total study
DESCRIPTION
orthodontic applicationTRANSCRIPT
CERTIFICATE
This is to certify that the dissertation entitled “LASER-BRAZING OF
NICKEL-TITANIUM AND STAINLESS STEEL ORTHODONTIC WIRES
USING Nd:YAG LASER WELDING MACHINE” submitted by Srinivasan.T,
postgraduate student (2004-2007) , Department of Orthodontics, Tamilnadu Government
Dental College and Hospital, Chennai, to the Tamilnadu Dr.M.G.R Medical University in
partial fulfillment of the Master of Dental Surgery, Branch-V, Degree examination in
2007 is a research work done under our guidance and supervision.
Dr.C.Karunanidhi, M.D.S., Dr.W.S.Manjula, M.D.S., Additional Professor, Professor and Head of the Department, Department of Orthodontics, Department of Orthodontics,Tamilnadu Govt Dental College Tamilnadu Govt Dental College& Hospital, Chennai-600 003 & Hospital, Chennai-600 003
CONTENTS
Sl.No TITLE Page No.
1. Introduction
2. Aims and Objectives
3. Review of Literature
4. Materials and Methods
5. Results
6. Discussion
7. Summary and Conclusion
8. Bibliography
INTRODUCTION
Orthodontics is a specialized dental procedure concerned with the
movement of teeth to achieve an effective occlusion and to provide a
pleasing facial contour and appearance of the teeth. The most common and
useful elastic element in modern orthodontics is an archwire.
At present, orthodontic wires and arches are mainly made of metal
alloys, such as nickel titanium and stainless steel. Nitinol was first alloyed
in the early 1960s by William Buehler at the US naval ordnance laboratory.
Super elastic nitinol archwires were introduced to clinical orthodontics by
Dr.George Andreasen in 1971. The development of nitinol archwires
represented a significant advance in orthodontic biomechanics and
treatment. The particular properties of super elasticity and shape memory
effect allow teeth to move under weak but constant continuous forces over
long treatment time and much larger displacement can be achieved.
However, the stiffness of Nickel titanium wires is small, which can result in
the loss of anchorage. On the contrary, the high stiffness of stainless steel
wires can offer adequate anchorage but its elasticity is low and can engender
excessive orthodontic forces preventing teeth from moving. If Nickel
titanium and stainless steel wires were bonded together and used in
orthodontic treatment, the advantages of the two materials could be
expressed by using stainless steel and Nickel titanium shape memory alloy
wires as anchorage parts and treatment parts respectively. This could greatly
shorten the period of orthodontic treatment and improve the quality.
A major limitation in the use of nickel titanium alloys has been the
difficulty of joining this material, both to itself, and to other materials. If at
all Nickel titanium and stainless steel were joined, maintaining the super
elasticity and shape memory effect of nickel titanium proves to be a
challenge.
By conventional joining methods, the heat affected zone is large in
area due to absence of point source of heat and increased duration of joining
process. The heat affected zone should be kept as minimal as possible to
maintain the shape memory effect and super elasticity of nickel titanium
wires.
Lasers, with their high power output and converge ability towards a
point target, found to be a good source of power for brazing stainless steel
and nickel titanium.
The present work investigates the properties of laser-brazed joints of
Nickel titanium and stainless steel wires using silver-based filler metal, at
different brazing parameters.
AIMS AND OBJECTIVES
The aim of the present study is to evaluate the properties of laser-brazed
joint of NiTi shape memory alloy and stainless steel orthodontic wires. Put
in detail, the objectives are:
1. Laser-brazing of stainless steel and Nickel-Titanium shape memory
alloy wire using silver-based filler metal at different brazing
parameters.
2. Performing tensile tests in the samples to evaluate breaking stress and
percent elongation.
3. Performing Elasticity tests in the samples to evaluate the
superelasticity loss of NiTi wire in the heat affected zone.
4. Performing Bending tests in the samples to evaluate the shape
memory effect loss of NiTi wire in the heat affected zone.
REVIEW OF LITERATURE
Primitive orthodontic appliances have been found with Greek and Etruscan
artifacts. Archaeologists have discovered Egyptian mummies with crude metal
bands wrapped around individual teeth. It is speculated that catgut was used to
close the gaps.
Historically, gold alloy wires were first used in orthodontic practice. The
gold alloy wire compositions were generally similar to those of the type IV gold
casting alloys, and their modulus of elasticity was approximately 100 GPa.
Pierre Fauchard (1728) described his ‘bandeau’ appliance, an expansion
arch consisting of a horse shoe-shaped strip of precious metal to which the teeth
were ligated by gold wires.
The technique of soldering, first used in the specialty was to fuse gold
alloy to gold bands by either silver or gold solder melted by fine flame from a gas
torch. This joining process resulted in sufficiently firm, well-bonded joint.
The late 1920s brought hard-drawn, austenitic, stainless steel wire to the
specialty. This wire alloy, with chromium and nickel in its metallurgy, was touted
as superior to its precious-metal predecessors because of its higher strengths,
greater elastic modulus, ductility, and its corrosion resistance in the oral
environment.
In the early 1930s annealed stainless steel strips were produced, and
fluoride fluxes were introduced, enabling successful soldering, and the gold,
silver, and platinum alloys began to disappear as appliance materials.
Vosmik and Taylor (1936), attempted to determine soldering temperatures
for stainless steel. They soldered joints in a dark room and estimated the
soldering temperature from the color of the wire.
During the 1940s, Perceival Raymond Begg developed the highly resilient,
stainless-steel “Australian” wire, along with Wilcock. This was introduced in
United States in late 1956. He was using titanium wire long before American
Orthodontists had discovered its unique properties.
Charles H. Tweed in 1941, gave an account on soldering technique for
steel arch wire. He described free hand soldering method for soldering
intermaxillary hooks made of brass wire over rectangular and round stainless
steel wire. Stops were soldered using similar method .He was one of the first to
note that the union obtained between the steel arch wire and the attachment by
soldering is a physical joint. Hence, in order that the attachments may withstand
the pressure of wire ligatures and rubber elastics, he recommended that the solder
must be flowed around at least three sides of the rectangular steel wire.
In 1950s, a cobalt-chromium-nickel orthodontic wire alloy (Elgiloy) was
developed by the Elgiloy Corporation The four tempers, color-coded by the
manufacturers have the same composition .Differences in mechanical properties
arise from variations in the wire processing.
William J. Buehler of U.S. Naval Ordnance Laboratory in 1959 discovered
acoustic damping property of equiatomic nickel-titanium composition alloys and
named his discovery NITINOL.
Skinner and Phillips (1960) gave melting range of 607-688 degree C for
low fusing silver solders available at that time. But ideal soldering temperature
range was nowhere mentioned.
John H.Parker in 1960, described an improved soldering technic. He used a
506 Rocky Mountain Welder at mode number 4 for all wire soldering. This
carbon tip soldering found major uses in soldering of prewelded chrome wires or
springs of any size directly to the archwire or band material of either gold or
chrome, reinforcement of band material surfaces or ends of wire and repairing
wires broken near acrylic retainers.
David S.Muzzey (1961) an associate technical director ,in a laboratory
management meeting of U.S.Naval Ordnance Lab applied heat from his pipe
lighter to the compressed folded fatigue-resistant strip of NITINOL sample
presented to him. To everyone’s amazement, the Nitinol stretched out
longitudinally. The accidental discovery of shape memory property of Nitinol
made.
