A Review on the Effect of Powder Oxidation, Internal

Porosity and Crystallographic Texture on the Charpy

Impact Energy of Ti-6Al-4V Specimen Fabricated using

Electron Beam Melting and Hot-isostatic Pressing Post

Processing

Abhinav Maurya Department of Manufacturing Process

and Automation Engineering

Netaji Subhas University of Technology

New Delhi, India

Akshay Dabas Department of Manufacturing Process

and Automation Engineering

Netaji Subhas University of Technology

New Delhi, India

Andriya Narasimhulu Department of Manufacturing Process

and Automation Engineering

Netaji Subhas University of Technology

New Delhi

Abstract — Electron beam melting (EBM) is an innovative

additive manufacturing process in which metal powder is

completely melted by a concentrated beam of electrons. In this

paper, the effect of powder oxidation, internal porosity and

crystallographic texture on the charpy impact energy of Ti-

6Al-4V specimen fabricated using electron beam melting and

hot-isostatic pressing post processing was reviewed. It was

observed that the charpy impact energy dramatically

decreases in a smooth but rapid fashion due to excessive

powder oxidation and has a deleterious effect due to the

internal porosity present in EBM Ti-6Al-4V specimen.

Crystallographic texture was also found to influence Charpy

absorbed energy. However, HIP post-processing significantly

increases the impact toughness for specimens in each case.

Keywords — Additive Manufacturing, Electron Beam Melting,

Hot-isostatic pressing, internal porosity, powder oxidation,

Crystallographic Texture

I. INTRODUCTION

Electron beam melting (EBM) is an innovative additive

manufacturing (AM) process in which metal powder or

filament is completely melted by a concentrated beam of

electrons. Production in a vacuum chamber ensures that

oxidation will not deteriorate highly reactive materials like

titanium. Vacuum production is also required so electrons do

not collide with gas molecules. Among other additive

manufacturing (AM) technologies, metallic powder bed fusion

electron beam melt (EBM) has recently gained considerable

attention in the medical, automotive, and aerospace

communities for the fabrication of production components

directly from 3D CAD files bringing enhanced design freedom,

minimal material waste, and little post-processing (1).

The electron beam technology began with the experiments by

physicists Hittorf and Crookes, who first tried to generate

cathode rays in gases (1869) and to melt metals (1879). The

heat created by electrons colliding had damaging effect and

attempts were made to inhibit this by means of cooling. In 1906,

physicist Marcello von Pirani first made use of this effect by

building a piece of apparatus for melting tantalum powder and

other metals using electron beams. In 1948, Dr. H.C. Karl-

Heinz Steigerwald built the first electron beam processing

machine in 1952. The initial development work was done in

collaboration with Chalmers University of Technology in

Gothenburg. In 1993, a patent was filed describing the principle

of melting electrically conductive powder, layer by layer, with

an electric beam, for manufacturing three-dimensional bodies.

In 1997 Arcam AB was founded and the company continued

the development on its own, with the objective to further

develop and commercialize the fundamental idea behind the

patent.

The EBM process involves spreading a layer of pre-alloyed

metallic powder in the evacuated build space, selectively

melting regions in the layer with an electron beam, spreading

another layer of powder and repeating the process until a three-

dimensional solid metal part is contained within the powder

cake (1). A tungsten filament in the electron beam gun is

superheated to create a cloud of electrons that accelerate to

approximately one-half the speed of light. A magnetic field

focuses the beam to the desired diameter. A second magnetic

field directs the beam of electrons to the desired spot on the

print bed.

Once a component or prototype has been printed, the build

envelope is removed and the build platform and attached object

are removed from the loose powder. Powder clinging to the

object or remaining in internal cavities is blown or blasted

away. Post-processing methods, including hot isostatic pressing

(HIP), heat treatment in inert gas or vacuum heat treatment may

be employed to release residual stresses and improve

mechanical properties.

Today, the potential of electron beam melting technology is

recognized and is used to print components used in aerospace,

automotive, military, petrochemical and medical applications.

