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Acta Metall Sin  2017, Vol. 53 Issue (9): 1065-1074    DOI: 10.11900/0412.1961.2017.00005
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Microstructures and Mechanical Properties of TC11 Titanium Alloy Formed by Laser Rapid Forming and Its Combination with Consecutive Point-Mode Forging
Mingzhe XI(), Chao LV, Zhenhao WU, Junying SHANG, Wei ZHOU, Rongmei DONG, Shiyou GAO
Key Laboratory of Advanced Forging & Stamping Technology and Science, Ministry of Education, Yanshan University, Qinhuangdao 066004, China
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Abstract  

The titanium alloy parts, which have been formed by traditional laser additive manufacturing (LAM) method, usually have obviously different microstructure from wrought microstructure of titanium alloy and show room temperature mechanical anisotropy. In order to make the LAMed titanium alloy parts get the same microstructure and mechanical properties as wrought titanium alloy, a new technology of LAM called consecutive point-mode forging and laser rapid forming (CPF-LRF) has been proposed. During CPF-LRF process, deposited TC11 titanium alloy by laser rapid forming (LRF) was deformed by consecutive point-mode forging (CPF), and then on the surface of the deformed TC11 titanium alloy, new LRF process started over again. Both LRF and CPF were performed alternatively throughout the process of the fabrication of a TC11 titanium alloy part. Microstructures and mechanical properties of the CPF-LRFed TC11 alloy sample have been investigated. The average grain size of equiaxed grains of the CPF-LRFed TC11 alloy sample is 48.7 μm. The equiaxed grains have continuous grain boundary α phase. The microstructure of the equiaxed grain is bimodal microstructure consisting of primary α phase lath and transformed β. During CPF-LRF process, being plastically deformed by CPF, the surface deformation zone of the thick-wall TC11 titanium alloy part is 1.5 mm depth and its deformation degree is 20%. During a new layer deposited on the surface of the CPF cold deformed TC11 titanium alloy part, when laser beam scans through, about 1 mm thick (four layers) cold deformed titanium alloy in the heat affected zone of laser melting pool is heated up above β-transus temperature of TC11 titanium alloy in which static recrystallization complete within time interval of 0.86 s. The mechanical properties indicate that compared with the tensile properties at room temperature of TC11 alloy forged piece, the CPF-LRFed TC11 alloy has higher strength and less ductility. Fracture analysis indicates that intergranular fracture is mainly responsible for the poor ductility of CPF-LRFed TC11 alloy.

Key words:  consecutive point-mode forging      laser rapid forming      TC11 titanium alloy      microstructure      tensile property     
Received:  06 January 2017     
ZTFLH:  TG132.3  
Fund: Supported by National Natural Science Foundation of China (Nos.51375426 and 51375245)

Cite this article: 

Mingzhe XI, Chao LV, Zhenhao WU, Junying SHANG, Wei ZHOU, Rongmei DONG, Shiyou GAO. Microstructures and Mechanical Properties of TC11 Titanium Alloy Formed by Laser Rapid Forming and Its Combination with Consecutive Point-Mode Forging. Acta Metall Sin, 2017, 53(9): 1065-1074.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2017.00005     OR     https://www.ams.org.cn/EN/Y2017/V53/I9/1065

Fig.1  Schematic of technical process of consecutive point-mode forging and laser rapid forming (CPF-LRF) (a) laser rapid forming (b) consecutive point-mode forging
Fig.2  Sampling location of tensile specimen (a) and gemometric size of tensile specimen (unit: mm) (b)
Fig.3  OM images of xz cross-section of TC11 alloy sample (a) overall view (b) layers N+5 and N+4, showing columnar grains (c~g) layers N+3, N+2, N+1, N and N-1, respectively, showing equiaxed grains
Fig.4  SEM images of layers (N+4)~(N-1) of xz cross-section of the CPF-LRFed TC11 alloy sample
(a) layer N+4, showing columnar grains (Inset shows widmanst?tten microstructure) (b~d) layers (N+3)~(N+1), showing equiaxed grains (Insets show Widmanst?tten microstructure )(e, f) layers N and N-1, showing continuous grain boundary α phase (Insets show special duplex microstructure consisting of lath αp and transformed β)
Fig.5  Microhardness of the top eight layers of xz cross-section of the CPF-LRFed TC11 alloy sample
Fig.6  Tensile fracture morphologies of CPF-LRFed TC11 alloy sample
(a) macro-fractography of the CPF-LRFed TC11 alloy sample (b) high magnified image of zone 1 in Fig.6a, showing steps and intergranular fracture zones (c) high magnified image of zone 2 in Fig.6a, showing trangranular fracture zone (d) high magnified image of zone 3 in Fig.6c, showing dimples
Manufacturing method σs / MPa σb / MPa Elongation / %
CPF-LRFed 1040±12 1146±11 6.2±0.8
β forged[25]
α+β forged[25]
1020
985
1110
1043
9.7
15.3
Table 1  Tensile properties of CPF-LRFed TC11 titanium alloy
Fig.7  Tensile stress-strain curves of CPF-LRFed TC11 titanium alloy at room temperature
Fig.8  Slip-lines field of rigid flat punch pressing in semi-infinite high billet (2b—width of flat punch, d1—depth of plastically deformed zone, σy—normal stress on the contact surface between flat punch and billet)
Fig.9  Schematics of evolution of bgrain morphology of TC11 titanium alloy during CPF-LRF (a~e)
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