Frederick E.Wang (1962) a crystal physics expert joined Buehler’s group
and scientifically explained that the properties of Nitinol are due to changes in
crystal phase transitions.
John V. Wilkinson (1962) found that the annealing range for hard drawn
stainless steel wire started at about 500 degree C. Since clinically satisfactory
solders had a flow point of at least 600 degree C, the wire was softened as a result
of soldering. The degree and extent of softening depended on the operator’s
control of heat during soldering.
John V. Wilkinson (1963) from his experiments, derived 4 main
conclusions.
1 .It is possible to solder stainless steel at temperatures 20 to 40 degree C
above the flow point of the solder, The soldering temperature need not exceed
690 degree C.
2. In the temperature range necessary for soldering stainless steel the time
of heating is not a critical factor for softening of hard drawn wire.
3. Soldering techniques for stainless steel should be directed towards
limitation of temperature rather than limitation of heating time.
4. Research into solders which have a flow point lower than that of those
currently available should prove profitable to the practice of orthodontics.
George Andreasen and Hilleman TB (1971) attracted by the unique
properties inherent in NiTi alloy such as high elastic limit and low elastic
modulus reported the results of their investigation for clinical use.
Subsequently, Unitek Corporation has produced this wire for the
profession under the trade name of Nitinol. Nitinol has an excellent springback
property, But it does not possess shape memory or super-elasticity because it has
been manufactured by a work-hardening process.
Furukawa Electric Company in 1978, produced a new type of Japanese
Niti alloy, possessing all three properties namely excellent spring back, shape
memory and super elasticity.
Hua-cheng Tien and colleagues (1978) developed Chinese Niti alloy wire,
a new superelastic Orthodontic wire at the General Research Institute for non-
ferrous metals in Beijing, China.
George Andreasen (1980) did a clinical trial of alignment of teeth using a
0.019 inch thermal nitinol wire and noted that nitinol can be used for longer
periods of time without changing it and it can shorten treatment time needed in
leveling the dentition.
Burstone and Goldberg (1980) introduced Beta-Titanium a new
orthodontic alloy to the profession. Its increased springback, reduced force
magnitudes, good ductility and weldability made it superior to stainless steel in
many aspects.
Charles J.Burstone (1981) in his account on variable-modulus
orthodontics, derived from equations that Nitinol wire with dimensions of 0.018
X 0.025 inch has a stiffness number of 251.4, which is similar to 0.016 inch steel
wire. The principle of variable modulus orthodontics reduces the number of
archwires needed for aligning since bracket play is eliminated by rectangular
dimension ‘light’ wires.
John W. Edie, George F. Andreasen, Mary P. Zaytoun (1981) studied
surface corrosion of Nitinol and stainless steel under clinical conditions and
found no significant difference.
Scott R .Drake, Donald M. Wayne, John M. Powers and Kamal Asgar
(1982) studied mechanical properties of orthodontic wires in tension, bending
and torsion. They concluded that in bending and torsion, the stainless steel wires
had the least stored energy at a fixed moment, whereas the nickel-titanium wires
had the most. Stored energy of Titanium-Molybdenum wire falls between
stainless steel and Nickel-Titanium. A Titanium-Molybdenum teardrop closing
loop delivered less than one half the force of a comparable stainless steel loop for
similar activations.
James L. Cannon (1983) obtained an US patent for inventing a new
Orthodontic archwire design. This wire have anterior and posterior segments of
different elasticity or stiffnes to enable early and simultaneous treatment of teeth
in the entire dental arch. The wire is preferably made by uniting a central
segment of relatively resilient wire with end or posterior segments of a different
and more rigid wire. Different cross-sectional shapes may also be used in the
several segments. The segments are joined or united by crimped tubes, welding,
brazing, soldering or any other technique compatible with the metal alloys
forming the several segments. The author mentioned that a crimped-tube
attachment is useful with an anterior segment made of Nitinol alloy which cannot
be readily welded. He claimed that the new composite archwire provides
significant benefits to both Orthodontist and patient in terms of shortening
treatment time, minimizing patient discomfort, and providing more precise
control of tooth movements.
James L. Cannon (1984) named his patented wire configuration as Dual-
Flex archwires. Intermaxillary hooks or cast ball hooks can be provided at the
junction of two segments just mesial to the cuspids to improve versatility of the
archwire. Dual Flex archwire 1 is especially useful with the lingual appliance,
where anterior interbracket width is greatly reduced.
Mark T. Donovan, John Jin-Jong Lin, William A. Brantley and John
P.Conover (1984) studied the weldability of beta titanium archwires by means of
four different commercial welders available to the Orthodontist. They concluded
that for each of the four welders there exist optimum conditions which provide
strong, clinically satisfactory joints.
Fujio Miura, Masakuni Mogi, Yoshiaki Ohura and Hitoshi Hamanaka
(1986) evaluated the super elastic property of the Japanese NiTi alloy wire for
use in Orthodontics. On studying the effect of heat on Japanese NiTi they noted
no significant change in mechanical property of wire at 200-300 degree C for
five min.At 500 degree C superelasticity was definitely decreased. At 600 degree
C for 5 min, superelasticity and good spring back property of the wire were
almost completely lost.
William J.Thompson (1988) on updating the mechanics of combination
anchorage technique, proposes the use of combination arch wires such as Dual
Flex 1 and 2. They had minimized or even eliminated the use of loops in the
phase I bite-opening, alignment and retraction mechanics. The Dual Flex 1
archwires are multisegment wires with round 0.016- inch stainless steel in the
posterior section and round 0.016- inch Nickel titanium (Titanal) in the anterior
segment. The rigid steel wire will assist in bite opening and molar control, while
the flexible anterior section is used for (1) rapid alignment, (2) leveling and (3)
retraction of the anterior teeth. In cases that require heavy anterior resistance to
minimize lingual movement of anterior teeth, a Dual Flex 2 wire can be used.
This wire has a round 0.018 inch stainless steel posterior segment and 0,016 X
0.022- inch Nickel-Titanium anterior segment from canine to canine. The steel
posterior segment is seated in the gingival slot where the resistance is minimal
and the 0.016 X 0.022- inch segment is used in anterior edgewise slots. The
light, flexible rectangular wire features greater bracket engagement, increased
frictional resistance and lingual root torque, and is used to establish increased
anterior resistance and facilitate incisor control as posterior teeth are moved
mesially in the light wire slot. Modification of the Dual Flex wire is needed when
the arch wire passes from the edgewise to the gingival slot.
John J. Hudgins, Michael D. Bagby, Leslie C. Erickson (1990) studied the
effect of long term deflection on permanent deformation of Nickel-Titanium
archwires and concluded that no clinically significant difference was found
between presently available nickel-titanium wires in terms of permanent
deformation, long- or short-term.
OrthoOrganizers in an attempt to produce optimal orthodontic forces to
different teeth in a single wire, put forth Multi-Force Nitanium archwires. Multi
modulus Nitanium have three force regions, anterior (mild), canine-premolar
(moderate), and molar (high). The applications are for leveling and aligning in
most cases. The manufacturers recommend the wire to be used with Delta force
bracket system.
OrthoOrganizers also deviced Nitanium adjustable Utility arch. The
Nitanium anterior segment and stainless steel posterior segments are joined by
inconel tubing which allows for precise adjustments in arch length. The 3 mm
step-down allows gentle forces to be applied to anterior teeth. The stainless steel
posterior segment provides the heavier forces required to rotate or upright molars,
open or close bites and perform other adjustments. Highly recommended for
mixed and adult dentition cases according to manufacturers’ claim.
