J O U R N A L O F M A T E R I A L S S C I E N C E L E T T E R S 2 1, 2 0 0 2, 1 6

Effects of crystal orientation on microstructure of molten pool in laserrapidly solidified DD2 single crystal

SEN YANGDepartment of Mechanical Engineering, Tsinghua University, Beijing 100084, Peoples Republic of ChinaE-mail: yangsen@chinaren.com

WEIDONG HUANGState Key Lab of Solidification Processing, Northwestern Polytechnical University, Xian 710072,Peoples Republic of ChinaE-mail: cgs@nwpu.edu.cn

WENJIN LIUDepartment of Mechanical Engineering, Tsinghua University, Beijing 100084, Peoples Republic of China

YAOHE ZHOUState Key Lab of Solidification Processing, Northwestern Polytechnical University, Xian 710072,Peoples Republic of China

Laser surface remelting, as a newly developed rapidsolidification technique, has been extensively used insurface modification of materials and studies on rapidsolidification theories for its unique advantages since1989 [17]. During laser surface remelting, the moltenpool is in contact with the base metal, and no nucleationis involved, the structure of the molten pool epitaxiallygrows at the partially melted grains along the fusionline [8, 9]. Generally, the average growth direction dur-ing solidification of the laser molten pool is approxi-mately perpendicular to the solid-liquid interface andanti-parallel to the heat flow direction. In fact, the devel-opment of grain structures is a very complex processingin the molten pool, which is not only affected by heatflow, but also affected by preferred crystallographic di-rection, namely the easy growth direction. In the sametime, the microstructures of molten pool have a greatimpact to its final properties, so it is very important tostudy mechanisms of microstructure formation in themolten pool.

In past researches and major engineering applica-tions, the selected base metal is polycrystalline in gen-eral. So it is difficult to identify the extent to whichgrowth crystallography and the direction of the ther-mal gradient influence the grain growth behavior, andit is inevitable to introduce complexity when determin-ing the crystal growth velocity, which will greatly af-fect the analysis results. It is well known that for cu-bic metals the six 100 crystallographic directions arepreferred growth directions. In most cases, the stalkslie between the direction of temperature gradient and100 directions. The stalks are very close to the di-rection of temperature gradient when growth velocityis very slow, and they turn initially to 100 very fastthen gradually slow down with increase of growth rate.Only if dendrites are well developed, can stalks growexactly along one of six 100. For cellular growth, gen-erally we consider that cellular grow along heat flow.For dendrite growth, there is preferred crystal orien-

tation, which makes crystals grow faster along certaindirection [10,11].

So in a polycrystalline metal, it is impossible to de-termine which of these six variants for dendrite growthis active or how they influence the development of grainstructure in molten pool.

In recent years, a few experiments and theoreticalinvestigations have been conducted on the formationmechanism of solidification structure in single crys-tal alloys subjected to relatively rapid solidification bylaser or electron beam welding. Narasimhan [12] inves-tigated the microstructure of weld fusion zone in laser-melted single crystal of Ni base superalloy Udimet 700,and reported the characteristics of the solidificationstructure in the zones. Rappaz [13, 14] performed elec-tron beam welding on the single crystal alloy of Fe-15Ni-15Cr. Katayama et al. [15, 16] used pulsed YAGlaser to irradiate to melt pure 99.99%Al, Al-2%Cu andAl-2%Zn single crystal to study the relationship be-tween microstructure and crystallographic orientation.However, further information and data to investigate themicrostructure formation during laser surface remeltingare needed.

TABLE I Chemical composition of DD2 single crystal

Elements Al Cr Co Mo Ti W Ta C Ni

wt% 5.6 8 5 6 1 8 6

Figure 1 Resolidified laser melted regions of [100] direction, (001) surface, with Vb = 5 mm/s. (a) Transverse section; (b) longitudinal section;(c) top surface.

In this letter, detailed laser surface remelting experi-ments were carried out to determine the effect of crys-tallography and heat flow direction on the developmentof laser molten pool by scanning along specific crys-tallographic direction on a well-characterized crystal-lographic face of single crystal with a continuous waveCO2 laser.

