Brazing and braze welds

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  • 1. 5.BRAZING AND BRAZE WELDS 5.1 INTRODUCTION Mostly products used in the common manufacturing process are assembled from more than one component. During assembly of structures joining plays an important role. The various joining processes can be broadly directed as adhesive bonding, soldering, brazing, pressure nonfusion welding. Each joining method has its own practical advantages and limits. Choosing one depends upon the technical demand, concerned economy and intended use. Among all brazing is a process which can be applied to have a permanent joint starting from identical adherents to dissimilar adherents. The adherents may be metals, alloys, ceramics, graphite and cemented carbides. 5.2 BRAZING AND ITS NATURE Brazing is defined as a joining process wherein coalescence is produced between the adherents by heating them to a suitable temperature above 450ºc and by using a filler nonferrous alloy having its liquidus temperature above 450ºC and below solidus temperature of used base metals. The filler alloy flows into the joint by capillary attraction and this necessitates that the clearance between the base materials in the joint region be kept very small. In brazing the parent materials are not fused, but since the temperature is high enough as appreciable diffusion and alloying action is possible between the brazing alloy and the parent material. The chief feature of brazing is that many dissimilar metals can be readily joined. 5.3 PRINCIPLE The joint is produced by diffusion of elements of filler metal into the base metal or vice versa. Diffusion of the elements creates bonds, which contributes to joint. Since the filler metal is in liquid state the diffusion rate is faster than in solids. The 109
  • 2. capillary action plays an important role in holding the liquid filler metal which would otherwise flow out. After soaking the samples for a long time at brazing temperature the samples are quenched to room temperature. The wetting angle depends upon the free surface energy of liquid-vapor interface, solid-vapor interface and solid-liquid interface. For a good wetting the wetting angle should be less than 90º. So the free surface energy of solid-vapor interface must be greater than solid-liquid interface. The presence of adsorbed molecules on a metal surface markedly decreases the surface energy of solid-vapor interface and thus increasing the contact angle. Therefore the brazing surfaces should be free from any oxide layer or impurity. Good wetting increases the brazing efficiency. 5.4 MECHANICS OF BRAZING Brazing involves a limited dissolution or plastic deformation of the base metal. Brazing comprises a group of joining processes in which coalescence is produced by heating to a suitable temperature above 450ºC and by using a ferrous or nonferrous filler metal that must have a liquidus temperature above 450ºC and below the solids temperature of the base metal. The filler metal is distributed between the closely fitted surfaces of the joint. We should take care to maintain a clearance between the base metals to allow capillary action to work most effectively. This means a close clearance. The following chart is based on brazing butt joints of stainless steel. It shows how the tensile strength of the brazed joint varies with the amount of clearance between the parts being joined. 110
  • 3. Brazing proceeds through four distinct steps • The assembly or the region of the parts to be joined is heated to a temperature of at least 450ºC. • The assembled parts and brazing filler metal reach a temperature high enough to melt the filler metal but not the parts. • The molten filler metal, held in the joint by surface tension, spreads into the joints and wets the base metal surfaces. • The parts are cooled or solidify, the filler metal, which is held in the joint by capillary attraction and anchors the parts together by metallurgical reaction and atomic bonding. Recommended pickling solutions for post-braze removal of oxides The pickling solutions recommended below may be used to remove oxides from areas that were not protected by flux during the brazing process. In general, they should be used after the flux residue has been removed from the brazed assembly. 111
  • 4. Application Formulation Comments Oxide removal from 10 to 25% hot sulphuric Pickling can be done at same time copper, brass, bronze, acid with 5 to 10% flux is removed. Will work on nickel silver and other potassium dichromate carbon steels, but if pickle is copper alloys added. contaminated with copper, the containing high copper will plate out on the steel and percentages of will have to be removed copper. mechanically. This sulphuric pickle will remove copper or cuprous oxide stains from copper alloys. It is an oxidizing pickle, and will discolor the silver filler metal, leaving it a dull gray. Oxide removal from A 50% hydrochloric A mixture of 1 part hydrochloric irons and steels. acid solution, used cold acid to 2 parts water can be used for or warm, More diluted Monel and other high nickel alloys. acid can be used (10 to Pickling solution should be heated to 25%) at higher about 180'F/80'C. Mechanical temperatures (140- finishing is necessary for bright 160°F/60-70°C.) finishes. This HCI pickle is not like bright dips on nonferrous metals. Oxide removal 20% sulphuric acid, 20% This pickle is followed directly by a stainless steels and hydrochloric acid, 60% 10% nitric dip, and then a clean alloys containing water, used at a water rinse. chromium. temperature of 170180°F(75-80°C.) 20% hydrochloric acid, This pickle is more aggressive than 10% nitric acid, 70% the sulphuric-hydrochloric mixture water, used at about listed above, and will etch both the 150°F(65°C.) steel and the filler metal. 