Bourauel C, Nolte LP, Drescher D (1992) noted that the loops made of
stainless-steel, cobalt chromium, titanium-molybdenum alloys follow a static
force system.The static force systems of fixed appliances made of these materials
are well known from experimental and numerical studies, but as they may change
during tooth movement, orthodontists are often confronted with problems in
therapy. The introduction of pseudoelastic nickel titanium alloys (NiTi) into
orthodontic treatment, offers the chance of improving the effectiveness and
reliability of orthodontic devices. In this study a plane finite element (FE) for the
analysis of orthodontic loops is presented. It enables the determination of the
nonlinear behaviour of pseudoelastic NiTi-alloys and is capable of simulating
large structural displacements and rotations accompanied by moderate strains. A
comparative numerical and experimental study shows the efficiency of this
element. The associated results reflect pseudoelastic effects on certain loop
designs, and reveal the benefits for the orthodontist and his patients.
Drescher D, Bourauel C, Their M (1992) proposed that during uprighting
inclined molars, to prevent occlusal trauma, an intrusive force has to be applied
to the molar in addition to the uprighting moment.Owing to their construction,
current mechanical devices for uprighting either do not meet this requirement, or
are difficult to adjust when in place. For this reason, an improved uprighting
spring is described by the authors which utilizes the properties specific to super-
elastic (pseudo-elastic) NiTi alloys. The most important property of super-elastic
wires is the fact that they produce constant forces or moments within a specific
deformation range. In order to utilize this useful property, certain design criteria
have to be met. Experimental measurements have shown that a super-elastic wire
(Sentalloy, 0.016” x 0.022”) having a length of 10 mm generates a constant
moment of 7 to 8 Nmm within a bending angle of 50 degrees to 180 degrees. On
the basis of these results, a table that permits the determination of the proper wire
length needed to provide a constant moment within a given range of bending
angles is proposed. The superelastic uprighting spring described here comprises
an NiTi wire segment having a length of 7 mm and a mesial and distal steel wire
segment. In the active state, the spring generates an uprighting moment of 8
Nmm and an intrusive force of 0.6 N.
Julie Ann Staggers and Dallas Margeson (1993) studied the effects of
sterilization on the tensile strength of Orthodontic wires. They concluded that
dry heat and autoclave sterilization significantly increased the tensile strength of
Sentalloy wires. The results of this study suggest that Orthodontists who choose
to recycle Nickel titanium or Beta titanium archwires do not need to be
concerned about reducing the wires’ ultimate tensile strength by sterilization
procedures.
Bourauel C, Drescher D, Nolte LP (1993) in their article stressed the
importance of keeping a constant moment-force ratio throughout space closure in
segmented arch technique using loops to achieve a purely translatory retraction.
The moment to force ratio (M/F) is determined by the occluso-gingival height,
but, as there are intraoral limitations to the spring height, the maximum possible
M/F is also limited. Consequently the M/F is no longer constant and activation of
the loop to achieve a specific M/F can become critical. This potential problem
can be overcome by the use of highly flexible wires, particularly those made of
the superelastic alloy nickel titanium. The data presented in this study result from
calculations arrived at with the help of a plane numerical model of
pseudoelasticity which in turn is based on the finite element method. The
calculations are compared with those resulting from experimentations using the
orthodontic measurement and simulation system. A variation in the different
parameters led to the design of a T-loop with a height and an apical length of 10
mm each. A T-segment made of superelastic NiTi alloy Sentalloy (0.016” x
0.022”) was joined to steel horizontal arms. This loop produces a constant M/F
ratio of -7 mm and requires no uprighting bends. The range of activation is
approximately 15 mm. A superelastic plateau was calculated between an
activation of 10.5 mm and 2.5 mm, with a distalizing force from 0.9 N to 0.5 N.
The experimental values corresponded to the numerical data. The clinical
application of the superelastic T-loop is thus demonstrated in this study.
Bourauel C,Drescher D (1994) based on the favorable results achieved by
the application of pseudoelastic T loops in the course of canine retraction,
investigated their application to the retraction of maxillary incisors. A modified
Burstone T loop was made of a pseudoelastic orthodontic nickel titanium wire
and then subjected to experimental testing. The results obtained were compared
with extensive numerical studies. The results of the tests showed that forces and
moments generated by the T loop are suitable for the retraction of upper incisors.
The clinical application of the pseudoelastic spring was performed using an
individualized retraction arch enclosing the whole front segment. Whereas during
canine retraction the experimental and clinical results corresponded very well,
such was not observed during the retraction of upper incisors. This result implies
that the location of the center of resistance of the upper incisors has not been
completely clarified. Thus the authors recommended that this matter should be
given further study.
Wichelhaus A and Sander FG (1995) in their study presented an
uprighting spring that consists of a combination of superelastic material which is
connected with a steel wire by means of a crimped connector. Pseudo-elastic
areas of such a spring can be used well by combining superelastic material with
steel. According to the authors the uprighting spring presented here yields the
following advantages: 1. The uprighting moment of the molar is between 10 and
20 N with a 40 degree tipping of molar.2. The uprighting springs exhibit a large
plateau in the area of 8 to 15 Nmm depending on a bending-in of an alpha-bend.
3. An intrusive force of approximately 0.5 to 1.0 N can be produced by varying
the alpha-bend. The preformed uprighting spring in combination with a cross
tube can be affixed without any problems, because only the alpha-activation must
be bent in. 5. Practically, a reactivation during uprighting is not required. 6. An
enlargement of the alpha-moment to produce an intrusive force makes great
demands on the anchoring element. For this reason, one must check in each
individual case, if an anchoring segment displays the required stability. 7. By
lengthening the SE material at the crimped connector, the alpha and beta-
moments become smaller, as does the intrusive (extrusive) force applied to
molars.
Wichelhaus A and Sander FG (1995) in their second part of the study used
the NiTi steel uprighting spring to upright 30 molars.The advantage of this spring
is that in large areas the pseudoelastic part of the spring transfers constant
moments and forces to the molars. In addition, the steel part makes it possible to
simply and easily adjust and fasten alpha-bends. Because of the relatively small
uprighting moments of 10 upto a maximum of 25 Nmm such an uprighting spring
can also be applied without any modifications in cases in which the molars are
tipped up to 50 degrees. The uprighting spring presented here proves to be an
effective method for achieving a fast and trouble free uprighting of molars.
Sun z and Ion (1995) in their review on laser welding noted that the ability
to manufacture a product using a number of different metals and alloys greatly
increases flexibility in design and production. Properties such as heat, wear and
corrosion resistance can be optimized, and benefits in terms of production
economics are often gained. Joining of dissimilar metal combinations is,
however, a challenging task owing to the large differences in physical and
chemical properties which may be present. Laser welding, a high power density
but low energy-input process, provides solutions to a number of problems
commonly encountered with conventional joining techniques. Accurate
positioning of the weld bead, rapid heating and cooling, low distortion, process
flexibility, and opportunities for product redesign are its principal characteristics.
The review also describes the principles underlying laser welding of dissimilar
metal combinations and highlights the above benefits in a number of practical
applications. It is concluded that there is potential for its application in many
industrial and medical sectors.
