UNIVERSIDADE ESTADUAL PAULISTA

“JÚLIO DE MESQUITA FILHO”

Instituto de Ciência e Tecnologia

Campus de São José dos Campos

ORIGINAL ARTICLE DOI: https://doi.org/10.4322/bds.2023.e3562

Braz Dent Sci 2023 Apr/Jun;26 (2): e3562

Evaluation of EDM inclusion into the conventional polishing of

printed Co-Cr alloy by selective laser melting

Avaliação da inclusão de EDM no polimento convencional da liga de Co-Cr impressa por fusão seletiva a laser

Mahmood Abdulla Ahmad AL NUAIMI

, Israa Mohammed HUMMUDI



1 - Middle Technical University, College of Health and Medical Techniques, Baghdad, Iraq.

How to cite: Nuaimi MAAA, Hummudi IM. Evaluation of EDM inclusion into the conventional polishing of printed Co-Cr alloy by

selective laser melting. Braz Dent Sci. 2023;26(2):e3562. https://doi.org/10.4322/bds.2023.e3562

ABSTRACT

Objective: The selective laser melting (SLM) technique used in manufacturing results in a rougher surface that

requires more satisfying processing than conventional hand-nishing operations. The electro discharge machine (EDM)

has various possibilities in the adjustment of surfaces. The present study assesses whether the participation of the

EDM technique with the conventional nishing and polishing methods enables surface improvement for the Cobalt-

Chromium alloy fabricated by SLM. Material and Methods: Twenty discs of cobalt chromium alloy were fabricated

by SLM, divided equally into two groups: (TF) control group for nishing and polishing in the conventional method

in accordance with the manufacturer’s recommendations; and (EF) group for conducting polishing incorporating the

EDM method. Results: The EF group recorded the lowest mean value of surface roughness and the highest mean

value of micro hardness compared to the TF group. Furthermore, statistically signicant differences (P < 0.05)

were found for surface roughness as well as micro hardness. Conclusion: Reliance of the electric discharge machine

proactively within nishing and polishing procedures promotes competence in the conventional polishing method

and improves the surface properties of cobalt chromium alloy printed by SLM technology.

KEYWORDS

Dental nishing; Cobalt-chromium alloy; Selective laser melting; Electric discharge machine; Additive manufacturing.

RESUMO

Objetivo: A técnica de fusão a laser seletiva (SLM) usada na fabricação resulta em uma superfície mais rugosa

a qual requer um processamento mais satisfatório do que o acabamento manual. A máquina de eletro descarga

(EDM) possui várias possibilidades no ajuste de superfícies. O presente estudo avalia se a participação da técnica

EDM associada aos métodos convencionais de acabamento e polimento possibilita a melhora da superfície da

liga Cobalto-Cromo fabricada através da SLM. Material e Métodos: Vinte discos de liga de cromo-cobalto foram

confeccionados por SLM, e divididos igualmente em dois grupos: (TF) grupo controle, realizado acabamento e

polimento pelo método convencional de acordo com as recomendações do fabricante; e (EF) grupo do polimento

associado ao método EDM. Resultados: O grupo EF registrou o menor valor médio de rugosidade supercial e o

maior valor médio de microdureza em relação ao grupo TF. Além disso, diferenças estatisticamente signicativas

(P < 0,05) foram encontradas para rugosidade supercial, assim como para a microdureza. Conclusão: A

conança na máquina de descarga elétrica proativamente nos procedimentos de acabamento e polimento

promove a competência no método de polimento convencional e melhora as propriedades de superfície da liga

de cromo-cobalto impressa pela tecnologia SLM.

PALAVRAS-CHAVE

Acabamento dental; Liga de cobalto-cromo; Fusão a laser seletiva; Máquina de descarga elétrica; Manufatura

aditiva.

Braz Dent Sci 2023 Apr/Jun;26 (2): e3562

Nuaimi MAAA et al.

Evaluation of EDM inclusion into the conventional polishing of printed Co-Cr alloy by selective laser melting

Nuaimi MAAA et al.

