International Journal of Refractory Metals and Hard Materials 116 (2023) 106364

Contents lists available at ScienceDirect

International Journal of Refractory
Metals and Hard Materials
journal homepage: www.elsevier.com/locate/IJRMHM

Damage characterisation of tantalum ion source electrodes andreconditioning by wire- and powder-based laser metal deposition
Tobia Romano a, b, *, Dagnija Kroģere b, Andris Ratkus b, c, Hannes Lauer d, Franz Marquardt d,Maurizio Vedani a, Diego Obradors e, Markus Weinmann f, Toms Torims b, ca

Department of Mechanical Engineering, Politecnico di Milano, Milan, Italy
Centre of High-Energy Physics and Accelerator Technologies, Riga Technical University, Riga, Latvia
The European Organization for Nuclear Research (CERN), Meyrin, SwitzerlanddFraunhofer Institute for Material and Beam Technology (IWS), Dresden, Dresden, GermanyeDepartment of Technology, Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Madrid, SpainfTaniobis GmbH, Goslar, Germanybc

A R T I C L E I N F O

A B S T R A C T

Keywords:
Laser metal deposition
Tantalum
Additive manufacturing
Repair
PIG ion source
Abnormal grain growth

The internal ion source of the Advanced Molecular Image Technologies (AMIT) superconducting cyclotron uses
cathodes made of pure tantalum to generate high energy H− ion beams for the production of isotopes for positronemission tomography. During service, the cathodes are impacted by high-energy ions from the plasma. The
resulting erosion creates craters that reduce the current density of the extracted beam. The cathodes eventuallyneed to be replaced when the ion source can no longer be activated. This research explores the possibility ofrepairing the tantalum cathodes used in the AMIT cyclotron through laser metal deposition additive
manufacturing. The damaged parts were first characterised by 3D imaging, scanning electron microscopy, andVickers microhardness to understand the damage mechanisms occurring during service and quantify the extent
of the damage. Different repair strategies were then tested employing both high-purity tantalum wire andpowder feedstocks and the properties of the reconditioned electrodes were determined. The ability of laser metal
deposition to restore the damaged cathodes for use in the AMIT cyclotron has been demonstrated.

1. Introduction
Due to its excellent high-temperature stability, corrosion resistance,and biocompatibility, tantalum is employed in specialised applicationsin the electronics, aerospace, nuclear, chemical, biomedical, and mili­tary fields [1,2]. Tantalum is also used to manufacture components forparticle accelerators [3], such as the cathodes used in the Penning Ion­
isation Gauge (PIG) ion source of the compact superconducting cyclo­tron developed within the Advanced Molecular Image Technologies(AMIT) project [4,5] (Fig. 1a). The PIG ion source used by the AMIT
cyclotron installed at Centro de Investigaciones Energéticas, Medi­
oambientales y Tecnológicas (CIEMAT) in Madrid consists of a hollow
anode made of a copper‑tungsten alloy and two electrodes (a cathodeand an anticathode) made of 99.95% pure tantalum. The ion source
operates in a pure hydrogen atmosphere to generate high-energy H− ionbeams for the production of 18F and 11C isotopes for positron emission
tomography (PET) [4,6]. Tantalum is employed owing to its highmelting point (3017 ◦ C), low discharge voltage (1700 V), low work
function (4.25 eV), and low sputtering coefficient in H2 (16 mg/Ah)[3,7,8].
During operation, the voltage applied between the electrodes causes
emission of electrons from the cathode. Emitted electrons are acceler­ated towards the opposite cathode by the potential difference, following
a helical orbit across the hollow anode due to an applied axial magnetic
field (Fig. 1b). Positive and negative hydrogen ions (H+ and H− ions,
respectively) are produced by collision of electrons with the gas mole­cules in the working chamber. As a result, a plasma is formed. H− ionsare extracted through a slit opening on the hollow anode by a puller
biased with an alternating potential, during the positive half periods ofthe potential. On the other hand, H+ ions are accelerated towards the
anticathode and impinge on it with a typical energy of several hundredeV. These collisions locally increase the temperature of the cathode andpromote the emission of secondary electrons by thermionic effect.
Although this is essential to sustain the glow discharge and produce the* Corresponding author at: Department of Mechanical Engineering, Politecnico di Milano, 34 La Masa, Milan 20156, Italy.
E-mail address: tobia.romano@polimi.it (T. Romano).
https://doi.org/10.1016/j.ijrmhm.2023.106364
Received 17 May 2023; Accepted 10 August 2023
Available online 17 August 2023
0263-4368/© 2023 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

