Studies On Formation Mechanism And Control Methods Of Cracking In Laser Additive Manufactured Inconel 718 Alloy

Due to the excellent mechanical property and antioxidation at high temperature,nickel based superalloy is widely used in the fields of aerospace,energy,petroleum,etc.While the manufacturing of nickel based superalloy components is very costly and therefore,periodic inspection and remanufacturing become the necessary mesures to extend the lives of the nickel based superalloy components.Laser additive manufacturing has widely been used in the remanufacturing of nickel based superalloy components because of advantages of low heat input,low deformation and well metallurgical bonding with substrate,etc.Unfortunatly,the cyclic rapid heating and cooling during laser additive manufacturing produce extremely high stress in the alloy,resulting severe cracking formation.As a result,the application of laser additive manufacturing in the remanufacturing of nickel based superalloy components is restricted greatly.Therefore,the hot cracking problem in the laser additive manufacturing of Inconel 718 alloy was studied systematically in this paper and a series of technical methods for hot cracking inhibition were also developed.The microstructure of laser additive manufactured Inconel 718 alloy consists most of fine and long columnar dendrites,as well as a small amont of equaxed dendrites at top of the alloy.The segregation phases of the alloy are NbC phase and Laves phase by eutectic reaction.During the multi-layer cladding process,the interdendritic low melting eutectic compounds can be remelted and re-segregated several times by the heat of laser.The cracks in the laser additive manufactured Inconel 718 alloy includ a small quantity of solidification cracking formed at top of the clad and also the hot cracking produced by the liquation or partial liquation of interdendritic region in the heat affected zone.During the multi-layer deposition,hot cracking in the heat affected zone normally initiates from the fusion lines and then propagates larger gradually by the increase of the deposition layers.Throuth the finite element simulation,the evolution of temperature field and stress field during laser additive manufacturing was analyzed.The results of temperature field simulation show that,althrough the cooling rate of laser additive manufacturing is very high,the temperature in the bottom of the first layer can reach 1000 °C in the continuous fifth layer's deposition attributed to the accumulation of heat.It means that the interdendritic region can still be liquated or partially liquated,resulting the formation of hot cracking.The results of stress field simulation show that the stress inner the first layer clad is compressive stress;when the deposition increases to 3 layers,the stress in the first layer clad turns to tensile;further increases the deposition layers,the tensile stress region expands upward gradually.The effects of laser scanning speed and heat input on the susceptibility to hot cracking in the laser additive manufacturing were analyzed.The increase of laser scanning speed increases the temperature gradient of molten pool and as a result,it generates higher thermal stress in the solidification.The increase of heat input extends the heat affect zone and also prolongs the high temperature time of liquation film in the interdendritic region.Consequently,under the same laser heat input,hot cracking increases with the increase of laser scanning and under the same laser scanning speed,hot cracking increases with the increase of laser heat input.The effect of directional base cooling on the growth of dendrites and the susceptibility to hot cracking in the laser additive manufactured Inconel 718 alloy was studied.The results show that the directional base cooling on the back of 3 mm thin substrate can increase the cooling rate from 2700 K/s to 3200 K/s at the initial stage,and also increase the cooling rate from 250 K/s to 500 K/s at the stable stage.The directional base cooling can effectively improve the crystal orientation of the alloy,and at the same time,reduce the 85% formation of hot cracking.The effect of grain boundary misorientation on the hot cracking susceptibility of laser additive manufacturing was studied.The decrease of grain boundary misorientation reduces the stability of liquation film on the grain boundary in the solidification.The local stress concentration on the grain boundary liquation film in the last stage of solidification can be avoided and the hot cracking formation can thus be depressed.The effect of laser input angle on the growth of dendrites and the susceptibility to hot cracking in the laser additive manufactured Inconel 718 alloy was studied systematically.When the laser beam turns clockwise to the laser scanning direction,most of the laser energy focuses on the front of molten pool,as a result,the lateral heat dissipation in the molten pool increases.On the one hand,this improves the growth of secondary dendrites in the solidification,producing many regular “cross bands” microstructure in the clad.The interdendrtic bonding is improved and therefore,the resistance to hot cracking is increased.On the other hand,the increase of lateral heat dissipation decreases the heating effect in the region beneath the molten pool and then narrows the heat affected zone.On the contrary,when the laser beam turns anticlockwise to the laser scanning direction,most of laser energy focuses on the back of molten pool,as a result,the vertical heat dissipation in the molten pool increases.On the one hand,this inhibits the growth of secondary dendrties and then weakens the interdendritic bonding.On the other hand,the increase of vertical heat dissipation improves the heating effect in the region beneath the molten pool and then extends the heat affected zone.Consequently,hot cracking decreases 65% and 56% when laser beam turns clockwise to 10° and 20°,respectively.Whereas,hot cracking increases 51% and 21% when laser beam turns anticlockwise to 10° and 20°,respectively.The carbon nanotube reinforced laser additive manufactured Inconel 718 composite alloy was fabricated successfully.Through the electroless plating treatment of nickel on the surfaces of carbon nanotube,the tube-like structure of carbon nanotube can be silightly mantainted.At the same time,the tube of carbon nanotube can be patrtially or fully opened,forming the structure of graphene nanosheet;the tube of carbon nanotube can be collapsed and also inter-bonded with the carbon nanotube and graphene nanosheet in the ambient region,forming the structure of carbon nanoribbon;the tube of carbon nanotube can also be curled and transformed into spherical diamondlike nano particle.The incorporation of these carbon nano structure increases the strentgh of the alloy and then reduces the local interdendritic strain under the same condition of thermal stress.It also improves the interdendritic bonding and stress transfer,avoiding local stress concentration formation in the interdendritic region.At the same time,through the bridging and pull-out mechanism of carbon nanotube,graphene nanosheet and carbon nanoribbon,the intrinsic resistance of interdendritic liquation film to thermal strain can also be improved.In the consequence,hot cracking in the alloy can be depressed 78% and 90%,and the tensile strength of the alloy can also be improved 25% and 16.7% attributed to the addition of 5 wt.% and 10 wt.% nickel plated carbon nanotube,respectively.Through the optimization of laser input anlge,modification of laser scanning path and addition of nickel coated carbon nanotube,large multi-pass and multi-layer crackfree bulk Inconel 718 alloy was manufactured successfully,offering more effective technical supports on the control of hot cracking during the remanufacturing of nickel based superalloy components by laser additive manufacturing.