Evaluation of EDM inclusion into the conventional polishing of

printed Co-Cr alloy by selective laser melting

INTRODUCTION

Selective laser melting (SLM) has become an

excellent approach for fabricating functional metal

structures, which represents one of the essential

additive manufacturing (AM) technologies in

dentistry. The printing procedure for metallic

alloys by the SLM technique is based on the emitted

structure of metal powder being constructed in a

layered pattern over the building plate using a

laser beam to induce localised melting by focusing

heat and prompt solidication of totally melted

particles [1,2]. Cobalt-chromium (Co-Cr) alloy is

regarded as an appropriate biomedical metallic

alloy for dental prosthesis applications as well as

a practical alloy in SLM printing with acceptable

mechanical properties [3,4]. However , the SLM

technique is conducive to high surface roughness

for the produced metal structures compared

to casting and block milling techniques [5,6].

Although factors such as build orientation and

process parameters can alter the printed alloy

properties towards acceptable performance , the

roughness of the surface is a perennial problem

in SLM manufacturing and may be difficult to

overcome in the settings of the alloy printing

phase [7,8]. There is still a need to refurbish how

to tackle surfaces for various metal prostheses

in dental applications, such as metal frames of

removable partial dentures, which have a wide

aspect of surface exposure [9]. particularly when

produced using the SLM technique, which may

require additional effort and efciency [10-12].

Given that conventional nishing and polishing

procedures may be difficult to predict their

effectiveness, this may necessitate a more efcient

alternative achieved on dental alloys printed by

SLM technology that have high surface roughness.

In this context, electric discharge machines

(EDM), which are referred to in dentistry as

“spark erosion,” may be used to alter the surface

of conductive materials. This erosion function is

caused by the electrical discharge’s high-frequency

spark between two electrodes, which affects the

substrate surface by removing excessive material

through electric erosion [13-15].

The feasibility of using EDM to nish hard

metals is considerable and can thus approach the

micron level of nishing when the spark intensity

and discharge electric current are precisely

regulated in the suitable gap between the work

piece and the terminal tool of the electrode, which

should be smooth and regular in shape [16]. It is

noteworthy that EDM applications in dentistry

are given particular attention as an opportunity

to benet in limited inter-removal tasks of metal

alloys to ensure precision tting prostheses and

superstructures interface over implants through

surface refinement by thermal energy that is

generated into electricity discharge [17-20]. From

the same perspective, the dental literature may

not give an accurate description of how to utilize

EDM in surface nishing and knowledge about

suitable parameters of this technique , although

it may be practicable for EDM inclusion within

conventional hand-nishing/polishing phases

on dental alloys printed by SLM, which may be

appropriate for the attainment of aspirations

through the conciseness of the nishing process

and participate treatment of SLM surface defects.

This study aims to determine whether

inclusion of the electro discharge machine

(EDM) in conventional nishing and polishing

procedures for dental cobalt chromium alloys

printed by SLM technology inuences surface

roughness and micro hardness.

The null hypothesis was that the inclusion

of the electro discharge machine (EDM) in

conventional nishing and polishing procedures

not inuences on tested properties.

MATERIAL AND METHODS

The commercial SLM processing dental alloy

that is used is cobalt-based Co-Cr (WIRONIUM®

RP, BEGO, Germany), 10-45 um grain size, ISO

22674 [21], with chemical compositions of (Co

66.2%, Cr 28.2%, Mo 5.5%, N < 1%) that are

specied by the manufacturer.

Preparation of specimens

The study included twenty specimens that

were designed from a computer-aided design

(CAD) in a disc shape, 2 mm thick and 15 mm in

diameter, utilizing application software (Rhino 7,

Robert McNeel & Associates, USA) in a standard

tessellation language (. STL) file [5], then

printed by machine (NCL M2150X) operating

with an ytterbium ber laser beam of 200 watts

of power (IPG Laser). The following settings

were adopted: a powder layer thickness of

0.03 mm, laser output of 195 watts, a scan speed

of 1200 mm/s, and track spacing of 0.09 mm.