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International Journal of Refractory Metals and Hard Materials 116 (2023) 106364

Fig. 1. Schematics of tantalum cathodes (a) and AMIT cyclotron ion source design (b), and photograph of the unused (Ta0) and worn (Ta1 and Ta2) cathodes (c).
Adapted from [6].

ion beam, after a variable running time a crater-shaped erosion profile
appears on the surface of the electrodes in the region that is subjected toion bombardment (Fig. 1c). This delays thermionic emission and reduces
the current density of the extracted beam [5]. When a relatively large
amount of material is removed, the ion source can no longer be oper­
ated. The current practice is to simply replace the worn cathodes withnew parts, since the damage mechanisms to which they are subjected
and how they affect their performance are not yet fully understood.
The need to periodically replace the cathodes increases the operating
costs of the AMIT cyclotron [5]. In addition, tantalum is considered a
critical raw material because it is widely employed in various industrial
fields, but its supply chain has often been subjected to fluctuations andFig. 2. Schematic and photograph of the LMD experimental setup using wire (a) and powder (b) feedstocks. Adapted from [23,24].
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Fig. 3. Schematic of repair strategies based on erosion crater filling (a) and full head rebuild (b).

instrument. The parts were previously covered by a thin graphite layer
to avoid excessive light reflection during the measurement. The surface
morphology and microstructure of the electrodes were studied using a
Nikon Eclipse LV150NL optical microscope and a Zeiss Sigma 500 fieldemission scanning electron microscope (FE-SEM) equipped with an
Oxford Instruments energy dispersive X-ray spectrometer (EDS) and anelectron back-scattered diffraction (EBSD) detector. For microstructuralobservation, the specimens were first sectioned by a precision diamondblade saw, mounted in thermosetting resin, grinded with SiC papers up
to 2500 grit, and finally polished with 10% water-based 0.5 μm alumina
and 0.03 μm silica suspensions. An additional vibratory polishing stepwas conducted for EBSD analyses using a 25% water-based 0.03 μm
silica colloidal suspension. Chemical etching was performed using a
fresh solution prepared with 10 ml H2SO4, 5 ml HF, and 5 ml HNO3.
Vickers microhardness measurements were performed on the pol­ished samples with a Future-Tech FM-700 microhardness tester. A loadof 500 g was applied for a dwell time of 10 s in each test repetition.

disruption [9]. Therefore, it would be not only economically advanta­
geous, but likewise sustainable, to restore the worn electrodes after they
have become inoperable, instead of replacing them with new parts.
In recent years, the ability of additive manufacturing (AM) tech­
nologies to repair and restore various kinds of industrial components has
been demonstrated [10–12]. Among these, laser metal deposition (LMD)
is particularly suited for repair, as it uses a robotic arm equipped with alaser source and a dispenser to add material to existing parts. Its highflexibility combined with the use of a highly focused laser beam enables
the repair of damaged structures with good dimensional accuracy,
minimal distortion, and excellent metallurgical bonding between thesubstrate and the added material [13]. AM of pure tantalum has been
mainly investigated in the context of laser powder bed fusion (LPBF) andelectron beam powder bed fusion (EB-PBF) for the manufacturing oforthopaedic implants [14–18], and wire arc additive manufacturing(WAAM) to produce larger components for the aerospace and chemicalindustries [7,19]. LMD has also been applied to fabricate pure tantalum
parts [20] and deposit tantalum coatings to improve the osseointegra­
tion of titanium implants [21]. However, to the best of authors'knowledge, no attempts to repair tantalum components have been re­ported in the literature so far.
In the present study, wire- and powder-based LMD technologies areproposed for repairing the worn tantalum cathodes used in the AMIT
superconducting cyclotron. Damaged parts were first characterised tounderstand the damage mechanisms occurring during service and toquantify the extent of the damage in order to devise appropriate resto­
ration strategies. After optimisation of the process parameters, repre­
sentative pure tantalum cathodes with different degrees of damage werereconditioned and then characterised to compare their properties withthose of the original parts.

