Principles, advantages and disadvantages, and process parameters of Laser Welding

Principle of Laser Welding
Laser Welding uses a high-energy laser pulse to partially heat the material in a tiny area. The laser radiation energy spreads to the material through heat conduction, and the material is melted to form a specific pool. It is a new welding method. Laser Welding is mainly used to weld Thin-Walled materials and precision parts. It can realize spot welding, butt welding, stacked welding, sealed welding, etc., with a high depth-width ratio, the weld width is small, the heat affected area is small, the deformation is small, the welding speed is fast, the weld is smooth, beautiful, no need to deal with after welding or only a simple treatment, the weld quality is high, no pores, precise Control, small focus points, high positioning accuracy, and easy automation.

Welding Characteristics

It is a type of melt welding. It uses a laser beam as the energy and impacts on the weld joint.
A laser beam can be guided by a plane optical element (such as a mirror) and then projected onto the weld using a reflection focusing element or lens.
Laser welding is a non-contact welding, the operation process does not require pressurization, but need to useInert gas to prevent oxidation of molten pool, sometimes used as filler metal.
Laser Welding can be combined with MIG welding to form a laser MIG hybrid welding to achieve deep penetration welding, while the heat input is much less than that of MIG welding.

Main advantages of Laser Welding

(1) the incoming heat can be reduced to a minimum requirement, the scope of metallographic change in the heat-affected zone is small, and the deformation caused by heat conduction is also the lowest.
(2) the welding process parameters of single-track welding with 32mm plate thickness have been verified, which can reduce the time required for Thick Plate Welding and even save the use of filler metal.
(3) There is no need to use electrodes, and there is no concern about electrode contamination or damage. Because it does not belong to the contact welding process, the wear and deformation of the machine can be minimized.
(4) laser beams are easy to focus, align, and guided by optical instruments. They can be placed at an appropriate distance from the workpiece and can be re-guided between machines or obstacles around the workpiece, other welding rules cannot be used due to space limitations described above.
(5) the workpiece can be placed in a closed space (under the control of vacuum or internal gas environment ).
(6) the laser beam can be focused in a very small area and can be welded to small parts with close intervals.
(7) The range of solderable materials is large, and various heterogeneous materials can also be joined to each other.
(8) Easy to automate high-speed welding, digital or computer control.
(9) When welding thin materials or fine diameter wires, it will not be as easy as the arc welding will have the trouble of re-fusing.
(10) It is not affected by the magnetic field (arc welding and electron beam welding are easy) and can be precisely aligned with the welding pieces.
(11) two metals with different physical properties (such as different resistors) can be welded.
(12) No vacuum or X-ray protection is required.
(13) If perforated welding is used, the width ratio of the weld channel depth can reach 10:1.
(14) A switching device can transmit a laser beam to multiple workstation.

Main disadvantages of Laser Welding

(1) welding piecesLocation must be very accurateMust be within the focus range of the laser beam.
(2) When welding pieces require sandwich fixtures, make sure that the final position of the welding pieces must be aligned with the impact solder joints of the laser beam.
(3) The maximum solderable thickness is limited to more than 19mm penetration thickness of the workpiece, the production line is not suitable for laser welding.
(4) high reflectivity and high thermal conductivity of materials such as aluminum, copper and its alloys, welding properties will be changed by the laser.
(5) When welding a laser with medium energy to high energy, the ionic gas around the pool should be evicted by the plasma controller to ensure the appearance of the weld track.
(6)Low energy conversion efficiency, usually less than 10%.
(7) Rapid Solidification of welding channels, possibleAir hole and brittle concerns.
(8) expensive devices.

Process Parameters of Laser Welding 
(1) power density. Power density is one of the most critical parameters in laser processing.With a high power density, the surface can be heated to the boiling point in microseconds, resulting in a large amount of evaporation.. Therefore, high power density is advantageous for material removal and processing, such as drilling, cutting, and engraving.For a low power density, it takes several milliseconds for the surface temperature to reach the boiling point. Before the surface evaporation, the bottom layer reaches the melting point, which is easy to form a good melt welding.. Therefore, in conducting laser welding, the power density ranges from 10 to 4 ~ 10 ^ 6 W/cm ^ 2.
(2)Laser Pulse Waveform. Laser pulse waveform is an important issue in laser welding, especially for Thin Sheet welding. When a high-intensity laser is routed to the surface of the material, the metal surface will be 60 to 60 ~ 98% of laser energy reflection is lost, and the reflectivity changes with the surface temperature. During a laser pulse, the metal reflectivity changes greatly.
(3)Laser Pulse Width. Pulse width is one of the important parameters for Pulse Laser Welding. It is not only an important parameter different from material removal and melting, but also a key parameter that determines the cost and volume of processing equipment.
(4) The effect of coke removal on welding quality. Laser Welding usually requires a certain degree of separation. Because the power density at the center of the laser focus is too high, it is easy to evaporate into a hole. The power density distribution is relatively uniform on the plane that leaves the laser focus. There are two ways to defocus: positive and negative. The focal plane is located at the top of the workpiece, which is positive and vice versa. According to the geometric optics theory, when the distance between the positive and negative focal plane and the welding plane is equal, the power density on the corresponding plane is approximately the same, but the actual shape of the molten pool is different. In case of negative defocus, a larger depth can be obtained, which is related to the formation process of the pool. Experiments show that laser heating is 50 ~ The 200us material began to melt, forming a liquid phase metal, and then formed a municipal pressure steam, which was sprayed at an extremely high speed and emits dazzling white light. At the same time, the high concentration of Steam makes the liquid metal move to the edge of the molten pool, forming a sag in the center of the molten pool. When the negative focus is removed, the internal power density of the material is higher than the surface, which is easy to form stronger melting and evaporation, so that light energy can be transferred to the deeper part of the material. Therefore, in practical application, when the penetration depth is large, the use of negative defocus; welding thin materials, the use of positive defocus.