By improving access to emerging high-growth submarkets,

electron beam melting technology offers a competitive edge to

progressive enterprises. In many applications, designers enjoy

unprecedented design flexibility. It produces parts with

properties similar to wrought parts and better than those of cast

parts. For many applications, EBM is a cost-effective process

that reduces inventory requirements and with build rates of

almost four times those of other AM technologies. The electron

beam melting process reduces residual stresses in a variety of

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ways. It is possible to control residual stress during the

preparation of CAD data, during printing and in post-

processing. During printing, residual stress is reduced by

preheating the print bed and by the heating of the material

before it is struck by the electron beam. To a degree, lower

residual stress is also a function of the process’ high build

temperatures and slower cool-down rates compared to laser-

based AM processes.

Electron beam melting requires the use of pure, unadulterated

metals. Any changes in powder properties such as powder

oxidation, internal porosity and crystallographic texture leads

to changes in material property and hence product properties.

Many researchers have investigated the effects of EBM

processing parameters on material properties, with the aim of

identifying the optimal parameters for near-fully-dense parts

[11-14]. The EBM process has many parameters that can be

altered by the user, but the EBM manufacturer provides

recommended settings for Ti-6Al-4V.

II. LITERATURE REVIEW

In this paper, the effect of powder oxidation, internal porosity

and crystallographic texture on the charpy impact energy of Ti-

6Al-4V specimen fabricated using electron beam melting and

hot-isostatic pressing post processing was reviewed. It was

observed that the charpy impact energy dramatically decreases

in a smooth but rapid fashion due to excessive powder oxidation

and has a deleterious effect due to the internal porosity present

in EBM Ti-6Al-4V specimen. Crystallographic texture was also

found to influence Charpy absorbed energy. However, HIP

post-processing significantly increases the impact toughness for

specimens in each case.

A. Effect of powder oxidation

Powder quality in additive manufacturing (AM) electron beam

melt (EBM) is crucial in determination of material properties.

The effect of powder oxidation on the Charpy impact energy of

Ti-6Al-4V specimen manufactured using EBM was studied by

considering oxygen content significantly higher than 0.2 %

mass fraction and precisely controlling the amount of oxygen

as a function of time and temperature [3].

Powder samples were prepared by artificially oxidising powder

in air at elevated temperature and then mixing it with low

oxygen content powder. Using this method, four batches of

powders were used (A) virgin powder (B) five-times reused

powder (C) marginally oxidized powder, made up of a mixture

of five-times reused powder mixed with artificially oxidized

powder and (D) highly oxidized powder (Table 1). These

powder samples were then used to fabricate Charpy specimen

with the EBM process in three distinct orientations. Further,

half of the blanks were post-processed using a standard hot-

isostatic pressing (HIP) procedure common for Ti-6Al-4V

castings and EBM produced components and then machining

them into Charpy specimens and testing at room temperature.

The comparison of the absorbed energies and fracture

appearances was done.

TABLE 1: POWDER LOT DESCRIPTIONS AND POWDER OXYGEN

CONTENTS [1] Identifier Powder Mixture

Description

Powder Oxygen Content (% wt)

(± 2.5 %.)

Before build After build

A 100% Virgin Powder 0.11 0.123

B 1005 5x Reused

powder

0.142 0.157

C Mixture of 96.5 % 5x

reused and 3.5 % aged at 650 °C for 4 h

0.340 0.292

D Mixture of 88.8 % 5x

reused and 11.2 % aged at 650 °C for 4 h

0.525 0.455

Fig. 1: SEM images and cross-sections of (a, b) virgin and (c, d) highly oxidized (650 °C for 4 h in air) powder. (b, d) [1]

for all test conditions. The HIP process is typically performed

at α+β annealing temperatures, and thus results in a coarsened

microstructure compared to the as-built condition.