5.5 TYPES OF BRAZED JOINTS A spot joint made at one point can be accomplished as easily by welding as by brazing. But a linear joint – all other things being equal – is more easily brazed than welded. Brazing needs no manual tracing. The filler metal is drawn through the joint area by capillary action, which works with equal ease on any joint configuration. 112
  • 5. There are many kinds of joints. But there are only two basic types – the butt and the lap. The rest are essentially modifications of these two. The butt joint, both for flat and tubular parts: The butt joint gives the advantage of a single thickness of the joint. Preparation of this type of joint is usually simple, and the joint will have sufficient tensile strength for a good many applications. However, the strength of the butt joint does have limitations. It depends, in part, on the amount of bonding surface, and in a butt joint the bonding area can't be any larger than the cross-section of the thinner member. If it is compared this with the lap joint, both for flat and tubular parts 113
  • 6. For a given thickness of base metals, the bonding area of the lap joint can be larger than that of the butt joint and usually is. With larger bonding areas, lap joints can usually carry larger loads. The lap joint gives a double thickness at the joint, but in many applications (plumbing connections, for example) the double thickness is not objectionable. And the lap joint is generally self-supporting during the brazing process. Resting one flat member on the other is usually enough to maintain a uniform joint clearance. And, in tubular joints, nesting one tube inside the other holds them in proper alignment for brazing. However, suppose we want a joint that has the advantages of both types; single thickness at the joint combined with maximum tensile strength. We can get this combination by designing the joint as a butt-lap joint. The butt-lap is usually a little more work to prepare than straight butt or lap, but one can wind up with a single thickness joint of maximum strength. And the joint is usually self-supporting when assembled for brazing. 5.6 ADVANTAGES Brazing has many distinct advantages: • Economical fabrication of complex and multicomponent assemblies • Simple method to obtain extensive joint area or joint length • Joint temperature capability approaching that of base metal • Excellent stress distribution and heat transfer properties 114
  • 7. • • • • • • • • • • • • Ability to preserve protecting metal coating or cladding Ability to join cast materials to wrought metals Ability to join nonmetals to metals Ability to join metal thickness that vary widely in size Ability to join dissimilar metals Ability to join porous metal components Ability to fabricate large assemblies in a stress-free condition Ability to preserve special metallurgical characteristics of metals Ability to join fiber- and dispersion-strengthened composites Capability for precision production tolerance Reproducible and reliable quality control techniques Strong, uniform, leak-proof joints can be made rapidly and inexpensively. Joints that are inaccessible can also be joined by brazing. • Brazed joints are high in strength. With properly designed and made brazing joint is as strong as brazed parent material. 5.7 LIMITATIONS A brazed joint is not a homologous body but rather is heterogeneous, composed of different phases with differing physical and chemical properties. In the simplest case, it consists of the base metal parts to be joined and the added filler metal. However, partial dissolution of the base metal, combined with diffusion processes, can change the composition and therefore the physical and chemical properties of the boundary zone formed at the interface between base metal and filler metal. In determining the strength of such heterogeneous joints, the simplified concepts of elasticity and plasticity theory no longer apply. In a brazed joint formed of several materials with different characteristics of deformation resistance and deformation speed, the stresses caused by externally applied loads are nonuniformly distributed. 5.8 APPLICATION Brazing gives a weaker bond than welding but brazing has its own applications. In welding the base metal also melts and there is local casting. Welding is applicable if both the components are of same materials. Brazing can be done for the components of different materials and the temperature required is less than required in welding as base metal does not melt. Brazing is widely used due to its numerous advantages. Brazing is used in aluminum and its alloy, magnesium and its alloy, nickel alloys 115
