Chapter 32 Barzing, Soldering, Adhesive-Bonding, and Mechanical Fastening Processes

Different from welding: 1-no diffusion takes place 2-filler metal is used to produce a strong bond 3- require lower temperatures than fusion welding

FIGURE 32. 1 Examples of brazed and soldered joints FIGURE 32.1 Examples of brazed and soldered joints. (a) Resistance-brazed light bulb filament; (b) brazed rocket tubing assembly; (c) soldered circuit board.

BRAZING 1- the two surfaces are cleaned 2 -A filler metal is placed at the interface 3-A flux is used to prevent oxidation 4-The temperature is then raised to melt the filler metal 450 C 5-joint clearance is filled by capillary action 6-The filler metal solidifies bonding the two parts

FIGURE 32.2 An example of furnace brazing (a) before and (b) after brazing. The filler metal is a shaped wire and the molten filler moves into the interfaces by capillary action, with the application of heat.

FIGURE 32. 3 Joint designs commonly used in brazing operations FIGURE 32.3 Joint designs commonly used in brazing operations. The clearance between the two parts being brazed is an important factor in joint strength; if the clearance is too small, the molten braze metal will not fully penetrate the interface, and if it is too large, there will be insufficient capillary action for the molten metal to fill the interface.

TABLE 32.1 Typical Filler Metals for Brazing Various Metals and Alloys

FIGURE 32.4 The effect of joint clearance on the tensile and shear strength of brazed joints; note that, unlike tensile strength, the shear strength continually decreases as the clearance increases.

FIGURE 32. 6 Examples of poor and good designs for brazing FIGURE 32.6 Examples of poor and good designs for brazing. Source: American Welding Society.

SOLDERING Filler metals (tin-lead, zinc-aluminum, tin-zinc) melt at a much lower temperature below 370 C Copper, silver and gold are easy to solder Iron and nickel are more difficult to solder

TABLE 32.2 A Selection of Common Solders and Their Typical Applications

FIGURE 32.8 Joint designs commonly used for soldering.

ADHESIVE BONDING Filler material is usually rubber or polymer Surfaces must be well cleaned Properties of adhesive material 1. Strength : shear and peel 2. Toughness 3. Resistance to various fluids and chemicals 4. Resistance to environment degradation (heat and moisture) 5. Capability to wet the surfaces to be bonded

TABLE 32.3 Typical Properties and Characteristics of Chemically Reactive Structural Adhesives

FIGURE 32.9 Common arrangements for evaluating adhesives: (a) tapered double cantilever beam, (b) peel test, and (c) wedge test.

FIGURE 32.10 Characteristic behavior of (a) brittle and (b) tough adhesives in a peeling test; this test is similar to the peeling of adhesive tape from a solid surface.

JOINT DESIGN 1. JOINTS ARE PREFERABLY SUBJECTED TO COMPRESSION OR SHEAR 2. Tapered joints should be used when feasible 3. Coefficient of thermal expansion of the two joined parts should be very close in order to avoid internal stresses during bonding

FIGURE 32.11 Various joint designs in adhesive-bonding; note that good designs require large contact areas between the members to be joined.

FIGURE 32.12 Desirable configurations for adhesively bonded joints.

FIGURE 32.13 Two examples of combination joints, for purposes of improved strength, air of liquid tightness, and resistance to crevice corrosion.

MECHANICAL FASTENING Some joined components may be taken apart during the product’s service life threaded fasteners:use of nuts and screws

RIVETS Rivet is placed in the hole, and its end is deformed by upsetting

FIGURE 32.14 Examples of rivets: (a) solid, (b) tubular, (c) split or bifurcated, and (d) compression.

FIGURE 32. 15 Design guidelines for riveting FIGURE 32.15 Design guidelines for riveting. (a) Exposed shank is too long; the result is buckling instead of upsetting. (b) Rivets should be placed sufficiently far from edges of the parts, to avoid stress concentrations. (c) Joined sections should allow ample clearance for the riveting tools. (d) Section curvature should not interfere with the riveting process. Source: After J.G. Bralla.

SEAMING

FIGURE 32.17 Stages in forming a double-lock seam.