Soldering, Brazing and Braze Welding

Soldering, Brazing and Braze WeldingAg Metals IWelding Principles & ApplicationsChapter 31ObjectivesDefine the terms soldering, brazing and braze weldingExplain the advantages and disadvantages of liquid-solid phase bondingDescribe the functions of fluxes in making proper liquid-solid phase bonded jointsIntroductionSoldering and brazing are classified by the AWS as liquid-solid phase bonding processesThis meansThe filler metals is meltedThe base material or materials is not meltedThe phase is the temperature at which bonding takes place between the filler and baseThe bond between the base material and filler material is metallurgical because no alloying or melting of the base metal occursIf done correctly, this bond results in a joint that has 5 Xs the tensile strength of that of the filler metalSoldering & Brazing SolderingTakes place at a temperature below 804 F BrazingTakes place at a temperature above 804 FThis is the only difference between the two

BrazingBrazingParts being joined must be fitted so the joint spacing is very smallThis small spacing allows capillary action to draw the filler metal into the joint when the parts reach the proper phase temperatureBraze WeldingDoes not need capillary action to pull filler metal into the jointAdvantages of Soldering and BrazingLow temperaturePermanent or Temporary JoiningDissimilar materials can be joinedSpeedLess chance of damageSlow rate of heating/coolingParts of varying thickness can be joinedEasy realignmentTensile StrengthThe joints ability to withstand being pulled apartBrazed joints can be made to have a tensile strength of 4-5 Xs higher than the filler metal itselfAs joint spacing decrease, surface tension increases the tensile strength of the joint

Shear StrengthA joints ability to withstand a parallel forceDepends upon the amount of overlapping area of the base partsThe great the area of overlap, the greater the strengthDuctilityThe joints ability to bend without failingMost soldering and brazing alloys are ductile metals making the joints they are made with ductile as wellFatigue ResistanceThe joints ability to be bent repeatedly without exceeding its elastic limit and without failureFairly low for most soldered or brazed jointsFatigue failures may also occur as a result of vibration

Corrosion ResistanceThe joints ability to resist chemical attackCompatibility of base metals to filler metals will determine corrosion resistance

Functions of FluxRemove any oxides that form as a result of heating the partsPromote wettingAid in capillary actionFlux in GeneralWhen heated to its reacting temperature must be thin and flow through the gap provided at the jointAs it flows through the joint it absorbs and dissolves oxides, allowing the molten filler metal to be pulled in behind it.Once the joint is complete the flux material should be completely removableTypes of FluxesSolidsPowdersPasteLiquidsSheetsRingsWashersThey are also available mixed with filler metal, inside the filler metal or on the outside of filler metal

Fluxing ActionWill remove light surface oxides, promote wetting, and aid in capillary actionBut they do not eliminate the need for good joint cleaningFlux will not remove oil, dirt, paint, glue, heavy oxides or other surface contaminants

Soldering & Brazing Fluxes Soldering Fluxes are chemical compounds such asMuriatic acid (hydrochloric acid)Sal ammoniac (ammonium chloride)RosinBrazing Fluxes are chemical compounds such asChemical compounds such asFluoridesChloridesBoric acidsAlkalies

What They DoReact to dissolve, absorb or mechanically break up thin surface oxides that are formed as the parts are being heatedMust be stable and remain active through the entire temperature range of the solder or braze filler metalChemicals react as either acids or basesSome dip fluxes are saltsGeneralMethods are grouped according to which heat is appliedTorchFurnaceInductionDipResistance

Torch Soldering and BrazingOxyfuel or air-fuel torchesAcetylene is the most often used but is not as preferable when compared to other fuel gasesThis is due to uneven heating

Torch Soldering and BrazingAdvantagesVersatilityPortabilitySpeedDisadvantagesOverheatingSkillFires

Furnace Soldering and BrazingParts are heated to their soldering or brazing temperature by passing them through a furnace

Furnace Soldering and BrazingAdvantagesTemperature controlControlled atmosphereUniform heatingMass productionDisadvantagesSizeHeat damage

Induction Soldering and BrazingUses high frequency electrical current to establish a corresponding current on the surface of the partThe current on the part causes rapid and very localized heating of the surface onlyLittle if any internal heating of the part except by conductivity of heat from the surfaceInduction Soldering and BrazingAdvantagesSpeedVery little time is required for the part to reach the desired temperatureDisadvantagesDistortionLack of temperature controlIncomplete penetrationDip Soldering and BrazingTwo typesMolten flux bathMolten metal bath

Molten Flux MethodSoldering or brazing filler metal in a suitable form is preplaced in the joint and the assembly is immersed in a bath of molten fluxThe bath supplies the heat to preheat the joint and fuse the solder or braze metal and it provides protection from oxidationMolten Metal MethodPrefluxed parts are immersed in a bath of fused solder or braze metal which is protected by a cover of molten fluxMethod is confined to wires and other small partsOnce removed from the bath, the ends of the wires or parts must not be allowed to move until the solder or braze metal has solidifiedDip Soldering and BrazingAdvantagesMass productionCorrosion protectionDistortion minimizedDisadvantagesSteam explosionsCorrosionSizeQuantity