Sander FG, Wichelhaus A, Schiemann C (1996) used the intrusion
mechanics according to Burstone with the NiTi- superelastic-steel uprighting
spring. Eleven important conclusions were made by the authors; 1. By applying
the NiTi-SE-steel uprighting spring, relatively constant forces can be exerted
over a large range of intrusion on both sides of the anterior tooth archwire. 2. By
bending a 150 degrees tip-back bend or a curvature into the steel portion, the
uprighting spring presented here is brought into the plastic range of the
characteristic curve of force. 3. Application of sliding hooks on the intrusion
spring permits readjustment for force transfer onto the anterior archwire. 4.
Connecting the anterior archwire with the posterior elements by means of a steel
ligature can be recommended only in some cases, because sagittally directed
forces may be produced. 5. The adult patients presented showed an average
intrusion of 0.6 mm/month, if a linear connection was presupposed. 6. An
intrusive effect on the incisors could first be detected clinically after 6 to 8
weeks. 7. Application of a torque-key proves especially useful in controlling the
incisor position during intrusion in order to avoid unnecessary radiography. 8.
Actual prediction of the centre of resistance with the help of a cephalometric
radiograph proved not to be feasible. 9. The calculated maximal intrusion of the
mandibular incisors was 7 mm. 10. The torque-segmented archwire with crimped
hooks and pseudoelastic springs between the molars and the crimped hooks
proved very effective for retrusion and intrusion of maxillary incisors. The
maxillary anterior teeth can be retruded by a total of 7 mm without readjustment.
11. Constant moments and forces could be transferred by applying preformed
arch wires and segmented arch wires.
Bourauel C, Drescher D, Ebling J, Broome D, Kanarachos A (1997) made
an experimental investigation of force systems on superelastic nickel titanium
alloy retraction springs.. A modified Burstone T-loop was used to construct an
experimental canine retraction spring 10 mm in height and 10 mm in length.
Twenty-five NiTi T-segments were hand made from the superelastic orthodontic
alloys Ormco NiTi and Soar Sentalloy (dimensions 0.016 x 0.022”). The T-
segments were equipped with arms made of rectangular standard steel wire
(0.017 x 0.025”). The following geometrical and mechanical parameters of the
retraction springs were analysed: radius and bending angles of the T-segments,
distalizing force and M/F ratio during activation and the force/deflection rate of
the springs. The error in the geometric parameters was in the range of 5-10 per
cent, irrespective of the alloy used to produce the T-segments. On the other hand,
the force systems of the springs were strongly influenced by the alloy and the
batch under investigation. There were differences in the distalizing force of up to
100 per cent, i.e. at the beginning of the unloading plateau the distalizing force
varied from 0.4 to 2.5 N. The force/deflection rate varied between a value of 0.06
and 0.15 N/mm, whereas the moment/force ratio reached values of 6.5-7.0 mm.
Within a single batch, a reproducibility of these mechanical properties of
approximately 5 per cent could be obtained. These results confirm that each
orthodontic device made of superelastic NiTi alloys has to be calibrated
individually. The authors recommended that the manufacturers should pay more
attention to keeping the material properties of their NiTi alloys constant.
Stanley Braun, Robert C. Sjursen, Harry L. Legan (1997) proposed a
Variable modulus orthodontics advanced through an auxiliary archwire
attachment.They noted that reducing the load deflection rates of orthodontic
springs is important, for it provides relative constancy of the moment-to-force
ratio applied to the teeth with concomitant, forecastable dental movement.
Increasing patient comfort and reducing the number of office visits while
lowering potential tissue damage are additional features of lower load deflection
rate springs. A simple auxiliary attachment, which can be crimped into position
on an archwire or onto segments of an archwire, is described. This attachment
permits the clinician to incorporate a relatively high rate stiff wire to enhance the
anchorage of the reactive teeth in one area of the dental arch, while allowing the
use of wire of lesser stiffness (lower load deflection rate spring) to engage teeth
targeted for movement. The auxiliary allows the clinician to choose various
stiffnesses through the use of wire of one modulus (stainless steel, for example)
in one area of the arch, and wire of a differing modulus (NiTi, for example) in
another area of the same arch. The advantages and disadvantages of choosing
wires of differing moduli are reviewed. Alternative methods of transforming the
spring rate through changes in wire cross-section or length are also reviewed.
Practical clinical applications of the auxiliary attachment are shown.
GAC introduced a new variety of orthodontic wires called ‘BioForce’. The
manufacturers claim that it is the only single strand, superelastic orthodontic
archwire that provides forces that range gradually from 80 grams in the central
incisors to 320 grams at the molars.
Ibe DM, Segner D (1998) in their study evaluated archwire materials
which are claimed to be capable of exerting different force levels within one arch.
Such orthodontic archwires are of importance because the forces they exert are
supposed to be better adapted to different physiological force levels as required
by teeth with different root surface areas. Undesired side effects such as root
resorptions and pain should thus be minimized. The purpose of this study was to
test these types of wires and to evaluate whether and, if so, to what degree the
claimed properties are present. Load-deflection curves of 6 superelastic nickel-
titanium orthodontic wires by 4 manufacturers were recorded using a simple
beam bending test at mouth temperature. Activation-deactivation cycles with
varying maximum deflection were achieved using a computer controlled stepper
motor. Each archwire was tested at 4 different points on the wire. The resulting
forces were evaluated metrically and graphically. Great differences were found
between the superelastic properties claimed by the manufacturers and the
observations of this study and also between the different products.
Devanathan Deva (1999) from his investigations performed at the
Research Laboratories of TP Orthodontics, Indiana , described the development
of a new titanium alloy wire called Timolium. This wire has slightly higher
stiffness than Beta titanium but almost half the stiffness of stainless steel.
Sander FG (2000) claimed that their unit had successfully developed a
retraction spring that shows virtually constant retracting forces up to an activation
of about 4.5 mm through the use of various non-linear materials. Compared to all
other known retraction springs, an actual bodily retraction is possible over a large
range for the first time. The clinical application requires no more than one
reactivation. The anti-tipping moment is 10 Nmm and is to be considered
constant over the entire activation range. This anti-tipping moment produces an
extrusive force for the canine and an intrusive force for the molar. This side-
effect can be avoided by bending a sweep into the steel portion or compensated
by bending a step into the steel portion of this retraction spring. The anti-
rotational moment is about 3 to 5 Nmm measured over the entire activation range
. In contrast to many other springs, the favorable M/F ratio for the anti-tipping
movement allows an actual bodily retraction of canines. Even when the retracting
force is no longer active, the moment that moves the root of the canine distally is
still acting, so that the spring can also be used for the root movement. The M/F
ratio for the anti-rotational movement is between 3 and 5 mm and therefore
allows retraction of the canine without causing major distortions. Customary
brackets with a .018” or .022” horizontal slot can be used, as the spring is
designed for a .018” x .018” vertical slot. Each spring can be used for both the
left and the right canines. The steel portion allows second-order and, if desired,
third-order bends to be made. The hybrid retraction spring can also be applied for
en masse retraction of incisors if a cross-tube is used for the anterior area.
Brian K. Rucker, Robert P. Kusy (2002) studied elastic flexural properties
of multistranded stainless steel versus conventional Nickel Titanium archwires
for initial alignment phase.They concluded that when compared to the elastic
properties of the conventional NiTi wires, the triple and coax SS wires generally
matched the stiffness, but had only one-third to one-half of the strength and
range. Superiority of Nickel-Titanium as initial archwire for alignment is
confirmed in this study.
