Laser welding 304 stainless steel application program

In the laser welding industry, there are many manufacturers that use laser welding of stainless steel workpieces, and the welding effect is also very good. It is also a good choice to use laser welding for 304 stainless steel.

　　The advantages of laser welding for stainless steel:

Compared with traditional welding, low-power pulsed laser seam welding has the advantages of high power density, energy concentration, small heat input, narrow weld seam and small deformation. After the laser beam is focused, a small spot can be obtained and precise positioning can be achieved. The characteristics make laser seam welding more suitable for welding small-sized workpieces than other welding methods. For the laser welding of ultra-thin stainless steel materials, because the material is very thin, it is easy to vaporize and perforate. In order to obtain a continuous, non-burn-through weld. The key is precise control of the parameters.

　　The main parameters that affect the quality of laser welding are welding current, pulse width, pulse frequency, etc. The main effects are as follows:

　　 (1) As the current increases, the width of the weld increases, and spatters gradually appear during the welding process, and the surface of the weld appears to be oxidized and has a rough feeling.

　　(2) As the pulse width increases, the width of the weld seam also increases. The change of pulse width has a significant effect on the laser welding effect of stainless steel ultra-thin plates. A slight increase in the pulse width may cause the sample to be oxidized and burned through.

　　(3) As the pulse frequency increases, the overlap rate of solder joints increases, and the width of the weld joint increases first. Basically remain unchanged afterwards. Observed under a microscope, the weld is getting smoother and more beautiful. However, when the pulse frequency increases to a certain value, the welding process spatters severely, the weld becomes rough, and the upper and lower surfaces of the welded parts are oxidized.

　　(4) Positive defocusing is suitable for laser welding of ultra-thin plate materials. Under the same defocusing amount, the weld surface obtained by positive defocusing laser welding is smoother and more beautiful than negative defocusing.

        Detailed explanation of laser welding technology

　 1. Basic principles of laser

What does 　　1, LASER mean

　　 Light Amplification by Stimulated Emission of Radiation (the abbreviation of Light Amplification by Stimulated Emission of Radiation).

　　2, the principle of laser generation

　　Laser-"stimulated radiation amplification" is to irradiate the laser-generating medium with strong light, so that the electrons of the atoms in the medium gain energy, and the electrons move orbits to migrate from low-energy state to high-energy state by being excited. The atoms in the excited state are induced by external radiation to make the atoms in the excited state transition to a low-energy state, and at the same time emit a beam of light; this beam of light is completely consistent with the incident light in terms of frequency, phase, propagation direction, and polarization. The light is stimulated emission light. In order to obtain a laser with high energy density and high directivity, it is necessary to have a resonant cavity that closes the light, so that the sightseeing beam oscillates back and forth between the mirrors placed on both sides of the laser generating medium, thereby increasing the light intensity and improving the direction of the light. .

　　 The laser light generated by the YAG crystal containing neodymium (ND) is a near-infrared light with a wavelength of 1.064um that is invisible to the human eye. This kind of light beam can realize continuous vibration even when it is weakly excited. YAG crystal is the abbreviation of gem yttrium aluminum garnet. It has excellent optical properties and is the best crystal for laser oscillation.

3. The main specialty of Ziguang

　　A. Monochromaticity-laser is not a mixture of many different lights, it is the purest monochromatic light (the length, frequency)

　　B. Directionality-Olive light basically does not diffuse outward when it propagates.

　　C, Phase Thousand Nature-Huiguang's phases (peaks and troughs) are very regular and have good coherence.

　　D. High output power. After focusing the laser with a lens, the energy density obtained is several hundred times that of sunlight.

　　二、YAG laser welding

　　Laser welding is to use the excellent directivity of the laser beam and high work density and other characteristics to work. The laser beam is focused in a small area through the optical system, forming a highly concentrated heat source area at the welded part in a very short time, so that the welded object is melted and a firm weld spot and weld seam are formed.

