SOLDERINGBRAZINGWELDING

Dr LAKSHMI RAVI M.D.SAsst ProfessorDept of OrthodonticsSt.G.D.C

DEFINITIONS

SOLDERING: It’s defined as the joining of metals by the fusion of filler metal between them, at a temperature below the solidus temperature of the metals being joined and below 450°C.

BRAZING: It’s a term used industrially. Soldering operations at or above 450°C is generally termed brazing. Most dental soldering procedures are actually brazing, but the names are used interchangeably in dentistry.

Brazing is defined as joining of metals by the fusion of a filler metal between them, at a temperature below the solidus temperature of metals being joined and above 450°C.

LIQUIDUS TEMPERATURE: the temperature at which metals of an alloy system begin to solidify on cooling or become totally liquid on cooling.

SOLIDUS TEMPERATURE: the temperature at which metals of an alloy system become completely solidified on cooling or start to melt on heating.

WELDING: The joining of two or more metal parts by applying heat, pressure or both, with or without a filler metal, to produce localized union across the interface through fusion or diffusion.

CAST JOINING Another type of metal joining procedure in dentistry. It is the process of combining two components of a fixed partial denture by means of casting molten metal into interlocking region between invested components. This is preferred for base metal alloys because of technique sensitivity of brazing or soldering these alloys.

Soldering / Brazing : The diff. is between the liquidus temp. of the filler metal.

Soldering & Brazing / Welding : Possible absence of the filler metal & the partial fusion of the parts joined by welding.

SOLDERING HISTORY The soldering technique has been known to man for hundreds,

maybe thousand years. However, with the industrial revolution in Europe as well as North America, the need for higher temperatures and more user-friendly tools emerged. The old methods of heating metals with coal fires etc. were no longer practical.

It was discovered that when the vapor from heated alcohol was ignited over a burning wick, it burnt with a very concentrated flame of high temperature very suitable for different heating purposes.

Many different designs emerged using this technique and these heating tools were generally called blow pipes. The first known patent is from France and is dated January 7, 1791.

During the early 1900`s, a great variety of blow lamps specially designed for different purpose and applications came on the market.

After the Second World War, the propane gas emerged as a cleaner and safer fuel for different heating purposes. The introduction of propane caused a lot of changes in the blow lamp industry world-wide.

The first appliances used had a metal frame work. The attachment of axillaries to bring about the different type of tooth movements required soldering of these parts. Welding in orthodontics became popular after the arrival of spot welders. It became popular because of the short time required, the ease of welding and the absence of elaborate equipments

Soldering is often used in construction of dental appliances. Large partial dentures are frequently cast in parts that are soldered together after carefully fitting them to master cast. In orthodontics soldering is used for joining wires, bands springs etc

The soldering process involves the substrate or the parent metals to be joined, soldering filler metal (usually called solder), a flux, and a heat source. All are equally important and the role of each must be taken in to consideration to solder metal components successfully.

COMPONENTS OF SOLDERED JOINT Parent metal Solder/filler metal Fluxes and Anti fluxes

PARENT METAL The parent metal is the metal or alloy to be joined. This is also known as a substrate metal or base metal. Soldering operation is the same for any substrate metal, but the ease of soldering is not same for any substrate metal.

The composition of parent metal determines- Melting range Oxide that forms on the surface during heating Wettability of the substrate by the molten solder. Soldering should take place below the solids temperature of the

parent metal.

Composition of alloy determines the oxides that form on its surface during heating. The flux used should be able to reduce these oxides, inhibit further oxidation and facilitate its removal.

Composition of alloy determines the wettability of the substrate by the molten solder alloy. The solder chosen must wet the metal at as low a contact angle as possible to ensure wetting of the joint area.

Manufacturer of the alloy should provide guidance and instruction regarding the flux to be used with that alloy.

A low temp soldering is preferred rather than the high temp soldering for Stainless steel wire to prevent carbide precipitation and to prevent an excessive softening of the wire. So silver solders are generally preferred.