B. Effect of internal porosity and crystallographic texture

The effects of internal porosity and crystallographic texture on

Charpy absorbed energy (over temperature range of -196 to 600

°C) were determined by two heat treatment conditions (As-

Built and Hot Isostatically Pressed (HIPed)) on Vertical and

Horizontal specimen orientations [4]. The specimens were

fabricated using an Arcam1 A1 EBM machine at the standard

Arcam A1 build theme for Ti-6Al-4V (accelerating voltage 60

kV, layer thickness 50 μm, speed factor 35, software version

3.2.132) and the standard Arcam Ti-6Al-4V gas atomized

powder (particle size range approximately 40–100 μm, average

approximately 70 μm). Charpy build volumes were built 5mm

above the build plate and connected to the build plate using

standard thin wafer supports. It is important to note that Chaput,

et al. [5] did not use supports and instead built parts directly

attached to the build plate.

Total twenty specimen for each orientation (Horizontal,

Vertical) were fabricated of which half were randomly assigned

for HIPing, and other half remained As-Built leading to four

testing conditions (As-Built/Horizontal, As-Built/Vertical,

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HIPed/Horizontal, and HIPed/Vertical). The standard Ti-6Al-

4V HIP cycle was used (2 h at 900 °C and 100 MPa in argon,

with 12 °C/min heating and cooling rates). Charpy tests were

performed at temperatures ranging from -196–600 °C (±3°C).

Wrought Ti-6Al-4V (mill-annealed condition) was also Charpy

tested to provide a non-AM comparison for Charpy properties

due to its different microstructure. Chemistry was measured for

wrought Ti-6Al-4V, As-Built EBM Ti-6Al-4V, and HIPed

EBM Ti- Al-4V and then compared to ASTM F2924 [6].

Internal porosity was non-destructively evaluated for As-Built

and HIPed material using a laboratory x-ray micro-computed

tomography (CT) instrument. Texture was measured using

electron backscatter diffraction (EBSD) on a scanning electron

microscope (SEM) operated at 20 keV. Samples were mounted

and polished with diamond slurry to 1 μm and then vibratory

polished with 0.05 μm colloidal silica for up to 8 h. Both β and

α texture were measured directly at 30 nm step size for 13 fields

of view per testing condition.

No pores were observed (1 μm voxel size) in the HIPed

condition and the pore size distribution for the as-built

condition agreed well with previous work on the same material

with pores up to 10 μm diameter in HIPed material (calculated

relative density as 99.8% dense) [7]. All observed porosity was

approximately spherical, indicating it is of the gas porosity, and

not the lack of fusion, variety [8-10].

Representative fracture surfaces for all conditions are shown in

Figure 3, displaying two main features of interest, internal pores

and ridges. Internal pores (Figure 3f and black arrows) were

observed on all As-Built fracture surfaces but not on HIPed

fracture surfaces. Ridges (Figure 3e and white arrows) were

observed on all Horizontal fracture surfaces but not on Vertical

fracture surfaces which ran perpendicular to the layers (X-Y

plane) and parallel to the build direction (Z).

Significant coarsening was observed due to HIPing (Figure 4)

whilst, α lath thicknesses did not change appreciably as a

function of distance from the bottom of the build volume. The

As-Built/Vertical testing condition had a majority of 001 β

orientation, but it was far from an exclusive 001 α fibre texture.

The Burgers relationship was found to hold for α orientation

with respect to β. It is apparent from Figure 5 that prior-β grains

are elongated in the build (Z) direction. In larger area texture

maps, no appreciable differences were perceived between the

between HIPed/Horizontal (Figure 5a, Figure 5c) and

HIPed/Vertical (Figure 5b, Figure 5d) conditions when

compared. However, considerable differences were observed

between the two planes (X-Y and XZ) due to the anisotropic

morphology of the elongated prior-β grains in the z-direction. It

has recently been shown that prior-β grain boundaries impede

dislocation motion as long as the two adjacent prior-β grains are

of different texture orientation [19].

