doi: 10.11900/0412.1961.2016.00417

Crystallization Behavior of Laser Solid Forming of Annealed Zr55Cu30Al10Ni5 Powder

Abstract:

Laser solid forming (LSF) provides an innovative way in building the bulk metallic glasses (BMGs) due to its inherently rapid heating and cooling process and point by point additive manufacturing process, which can eliminate the limitation of critical casting size of BMGs. The annealed powder has been demonstrated to be applicable to the preparation of BMGs with high content of amorphous phase using LSF. In this work, the plasma rotating electrode processed (PREPed) Zr55Cu30Al10Ni5 (Zr55) powders annealed at 1000 K are used for LSF of Zr55 BMGs. The influences of powder size and laser processing parameter on the crystallization characteristic of the deposit are investigated, and the crystallization behavior of the remelted zone (RZ) and heat affected zone (HAZ) is analyzed. It is found that the microstructures of the pre-annealed Zr55 powders are composed of the Al5Ni3Zr2, CuZr2 and Al2Zr3 phases. As the heat input increases from 7.0 J/mm to 15.7 J/mm, the every deposited layer presents a periodic repeating gradient microstructure (amorphous, NiZr2 nanocrystal, CuZr2+ZrCu dendrite-like eutectic, CuZr2+ZrCu spherulite-like eutectic) from the molten pool to the HAZ. The size of the eutectic phase in the HAZ decreases as the increase of distance from the featureless amorphous zone. On condition that the laser heat input is less than 7.0 J/mm, the deposits contain a high content of amorphous phase. As the increase of laser heat input, the crystallization degree of HAZ does not increase obviously for the deposit prepared by the powder with size range of 75~106 μm. However, the crystallization degree of HAZ increases significantly for the deposit prepared by the powder with size range of 106~150 μm. That is because the lower overheating temperature and shorter existing time of the molten pool enhances the heredity of Al5Ni3Zr2 clusters and other intermetallic clusters in remelted alloy melt during LSF of coarser powder, which decreases the thermal stability of the already-deposited layer and induces the severe crystallization. It is deduced that the raw state of annealed powders has a minimal impact on the crystallization behavior of the Zr55 deposited layers when the content of Al5Ni3Zr2 phase is same in different sizes of annealed powders. The thermal history of RZ and HAZ during deposition is the primary factor to affect the crystallization behavior in the Zr55 deposits fabricated by different powder sizes.

Key words: ZrCuAlNi bulk metallic glass ; laser solid forming ; powder state ; crystallization

1 实验方法

Table 1 Measured and nominal compositions of the Zr55Cu30All0Ni5 (Zr55) alloy powder (mass fraction / %)

Table 2 Parameters of laser solid forming (LSF)

2 实验结果

Fig.1 SEM-BSE images of original (a, c) and annealed (b, d) Zr55 powders with the size range of 106~150 μm (a, b) and 75~106 μm (c, d) (Inset in Fig.1b shows the enlarged view of faceted phases)

Fig.2 XRD spectra of original and annealed Zr55 powders

Fig.3 Cross-sectional OM images of the LSFed Zr55 deposits prepared by the annealed powders with different sizes(a) specimen 1 (b) specimen 2 (c) specimen 3 (d) specimen 4 (e) specimen 5 (f) specimen 6

Fig.4 XRD spectra of the LSFed specimens 1~6

Fig.5 SEM images of crystalline band between the adjacent tracks in specimen 5 (a) and specimen 6 (b) (Insets show the enlarged views of square areas, the white arrows indicate the directions from RZ to HAZ, RZ—remelted zone, HAZ—heat affected zone)

Fig.6 TEM images and corresponding SAED patterns of dendrite zone (a) and spherulite zone (b) in specimen 3

3 分析讨论

Fig.7 Simulation results of the thermal field in the deposit during one layer single-track deposition(a, b) temperature distributions for 0.06 s after irradiation with 7.0 J/mm (Points A and B are located at the surface of the molten pool zone, points C and D are located at the top of HAZ, points E and F are at the boundary between the HAZ and already-deposited amorphous zone; Tm is the melting temperature, Tx is the onset crystallization temperature, Tg is the glass transition temperature)(c) continuous heating transformation (CHT)curves for Zr55 bulk metallic glass (BMG) according to thermal cycles at the boundaries (Curves 1~3 denote the temperature profiles during deposited the coarser powder with laser heat input of 7.0, 10.8 and 15.7 J/mm, curves 4~6 denote the temperature profiles during deposited the finer powder with different heat inputs)

$τ = 7.2 R g f θ 1 - cosθ ∙ a 4 d α 2 C 0 ∙ T t D S m ∆ T t 2$ (1)

$d α = W m N A ρ 1 3$ (2)

Fig.8 Incubation time of CuZr2 phase and ZrCu phase nucleated from Zr55 melts as function of undercooling (ΔTc—critical undercooling degree)

Fig.9 Calculated growth rates of lamellar eutectic and dendrites in Zr55 alloy versus undercooling

4 结论

(1) 尺寸为75~106 μm以及106~150 μm的Zr55Cu30Al10Ni5 (Zr55)退火态粉末组织均由Al5Ni3Zr2、CuZr2和Al2Zr3相组成。经不同激光线能量(7.0、10.8和15.7 J/mm)熔覆沉积后,熔覆层的熔池区都能保持非晶态。

(2) 不同激光线能量下所制Zr55熔覆层组织从熔池到热影响区依次分布着非晶、NiZr2纳米晶、CuZr2+ZrCu枝状共晶、CuZr2+ZrCu球粒状共晶。随着距熔池距离的增加,共晶晶粒的尺寸逐渐减小,而数量增多。

(3) 随着激光线能量的增大,采用尺寸为106~150 μm的退火态粉末所制试样的热影响区的晶化程度加重,而尺寸为75~106 μm的退火态粉末所制试样中热影响区的晶化程度无明显变化。理论推导了Zr55熔覆沉积层非晶区的连续加热相变曲线,证明熔覆小尺寸Zr55粉末时熔池区较高的过热度导致已沉积非晶区具有较高的起始晶化温度和热稳定性。

The authors have declared that no competing interests exist.

PDF下载数
RichHTML 浏览数

 相关文章:

ZrCuAlNi块体非晶合金

ZrCuAlNi bulk metallic gl...
laser solid forming
powder state
crystallization

ZHANG Yuanyuan
LIN Xin
WEI Lei
REN Yongming