FLUX In Latin flux means “to flow”.

Purpose of flux is to remove any oxide coating on the substrate metal surface when the filler metal is fluid and ready to flow into place.

They protect the alloy surface from oxidation during soldering and dissolve metallic oxides as they are formed.

The resulting solution of oxides or other extraneous matter in flux constitutes “slag”.

CLASSIFICATION OF FLUX1.According to their primary purpose / activity

Surface protection type: - This type of flux covers the metal surface and prevents access to oxygen, so that no oxides can form.

Reducing agent type: - This type reduces any oxides present and exposes clean metal.

Solvent type: - This type dissolves any oxides and drives them away.

The composition of most commercial fluxes is formulated to accomplish two or more of these purposes.

2.According to their composition Borax fluxes Fluoride fluxes

3. According to the pH of the flux Acidic fluxes – SiO2 Basic fluxes – CaO, lime CaCO3 LIMESTONE Neutral – Fluorspar (Ca.F2),Borax (Na2B4O2)

BORAX FLUXES Borax from Persian burah Also called sodium borate, or sodium tetraborate, or disodium Tetraborate. They are based on boric or borate compounds such as boric acid/boric anhydrate and borax. It is usually a white powder consisting of soft colorless crystals that dissolve

easily in water.

Borax has a wide variety of uses:-1. It is a component of many detergents, cosmetics, and enamel glazes. 2. It is also used to make buffer solutions in biochemistry3. as a fire retardant4. as an insecticide5. as a flux in metallurgy6. They act as protective fluxes and reducing fluxes for low stability. oxides such as copper oxide. And are used for noble metal alloys.

BORAX

They are available in

Liquid form: Solution of borax/boric acid in water. Indicated for soldering of orthodontic appliances and bridges in which minimum amount of flux is required.

Paste form: Formed by mixing borax with petroleum jelly. Required when fluxes are needed in large quantity.

Powder form: Contains a mixture of borax, basic acid, silica flour and finely divided charcoal. Charcoals reducing agent and silica holds molten flux in surface of hot metal. This is usually used for casting operation.

FLUORIDE FLUXES

Composition:- Potassium fluoride – 50-60% Boric acid – 25-35% Borax glass - 6-8% Potassium carbonate – 8-10%

As the choice of flux is dictated by the type of alloys to be soldered, the fluoride flux is used with alloys containing base metals even if a gold/silver solder is used. Some fluoride containing fluxes involve toxic fluorides when heated, so inhalation of fumes should be avoided.

POTASSIUM FLUORIDE

The chemical compound with the formula KF. After hydrogen fluoride, KF is the primary source of the fluoride ion for applications in manufacturing and in chemistry. It is an alkali metal halide and occurs naturally as the rare mineral carobbiite(potassium-67.30% + fluorine-32.70%)

is a colorless cubic mineral. It is found at Monte Somma, Somma-Vesuvius Complex, Province of Naples, Campania, Italy. It was discovered in 1956.

Aqueous solutions of KF will etch glass due to the formation of soluble fluorosilicates.

SUPER FLUX

A combination of high melting salts is used as fluxes to combine the good characteristics of each ingredient and create superior flux.

A formula for efficient flux is Borax glass – 55 parts

Boric acid – 35 parts Silica - 10 parts

The ingredients may be fused together and then crushed to fine powder.

APPLICATION OF FLUX Painted on to the substrate metal at the junction of pieces to be joined.

Fused on to the surface of the filler metal strip.

Whatever be the technique used the most important thing to consider is the amount of flux used.

Too little flux tends to burn off and will be ineffective.

Excess flux remains trapped within filler metal and cause a weakened joint.

Flux combined with metal oxides forms a glass during soldering process that is difficult to remove completely.

A two step method for removing residual flux Blast joint immediately after removal from investment with alumina abrasive particles followed by boiling in water for about 5 minutes.

