Magnesium alloy castings often have defects such as pores and inclusions, and these defects are usually discovered after the parts are processed to the required size, thus resulting in a low yield of magnesium alloy castings. In the repair process of magnesium alloy defects, the following problems are faced:
(1) Coarse crystal problem: The melting point of magnesium is low (651 ° C), but because magnesium conducts heat quickly, it is necessary to use a heat source of a larger power, which makes the magnesium alloy susceptible to overheating and grain growth.
(2) Oxidation and evaporation: Magnesium is extremely active, and is easily oxidized to form magnesium oxide at high temperatures. Its melting point is high (2500 ° C), density is high (3.2 g / cm 3 ), and it is easy to form fine flakes in the molten pool. Solid slag. Moreover, magnesium alloys are easily burned without isolating oxygen. At high temperatures, magnesium is also easily combined with nitrogen in the air to form a nitride of magnesium, which deteriorates the properties of the melt zone after cooling. Magnesium has a low boiling point (1100 ° C), and magnesium is easily evaporated at high temperatures. Therefore, magnesium alloys need to be strictly protected during melting.
(3) Thermal stress: Magnesium and its alloys have a large thermal expansion coefficient, which is about twice that of steel and 1.2 times that of aluminum. Therefore, it is easy to cause large thermal stress, which aggravates the generation of cracks and causes deformation of the workpiece.
(4) Crack: Magnesium easily forms low-melting eutectic with some alloying elements (such as Cu, Al, Ni, etc.), so the brittle temperature range is wide and it is easy to form hot cracks.
(5) Porosity: Hydrogen pores are easily generated, and the solubility of hydrogen in magnesium decreases sharply with decreasing temperature. When there are many sources of hydrogen, the tendency of pores to appear is large.
(6) Control of the heat source: The heat source used must have sufficient energy rate, otherwise the heat will be quickly transferred to the substrate during heating, and the molten layer will be too deep, and the entire substrate will be melted.
This makes the repair of magnesium alloys more difficult than ordinary materials.
The company uses a YAG laser with a wavelength of 1.06μm. Under the protection of the special air curtain, the oxidation of the magnesium alloy during laser processing is effectively avoided, and the laser repair of the magnesium alloy is successfully realized. The figure below shows the photo of the single-pass and multi-channel laser repair. From the cross-sectional analysis, there is no porosity and cracks in the repair layer, and it has a good metallurgical bond with the substrate, and the thermal influence on the substrate is extremely small.
Application areas: repair of magnesium alloy casting defects, such as notebook casing, magnesium alloy instrument panel, magnesium alloy automotive parts.
Figure 1 Laser multi-channel repair
Figure 2 Laser single-channel repair
Figure 3 Cross-sectional morphology of the laser repair layer
Figure 4 Interface morphology between the laser repair layer and the substrate
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