2.2. Development of LMD repair strategies
LMD repair tests were conducted using wire and powder feedstocks,
respectively. The wire-based process was carried out using the COAX­
wire mini laser deposition head developed at Fraunhofer Institute forMaterial and Beam Technology IWS (Dresden) (Fig. 2a) [22]. The LMD
machine was equipped with a 1 kW TRUMPF TruDisk 1000 laser having
1064 nm wavelength and fed with a 99.95% pure tantalum wire 0.5 mm
in diameter. In the powder-based process, a COAX14 annular gap nozzle
and a Laserline LDF laser with 1080 nm wavelength and 1.1 mm spot
size were used for material deposition (Fig. 2b). The AMtrinsic® 99.9%
pure tantalum powder provided by Taniobis, having sphericalmorphology and 10–63 μm particle size, was used as feedstock material.
In both cases, argon shielding gas was continuously delivered to the melt
region.
Two different repair strategies were tested in preliminary LMD trials.
The first approach was to deposit the material inside the erosion craters
to restore the original electrode morphology (Fig. 3a). Due to the limited
availability of representative tantalum test pieces, it was not possible toconduct a robust study for the optimisation of all the process parameters.
However, the effects of wire feed rate, oxygen content in the local
shielding atmosphere, and substrate preheating condition were evalu­
ated in the wire-based process in order to obtain high-quality deposits.

2. Materials and methods
2.1. Electrode characterisation
Initial characterisation analyses were performed on a reference un­used tantalum electrode (Ta0) for comparison purposes, on a slightly
worn electrode having a shallow crater (Ta1), and on a heavily wornelectrode with a deeper crater (Ta2), as shown in Fig. 1b.
The shape of the crater of each worn cathode was determined by
optical 3D measurements, employing an Alicona InfiniteFocus
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same procedures described above. A part used in one of the powderbased LMD preliminary trials was grinded using a 320-grit SiC abra­
sive paper to demonstrate that a surface finishing similar to that of theoriginal electrodes can be achieved through a simple post-processing
operation.

Table 1
Process parameters used in multilayer powder-based LMD tests.
Parameter

Value

Laser power (W)Powder feed rate (g s− 1)
Carrier gas flow rate (l min− 1)
Deposition speed (mm min− 1)
Layer thickness (mm)
Cooling time between layers (s)

1200
2.55
6
300
0.3
30

Table 2
Process parameters for wire-based LMD repair.

The microstructure of specimens manufactured under the different
experimental conditions was characterised using an Olympus GX51
optical microscope. Metallographic preparation involved grinding with
220, 500, and 1200 grit abrasive papers and polishing using a compound
prepared with 200 ml CH3OH, 40 ml H2O2, 1 ml HNO3, and few drops of
HF. The etching solution was prepared with water, HCl, HNO3, and HF
in equal parts. The second strategy was to cut off the damaged head ofthe electrodes and build a new one starting from the top end of theremaining stem (Fig. 3b). Multilayer build-up tests were conducted
through powder-based LMD using the parameters that resulted in single
track deposits free of interface and lack of fusion porosity (Table 1).

Parameter
Laser power (W)Wire feed rate (mms− 1)Shielding gas pressure(bar)Oxygen content (ppm)
Preheating time (s)Deposition time (s)Electrode
W1

W2

W3

1000
50 (start/end
speed)
20 (normal
speed)

1000
50 (start/end
speed)
20 (normal
speed)

1000
50 (start/end
speed)
20 (normal
speed)

2.5

2.5

2.5

300
1
0.2

300
1
0.4

300
1
0.6

P1

P2

P3

800
2

800
2

800
2

6

6

6

12

12

12

400 (inner
circle)600 (middle
circle)600 (external
circle)0.4 (inner
circle)0.9 (middle
circle)1.5 (external
circle)400 (inner
circle)600 (middle
circle)600 (external
circle)0.4 (inner
circle)0.9 (middle
circle)1.2 (external
circle)400 (inner
circle)600 (middle
circle)600 (external
circle)0.4 (inner
circle)0.9 (middle
circle)1.2 (external
circle)––0.5

