Study On Microstructure And Mechanical Properties Of Nickel-based Single Crystal Superalloy By Laser Additive Manufacturing

Single crystal(SX)blades are often worn out,cracked,and locally ablated due to abrasion,high-temperature gas erosion,etc.,and can no longer be used.The tip repair work of SX blades has always been the short board in China.Aiming at the problem of microstructure control in the repairing process of SX blade tip,this paper uses pulsed laser additive manufacturing technology,combined with experimental exploration and theoretical simulation,to study the microstructure control of laser additive manufacturing of SX superalloys.The tensile property and stress rupture property were characterized.The main findings are as follows:(1)Aiming at the problem of missing pulse laser additive manufacturing process codes,this paper proposes a modular design method for pulse laser additive manufacturing code generation.The pulse laser additive manufacturing code generation software is composed of four modules:line segmentation module,G-code formatting module,file I/O module and external data transmission interface module,realizing single point code,thin wall code,cuboid code and conversion code generation function.The software uses convenient and quick visual interface program programming.In the single-point code,the pulse parameters and the layer thickness are coupled to make the laser pulse width and period match the Z-axis movement.In the thin-wall code,the software sets the X-axis direction,the Y-axis direction and the thin-wall in any direction.The software automatically adjusts the input overlap rate,thin wall length and laser beam diameter.In the cuboid code,the software divides the cuboid with vertical X-axis and Y-axis line segments;the overlap ratio in the X-axis direction and the Y-axis direction will be adaptively adjusted according to the rationality of the parameter settings.In the conversion code,input the pulse laser related parameters,the continuous laser additive manufacturing code can be converted into the pulse laser additive manufacturing code,and the overlap rate can be adjusted adaptively;(2)Aiming at the problem of microstructure control in pulsed laser additive manufacturing,this paper conducts a systematic study on the evolution of microstructure formation through laser experiments and numerical simulations.A numerical model of primary dendritic arm spacing(PDAS)during pulsed laser remelting of nickel-based SX superalloys is established.The numerical model combines the theoretical PDAS model with the temperature field calculation model.The calculated PDAS is in good agreement with the experimental PDAS.PDAS increases with the increase of laser power(P)and pulse width(t);the average temperature gradient(G)and average dendrite growth rate((?))increase with the increase of P and t.The cladding experiment quantitatively analyzed the effects of laser power(P),pulse width(t)and powder feed rate(m)on epitaxial growth and cladding efficiency.In order to visually express the influence degree and trend of each process parameter on the index,a radar chart reflecting the influence degree and trend of each process parameter on the corresponding index is established.The increase of P,t and m increases the size of the laser cladding layer,and the effects of t and m are significant.Low P,t and m are conducive to dendrite epitaxial growth.In order to ensure the epitaxial growth and repair efficiency in the actual pulsed laser additive manufacturing process,m must be adjusted first.In order to ensure the width of the cladding layer,P or t should be adjusted first.On this basis,a set of process parameters for repairing DD432 haa been optimized(P:2000 W?2200 W,t:0.16 s,m:11 g/min);(3)Using pulsed laser additive manufacturing process,a SX superalloy without cracks has been successfully prepared on a SX substrate,and the structure of the repaired superalloy is characterized.The primary dendrite spacing(17.2 ?m),secondary dendrite spacing(3.3 ?m),porosity size(3.2 ?m)and ?' size(39.9 nm)of the deposited area are all smaller than those of the substrate area.The porosity fraction of the deposition area(0.19%)is higher than that of the matrix area(0.11%);the initial cladding layer has a fast cooling rate,which inhibits the formation of(?+?')eutectic.As the number of cladding layers increases,the cooling rate of high-cladding layers gradually decreases,resulting in an increase in(?+?')eutectic.Due to the existence of the heat affected zone(HAZ),the(?+?')eutectic is partially dissolved,and the(?+?')eutectic at the bottom of the Nth layer is more than that on the top of the N+1th layer,4)The tensile properties of the SX by pulsed laser additive manufacturing are analyzed.The results show that under the condition of 660 to 760?,the fracture mode of the sample is cleavage fracture mode,and the coarse carbide in the SX substrate area cracks,which provides a crack propagation path and causes the substrate area to fracture preferentially.At this time,the movement mechanism of dislocations is mainly shear type.Under the condition of 900 to 1100?,the fracture mode is microporous aggregation fracture.The eutectic and pores in the cladding zone(?+?')deteriorate the high-temperature performance,and the cladding zone preferentially fractures.At this time,the main mechanism of dislocation movement is that Orowan bypasses,and a large number of irregular dislocation networks are formed;There is no fracture at the interface between the base area and the cladding area,which is related to the absence of the coarse MC carbides in the base area and the(?+?')eutectic of the high cladding layer.(5)Under the condition of 1000 ?/280 MPa,the stress rupture property of pulsed laser additive manufacturing and cast DD432 SX samples after standard heat treatment are studied.The average creep life of pulsed laser additive manufacturing samples(65.4 h)is longer than that of cast samples(59.1 h).The pulsed laser additive manufacturing DD432 SX sample has fine carbides,small dendrite arm spacing,and small component segregation;the pulsed laser additive manufacturing sample and the cast sample have the same mean ? channel thickness(288 nm).However,the width of the ? channel between the interdendritic zone and the core dendrite zone in the cast sample is not uniform;the topological close packing(TCP)phase is formed in the cast sample,but no TCP phase is observed in the repaired area;the cast sample The carbides in the matrix fracture or separate from the matrix,forming cracks,while the carbides in the pulsed laser additive manufacturing samples hardly fracture or separate from the matrix.