Laser sensor welding technology

1. Sensor

According to the national standard GB7665-87, a sensor is defined as a device that can sense the specified measured quantity and convert it into a usable output signal according to a certain rule. As a detection tool, the sensor is required to detect the physical or chemical information of the research object. Its working process requires stability, reliability and high precision, so there are several requirements for the sensor:

(1) Strong ability to adapt to harsh environments

Sensors generally work in a wide range of environments, from extremely cold to extremely hot areas. Many work in open air environments and must be resistant to flying sand and rocks, dust, and moisture. They must also have a high resistance to salt corrosion and acid corrosion, as well as the ability to resist interference from polluted gases. They must be able to adapt to high temperature, extreme cold, strong vibration, impact, and the ability to work normally under other conditions. They must also have strong noise resistance and a high signal-to-noise ratio.

(2) The price is moderate and suitable for mass production.

The sensor is required to have good consistency and be suitable for automated mass production. It also has high requirements for processing equipment in order to eliminate inconsistencies and errors caused by manual operation.

(3) High stability and reliability

Sensors are high-precision detection instruments. They have strict requirements for use in military, aviation, and aerospace applications. Products must undergo rigorous testing before they can be used. Therefore, sensor production is a specific application and embodiment of high-tech. Whether a sensor has a high technical added value is reflected in the technical content and processing technology involved. Some sensors require metal packaging due to the conditions of their application environment. They are generally sealed by welding, such as pressure sensors, force sensors, Hall sensors, photoelectric sensors, temperature sensors, etc. These sensors have sensitive components and integrated circuits inside, which are filled with inert gas or vacuumed to isolate them from the outside world. They have pressure resistance and airtightness requirements, as well as welding strength requirements and leakage rate requirements. They have high requirements for welding quality, and require small deformation during welding, and must not damage internal components and microcircuits. At present, sensor sealing welding includes resistance welding, tungsten inert gas welding, plasma arc welding, electron beam welding, and laser welding.

2. Laser welding

1. Principle of laser welding

Laser is the abbreviation of stimulated emission of radiation. Due to its unique high brightness, high directionality, high monochromaticity and high coherence, it has been widely used in industrial processing since its birth and has become a common processing method for future manufacturing systems. Laser welding uses a high-intensity laser beam. After the laser beam is focused by the optical system, the power density of the laser focus is 104~107W/cm2. The workpiece is placed near the laser focus for heating and melting. Whether the melting phenomenon can occur and the intensity of the melting phenomenon mainly depends on the time, power density and peak power of the laser acting on the surface of the material. By controlling the above parameters, laser can be used for various welding processes.

2. General characteristics of laser welding

Laser welding is a thermal processing technology that uses a laser beam as a heat source. Compared with electron beam plasma beam and general mechanical processing, it has many advantages:

(1) The laser beam has a small laser focus spot and high power density, and can weld some high melting point and high strength alloy materials.

(2) Laser welding is a non-contact process, without tool loss and tool replacement. The laser beam energy and moving speed are adjustable, and a variety of welding processes can be performed.

(3) Laser welding has a high degree of automation and can be controlled by computer. It has a fast welding speed and high efficiency, and can easily weld any complex shape.

(4) The heat affected zone of laser welding is small, the material deformation is small, and no subsequent processing is required.

(5) Laser can weld workpieces in vacuum containers and workpieces located inside complex structures through glass.

(6) The laser beam is easy to guide and focus, and can be transformed in various directions.

(7) Compared with electron beam processing, laser welding does not require a strict vacuum equipment system and is easy to operate.

(8) Laser welding has high production efficiency, stable and reliable processing quality, and good economic and social benefits.

3. Process characteristics of laser welding in sensor production.

Laser welding of sensor metal housing is currently the most advanced processing method, mainly based on the following characteristics of laser welding:

(1) High depth-to-width ratio. The weld is deep and narrow, and the weld is bright and beautiful.

(2) Minimum heat input. Due to the high power density, the melting process is extremely fast, the heat input to the workpiece is very low, the welding speed is fast, the thermal deformation is small, and the heat affected zone is small.

