Dynamic mechanical properties and failure characteristics of electron beam melted Ti-6Al-4V under high strain rate impact loadings

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Highlights

  • Ti64 rods were printed using EBM technique in horizontal and vertical directions.

  • Dynamic impact tests (1150s−1 to up to fracture) were conducted using SHPB.

  • Effect of build direction on the dynamic deformation behaviour was reported.

  • Flow softening is primarily due to the formation of voids and adiabatic shear bands.

  • Predictions of the constitutive model were in agreement with experimental data.

Abstract

This study presents an investigation on the effects of building direction on microstructure, dynamic mechanical properties, and deformation mechanisms of electron beam melted Ti-6Al-4V (EBM-Ti64) cylindrical rods. Microstructural features were characterized using optical microscopy (OM) and scanning electron microscopy (SEM). The initial microstructure in both directions consists of transformed α+β phases and grain boundary-α (αGB)along prior β-grain boundaries. The vertically built cylindrical specimens have a finer grain structure, including lower interlamellar spacing and finer α-laths compared to the horizontally built ones. Dynamic impact tests using Split-Hopkinson Pressure Bar (SHPB) were conducted on both horizontally and vertically built samples at strain rates ranging between 1150 and 2700 s−1. Dynamic mechanical properties are strain-rate sensitive; the maximum flow stress of 1960 MPa (at 2100 s−1) and 2160 MPa (1650 s−1) were obtained for horizontal and vertical specimens, respectively. Horizontal specimens fragmented when deformed at 2700 s−1, whereas the vertical specimens failed at a much lower strain rate (1900 s−1). At a given strain rate, vertical specimens exhibited better dynamic strength and lower strain (total strain) due to their finer microstructure. The temperature rise during deformation primarily governs flow softening at all conditions, which led to the formation of adiabatic shear bands (ASBs). Microstructures of deformed specimens revealed thermal softening features such as voids formation. These voids coalesced and grew, leading to crack initiation and propagation along ASBs. Fractographic examination of the fragmented specimens under impact loading revealed ductile dimples and smoother surfaces, which indicate a combination of both ductile and brittle fracture. The contribution of twinning and pyramidal <c+a> slip systems is the primary deformation mechanism during high strain rate impact loading of EBM-Ti64. The experimentally obtained flow curves are in good agreement with the Chang-Asaro equation-based constitutive modeling results.

Introduction

Titanium alloys exhibit outstanding properties such as high strength-to-weight ratio, toughness, good elevated temperature performance, biocompatibility, and excellent corrosion resistance [1,2]. These properties make them desirable for a wide range of industries from aerospace, automotive, marine, chemical and energy industries, and also in biomedical applications as surgical implants [[3], [4], [5], [6]]. Ti-6Al-4V (hereafter Ti64) is one of the most commonly used titanium alloys as it exhibits the best combination of mechanical properties with a balance between strength and ductility [7]. The microstructure of Ti64 alloy consists of two phases of α and β. Its strength comes from the solid solution strengthening by aluminum in the α-phase, whereas the ductility and toughness are attributed to the β-phase. However, titanium has a high affinity for oxygen [8], and studies have shown that the increase in oxygen content reduces toughness and high-temperature corrosion behavior [9,10]. Generation of a considerable amount of waste or scrap during conventional processing and the tendency to pick up oxygen during welding at high temperatures are some of the concerns with the subtractive manufacturing routes.

Additive manufacturing (AM) is a unique technique that involves the fabrication of complex or desirable engineering components from a 3D design in short production time without the need for expensive tools (such as dies and cast molds) for high geometrical freedom [11]. Additive manufacturing is based on layer-by-layer stacking of materials leading to effective and economical material utilization, which is in contrast to bit by bit material removal in the conventional machining processes [12]. Based on material feedstock, metal additive manufacturing technologies are broadly divided into two types; powder bed-based and powder/wire feed [13]. A further classification in powder-based fusion technology is based on the used energy source including laser beam (e.g., selective laser melting – SLM, Direct Metal Laser Sintering – DMLS), electron beam (e.g., Electron Beam Melting – EBM), and electric arc sources (e.g., wire arc additive manufacturing – WAAM) [[14], [15], [16]].

