Brazing Handbook 1963 PDF

6 CHAPTER 1—BASICS OF BRAZING AWS BRAZING HANDBOOK joint dimensions for optimal brazing strength— to permit very accurate control of the heating and 0.000 inch (in.) to 0.002 in. with little or no solubil...

6 CHAPTER 1—BASICS OF BRAZING AWS BRAZING HANDBOOK joint dimensions for optimal brazing strength— to permit very accurate control of the heating and 0.000 inch (in.) to 0.002 in. with little or no solubil- melting cycles. ity of the base metal; the 0.002 in. to 0.005 in. for The more than 80 alloys currently listed as braz- mutual solubility; and 0.006 in. to 0.025 in. for alu- ing filler metals substantiate the amount of research minum mutual solubility. The best joint design for that was involved to provide the multitude of design brazing was identified as the shear joint for maxi- options that exist today. Each brazing filler metal has mum strength. a characteristic with respect to mechanical proper- During the 1950s, brazing found new uses in the ties, service temperature, assembly and fixturing, repair of aircraft engine parts, combustion chambers, fatigue, stress distribution, stress rupture, and creep and transition ducts. For example, in 1952, the U.S. strength. Air Force tested and inspected 250,000 J-33 engine In the early 21st century, brazing still has not jet engine parts. It was found that these parts, which reached its plateau, as new and exciting heat sources had been originally manufactured with silver brazing and brazing processes are being developed and filler metal, were repaired successfully by replacing exploited. the silver filler metal with a nickel brazing filler metal, AWS BNi-1, thus extending the life of the components. In 1958, Robert Peaslee proposed the testing of brazed joints in an article in the Welding Journal, PHYSICS OF BRAZING and the AWS Committee of Brazing and Soldering, founded by Peaslee, began round-robin testing the In brazing, the temperature of the components to strength of brazed joints. Later, in 1963, the AWS be brazed, termed the assembly, is raised to the point Committee published the standard AWS C3.2-63, at which the brazing filler metal becomes molten and Standard Method for Evaluating the Strength of fills the joint clearance between the base materials. Brazed Joints.14 The assembly is then cooled to solidify the brazing In 1960, cold-wall vacuums were improving in filler metal, which is held in the joint by capillary design and pushing new brazing filler metal combi- action, anchoring the component parts together by nations to higher brazing and remelt temperatures. means of metallurgical reaction and atomic bonding. The modern microprocessor controls were developed This process is illustrated in Figure 1.2. In most cases, the interaction between the molten brazing 14. American Welding Society Committee on Brazing and Solder- filler metal and the base materials results in the ing, 1963, Standard Method for Evaluating the Strength of Brazed establishment of a metallurgical bond when the braz- Joints, AWS C3.2-63, New York: American Welding Society. ing filler metal solidifies. BASE MATERIAL CONTACT ANGLE SOURCE OF LIQUID BRAZING FILLER METAL JOINT CLEARANCE DIRECTION OF FLOW BASE MATERIAL Figure 1.2—Schematic Illustration of a Brazed Joint AWS BRAZING HANDBOOK CHAPTER 1—BASICS OF BRAZING 7 Although the representation of brazing depicted iar problem of a liquid droplet in contact with a flat in Figure 1.2 is relatively simple, basic and often solid surface. In the ideal case in which no chemical complex metallurgical and chemical processes take reactions occur between the solid, liquid, and vapor place within the joint and on the surfaces of the phases and gravitational forces can be ignored (e.g., materials involved. An appreciation of the complex- for relatively small droplets), the liquid droplet ity of the process is necessary in order to design and assumes an equilibrium configuration dictated by produce brazed joints with closely controlled physi- surface free energy considerations. The shape of the cal and chemical properties. liquid droplet is uniquely characterized by θ, its con- The phenomenon known as wetting, whereby a tact angle with the solid as shown in Figure 1.3. liquid brazing filler metal or flux spreads and The relationship between the contact angle and adheres in a thin, continuous layer to a solid base the surface free energies at the liquid-vapor, solid- metal,15 and the flow of the brazing filler metal, also vapor, and the solid-liquid interfaces is expressed as known as capillary action, the force by which a liq- follows: uid that is in contact with a solid is distributed between the closely fitted faying surfaces of the joint to be brazed,16 are basic to most models developed (γ SV − γ SL ) to describe the formation of a brazed joint. The wet- cos θ = (1) γ LV ting of the base materials by the brazing filler metal is required to provide intimate contact between the base materials and develop the bonding necessary to where form the joint. The driving forces behind wetting and capillary θ = Contact angle, degrees; action are characterized by the thermodynamic con- γSV = Solid-vapor interface; cepts of surface free energy