Investigation On Laser Welding Technology Of Al₂O₃ Ceramic Liners In Composite Steel Pipes

Al₂O₃ ceramic composite steel pipes with its wear resistance, high temperature resistance,corrosion resistance, long service life, low cost characteristics, are widely used in fields of machinery, petroleum, paper, metallurgy. In the connection process of ceramic-lined composite steel pipes, due to poor welding performance of Al₂O₃ lined-layer, the density and strength of connected regions are unable to meet the application requirements of transporting gas and liquid by the way of district joining, which makes the practical application of the ceramic-lined steel composite pipes be limited. In order to solve this problem, it is necessary to carry out the research on the laser welding technology of Al₂O₃ ceramic liners in composite steel pipes.Firstly, the laser filler welding technology was used to connect the Al₂O₃ ceramic liners.According to the composition of the substrate and the performance requirements of the welding parts, the SiO₂ and Al₂O₃ were selected as the basic components of the fillers.Setting the laser parameters, defocus f=0mm, the beam incident angle ? = 0o, laser spot diameter D=3 mm, the solders with different widths x?1, 1.5, 2, 2.5, 3, 3.5 mm? for welding experiments. Comparing morphology and weld forming quality of welding joints with different widths solders. The results of experiments showed that when the width of solder is equal to the diameter of laser spot size?3mm?, laser beam irradiates exactly in the surface of solder, obtaining the less defects after welding in the interlayer, no crack formation in the substrate and the best weld formation quality. To determine the optimum parameters, the effects of laser power and welding speed on the welds penetration and welds width were analyzed. When the laser power P=700W, welding model is from heat conduction welding transiting into deep penetration welding. At this time, laser utilization rate of solders is the maximum. When the welding speed v= 0.8¹mm/s, the effects of welding speed on weld depths and weld widths size are drastic, leading to deep penetration welding. The results showed that when the solder width is 3mm, the laser power and welding speed are 700 W and1mm/s, the Al₂O₃ ceramic liners can be joined with good morphology.Secondly, to study the effects of SiO₂ content on properties of solders and welding performances. Preparing the solders containing SiO₂ contents of 10, 20, 30, 40, 50 wt.%respectively, with certain laser parameters?P=700W,?=1mm/s,f=0mm,?=0o,D=3 mm,x=3mm?, the effects SiO₂ content on laser absorption rate and interfacial wettability of solders were discussed. With the increase of SiO₂ content, laser absorption rate of solders increases first and then decreases, at the 30 wt.% reaching the maximum value of 8.4%. The wetting angle between the solders and substrates with the increase of SiO₂ decreases gradually, at 50 wt.% reaching the minimum value of 29.8°. In addition, analyzing the compositions, microstructures and element distributions in the interlayer, the effects of SiO₂ content on the strength, density and corrosion resistance of the joints were studied. The results of experiment indicated that when the SiO₂ content is 30 wt.%, there are few the pores and cracks in the interlayer and a lot of mullite phase generating, which can retard the crack growt.h and make the bending strength of the joints reach the maximum value of 173 MPa.The relative density of joints is 95.3%, the acid resistance of welds is 2.45 times of that of the alumina liner and alkali resistance is 0.22 times of that of alumina liner at 30 wt.% SiO₂.Finally, to improve quality of the welds and enhance connection strength of the joints, the experiments of adding the MgO to Al₂O₃-SiO₂ solders were implemented. Preparing the solders with the MgO content of 0, 5, 10, 15, 20 wt.%. Exploring the effects of MgO content on laser absorption rate and interfacial wettability of solders. After the addition of MgO, the laser absorption rate and interfacial wettability of the solders are increasing. Overmuch addition of MgO causes the high melting temperature of the solder, so that the wetting angles bewt.een the solders and the substrates increase. When the MgO content is 15 wt.%, the absorption rate of the solder increases to 14.82%, and the wetting angle reduces to 29°. In addition, analyzing the compositions, microstructures and element distributions of the interface elements, the effects of MgO content on the density, joining strength and corrosion resistance of the joints were studied. The experimental results showed that the MgO promotes the diffusions of atoms between the solders and the substrates, thus porosity andother defects of interlayers reduce. Mg2+ and Fe2+ coming from substrate have generated spinel mixed crystal, to increase the flexural strength of the joints. At 15 wt.% MgO content,the welded joints have the greatest flexural strength of 257 MPa, the relative density increases to 96%, the acid resistance of welds is 1.43 times of that of the alumina liner, the alkali resistance is 0.67 times of that of alumina liner. In conclusion, the joining with Al₂O₃-SiO₂-MgO solders obtains more ideal connection strength, density and alkali resistance than with Al₂O₃-SiO₂ solders.