Acta Metall Sin  2019, Vol. 55 Issue (6): 692-700    DOI: 10.11900/0412.1961.2019.00007
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Microstructure and Tensile Property of TC4 Alloy Produced via Electron Beam Rapid Manufacturing
Zheng LIU1,2,Jianrong LIU1(),Zibo ZHAO1,Lei WANG1,Qingjiang WANG1,Rui YANG1
1. Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
2. School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China
Abstract

Electron beam rapid manufacturing (EBRM) is one of the 3D printing technologies. The main attractions of EBRM technology are its high efficiency and economy in fabricating large, complex near net shape components dielessly and only needing limited machining. In general, the microstructure and texture of titanium alloy can play a significant role in determining its mechanical behaviors. In the present work, the microstructure, texture and tensile property of TC4 alloy produced by electron beam rapid manufacturing (EBRM) are investigated. Results show that the microstructure is comprised of columnar prior β grains that orient parallel to the building direction. The width of the columnar β grains increased rapidly at the initial several build layers, and the subsequent increase rate of the width of the columnar β grains tends to slow down. Fine α lamellae with gradient size are observed inside the columnar prior β grains, which occur because the alloy experiences different complex thermal histories during the EBRM-produced process. The size of α lamellae tends to decrease with the increase of build layers. The XRD result shows that the TC4 alloy has a typical α phase texture, (the c-axes are either concentrated at about 45° or are perpendicular to the building direction). At the same time, the <$10\bar{1}0$> poles are relative to random distribution. For the tensile samples along the electron beam scanning direction, the yield strengths do not show significant change with the increase of build layers, but the tensile strengths increase. The ductility of the alloy also has an upward trend, despite of a slightly decreasing ductility in the top sample. The tensile samples at the bottom of the alloy (10 mm and 20 mm away from the substrate) have similar work hardening exponents, which are lower than the top sample. The top sample shows the highest work hardening exponent. This difference in the tensile properties can be highly attributed to the gradient microstructure. The alloy also presents obvious anisotropy in tensile strength. The tensile sample along the 45° direction has a higher strength than the sample along the X direction, while the tensile sample along the Z direction shows the lowest strength. This anisotropic strength is strongly associated with the α phase texture. When the loading direction is 45° to the building direction, most of the c-axes of α phase are about parallel to the loading direction, showing a "hard" orientation, leading to a higher strength than other oriented samples. Conversely, when the loading direction is along the building direction, most of the α phase present a "soft" orientation, resulting in lower strength compared to the tensile samples along the 45° or the X direction.

 ZTFLH: TG14
Fund: National Key Research and Development Program of China(No.2017YFB1103100);AVIC Science Foundation of China(No.20175492002)
Corresponding Authors:  Jianrong LIU     E-mail:  jrliu@imr.ac.cn
 Fig.1  Schematics of the TC4 alloy produced by electron beam rapid manufacturing (EBRM) (a) and sampling location of tensile specimens (b) (X direction is the electron beam scanning direction, Z direction is the building direction) Fig.2  OM images of substrate and bottom (a), and top (b) of the TC4 alloy produced by EBRM in the X-Z plane (Inset show the high magnified image of square area) Fig.3  The changes of the prior β grain size and the width of α lath along the building height Fig.4  SEM images of bottom (a), middle (b) and top (c) of the TC4 alloy produced by EBRM Fig.5  Pole figures of {0001} (a) and {10$1ˉ$0} (b) for the α phase in the TC4 alloy Fig.6  Tensile properties of samples along the X direction at different Z direction height Fig.7  True stress-true stain curves plotted on logarithmic axes in plastic portion between 2.5% and 4.6% strain for the tensile samples at different Z direction height and work hardening exponent (σ—true stress, ε—true strain, n—work hardening exponent) Fig.8  TEM images of bottom (a), middle (b) and top (c) of the TC4 alloy produced by EBRM Fig.10  Schematics of slip length along three different loading directions Fig.11  Inverse pole figures of TC4 alloy along X direction (a), 45° angle to X direction (b) and Z direction (c) (The black and blue contour lines are the Schmid's factors for basal and prismatic slip in orientation triangle, respectively)