microscopic analyses of welding defects in rpd technology

orIgInal arTIClES
Microscopic AnAlyses of Welding defects in
rpd technology
Bortun Cristina 1 , Ghiban Brandusa 2 , Barlea Romeo 3 , Gombos Otilia 4
reZUMAt
Introducere: Aliajele pe bază de cobalt sunt frecvent folosite la realizarea scheletelor metalice ale protezelor parţiale mobilizabile scheletate (PPMS).
Stresul mecanic, la care acestea sunt expuse, poate genera apariţia unor fisuri. Autorii au studiat o metodă de reparaţie, şi anume sudura cu laser întrun
sistem pulsatoriu. Obiective: creşterea duratei de viaţă a PPMS, reparaţia şi analiza cauzelor eşecului la protezele sudate, realizate din aliaje de
cobalt. Material şi metodă: 20 suprafeţe ale unor schelete metalice sudate au fost analizate la microscop. Procedeul de sudură s-a realizat cu sau
fără material adiţional. S-au utilizat aliaje de CoCrMo: aliaj “C” (Vaskut Kohàszati Kft – Ungaria) şi WIRONIT (Bego - Bremen, Germania). Calitatea sudurii
(realizată în laboratorul de tehnică dentară cu Mini Laser XXS Model.Orotig Italia) a fost observată cu un microscop metalografic (OPK-6A BEL PHOTONICS
Bel Engineering SRL, Italia).Rezultate: La analiza microscopică au fost descoperite defecte ale sudurii şi fisuri, care sunt legate în special de mărimea
spotului şi puterea laserului de sudură. Majoritatea fisurilor apar în cazul combinaţiei spot mare/putere înaltă, în cursul procesului de netezire/finisare a
sudurii. Folosind un spot şi o putere mai mică decât cea stabilită de producător, putem obţine puncte de sudură fără defecte.Concluzii: Alegerea unei
combinaţii adecvate- energia impulsului, durata acestuia şi putere mai slabă pentru fiecare etapă a sudurii, este decisivă pentru succesul acesteia. Este
important să cunoaştem calitatea şi defectele structurale ale reparaţiilor, pentru a obţine proteze cu o rezistenţă superioară.
Cuvinte cheie: eşec, sudura aliajelor de CoCrMo, proteze parţiale mobilizabile scheletate.
ABstrAct
Introduction: Cobalt based alloys are frequently used in manufacturing metallic framework of removable partial dentures. Because of mechanical stress
to which they are exposed, cracks in dental prostheses can develop. The authors studied a repairing method, namely welding through a laser beam
in a pulse operating system. Objectives: to increase the rpd lifetime, repair and analysis of the causes of failure for welded cocrmo alloy prostheses.
Material and methods: Surfaces of 20 welded metallic frameworks were microscopic analyzed. The welding procedures were: butt joint with or without
filler materials. There were used CoCrMo alloys: “C” alloy (vaskut kohàszati kft – hungary), wironit (bego - bremen, germany). The quality of welding
(made in dental laboratory with mini laser xxs model, orotig italia), was observed using an inverted metallurgical microscope (opk-6a bel photonics bel
engineering srl, italy).Results: at microscopic analysis, we discovered welding defects and cracks, which are mainly connected to spot size and welding
power. Most of the cracks appear at large spot size/high welding power combination, in the process of smoothing the welding. Using of smaller spot size
and lower power than the one established by the producer can generate faultless welding points.Conclusions: Selecting the adequate combination of
pulse energy, pulse duration and peak power for each welding step is decisive for the success of welding procedure. It is important to know the quality
and structural defects of our repairs, in order to obtain high resistance prostheses.
Key words: failure, welding CoCrMo alloys, removable partial dentures
1 Department of Removable Partial Dentistry Technology, Faculty of
Dental Medicine, “Victor Babeş” University of Medicine and Pharmacy
Timişoara, Dental Technology Specialization
2 Politehnica University Bucharest
Correspondence to:
Borţun Cristina
Department of Removable Partial Dentures Technology, Dental Technology
Specialization, Blv. Revoluţiei 1989, no. 9
Phone: 0745378254
E-mail: cristinabortun@yahoo.com; cristinabortun@gmail.com
Received for publication: Oct. 03, 2009. Revised: Nov. 14, 2009.
introdUction
CoCrMo alloys are frequently used for
manufacturing metallic framework of removable
partial dentures and for their rehabilitation. 1 These
dentures are used for treatment of patients with
medium incomes, who are between 50 and 65 years old.
