UNIT 8: Joining Processes and Equipment Unit 8 Copyright © 2014. MDIS. All rights reserved. 1 Modern Manufacturing

UNIT 8: Joining Processes and Equipment

Unit 8 Copyright © 2014. MDIS. All rights reserved. 1

Modern Manufacturing

2

Objectives

Unit 8 Copyright © 2014. MDIS. All rights reserved.

• Understand the reasons when parts needed to be joined.• Understand the various joining processes and methods.• Compare the various welding methods and technologies.• Understand the brazing, mechanical fastening and adhesive bonding

methods and techniques.

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Joining Processes


• Except the simplest of products, almost all products re assembled from components that have been manufactured as individual parts. Even relatively simple products consist of at least two different components joined by various means.

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Joining Processes


• Manufacturing the product as a single piece impossible or uneconomical.• The need to be designed to be able to be taken apart for maintenance or

replacement of their parts.• Different properties may be desirable for functional purposes of the product.• Transporting the product in individual components and assembling them later

may be easier and less costly than transporting the completed item.

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Comparison of Various Joining Methods


Fusion-WeldingProcesses

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Welding


• The welding processes described in this chapter involve the partial melting and fusion between two members to be joined. Here, fusion welding is defined as melting together and coalescing materials by means of heat. Filler metals, which are metals added to the weld area during welding, also may be used. Fusion welds made without the use of filler metals are known as autogenous welds.


Oxyfuel-gas Welding

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• Oxyfuel-gas welding (OFW) is a general term used to describe any welding process that uses a fuel gas combined with oxygen to produce a flame. The flame is the source of the heat that is used to melt the metals at the joint. The most common gas welding process uses acetylene; the process is known as ox;/acetylene-gas welding

• (OAW) and is typically used for structural metal fabrication and repair work. • The low equipment cost is an attractive feature of oxyfuel-gas welding. Although

it can be mechanized, this operation is essentially manual and, hence, slow. However, it has the advantages of being portable, versatile, and economical for simple and low-quantity work.


Arc-welding Processes

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• In arc welding, the heat required is obtained from electrical energy. The process involves either a consumable or a non-consumable electrode. An AC or a DC power supply produces an arc between the tip of the electrode and the workpiece to be welded. The arc generates temperatures of about 30,000°C, which are much higher than those developed in oxyfuel-gas welding.Arc welding may employ consumable electrodes or non-consumable electrodes.

• There are various variations of Arc welding processes under the two categories.


Electron-beam Welding

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• In electron-beam welding (EBW), heat is generated by high velocity narrow-beam electrons. The kinetic energy of the electrons is converted into heat as they strike the workpiece. The process requires special equipment to focus the beam on the workpiece, typically in a vacuum. The higher the vacuum, the more the beam penetrates, and the greater is the depth-to-width ratio; thus, the methods are called EBW-HV (for “high vacuum”) and EBW-MV (for “medium vacuum”); some materials also may also be welded by EBW-NV (for “no vacuum”)


Electron-beam Welding

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• The EBW process has the capability of making high-quality welds that are almost parallel sided, are deep and narrow, and have small heat-affected zones. Depth-to-width ratios range between 10 and 30. The sizes of welds made by EBW are much smaller than those of welds made by conventional processes. With the use of automation and servo controls, parameters can be controlled accurately at welding speeds as high as 12 m/min.


Laser-beam Welding

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• Laser-beam welding (LBW) utilizes a high-power laser beam as the source of heat, to produce a fusion weld. Because the beam can be focused onto a very small area, it has high energy density and deep-penetrating capability.

• The beam can be directed, shaped, and focused precisely on the workpiece. Consequently, this process is suitable particularly for welding deep and narrow joints with depth-to-width ratios typically ranging from 4 to 10.


Laser-beam Welding

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• Laser-beam welding has become extremely popular and is used in most industries. In the automotive industry, welding transmission components are the most widespread application.

• The major advantages of LBW over EBW are the following:• A vacuum is not required, and the beam can be transmitted through air.• Laser beams can be shaped, manipulated, and focused optically (by means of

fibre optics), so the process can be automated easily.• The beams do not generate X-rays.• The quality of the weld is better than in EBW; the weld has less tendency

toward incomplete fusion, spatter, and porosity; and there is less distortion.