  • 8. and those of copper such as brasses, bronzes, copper beryllium and the copper silicon alloys. Among the ferrous and rarer metals the list includes carbon and low alloy steels, stainless steel, high speed steel, cast iron, cemented carbides, zirconium, tungsten and molybdenum. 5.9 COMPARISION OF JOINING METHODS Parameter Soldering Joint formed Mechanical Filler metal <450 melt temp. (ºC) Brazing Metallurgical >450 (less than m.p. of base metal) Base metal Fluxes Heat sources Does not melt Optional Furnace;torch; Induction; inFrared Atypical Tendency burn Does not melt Required Soldering iron; ultraSonics;oven to Atypical Welding Metallurgical >450 (less than or equal to m.p. of base metal) Melts Optional Plasma;laser Resistance; Electron beam Potential distortion 5.10 BRAZING PROCESSES Brazing processes are classified by the method used to heat the assembly. Selection of the method for a particular job is determined by the type of equipment available, 116
  • 9. the skill of the operator, the nature and working place of the parts to be brazed, the relative costs of labor and materials. The processes are: • Torch brazing • Furnace brazing • Vacuum brazing • Dip brazing • Salt-bath brazing • Infrared brazing • Electric blanket brazing • Induction brazing • Resistance brazing • Exothermal brazing 5.10.1 Vacuum Brazing Vacuum brazing is done by keeping the components in an evacuated chamber with low pressure and then applying heat. It has its own benefits. Vacuum brazing is well suited for heat resistant nickel- and iron based alloys that contain aluminum or titanium, reactive metals, refractory metals and ceramics. The filler metal can be used as a sheet, wire or powder paste or molten rod in the joint area. Two basic types of equipment can be used, one consists of the hot wall while another cold wall chamber. Vacuum conditions are well suited for brazing large area where solid or liquid fluxes cannot be removed adequately from the brazing interface. The atmospheric gases may result in embrittlement and sometimes disintegration at brazing temperature. Many oxides disintegrate in vacuum and thus give good wetting. Vacuum brazing has its own advantages: 1. Vacuum removes all gases and thus reduces the chance of oxidation. The actual pressure used depends upon the base metal, the filler metal and the degree to which gases are expelled. 2. Removal of Oxides of most metals increases the brazing efficiency. Oxides are removed by dissociation, diffusion or chemical reaction. 3. The low pressure around the interface removes volatile gases and impurities from the metals. It improves frequently the properties of metals being brazed. One of the disadvantages of vacuum brazing is that if the filler metal is volatile then vacuum has to be maintained at low level to prevent gasification of filler metal. 117
  • 10. 5.10.2 Role of Flux Fluxes are applied to the brazing surface to make wetting efficient and hence brazing. The flux used should decompose oxides without corroding the base metal or the brazing filler metal, should be extremely active because or the short brazing times employed, and should be easy to remove after brazing. The flux must be capable of dissolving any oxide remaining on the base metal after it has been cleaned and any oxide films on the liquid filler metal. Fluxes serve to suppress the volatization of high vapor pressure constituents in brazing filler metal. To effectively protect the surfaces to be brazed, the flux must completely cover, be applied as an even coating and protect them until the brazing temperature is reached. It must remain active throughout the brazing cycle. Reaction rates of the flux with oxygen, base metals, brazing filler metals, and any foreign materials present increase with temperature. Composition of the flux must be carefully tailored to suit all the factors of the brazing cycle, including dwell time. Attack of the flux on the metals must be limited, because the flux must react promptly with metal oxides or other tarnish to enable the joint to be satisfactorily formed. Active halides, such as chlorides and fluorides, are necessary for alloys containing aluminum or other highly electropositive metals. 5.11 IMPORTANCE OF O.F.E. COPPER Its common name is Oxygen free electronic copper and it is designated by C10100. It contains 99.99 percent Cu min. OFE copper is high conductivity electrolytic copper produced without use of metal or metalloid deoxidizers. Physical properties of copper are significantly affected by small amounts of impurity elements. The oxygen is relatively insoluble in copper and it is present in the form of an oxide. Large amount of oxygen ( more than 1%) can make copper brittle. Even trace amount of oxygen can cause embrittlement over about 370ºC. Oxygen free coppers are not subjected to embrittlement at elevated temperatures and can be readily welded. Pure non-phosphorus oxygen free electrolytic copper has high degree of electron mobility. In comparison of welding, brazing of copper is a widely used process. The brazing of copper to copper is applicable in manufacturing 118