Cobalt based alloys are used in dentistry with
great interest, due to their simultaneous properties,
such as: high mechanical characteristics (yielding
strength, ultimate tensile strength and hardness),
biocompatibility or wear resistance. 2 One can notice
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Cristina Bortun et al 23

a significant dental alloys’ diversification in time, in
connection with manufacturing technologies. The field
of Dental Technology uses especially CoCr alloys, 3-5
which have ISO 22674(2006) guidelines: Type 4- for
dentures with small sections subjected to great forces
and Type 5- for dentures that need a great rigidity and
mechanical resistance, also for RPD. The difference
between these two types is related to the elasticity
conventional limit, which is higher at type 5 (360
towards 500). Knowledge of dental alloys’ structure
is necessary in order to optimize some casting and
welding technologies.
In order to reduce the costs and to increase the
dentures lifetime, there appeared the reoptimization
of metallic components with help of laser welding (the
method of combining two materials or assembling
of two materials in plastic or fluid phase, through
their local marginal fusion, with or without material
addition, with help of optic energy).
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24 TMJ 2010, Vol. 60, No. 1
Figure 1. Materials and equipment used for our experiment: a,b. Alloys c.
Welding equipment; d. Welded RPD Framework; e. Inverted Metallurgical
Microscope OPK-6A BEL
For about 15 years, one can talk about the
advantageous and safe run of the laser welding in
dental technology. 6-12 Laser, refused in the past because
of high costs and lack of knowledge, has become
today an aim of dental technology. The modern lasers,
like those belonging to: BEGO (LaserStar T plus,
LaserStar PW, LaserStar PW LYNX), GIRRBACH
DENTAL SYSTEM (Neolaser L 126500), VISION

INDUSTRY (LWI 4th generation), INTERDENT
Laser System (Herculea), ROFIN-SINAR Hamburg/
Germany (Macro, micro), MANFREDI, have
electronic programming, microscopic visualization
system and laser parameter adjustment, based on the
welded alloy. The welding has maximum stability, gives
a biocompatible zone with mechanic resistance and
can be done very fast, with maximum accuracy, at cold.
Bertrand 6,7 synthesized the advantages of
laser welding: the welding is done directly on the
cast, which permits a right aligning of the metallic
framework fragments; it is possible to weld very close
to the acrylic resin or ceramic, without any cracks or
color damage; potentially, all metals can be joined
but particularly titanium alloys; laser welding joints
have a high strength for all metals, consistent with
that of the substrate alloy; high mechanic resistance;
reduced thermal influence, which involves minimum
deformation; it can be used in different working
phases in dentures technology. If one takes into
account the right adjustment of these parameters, one
can obtain breaking resistance close to or even higher
than that of casted pieces.
The estimation of welded joints quality for some
alloys used in dental technology can be done using
destructive and non-destructive methods. Among
the non-destructive methods are: spectrographic,
microscopic or radiological analyses; metallographic
analysis and micro hardness belong to destructive
methods.
The aim of the study was to analyze the causes
of failures for welded CoCrMo alloy prostheses,
using non-invasive, rapid testing methods, like optic
microscopy. The purpose is to increase the use of
dentures.
MAteriAls And Methods
We have studied 20 removable partial denture
(RPD) metallic frameworks, rehabilitated through laser
welding with or without filler materials (Fig. 1.d). The
welding has been done following the Kou 8 specification
from Welding in Metallurgy (2003) manual. CoCrMo
alloys were used, such as: “C” alloy from Vaskut
Kohàszati Kft - Hungary, WIRONIT from Bego
- Bremen, Germany (Fig. 1. a,b). We used a type 4
laser- Mini Laser XXS Model Orotig Italia (Fig. 1.c).
The quality of welding, made in dental laboratory, was
observed using an Inverted Metallurgical Microscope
OPK-6A BEL PHOTONICS (Bel Engineering SRL,
Italia - Fig. 1. e), which has an image capture system
connected to PC.
resUlts
The use of metallographic microscope permitted
non-invasive evaluation of the investigated metallic
surfaces. Thus, one can observe different processing
aspects of the metallic components (Fig. 2), which are
relevant for preparing the welding procedure.