The DD2 single crystal alloy used in the present workwas prepared by Bridgman technique, whose compo-sition is listed in Table I. The specimens of 6 6

Figure 2 Resolidified laser melted regions of [100] direction, (001) surface, with Vb = 24 mm/s. (a) Longitudinal section; (b) top surface.

30 mm used for laser resolidification were carefullyoriented prior to cutting from the as-grown crys-tal by means of Laue back-reflection X-ray tech-niques so that the major surface of the specimenscorresponded to the (001) and (110) crystallographicplane respectively. The [100], [110] and [ 110] direc-tions lying in the plane of the specimens were also iden-tified from Laue-reflection photographs so that laserbeam can scan along these directions. In order to min-imize the reflection for the laser beam and obtain a

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Figure 3 Resolidified laser melted regions of [110] direction, (001) surface, with Vb = 5 mm/s. (a) Top surface; (b) transverse section; (c) localmagnification of (b); (d) longitudinal section.

similar surface quality for each specimen, all the spec-imens were ground up to 800 grit SiC paper and thor-oughly cleaned in methanol prior to laser surface reso-lidification.

Laser resolidification experiments were conductedon a 5 kW CO2 laser (Rofin-Sinar RS 850). The nor-mally incident laser beam was focused to a spot diame-ter of 0.3 mm. During the laser treatment, a continuousflow of 51/min of helium was blown to the melted zoneto prevent heavy oxidation. Output power of the laser

was fixed at 1.2 kW. Table II gives the laser processingconditions. The microstructure analysis was performedby standard metallographic techniques after carefullysectioning the specimens and etching them in a solu-tion containing 21 ml of H2O, 14 ml of HCl, and 8 gof FeCl3. Top surface, transverse and longitudinal sec-tions of laser-melted zones were examined by opticaland scanning electron microscopy.

Fig. 1 shows the transverse section, longitudinal sec-tion and top surface microstructures, respectively, of

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the molten pool of a DD2 single crystal scanned onthe (001) face along the [100] direction at a velocityof 5 mm/s. The molten pool is obviously composed ofthree zones with different growth directions. There isa nonstructural layer of approximately 510 m widebetween molten pool and base metal, which is due to thefact that the maximum temperature gradient, and mini-mum growth rate in the edge of the molten pool lead tosolid-liquid interface grow in the planar form. The firstzone solidified by the growth of cell-like dendrites in thebottom of the molten pool, which grew in the plane ofthe transverse section along [001] direction (see Fig.1aand b). The second zone located in the two sides of themolten pool, which grew in the plane of the transversesection along [010] direction (see Fig. 1a and c). Thethird zone or innermost zone located in the upper cen-ter of molten pool, which grew perpendicular to planeof the transverse section along [100] direction and par-allel to the laser beam scanning direction. As one cansee in Fig. 1, the dendrites are only slightly branchedand thus have a noticeable cell-like character. Normally,for cellular growth, the cellular orientation is primarilydictated by the heat flow direction and not by the crys-tallographic orientation [17]. However, as is apparentin Fig. 1, crystallography dictated the orientation of thecell-like dendrite while the shape of the molten pooland, accordingly, the heat flow direction had a negligi-ble effect. Although the cell-like dendrites observed inFig. 1 have their trunks closely aligned along one of the100 directions regardless of the heat-flow direction, itis strongly influenced by the pool shape and heat flow ifthat the specific 100 dendrite appears or not. With theincrease of the scanning velocity, the molten pool be-comes shallow. Under this condition, only did the sizeof the zone change; the growth directions of dendrite ineach zone have not changed. In other words, technologyparameter will decide which 100 dendrite will appearor not. Fig. 2 shows the longitudinal section and top sur-face microstructures, respectively, of the molten poolof a DD2 single crystal made on the (001) plane alongthe [100] direction at a scanning velocity of 24 mm/s.As one can see in Fig. 2, dendrite grown along [100]direction did not occur on the top surface of the moltenpool again.

Figure 4 Micrographs of laser melted regions of [110] direction, (001) surface, with Vb = 24 mm/s. (a) Longitudinal section; (b) top surface.

