Research On The Mechanism Of Diode Laser Joining Al₂O₃ Ceramic Liners From Ceramic Lined Composite Pipes

The alumina ceramics owe uncomparable application value in many fields such as aerospace,petrochemical industry,metallurgy and chemical industries.However,the connection problem of ceramic composite pipes,especially for Al₂O₃ ceramic liners,has restricted the development of alumina-lined composite pipes for oil and gas transportation.The joining method in a furnace restricted by temperature and dimension cannot be qualified for the joining of Al₂O₃ liners.Under the uneven distribution of welding temperature field,ceramics are prone to thermal cracking because of the brittleness and poor heat resisitance.Therefore,this paper innovatively carried out the research of diode laser joining Al₂O₃ ceramic liners.The research work in this paper has positive theoretical and practical value in promoting the development of "The Belt and Road"in China's petroleum industry.The main research contents are as follows:?1?The experimental and theoretical method of Al₂O₃ joining based on diode laser was proposed.XRD and EDS results indicated that the crystalline phases of liners were Al₂O₃,SiO₂,Al₈₂Fe₁₈ and a few FeO·Al₂O₃.The mullite filler with the quality ratio of 70wt.%Al₂O₃-30wt.%SiO₂ was determined as the initial filler.The influence of fillers width on morphology of weld was studied.When the width of fillers was less than 2mm,macroscopic cracks appeared in the welding seam.When the width of fillers was 2mm-2.5mm,no macroscopic cracks but defects such as pores were found in the welding seam.That was because part of the laser energy directly irradiated on the brittle liners,thus leading to defects in the weld zone.When the width of fillers was the same as the laser spot diameter?3mm?,laser irradiation was completely on the surface of solder,and it was found that the weld zone was free from pores and macro cracks.The effect of laser power and joining speed on the weld depth and weld width was investigated.With the increase of laser joining heat input,weld penetration increased slowly and then sharply.The experimental results indicated that good morphology of the weld can be formed with the fillers width of 3mm,laser power of 700W and welding speed of 1mm/s.?2?The transient multiple physical model of diode laser joining Al₂O₃ ceramic liners was built by Multiphysics Comsol.The"sandwich"structure model with mullite filler was established.The load of heat souce on the model and the boundary conditions of the model were analysed.The coupling of temperature and stress field was carried out.The thermodynamic properties of the Al₂O₃ liners and fillers were analyzed and tested.The effect of fillers width,power of laser irradiation,speed of joining on the penetration of weld and stress of seam zone was primary studied.Numerical results showed that the increase of fillers width would effectively reduce the thermal stress of the seam zone.With the increase of laser power or the decrease of joining speed,the temperature and thermal stress of the seam zone increased gradually.Good joining parameters could be obtained by the analyze and comparion of simulation results.?3?For the first time,the ternary system Al₂O₃-MgO-SiO₂ was introduced as fillers for diode laser joining Al₂O₃ liners.The effect of MgO content on properties of fillers were studied.Experimental results showed that,the addition of MgO to mullite fillers could promote grains growth,and increase the density of fillers and reduce porosity.The crystalline phase of the fillers moved away from mullite and tended to spinel and sapphire phase region gradually,and Mg₄Al₁₀Si₂O₂₃ and MgAl₂O₄ gradually increased with the increase of MgO content.When the content of MgO was 15wt.%,the relative density of fillers reached to 93.4%,the laser absorption rate of fillers increased to 14.82%,the wetting angle of fillers on the liners achieved29°,thermal expansion coefficient of fillers from room temperature to 1000?was about 8×10^-6/?which was close to that of liners(8.0-8.3×10^-6/?),and the flexural strength of the filler was about 100MPa.?4?The effect of MgO content in Al₂O₃-MgO-SiO₂ fillers on joining performance was studied.Best composition of fillers was found by changing the addition of MgO.Mutual diffusion and substitution reaction in the process of joining were significantly strengthened with the addition of MgO.The eutectic spinel phase?Mg,Fe?Al₂O₄ formed in the joint could improve the quality of joint.The weld penetration maintained and then decreased sharply.Notably,with 20 wt.%MgO addition,the depth of fusion is only 3.42mm,which was cause by the high melting point?²000??and relatively low absorbance of laser?⁸.5%?.The joining performance corresponding to the fillers with 0-15wt.%MgO content was studied.It was found that with the increase of MgO content,the crystallinity of the joint increased gradually,and the grain boundary in the joint was gradually blurred,thus increasing the density of the joint.It was demonstrated that joints corresponding to fillers composed of 70wt%Al₂O₃-15wt%MgO-15wt%SiO₂ owed optimal performance,and the compactness of joints reached to 96%.The flexural strength of joints reached to 168MPa which was over 80%of the Al₂O₃ liner?200MPa?.The weight loss of joint interlayer in HCl?pH=0.45?for 24 hours was 0.62mg/cm² which was 0.16 mg/cm² lower than that of the liner.Besides,the softening temperature of joint interlayer exceeded to 1500?.As a result,the reliability of Al₂O₃-lined joints could be maintained during the sequential process of arc welding for outer steel pipe layers and the corrosion of pipe layers could be effectively avoided.