J Dutta Muzumdar and I Manna (2003) in their thesis made a reference that
under gravity conditions Nickel Titanium laser welding is possible. The laser
used was CW–CO2.They observed a decrease in Ms -start temperature and
ductility, and increase in amount of B2 phase and strength in the weldments .But
no change in shape memory effect are observed.
Edison Welding Institute (2003) claimed that it has developed and
demonstrated a technique for welding nitinol to stainless steel. Its approach was
aimed at overcoming some of the known difficulties, such as the formation of
brittle intermetallic phases usually found in nitinol-stainless steel fusion welds.
The process has been successfully demonstrated in laboratory settings with wire-
to-wire butt joints. The weld strengths are high enough to allow extensive
superelastic bend of the nitinol at the weld.
Vinod Krishnan and K.Jyotindra Kumar (2004) studied weld
characteristics of Orthodontic archwire materials and concluded that Beta
titanium alloys have superior weld characteristics followed by stainless steel.
Whereas Timolium exhibited poor weld characteristics.
Wichelhause A, Sander C, Sander FG (2004) in an attempt to further
develop and improve both the clinical and the biomechanical properties of the
conventional transpalatal arch made an investigation on new design called
compound palatal arch. The biomechanical effects of the newly developed
Compound palatal arch were verified by comparing them with those of
commercially available conventional transpalatal arches. The recently developed
Compound palatal arch is made of one compound element:
nickel-titanium/stainless steel. The specific dimensions and design of the nickel-
titanium element are aimed at exploiting its superelasticity, even during active
molar movement. The newly developed Compound palatal arch showed
substantial advantages in molar derotation compared with conventional
transpalatal arches. Superelastic properties were achieved through the design and
positioning of the nickel-titanium element. Expansion with the Compound palatal
arch was comparable with that with conventional transpalatal arches. The clinical
advantage is in the fact that this appliance can be reactivated and that dental
asymmetries can be treated.
Kum M, Quick A, Hood JA, Herbison P (2004) in their in vitro study
investigated the loads (“forces”), moments and moment:force ratios (M:F)
generated during activation and deactivation of three closing loop designs
constructed from two different orthodontic wire alloys.In their study the forces
and moments of non-preactivated vertical U-Loops, symmetrical T-Loops, and
asymmetrical T-Loops (X-Loops) made from titanium molybdenum alloy (TMA)
and Japanese nickel titanium alloy (NiTi) were measured at 35.6 degrees C +/-
0.5 degrees C. The M:F ratios generated during activation and deactivation were
calculated for each loop. Analysis of covariance was used to identify statistical
differences between loop material and design. Their results showed that the
forces, moments and M:F ratios produced by the NiTi closing loops were
significantly less than those from the TMA loops. NiTi T-Loops produced a
relatively constant force during activation compared to the same design in TMA.
Garrec P, Tavernier B, Jordan L (2005) studied the evolution of flexural
rigidity according to the cross-sectional dimension of superelastic nickel titanium
orthodontic wire This study compared bending in 10 archwires made from NiTi
orthodontics alloy of two cross-sectional dimensions. The results were based on
microstructural and mechanical investigations. With conventional alloys, the
flexural rigidity was constant for each wire and increased largely with the cross-
sectional dimension for the same strain. With NiTi alloys, the flexural rigidity is
not constant and the influence of size was not as important as it should be. This
result can be explained by the non-constant elastic modulus during the martensite
transformation process. Thus, in some cases, treatment can begin with full-size
(rectangular) wires that nearly fill the bracket slot with a force application
deemed to be physiologically desirable for tooth movement and compatible with
patient comfort.
Peter C Hall (2005) of Edison Welding Institute USA obtained a patent
describing a method of welding titanium, and titanium based alloys, to ferrous
metals. Welding of titanium, and titanium based alloys, is plagued with poor
quality and highly brittle welds, substantially due to formation of Ti—Fe
intermetallics in the weld pool. The instant invention provides supplementary
filler material to alter the proportions of various elements in the weld pool.
Certain fillers, such as nickel or iron, added to the weld pool enable high quality
welds to be fabricated utilizing a wide variety of fusion welding techniques,
including laser welding, between titanium, or titanium based alloys, and ferrous
metals, including but not limited to the welding of nickel-titanium and stainless
steel. Filler material may be supplied in various forms, including foil, wire,
powders, preformed gaskets, and numerous others. Optionally, the titanium or
titanium based alloy may be stress relieved to achieve full recovery of the shape
memory strain prior to welding.
Martin Geiger, Juergen Schneider and Franz G. Sander (2005) made a
finite element calculation of bone remodeling in orthodontics by using forces and
moments. In their work they simulated the orthodontic long-term tooth
movement of the canine retraction using the new hybrid retraction spring made of
Nickel-titanium. They concluded that this new hybrid retraction spring allows a
well- defined adjustment of the acting force system.
Laservall (2005) has come up with a basic manual laser spot welding
system that is available in both benchtop and floorstand configurations, and in
four power levels tailored to specific applications: orthodontic work/dental
laboratories, jewellery workshops (repair work and bespoke design), and
industrial applications. This 2 nd generation system features a particularly
ergonomic glove box that permits use for long periods without operator fatigue.
M.G.Li, X.M.Qui, D.Q.Sun, S.Q.Yin (2005) studied the properties of
laser-brazed joint of NiTi shape memory alloy and stainless steel orthodontic
wires. They concluded that the tensile strength of the laser-brazed joint could
reach to 340 MPa while the loss of superelasticity and shape memory effect of
NiTi heat affected zone is relatively low by strictly controlling brazing
parameters.
Shivananda pai Mizar (2005) stated that recent advances in materials
engineering have given rise to a new class of materials known as active materials.
These materials when used appropriately can aid in development of smart
structural systems. Smart structural systems are adaptive in nature and can be
utilized in applications that are subject to time varying loads such as aircraft
wings, structures exposed to earthquakes, electrical interconnections, biomedical
applications, and many more. Materials such as piezoelectric crystals,
electrorheological fluids, and shape memory alloys (SMAs) constitute some of
the active materials that have the innate ability to response to a load by either
changing phase (e.g., liquid to solid), and recovering deformation. Active
materials when combined with conventional materials (passive materials) such as
polymers, stainless steel, and aluminum, can result in the development of smart
structural systems (SSS).
Girish P. Kelkar (2006) in his review on resistance and laser welding for
medical devices, describes laser welding is a non contact process. The properties
of monochromatic and coherent light energy allow the laser to be focused on a
very small spot with sufficiently high energy density to melt metals. The laser
source most commonly used for pulsed welding is an Nd:YAG laser, which emits
a near-infrared wavelength of 1.064 micro meter. Most metals that are not highly
electrically conductive or reflective, such as titanium and stainless steels, absorb
laser light reasonably well and therefore laser manipulation is not a problem.
Shaw and Grummons (2006) invented a new method (patent pending) to
construct light-weight, cellular structures from wrought Nickel-Titanium
(Nitinol) shape memory alloy (SMA) elements. The method consists of a
reactivebrazing process that creates a robust metallurgical bond between Nitinol
and itself that solves many longstanding difficulties in the fabrication of SMA
structures, and it facilitates the fabrication of a variety of built-up sparse
structures, including SMA foams, honeycombs, and meshes. Such structures can
be designed to be arbitrarily light-weight, yet retain the adaptive properties of the
underlying SMA material (shape memory effect and superelasticity). This
combination of sparse topology and adaptive properties represents a new class of
materials that can be used as multifunctional structural elements that respond to
changes in external loads and temperature. In addition, the sparse topology leads
to an order-of-magnitude improvement in recoverable strain capability and
thermal response time, as compared to monolithic Nitinol. Potential applications
are numerous, including reusable energy-absorbing structures, highly resilient
structures, lightweight armor, thermal actuation materials, vibration isolation, and
biomedical implants. These materials have broad application possibilities in the
aerospace, automotive, energy, and biomedical industries.