Then the stress-relieving thermal treatment was

carried out for specimens that had been built

Braz Dent Sci 2023 Apr/Jun;26 (2): e3562

Nuaimi MAAA et al.

Evaluation of EDM inclusion into the conventional polishing of printed Co-Cr alloy by selective laser melting

Nuaimi MAAA et al.

Evaluation of EDM inclusion into the conventional polishing of

printed Co-Cr alloy by selective laser melting

in vertical orientation (6) and fastened to the

platform surface by beds heated in a furnace to

800 degrees Celsius for 45 minutes as per the

instructions of the alloy powder manufacturer.

Surface processing of specimens

Surface processing divided the specimens

into two main groups (n = 10) based on the

technique of nishing and polishing.

In the conventional nishing and polishing

control group “TF”, carried out in accordance with

recommendations specied by the manufacturer

in the rst step, ne grit stones with a high cutting

capacity (REF 43160, BEGO) and a 2.35 mm

shank are used for efcient grinding of dental

alloys. The specimens were mounted on a dental

surveyor (RD, M-103) with a hand piece (Kavo

Gmbh, Biberach, Germany) rotated at a rotational

speed of 30.000 rpm at one-minute intervals

for each specimen. Then a fine sandblasting

(Basic Eco, Renfert) unit, nozzle 0.8 mm, and

particles (Perla Blast glass bead blasting material,

50 microns, BEGO) were used to sandblast the

specimens. mounted at a distance of 10 cm

for 10 seconds under a pressure of 4 bar [22].

Thereafter, they electro “polished” for 5 minutes

with 12 V of electric current, preparatory to the

rubber-polishing phase too, which encompassed

rubber wheels (AB201, AB202-Dentorium,

USA) at a rotational speed of 10,000 rpm at

two-minute intervals, respectively. Ultimately,

the specimens were polished with paste (REF

52310, BEGO, Germany) by a buff brush (Wool

Buff, Attenborough Dental, UK), and the ensuing

leather deer mop (90 mm with Leather Strips,

Vertex, Netherlands) by means of the polishing

lathe (UNIVERSAL POLISHING BENCH, Balkan

Motor San, Turkey). The specimens were

postured on a special holder at successive speeds

of 1500 and 3000 rpm.

The EDM group “EF” was conducted with

a portable polishing machine (YJCS-5B, YIJING

ELECTRIC Co., Ltd., China) that emits an electric

spark at a direct current (DC) of 90 volts and

a maximum discharge current of 1.83 amps.

The machine discharges a high-frequency electric

spark to remove excessive material during EDM

erosion at an adjustable power level. In terms

of control power, grade one has the lowest

discharge at 0.25 amps, while grade nine has

the highest discharge at 1.83 amps without

change in voltage, which remains persistent

at 90 volts. The presumption of reaching low

surface roughness by the nishing process by

EDM necessitates a low discharge current through

a suitable spark of intensity [23]. Commensurate

with dental alloy, the surface alteration process

for fitting the superstructure on the implant

through EDM is proceeded at 1.5A in the rst

stage of inter-surface removal task and final

fitting at 0.5A to achieve optimal precision

contact of clear surfaces [13]. In accordance

with the manufacturer’s instructions, grade

“nine” is recommended for coarse surfaces and

major removal tasks, then gradually proceed to

the lowest grade until it reaches acceptable ne

surfaces; that is, it is intended to be practicable to

obtain a surface roughness of around Ra0.3 as a

result of applying grade “one” power on reasonably

smooth surfaces.

According to the pilot sample studied and

previous instructions, the aspiration was to obtain

a particular grade that corresponded to the phases

of the standard nishing process, which resulted

in the electric current selection (0.68 , 0.63, 0.46,

0.3, 0.25) of “ve” to “one,” operating grades,

consecutively, one-minute duration per specimen

that was attached to the power circuit with an

electrode of anode and an electrode of brass rod

H62 cathode, immersed in the water basin at a

depth of 5 mm with a suitable effective spark gap

for all held by a dental surveyor. After the EDM

phase, all specimens in this group are polished by

the rubber wheel polishing and eventually by the

lathe polishing machine, as indicated in “TF.”, in

order to attain a lustrous surface.

