Elsevier

Materials Science and Engineering: A

Volume 713, 24 January 2018, Pages 125-133
Materials Science and Engineering: A

Strain rate effect on the tension and compression stress-state asymmetry for electron beam additive manufactured Ti6Al4V

https://doi.org/10.1016/j.msea.2017.12.062Get rights and content

Abstract

The present experimental investigation lays the groundwork for coupling the plastic flow stresses of an Additive Manufactured (AM) Ti6Al4V with the corresponding strain hardening rate responses and fracture morphologies for this material at varying strain rates and stress states for the first time. The macroscopic response was obtained under different stress states (uniaxial tension and compression) and deformation rates (ranging from quasi-static at 0.001 s−1 to the high rate domain at 1500 s−1) to elucidate the tension-compression asymmetry and strain rate dependence of the material. The results identified that, (a) compressive yield strengths are higher than their tensile counterparts and (b) the strength differential effect is more prominent under high strain rates. The mechanical behavior is explained in terms of deformation mechanisms reasoned from close inspection of strain hardening rate curves. Dislocation glide and mechanical twinning contribution as dominant plastic units were identified in stages and their relation with deformation mode and rate was established. Based on fractography analyses of the tensile samples, a rate dependence showed a shift from classic cup-cone fracture at quasi-static rates to fracture along the shear plane at high strain rates. The high strain rate compression samples also show an interesting fracture morphology with high speed video capturing adiabatic shear localization.

Introduction

Although rapid prototyping and related layer manufacturing processes have been developed over the past several decades, the recent availability of commercial systems have posted revolutionary manufacturing concepts [1]. Additive Manufacturing (AM), termed by some researchers as “the next renaissance in manufacturing,” [2] offers many promising benefits such as reducing the need for tooling accessories such as molds and jigs and allowing for more optimized and complex components to be produced than conventional manufacturing techniques. These attributes make AM especially applicable to the automotive, aerospace, defense, electronics and medical/biomedical industries. Specifically, Electron Beam Melting (EBM) is an AM Powder-Bed Fusion (PBF) process that uses a high-power electron beam to generate the energy needed for melting conductive pure and/or alloy metal precursor powders. Examples of materials used for EBM manufacturing includes Ti6Al4V, Ti48Al2Cr2Nb, CoCr alloys, H13 steel, Inconel 625 and 718 alloys, Rene 142 alloys, Nb and Fe [3], [4], [5] with Ti6Al4V being the most widely investigated because of its high strength to weight ratio, machinability, heat treatability and excellent corrosion resistance [6], [7]. Titanium, an allotropic element, and the respective titanium alloys are well suited for aero-engines, energy generation turbines, armor structures and/or biomedical applications owing to intrinsic material property behaviors.

Of paramount importance is the meticulous mechanical characterization of materials intended for high velocity impact, shock waves and/or ballistic service conditions. Consequently, the response of Ti and Ti alloys, and their sensitivity to deformation rate, temperature and stress state, has been extensively reported in literature from watershed publications by Meyers et al. [8], da Silva and Ramesh [9] Liao and Duffy [10] and Nemat-Nasser and coworkers [11] to recent articles centered on characterizing Ti6Al4V [12], [13], [14], [15], [16] and AM Ti6Al4V [17], [18], [19], [20], [21]. In addition to examining deformation rate, temperature and stress state influences, several researchers have investigated the flow stress dependence on initial microstructures [22], [23], [24]. In fact, studies have shown that the strain rate dependence on flow stress for Ti6Al4V is different when comparing stress states [25].

In separate studies, Nemat-Nasser et. al [26] and Chichili and coworkers [27] reported the dynamic flow stress curves of α-Ti. While Chichili and collaborators [27] shed light to the strain rate sensitivity and work hardening of α-Ti plastic flow, Nemat-Nasser's [26] thermomechanical studies correlates temperature and strain rate to a three-stage strain hardening response. Furthermore, both investigations identified the underlying deformation micro-mechanisms controlling the macroscopic dynamic response. For instance, at the submicron level Chichili observed both dislocations and twins [27]. Chichili also pointed out that twin density increased with strain and deformation rate, and although that dislocation glide accounted for the majority of the plastic strain, twin-dislocation interaction plays an important role in strain hardening [27]. Meanwhile, Nemat-Nasser et al. [26] identified a three-stage deformation pattern at a temperature range from 23 to 525 °C which was ascribed to dynamic strain aging of dislocations caused by solute atoms. Nevertheless, this explanation does not apply to ambient temperature deformation since dynamic strain aging is only essential in Ti in the 225–525 °C temperature range [28]. As later researched in Ti plasticity, a second strain hardening stage usually coincides with the onset of twinning and their resistance to dislocation glide, an initial domain signifies dynamic recovery (typical of metals with high stacking fault energy) and a final-third regime reflects flow saturation [29], [30].

Kapoor and Nemat-Nasser measured the fraction of energy converted into heat during high rate plastic deformation on commercially pure Ti [31]. Based on infrared detector thermal signals, the researchers reported that nearly all the work done during high strain rate plastic deformation was converted into heat, with the temperatures quantified being strongly dependent on strain rate. In spite that the empirically determined cooling rate lies within the 107 °C s−1 range, the rate of heat generation due to plastic flow exceeds the rate of heat dissipation to the surrounding material [10], [32]. When localized, such a mismatch produces plastic flow instabilities that are manifested as zones of highly localized deformation commonly referred to as Adiabatic Shear Bands (ASB). The ASB phenomena has been noted as the dominant failure mode for metals and alloys subjected to high rate loading conditions observed in ballistic impact, explosive fragmentation and high velocity fabrication [17], [33]. Additionally, several authors have documented that the localization occurs more easily in materials with low thermal conductivity and a high thermal softening rate [17], [18], [34] thus demonstrating that Ti alloys are susceptible to the formation of and subsequent failure by ASB.