  • 11. heat exchanger equipment refrigeration and air conditioning industry, and in aerospace industry. OFE copper is used in busbars, waveguides, lead-in wire, anodes, vacuum seals, transistor components, glass-to-metal seals, coaxial cables, klystrones, microwave tubes. Heating in oxidizing atmospheres should be avoided while using. Melting point of OFE copper is 1083ºC and its coefficient of linear thermal expansion is 17μm/m.k at 20 to 100ºC. Its specific heat is 385J/kg.k at 20ºC and thermal conductivity is 391 W/m.k at 20ºC. OFE copper can be readily soldered, brazed, gas tungsten arc welded, gas melt arc welded. Its capacity for being oxyfuel gas welded is fair. Shielded metal arc welding methods are not recommended. 5.12 BRAZING ALLOYS Brazing filler alloys are largely classified on basis of their chemical composition rather than mechanical properties requirement. The major categories have various classification in each category and these are: • Aluminum silicon • Copper and copper-zinc-tin • Copper-phosphorus with or without silver • Cobalt • Gold-copper and gold-nickel-palladium • Magnesium-aluminum-zinc • Nickel • Silver-copper with or without zinc Silver based brazing alloys play a major role in the field of metal joining. This is because of their low melting point. They also have wide metallurgical compatibility. Circumstantially these can be used for most of the engineering materials with the exception of aluminum and its alloys, magnesium and its alloys with all methods of heating. 5.13 DESCRIPTION OF ANALYTICAL EQUIPMENTS 5.13.1 Furnace : The furnace used is high vacuum high temperature furnace. It has a digitally controlled programmer. One can fix the heating rate and soaking temperature. The furnace is double walled and can sustain a maximum of 1600 ºC and can produce a vacuum of 10-6 torr. The furnace also has a provision of quenching the samples in gas. It has a heat exchanger, which cools the gas. The furnace is water-cooled with 119
  • 12. three sets of heater. Three sets are provided for homogeneous heating area inside the furnace. The heater and the shields are made of molybdenum, as molybdenum is very good reflector of heat and can bear high cyclic temperature in inert or vacuum environment. Three vacuum pumps are attached to the furnace to give a good vacuum. The pumps are namely: Roughening pump, Root pump, diffusion pump. The valves attached to the furnace work with pneumatic principle. The temperatures are reached with the help of corresponding appropriate thermocouple. 5.13.2 Tensile testing machine : The universal tensile testing machine that we used to test the tensile samples of two brazed and one unbrazed (reference) samples of copper. It is an automatic tensile testing machine. The software it works on directly measures the engineering stressstrain of the test-samples. The machine is capable of performing compression as well as the bend tests. Experimental details : The samples for vacuum brazing were made from the cold drawn OFE copper rod having diameter 15 mm. The brazing samples had length of 40 mm. One sample of length 80 mm was also cut for reference. The samples for brazing were turned and made planar in C.N.C lathe. Then the samples were polished to give a perfect planar surface. The filler metal chosen was silver copper eutectic alloy. Eutectic alloy was used as it melts congruently and has lower melting point than copper. The samples to be brazed were fixed in the jig made from stainless steel. Stainless steel was chosen as it has higher melting point than both the filler and base metal and has coefficient of linear expansion less than copper so the samples would not slip from its position during brazing. After fixing the samples in the jig the whole combination was put in the furnace for brazing at a high vacuum. The brazing temperature was 800 ºC and the soaking time was 25 min, after the soaking time the samples were quenched with argon gas. The brazed samples and the reference were turned in C.N.C lathe to get A.S.T.M standard tensile specimen. The tensile testing was carried out and the different mechanical properties were obtained. The filler metal and the base metal were also seen under microscope. Results : Sample Usual OFE cold worked UTS (Mpa) 359 Y.S. (Mpa) 103 Ductility (%) 17.6 Breaking stress (Mpa) 57.8 120
  • 13. Vacuum brazed OFE 202.23 87.07 21.4 52.3 Joint efficiency = strength of brazed joint / strength of parent metal = 202.23 / 359 = 56.33% Conclusion : With this tensile joint design it is found that the ultimate tensile strength of cylindrical oxygen free electrolytic copper, brazed at 800°C for 25 min, is 202 Mpa where as that of parent metal is 359Mpa. The joint efficiency of the brazed sample at 800°C is 56.33%. Investigation shows a number of plastic instability on the brazed tensile sample compare to parent sample. Failure occurred at the brazed joint indicating a weak region. As ductility concerns, the brazed joint exhibited 21.41% elongation compare to that of pure copper(17.6 %).Tensile testing of brazed sample showed a abrupt decrease in strength. This indicates there is a possibility of improving the designed joint strength with further process optimisation. 121