2.3. LMD repair and characterisation of reconditioned electrodes
Table 3
Process parameters for powder-based LMD.

The strategy adopted to refurbish representative damaged electrodesconsisted of depositing material inside the erosion craters to restore theoriginal electrode morphology. Three tantalum cathodes with varying
extent of damage were reconditioned with each LMD technique (Fig. 4).
Regarding the wire-based process, the erosion craters were simply
filled with a single dot of material, as shown in Videos 1–3. Since thevolume of deposited material depends on the duration of the process, ashort deposition time of 0.2 s was used to refurbish the electrode withthe smallest crater (W1). The deposition time was conveniently
increased for the electrodes with medium and large crater size (W2 andW3, respectively). The main process parameters for the wire-based
repair are listed in Table 2.
The scanning strategy selected for the powder-based process
involved the deposition of three concentric rings inside the erosion
crater, starting from the inner circle. The radius of the external circle
was set depending on the extension of the damaged zone (larger forelectrode P1). On the other hand, the deep crater of electrode P3 waspartially filled by depositing two dots of material before building theconcentric rings. Table 3 reports the main process parameters used in the
powder-based repair operations.
After repair, the electrodes W2 and P2 were cut, mounted in resin,
and polished for SEM, EBSD, and microhardness analyses following theParameter
Laser power (W)Powder feed rate (g s− 1)
Carrier gas flow rate (lmin− 1)
Shielding gas flow rate(l min− 1)
Deposition speed (mmmin− 1)

Circle radius (mm)Deposition time for dots(s)ElectrodeFig. 4. View of electrodes repaired with wire- (a) and powder-based LMD (b).
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Fig. 5. 3D images of the unused and worn tantalum electrodes.

arrows in Fig. 6 may have been caused either by shrinkage stress upon
re-solidification of the small molten volume or by volume expansion
produced by hydrogen absorption [25], since the cathode was kept in apure hydrogen atmosphere during operation in the cyclotron.
While the initial material exhibits a homogeneous equiaxed micro­
structure with average grain size of about 8.5 μm (Fig. 7), the head of theworn electrodes comprises of few extremely coarse grains, as high­lighted in the EBSD images of Fig. 8. Pedrazas et al. [26] demonstrated
that pure tantalum undergoes dynamic abnormal grain growth (DAGG)
when plastically deformed at temperature higher than 1450 ◦ C. The fastgrain boundary migration leads to the formation of abnormally coarse
grains, with size up to several tens of millimetres. Abnormal grain
growth may have been promoted by the high temperature and theplastic deformation experienced by the electrodes due to the impact ofhigh-energy ions. The macroscopic deformation of the head of thecathode Ta2 was accompanied by the formation of a large twin visible in
the inverse pole figure (IPF) map of Fig. 8b, as identified by themeasured 60◦ misorientation angle about 〈111〉 direction (inset of
Fig. 8b), which is typical of body-centred cubic crystals [27].
A progressive increase in the grain size was observed along the stem
length of the electrodes Ta1 and Ta2, from about 8.5 μm at the base, tomore than 100 μm in the regions just below the head (Fig. 9a-f), up to theabnormally coarse grains at the cathode head (Fig. 8). This variation is
attributed to the higher temperature experienced by the material near
the head of each electrode, where ion bombardment concentrated dur­ing operation in the cyclotron.
Although a larger grain size is normally associated with a lower
hardness, the Vickers microhardness progressively increases from about
100 HV0.5, corresponding to the hardness of the original material, toover 700 HV0.5, as shown in Fig. 9g. Since no hard second phases nor asignificant increase in the content of strengthening interstitial elements[20,28] were detected, this trend may be attributed to hydrogen- andion-irradiation-induced hardening. Previous results showed thathydrogen can have a hardening effect in other metals featuring a bodycentred cubic crystal structure, like pure iron [29] and ferritic-pearlitic
steel [30], when its content is high and hydrogen atoms are trapped bystrong trapping sites. Under these conditions, hydrogen atoms tend tosegregate to form atmospheres around dislocations, producing a pinningand dragging effect that hinders the motion of dislocations, thusincreasing the hardness of the material [29–31]. Although this effect ismore likely to occur when large amounts of hydrogen are introduced byelectrochemical charging than by simple exposure to a hydrogen gas
environment [30], Takagi et al. [32] demonstrated that vacancy clusters
acting as strong trapping sites for deuterium atoms can be introduced intantalum by bombarding it with high-energy ions. Therefore, ion
bombardment during cyclotron operation may have introducedhydrogen traps in the tantalum electrodes, promoting hydrogenentrapment and resulting in the increasing hardness measured along the
body of the serviced cathodes. In addition, sessile dislocation loops
produced in pure tantalum by H+ ion (i.e., proton) irradiation [33] caninteract with edge dislocations, reducing their mobility and leading toan increase in hardness [34].

