Effect of Laser Oscillation on the Microstructure and Mechanical Properties of Laser Melting Deposition Titanium Alloys
FANG Yuanzhi1, DAI Guoqing1, GUO Yanhua1, SUN Zhonggang1(), LIU Hongbing2, YUAN Qinfeng3
1.Tech Institute for Advanced Materials, College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China 2.School of Materials Engineering, Shanghai University of Engineering Science, Shanghai 201620, China 3.Zhejiang Shenji Titanium Industry Co., Ltd., Huzhou 313306, China
Cite this article:
FANG Yuanzhi, DAI Guoqing, GUO Yanhua, SUN Zhonggang, LIU Hongbing, YUAN Qinfeng. Effect of Laser Oscillation on the Microstructure and Mechanical Properties of Laser Melting Deposition Titanium Alloys. Acta Metall Sin, 2023, 59(1): 136-146.
Laser melting deposition (LMD) combines the laser cladding and rapid prototyping manufacturing technologies, and can be used for swift prototyping of complex parts with excellent comprehensive properties. However, due to its unique metallurgical conditions, it is easy to develop penetrating columnar crystals and coarse primary grains along the building direction. This remarkably reduces the mechanical properties of the alloy. The root cause of this issue can be traced back to the thermodynamic and dynamic metallurgical processes. Thus, this study proposes an oscillating laser melting deposition (OLMD) based on laser oscillating welding technology, and aims to elucidate the metallurgical structure and defects of laser melt deposition. OLMD modifies the motion trajectory of the molten pool using a laser in situ oscillation, and directly impacts the temperature gradient and solidification rate, thus improving the microstructure of titanium alloy by LMD. Furthermore, the microstructure evolution and mechanical properties of TC4 titanium alloy produced using OLMD were studied using OM, SEM, EBSD, and a Vickers hardness tester. The results indicate that the optimum process parameters of laser melting deposition without oscillation are as follows: the laser power is 1000 W, scanning rate is 8 mm/s, and powder feeding rate is 6.92 g/min. The optimum technological parameters of linear oscillation are as follows: the frequency is 200 Hz and the oscillation amplitude is 1.5 mm. Addition of linear laser oscillation considerably improved the morphology of the molten pool, and defects such as porosity and cracks were not observed. The overall number and size of columnar crystals reduced, and the grains were equiaxed. When compared to the sample without oscillation, the average grain size of Ti-6Al-4V alloy with linear oscillation decreased from 5.20 μm to 4.37 μm, while hardness increased from 418.00 HV to 428.75 HV.
Fund: National Natural Science Foundation of China(51875274);Key Research and Development Project of Zhejiang Province(2021C01085);Priority Academic Program Development of Jiangsu Higher Education Institutions
About author: SUN Zhonggang, professor, Tel: 15921177783, E-mail: sunzgg@njtech.edu.cn
Table 1 Process parameters of single-track samples without laser oscillation
Fig.1 Schematics of oscillating laser melting deposition (a) oscillating laser melting deposition (?—diameter) (b) trajectory of linear oscillation (c) transverse section of the specimen (d) longitudinal section of the specimen
Sample No.
f / Hz
A / mm
L1
150
0.5
L2
150
1.0
L3
150
1.5
L4
150
2.0
L5
200
0.5
L6
200
1.0
L7
200
1.5
L8
200
2.0
L9
250
0.5
L10
250
1.0
L11
250
1.5
L12
250
2.0
Table 2 Process parameters of single-track samples with linear laser oscillation
Fig.2 OM images of macro morphologies (left) and transverse section (right) of single-track samples Nos.1-9 (a-i) without laser oscillation (H—height of the molten pool, D—depth of the molten pool, W—width of the molten pool)
Sample No.
H / mm
D / mm
W / mm
1
0.373
1.216
3.357
2
0.412
1.104
3.184
3
0.467
0.987
2.973
4
0.554
1.295
3.704
5
0.564
1.195
3.574
6
0.331
1.201
3.390
7
0.666
1.538
4.190
8
0.419
1.475
4.191
9
0.488
1.303
3.802
Table 3 Molten pool sizes of single-track samples without laser oscillation under different process parameters
Fig.3 OM images of macro morphologies (left) and transverse section (right) of single-track samples Nos.L1-L12 (a-l) with linear laser oscillation
Fig.4 Longitudinal section OM images of single-track samples Nos.L1-L12 (a-l) with linear laser oscillation under different frequencies and amplitudes
Fig.5 Molten pool sizes of single-track samples with linear laser oscillation under oscillation frequen-cies of 150 Hz (a), 200 Hz (b), and 250 Hz (c)
Fig.6 Low (a, b) and high (c, d) magnified OM images of single-track samples No.5 (a, c) and No.L7 (b, d) under optimum process parameters (CG—columnar grain, EG—equaixed grain, HAZ—heat affected zone, HAZ (α + β)1—primary (α + β) heat affected zone, HAZ (α + β)2—secondary (α + β) heat affected zone)
Fig.7 OM images of three-track samples without laser oscillation (a) and with linear laser oscillation (b)
Fig.8 EBSD analyses of laser melting deposition (LMD) overlap specimens without laser oscillation (a) inverse pole figure (IPF) of overlap zone (b) texture intensity of overlap zone (TD—transverse direction, RD—rolling direction) (c) distribution of misorientation difference of overlap zone (LAGB—low-angle grain boun-dary, HAGB—high-angle grain boundary)
Fig.9 EBSD analyses of LMD overlap specimens with linear laser oscillation (a1, a2) IPFs of single-track and overlap zones, respectively (b1, b2) texture intensities of single-track and overlap zones, respectively (c1, c2) distributions of misorientation difference of single-track and overlap zones, respectively
Fig.10 Vickers hardnesses of Ti-6Al-4V alloys without laser oscillation and with linear laser oscillation
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