this technology and the continued reduction of equipment costs, the use of portable laser welding can reduce costs, improve efficiency and improve customer satisfaction.
It can be predicted that manual arc welding will be gradually replaced in stainless steel car bodies in the future.