Research conducted by Liao and Duffy [10], Rittel and Wang [14] and Landau et al. [15] presents a comprehensive examination of the localized shear zone's microstructural evolution in dynamically-loaded Ti6Al4V. Liao and Duffy [10] reported local strain and strain rate values (75–350% and 8000 s−1 respectively) on patterned thin-walled tubular specimens tested in a torsional Kolsky bar. In addition, the authors used an array of infrared detectors to interrogate the local temperature rise during the deformation process; temperature readings within the 440–550 °C range were recorded inside the sheared region. Based on the measured temperature raise and fractography analysis, the authors concluded that there was no clear evidence to suggest that the material within the shear band region had undergone phase transformation. Subsequently, the research by Rittel and Wang [14] leads to the identification of the three distinct stages in ASB formation (homogeneous, inhomogeneous and localized strain); their findings concur with earlier experimental results reported by Marchand and Duffy [35]. While the latter based their conclusion on high-speed photography of a fiducial grid pattern, the Rittel and Wang [14] research relied solely on infrared radiometer measurements. Landau and coworkers [15] expand on the work of Rittel and Wang [14] by examining the shear concentration zone at its vicinity via Transmission Electron Microscopy (TEM) analysis. Both, interrupted and fractured shear compression specimens, presents pronounced microstructural refinement from dislocation cells with increasing misorientation until the formation of dynamically recrystallized grains (DRX).

Although, as shown in the referenced work, there has been a number of dynamic studies on Ti and its alloys, however, there is scarce work in Ti systems manufactured by AM methods. In one study, uniaxial static and dynamic compression experiments coupled with microstructural analyses by Biswas et al. [18] identified ASB as the predominant failure mode on Laser-Engineered-Net-Shaped (LENS™) porous Ti6Al4V. By similar analyses Li and collaborators [19] reported the presence of ASB on dynamically loaded laser-deposited Ti6Al4V. Additionally, the tension-compression asymmetry under high strain rate conditions was discussed. Interesting to note, these studies show that the AM process does indeed affect the microstructure and subsequent mechanical properties. Given the wide breath of dynamic reporting of wrought Ti6Al4V in literature, and the current rise of AM methodologies of Ti6Al4V, there exists much to understand regarding the dynamic structure-property relations of AM produced Ti6Al4V.

As previously mentioned, the EBM AM process is an attractive AM process that warrants further investigation of the dynamic response of Ti6Al4V; mainly, since EBM has already been successfully utilized in other Ti6Al4V studies [3], [4], [5], [6], [7]. In these studies, EBM has shown uniqueness in microstructures that are heavily dependent on the AM input parameters. Furthermore, the prior work by the authors [20], the first study on dynamic tensile response of EBM AM Ti4Al4V, only focused on a single stress state and therefore lacks information on how the stress state asymmetry may influence the performance of components made by EBM. Being that the EBM process has already been shown by the authors to present interesting dynamic properties, connecting this unique microstructure to the stress state dependent dynamic mechanical properties, especially damage and failure, is of great advantage to AM manufacturing of components that may undergo dynamic events in service. As referenced previously, Ti6Al4V has already been shown to have significant strain rate dependence on stress state asymmetry [25].

With the aforementioned in mind, this research effort attempts to develop an understanding of the stress state and strain rate dependence of EBM AM Ti6Al4V for the first time. The emphasis is placed on the correlation of the macroscopic flow response to failure where the possible mechanism controlling damage and microstructural aspects of fracture behavior are discussed.

Section snippets

Materials and methods

An ARCAM S12 EBM system located at NASA Marshall Space Flight Center was used to fabricate the samples for this study. Fine pre-alloyed bimodal (α + β) Ti6Al4V powder, with particle diameter between 45 and 100 µm was used as feedstock precursor. The nominal chemical composition of the as-supplied powder was 6Al, 4V, 0.03C, 0.10Fe, 0.15O, 0.01N, 0.003H, balance Ti (w.t.%). Target pre-heat temperature was set to 730 °C. Scan speed was 0.376 m/s with a beam current of approximately 6 mA. Powder layer

Results and discussion

The as-built microstructure is presented by the bright field 3D optical reconstruction in Fig. 1. SEM insets illustrate higher magnification micrographs for different planar orientations of the as-built unit cell. Notice the orientation dependence of the initial microstructure with respect to the built axis (z-axis) that correlates to prior AM Ti6Al4V investigations [45], [46], [47]. The equiaxed grain morphology (49.33 µm ± 15.22 µm) observed in the planar region XY resulted from the

Conclusions

To conclude, the macroscopic flow stress response of an AM-EBM Ti6Al4V at varying stress states and deformation rates was coupled with its corresponding strain hardening rates and fracture morphologies with the goal to elucidate the underlying deformation mechanisms. Specifically, the research identified the following:

  • 1.

    While deformation rate effect on tensile flow stress is negligible, the influence is reflected on the fracture path and morphology. Fractography analyses revealed cup-and-cone

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