REAL-TIME OBSERVATION OF SOLIDIFICATION MICROSTRUCTURE IN LASER REMELTING POOL
WANG Lilin(), LIN Xin, WANG Yonghui, YU Honglei, HUANG Weidong
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072
Cite this article:
WANG Lilin, LIN Xin, WANG Yonghui, YU Honglei, HUANG Weidong. REAL-TIME OBSERVATION OF SOLIDIFICATION MICROSTRUCTURE IN LASER REMELTING POOL. Acta Metall Sin, 2015, 51(4): 492-498.
The final quality of parts fabricated by high energy beam (laser, electron beam and arc) processing technology is determined by solidification microstructure formation in the molten pool, which attracts lot of attention of researches. However, real-time observation of solidification microstructure formation in the molten metal pool is very difficult because of its high temperature, rapid solidification and opacity. In this work, using a transparent model alloy of succinonitrile-2.0% (mass fraction) ethanol (SCN-2.0%Eth), the solidification microstructure evolution in the molten pool during laser surface remelting (001) crystal plane of a single-crystal substrate was real-time observed as the laser scanning direction deviated different angles from [100] crystal orientation of the substrate. It was found that and dendritic columns grow symmetrically in the molten pool when the scanning direction parallels to the [100] crystal direction. Dendritic columns grow asymmetrically in the molten pool when the scanning direction deviates an angle of 20° from the [100] crystal orientation. Specifically, dendritic columns always grow at one side of the molten pool while [100] and [010] dendritic columns compete to grow alternately at the other side. [100] and dendritic columns grow perpendicular to each other in the molten pool when the scanning direction deviates an angle of 45° from the [100] crystal orientation. According to the preferential growth criterion of dendrite, a model describing the dendritic growth behavior in laser remelting pool was established. It can explain the experimental results well. The results showed that the solidification microstructure formation in laser remelting pool is influenced by both pool morphology and crystal orientation of the substrate.
Fund: Supported by National Natural Science Foundation of China (Nos.51271213 and 51323008), National Basic Research Program of China (No.2011CB610402), China Postdoctoral Science Foundation (No.2013M542384) and Specialized Research Fund for the Doctoral Program of Higher Education (No.20116102110016)
Fig.1 Schematic of real time observation apparatus of laser remelting process (x, y—coordinates of the motion platform, v—scanning velocity)
Fig.2 Macroscopic morphology evolution of laser remelting pool with laser scanning time t =1 s (a), 3 s (b) and 51 s (c)
Fig.3 Dendritic growth evolution in laser remelting pool with t =5 s (a) and 9 s (b) when the scanning direction parallels to [100] crystal orientation
Fig.4 Dendritic growth evolution in laser remelting pool with t =3 s (a), 7 s (b), 8 s (c), 14 s (d), 19 s (e), 21 s (f) and after remelting (g) when the scanning direction deviates 20︒ from [100] crystal orientation
Fig.5 Dendritic growth evolution in laser remelting pool with t =7 s (a) and 22 s (b) when the scanning direction deviates 45° from [100] crystal orientation
Fig.6 Schematic of dendritic growth in laser remelting pool when the scanning direction deviates ? from [100] crystal orientation ( 2θ —rear angle of the molten pool, w—angle between the scanning direction and the normal direction of solidification interface, ? —angle between the scanning direction and [100] crystal orientation, vi —normal velocity of solidification interface, vU[100] —[100] dendrite growth velocity at the up side, vU[010] —[010] dendrite growth velocity at the up side, vD[100] —[100] dendrite growth velocity at the down side, vD[01ˉ0] — [01ˉ0] dendrite growth velocity at the down side)
Fig.6 Schematic of dendritic growth in laser remelting pool when the scanning direction deviates ? from [100] crystal orientation ( 2θ —rear angle of the molten pool, w—angle between the scanning direction and the normal direction of solidification interface, ? —angle between the scanning direction and [100] crystal orientation, vi —normal velocity of solidification interface, vU[100] —[100] dendrite growth velocity at the up side, vU[100] —[010] dendrite growth velocity at the up side, vU[100] —[100] dendrite growth velocity at the down side, vD[01ˉ0]—[01ˉ0] dendrite growth velocity at the down side)
Fig.7 Dendritic growth diagram of laser remelting pool
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