ANTI FLUX

Materials used to restrict flow of solder are known as anti flux. It is applied on the surface of specific area where the solder

should flow into. It is applied before applying flux or solder.

E.g.: Graphite in the form of lead pencil. Disadvantage of graphite is that it can burn off on prolonged heating at high temperature.

In such cases whiting (CaCO3 in alcohol and water suspension) is used.

FILLER METAL/SOLDER Qualities of an ideal solder Ease of flow at relatively low temperature.

Sufficient fluidity to freely flow when melted.

Ability to wet substrate metal.

Strength compatible with that of the structure being joined.

Resistance to tarnish and corrosion.

Acceptable colour to give an inconspicuous joint.

Resistance to pitting during heating.

FLOW TEMPERATURE

The temperature at which the filler metal wets and flows on the substrate metal and produces a bond. It is usually higher than the liquidus temperature.

ISO 9333 requires that the flow temperature of the filler metal should be lower than the solidus temperature of the substrate metals. A rule of thumb is that flow temperature of the filler metal should be 56°C (100°F) lower than the solidus temperature of the substrate metal.

If the flow point of the filler metal is close to or above the solidus of either substrate alloying can take place. An alloy formed through diffusion can have properties different from filler metal and substrate metal.

CLASSIFICATION OF SOLDERS

l. Soft solders Hard solders

II. Precious metal solders Non precious metal solders

SOFT SOLDERS They are lead- tin eutectic alloy with a low melting

point. Sometimes called as plumbers solder. They have low fusion range of about 260°C or less. Soft solders lack corrosion resistance, so they are impractical for dental use.

HARD SOLDERS Hard solders have higher meting temperature &

possess greater hardness and strength. Heating is done with gas torch or special devices. Two types of hard solders are used in dentistry

Gold solders

Has good tarnish and corrosion resistance Extensively used for crown and bridge applications.

Composition Gold – 45-81 wt % Silver - 8-30 wt % Copper -7-20 wt %

with small amounts of Tin, Zinc and Phosphorus to modify

fusion temperature and flow qualities. They are high fusing with a fusion temperature range of 750- 900° C.

Silver solders Used in orthodontic appliances They are low fusing –fusion temp-600-750°C Used with stainless steel or other base metal alloys Resistance to tarnish and corrosion is not as good as gold

solders But have strength comparable to gold solders Composition Silver -10-80 % Copper -15-30% Zinc -4-35% with small amounts of cadmium, tin and phosphorus. The formation of silver-copper eutectic is responsible for

the low melting range of silver solder.

SILVER SOLDER

HEAT SOURCE

The most common instrument used as heat source is gas- air or gas- oxygen torch.

The type of torch depends on the type of fuel.

The fuels used are :-

Hydrogen-low heat content, so heating is slow.

Natural gas- heat content is four times that of hydrogen.

Acetylene - high flame temperature, but variation in temperature from one part of the flame to the other part is more than 100°C. So positioning of the torch is critical. It is chemically unstable gas, decompose to carbon and hydrogen. carbon can get incorporated in to nickel and palladium alloys.

Propane - is the best choice. Have highest heat content& good flame temperature.

Butane - has similar flame temperature and heat content. Both are readily available. Uniform in quality, virtually water free and burn clean.

FLAME The flame can be divided in to four zones

1. Cold mixing zone (unburned gas)2. Partial combustion zone (oxidizing)3. Reducing zone4. Oxidizing zone (burned gas).

The portion of the flame that is used to heat the soldering assembly should be the neutral or slightly reducing part, because this produces the most efficient burning process and most heat.

Improperly adjusted torch or improperly positioned flame can lead to oxidation of the substrate or filler metal and result in a poorly soldered joint.

If unburned portion of flame is used carbon may be introduced to the substrate or filler.

To prevent oxide formation the flame should not be removed once it has been applied to the joint area until soldering process has been completed.

OVEN (FURNACE) SOLDERING

A furnace with enough wattage to provide heat

required to raise the temperature of the filler

metal to its flow point.

