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Research Papers

Dynamic Tensile Response of Additively Manufactured Ti6Al4V With Embedded Spherical Pores

[+] Author and Article Information
Refael Fadida

Rafael,
POB 2250,
Haifa 3102102, Israel
e-mail: fadidarafi@gmail.com

Amnon Shirizly

Rafael,
POB 2250,
Haifa 3102102, Israel

Daniel Rittel

Faculty of Mechanical Engineering,
Technion,
Haifa 3200003, Israel

1Corresponding author.

Contributed by the Applied Mechanics Division of ASME for publication in the JOURNAL OF APPLIED MECHANICS. Manuscript received November 7, 2017; final manuscript received January 14, 2018; published online February 2, 2018. Editor: Yonggang Huang.

J. Appl. Mech 85(4), 041004 (Feb 02, 2018) (10 pages) Paper No: JAM-17-1620; doi: 10.1115/1.4039048 History: Received November 07, 2017; Revised January 14, 2018

The dynamic tensile response of additively manufactured (AM) dense and porous Ti6Al4V specimens was investigated under quasi-static and dynamic tension. The porous specimens contained single embedded spherical pores of different diameters. Such artificial spherical pores can mimic the behavior of realistic flaws in the material. It was found that beyond a certain pore diameter (Ø600 μm), the failure is determined according to the pore location, characterized by an abrupt failure and a significant decrease of ductility, while below that diameter, necking and fracture do not occur at the pore. The dynamic tensile mechanical behavior of the additively manufactured dense material was found to be similar to that of the conventional equivalent material, but the ductility to failure of the latter is observed to be higher.

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Figures

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Fig. 5

Additively manufactured Ti6Al4V compared to conventional material in quasi-static tension at nominal strain rate of 2 × 10−4 s−1. The graph represents the uniform elongation part, prior to necking.

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Fig. 4

The pore is concentric to a theoretical sphere, which determines the volume fraction of the pore

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Fig. 3

A set of additively manufactured specimens, which contain an enclosed pore (outlined) with different diameters. The pore is located at the geometrical center of the gauge section. Note the shape of the pore, which is relatively spherical.

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Fig. 2

A reference specimen for evaluating the optimal pore shape and diameter. (a) The cross section of the CAD model, (b) the X-ray imaging, and (c) the Ø300 and Ø400 μm pore at higher magnification. Note that Ø300 μm is the lower limit of feasible pores in the current setup.

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Fig. 1

A sketch and typical printed tensile specimen after machining

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Fig. 6

Additively manufactured Ti6Al4V compared to conventional material in dynamic tension at nominal strain rate of 1 × 103 s−1. The graph represents the uniform elongation part, prior to necking.

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Fig. 7

A single pore specimens in quasi-static tension with different pore diameter at nominal strain rate of 2 × 10−4 s−1. (a) All the Ø300 μm specimens and one Ø400 μm specimen did not fail at the pore (similar to the dense specimens behavior). (b) One Ø400 μm specimen and all the Ø500, Ø600, Ø700, and Ø1000 μm specimens failed at the pore.

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Fig. 8

Load displacement curves of all single pore specimen type i.e., Ø300, Ø400, Ø500, Ø600, Ø700, and Ø1000 μm at nominal strain rate of 1 × 103 s−1. The red line in each figure represents the mean curve of all tests. Above Ø500 all specimens failed at the pore without exceptions. The displacement to failure was determined by drawing a line, which is parallel to the linear elastic response, at the point before the force rapidly drops.

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Fig. 9

X-ray image of an additively manufactured specimen contains Ø500 μm pore. Two deformed specimens on the right, compared to an undeformed specimen on the left. Note that the fracture occurred not at the pore.

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Fig. 10

A SEM image of a specimen contains Ø600 μm pore after failure. (a) The trace of the pore in the top of the cone can be clearly seen. (b) A typical view of the cone surface characterized by dimples indicates ductile fracture.

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Fig. 11

A typical fractography of a specimen contain Ø600 μm pore after tensile test. (a) Quasi-static loading. (b) Dynamic loading.

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Fig. 12

High speed camera image of specimens at dynamic tensile test. (a) Fully dense AM Ti6Al4V. The fracture occurred at the impacted side. (b) A Ø1000 μm pore at the moment of fracture. Note the residual powder that was released out, which proves that the pore content is not solid (i.e., in powdery state).

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Fig. 13

Single pore specimens in tension with different pore diameter at nominal strain rate of 1 × 103 s−1. (a) All the Ø300, Ø400, and most Ø500 μm specimens failed away from the pore. Note that all the curves tend to cluster around the dense specimen curves. (b) Only one Ø500 μm specimen and all the Ø600, Ø700, and Ø1000 μm specimens failed at the pore. (The arrows indicate a typical curve based on the average curve represented in Fig. 8). Here, a clear decline in displacement to failure can be noticed with increasing pore diameter.

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Fig. 14

Displacement to failure as a function of pore volume fraction for a single pore specimen under dynamic loading. The vertical dashed line separates between the specimens that failed at the pore and the ones who did not. The displacement to failure at zero porosity represents the dense material.

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Fig. 15

The mesh of the numerical model. The specimen (additively manufactured Ti6Al4V), which contain a single pore Ø600 μm and the incident and the transmitted bars (maraging 300).

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Fig. 16

The numerical simulation results for the fully dense and the single-pore specimens. (a) The Ø300 and Ø400 μm specimens failed away from the pore. (b) The Ø500, Ø600, Ø700, and Ø1000 μm specimens failed at the pore.

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Fig. 17

At the top, a Ø600 μm pore at the moment of fracture. Note that the simulation captures the cup and cone pattern that was observed in experiments. The neck in the simulation is noticeable, although in practice it is quite limited (as can be seen at the bottom). This can explain the bias between simulation and experimental results.

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Fig. 18

The simulation corresponded well with the force–displacement response of the dense specimens, but from Ø600 μm and above some deviation is observed regarding final specimen failure

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