The introduction of the EBM technique has brought remarkable changes to the manufacturing industry, especially in the aeronautical and biomedical sectors. Lightweight EBM-produced complex parts with optimized microstructure and good strength are desirable for fuel conservation in the transportation sector. EBM-produced porous titanium structures are proved to be competent to accelerate the process of bone healing by stimulating the growth of bone tissue in the liquid environment contacted by the implant in the human body [17]. An EBM system is generally equipped with a high energy source (tungsten filament) electron beam and vacuum device. Owing to high beam intensities that are inertia-free and controlled by electromagnetic lenses, the EBM process produces high scan velocities, which leads to lower fabrication time and, consequently, to low production costs, especially for complex shapes [18,19]. EBM process is performed in a vacuum chamber (10−4-10−5 mbar), which prevents the entrapment of gases like oxygen and nitrogen in titanium and aluminum alloys. EBM has attractive unique features such as pre-heating procedures and a high processing temperature environment. It is reported that EBM-produced Ti64 parts are fully dense, and they have fewer defects and less residual stresses compared to the SLM-produced counterparts [20].

For structural engineering applications, where EBM-Ti64 components are subjected to impact and dynamic loading conditions, it is imperative to understand the response of this alloy for optimum design and production requirements. There have been few studies on the dynamic deformation behavior of conventional and AM-produced Ti64 [[21], [22], [23], [24], [25], [26], [27], [28]]. Most of these studies focused on the dynamic behavior at a macroscopic level or the effect of microstructural variation with processing or heat treatment conditions on such behavior. To the authors' best knowledge, there has been no work done on the effect of build direction on the dynamic response of the EBM-Ti64. In the authors' previous work in Ref. [29,30], EBM-Ti64 rods were printed in horizontal and vertical directions, and the influence of build direction on tensile behavior and hardness was studied. The effect of the build orientation on the dynamic response of the EBM-Ti64 at elevated strain rates up to 1100 s−1 was the subject of another study.

The objective of this study is to investigate the difference in the dynamic deformation behavior of horizontally and vertically produced EBM-Ti64 under dynamic loadings at a range of high strain rates (>1100 s−1) until fragmentation occurs. Both macroscopic and microscopic behaviors (compressive properties and fracture features), and the underlying deformation mechanisms are discussed. Mathematical modeling based on the Chang-Asaro constitutive equation is used to evaluate and predict the dynamic flow behavior.

Section snippets

EBM process and fabrication of Ti64 rods

Cylindrical Ti64 rods of 120 mm length and 9.5 mm diameter were printed through the EBM process using an Arcam EBM Q10 additive machine at Woburn, MA, USA. The dimensions of a stainless steel base plate are 200 mm × 200 mm × 180 mm. A single crystalline cathode with a maximum beam power of 3000 W was used in a vacuumed chamber (5 × 10−4 mbar). Optimum process parameters, as recommended by Arcam, shown in Table 1, were used to fabricate the rods to achieve high density product. Powder bed

Powder characterization

Fig. 4(a)–(b) shows the morphology of the Ti64 Grade 5 virgin powder at low and high magnifications, respectively. Almost all particles exhibited globular morphology and relatively smooth surfaces. The size of the powders varied from 40 to 100 μm with an average diameter of 55 μm. Some satellite-like particles are also present in the powder, which are smaller in size.

Microstructural features of as-built (initial) EBM-Ti64 rods

Typical microstructures of the as-built horizontal and vertical EBM-Ti64 samples are presented in Fig. 5(a)–(b), respectively. In

Conclusions

Dynamic deformation behavior of horizontally printed (at the strain rates of 1150 s−1, 1650 s−1, 2100 s−1, 2700 s−1) and vertically printed (1150 s−1, 1500 s−1, 1650 s−1, 1900 s−1) EBM-Ti64 rods were evaluated using a split-hopkinson pressure bar system. Microstructural features were characterized using optical and electron microscopy. The conclusions made from research findings are summarized as follows:

  • 1.

    The typical initial microstructures of EBM-Ti64 in both build directions consist of αGB

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

CRediT authorship contribution statement

R. Alaghmandfard: Investigation, Data curation, Writing - original draft. C. Dharmendra: Formal analysis, Visualization, Writing - review & editing. A.G. Odeshi: Methodology, Investigation. M. Mohammadi: Conceptualization, Supervision, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to thank Natural Sciences and Engineering Research Council of Canada (NSERC) grant number RGPIN-2016-04221, New Brunswick Innovation Foundation (NBIF)grant number RIF2017-071, Atlantic Canada Opportunities Agency (ACOA)- Atlantic Innovation Fund (AIF) project number 210414, Mitacs Accelerate Program grant number IT10669 for providing sufficient funding to execute this work. The authors acknowledge the GE/Arcam in Woburn, MA, USA, for fabricating the Ti-6Al-4V cylinders

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