The Effects Of Activating Flux On The Properties Of Hardfacing Alloys Deposited With Composite Powder Particles And Solid Wire

Arc additive manufacturing technology（AAMT）,in which powder particles are used as raw materials and are added layer by layer,faces such a challenge as insufficient raw powder available.Although composite powder particles（CPPs）settle a problem of too low alloying element transition coefficient for AAMT,it still encounters such a problem as insufficient powder filling amount due to their composition dilution induced by base metals and solid wire compared to that of flux-cored wire.As an efficient and economic welding method,activated welding has been proved to improve the properties of weld metals with advancing the penetration of weld beads,but there are few reports on their effects on the microstructure ant the properties of arc hardfacing alloys.In view of this,active welding was introduced into the field of hardfacing in this paper on the base of constantly innovating and exploring.The addition style of activating flux was improved and such a new concept as"active composite powder particle","active blocks"were proposed in the paper to advance the transition coefficient of alloying elements and the decrease of composition dilution effects.The researched objects in this paper are as follows:（1）The effects of activing flux on arc morphology and its stability.（2）Effect of activing flux on the microstructure and the properties of high chromium hardfacing alloy.High speed camera and current and voltage sensor were used to investigate the arc shape and electrical signal.The effects of series of activating flux on the microstructure and properties of open arc self-shielded hardfacing alloy were studied by super field depth optical microscopy,X-ray diffractometer（XRD）,scanning electron microscopy（SEM）,energy dispersive spectrometer（EDS）,Rockwell hardness and Vickers hardness tester.With the addition of CaCO₃ into CPPs,the results show that,the arc expands outward,and the arc changes from a conical shape to a flat bell jar one,which significantly increases the powder filling amount of hardfacing alloys and reduces the dilution ratio.Ca CO₃ has no obvious effect on arc current and voltage,but significantly improves arc stability.With the increase of Ca CO₃ doping amount,the bowl penetration of weld bead is eliminated,and the microstructure of hardfacing alloys change from hypoeutectic to hypereutectic.The macro hardness is greatly improved,up to 62.3HRC,in which the matrix is dominated byγ-Fe.It forms a cooperative structure of wear-resistant phase and plastic ductile phase.Their abrasion resistance is significantly improved and the maximum increase at least 2.3times in net.With the addition of Na₂CO₃ into CPPs,the arc"drifts"forward and the root of the arc expands slightly,which enhances the preheating effect of the arc on CPPs and improves the filling amount of the composite powder.The dilution ratio was reduced.The arc length got larger and heat input was advanced,but the arc stability is improved with the doping of Na₂CO₃.The microstructure of high chromium hardfacing alloys changed from hypoeutectic to hypereutectic and the carbides were refined,but the volume fration ofγ-Fe grew up.At 2g Na₂CO₃ per 100g alloy powder their abrasion resistance was improved about 1.87 times in net.With the addition of CeO₂,the arc expands outward and increases the arc length.It led to an increase of powder filling amount and a decrease of dilution ratio.Proper amounts of the cerium element transition to weld metal can effectively refine weld microstructure and promote carbide nucleation.The increase ofγ-Fe content significantly improves their abrasion of hardfacing alloys.At the doped amount of4%CeO₂,their abrasion resistance is best,which is 3.24 times that of the undoped Ce O₂.With the doping of CaF₂ into CPPs,the arc column shrinked,but its root enlarged slightly.It advanced the arc length.It improved the arc stiffness and advanced the melting amount of composite powder particles.The volume fraction of the carbides gradually increased and their wear resistance had improved.When Ca F₂doping amount is too much,the increasing amount of the melt of solid wire,the dilution effect got enlarged.Although their carbide got refined,the abrasion resistance decreased slightly.At the doped amount of 5g Ca F₂ per 100g alloy powder,their abrasion resistance was best,which is 2.22 times that of undoped Ca F₂.The experimental results of active blocks showed that K₂CO₃ on their lateral makes the arc expanded outward and changed from a bell shape to a hemispherical one.The filling amount of powder particles in hardfacing alloys increases and the dilution ratio decreased.With the increase of K₂CO₃ content in the block,the arc stability was improved,but when the K₂CO₃ content is too much,the decomposed gas has an impact on the arc and reduced its stability.The setting of the block and its K₂CO₃ active layer can result into the the macro hardness of high chromium alloys firstly increased rapidly from 55.2HRC to 61.8HRC,and then decreased to 59.5HRC.Their abrasion resistance firstly improved significantly and then decreased.The improvement of their abrasion resistance is mainly related to the comprehensive effects of the microstructure of high chromium alloys from hypoeutectic to hypereutectic.The decreased result subsequently was attributed to the refinement of the carbides.The main wear mechanism of high chromium alloy is micro cutting of abrasive particles.With the introduction of feldspar active layer into the block,the arc expanded obviously,and its shape changed from a typical bell jar shape to a hemispherical one.With the increase of feldspar amount in the block,the arc length increased and the arc stability has been improveds.The setting of the block and its active layer led to a decreaseof dilution ratio and as increase of powder filling amount.However,excess feldspar led to an increase of the melt of solid wire.Also,the macro hardness of high chromium alloys firstly increased rapidly from 55.5HRC to 59.1HRC,and then decreased to 56.9 HRC.Their wear resistance firstly improved significantly and then decreased.The increase is mainly related to an increase of the size and the volume fraction of primary carbides of high chromium alloys.Its subsequent decrease is due to the refinement of the carbides.The above results showed that activing flux have a significant effect on the microstructure and the properties of hardfacing alloys by the function on arc shape and CPPs.A proper addition amount of activating flux can significantly improve the the microstructure and the properties of hardfacing alloys at a very low cost.It makes a significant sense for the advance of utilization rate of raw powers and it meets the requirements of green environmental protection.