III. RESULTS

A. Effect of Powder Oxidation

For a given oxygen content, no differences were observed in the

overall roughness and appearance between the samples tested

in different orientations. For the five-times reused and

marginally oxidized samples, the fracture surface features had

characteristics of both the virgin and highly oxidized samples,

with more brittle-type features in the artificially oxidized

samples and more ductile-type features in the five-times reused

samples. However, voids were always detected on the fracture

surfaces of the non-HIP specimens but were not observed on

the HIP surfaces, irrespective of their oxidation levels and

orientations. This was found to be true for the X-ray Computed

Tomography (CT) scan results of HIP and non-HIP specimens.

Figure 6 and figure 7 show the comparison between Charpy

absorbed impact energies and Hardness Rockwell C values

from the ends of the Charpy specimens (respectively) for the

Fig. 2: SEM images of Charpy specimen fracture surfaces made from virgin powder in the (a, d) X-Y, (b, e) X-Z, and (c, f) Z-X orientations with (a, b, c)

HIP or (d, e, f) non-HIP post-processing (arrow/arrow point indicates the build direction Z) [1]

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Fig. 4: EBSD images showing representative direct β texture measurements (bright green) using 30 nm step size for a) As-Built and b) HIPed conditions

[2]

Fig. 5: EBSD larger area α texture maps for a) HIPed/Horizontal X-Y plane, b) HIPed/Vertical X-Y plane, c) HIPed/Horizontal X-Z plane, and d) HIPed/

Vertical X-Z plane [2]

four oxygen levels including the effects of the three specimen

orientations and non-HIP versus HIP post-processing. Figure 8

shows the charpy absorbed impact energy as a function of

consolidated material oxygen content, including the effects of

the three specimen orientations and non-HIP versus HIP post-

processing

It was perceived from the figures 6-8 that the Charpy absorbed

impact energies decreased dramatically with increasing oxygen

content in case of HIP and that HIP post-processing always

Fig. 3: SEM images of fracture surfaces of a) As-Built/Horizontal, b) As-Built/Vertical, c) HIPed/Horizontal, and d) HIPed/Vertical [2]

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Fig. 6: Charpy absorbed impact energies for the four oxygen levels including

the effects of the three specimen orientations and non-HIP versus HIP

post-processing [1] (Error bars represent ± 1 standard deviation)

Fig. 7: Hardness Rockwell C values from the ends of the Charpy specimens

for the four oxygen levels including the effects of the three specimen

orientations and non-HIP versus HIP post-processing [1] (Error bars represent

± 1 standard deviation)

improved the impact energy compared to as-built samples. The

orientation effects were found to be much less discernible for

the highly oxidized specimens and the Z-X orientation was

found to be the toughest. Additionally, the instrumented striker

force-time histories indicated relative differences in ductility

between the specimens, where the specimens with the lowest

oxygen contents, the most brittle behavior by the highly

oxidized specimens exhibited the most ductile behavior.

It was observed that HIP post-processing always improved the

impact energy (as expected) compared to as-built samples and

that reduces the effects of orientation in the most severely

oxidized samples due to the dropped impact energies. In

addition, HIP post-processing removes the typical micro-voids

encountered and improves the ductility of EBM Ti-6Al-4V

while maintaining the strength at acceptable levels [15-16].

B. Effect of internal porosity and crystallographic texture

The results suggest that internal porosity has a deleterious effect

on Charpy absorbed energy, which has been shown previously

for other material systems too [17]. X-ray CT measurements

showed internal porosity in the As-Built condition (99.8%

dense), but not in the HIPed condition which was supported by

the fractography (Figure 3 through evidence of pores on the

fracture surfaces of As-Built Charpy specimens. In Charpy

results (Figure 10), HIPed conditions have higher absorbed

energy compared to As-Built conditions and was attributed to

the effect of internal porosity. α lath thicknesses as a function

of distance from bottom of build volume are shown in Fig. 9.

The observed coarsening of α laths due to HIPing (Figure 4,

Figure 9) also contribute to this trend as it is known that coarser

Ti-6Al-4V microstructures have higher Charpy absorbed

energy [18].

