Microstructure transformations of laser-surface-melted near-alpha titanium alloy

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Microstructure transformations of laser-surface-melted near-alpha titanium alloy G.X. Luo, G.Q. Wu⁎, Z. Huang, Z.J. Ruan Department of Material Science and Engineering, Beijing University of Aeronautics and Astronautics, Xueyuan Road 37, Haidian District, Beijing 100191, PR China

AR TIC LE D ATA

ABSTR ACT

Article history:

Microstructural transformations of near-alpha titanium alloy under laser-surface -melted

Received 22 March 2007

and post-heat treated conditions were systematically investigated. The results show that,

Received in revised form

after laser surface melting treatment, a melted zone and a heat-affected zone are formed in

11 November 2008

the titanium alloy. The melted zone consists of columnar grains, which are characterized by

Accepted 10 December 2008

fine a″ phase, distributed dispersively in acicular martensite phase α′. Martensite

Keywords:

laser-surface-melted zone is further refined during the recrystallization, and a fine

Titanium alloy

dispersive α + β microstructure is formed when experienced post-heat treatment at an

Laser processing

elevated temperature.

transformation occurs at a lower temperature in post-heating process. The α phase in the

Heat treatment

© 2009 Elsevier Inc. All rights reserved.

Microstructure

1.

Introduction

Titanium alloys for structure application at high temperature are generally near-α alloys which have good elevated temperature properties and weldability similar to α-titanium alloy and good processing plasticity comparable to α + β alloy [1]. Microstructural feature of titanium alloys, which is one of important factors controlling mechanical properties, are sensitive to the heating temperature, holding time and cooling rate of heat treatment [2,3]. Nowadays, high power lasers are frequently used in the field of surface treatments including laser melting, laser alloying and laser cladding. The common feature of these processes is to improve the surface properties by modifying the microstructure feature and (or) composition of the work-piece while maintaining other original properties [4–6]. With high heating rate and cooling rate, laser surface processing generally results in the obvious refinement of the surface microstructure compared to conventionally solidified materials [7–9]. It has been proved that fine microstructures are beneficial to the acceleration of the

diffusion bonding process and the simultaneous decrease of the bonding temperature [10–12]. Accordingly, laser surface modification is an effective technique to improve the diffusion bond weldability of dissimilar materials. In our study, laser surface melting (or cladding) technique is applied to dissimilar diffusion bonding of near-alpha titanium alloys to other high temperature structural materials, to gain sound joints at lower bonding temperature in less bonding time. The present paper is mainly aimed at investigating microstructural characteristics of laser surface melted titanium alloy and its transformation during post-heat treatment.

2.

Experimental Procedure

The substrate material is BT20 near-alpha titanium alloy with the nominal composition (wt.%) of 5.5~7.0Al, 1.4~2.5Zr, 0.5~1.8Mo, and 0.8~2.3 V in the form of 50×20×6 mm rectangular plate. Surface-melting experiments were performed using a continuous wave transverse flow CO2 laser material pro-

⁎ Corresponding author. Tel./fax: +86 108 231 3240. E-mail address: [email protected] (G.Q. Wu). 1044-5803/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2008.12.009

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3.

Results and Discussion

The cross-sectional macrograph of laser-surface melted BT20 alloy is shown in Fig. 1. It can be seen that the specimen is divided into three zones, namely laser-melted zone (LMZ), heat-affected zone (HAZ) and substrate. The BT20 substrate in

Fig. 1 – Cross-sectional micrograph of laser surface melted BT20 titanium alloy (the arrows marking the columnar grains nucleated at coarse grains in HAZ and grown vertically to molten pool bottom).

cessing system. Laser processing parameters are: laser power 0.8 ~ l kW, beam diameter 3 mm, laser scan speed 150 ~ 250 mm/min. A shielding gas of argon was used to prevent the specimen from oxidizing during laser processing. After laser surface melting treatment, the specimens sealed in quartz tube under vacuum condition were annealed at different temperatures for 60 min and cooled with quartz tube under the air condition. Microstructures are investigated using optical microscopy (OM), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) with a secondary electron detector. The phases were examined by X-ray diffraction (XRD) using a D/max2200PC Rigaku X-ray diffractometer with Cu–Kα radiation. Differential scanning calorimetry (DSC), typed of a Perkin–Elmer DSC-6 differential scanning calorimeter, was adopted to measure the microstructural transformation behaviors of the laser melted layer at a heating rate of 0.33 c/ s under an argon flow. The sample weights 15 mg, and the test temperature range is 20 ~ 1200 °C.

Fig. 2 – SEM image by secondary electron detector of BT20 alloy substrate.

Fig. 3 – XRD patterns of laser surface melted BT20 alloy: (A) substrate, (B) as-melted, (C) as-annealed at 850 °C for 1 h, (D) as-melted by slow scanning rate and (E) as-annealed at 850 °C for 1 h by slow scanning rate.

