Effects of scanning speed on microstructure in laser surface-melted single crystal superalloy and theoretical analysis

doi:10.1016/j.jmst.2017.11.027

Abstract

Abstract:

Scanning speed is a critical parameter for laser process, which can play a key role in the microstructure evolution of laser melting. In the laser melting of single crystal superalloy, the effects of scanning speed were investigated by experimental analysis and computational simulation. The laser was scanning along [71ˉ0] direction on (001) surface in different speeds. Solidification microstructures of dendrites growth direction and the primary dendritic spacing were analyzed by metallograph. Besides, a planar interface during solidification was taken into attention. Experiment results indicated that the primary dendritic spacing and thickness of planar interface decrease with the increase of speed. Through simulation, distribution of dendrites growth velocity and thermal gradient along dendrite growth direction were calculated, and the simulation of dendrites growth direction agreed with the experiment results. Additionally, a constant value was acquired which can be used to predict the primary dendritic spacing. Moreover, according to curve-fitting method and inequality relation, a model was proposed to predict the thickness of planar interface.

Key words: Scanning speed, Laser remelting, Single crystal superalloy, First dendrites spacing, Planar interface

Guowei Wang, Jingjing Liang, Yanhong Yang, Yu Shi, Yizhou Zhou, Tao Jin, Xiaofeng Sun. Effects of scanning speed on microstructure in laser surface-melted single crystal superalloy and theoretical analysis[J]. J. Mater. Sci. Technol., 2018, 34(8): 1315-1324.

Figures/Tables 14

Fig. 1. Schematic illustration of laser moving along the [70] orientation on the (001) surface.

Table 1 Selected scanning velocities for laser surface-melting.

	A	B	C	D
V_b (mm/s)	5	15	25	35

Fig. 2. Schematic representation of cross-sectional microstructure.

Fig. 3. Optical micrograph of cross-sectional microstructure by laser surface-melting with different scanning velocities: (a) 5 mm/s, (b) 15 mm/s, (c) 25 mm/s, (d) 35 mm/s.

Table 2 Measured parameters about microstructure from metallograph.

	A	B	C	D
W_pool (μm)	1296.1	999.0	875.8	773.3
H_pool (μm)	301.7	214.0	176.1	151.5
H₀₀₁ (μm)	123.5	116.5	124.2	124.9
H₀₀₁/H_pool	0.4093	0.5444	0.7053	0.8244
S₀₀₁/S_pool	0.3954	0.4787	0.5886	0.6611
λ₁ (μm)	5.3	4.1	3.5	3.1
T_planar (μm)	9.3	4.7	3.7	3.1

Fig. 4. Metallograph of partial enlargement at the [100] region: (a) 5 mm/s, (b) 15 mm/s, (c) 25 mm/s, (d) 35 mm/s.

Table 3 Calculated parameters about microstructure and thermal condition by simulation.

	A	B	C	D
H₀₀₁ (μm)	130.0	131.3	118.7	102.6
H₀₀₁/H_pool	0.4259	0.6022	0.6714	0.6921
S₀₀₁/S_pool	0.3937	0.5011	0.5492	0.5797
G_blue (×10⁶K/m)	1.182	1.422	1.286	1.263
V_blue (mm/s)	5.052	15.156	25.178	35.393
G_red (×10⁶K/m)	1.349	1.999	2.377	2.640
V_red (mm/s)	0.893	1.910	3.112	4.256

Fig. 5. Solidification microstructure evolved from planar interface to dendritic interface: (a) 5 mm/s, (b) 15 mm/s, (c) 25 mm/s, (d) 35 mm/s.

Fig. 6. Simulation results of dendrites growth direction by laser surface-melting with different scanning velocities: (a) 5 mm/s, (b) 15 mm/s, (c) 25 mm/s, (d) 35 mm/s.

Fig. 7. Components of thermal gradient along dendrite growth direction (Gd) by calculation: (a) 5 mm/s, (b) 15 mm/s, (c) 25 mm/s, (d) 35 mm/s.

Fig. 8. Calculated ratio of dendrites growth velocity and laser scanning speed (Vd/Vb): (a) 5 mm/s, (b) 15 mm/s, (c) 25 mm/s, (d) 35 mm/s.

Fig. 9. The schematic diagram of evolving from planar interface into dendritic dendrites during solidification.

Fig. 10. The calculation results of the product of Gd and Vd (Gd·Vd): (a) 5 mm/s, (b) 15 mm/s, (c) 25 mm/s, (d) 35 mm/s.

Fig. 11. Values of dE, TE and their cross point (data comes from sample A).

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