As an additive manufacturing technology, selective laser melting (SLM) process can solve the manufacturing difficulty of Ti-5Al-2.5Sn (TA7) easily. But the low building efficiency of SLM retards its wide applications in aviation, petrochemical and other fields. In order to solve the above problem, the influence of layer thickness on relative density, microstructure and mechanical properties of SLMed TA7 samples were studied in this work. The results show that when the laser power and hatching space are constant, the relative density gradually increases with the decrease of the laser volume energy density under the layer thicknesses less than or equal to 40 μm, whereas first increases and then declines with the decrease of the laser volume energy density under the layer thicknesses larger than 40 μm. At the same time, with the increase of layer thickness and the decrease of scanning velocity, the cooling rate gradually decreases during the SLM processing, when the cooling rate is lower than 6.8×107 K/s, the microstructure will gradually transform from acicular martensite α' to massive αm. Through the optimization of SLM parameters, the dense TA7 bulk specimens with higher microhardnesses, yield strengths and ultimate strengths in comparison to the as-cast and deformed TA7 alloys can be obtained under all layer thicknesses (20~60 μm). While when the layer thicknesses are not larger than 40 μm, the ductility of the SLMed TA7 is also superior to that of the as-cast TA7 and comparable to that of the deformed TA7. Finally, the optimal layer thickness and combination of SLM process parameters are successfully determined to balance the building efficiency, metallurgical quality and mechanical properties of the TA7 alloy parts.
Fund: Supported by National Basic Research Program of China (No.613281), Fundamental Research Funds for the Central Universities (No.2016XYZD005) and Technical Basis Project of State Administration of Science and Technology and Industry for National Defense (No.JSCG2016204B001)
Table 1 Parameters for selective laser melting process
Fig.1 SEM image (a) and particle size distribution (b) of TA7 alloy powders (D10, D50 and D90 are the diameters at which 10%, 50% and 90% of the samples' mass are comprised of particles with a diameter less than these values)
Fig.2 The variation of the relative density and volume energy density with the process parameters under layer thicknesses δ=20 μm (a), δ=30 μm (b), δ=40 μm (c), δ=50 μm (d) and δ=60 μm (e) (Insets a1 shows the molten pool morphology of a single track, the dotted line represents the boundary of the molten pool, the pore in the molten pool shows the 'keyhole effect'; insets a2, b1, c1, d1 and e1 show the circular pores in the samples; insets c2, d2 and e2 show the irregularly unmelted voids in the samples)
Sample No.
δ / μm
V / (mms-1)
S / mm
EV / (Jmm-3)
Relative density / %
1
20
1000
0.08
125.00
99.95
2
30
1000
0.06
111.11
99.94
3
40
1000
0.08
62.50
99.92
4
50
800
0.08
62.50
99.76
5
60
800
0.06
69.44
99.38
Table 2 The relative density and process parameters of the densest TA7 samples under different layer thicknesses
Fig.3 OM image (a) and SEM image (b) of the acicular α′ martensites and EDS analysis of the position P in Fig.3b (c) (δ=20 μm, V=1000 mm/s, S=0.08 mm)
Fig.4 OM image (a) and SEM image (b) of the acicular α′+massive αm microstructures and EDS analyses of the position P1 (c) and position P2 (d) in Fig.4b (δ=40 μm, V=600 mm/s, S=0.08 mm)
Fig.5 OM image (a) and SEM image (b) of the massive αm microstructures and EDS analysis of the position P in Fig.5b (c) (δ=60 μm, V=400 mm/s, S=0.08 mm)
Fig.6 XRD spectra of the TA7 bulk samples corresponding to three types of microstructures respectively
Fig.7 Relationship between metallographic structures of TA7 samples and process parameters (P=200 W, S=0.06 mm, 0.08 mm and 0.10 mm)
Fig.8 The variation of diameters of the molten pools with the layer thickness and scanning velocity
Fig.9 The variation of cooling rates of the molten pools with the layer thickness and scanning velocity
Fig.10 Microstructures of the densest TA7 samples under layer thicknesses δ=20 μm (a), δ=30 μm (b), δ=40 μm (c), δ=50 μm (d) and δ=60 μm (e) (ΔT—variation of temperature, Δt—variation of time, ΔT/Δt—cooling rate of the molten pool)
Fig.11 Trends of microhardnesses and tensile properties with layer thickness for the five densest TA7 samples
Processing method
Microhardness
Elongation
Yield strength
Ultimate strength
HV
%
MPa
MPa
Deformed TA7[20,30]
300~331
8~20
680~730
765~930
As-cast TA7[20,31]
200~310
5~8
700~725
760~795
Table 3 Mechanical properties of the conventional deformed and as-cast TA7 alloy[20,30,31]
Fig.12 SEM images of tensile fracture morphologies of the densest TA7 tensile samples deposited by layer thicknesses of δ=20 μm (a), δ=30 μm (b), δ=40 μm (c), δ=50 μm (d) and δ=60 μm (e)
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