Fig. 6. Evidence of local melting on electrode Ta1.

Fig. 7. Fine equiaxed microstructure of the unused electrode Ta0.

3. Results and discussion
3.1. Characterisation of unused and damaged electrodes
Figure 5 shows the 3D reconstruction of the head region of thecathode samples. A shallow erosion crater displaced from the headcentre is observed on both worn electrodes Ta1 and Ta2. This indicatesthat ion sputtering was concentrated on a limited area during operationin the cyclotron. The regions around the crater appear severely pitted
from the impact of ions. In addition, the presence of droplets of resolidified material (Fig. 6) indicates that the material reachedextremely high local temperatures under the action of the high-energy
ions impinging on the electrode surface. The cracks highlighted with5

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International Journal of Refractory Metals and Hard Materials 116 (2023) 106364

Fig. 8. Abnormally coarse grain structure at the head of electrodes Ta1 (a) and Ta2 (b).

Fig. 9. Grain size variation along the body of electrodes Ta1 (a-c) and Ta2 (d-f) and corresponding microhardness profile (g).

Fig. 10. Microstructure of deposits produced by wire-based LMD without (a) and with laser substrate preheating (b), and effect of insufficient start wire feed rate (c).

3.2. Development of LMD parameters

observed that the substrate material needs to be preheated prior to thedeposition process to avoid excessive residual porosity at the interface
and improve dilution with the substrate. A 1000 W laser preheating of 1s effectively eliminated the interface porosity, reduced the clad angle(ϑ), and improved the metallurgical bonding between the LMD deposit
and the substrate, as shown in Fig. 10a-b. The grain structure of the

Although an in-depth optimisation study was not conducted due tothe few available tantalum test samples, the effects of experimental
conditions on the quality of LMD deposits were investigated by varying
the main process parameters. As regards the wire-based process, it was
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Fig. 11. Near defect-free single track (a) and multilayer structures deposited by powder-based LMD (b).

Fig. 12. EBSD (a) and SEM (b) images of the electrode W2 repaired by wire-based LMD.

dome deposited with laser preheating (Fig. 10b) featured almost equi­axed grains, which initially grew epitaxially from the grains of thesubstrate. However, it was noted that the substrate should not be
exposed to a laser power of 1000 W for more than 1.5 s to avoid melting.
An oxygen content of ~12% in the local shielding atmosphere led to theformation of a distinct oxide layer on the surface of the deposits, as
highlighted in Fig. 10a-b. Therefore, the oxygen content was reduced to
300 ppm in the actual repair operations by generating a shielding gas
bell around the melt pool region. Regarding the deposition strategy, it
was observed that the wire must be fed at a higher rate at the beginningand end of the process to prevent it from melting before it has reached
the substrate, generating a ball of melted and resolidified material thatcannot be deposited correctly (Fig. 10c and Video 4).
Laser power, powder feed rate, and scanning speed were adjusted in
the powder-based process by depositing several clad beads and domes
consisting of concentric circles. The combination of parameters leading
to deposits with no lack of fusion porosity at the interface with thesubstrate (Fig. 11a) was employed in multilayer build-up tests, as indi­cated in Table 1. Based on previous experience, a cooling time of 30 s
was applied between the deposition of two successive layers to avoid
overheating and keep the interlayer temperature below 250 ◦ C. Despite
this, after depositing the first ten layers, the material underwent creep
flow and the target geometry could not be attained (Fig. 11b). Guanet al. [20] successfully manufactured pure tantalum thin-walled plates