　　 There are two commonly used laser welding methods: pulse laser welding and continuous laser welding. The former is mainly used for single-point welding of continuous and thin materials. The latter is mainly used for welding and cutting of large and thick parts.

　　1. Features of laser welding processing method

　　A, non-contact processing, no need to pressurize the workpiece and surface treatment.

　　B, small solder joints, high energy density, suitable for high-speed processing.

　　C, short-time welding, not only has no thermal influence on the outside world, but also has a small thermal deformation and heat-affected zone on the material itself. It is especially suitable for processing high melting point, high hardness, and special materials.

　　D, there is no need to fill metal, no vacuum environment (can be done directly in the air), and there is no danger of X-rays in the air like electron beams.

　　E, compared with the contact welding process. No wear and tear of electrodes, tools, etc.

　　F, no processing noise, no pollution to the environment.

　　G, small workpieces can also be processed. In addition, welding can also be carried out through a wall of transparent material.

　　H, it can realize long-distance, difficult-to-reach parts by ordinary methods, multi-channel simultaneous or time-sharing welding through optical fiber.

　　I, it is easy to change the laser output focal length and solder joint position.

　　J, it is easy to mount on automatic machines and robots.

　　K, the conductor with insulation layer can be directly welded, and the dissimilar metals with large differences in performance can also be welded.

　　2. The mechanism of pulsed laser welding

　　Pulse laser welding can be divided into heat transfer melting welding and deep penetration melting welding

Heat transfer melting welding refers to when the laser beam is irradiated on the surface of the material, the material absorbs light energy and heats and melts. The heat of the surface layer of the material continues to be transferred to the depth of the material in a conductive manner until the contact surfaces of the two parts to be welded are mutually dissolved and welded together.

Deep penetration fusion welding means that when a laser beam of higher power density is irradiated on the material, the material is processed and melted and even vaporized, resulting in a larger vapor pressure. Under the action of the steam pressure, the molten metal is squeezed around. Make the irradiated place (melt pool) present a pit. As the laser beam continues to irradiate, the pit becomes deeper and deeper and penetrates into another workpiece. After the laser stops irradiating, the molten metal squeezed around the pit flows back into the pit again, and the workpieces are welded together after solidification.

　　 These two laser welding mechanisms are related to factors such as power density, irradiation time, material properties, and welding methods. When the power density is low, the irradiation time is long and the weldment is thinner, the heat transfer melting mechanism is usually the main method. On the contrary, it is based on the deep penetration melting mechanism.

　　 3. Process characteristics and influencing factors

　　 1. The input energy density of the laser. The main methods of adjusting the energy density of laser irradiation are:

　　A, adjust laser output energy (adjust excitation voltage)

　　B, adjust the size of the light spot (adjust the focal length)

　　C, change the energy distribution in the spot (change the fiber type: peak output type-GI type fiber, trapezoidal output type-SI type fiber)

　　D, change the width and waveform of the emitted pulse

　　2, material reflectivity

　　Most metals will reflect most of the laser energy when the laser starts to irradiate. Therefore, at the beginning of the welding process, the power of the beam should be increased accordingly. When using the pulsed laser to sew the second art, the ignition arc board can be connected to ensure the quality consistency of the entire welding section. When the metal surface starts to melt or vaporize, its reflectivity decreases rapidly.

　　 Fourth, the main factors affecting the absorption of the laser beam by the material

　　1, temperature

　　At room temperature, the absorption rate of the two lasers of metal materials is generally below 20℃; when the metal temperature reaches the melting point, the absorption rate rises to 40-50% after melting and vaporization; when it is close to the boiling point, the absorption rate can be as high as 90%.

　　The DC resistivity of the material

　　The absorptivity of the material to the laser is proportional to the square root of the DC resistivity of the material, and inversely proportional to the square root of the laser's length.

　　2, the incident angle of the laser beam

　　The larger the incident angle, the smaller the absorptance. When the laser is irradiated perpendicular to the metal surface, the absorption rate of the metal to the laser is the largest. However, in order to protect the laser exit lens, it is necessary to maintain a certain incident angle.

