Chapter 30

Brazing, Soldering, Adhesive, Bonding

& Mechanical Fastening process

Brazing

• Joining process

• A filler metal is placed between two workpieces and heated until melted

• Two main types of Brazing– Ordinary– Braze welding

• Use of flux is very important

Filler Metals• Available in a wide range of brazing temperatures

• They come in a wide range of shapes

• Choice of the filler metal and its composition are important

• Diffusion of the filler metal in to the workpeice is an important consideration

Fig: a) Brazing b) Braze welding operation

Fluxes

• The use of flux in brazing is very important

• Generally made of:– Borax– Boric acid– Borates– Fluorides– Chlorides

• Wetting agents may also be added

Brazing Methods

• Torch Brazing

– Performed by heating the joint with a torch

– Depositing the filler metal in the joint

– Suitable part thickness (0.25 – 6.0)mm

– Not a automated process

– More than one torch can be used in this process

• Furnace Brazing– Precleaned & Preloaded with brazing metal– Heated in a furnace

Fig:An example of furnace brazing a)before b) after

Brazing Methods

Other Types Of Brazing

• Induction Brazing

• Resistance Brazing

• Dip Brazing

• Infrared Brazing

• Diffusion Brazing

Braze Welding

• Prepared like fusion welding

• Filler metal is deposited at the joint with the use of an oxyacetylene torch

• Considerably more filler is used

• Temperature is minimal compared to that of fusion welding; part distortion is minimal

Brazing Process Capabilities

• Dissimilar metals can be assembled with good joint strength

• Shear strength of brazed joints can reach 800Mpa

Fig: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 to small, the molten braze metal will not fully penetrate the interface.

Good/Poor Designs

Soldering

• Different types of soldering

– Torch– Furnace– Iron– Induction– Resistance– Dip– Infrared– Ultrasonic– reflow (paste)– Wave

Reflow Soldering

• Solvents present in the paste are evaporated

• The Flux in the paste is activated and the fluxing action occurs

• The components are carefully preheated

• The solder particles are melted and wet the joint

• The assembly is cooled

Wave Soldering• Popular approach to attaching circuits to circuit boards

Fig: a)Screening or stenciling paste onto a printed circuit board: 1) Stenciling process 2) a section of a typical stencil pattern b) wave soldering process

Types Of Fluxes

• Inorganic acids or salts – clean the surface rapidly

• Noncorrosive resin-based – used in electrical applications

• Soldering is used extensively in electronics industry

Adhesive Bonding

• Products are joined and assembled by the use of Adhesives

• Adhesives properties to be considered– Strength– Toughness– Resistance to various fluids– Ability to wet the surface to be bonded

Types of adhesives

Surface must be clean for joining parts

• Should avoid joints that might be subjected to peeling forces• Design for adhesive bonding

Adhesive Peeling Test

Fig : Characteristic behavior of (a) brittle (b) tough adhesive in a peeling test

Joint Design in Adhesive Bonding

Fig:Various joint design in adhesive bonding.

Configurations for adhesive bonds

Fig : (a) single lap (b) double lap (c) scarf (d) strap

Mechanical Fastening

• Threaded Fasters– Bolts– Screws– Nuts

• Other Fastening Methods– Stapling– Crimping– Snap-in Fasteners– Shrink and press fits

Rivets

Fig:a)solid b)tubular c) split (bifurcated) d) compression

Fig : 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 to avoid stress concentrations (c)Joined sections should allow ample clearance for riveting tools (d) section curvature should not interfere with the riveting process

Joining Plastics

• Heat softens the plastic to a molten state

• Then pressure added & fusion takes place

• External Heat Sources– Hot air– Heated tools & dies– Electrical-Resistance– Lasers

• Internal Heat Sources– Ultrasonic welding– Friction welding

THE END