The results also suggested that crystallographic texture has an

effect on Charpy absorbed energy. For the HIPed condition, the

Vertical orientation exhibited higher absorbed energy

compared to the Horizontal orientation but for the As-Built

condition, there appeared to be no difference (Figure 10). Even

Fig. 8: Charpy absorbed impact energy as a function of consolidated material oxygen content, including the effects of the three specimen orientations and

non-H IP versus HIP post-processing [1]

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Fig. 9: α lath thickness as a function of distance from the bottom of the build

volume [2]

Fig. 10: Charpy absorbed energy for all four EBM Ti-6Al-4V test conditions as well as Wrought Ti-6Al-4V (mill annealed) [2]

though the crack pathway for the Vertical orientation crosses

more prior-β grain boundaries than the crack pathway for the

Horizontal orientation, the latter was found to be tougher for the

As-Built conditions. It is hypothesized this is because of the

differing texture (i.e., predominantly 110 β for Horizontal and

predominantly 001 β for Vertical). The fact that the

predominantly 001 β texture has lower toughness compared to

the predominantly 110 β texture [20] led to the negligible

difference in Charpy absorbed energy for As-Built/Horizontal

and As-Built/Vertical test conditions.

Figure 10 shows the charpy absorbed energy for all four EBM

Ti-6Al-4V test conditions as well as Wrought Ti-6Al-4V (mill

annealed). The results suggest that HIPed conditions have

higher absorbed energy than As-Built conditions (Figure 10).

For the HIPed condition, the Vertical orientation appear to have

higher absorbed energy than the Horizontal orientation.

However, this apparent trend did not seem to exist for the As-

Built condition. All of these observed differences appear to be

larger at higher temperatures. HIPed EBM Ti-6Al-4V compares

well with Wrought Ti-6Al-4V, despite differences in

microstructure (mill annealed) and chemistry (e.g. oxygen

content).

IV. CONCLUSION

It was found that excessive powder oxidation (oxygen mass

fraction above 0.25 % and up to 0.46 %) dramatically decreases

the impact energy, about seven times at the room temperature.

As the powder oxygen mass fraction increased from 0.11 % to

0.53 %, Charpy impact energy of Ti-6Al-4V decreased in a

smooth but rapid fashion (depending on orientation and post-

processing). In addition, HIP post-processing significantly

increases the impact toughness, especially for specimens with

lower or normal oxygen content. The specimen orientation

effect was found to be more significant for low oxidation levels.

HIP post-processing increased toughness of the alloy in all

directions, especially for low and medium oxygen content. Both

the HIP post-processing and orientation effects on toughness

largely disappeared at the 0.5 % mass fraction level.

Results suggest that internal porosity has a deleterious effect on

the Charpy absorbed energy of EBM Ti-6Al-4V. This was

evident as HIPed material displayed higher Charpy absorbed

energy over a range of temperatures (-196 to 600 °C) compared

to As-Built material, and spherical internal porosity was found

(x-ray CT, fractography) in the As-Built condition (99.8%

dense) but not after HIPing. Crystallographic texture was also

found to influence Charpy absorbed energy along with the

anisotropic grain morphology (i.e., prior-β grains elongated in

the build (Z) direction). It was observed that for similar texture,

crack pathways that cross more prior-β grain boundaries lead to

higher Charpy absorbed energy. However, differing textures

were found to negate the prior-β grain boundary strengthening

effect in some cases, emphasizing the influence of texture and

variations in texture on Charpy absorbed energy. For the four

testing conditions (As-Built/Vertical, As-Built/Horizontal,

HIPed/Vertical, and HIPed/Horizontal), types of texture varied

from predominantly 001 β to predominantly 110 β, but none

matched the most commonly reported texture i.e. 001 β-fibre.

V. FUTURE SCOPE

Apart from the many advantages of EBM, it has many

challenges which are all related to powder used (as powder is

the raw material for EBM). So far, researchers have

qualitatively reviewed the (limited) parameters related to

powder i.e. powder oxidation, internal porosity and

crystallographic texture and their effect on the charpy impact

energy only. Quantification of the respective contributions of

porosity and coarsening haven’t been done till now. In addition,

a deeper understanding of the influences of texture processing

is necessary and needs to be addressed in future work.

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