Fig. 13. SEM images of the electrode P2 repaired by powder-based LMD.
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Fig. 14. Phase and elemental maps of powder-based LMD deposit.

by depositing several layers using powder-based LMD. However, thesmall size of the multilayer deposits tested in the present study probably
resulted in a poorer dissipation of the input heat during the deposition
process, giving rise to the observed flowing phenomenon. Therefore,
further research efforts need to be dedicated to better adjust the processparameters to pursue the repair strategy based on printing a new elec­
trode head to replace the damaged one.

Table 4Vickers microhardness of deposits made by wire- and powder-based LMD incomparison with the original material.
Electrode
Microhardness (HV0.5)

3.3. Characterisation of repaired electrodes

Ta0

W2

P2

98.9 ± 2.4

315.3 ± 10.3

665.4 ± 19.0

tantalum parts fabricated by multilayer LMD [20] were not observed
because no significant competitive growth occurred by depositing
concentric rings in the same plane.
Observation of high magnification SEM images revealed that fineprecipitates decorate the grain boundaries and extremely fine particles(less than 300 nm in diameter) are also dispersed within the grains, as
depicted in Fig. 13b. The phase and elemental maps collected from the
EBSD and EDS analyses (Fig. 14) indicate that these scales are Ta2O5
particles, possibly formed due to turbulence in the shielding gas flow
during the LMD process caused by the carrier gas conveying the powder
particles towards the melt pool [39]. The overall oxygen content
measured by EDS is around 3 wt%. In addition, a distinct oxide layer
about 60 μm in thickness continuously covers the external surface of thedeposited dome, as highlighted in Fig. 13a. This must have formed at theend of the deposition process, when the already solidified but still hot
metal was exposed to environmental oxygen once the shielding gas flow
was turned off. This oxide layer may have a beneficial effect byincreasing the secondary emission coefficient of the material (i.e., theratio of the emitted secondary electrons to the incident primary elec­
trons), thus helping to sustain the glow discharge and produce the ion
beam [3,40]. However, its actual effect on the electrode performancestill needs to be evaluated by testing the restored cathodes under service
conditions. The round pores located at the interface between the sub­strate and the deposited dome (Fig. 13a) may have been generated bygases trapped inside the molten bath during the deposition process [38].
Table 4 compares the Vickers microhardness of the repaired elec­
trodes with that of the original material. The microhardness of the de­posit made with the wire-based process is similar to that reported fortantalum parts and coatings fabricated by direct laser deposition [20,21]
and slightly lower than pure tantalum processed through LPBF [14].
This can be attributed to the slower cooling rates of LMD compared to
LPBF [38], typically resulting in a coarser microstructure, and to thevery small size of the electrodes, leading to poor dissipation of theprocess input heat. The high hardness of the deposit made using thepowder feedstock can be primarily attributed to the fine oxide particles
observed both at the grain boundaries and dispersed within the grains,providing a precipitation hardening effect. Another contribution is due