and the free energy from γSL = Solid-liquid interface; and formation of phases that may result from the diffu- sion that occurs during brazing. After wetting condi- γLV = Liquid-vapor interface. tions are established, capillary forces produce the flow of the liquid brazing filler metal and act to fill The boundary between wetting and nonwetting the joint clearance with the molten metal. Both wet- conditions is generally taken to be θ = 90º. For θ < ting and flow are strongly influenced by the chemical 90º, wetting occurs, while θ > 90º represents a condi- reactions occurring at the interfaces and within the tion of nonwetting. The term spreading is defined as brazing filler metal itself as well as by the geometry the condition in which the liquid completely covers of the joint. The quality of the wetting strongly influ- the solid surfaces. This condition occurs when θ ences the final properties of the joint. approaches the value of 0º. An exhaustive description of the theories of wet- For most brazing systems, the optimum value of θ ting and capillary action as they apply to brazing is is in the range of 10º to 45º. This is determined by beyond the scope of this volume. However, a brief joint clearance or thickness—a small contact area for introduction to these concepts is presented below as very thin joints, for example. It should be noted that this information is useful when outlining the general when appreciable chemical reactions occur during characteristics of solids and liquids, which influence brazing, the equation relating the contact angle to the process. the surface energies is of qualitative value only. It is also important to understand that the liquid and solid-surface free energies can be markedly low- WETTING ered by the absorption of surface-active impurities at The factors that are important when determining any of the three interfaces shown in Figure 1.3. All the extent of wetting can be illustrated by the famil- real surfaces of liquids and solids are modified to some extent by the absorption of surface-active ele- 15. American Welding Society (AWS) Committee on Definitions, ments and particularly oxidation. Indeed, the pres- 2001, Standard Welding Terms and Definitions, Including Terms ence of oxide on a solid metal surface suppresses for Adhesive Bonding, Brazing, Soldering, and Thermal Spraying, wetting and inhibits the spreading of liquid metal AWS A3.0:2001, Miami: American Welding Society, p. 44. over the surface. Therefore, much of the technology 16. American Welding Society (AWS) Committee on Definitions, 2001, Standard Welding Terms and Definitions, Including Terms of brazing focuses on eliminating the potentially det- for Adhesive Bonding, Brazing, Soldering, and Thermal Spraying, rimental effects that the presence of oxide may have AWS A3.0:2001, Miami: American Welding Society, p. 6. on wetting. 8 CHAPTER 1—BASICS OF BRAZING AWS BRAZING HANDBOOK γ SV – γ SL cos θ = γ LV θ > 90° θ < 90° γ LV γ SL θ γ SV (A) (B) θ = 0° (C) Key θ = Contact angle, degrees γSV = Solid-vapor interface γSL = Solid-liquid interface γLV = Liquid-vapor interface Figure 1.3—Contact Angle, θ, for a Liquid Droplet on a Solid Surface: (A) θ > 90°; (B) θ < 90°; and (C) θ = 0° CAPILLARY ACTION brazing filler metal flow can be markedly enhanced by an increase in brazing temperature until the base No simple method exists to describe the capillary metal begins to erode. action of the brazing filler metal in brazing. Explicit flow rate expressions for even simple Although the field of fluid dynamics provides a basis geometries are complex and difficult to verify by for quantitative insight, the complexities of this phe- experimentation. However, analyses and experi- nomenon lie beyond the scope of this introductory ments indicate that flow rates can be high, and they discussion. Thus, only a qualitative treatment of the tend to increase with the magnitude of the relation- flow by means of capillary action is presented here. ship specified below: Experience has shown that brazing filler metal flow is a function of (1) the capillary action driving (γ LV )(cos θ) force, (2) the viscosity and density of the molten (2) η metal, and (3) the geometry of the joint. The vis- cosity of liquid metals cannot be given quantitatively by a single formula applicable for all pure metals where and alloys. In a most general way, it degreases with temperature as complex exponential function param- γLV = Geometric function, eters which are more or less specific for each particu- θ = Contact angle, degrees; and lar metal and alloy. η = Viscosity, lb/ft/s × 0.0672 (g/cm/s × 100). Figure 1.4 shows the measured viscosity of iron, nickel, and copper as a function of temperature up to Unless impeded by other factors such as base the point when the erosion of the base metal begins. metal surface roughness and chemical reactions, the This behavior is typical of other metals and alloys. flow by means of capillary action is expected to be The nearly linear dependence indicates that tempera- rapid, and the flow rate is not of concern in the time ture has a strong influence upon viscosity and that frame of most braze processing. AWS BRAZING HANDBOOK CHAPTER 1—BASICS