| Underwater friction taper plug welding(FTPW)belongs to solid phase welding.It has the advantages of being unaffected by water depth,low heat input,being green and environmentally friendly,and easy to realize automation.It is considered to be one of the most promising underwater joining and repairing technologies.Based on the underwater friction taper plug welding equipment independently designed and developed by Tianjin University,some basic scientific issues related to underwater friction taper plug welding for X52 pipeline steel,such as the formation of FTP weld,microstructure characteristics and toughness mechanism,were studied in this thesis by welding experiments,experimental observation,theoretical analysis and numerical simulation,to provide theoretical and practical basis for the industrialization of underwater friction taper plug welding.The innovative research work of this thesis includes:(1)The thermal-mechanical coupling finite element models of underwater FTPW process were established.The thermal cycle process and material flow characteristics in the welding process were studied by a combination of experimental measurement and simulation calculation.The peak temperature of the underwater FTPW joint can reach 1300℃,and the temperature gradually increases from weld center to the bonding zone,and the temperature gradually decreases from the bonding zone to the heataffected zone.The closer to the upper surface,the lower the temperature of the plug center;the closer to the plug edge,the faster the heating rate and the higher the peak temperature.The bottom of the weld is heated first,the middle position is followed,and the top is the last.As the axial force and rotational speed increase,the welding time is shortened and the temperature of the joint increases.In the underwater FTPW process of through-hole type,the plug quickly contacts with the side wall of hole,and the temperature of the substrate side rises faster than that of the blind hole type.The maximum cooling rate of the joint is greater than 200°C/s.The equivalent strain of the blind-hole type underwater FTPW weld mainly occurs at the bottom and edge of the plug.Except for the obvious equivalent strain at the flash on the upper surface of the substrate,the equivalent strain in other areas is very small.The bottom end of the plug during the underwater FTPW process with through-hole substrate is squeezed and has a larger equivalent strain.The bottom and surface of the base plate are deformed obviously and the equivalent strain in the middle area is smaller.The plug in the through-hole type underwater FTPW process only frictionally presses against the side of the substrate to form a ferrite band to connect the plug and the substrate.(2)The existence form,forming mechanism of welding defects and joint connection mechanism of underwater FTPW joint were studied by welding experiments.It is found that the welding defects of blind-hole type underwater FTPW joints mainly appear in the fillet transition zone and the friction interface,while the welding defects of the through-hole type FTPW joints exist in side wall and the friction interface.The welding defects of the fillet transition zone are related to the material mechanical properties of plug and welding parameters.The welding defects of side wall depend on the stress state of the weld.The small axial force,large cooling rate and the thermal expansion and cooling shrinkage are the main causes of defects at the friction interface.A metallurgical connection under the combined action of pressure and frictional heat is formed at the bottom of the blind hole type underwater FTP weld with a larger axial force.The plug and the hole side are connected by a thin plastic metal layer.(3)The microstructure characteristics of underwater FTPW joints were studied by OM,SEM,TEM methods and so on.The influences of welding parameters(axial pressure and welding speed),plug material,post-weld heat treatment and continuous welding on the microstructure of underwater FTPW joints were discussed.The weld center is mainly composed of hardened martensite,and the bonding zone is composed of a band of fine ferrite.Due to the difference in the severity of thermal cycling and the plastic deformation at different positions of the weld,the microstructure of the underwater FTPW weld is not uniform.Compared to the rotational speed,the axial force has a huger effect on the joint microstructure.The impact of plug material on the microstructure of the weld is obvious and the post-weld heat treatment with appropriate temperature can improve the microstructure uniformity of the weld.The seam formed by friction taper stitch welding(FTSW)is affected by the thermal cycle of the subsequent welding process,leading to the microstructure of the weld center is mainly tempered martensite.Due to the restriction of the plug hole,the overheated metal formed in the FTPW process becomes a part of the weld and the weld contains coarse microstructure.Therefore,the concept of "closed" mode friction welding is put forward in this thesis.(4)The inhomogeneity of the mechanical properties of the underwater FTPW joint was studied in detail by using microhardness,tensile and bending tests.The hardness distribution curve of the blind hole type underwater FTPW joint in the thickness direction is displayed as "M" or "n" shape.The hardness at edge of plug is as high as 400 HV,showing an obvious hardening trend,and the bonding zone is a softened zone with a width of tens of microns.The underwater FTSW seam center has a more uniform hardness distribution than the single-hole underwater FTPW weld center.The upper part of the blind-hole type underwater FTPW weld has a better mechanical performance than its bottom.The through-hole type underwater FTPW weld is also hardened and the tensile strength is in the range of 344～590MPa.Post-weld heat treatment significantly reduces the degree of heterogeneity.(5)Based on the requirements for toughness of marine engineering structural parts and the characteristics of the underwater FTPW process,the toughness and fracture behavior of underwater FTPW joints were investigated by notch impact test and three-point bending test.It is shown that the impact absorbed energy of underwater FTPW weld is in the range of 20～65J.The crack tip opening displacement(CTOD)of the weld is below 0.15 mm.The fracture toughness of weld center and bonding zone is far smaller than that of base material.The fracture toughness of the underwater FTSW seam center is better than that of the underwater FTPW joint center.(6)The methods of improving the toughness of underwater FTP welds were studied by adjusting the welding parameters(welding speed and axial force),changing plug material and post-weld heat treatment and the industrial application of the underwater FTPW technology was discussed.Within the range of equipment adjustable welding process parameters,the welding parameters have little effect on the fracture toughness of the joint.After being heat treatment at 800℃ for 2h,the CTOD value of the weld center reaches 89% of the base metal showing the toughness of the weld center can be significantly improved.Due to the unique connection mode of the underwater FTPW weld and the inclusions in the bonding zone,it is difficult to significantly improve the fracture toughness of the bonding zone.The application of underwater FTPW technology to carbon steel and low alloy steel has limitations.For stainless steel(similar to 316L)that does not undergo phase transformation at high temperatures and has high strength,the underwater FTPW technology has broad application prospects. |
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