Eventually, both groups (n = 20) were

cleaned with deionized water and 70% propanol

in an ultrasonic bath (CD-4860, CODYSON)

for ten minutes [5] in readiness for the surface

roughness and hardness tests.

Surface Roughness

Specimens were measured using a

profilometer (Taylor HOBSON, Leicester,

UK) according to the stander test method

(ASTM-D7127-17) [24]. The test style was two

diagonal lines crossed perpendicularly, calibrated

at a length of 8 mm “CUT-OFF”, and the adoption

of Mean Value Ra (µm) on each specimen.

Hardness

The Micro Vickers Hardness HV tester

(TH710, Beijing TIME High Technology Ltd.)

Braz Dent Sci 2023 Apr/Jun;26 (2): e3562

Nuaimi MAAA et al.

Evaluation of EDM inclusion into the conventional polishing of printed Co-Cr alloy by selective laser melting

Nuaimi MAAA et al.

Evaluation of EDM inclusion into the conventional polishing of

printed Co-Cr alloy by selective laser melting

was used to measure specimens in accordance

with the ASTM E 92 Standard [25] test method

for Vickers Hardness of metallic materials at a

setting dwell time of 15 seconds, an applied force

of 4.904 (N), and the mean of three indentations

for each specimen was recorded.

The Statistical software (SPSS) version

(22.0) was used to evaluate the study’s ndings

and apply the independent-samples t-test analysis

to analyse the difference between the two groups.

RESULT

Table I summarizes the surface roughness

testing (Ra) and Table II summarizes the micro

hardness measurements (HV) of two independent

groups: the conventional finishing/polishing

group “TF” and the (EDM) group “EF.” With the

inclusion of the electro discharge machine, the

results show that the “Surface Roughness test”

accounted for a low level of readings (Ra) and

a high level of readings (HV) in the “EF” group

of (EDM).

With respect to testing statistical hypotheses,

and that should be proved according to the

test of equal variances, as well as equal mean

vales are assumed through Levene and t-test,

respectively, the (Ra) results showed that no

significant difference was accounted for at

P > 0.05 regarding the test of homogeneity of

variances, as well as a signicant difference

was accounted for at P < 0.05 with reference

to testing of mean values, as illustrated in

Table III.

Furthermore, the (HV) results showed that

a significant difference was accounted for at

P<0.01 regarding the test of homogeneity of

variances, as well as a signicant difference was

accounted for at P<0.05 with reference to testing

of mean values, as illustrated in Table IV.

Table I - Mean values, standard deviation, standard error, and 95% conﬁdence interval of mean parameters concerning mean values of the

Surface Roughness Test for the studied group’s readings and the two extreme values (min. and max.)

Test Groups No. Mean SD SE

95% C.I. for Mean

Min. Max.

L.b. U.b.

Surface

Roughness

TF 10 0.303 0.124 0.039 0.21 0.39 0.19 0.60

EF 10 0.194 0.102 0.032 0.12 0.27 0.09 0.46

SD: Standard Deviation; SE: Standard Error; C.I.: Conﬁdence Interval; L.b: Lower Bound; U.b: Upper Bound; 95% conﬁdence interval (CI) at the

lower bound (L.b.) and upper bound (U.b.).

Table IV - Testing equal variances and equal mean values for the HV Micro Hardness test concerning the studied groups TF and EF

Micro Hardness

Testing Homogeneity of Variances T-Test- Testing Equality of Means

Levene Statistic Sig.

(*)

T-test Sig.

(*)

TF and EF 22.110 0.000 (HS) -2.871 0.017 (S)

(*)

HS: Highly Sig. at P<0.01; S: Sig. at P<0.05.