After welding, the chemical composition of
the alloys changes a little- by decrease of the basic
components. The alloy becomes harder in the welded
zone. In the heat affected zone (HAZ) cracks can
appear, caused by the alloy rapid cooling after welding:
Co from 65% to 64.1%; Cr from 29% to 27.4%; Mo
from 5% to 4.1%.
The CoCr alloys presented a good weldability.
In the first phase we searched the optimal working
parameters, which were correlated to alloy type, its
chemical composition, fault- crack type, fracture,
the fault size (length and thickness), working phase
(junction, deposition, smoothing). The adequate
combination of working parameters can be customized
with the help of analyses, for its reproducibility and
practice application. The mechanic resistance of
welded joints depends on the welding spots integrity.
Thus, we focused on the welding soldered joint aspect,
on the faults that can appear in the welding points and
compromise our reparation lifetime.
Choosing of optimal parameters for our laser
device is connected to the existence of a Nd:YAG laser
with pulsed functioning, 25J maximum energy, 6-10A
Input, 1064 nm wavelength; the clearance of laser
radiated surfaces and interfaces is compulsory; the
faults that are bigger than 0.2-0.5 mm need addition
of a foil or CoCr wire.
Working parameters for our laser device:
• 8 spot types between 0.3 and 1.8 mm;
• Power of the laser pulse 0.5-3 Kw;
• Pulse duration is 0.5-8 msec;
• The radiation diameter no smaller than 1.5 mm
for non precious dental alloys;
• 0.8 mm penetration depth is enough for
obtaining a good braking resistance, in case of
long bridges; in case of higher forces, welding is
done also on the opposite part;
• The superposition of successive welded
surfaces has to be 60-70%;
• Pulse frequency no higher than 2 Hz, especially
in case of small dimension pieces (for example
clasps, precision attachments, individual milled
attachments);
• One can give up to inert protection gas.
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Cristina Bortun et al 25

Figure 2. Aspects of RPD metallic surfaces processing (resolution 6 MP,
objective 40X / magnification 400X): a. sandblasting; b. mill processing;
c. polipanto processing; d. polishing
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26 TMJ 2010, Vol. 60, No. 1
Laser welding parameters mixing up is a very
complex operation, which depends on alloys
composition, welding procedure, thickness and profiles
of cast samples. In fact, some of the macroscopically
faultless welding contained hidden pores or cracks,
which compromised the entire joint. Welding defects
and cracks were discovered only at the microscopic
analyses.
Figure 3. Aspects of welding spots (power/ time/ frequency) at different
parameters (resolution 6 MP, objective 20X, magnitude of the zoom
200X): a. little weld spot 1,6/1,4/1; b. little weld spot 1,7/1,4/1; c.
medium spot: 1,7/1,6/1; d. medium spot ; e,f. welding with filler material
1,9/1,6/1; g. big spot 2/1,7/1; h. big spot 2,1/1,7/1; i,j. fracture in
welding point after welding with three variants of spots
These defects are mainly connected to spot size
and welding power. So, the use of a smaller spot size
and a lower power than the one established by the

producer can generate faultless welding points. Most
of the cracks appear at a large spot size/ high welding
power combination, in the process of smoothing the
welding. When cracks continue from one welding
point to another, it generally determines the break of
the entire joint. Therefore, the highest attention is to
be paid for testing a faultless welding.
Figure 3 shows images captured with help of
metallographic microscope video camera, which
permitted the welding quality evaluation.
In 72% of the cases, the cracks were found inside
the welding points, but at least apparently, not always
affected the weld mechanic resistance.
discUssion
Wulfes, Wataha, 1,4 who have vast experience,
admitted the good weldability of CoCr alloys. They
used welded joints for the RPD components, with
clasps and also with different precision attachments,
individual milled attachments. Two thirds coverage
of the previous spot is ok. One considers that the
complementary material has to be 45 0 headed towards
the laser beam and the processed object. Also, the
absorption and reflection phenomena influence the
weld efficacy with chosen parameters. The blind, non
retro-reflecting surfaces increase the efficiency, while
those retro-reflecting (polished crowns) decrease it. In
order to obtain good results, the weld parameter has to
be set for each situation.
Bertrand 6,7 studied 3 alloys for different types of
dentures; his results showed a maximum penetration
depth of 1-2 mm, which represents one of the most
usual depths of the components that must be repaired.
The surface structure influences the penetration rate- a
shiny surface decrease laser beam penetration. Because
of the laser effect, the weld zone suffered an important
microstructure change- micro hardness increasing.