MATERIALS AND METHODS
Specimen Preparation:
Neo Sentalloy (Nickel-Titanium shape memory alloy) orthodontic
wires were obtained from GAC International, Inc. Japan.The dimensions as
obtained from manufacturers are
0.018” x 0.025” x 7”
(0.46 x 0.64 x 177.8 mm)
The Stainless steel orthodontic wires were obtained from American Braces
and Components, USA. The dimensions are
0.018” x 0.025” x 15”
(0.46 x 0.64 x 381 mm)
A silver-based filler metal was adapted to braze Nickel-titanium and
stainless steel wires with the composition of 50% silver,24% copper, 18%
zinc, and 8% tin.
The lengths of Neo Sentalloy and Stainless steel wires were reduced to 55
mm and a total of 45 specimens were prepared in each type of base metal
alloy.
Prior to brazing, the brazing surfaces of base metals were polished with 600-
grit silicon carbide abrasive strips. Then the specimens were cleaned
ultrasonically in an acetone bath.
A fixture was made to hold the specimen passively along their butt ends.
The fixture further stabilized by an overlay hinged load in the vertical plane.
Nickel-titanium and stainless steel were brazed using a Nd:YAG laser
welding machine . The Brazing heat input was controlled by laser output
power and brazing time.
Q = Pt
Where, Q- Brazing heat input
P- Laser output power
t- Brazing time
Three brazing parameters were chosen for the investigation:
Group A – 50 W laser output power for 10 seconds brazing
Group B - 60 W laser output power for 15 seconds brazing
Group C - 70 W laser output power for 20 seconds brazing
Brazing was done with 15 sample pairs in each group. After Brazing , three
tests were performed namely i) Tensile test ii) Elasticity test and iii) Bending
test in these groups (five samples from each group for every test).
Elasticity and Bending tests were performed in five Nickel-titanium base
metal segments as control group to compare with brazed segments.
TENSILE TESTING:
Tensile testing was conducted at room temperature with a universal
testing machine at a cross head speed of 2 mm/min and a gauge length of 6
mm. In the tensile test the ends of test piece are fixed into grips, one of
which is attached to the load measuring device on the tensile machine and
the other to the straining device. The brazed joint was positioned such that it
occupies the centre portion of the gauge length segment. The values of
breaking stress and percent elongation were recorded.
ELASTICITY TESTING:
The superelasticity of heat affected zone of the Nickel-titanium
segment was investigated by stress-strain measurements carried out at room
temperature using the same universal testing machine. The gauge length of
3 mm was chosen in this investigation. The brazed joint along with stainless
steel segment, is inserted in the lower part of the tester. The heat affected
zone on the Nickel-titanium segment was exposed as gauge length. The
residual strain of Nickel-titanium shape memory alloy heat affected zone
was recorded after loading and then unloading with the maximal strain of
4%.
BENDING TEST:
The shape memory effect was measured by the cantilever bend test to
evaluate the shape recovery ratio (SRR) of Nickel-titanium heat affected
zone. The Nickel-titanium side of joint specimen was bent to 90 degree (D)
for 300 seconds. After this time an angle d1 was retained. Then the
specimen was put in boiling water (100 degree C) for 5 seconds. There was
further recovery in the angle d2. The shape recovery ratio of Nickel-
titanium heat affected zone was determined by the following formula
d1 – d2
SRR = ____________ X 100%
d1
STATISTICAL ANALYSES:
All of the test values were statistically analyzed by ANOVA [analysis
of variance]. Turkey’s test (+/-5) was chosen as the following multiple-
comparison technique when necessary.
TENSILE TESTING RESULTS
BEND TEST RESULTS
Group Samples Breaking stress Elongation (%)
A 1 183 4
2 168 3.8
3 180 4.3
4 171 4.5
5 179 5.2
B 1 221 8.1
2 230 7.5
3 198 7.3
4 228 8.4
5 220 8.2
C 1 286 10.3
2 295 10.6
3 279 10.5
4 284 11
5 290 11.3
Group Sample no.
After unloading (d1)
After kept in boiling water t=5 sec (d2)
Shape recovery ratio (%)
Control 1 47 3 93.61
2 45 3 93.33
3 48 2 95.83
4 54 4 92.59
5 51 3 94.11
A 1 40 6 85
2 38 10 74
3 42 8 80
4 45 10 77
5 40 10 75
B 1 62 15 75.8
2 60 18 70
3 58 20 65.5
4 52 22 57.6
5 61 20 67.2
C 1 72 30 58.3
2 70 32 54.28
3 # # #
4 84 45 46.42
5 75 38 49.33
CBAControl
Mea
n S
hape
Rec
over
y R
atio
100
90
80
70
60
50
40
CBA
Mea
n B
reak
ing
Str
ess
in M
Pa
300
280
260
240
220
200
180
160
CBA
Mea
n E
long
atio
n in
%
12
10
8
6
4
2
INTERPRETATION OF RESULTS:
Tensile test;
Group C has the maximum mean breaking stress of 286.8 MPa followed by group B (219.4) and group A (176.2)
As the breaking stress increases, the percentage elongation also increases.
As the output power increases the standard deviation in percentage elongation of test specimens decreases.
The difference between the groups is statistically significant (p<0.01)
Elasticity test;
During elasticity test, the martensitic transformation occurred at mean stress of 180 MPa.
A flat plateau upto maximum strain of 3.6% obtained. A residual strain of 0.2% obtained for NiTi base metal alloy after unloading.
Whereas the residual strain values for Group A,B,C are 0.5%, 0.9% and 2.7% respectively.
Bend Test;
Shape recovery is maximum (93.89 %) in control group.
As the brazing power is increased the amount of permanent deformation also increased in the test specimen.
Among the test specimen Group A has the maximum shape recovery ratio (SRR) of 91.6% and Group C has the minimum SRR of 62.1%. During the bend test sample number 3 of Group C fractured near the heat affected zone of nickel-titanium segment.
DISCUSSION
A wire is a flexible structure or machine component having a working
length many times that of its cross-sectional dimension and the capability of
transmitting force along that length.
Wires have substantial structural presence in active and retentive
orthodontic therapy. Wires and auxiliaries fabricated from wire may deliver
force to produce dental displacements, they may attempt to prevent
unwanted displacements, or they may carry force from one location to
another within the dentofacial complex.
Burstone and Goldberg considered that an active orthodontic wire
should have three important characteristics – 1. High deflection without
permanent deformation (large spring back), 2. Low stiffness to produce light
force and 3. High formability.
The noble alloy wires used initially in the profession are too soft for
nearly all dental purposes. The constant increase in price and application of
more versatile stainless steel wires made the use of gold and silver alloy
wires obsolete for orthodontic purposes.