(3) High density. During the weld formation process, the molten pool is constantly stirred, and gas is easily released, resulting in a pore-free, fully penetrated weld. The high cooling rate after welding can easily make the weld microstructure fine, and the weld strength, toughness and comprehensive performance are high.

(4) Strengthen the weld. The high temperature heat source and the full absorption of non-metallic components produce a purification effect, reducing the impurity content, changing the size of inclusions and their distribution in the molten pool. No electrodes or filler wires are required during welding, and the molten zone is less contaminated, making the weld strength and toughness at least equivalent to or even exceeding that of the parent metal.

(5)精确控制。因为聚焦光斑很小，焊缝可以高精度定位，光束容易传输与控制，不需要经常更换焊炬、喷咀，显著减少停机辅助时间，生产效率高，光无惯性，还可以在高速下急停和重新启始。用自控光束移动技术则可焊复杂构件。

(6) Non-contact, atmospheric environment welding process. Because the energy comes from the laser, there is no physical contact between the workpiece and the workpiece, so no force is applied to the workpiece. In addition, magnetism and air have no effect on the laser.

(7) Due to the low average heat input and high processing accuracy, reprocessing costs can be reduced. In addition, the operating costs of laser welding are low, which can reduce the cost of workpieces.

(8) It is easy to automate and can effectively control the beam intensity and fine positioning.

3. Comparison between laser welding and existing welding methods

At present, the methods used for sensor sealing welding include: resistance welding, argon arc welding, electric beam welding, plasma welding, etc.

1. Resistance welding: It is used to weld thin metal parts. The workpiece to be welded is clamped between two electrodes and a large current is passed to melt the surface contacted by the electrodes, that is, welding is carried out by heating the workpiece with resistance. The workpiece is easy to deform. Resistance welding welds both sides of the joint, while laser welding is only performed from one side. The electrodes used for resistance welding need to be maintained frequently to remove oxides and metal adhered from the workpiece. Laser welding thin metal lap joints does not contact the workpiece. Moreover, the beam can enter areas that are difficult to weld with conventional welding, and the welding speed is fast.

2. Argon arc welding: It uses non-consumable electrodes and shielding gas and is often used to weld thin workpieces. However, the welding speed is slow and the heat input is much greater than that of laser welding, which is prone to deformation.

3. Plasma arc welding: Similar to argon arc, but its welding torch produces a compressed arc to increase the arc temperature and energy density. It is faster and has a deeper penetration than argon arc welding, but inferior to laser welding.

4. Electron beam welding: It relies on a beam of accelerated high-energy density electron flow to hit the workpiece, generating huge heat in a small area on the surface of the workpiece, forming a "pinhole" effect, and thus implementing deep melting welding. The main disadvantages of electron beam welding are that it requires a high vacuum environment to prevent electron scattering, the equipment is complex, the size and shape of the weldment are limited by the vacuum chamber, and the assembly quality requirements for Korean parts are strict. Non-vacuum electron beam welding can also be implemented, but the effect is affected by poor focusing due to electron scattering. Electron beam welding also has magnetic offset and X-ray problems. Since electrons are charged, they will be affected by magnetic field deflection, so it is required that the workpiece be demagnetized before electron beam welding. X-rays are particularly strong under high pressure, and operators need to be protected. Laser welding does not require a vacuum chamber or demagnetization of the workpiece before welding. It can be carried out in the atmosphere and there is no problem of X-ray protection, so it can be operated online in the production line and can also weld magnetic materials.

4. Application prospects of laser welding in sensor production

Laser welding technology is a high-tech technology. Due to its unique characteristics, it is particularly suitable for use in sensor sealing welding. At present, many foreign sensor manufacturers use laser welding technology to produce sensors, while there are not many domestic manufacturers using this technology. It is mainly some manufacturers of military sensor products and some scientific research institutions that use this technology, and most of them use foreign laser welding machines. At present, the performance of domestic laser welding machines is not far behind that of foreign products, and they can fully meet the process requirements of domestic sensor production, but the price is 1/3-1/5 of similar foreign products. In order to improve the overall level of domestic sensors and develop the national laser industry, my country's sensor manufacturers should use domestic laser welding machines to produce and process sensors as soon as possible to increase product competitiveness and open up the international market.