Craig 2 opined that, „brazing is a process in which
a molten filler metal wets and fills the gap between
the parent metal surfaces. The filler metal has a lower
melting point than the parent metal. In welding, the
parent metals fuse and form the joint with or without
a filler alloy”. When using filler material, half of the
laser beam must fall on the wire, the rest of it- on
the parent metal. Thus, we can obtain a blend of
homogeneous molten material. In order to avoid
cracks, pulse duration is reduced. If that is not enough,
one decreases the electric tension and increases the
focal diameter.
Kakimoto Kazutoshi 9 evaluated the laser
weldability of 7 dental CoCr alloys. Spot welds were
formed on the surfaces of specimens with a pulsed
YAG laser. Observation of the microstructure and
the fracture surface suggested that the cracks were
solidification cracks. These cracks were probably
caused by the addition of elements, like silicon and
carbon. Although the alloy laser weld had decreased
hardness, neither cracking nor porosity was generated.
The determiner factors for welding quality,
respectively the operator ability of choosing the
welding parameters (power, pulse duration, energy)
were investigated also by Watanabe. 13-15 Regarding the
inert gas atmosphere- argon, there are studies that
show it isn`t absolutely necessary in case of CoCr alloy
welding.
The same author opine that, welding without filler
material is recommended only if the gap is smaller
than 0.5 mm. Cleaning of surfaces, their abrasion (2-
3mm) and filler material adjustment, is indispensable
for obtaining a perfect joint. The increased thickness
of the pieces to be welded has a negative influence
on the weldability, because a large metallic mass
cause rapid cooling of the weld and HAZ, which can
generate fragile structures or even cracks. The big
internal tensions (result of contraction) can lead to
cracks or braking during the function in the oral cavity.
Frentzen, Koort, Yamagishi, Neumann and
Dobberstein 8,15-17 examined the fracture surfaces and
showed that the laser welding technique was much
more effective in the peripheral than in the central
parts of the specimens. The tensile strength of the
laser-welded joints was significantly lower than that
of the brazed joints, mainly due to the smaller crosssection
of the welded joints and partly due to the
relatively strong brazed joints. SEM analysis revealed
localized phenomena, such as pitting near the joints
and pronounced corrosion in some defects on surfaces.
In case of localized corrosion, and over longer periods
of time, the process could become autocatalytic and
more pronounced than in the study. The laser welding
process could be improved by increasing the weld
penetration depth. Preparation of the areas to be
welded can reduce laser beam reflection and probably
improve the welding efficiency.
Zupancic et al. 5 opined that, large joint surfaces
might ensure sufficient strength and limited thickness
might enable complete joining with minimum porosity.
The longevity of RPD frameworks is limited due
to the mechanical or corrosive failure of the joints.
The purpose of their study was to determine which
joining method offers the best properties for CoCr
alloy frameworks. When laser welding was used,
successful joining was limited to the peripheral aspects
of the weld. The welding technique did not affect
significantly the joint tensile strength. Electrochemical
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Cristina Bortun et al 27

measurements indicated that, the corrosion resistance
of the laser-welded joints was better than that of the
brazed one.
Reclaru 3 shows that, precious metal based dental
alloys generally have a superior corrosion resistance,
in particular enhanced resistance to pitting and crevice
corrosion, compared to non-precious metal based
alloys such as CoCr alloys. A new generation of Co-Cr
alloys enriched with precious metals (Au, Pt, Ru) have
now appeared on the market.
Burkhardt, Reichert 11,18 compared laser and
microimpulse welding. They concluded that, laser
welding is superior, but more expensive due to the
working equipment.
conclUsions
Because of mechanical stress to which they are
exposed, cracks can appear in dental prosthesis. The
repairing method studied by the authors is welding
through a laser beam in a pulse operating system. Since
the cracks that tend to appear do not have a regular
shape, it is highly recommended the use of welding
by hand.
Laser welding is a performing method for metallic
prostheses repairs. Selecting the adequate combination
of pulse energy, pulse duration and peak power for
each welding step is decisive in the success of welding
procedure.
It is important to know the quality and structural
defects of our repairs, both base metal and welded
metal, in order to obtain high resistance prosthesis.
Microscopically non-invasive analyses and tests show
good or poor quality of the welded joints.
AcKnoWledgeMents
This study was supported by the CNCSIS Grant
Ideas, 1878/2009, from the Ministry of Education
and Research of Romania
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28 TMJ 2010, Vol. 60, No. 1
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