OF BRAZING 9 TEMPERATURE, °F 2200 2300 2400 2500 2600 2700 2800 2900 3000 60 403 50 336 VISCOSITY, g/cm/s × 100 VISCOSITY,lb/ft/sec 40 269 30 202 20 134 COPPER NICKEL 10 IRON 67 0 0 1150 1200 1250 1300 1350 1400 1450 1500 1550 1600 1650 1700 TEMPERATURE, °C Source: Adapted from Cavalier, C., 1963, Measurement of Viscosity of Iron, Cobalt, and Nickel, Academy of Science: 256(6). Figure 1.4—Variations in Viscosity with the Temperature of Pure Iron, Nickel, and Copper can be caused by oil lubricants and cutting oils as well FACTORS CONTROLLING as by the sodium silicate used in cleaning solutions. Other sources of contamination that should be THE PROPERTIES OF THE removed as part of surface preparation include (1) BRAZEMENT the residue of tumbling or vibratory polishing with stone media, (2) blasting media that contains alumi- num oxides or is contaminated from other use, (3) With respect to obtaining the desired properties in nonmetallic grits, and (4) high-temperature forming the brazement, the importance of factors such as the lubricants. Sanding or surface finishing with alumi- selection of joint design, brazing filler metals, and num oxide can also be detrimental to the braze joint process variables is determined by the service condi- by preventing the capillary flow of the brazing filler tions anticipated for the brazement. These factors metal. have a significant effect on the geometries and micro- structures of braze joints, thus determining the prop- erties of fabricated (brazed) joints. The factors that JOINT DESIGN AND CLEARANCE influence the properties of brazed joints are discussed briefly below, followed by a more thorough treat- Braze joint design is an important consideration ment in subsequent chapters. in constructing an assembly to meet the established criteria. The effect of one variable of joint design—joint SURFACE PREPARATION clearance—on the tensile strength of brazed joints dramatically illustrates the influence of design on the Surface preparation is a very important factor in properties of brazed joint. Figure 1.5 demonstrates successful brazing. If the surface of the braze joint is the variation of tensile strength with joint clearance not clean, contamination prevents the proper wetting for butt joints of stainless steel brazed with brazing and flow of the brazing filler metal. Contamination filler metal AWS BAg-1a. These data show that at 10 CHAPTER 1—BASICS OF BRAZING AWS BRAZING HANDBOOK THICKNESS OF JOINTS, mm 0.076 0.152 0.228 0.305 0.381 0.457 0.533 0.609 140,000 965 120,000 862 TENSILE STRENGTH, MPa TENSILE STRENGTH, psi 100,000 690 80,000 552 60,000 414 40,000 276 0.003 0.006 0.009 0.012 0.015 0.018 0.021 0.024 THICKNESS OF JOINTS, in. Source: Adapted from Bredzs, N., 1954, Investigation of Factors Determining the Tensile Strength of Brazed Joints, Welding Journal 33(11), Figure 1; based on Leach, R. H., and Edelson, L. 1939, n.t., Journal of the International Society of Naval Engineers, 51(60): n.p. Figure 1.5—Relationship of Tensile Strength to Joint Clearance in Stainless Steel Brazed with AWS Brazing Filler Metal AWS BAg-1a small joint clearances—i.e., those below 0.006 in. be deduced from the phase diagram for the commer- [150 micrometers (µm)], joint tensile strength is quite cially important silver (Ag)-copper (Cu) system shown high and even approaches that of the stainless steel. in Figure 1.6. This occurs even though the intrinsic strength of the Except for the eutectic composition 72 Ag-28 Cu AWS BAg-1a is only in the range of 40 kips per weight percent (wt %), the melting of silver-copper square inch (ksi) [275 megapascals (MPa)]. alloys occurs over a range of temperatures, as illus- The reason the joint strength can be so much trated for the 50 Ag-50 Cu wt % compositions. higher than that of the brazing filler metal is that the Melting, which begins at solidus temperature of necking (stretching of the thin brazing filler metal 1435ºF (780ºC), is not complete until a temperature layer) is suppressed. During tension testing, the layer in excess of 1560ºF (850ºC) is reached. Within that of brazing filler metal approaches a stress state of temperature range, a mixture of liquid and solid wets very high triaxial tension, which effectively increases and flows in a manner distinctly different from that its tensile strength. As the joint clearance increases, exhibited by the liquid alloy. the suppression of necking is reduced or eliminated, The flow by capillary action is reduced when the and the joint strength approaches the intrinsic brazing filler metal is in the partially melted state, strength of the brazing filler metal. and the wetting and spreading behavior of the low- melting liquid phase in the brazing filler metal sepa- rates from the solid constituents. This behavior, in BRAZING FILLER METALS turn, can result in the formation of joints possessing discontinuities because of insufficient or nonuniform The brazing filler metals used in brazing can be joint filling. complex alloys whose melting occurs over a range of Figure 1.7 illustrates the differences in flow by temperatures. The implications of such behavior can capillary action between a brazing filler metal with a AWS BRAZING