Study On Process Conditions Of Laser Cladding Of Al₂O₃Coatings On Aluminium Alloy

The alumina gel coatings and the CNT/alumina gel composite coatings are fabricated on the surface of ZL109aluminum alloy by the non-aqueous sol-gel method using aluminum isopropoxide [Al(OC3H7)3] as the precursor. The continuous Nd-YAG laser was employed to treat the alumina gel coatings and the CNT/alumina gel composite coatings. This article investigated the effect on the micro structure and properties of the cladding layers caused by laser power, scanning speed, the concentration of carbon nanotubes. The microstructure, the morphology, and the hardness of the cladding layers were characterized by the X-ray diffraction, scanning electron microscopy and micro-hardness tester.The laser cladding results of the alumina gel coatings showed that the cladding layers were composed of α-Al2O3phase and γ-Al2O3phase. Under the scanning speed200mm/s and the laser power500W. It showed that the cladding layer is compact but not uniform, and present a small amount of uncrystallized spherical gel particles. Under the scanning speed200mm/min, the laser power600W, the alumina gel particles disappeared and the cladding layer became more compact and continuous, the α-Al2O3phase diffraction peaks were significantly enhanced, when the scanning speed added to400mm/min, the degree of γ-Al2O3phase transformed to α-Al2O3phase are decreased, the structure of cross-section was more loose, and part of the coating appeared to peel off. Under the condition of laser power800W, scanning speed200mm/min and400mm/min, the surface quality of aluminum oxide cladding layer were poor, the aluminum alloy needles organization appeared on the surface, the cladding layer quality were not continuous and the thickness were not even, and the alumina coating may loss due to evaporation.The Al/C molar ratio of8:1and2:1CNT/Al2O3composite gel pre-coating are fabricated on the ZL109aluminum alloy surface. The laser cladding experiment are carried out under the condition of scanning velocity of400mm/min but different laser power. For the Al/C molar ratio of8:1, the laser power of600W. The CNT/Al2O3cladding layer is composed by α-Al2O3phase and γ-Al2O3phase, and there is a uniform dispersion of carbon nanotubes. While the laser power increased to900W, the alumina sol reacted with CNT, and generated Al2OC phase in the cladding layer, the cladding layer became smooth and dense, uniform continuous. For the Al/C molar ratio of2:1, the cladding layer is composed of α-Al2O3, 7-Al2O3and Al2OC phase, uncrystallized spherical alumina gel particles exist in the cladding layer under the laser power of600W. The laser power increased to900W, the cladding layer was wholly composed of spherical particles of several tens of nanometers in diameter, and plenty of pores existed between the particles.Microhardness of cladding layer depends on the phase structure and surface morphology The hardness of cladding layer with200mm/min scanning speed is significantly higher than400mm/min scanning speed. Microhardness of the alumina cladding layer was first increased and then decreased with the increase of laser power under the constant scanning speed. The mircohardness of alumina cladding is observed to be the highest up to264HV0.025under the condition of200mm/min scanning speed and600W laser power, which is five times of aluminum matrix. While the friction coefficient is0.41. The mircohardness of CNT/AI2O3cladding layer is lower than the mircohardness of alumina cladding layer. For the Al/C molar ratio of2:1, the mircohardness of cladding layer can reach136HV0.025, for the Al/C molar ratio of8:1, the mircohardness of cladding layer can reach238HV0.025at most. The presence of CNT the friction coefficient of CNT/Al2O3cladding layer fell to0.17due to the present of CNT.