Table II - Mean values, standard deviation, standard error, and 95% conﬁdence interval of mean parameters concerning mean values of the HV

Micro Hardness Test for the studied group’s readings and the two extreme values (min. and max.)

Test Groups No. Mean SD SE

95% C.I. for Mean

Min. Max.

L.b. U.b.

HV Micro

Hardness

TF 10 328.48 95.405 30.17 260.23 396.73 172.3 452.2

EF 10 416.52 17.383 5.497 404.09 428.96 381.4 432.8

SD: Standard Deviation; SE: Standard Error; C.I.: Conﬁdence Interval; L.b: Lower Bound; U.b: Upper Bound; 95% conﬁdence interval (CI) at the

lower bound (L.b.) and upper bound (U.b.).

Table III - Testing equal variances and equal mean values for the Surface Roughness test concerning the studied groups TF and EF

Surface Roughness

Testing Homogeneity of Variances T-Test- Testing Equality of Means

Levene Statistic Sig.

(*)

T-test Sig.

(*)

TF and EF 0.418 0.526 (NS) 2.144 0.046 (S)

(*)

S: Sig. at P<0.05; NS: Non Sig. at P>0.05.

Braz Dent Sci 2023 Apr/Jun;26 (2): e3562

Nuaimi MAAA et al.

Evaluation of EDM inclusion into the conventional polishing of printed Co-Cr alloy by selective laser melting

Nuaimi MAAA et al.

Evaluation of EDM inclusion into the conventional polishing of

printed Co-Cr alloy by selective laser melting

DISCUSSION

This study aimed to determine the inuence

of surface roughness and micro hardness by the

inclusion of the electro discharge machine (EDM)

in traditional nishing and polishing procedures

for dental cobalt chromium alloys printed by

SLM technology. According to the results, the

null hypothesis was rejected given the inuence

of incorporating the electro discharge machine

(EDM) into conventional nishing and polishing

procedures, which triggered decreased surface

roughness and a rise in hardness value.

In practical terms, the presented ndings

are consistent with previous studies [26,27] that

employed the EDM method as a nishing process

conducive to surface improvement, accompanied

by an obvious reduction in roughness value.

Conversely, there is no agreement with studies by

Wandra et al. [28] and Kushwaha et al. [29] as

they reported dissatisfactory levels of roughness

on the surface by EDM application.

The employment of EDM in the surface

nishing demands consistent parameters with

considering that current is the primarily affecting

factor in electric discharge power for emitted

spark [30,31]. This shows the operation of

the EDM in group EF at an acceptable electric

current of 0.68 A for surface nishing, followed

by gradually diminishing the current to reach

a minimum current of 0.25 A that ensures the

surface roughness is obtained at a low value.

This result is consistent with the study of Wang

and Han [32], which reports that the progression

of the decrease in the surface roughness for

surface nishing purposes by the EDM operation

is dependent on a gradually decreasing electric

current parameter.

The decreasing level of roughness caused by

reliance on the EDM technique may give priority

to how to achieve dispensing with a series of initial

finishing-phases: grinding, sandblasting, and

electro-polishing, which are likewise preparatory

in fostering subsequent polishing [33,34]. It is

conceivable that the material removal rate (MRR)

produced by EDM on the surface was stable and

coordinated at an appropriate discharge energy

level, resulting in a decrease in surface roughness

on the remaining substrate [30].

In this study, the specimens were printed

in the optimal vertical orientation in the

SLM recommended methods, which obtain

low roughness results by stability in layering

formative. However, they are not asserted to be

released from disparity in melting entirely at the

consolidation of powder particles as a result of

turbulence in maintaining the necessary heat in

wetting the previous layer and applying the heat

necessary to melt the subsequent layer, which

causes surface defects [8].

There might be an assignment for EDM

to upgrade finishing/polishing in EF group to

overcome the defects of turbulent melting on

the substrate layer of the SLM process through

recasting the surface, which has not been obtained

by the initial phases of the TF group [26].