HANDBOOK CHAPTER 1—BASICS OF BRAZING 11 COPPER, ATOMIC % 1200 2192 1084.87°C (1984.77°F) 50% Ag + 50% Cu 1000 961.93°C 1832 72% Ag + 28% Cu ID US (1763.47°F) EUTECTIC LIQU COMPOSITION LIQUID + SOLID TEMPERATURE, °C TEMPERATURE, °F 800 780°C (1436°F) 1472 (Ag) (Cu) SOLIDUS MELTING RANGE 600 1112 SOLID 400 (Ag) + (Cu) 752 200 392 0 10 20 30 40 50 60 70 80 90 100 Ag COPPER, wt % Cu Figure 1.6—Silver-Copper Phase Diagram narrow melting range and a brazing filler metal with which the melting points of the brazing filler metal a wide melting range. In Figure 1.7(A), a slow fur- and the base material are affected and (2) the ten- nace heating of brazing filler metal AWS BAg-2 pro- dency for the formation of new phases. The liquid duced a nonuniform flow because of liquation or brazing filler metal can cause excessive dissolution diffusion of the low melting constituents from the (erosion) of the base material, resulting in significant brazing filler metal. In Figure 1.7(B), the rapid heat- changes in the composition of the brazing filler metal ing of BAg-2, which has a melting range between the and its melting characteristics. liquidus and the solidus of approximately 170°F Significant changes in the composition of the base (95°C), results in a better flow by capillary action. In material near the joint surfaces may be produced by Figure 1.7(C), a slow furnace heating of AWS BAg-1, the diffusion of elements from the brazing filler which has a narrower melting range (20°F [11°C]) metal. The extent of the effect of alloying on the base than BAg-2 results in a uniform flow by capillary material depends upon factors such as the (1) solubil- action. The heating rate through the melting tempera- ity of the brazing filler metal elements in the base ture range, the brazing temperature, and the brazing material, (2) brazing time and temperature, (3) kinet- time are important process variables that are adjusted ics of solid-state diffusion, (4) grain size in the base accordingly to avoid these types of problems. material, and (5) composition of the base material. In addition to the concerns associated with the Difficulty with wetting of the braze joint or the melting characteristics of brazing filler metals, alloy- mechanical properties of the brazed joint can result ing can occur between the liquid brazing filler metal from these interactions. and the base metal during brazing. The wetting and As alloying occurs to some extent in most braze flow of a brazing filler metal can be markedly influ- processing, it is generally desirable to control it in enced by alloying depending on (1) the extent to order to avoid deleterious effects on joint microstruc- 12 CHAPTER 1—BASICS OF BRAZING AWS BRAZING HANDBOOK (A) (B) (C) Courtesy of Handy and Harman Figure 1.7—Difference in Flow Properties between Eutectic and Noneutectic Brazing Filler Metals: (A) Slow Furnace Heating of AWS BAg-2 Produces a Nonuniform Flow by Capillary Action due to Liquation; (B) Rapid Heating of AWS BAg-2 Results in a Better Flow by Capillary Action, and (C) Slow Furnace Heating of AWS BAg-1 Results in Uniform Heating As No Liquation Occurs in Brazing Filler Metals with Eutectic Compositions ture and properties. Under certain circumstances, ASSEMBLY, FIXTURING, AND BRAZING however, alloying can have a beneficial effect by either increasing the solidus temperature of the braze FILLER METAL PLACEMENT layer or by improving the intrinsic mechanical prop- To fabricate a successful joint, the assembly and erties of the brazing filler metal. fixturing methods employed to align the joint com- ponents must provide the correct relative positioning throughout the braze cycle. The positioning is speci- RESIDUAL STRESSES fied by customer tolerances and specifications. Fix- turing is added to ensure that the assembly position When two different base materials—for example, is maintained in the brazing operation. In addition, a ferritic steel and an austenitic steel, or an austenitic the placement of the brazing filler metal must pro- steel and a ceramic—are to be joined by brazing, sig- vide faying contact to permit adequate flow and nificant residual stresses may form in the brazement ensure the complete filling of the joint. because of the difference in the thermal expansion For assemblies containing joints between dissimi- coefficients between the two materials. The residual lar materials, the differences in the thermal expan- stresses are produced during cooling from the braz- sion coefficients must also be considered for the proper selection of component tolerances, fixturing, ing temperature as one component of the assembly and brazing filler metal placement. shrinks at a different rate than the other. When the thermal expansion coefficients of the materials being joined are very different, these resid- ual stresses can be large enough to cause localized THE FIVE ELEMENTS OF deformation or cracking in the materials or warping of the brazed assemblies. Residual stresses can be BRAZING controlled to some extent by programmed cooling from the brazing temperature to promote stress Table 1.1 illustrates that the brazing processes relaxation. offer the user the flexibility to draw upon an array of