Dilution
Dilution is an important parameter to measure the quality of the laser cladding process, in order to quantitatively describe the degree of composition change caused by the melting of specific materials into the coating.
The dilution rate can be calculated by the geometric method of measuring the cross-sectional area of the coating: dilution rate = matrix melting area /(coating area + matrix melting area) x100%.
When the coating thickness and power density are constant, the dilution increases with the increase of laser energy density. The greater the energy density and the more melting areas of the substrate, the greater the change of coating composition. When the thickness of powder coating decreases, the dilution of coating increases. On the contrary, with the increase of powder coating thickness, the dilution decreases, because the increase of powder coating thickness limits the increase of matrix melting depth. The results show that the dilution decreases with the increase of scanning speed and powder feeding speed when the laser processing parameters remain unchanged. In order to ensure high coating performance, except for individual special cases, the dilution is generally required to be as low as possible. When laser spraying, it is usually controlled at about 5%. For laser cladding, the dilution should also be less than 10%, preferably about 5%.
Problems with pores and cracks
One of the important obstacles to the industrialization of laser cladding technology is the instability of the quality of the cladding layer, and its defects are mainly porosity, cracks, deformation and unevenness. One of the most difficult problems is the crack, and the formation and expansion of the crack is related to the existence of pores.
The main cause of the crack is the residual stress in the cladding layer due to rapid heating and cooling. In addition, the formation of cracks is also affected by many factors such as process parameters, cladding layer and base material, cladding layer thickness, etc. The main methods to inhibit crack formation are:
(1) Preheating and slow cooling are used to reduce the possibility of crack formation and relaxation stress.
(2)The gradient cladding layer is designed, and the transition cladding layer is selected between the matrix material and the cladding layer to alleviate the stress and reduce the crack generation.
The occurrence of porosity is caused by the presence of aerosols in the process of laser melting of the welding materials, and is caused by the rapid condensation of the gas, which is usually due to the carbon and oxygen reaction of the metal or the reaction of the metal oxide by the carbon reduction, and the volatile and the non-reactive air hole of the fixed matter. Not only does the stomhole itself reduce the coating performance, but it is also easy to be the aggregation point of crack initiation and extension, so it is also one of the important measures to prevent the crack initiation and expansion of the sealing layer and the important measures to prevent the crack. The control of the vent mainly comes from two aspects: the first is to take precautions to limit the source of the gas (such as the loss of the powder before the powder is used to dry the wet and the process of the melting process). The second is to adjust the process parameters to slow down the cooling crystallization rate of the molten pool to facilitate the gas escape.
Xi'an Guosheng Laser Technology Co., Ltd. is a high-tech enterprise specializing in R&D, manufacturing and sales of automatic laser cladding machine, high-speed laser cladding machine, laser quenching machine, laser welding machine and laser 3D printing equipment. Our products are cost-effective and sold domestically and abroad. If you're interested in our products, please contact us at bob@gshenglaser.com.