Advantages: Uniform temperature Close monitoring is possible Temperature is known Application of vacuum control oxidation

INFRARED SOLDERING The unit uses light from a 1000 watt Tungsten

filament quartz- iodine bulb which is mounted at the primary

focal point of a gold plated elliptical reflector. The material to be soldered is placed at the

reflectors secondary focal point at which the reflected infrared energy of Tungsten light source is focused.

This is used for high temperature soldering.

TECHNIQUES OF SOLDERING

Investment soldering Free hand soldering

INVESTMENT SOLDERING Used when very accurate alignment of parts to be

joined is needed. The parts are placed on the master cast with a gap of

at least 1mm. The parts are fastened with sticky wax before

placing soldering investment. Anti flux is applied to confine the flow of solder. The investment is preheated to eliminate moisture. Flux can be applied before or after heat treatment. Soldering is carried out with reducing flame at 750-

870°C. The investment is cooled 5 min before quenching. Flux will cool to a glass which is removed by

pickling.

FREE HAND SOLDERING

Free hand soldering is used for soldering orthodontic appliances.

Orthodontic torches can be placed on a bench so that both hands can be used to hold the parts in position.

SOLDER JIONT GAP

If the gap is too great the strength will be controlled by the strength of the filler.

If the gap is narrow the strength will be limited by the flux inclusions& porosities by the incomplete flow of the filler, metal.

STEPS IN SOLDERING Cleaning and preparing the surfaces to be joined Assembling the parts to be joined Preparation and fluxing of the gap surfaces between

the gaps Maintaining the proper position of the parts during

procedure. Control of proper temperature Control of time to ensure adequate flow of the

solder& complete filling of the solder joint

ORTHODONTIC SOLDERING

In orthodontic applications low temperature soldering is used to prevent carbide precipitation and to prevent excessive softening of the wire.

Low fusing silver solders are used with a soldering temperature range of 620-655°C.

Fluoride fluxes are used for orthodontic stainless steel and other base metal alloys.

Free hand soldering technique is employed with a needle like non luminous gas air flame is used.

The work should be held 3mm beyond the tip of the blue cone in the reducing zone of the flame.

Soldering should be observed in a shadow, against a black back ground, so that the temperature can be judged by the color of the work. The color should never exceed a dull red.

Flux must cover all the areas to be joined before heat is applied.

As soon as the flux fuses solder is added and heating is continued until metal flows around the joint. The work is then removed from the heat and quenched in water.

SOLDERING APPLICATIONS IN ORTHODONTICS

Wire to wire

Tubes can be soldered to the bridge of the Adams clasp.

Attachment of springs to arch wire. When soldering an auxiliary spring to arch wire, the solder must be a gold one with a melting point below 800°C.

Soldering lingual arch or palatal arch: to hold the arch during soldering position it on the model and place a blob of wet pumice over the middle portion of the model. Water is immediately soaked on to the cast leaving dried pumice which is firm enough to secure the arch during soldering.(F.G. Thompson, JCO 1969 April)

SOLDERING FAILURES Are due to: Failure to clean the parts to be joined

Improper fluxing

Poor flow of solder

Over heating of the solder can lead to pitted joint of low strength

Besides porosities and brittleness from oxides, gases, or foreign matter resulting from the soldering procedures as factors for increasing the incidences of failure of soldered joints.

Creep, corrosion, stress corrosion cracking, corrosion-fatigue, and corrosion-erosion.

Gas embitterment can also be generated by gases formed from electrochemical processes. Hydrogen embitterment from corrosion is a very well-known phenomenon that occurs with some material-solution combinations and at temperatures comparable to physiologic conditions.

CORROSION OF SOLDERED JOINTS A consideration of the composition of silver solders reveals that any

material containing up to about 20 percent zinc and 20 to 30 percent copper with additions in some cases of low cadmium and tin cannot remain inactive to physiologic solutions.