The austenitic stainless steel have a high value of yield strength (50,000
to 2,80,000 psi) and very high modulus of elasticity (23000000 to 29000000
psi).The high modulus necessitates the use of smaller diameter wires for
alignment procedures where lower forces are indicated. Unfortunately,
decreased wire size results in poorer fit in the bracket and loss of control. So
increasingly heavier wires are needed to be replaced thereby eliminating the
play between the wire and the bracket. This method is called variable-cross-
section orthodontics in which stainless steel wires of low load-deflection rate
to high load-deflection rate are sequentially replaced one after in appropriate
time.
The cobalt-chromium alloys developed in 1950s have stiffness
properties similar to stainless steel. One added advantage is that the strength
and formability of this alloy can be modified by heat treatment.
After the discovery of unique potential of equiatomic Nickel-Titanium
by Buehler, George Andreasen introduced Nitinol to the orthodontic
profession in 1971. Nitinol has excellent spring back property. But it does
not possess shape memory or super-elasticity because it has been
manufactured by a work-hardening process.
Shape memory refers to the ability of the material to remember, its
original shape after being plastically deformed while in the martensitic form.
Super elasticity refers to a very large reversible strains and a non-elastic
stress-strain curve. A super elastic archwire should exert the same amount
of force independent of the degree of activation within a wide range, a
phenomenon due to a stress-induced martensitic transformation from an
austenitic phase.
Nickel-titanium wires have limited formability, which contraindicates
its use for situations where bends with small radius are required. Its
springback properties are decreased after bending. These wires are not
amenable to conventional joining operations.
After a thorough review of orthodontic archwires many authors have
finally concluded that NO SINGLE ARCHWIRE IS IDEAL. In search of
single ideal archwire, GAC introduced BioForce wires. The manufacturers
claim that it is the only single strand, superelastic orthodontic archwire that
provides forces that range gradually from 80 grams in the central incisors to
320 grams at the molars.
OrthoOrganizers in an attempt to produce optimal orthodontic forces to
different teeth in a single wire, put forth Multi-Force Nitanium archwires.
Multi modulus Nitanium have three force regions, anterior (mild), canine-
premolar (moderate), and molar (high). But in vivo studies proved that these
wires are not as efficient as the manufacturers’ claim.
In this scenario, joining of stainless steel and Nickel-titanium has
multiple applications and advantages.
James L. Cannon noted that small gauge, flexible archwires that
produce light forces over a large working range are useful during the initial
alignment of crowded, malposed teeth.
However wire spans at extraction sites are longer and more flexible,
and space closure with inter or intramaxillary traction and overbite
correction requires stability in the buccal segments. Space closure should be
delayed until a stiffer archwire can be used to prevent uncontrolled
mesiolingual rotation and mesial tipping of first molars due to lack of
archwire support with labial and lingual appliances. Cannon crimped the
stainless steel buccal segment with Nickel-Titanium anterior segment. The
rigid steel wire assist in bite opening and molar control, while the flexible
anterior segment is used for rapid alignment, leveling and retraction of
anterior teeth. He obtained patent for this design and named them Dual-Flex
wires.
Thompson used Dual-Flex wires in his combination anchorage
technique. Dual-Flex 1 has 0.016 inch stainless steel in the posterior section
and 0.016 inch Nickel-titanium (Titanal) in the anterior segment. In cases
that require heavy anterior resistance to minimize lingual movement of
anterior teeth, a Dual Flex 2 wire can be used. This wire has a round 0.018
inch stainless steel posterior segment and 0,016 X 0.022- inch Nickel-
Titanium anterior segment from canine to canine. The steel posterior
segment is seated in the gingival slot where the resistance is minimal and the
0.016 X 0.022- inch segment is used in anterior edgewise slots. The light,
flexible rectangular wire features greater bracket engagement, increased
frictional resistance and lingual root torque, and is used to establish
increased anterior resistance and facilitates incisor control as posterior teeth
are moved mesially in the light wire slot.
Composite molar uprighting spring is a 7 mm segment of Sentalloy in
the tilted molar region joined with stainless steel in the remainder of
archwire. In the active state, the spring generates an uprighting moment of 8
Nmm and an intrusive force of 0.6 N. This simultaneous intrusion and
uprighting prevent occlusal trauma, provide gentle forces from Nickel-
titanium and it is a fail-safe procedure since full slot engagement is
frequently achieved in a continuous wire.
The Nitanium adjustable utility arch uses NiTi anterior segment for
intrusion and the 3 mm step down posterior segment used to deliver heavier
forces required to rotate or upright molars, open or close bites and perform
other adjustments.
The T-loop canine retraction spring joined to steel horizontal arms
found to deliver a constant moment-force ratio of 7 mm. Single activation
of 15 mm completes the whole space closure procedure. No uprighting
bends need to be given.
NiTi-superelastic-steel uprighting springs when incorporated in
Burstone intrusion mechanics, found to produce 0.6 mm/month average
intrusion. They also produced maximum intrusion of 7 mm in mandibular
incisors. The maxillary anteriors can be retracted by a total of 7 mm without
readjustments.
When superelastic segments are joined as auxiliaries in Variable
Modulus Orthodontics, a constant moment-force ratio is achieved. This
improves patient comfort, reduces number of office visits and lowers the
potential tissue damage.
Composite transpalatal arches made of Nickel-titanium and stainless
steel proved to be superior in molar derotation and unilateral expansions or
contractions can be achieved according to patient’s treatment need. Dental
asymmetries can thus be easily corrected.
But joining of Nickel-titanium and stainless steel by crimps produce
weak, bulky joints. They often get detached in patient’s mouth. The size of
crimp is controlled by interbracket span.
Laser, an acronym for light amplification by stimulated emission of
radiation, is essentially a coherent, convergent and monochromatic beam of
electromagnetic radiation with wavelength ranging from ultra-violet to
infrared. Laser can deliver very low (_mW) to extremely high (1–100kW)
focused power with a precise spot size/dimension and interaction/pulse time
(10-3 to 10-15s) on to any kind of substrate through any medium . Laser is
distinguished from other electromagnetic radiation mainly in terms of its
coherence, spectral purity and ability to propagate in a straight line. As a
result, laser has wide applications. The initial foundation of laser theory was
laid by Einstein. Subsequently, Kopfermann & Ladenburg presented the first
experimental confirmation of Einstein’s prediction. In 1960, Maiman
developed a ruby laser for the first time. This was followed by much basic
development of lasers from 1962 to 1968. Almost all important types of
lasers including semiconductor lasers, Nd:YAG lasers, CO2 gas lasers, dye
lasers and other gas lasers were invented in this era. After 1968, the existing
lasers were designed and fabricated with better reliability and durability. By
mid 1970s more reliable lasers were made available for truly practical
applications in the industrial applications such as cutting, welding, drilling
and marking.
Girish P. Kelkar recommends Nd:YAG laser for pulse welding, which
emits a near-infrared wavelength of 1.064 micro meter. Most metals that are
not highly electrically conductive or reflective, such as titanium and
stainless steels, absorb laser light reasonably well and therefore laser
manipulation is not a problem. Manna and Muzumdar observed no change in
shape memory effect when Nickel-Titanium is laser welded.
But welding of titanium and titanium based alloys, to ferrous metals is
plagued with poor quality and highly brittle welds, substantially due to
formation of Ti-Fe intermetallics in the weld pool. Only Peter C. Hall of
Edison welding Institute holds a patent for the solution overcoming this
problem. His invention provides supplementary filler material to alter the
proportions of various elements in the weld pool.
Brazing is a joining process whereby a non-ferrous filler metal and an
alloy are heated to melting temperature above 450 degree C and distributed
between two or more close-fitting parts by capillary action.