The EBSD image of Fig. 12a shows the cross-section of the electrodeW2 repaired with the wire-based process by depositing one dot of ma­
terial inside the crater. The LMD deposit, highlighted by the dotted line,
is composed of few large grains grown epitaxially from the substrate, as
they have the same crystallographic orientations as the grains beneath.
Additively manufactured tantalum typically exhibits coarse columnar
grains with a 〈111〉 preferential orientation aligned along the build di­rection [19,35]. This microstructure develops in pure tantalum because
favourably oriented 〈111〉 grains [36,37] tend to grow along the direc­
tion of maximum heat flow towards the substrate [38], that acts as aheat sink. In the present case, large columnar grains could not develop
because only one dot of material was deposited. Therefore, edge effects
dominated the solidification [35]. Only few dispersed pores smaller than
10 μm in size can be seen in the micrographs. In addition, there are no
distinct signs of oxide layers (Fig. 12b), indicating that the employedshielding gas bell effectively protected the material against oxidation athigh temperature during the repair process. An oxygen content of about
0.8 wt% was detected by EDS analysis. Although this technique cannot
provide a very precise measure of the content of light elements, thisvalue is comparable to that measured for the unused electrode (~0.5 wt%). More accurate measurements could not be performed due to thesmall size of the specimen.
Figure 13a shows the cross-section of the electrode P2 repaired withthe powder-based process. The LMD deposit is composed of randomly
oriented equiaxed grains with average size around 52 μm, as shown in
the EBSD image in the inset of Fig. 13a. Unlike the wire-based process,the substrate could not be preheated in powder-based LMD because thelaser could only be turned on after the powder flow had already been
established. Hence, the deposited material in contact with the cold
substrate experienced a higher cooling rate, generating the finer grainsobserved. Balla et al. [21] found a similar microstructure in tantalum
coatings of comparable thickness applied to titanium substrates by laser
deposition, but with a slightly larger grain size (about 70 μm). The lower
average grain size observed here can be attributed to the higher thermal
conductivity of the tantalum substrate compared to titanium, resulting
in a faster cooling. The large columnar grains that develop in pure8

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International Journal of Refractory Metals and Hard Materials 116 (2023) 106364

Fig. 15. Photographs and SEM images of a powder-based LMD specimen before (a) and after (b) the grinding post-processing operation.

to the interstitial oxygen detected by local EDS performed in areas
devoid of oxide particles, which can provide strong solid-solution
hardening in pure tantalum [20,28]. The finer grain size of the deposit
made with the powder-based process is also responsible for the increasein hardness compared to the other specimen through the Hall-Petchmechanism. Zhou et al. [14] showed that an increase in the hardnessof pure tantalum processed through LPBF is associated with improved
wear resistance. Therefore, the high hardness attained after repair byboth the wire- and powder-based process compared with the initialmaterial may be beneficial to the durability of the electrodes used in the
AMIT cyclotron.

oxygen content of ~0.8 wt%, similar to that of the original ma­
terial (~0.5 wt%).
(3) Concentric rings were deposited in the powder-based process.
Instabilities in the shielding gas flow resulted in ~3 wt% dis­solved oxygen and finely dispersed oxide particles, leading to a
microhardness of about 665.4 HV0.5 due to solid-solution andprecipitation strengthening, respectively.
(4) A simple post-processing operation effectively removed theexcess material deposited during repair, resulting in a surface
quality comparable to that of the original electrodes.
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.ijrmhm.2023.106364.

3.4. Improvement of surface finish

Declaration of Competing Interest

Both the wire- and the powder-based processes created a prominent
dome at the head of the repaired cathodes, as shown in the photographsof Fig. 4. In principle, a protruding surface could increase the local
electric field at the electrode head compared to a flat surface [41], thus
promoting the emission of electrons when the operating voltage isapplied between the electrodes of the ion source. In addition, it could
increase the service life of the electrodes by providing additional ma­
terial against erosion crater formation. The main disadvantage of thisdesign is the possible occurrence of short-circuits, which needs to beassessed by testing the restored cathodes in the AMIT cyclotron. The
possibility of obtaining a flat surface with a finish comparable to that of
the original parts was still demonstrated by removing the excess mate­rial from a test piece through a simple grinding operation (Fig. 15).

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.
Data availability
Data will be made available on request.
Acknowledgements
This work was carried out within the frame of the I.FAST project andhas received funding from the European Union's Horizon 2020 Researchand Innovation programme under grant agreement No 101004730.
Open access funding was provided by Politecnico di Milano within the
CRUI-CARE Agreement.
Authors would like to acknowledge the support given by Rasheed
Michael Ishola and Ludovica Rovatti for SEM and EBSD analyses.

4. Conclusions
This work investigated the damage mechanisms to which thetantalum cathodes used in the ion source of the AMIT superconducting
cyclotron are subjected during service and the possibility to restore themthrough wire- and powder-based laser metal deposition. The main out­
comes can be summarised as follows:

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