For the surface condition of 　　mura material, in order to have low reflectivity, a thin layer of all powder can be coated on the metal surface, but the two must be able to form an alloy. For example, rice, gold, and silver can be covered with a thin sharp layer. At this time, under the same penetration depth, the energy required for welding is about one-fourth of the original copper, gold, and silver.

　　3, focus and defocus

　　High-quality YAG laser welding device, its focus (spot size) is ensured by the coaxial precision of the optical path of the device itself, the imaging ratio of the output fiber and the output head, etc.

The position when the laser exit focus is exactly on the job is zero. Defocus refers to the distance of the focal point from this zero point. When the focus position exceeds the zero position, it is called negative defocus (the focus goes deep into the workpiece), and the distance value is the negative defocus amount. On the contrary, the value of the distance between the focal point and the zero point is the positive defocus amount. To obtain a larger penetration depth, the focus position can be selected at a certain position inside the workpiece, that is, a negative defocus amount is used for welding.

　　4, penetration depth of welding

　　Pulse laser welding is mainly carried out by means of heat transfer and melting. The direct penetration depth of the laser beam itself into the metal is limited, which mainly depends on the temperature coefficient of the material (the larger the temperature coefficient, the greater the penetration depth), rather than the power of the laser.

　　 Five, maintenance and maintenance

　　 1. Replacement of consumables

　　Pure water, ion exchange resin, water filter, excitation light, protective lens

　　2, check

　　A, laser vibration adjustment

　　 B, laser incident adjustment

　　C, fiber incident adjustment

　　D, energy balance adjustment

　　 6. Welding quality inspection

　　 Welding quality inspection generally has two methods: visual inspection and destructive inspection.

　　Visual inspection is to inspect each item shown in Figure 1. If the microscopic (mirror) photograph is used for metallographic inspection, it is necessary to cut and extract the weld nugget and grind and corrode (see Figure 2). However, it is not enough to draw a conclusion after only the appearance inspection, so please be sure to conduct a destructive experiment.

　　 Destructive testing is usually a tearing test, as shown in Figures 3 and 4, tear off the welding base metal for confirmation (round holes on one side and button-like residues on the other side)

　　 In addition, there is also a method for testing tensile strength using a tensile tester.

Seven, quality assurance methods

Although the resistance spot welding method is the most suitable welding method for mass production, improper quality management can cause huge losses. At present, since online non-destructive welding quality inspection cannot be realized, it is necessary to strengthen the management of quality assurance.

　　1, pressure detection

　　Welding heat is greatly affected by the contact resistance between the electrode and the workpiece. During the welding process, the pressure must remain constant, so it is necessary to frequently use a pressure tester to weld

　　2, electrode grinding

　　The increase in the number of welding will increase the wear of the electrode surface. The rough surface of the electrode will cause splashes and rough marks on the surface of the workpiece, which will affect the appearance of the workpiece. Therefore, it is necessary to prepare more ground electrodes and replace the electrodes appropriately according to the number of welding. Before using a new electrode, it is better to debug it as a waste workpiece.

　　3, the electrode is overheated

　　 Electrode overheating will not only shorten the life of the electrode but also cause uneven welding quality of the workpiece.

　　4, workpiece accuracy

　　 The phenomenon of defective welding products often occurs due to ignoring the changes in the thickness of the workpiece, the thickness of the plating layer, and the metal composition. Whether the quality of the workpiece itself is stable is also an important factor affecting welding quality.

　　 5. Current monitoring

　　Current monitoring is essential for welding. The main factors that affect the current change are: power supply voltage fluctuations, overheating caused by the overload of the welding machine, which reduces the current output, poor contact of the workpiece, and the current reduction, and the performance of the welding machine is poor.

　　 In order to prevent poor welding results caused by the above-mentioned reasons, it is necessary to monitor the welding current frequently. If the monitoring of welding current can be ensured, it will be easier to find other causes of changes that affect welding quality, thereby further improving the reliability of welding quality.