When laser beam scanning along another specificcrystallographic direction on the same crystallographicface (001), such as [110] direction, the micrographsof the molten pool are apparently different with thatof scanning along [100] direction. Fig. 3 shows thetransverse section, longitudinal section and top surfacemicrostructures, respectively, of DD2 single crystal al-loy made on the (001) plane along the [110] direction ata velocity 5 mm/s. In Fig. 3a, [100] and [010] cell-likedendrites symmetrically grow and meet at symmetricalline; this symmetrical line just parallels to the scan-ning direction. These dendrites are very well orientedas shown by the symmetry of the micrograph, indicat-ing that the resolidification maintains the single crystalnature of the base material.

In general, heat flow is perpendicular to the edge ofthe molten pool, so the structure should grow perpen-dicular to the edge or have a large angle between thegrowth direction and the edge of molten pool, but thestructure would rather grow along a certain directionthan along the heat flow. As found for the case of [100]remelting, the dendrite orientation is dictated by thecrystallography and not by the heat flow direction.

As can be seen from the transverse section micro-graph (Fig. 3b and c), the [100] and [010] dendritetrunks are cut at about 45 deg in this section. There-fore, they appear to be elliptical in shape, forming ahoneycomb structure, with their horizontal length about

2 larger than the vertical one, and [100] and [010]dendrites are only present at the top of the moltenpool. In the bottom of the pool, [001] dendrites favorgrowth. At higher laser beam scanning velocity, the tipradius and spacing of dendrites become smaller andless branched (see Fig. 4). Just like seen in laser beamscanning along [100] direction on (001) plane, increaseof scanning velocity only changes the size of each den-drite growth zone, and does not change the dendritegrowth direction. One can see in Fig. 4b that [100]and [010] dendrites do not meet at the symmetric lineon the top surface, while in the center of the moltenpool, the region of [001] dendrites extends almost tothe top surface. This is due to the fact that the moltenpool is more elongated and shallow at higher scanningvelocity.

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Figure 5 Resolidified laser melted regions of [110] direction, (110) surface, with Vb = 24 mm/s. (a) Transverse section; (b) top surface;(c) longitudinal section.

From the microstructure results of the resolidifica-tions made in two orientations on the same crystallo-graphic face, a comment can also be made with regardto the effects of scanning orientation on the microstruc-ture in molten pool. Comparing Fig. 1 and Fig. 3, andFig. 2 and Fig. 4, i.e., remelting made at 5 mm/s and24 mm/s, but in the [100] and [110] orientations, respec-tively, it is seen that the microstructure of the moltenpool is strongly affected by the scanning direction, butthe pool shape (size-the maximum depth and width)is independent of the scanning direction. As expected,the pool shape (size) is primarily controlled by the tech-nique parameters.

When laser beam scanning on another crystallo-graphic plane, the microstructures of the molten poolalso took place change. Fig. 5 shows the microstruc-tures of the molten pool obtained from top surface,transverse section and longitudinal section respectivelywhen laser beam scanning along [110] direction onthe (110) plane of DD2 single crystal alloy at a ve-locity of 24 mm/s. As can be seen from Fig. 5a andb, [001] and [001] dendrites grow in the two sides ofthe molten pool symmetrically, which are perpendic-ular to the scanning direction, while [100] and [010]dendrites symmetrically grow from bottom to top inthe major of the molten pool (see Fig. 5c). Generally,microstructure grows anti-parallel to heat flux. Due

to heat flux perpendicular to solid-liquid interface inthe molten pool, microstructure should grow perpen-dicular to the boundary in the bottom of the moltenpool, but in fact, [100] and [010] dendrites favor grow-ing along a fixed angle between growth direction andboundary of the molten pool, which is 45 deg. So it canbe concluded that the effects of favoring growth aremore than that of the heat flux when crystal grows indendrites.

In conclusion, the microstructure of the molten poolduring laser surface remelting is strongly affected byorientation of base metal and scanning direction. Onthe same crystal face, the microstructures in the moltenpool are obviously different when laser beam scanningalong different crystal orientation. When laser beamscanning along a fixed crystallographic orientation ondifferent crystal face, the microstructures are also dif-ferent. The experimental results show that crystallo-graphic effects can influence dendrite growth by favor-ing growth along a preferred crystallographic direction,and crystallographic effects are greater than that of heatflux during dendrite growth.

AcknowledgmentThe authors would like to express their gratitude toNational Natural Science Foundation of China GrantNo. 59771054 for financial support.

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Received 12 Marchand accepted 22 October 2001

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