Resistance Soldering and BrazingElectric current is passed through the partResistance of the part to the current flow results in the heat needed to produce the bondFlux is usually preplacedMaterial must have sufficient electrical resistance to produce the desired heatingMachine used in this process resembles a spot welderResistance Soldering BrazingAdvantagesLocalized heatingSpeedMultiple spotsDisadvantagesDistortionConductorsJoint DesignSpecial MethodsUltrasonic methodUses high-frequency sound waves are used to produce the bond or aid with heat in the bondingDiffusionUses pressure and may use heat or ultrasound to form a bondInfrared LightUses infrared light to heat the part for soldering or brazingGeneralSoldering and brazing metals are alloys or two or more metalsEach alloy is available in a variety of percentage mixturesAlmost all have a paste rangeA paste range is the temperature range in which a metal is partly solid and partly liquid as it is heated or cooledIt is important that joints not be moved during this stage, if they are they may crumble like dry clay and destroy the bondSoldering AlloysUsually identified by their major alloying elementsThe major types of solder alloys areTin-leadTin-antimonyCadmium-silverCadmium-zincTin-leadMost popularLeast expensive61.9% tin and 38.1% leadMelts at 362FNo paste rangeMost commonly used on electrical connectionsMust never be used for water piping Also not allowed by most codes for use on water or food handling equipmentTin-antimonyHigher tensile strength & lower creepMost common is 95/5 or 95% tin, 5% antimonyMost commonly used in plumbing because it is lead free

Cadmium-silverExcellent wetting, flow and strengthExpensiveHigh temp solders because they retain their strength at temperatures above other soldersUsed to join aluminum to itself or other metalsMost often seen used in piping for air conditioning equipmentBrazing AlloysDenoted by the letter B to indicate the alloy is used for brazingNext series of letters in the classification indicates the atomic symbol of metals used to make the alloyCopper-zincMost popular brazing alloyAvailable as regular and low-fumingZinc in the braze metal has a tendency to burn out if overheatedOverheating is indicated by a red glow on the molten pool which gives off white smokeThe white smoke is zinc oxide, if breathed in it can cause zinc poisoning. Use of low fuming alloy helps eliminate this problemExamples of low fuming alloys are RCuZn-B and RCuZn-CCopper-zinc & Copper-phosphorus A5.8Copper-zinc filler rods are often grouped together and called brazing rod5 classificationsCopper-zincNavel Brass Manganese-BronzeHigh silicon-BronzeNickel-BronzeCopper-phosphorusReferred to as phos-copperGood fluidity and wettabilityUsed in A/C and plumbing to join copper pipingCopper-phosphorus-silverReferred to as sil-phosSimilar to copper-phosphorus except the silver gives the alloy better wetting and flow characteristicsNot necessary to use flux when joining copper pipeMost common brazing alloy used in A/C compressor fittingsSilver-copperCan be used to join almost any metal, except aluminum, magnesium, zinc and a few other low-melting metalsOften referred to as silver brazeMost versatileAmong most expensive alloys except goldNickelUsed for joining materials that need high strength and corrosion resistance at elevated temperaturesApplications includeJoining turbine blades in jet enginesTorch partsFurnace partsNuclear reactor tubingWhen used on copper based alloys, the nickel may diffuse into copper, stopping its capillary flowNickel and Nickel Alloys A5.14Increase being used as a substitute for silver-based alloyMore difficult to use than silver due to lower wetting and flow characteristicsHigher strength than silver7 classesBNi-1: high strength, heat resistant, used in jet engine partsBNi-2BNi-3: high flow rate, excellent for close fitted jointsBNi-4: higher surface tension than other nickel filler rods, allows larger fillets and poor-fitted joins to be filledBNi-5: high oxidation resistance and high strength at elevated temps, can be used for nuclear applicationsBNi-6: extremely free flowing, good wetting characteristics, high corrosion resistanceBNi-7:high resistance to erosion and can be used for thin or honeycomb structures

Aluminum-siliconUsed to join most aluminum sheet and cast alloysAWS type 1 flux must be usedMust guard against overheatingCopper and copper alloys A5.7BCu-1Used to join ferrous, nickel and copper-nickelBCu-2Similar applications to 1Contains organic compounds to tie up porosity

Silver & GoldUsed in small quantities when joining metals that are under corrosive conditions and high joining ductility is needed or low electrical resistance is importantIncreasing price and decreasing availability

Joint DesignSpacing between the parts being joined affects tensile strengthStrongest joints are obtained when the parts are lappedButt joint strength can be increased by increasing the area being joinedJoint preparation is also very importantSurfaces must be clean and free of oil, dirt, paint, oxidesSoldering or brazing should begin as soon the parts are cleaned to avoid further contaminationBuilding Up Surfaces and Filling HolesBraze metal can be used to build up worn partsIdeal for parts that receive limited abrasive wear because buildup is easily machinableHas no hard spots to make remachining difficultGood for both round and flat stockLow temperature used does not tend to harden the base metalHoles in light gauge metal can be filled and ground flush leaving a strong patch with minimum distortionSummaryBrazing and soldering are process that have many great advantages that are often overlooked.They are an excellent process for portable applications and the versatility makes them great choices for many jobs.Their ability to join may different materials with a limited variety of fluxes and filler metals reduces the need for a large inventory of materials.