The thermal energy generated through electrical

discharge power by EDM serves as a heat treatment

by recasting the surface into a more consolidated

layer by means of addressing the defects such as

porosity and cracking in the substrate [35,36]. This

result essentially correlates with the low electric

current extent possible to the completion of the

nishing process through the thermal propagation

of spark discharge, which permits the partially

melted particles to evaporate and collapse the peak

of sharpness irregular ridges by means of melting

and subsequent solidication, which performs as

a surface modulation [23].

Consequently, recasting assists in eliminating

surface meanders and surface protrusions,

which might facilitate the finishing process

of SLM surface by providing an accessible

posture of renement by altering the surface to

accommodate the polishing phase. This shows

that the results of the EF group had a signicant

difference in comparison with the TF because of

the effectiveness of the polishing phases by the

rubber wheel polishing and the lathe polishing

machine, as illustrated in Table III.

This reveals mastery of the EDM finishing

process on surface renement was not implemented

by sandblasting with glass bead blasting material,

50 microns preceded by stone bur nishing according

to the manufacturer’s recommendations. That might

be construed as the existence of extra edges of the

SLM surface arising from the partial melting of

particles along the melting path, obstructing the

striking of sanding glass beads, leaving the beads in

a stacked state at surface scratches [37,38].

Concerning micro hardness, the EDM

technique offered a rise in hardness value recorded

in the EF group that was not attained with

the exception of including EDM as in the TF

Braz Dent Sci 2023 Apr/Jun;26 (2): e3562

Nuaimi MAAA et al.

Evaluation of EDM inclusion into the conventional polishing of printed Co-Cr alloy by selective laser melting

Nuaimi MAAA et al.

Evaluation of EDM inclusion into the conventional polishing of

printed Co-Cr alloy by selective laser melting

group. This finding was in agreement with

Mahajan et al. [14] and Adarsh et al. [29] as they

reported an increase in hardness value correlating

with the surface treatment by the EDM. This

technique fundamentally improves the hardness

of the surfaces when the energy of the spark

tends to generate heat that causes, in return,

melting of the external layer surface followed by

re-solidication by quenching in the same electric

water basin, which modies the substrate surface

by recasting the supercial layer into a more dense

layer [35]. The aforementioned demonstrate the

qualications of the EDM method in compensating

for conventional nishing in the initial phases in

this sense, the reliability of EDM inclusion within

conventional hand-nishing/polishing operations

on dental alloys printed by SLM may be appropriate

for the attainment of aspirations through the

conciseness of the finishing process. It implies

seizing the opportunity of recasting the surface by

EDM for a surface adjustment that may be decient

in resorting to conventional nishing techniques

and, furthermore, revealing the efcaciousness

of nal polishing phases for the same technique.

CONCLUSION

Within the limitations of this study, it

demonstrates the potential of including EDM

proactively within conventional finishing and

polishing procedures for dental cobalt chromium

alloys that are printed by SLM technology. resulting

in a reduction in roughness with an increase in micro

hardness for the surface.

Author’s Contributions

MAAAN: Methodology, Writing – Original

Draft Preparation. IMH:Supervision.

Conict of Interest

We have no conicts of interest to disclose

regarding this article. The opinions expressed

are solely those of the authors and have not

been influenced by any financial or personal

relationships.

Funding

The authors received no nancial support

for the research, authorship, and/or publication

of this article.

Regulatory Statement

This article did not involve the use of any

hazardous materials, living organisms, or any

procedures that could harm the environment. There

was no need to comply with any specic regulatory

laws or regulations regarding occupational health

and safety or the environment. All necessary

measures were taken to ensure compliance with

ethical research practices and laboratory safety.

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Date submitted: 2022 Jun 22

Accept submission 2023 Jan 12

(Corresponding address)

Mahmood Abdulla Ahmad Al Nuaimi

Middle Technical University, College of Health and Medical Techniques,

Baghdad, Iraq.

Email: mhmoodabdallaah@gmail.com