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Fig. 4 – Micrographs of laser melted zone of BT20 alloy: (A) SEM image, (B) TEM image, (C) selected-field diffraction pattern of –– orthorhombic phase (beam parallel to [3 1 2]α″).

annealed condition consists of 10 ~ 30 μm primary equiaxed α phase, second α phase and reticular residual β phase with an extent of 2 ~ 5 μm, whose microstructure and XRD spectrum are given in Figs. 2 and 3(A). There is no definite boundary between the substrate and the HAZ, whose microstructure varies gradually with the distance from laser-melted zone. The microstructure of HAZ near laser-melted zone is recrystallized to coarse equiaxed β grains with thin α-layer at grain boundaries, which is marked by skew arrows in Fig. 1, containing acicular martensite in the β grains. The laser-melted zone consists of large columnar β grains of 50 ~ 250 μm in diameter. These columnar grains are nucleated at coarse grains in HAZ and grown vertically to molten pool bottom, which is typically marked with vertical arrows in the Fig. 1. General XRD results (Fig. 3(B)) indicate that the laser-melted zone contains martensite phase α′ (α″). Meanwhile, martensite phase α″ exists in the laser-melted zone, which can be seen from the result (Fig. 3(D)) of X-ray diffraction with slow scanning rate. The microstructures of the laser-melted zone are illustrated in Fig. 4(A, B). The results of selected area diffraction measurement (Fig. 4(C)) indicate that the black lens-like phase in the TEM micrographs, which corresponds to the bright white phase in the SEM micrographs, is an orthorhombic α″ phase. On the other hand, a

segregation of the alloying elements was not detected in the laser melted zone. The martensite α″ needles, which are 200 ~ 500 nm in thickness and from several hundreds nanometers to several microns in length, are dispersively distributed in martensite phase α′ matrix. The martensite α′ needles are smaller in thickness and longer in length than those of martensite α″ needles.

Fig. 5 – DSC curve of laser-melted layer of BT20 alloy.

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Due to the very high heating and quenching rate, the microstructure formed by laser-melting is metastable and can be subsequently transformed into stable by post-heating treatment [13]. The DSC results of laser-melted layer in BT20 alloy indicate that there are three exothermic peaks at 479 °C, 712 °C and 1002 °C respectively in the DSC curve (Fig. 5). The peak of 1002 °C corresponds to the phase transformation (α → β) in BT20 alloy, which agrees well with literature data [14]. According to reference [15], decomposition of martensite occurs strongly when heated to 300 ~ 400 °C, so the transformation of martensite α′ and α″ into α + β phase may contribute to the 479 °C peak. Recrystallization occurs in the melted zone at 712 °C approximately. As shown in Fig. 3(C, E), after postheat treatment at 850 °C for 60 min, the laser-melted layer consists of α + β phases. The size and shape of large columnar grains in the laser melted zone of BT20 alloy have no obvious variation. The extent of intergranular α-layer increases with the increase of post-heating temperature. Microstructures of columnar grains after post-heat treatment at different temperatures for 1 h are presented in Fig. 6. The specimen post-heat treated at 650 °C for 1 h has a similar microstructural pattern (Fig. 6(A)) with the as-melted one. Obvious microstructural transformation occurs in the laser-melted zone of specimens post-heat treated at higher temperature. Ultra-fine granular β phase

are detected to precipite in α lamellae matrix in the 700 °C specimen. The size of granular β phase increases slightly when post-heat treated at 800 °C and 850 °C, and a fine dispersive (α + β) microstructure is formed in the BT20 melted zone. TEM micrographs of specimens post-heat treated at 700 °C and 850 °C are given in Fig. 7(A, B). It can be seen that, the long α lamellae has the tendency of being cut off by the recrystallized β phase after annealed at 700 °C for 60 min, and α phase with nano-grade extent is formed in the specimen post-heat treated at 850 °C. Besides the lens-like lamellar cross-section, near-globular cross-section of α phase (marked with arrow in Fig. 7(B)) is observed in TEM micrograph of laser melted zone post-heat treated at 850 °C.

4.

Conclusions

Laser melted zone, developed by laser surface melting, has a microstructure obviously different from the BT20 substrate alloy. The laser melted zone is directionally solidified into columnar grains with diameters of about 50 ~ 250 μm. In the columnar grains, there are fine acicular martensite α′ phase and α″ phase with thickness of about 200 ~ 500 nm, which are metastable and transformed into (α + β) phase during the postheat treatment at lower temperature. The (α + β) phase in laser

Fig. 6 – SEM micrographs of laser surface melted BT20 alloy post-heat treated at different temperature: (A) 650 °C, (B) 700 °C, (C) 800 °C and (D) 850 °C.

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Fundamental Science Foundation of China (01 H51004) for their financial support.

REFERENCES

Fig. 7 – Bright-field TEM micrographs of laser surface melted BT20 alloy post-heat treated at different temperature: (A) 700 °C, (B) 850 °C (the arrow marking α phase with globular cross-section).

melted zone is further refined during the recrystallization and characterized by α lamellae phase with nano-grade extent and small length-width ratio after post-heat treatment at an elevated temperature.

Acknowledgements The authors would like to acknowledge the National Natural Science Foundation of China (59971004) and the Aeronautics

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