At its liquid temperature, the molten filler metal interacts with a thin
layer of the base metal, cooling to form an exceptionally strong, sealed joint
due to grain structure interaction. When properly designed, a brazed joint
will yield a very high degree of serviceability under concentrated stress,
vibration, and temperature loads. In a properly designed brazement, any
failure will mostly occur in the base metal, and not in the joint.
The filler, which has a lower melting point then the metals to be joined,
is either pre-placed or fed into the joint as the parts are heated. In soldering,
a related process, the filler metal remains below 450 degree C. Brazed joints
are usually stronger than soldered joints.
In brazing, unlike welding, the parts are not melted. Brazing is best for
dissimilar or thinner metal parts and for parts difficult to weld or solder.
Although brazed joints are generally not as strong as welded joints, there are
certain advantages present in brazed joints over welded joints.
- the lower temperature of brazing is less likely to distort the
work piece.
- Brazing does not significantly change the crystalline structure
(creating a heat affected zone)
- Brazing does not induce thermal stress
- For thin work pieces, brazing is less likely to result in burn-
through
- Braze welded joints generally have smooth attractive beads that
do not require additional grinding or finishing.
M.G.Li et al studied the properties of laser-brazed joint of NiTi shape
memory alloy and stainless steel orthodontic wires. They concluded that the
tensile strength of the laser-brazed joint could reach to 340 MPa while the
loss of superelasticity and shape memory effect of NiTi heat affected zone
is relatively low by strictly controlling brazing parameters.
The heat-affected zone (HAZ) is the area of base material, either a
metal or a thermoplastic, which has had its microstructure and properties
altered by welding. The heat from the welding process and subsequent re-
cooling causes this change in the area surrounding the weld. The extent and
magnitude of property change depends primarily on the base material, the
weld filler metal, and the amount and concentration of heat input by the
welding process.
The amount of heat inputted by the welding process plays an important
role, as processes like oxyfuel welding use high heat input and increase the
size of the HAZ. Processes like laser beam welding give a highly
concentrated, limited amount of heat, resulting in a small HAZ.
When orthodontic wires are bowed and seated in orthodontic brackets
attached to teeth, fatigue fractures often occur after repeated loading and
unloading in modes similar to brittle fracture. As a result, the tensile strength
and flexural strength of the composite orthodontic wires made of NiTi shape
memory alloy and stainless steel are required to withstand this stress. In this
study, the high stiffness of stainless steel and the super elasticity and shape
memory effect of NiTi shape memory alloy were required, so the loss of
super elasticity and shape memory effect in NiTi heat affected zone after
brazing must be minimal and the heat affected zone width of the two base
metals must be narrow.
In this investigation,Laser-brazing of stainless steel and Nickel-
Titanium shape memory alloy wire using silver-based filler metal at
different brazing parameters was performed. Tensile tests were done in the
samples to evaluate breaking stress and percent elongation. Performing
Elasticity tests in the samples revealed the superelasticity loss of NiTi wire
in the heat affected zone. Bend test in the samples made, to evaluate the
shape memory effect loss of NiTi wire in the heat affected zone.
The results of tensile testing showed that the combined strength of the
SS and the filler metal were high, resulting in all fractures of the specimens
occurring in the center of the brazing seam; in the NiTi SMA HAZ; or at the
interface layer between NiTi SMA and the filler metal. The stiffness in the
SS HAZ was slightly influenced by the narrow width of the SS HAZ of 1
mm. Since the width of SS HAZ was narrow (only 1 mm), attention was
mainly focused on the changes of properties of NiTi SMA HAZ of the laser-
brazed joint.
Newman et al. reported a maximum clinical load of 1.82 kg can be
applied without preventing blood circulation in the periodontal ligament.
The minimal tensile strength of the specimens after brazing was about 168
MPa (about 3.5 kg), bigger than 1.82 kg, satisfying the clinical requirements.
As a result, the main factors influencing the clinical application of the
composite archwires were flexural strength and the loss of super elasticity
and shape memory effect of NiTi heat affected zone.
SME and SE (or transformation pseudo- elasticity) are always related to
the thermo-elastic martensitic transformation from the B2 parent phase to
the B19’ monoclinic phase in an approximate equiatomic NiTi SMA . One of
the properties of the SME and SE of Niti SMA is the reversibility of
martensitic transformation in crystallography, i.e., the corresponding
relationship between the B2 parent phase and the B19 martensite must be
maintained. The interfacial energy of the coherent interface between B2
phase and B19’ martensite of NiTi SMA is very low but its elastic
distortional potential is high due to the distortion on the interface. This
maintains the corresponding relation on the coherent interface. When NiTi
SMA and SS were laser-brazed, the brazing heat cycle was fast and the
brazing temperature was high. The constant growing of new phases made
the elastic distortional potential increase consistently by the thermal effect.
The corresponding relation between B2 phase and B19’ martensite would be
destroyed by the plastic deformation due to the increasing elastic distortional
potential beyond the yield limit of the parent phase. As a result, the
corresponding relation between the parent phase and the martensite of NiTi
SMA were partially destroyed after NiTi SMA and SS wires were laser-
brazed at high brazing heat input, i.e., the SME and SE of NiTi SMA HAZ
were partially destroyed.
Quantifying the amount of super elasticity loss and shape memory loss
in the heat affected zone is very important.
In Group A (50W/10s) although shape memory effect and super
elasticity were relatively well maintained in the heat affected zone, the yield
strength is least compared to other groups. In Group C (70W/20 s) where
the breaking stress values are very high, the properties of shape memory
effect and super elasticity are drastically reduced in the heat affected zone.
One specimen from Group C fractured during the bend test. These results are
similar to those obtained by Li et al. But there is statistically significant
difference between all the three groups in all the three tests (Tensile,
elasticity and bend). This differs from the result of previous study.
From the obtained results it is found that if the clinical situation
requires moderate superelsticity and shape memory effect in the NiTi
segment and overall high breaking stress in the composite archwire, a
relatively high brazing heat input is required. When the NiTi segment need
more amount of bending to get engaged in the archwire (example-mesially
tilted molars), the composite archwire should be fabricated in a low brazing
heat input setup.
The above findings need to be confirmed by clinical studies and the
study can be further continued using NiTi and stainless steel wires of
different dimensions in the same archwire.
SUMMARY AND CONCLUSION
The clinical advantages of composite wires made of nickel-titanium
and stainless steel are well established in the literature by various authors.
They include high versatility of appliance system, decreased chair side time,
decreased number of appointments, increased comfort to the patients,
improved control of tooth movement, and decreased treatment time.
In this study, laser-brazing - a relatively new method of joining
stainless steel and nickel-titanium, is investigated under varying braze
parameters. It was found that composite nickel-titanium and stainless steel
wires with sufficient tensile strength for clinical use can be produced by this
method. The loss of super elasticity and shape memory effect in the heat
affected zone of nickel-titanium segment could be kept relatively low by
strictly controlling the brazing heat input.
The combination of active material (Nickel-titanium) with a passive
material (stainless steel) forms a smart structural system (SSS). Such
systems are adaptive in nature and can be utilized in applications that are
subject to time varying loads. These composite archwires can also be used to
make our statically indeterminate force systems more predictable because of
their constant moment-force ratio for a larger range. Thus composite wires
made of Nickel-titanium and stainless steel has good prospects for
orthodontic applications.
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