Weak corrosion-prone micro structural phases composed mainly of copper and zinc has been shown to occur within the solder itself. It is known that corrosion occurs when an electrolyte comes into contact with a soldered joint.

The silver solders react readily to chemical attack.

The breakdown reaction between silver-soldered stainless steel joints is an electrochemical process with no initial evidence of gross macroscopic corrosion.

After a time, many silver-soldered joints exhibit a change in appearance such as darkening to resemble a tarnished, corroded surface.

BIOCOMPATIBILITY

Besides the deterioration of the soldered joint, concern must be given to the toxicological importance of the released corroded agents.

For Cadmium-containing solders, because of Cadmium's toxicity, a continual appraisal must be made regarding Cadmium's fate biologically. The release of Cadmium from dental alloys has been the subject of several reports.

Even in the case of such nontoxic elements as Zinc, Copper, Tin, and Silver, the introduction of higher concentrations of these elements via soluble corrosion products can alter their behaviour

Causing biologic imbalances with further biologic consequences.

It is believed that possible allergies to nontoxic metals released from dental alloys may be formed.

Metabolic and bacteriologic participation can also occur in response to

corroded metallic agents. Furthermore, the penetration and staining of hard dental tissues due to the release of metallic ions from solders or any biomaterial are definitely to be avoided.

Laboratory tests indicate that silver-soldered stainless steel joints degrade in a saliva substitute and other prepared solutions.

Corrosion products containing oxides, hydroxides, and chlorides of zinc, copper, tin, and cadmium can be easily identified. Silver is also attacked

Besides the oral physiologic fluids, additional chemical agents contained in mouth rinses and in toothpastes for oral antiseptic, need careful appraisal for resistance to the degradation and corrosion of dental materials.

Many commercial mouth rinses contain active chlorides and additional components. The chlorides are notorious for their depassivation tendencies of metallic materials.

NEWER SILVER SOLDERS

For joining stainless steel, additional alloys with improved corrosion resistance

classifications of silver soldering alloys (referred to here as brazing alloys), including BAg-18 and BAg-21 (American Welding Society), can be used very effectively.

These silver soldering alloys have silver contents at about the same level as the solder products presently employed, but they have slightly higher copper contents, with additions of up to about 10 percent tin for wetting stainless steels and up to about 3 percent nickel for immunity to crevice corrosion.

The important fact is that both cadmium and zinc are removed from these alloys. The soldering (brazing) temperature range is between 700 and 900°C, in some instances about 200°C higher than the presently employed products.

Because of the non-free-flowing characteristics of these proposed soldering alloys for dental applications, familiarization with their properties and handling characteristics is advisable.

WELDING Welding is the process by which the surfaces of

metals are joined by mixing, with or without the use of heat.

Cold welding is done by hammering or pressure. An example of cold welding is the gold foil filling.

Hot welding uses heat of sufficient intensity to melt the metals being joined. The heat source is usually an oxyacetylene flame or high amperage electricity.

TYPES OF WELDING

SPOT WELDING PRESSURE WELDING LASER WELDING PLASMA WELDING

LASER WELDING

PLASMA WELDING

PRESSURE WELDING

SPOT WELDER

THEORY Orthodontic spot welders employ the electrode

technique and are used instead of soldering in cases where the heating cycle must be very short, in order to prevent changes in the physical properties of the components being joined.

Orthodontic welding is achieved by passing a large amount of current for a very short duration through an area of high resistance. Heat is generated of a magnitude great enough to cause melting at the interface in contact.

Copper electrode - Low resistence As sufficient heat is generated at the weldmate

interface, the stainless steel components soften, flow and fuse together under the influence of mechanical pressure, forming a weld nugget

In spot welding the following three properties of the metal are favorable:

A comparatively low melting point (approximately 1370° C.),

high electric resistance, And low conductivity of heat.

VARIABLES AND THEIR APPLICATION

Welding of stainless steel depends on the proper use of each of the following three variables:

1. The current flowing through the circuit. 2. The time during which the current is

allowed to flow. 3. The mechanical pressure applied at the

welding head.

