Study On Key Technology For Fabricating Multi-directional Pipes Joints Of High Buildings By Wire And Arc Additive Manufacturing

Multi-directional pipes joints of high buildings(MDPJHB)is the key component of building steel structure,which is usually composed of sevel cylindrical or box pipes.The orientations and sizes of pipes are usually different,and they are connected with each others by spatially intersecting.The structure of MDPJHB is very complicated.MDPJHB is generally used to withstand the forces from different directions,and bear the loads for long periods of time.The strength and impact toughness of MDPJHB must be high.Nowadays,the mains methods to fabricate MDPJHB are welding and casting.However,due to the structural characteristics and mechanical properties requirements,it is difficult to manufacture high-quality and high-performance MDPJHB by welding and casting.Wire and arc additive manufacturing(WAAM)selets arc as the heat resource to melt metal wire and fabricates the component through depositing metal layer by layer.WAAM is an effective method for fabricating MDPJHB.In this paper,WAAM is used to manufacture MDPJHB with strength of 460MPa.The five aspects of research have been systematically studied,including the development of special wire materials,the processing optimization of space surface depositing,the microstructure transformation of high-strength low-alloy steel,Zr alloying and the forming of 9-directional pipe joints.The main research results are as follows:Based on the chemical composition and mechanical properties of 460 MPa steel,and combined with physical and chemical metallurgical of WAAM,the special metal wire is developed.The Mg powder is added into the metal wire to stabilize the arc.Under the different depositing parameters(current of 140?220 A and voltage of 16?20 V),the arc and droplet transfer of metal wire are stable,the splash rate is low(?2%),and the forming quality is excellent.The tensile strength of depositing metal is 659.7 MPa,and the impact toughness at room temperature is 141.7 J.The mechanical properties satisfy the requirement of 460 MPa steel.Based on the single pendulum theory and mathematical model of space surface,the curves of space surface could be decomposed into the circumflex line in X-Y plane and trigonometric function curve in the X-Z or Y-Z plane.The force model of molten pool is analyzed in different conditions:plane and curved surface.Combined with theoretical calculation and experimental verification,it is can be concludes that when the arc gun offset angle(₂)and the forming position(inclination angle₁)satisfies the relationship:sin(₂-₁)=0.06sin₁,the forming quality is good.At the same time,the influence of processing parameters on the accuracy of space depositing have been studied.The microstructure of depositing metal could be divided into four parts along the direction of depositing,which are solidifaction zone,complete austenitizing zone,partial austenitizing zone and tempering zone.Each part of depositing metal will undergo the four stage.The solidification zone is solidified from the molten pool,which is characterized by typical columnar crystals.After heated to over 1100?,the solidification zone transforms to be the complete austenitizing zone,and the typical columnar crystals are difficult to be observed in the complete austenitizing zone.When heated to the temperature between Ac1 and 1100?,the complete austenitizing zone transforms to be the partial austenitizing zone,and the crystal characteristics of the partial austenitizing zone are equiaxed grains.After several thermal cycles with peak temperature of 600?,the partial austenitizing zone transforms to the tempering zone,where the crystal characteristic has been fully equiaxed grains.From the solidification zone to the tempering zone,because of the precipitation of composite inclusions(Zr O₂·Al₂O₃·Mn O·Si O₂·Mn S)and the presence of ferrite nucleus,the contents of pro-eutectoid ferrite(PF),ferrite side plate(FSP)and acicular ferrite(AF)decreases,whereas the contents of fine grain ferrite(FGF)increases.In addition,the average size of grains decreases from 75?m to 20?m.With the addition of Zr,a large amount of composite inclusions will form in the depositing metal,and the core of composite inclusions is Zr O₂.The composite inclusions could refine the size of inclusions,and enhance the stablity of inclusions.Under the reciprocating heating of WAAM,the composite inclusions can effectively hinder the growth of prior austenite grains and induce the nucleation of AF.The mechanism of inducing AF mainly includes three forms:inclusion-inducing nucleation,ferrite-nuclei inducing nucleation and sympathetic nucleation.There are a large amount of AF formed in depositing metal.The formation of AF will refine the grain size and improve the mechanical properties of forming component.When the content of Zr is0.015 wt.%,the content of AF is at most 62%,and the average size of the depositing metal is at least 12.3?m.Compared with the depositing metal without Zr,when the content of Zr is 0.015 wt.%,the tensile strength of depositing metal has increased by36%,and the impact toughness at room temperature increases by 27%,and the difference of mechanical properties in vertical and horizontal are within±5%,respectively.Based on the structural characteristics of intersecting pipes,a surface layer slicing method is proposed,and the different path planning methods are used to fabricate different intersecting structures.For the saddle-shaped surface,the equal thickness offsetting is used to form rotating approaches;For the non-complete intersecting curved surface with inflection point,the zigzag offsetting is used to form raster approaches.The 9-directional pipe joint is manufactured by WAAM.The dimensional error of 9-directional pipe joint is not more than±0.5 mm,and the intersecting angle error value doesn`t exceed±0.4°(24?).Compared with the casting,the tensile strength of 9-directional pipe joint increases by 13.8%,and the impact toughness at room temperature increases by more than 100%.Rare defects such as pores and cracks were found,and the mechanical properties of 9-directional pipe joint are excellent.