| .With the development of economy and society,our country's steel structure buildings are developing towards large-scale,complex and individualized development.Steel structure buildings are widely used due to their high structural strength,light weight,energy saving and environmental protection.Building multi-directions steel node,as a key component connecting different shapes of steel structures,play a pivotal role in steel structure buildings.20 Mn steel has good strength,toughness and process performance.It is widely used in the manufacturing of multi-directions steel nodes.The manufacturing methods of 20 Mn steel building multi-directions steel node mainly include casting and welding.The multi-directions steel nodes manufactured by these two methods may have defects,which make steel structure buildings have safety hazards.WAAM technology is an emerging advanced manufacturing technology,which uses arc heat to melt metal wires,and then build complex metal components layer by layer according to a preset path.WAAM technology has the advantages of high efficiency and excellent mechanical properties of formed metal components.The use of WAAM methods to manufacture multi-directions steel node can effectively avoid the defects of welding and casting multi-directions steel node.WAAM technology is an more effective method for multi-directions steel node.Based on the chemical composition and mechanical performance requirements of20 Mn steel,a flux-cored wire that can be used for WAAM of multi-directions steel node has been developed.The metal deposited by the developed flux cored wire is flat on both sides under different process specifications and swing conditions,and its surface is flat,without cracks,pores and other defects,and has good formability;the current fluctuation range during the accumulation process does not exceed ±3.5%,the voltage fluctuation range does not exceed ±4.0%,the fluctuation is small,the arc is stable,and the spatter rate is less than 2.5%;the microstructure of the deposited metal is fine-grained ferrite and pearlite.The tensile strength,elongation and 20 ? impact energy are 512 MPa,25% and127 J along the depositing direction,and 519 MPa,24% and 134 J along the direction perpendicular to the depositing direction.According to the temperature field and thermal cycle characteristics of oscillating wire arc additive manufacturing,the metal deposited in the oscillating process is divided into 4 areas: SZ,OZ,NZand TZ.The SZ is obtained by the direct solidification of the liquid metal.During the solidification process,the liquid metal forms columnar crystals through epitaxial crystallization.The massive ferrite and side lath ferrite precipitate at the austenite grain boundaries,and acicular ferrite nucleates and grows radioactively with composite inclusions as the core.The SZ transforms into the OZ under the action of thermal cycling with a peak temperature greater than 1100?.Residual ferrite nuclei,the high-density dislocation field and the induced nucleation mechanism of the ferrite grain boundary jointly promote the ferrite nucleation and refine the grain of the OZ.The OZ transforms into the NZ under the action of thermal cycling at the peak temperature Ac1?1100?.The smaller size of the original austenite grains is decomposed into FGF and P,and under the action of dynamic recrystallization,RG are formed,which makes the grain of the NZ more refined than the OZ.The NZ is transformed into the TZ under the action of multiple peak temperatures less than Ac1 thermal cycle.Due to the increased dynamic recrystallization amplitude,more RG is formed,moreover,composite inclusions and nano-level VC hinder the growth of grains,making the grain of the TZ smaller than that of the NZ.From the CCT curve of the deposited metal of the 20 Mn steel flux-cored wire,it can be seen that in the cooling rate range of 0.1-10?/s,the deposited metal undergoes phase transformation of ferrite and pearlite,and as the cooling rate increases,the phase transformation temperature decreases.Taking the oscillating process parameters as input,and the forming size of deposited metal as output,a 3D response model between them is established.The established 3D response model is used to optimize the process parameters of the six-directions steel node by oscillating wire arc additive manufacturing.According to the structural characteristics of the six-directions steel node,it is manufactured using the strategies of zone forming,plane slicing,oscillating and offset filling.It is divided into the straight wall cylinder area and the intersecting area.The intersecting area includes two types: two-pipe intersection and three-pipe intersection.Straight wall cylinder areas are deposited by the optimized oscillating process parameter,and the two-pipe and three-pipe intersection areas are filled with contour offset filling and mixed path respectively.The overall average size deviation of the six-directions steel node by the oscillating wire arc additive manufacturing is±1.30 mm.Using finite element analysis and bench expertiment to test the load-bearing performance of the six-directions steel node by oscillating wire arc additive manufacturing,The results show that the maximum stress of six-directions steel node by oscillating wire arc additive manufacturing is 36.7MPa under actual working conditions,which is much smaller than the yield strength of the deposited metal.It is always in an elastic state,has good load-bearing performance,and can meet the requirements of actual use. |
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