Metallurgical Cracking Behavior And Strengthening Mechanism Of IN-738LC Alloy Prepared By Laser Solid Forming

Laser solid forming(LSF)is a typical additive manufacturing technology with synchronous material feeding.It can fabricate the high-performance and complex structural parts with near net shape.But it still faces challenges in the field of superalloys.Taking??-strengthening superalloys as an example,they have excellent high-temperature properties and are widely used in industrial applications.However,since?'forming elements are also forming elements of low-melting eutectic,the weldability of??-strengthening superalloys is relatively poor when the content of those elements is high.Thus,these kind superalloys are easily cracking under the high stress during LSF,which limits the excellent performance of the LSFed parts.IN-738LC alloy is a typical high(Al+Ti)content superalloy with excellent high temperature corrosion resistance and creep properties,and is widely used for the material of aircraft engines and gas turbines.The precipitates in IN-738LC alloy basically cover the main phases(carbide,?-?'eutectic,boride and?'phase)in the??-strengthening superalloys,which exhibits certain representativeness.The in-depth study of this alloy can promote the understanding of the cracking behavior and microstructural evolution mechanisms of these kind superalloys,and thus lay an important scientific foundation for the final realization of the control and optimization of the microstructure and mechanical properties.Therefore,this paper took IN-738LC alloy as the research object,and studied the phase and microstructure formation behavior and the metallurgical cracking behavior during LSF.On this basis,this paper further investigated the effect of preheating on the cracking and microstructure formation and evolution mechanism during LSF,and revealed the relationship between the mechanical properties and microstructure of LSFed IN-738LC alloy.The main findings of this paper are as follows:(1)Metallurgical cracking behavior during the LSF of IN-738LC alloy.Based on the local composition measurement,and combining the calculation of the Giovanola–Kurz(GK)model and the Scheil model,the solute segregation under the near rapid solidification of the molten pool was found to be the main reason for the formation of the continuous?-?'eutectic at the grain boundary(GB)of the LSFed IN-738LC alloy;the subsequent laser deposition could cause the melting of the?-?'eutectic in the previous layer,and then forming a continuous GB liquid film.The thermodynamic calculation results showed that a small amount of B(0.001-0.003%)can significantly reduce the liquation temperature of the GB(about 20?),as well as the?-?'eutectic forming elements enriched at the GB.However,the?'phase of the LSFed IN-738LC was completely coherent with the?matrix,which was found can inhibit the constitutional liquation of the GB?'phase during rapid heating.The crack propagation mechanism was studied,the results show that:(i)there were both liquation cracks and solidification cracks in the LSFed IN-738LC,and the solidification cracks in the subsequent layer can be directly formed by the epitaxial expansion of the liquation cracks in the heat affected zone(HAZ)in the former layer;(ii)the final solidification temperature of the high-angle GB is lower,and its wettability is better than those of the small-angle GB,which lead to the crack always forming and propagating along the high-angle GB.The heat flow direction on the left and right sides in the overlapping area was usually complicated,which is easy to cause the formation of the high-angle GB during the solidification.Besides,the residual stress in the overlapping zone was found to be about300 MPa higher than that in the inner-track region,which means the cracking driving force of the overlapping zone was high.(2)The effect of preheating on the cracking behavior and microstructure evolution of LSFed IN-738LC alloyThe non-preheated deposition specimens mainly consisted of epitaxially grown columnar grains,and the size of the?'phase from bottom to top gradually increased while the trend was opposite when adding preheating.With the increase of preheating temperature,the size of?'phase increased,the columnar-to-equiaxed transition(CET)tends increased during the solidification of the molten pool,and the size of?'phase in the whole specimen all increased;also,the dendritic spacing,microsegregation and the size of carbide and?-?'eutectic increased;the morphology of the carbide changed from rod to block.The microhardness at the middle and top first increased and then decreased and the microhardness at the bottom of the specimens gradually decreased,which result from the?'strengthening and the dilution by the substrate.A relationship map between the cooling rate of the induction heat source and?'phase morphology was established,where with that cooling rate decreased,the?'phase morphology would change:spherical?cubic?cube with concave?octet of cube?solid-state dendritic.The residual stress in the LSFed specimen preheating at 700? was lower(about 300MPa)than that in the specimen without preheating,and small crack-free specimens with multi-layer and two-track deposition can be obtained when preheating at 700?.But to obtain a crack-free block specimen with multi-layer and multi-track deposition,the preheating temperature needs to be raised to 1050?.The preheating parameters which can effectively form a crack-free block specimen were obtained:1050?(preheating temperature)and40?/min(cooling rate of induction heat source).(3)Microstructure and strengthening mechanism of the LSFed IN-738LC alloy with high temperature preheatingWhen the preheating at 1050?,the as-deposited and heat-treated specimens were all mainly composed of epitaxially grown columnar grains.The?'phase varied in size,but the difference in radius was less than 40 nm in the as-deposited sample.A double-sized?'phase(large-sized near-cubic and fine spherical)with a difference in radius larger than 130 nm appeared in the incompletely solution-treated specimens due to the further coarsening of the relatively larger?'phase in the as-deposited specimen.That coarsening rate of the relatively larger?'at 1120? and 1070? were about 0.17 nm~3/s and 0.068 nm~3/s,respectively.The?'phases with a relatively uniform double-size can be obtained when solution treated at 1070?.The solid solution rate of the small-sized?'phase(r?46 nm)was found to be 1.6 times that of the large-sized?'phase(r?83 nm)when solid solution at 1160?.After that complete solution treatment,the sample exhibited the single-size small spherical?'phase.The columnar grain width and the yield strength(YS)and ultimate tensile strength(UTS)of the standard heat-treated(SHT)specimens followed the Hall-Petch relationship.The relationship between the strength and the?'phase size of the IN-738LC alloy was established,which was concluded that dislocation cutting was the mainly interaction mechanism between the?'phases and the dislocations at the room temperature.A good room temperature strength and plasticity matching can be obtained when heat treated at 1070?/2h/air cooling+845?/24h/air cooling.After the hot isostatic pressing(HIP),the UTS and elongation(EL)of the specimen were further increased due to the increasing number of the near-cubic?'phases with good deformation ability,and the deformation of a certain amount of the fine spherical?'phases.The room temperature tensile properties of the LSFed IN-738LC exhibited anisotropic.The YS and UTS of the as-deposited specimens in the horizontal direction are higher than that in the vertical direction,which was mainly attribute to the presence of the columnar grains.After SHT,the UTS in the vertical direction was found to be slightly higher than that in the horizontal direction,which was mainly attribute to the deformation strengthening of the near-cubic?'phase.When deformed at 850?+365 MPa,the dislocation cutting,Orowan bypass,and dislocation climbing?'particle can all happen,while the former two were dominant.After the solution and aging treatment,continuous Ni-Cr-Al-Ti brittle phase would precipitate along the GB,which was mainly result from the dissolution of the GB?-?'eutectic during the solution treatment.That GB brittle phase was found to be harmful to the specimen plasticity,and it can easily become the crack source during the high temperature deformation process,which was the main reason for the low stress-rupture life.