The improper application of these variables can result in either over- or under welding

UNDER WELDING - Insufficient current - The current passed for an insufficient amount of time - Pressure applied inadequate in approximation.

OVER WELDING - Yield weak a joint as under welding. - Progressive corrosion. This occurs when chromium is precipitated at the grain

boundaries of each crystal. This process is known as weld decay.

A satisfactory welded joint is one which is strong, has not undergone oxidation (blackening), and has not been over compressed during fusion.

CLINICAL APPLICATION The use of the spot welder in orthodontics is so common that it

is almost a reflex.

1.The welding surfaces should be clean of all debris materials and oxides.

2.The surface of each electrode must be smooth, flat, and perpendicular to its long axis. When the electrodes are together, they should be in total contact. If not, they should be filed until total contact is achieved. Sparking and localized over welding will result if interface contact is not uniform.

3.Adjust the welder to settings recommended by the manufacturer.

4. Select the proper electrode for the thickness or shape of the material to be welded. A broad electrode should be used for thin material and a narrow one for thick material. This will allow sufficient heat to reach the weld area, but not over weld or oxidize the weldmates.

If too narrow an electrode is used in welding a bracket (thick) to a band (thin), localized over welding will occur in the thin material and under welding in the thick material

Proper electrode selection— a broad electrode for thin material in conjunction with a narrow electrode for the thicker material— will result in an even distribution of the weld nugget

5.Insert the weldmates between the electrodes, close them together, and depress the weld button.

If sparking is observed, localized over welding has occurred. The electrodes should be checked for size and/or contact. If black areas are seen at the points where the electrodes contacted the weldmates, over welding has occurred.

(JCO-Volume, 1976 Feb Orthodontic welding-Robert E. Binder. DMD)

A CHECKLIST FOR SUCCESSFUL WELDING

Mount the electrodes correctly in the welder and adjust with parallel, precision filing to eliminate gaps.

Weld together only wires of the same material. Place the thinner wire in the groove of the lower

electrode. Use 1540 Newtons of pressure. Set the voltage according to the operating

instructions. Use a single electrical impulse.

CONCLUSION The choice of solder material has extreme importance in determining the

properties of the soldered joints. In orthodontics, silver solders are popular because of their lower fusion temperatures and easy-handling characteristics. It is also rationalized that both fixed and removable orthodontic appliances are not meant to be permanent; therefore, solders with properties inferior to some of the higher-fusing solders can be tolerated.

Soldering is still a useful and needed procedure for the joining of metallic parts. The choice of whether free-hand or investment, torch or oven, or pre or post ceramic soldering techniques are used, as well as variations in gap distance and high- or low-fusing soldering alloys, is in many instances up to the discretion of the orthodontist and technician.

The breakage of soldered components is one of the pitfalls associated with this joining procedure. Even though following guidelines should theoretically prevent them from occurring, solder failures are not rare.

Spot welding is suitable only when the thickness of the band or flange more or less corresponds to that of the sheet to which it is to be welded, and should not be used to join auxiliary springs and arch wires. One kind of electrode is ample for spot welding in the construction of orthodontic appliances.

Although welding is one of the technical procedures most commonly used by orthodontists, the process is usually poorly understood and not employed efficiently.

In the final analysis, however, the combination of techniques which offer optimum mechanical, physical, and chemical properties or offer the desired property with the most favorable must be selected.

REFERENCE

Phillip’s science of dental materials- Anusavice dental materials –A programmed review of selected topics-

W.J.O’Brien Dental Materials – Craig. removable appliances – PC Adams American journal of orthodontics 1937 may American journal of orthodontics 1982 February journal of clinical orthodontics 1969 April journal of clinical orthodontics 1969 November journal of clinical orthodontics1976 February journal of clinical orthodontics 1987 September journal of clinical orthodontics 1991 January journal of clinical orthodontics 2000 may