The Weld joint, Quality, and Testing

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• Three distinct zones can be identified in a typical weld joint:• I. Base metal• 2. Heat-affected zone• 3. Weld metal.• The metallurgy and properties of the second and third zones depend

strongly on the type of metals joined, the particular joining process, the filler metals used (if any), and welding process variables. A joint produced without a filler metal is called autogenous, and its weld zone is composed of the re-solidified base metal. A joint made with a filler metal has a central zone called the weld metal and is composed of a mixture of the base and the filler metals.


Weld Quality

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• The major discontinuities that affect weld quality are:• Porosity. • Slag inclusions.• Incomplete Fusion and Penetration.• Poor Weld Profile.• Cracks.• Lamellar Tears.• Surface Damage.• Residual Stresses.


Weld Quality

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• Porosity in welds may be caused by• Gases released during melting of the weld area, but trapped during

solidification.• Chemical reactions during welding.• Contaminants.

• Slug inclusions are compounds such as oxides, fluxes, and electrode coating materials that are trapped in the weld zone.

• Incomplete fusion produces poor weld beads. • Weld profile is important not only because of its effects on the strength and

appearance of the Weld, but also because it can indicate incomplete fusion or the presence of slag inclusions in multiple-layer welds.


Weld Quality

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• Cracks may occur in various locations and directions in the Weld area. Typical types of cracks are longitudinal, transverse, crater, underbead, and toe cracks.

• Lamellar tears may develop because of shrinkage of the restrained components of the structure during cooling.

• Surface discontinuities thereby produced may be objectionable for reasons of appearance or of subsequent use of the welded part. If severe, these discontinuities adversely may affect the properties of the welded structure, particularly for notch-sensitive metals.

• Because of localized heating and cooling during welding, the expansion and contraction of the weld area causes residual stresses in the workpiece.


Welding Joint Design and Process Selection

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• The general design guidelines for welding may be summarized as follows:

• Product design should minimize the number of welds because, unless automated, welding can be costly.

• Weld location should be selected so as to avoid excessive stresses or stress concentrations in the welded structure and for appearance.

• Weld location should be selected so as not to interfere with any subsequent processing of the joined components or with their intended use.

Unit 8 Copyright © 2014. MDIS. All rights reserved.19


Solid-State welding

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• Solid State Welding is where joining takes place without fusion at the interface of the two parts to be welded. Unlike the situation with the fusion-welding processes, in solid-state welding no liquid or molten phase is present in the joint.

• Solid-state bonding involves one or more of the following phenomena:

• Diffusion.• Pressure.• Relative interfacial movements.

Brazing, Soldering,Adhesive-Bonding,and Mechanical-

Fastening Processes

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When welding is not suitable

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• Welding cannot be used on materials that cannot withstand high temperatures, such as electronic components, when the parts to be joined are fragile, intricate, or made of two or more materials with very different characteristics, properties, sizes, thicknesses, and cross sections.


Brazing

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• Brazing is a joining process in which a #Her metal is placed between the faying surfaces to be joined (or at their periphery) and the temperature is raised sufficiently to melt the filler metal, but not the components (the base metal).


Brazing

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http://www.abbottfurnace.com/images/technology/fundamentals/brazing/fundamentals-brazing-fig-2.gif


Filler Material

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• Several filler metals (braze metals) are available with a range of brazing temperatures.

• Unlike those for other welding operations, filler metals for brazing generally have a composition that is significantly different from those of the metals to be joined. They are available in a variety of shapes, such as wire, rod, ring, shim stock, and filings. The selection of the type of filler metal and its composition are important in order to avoid enbrittlement of the joint by (a) grain-boundary penetration of liquid metal; (b) the formation of brittle intermetallic compounds at the joint; and (c) galvanic corrosion in the joint


Filler Metals

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Flux in Brazing

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• The use of a flux is essential in brazing; a flux prevents oxidation and removes oxide films. Brazing fluxes generally are made of borax, boric acid, borates, fluorides, and chlorides. Wetting agents may be added to improve both the wetting characteristics of the molten filler metal and the capillary action.


Soldering

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• In soldering, the filler metal (called solder) melts at a relatively low temperature. As in brazing, the solder fills the joint by capillary action between closely fitting or closely placed components. Two important characteristics of solders are low surface tension and high wetting capability. Heat sources for soldering are usually soldering irons, torches, or ovens.


Soldering

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Soldering

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http://www.ami.ac.uk/courses/topics/0225_wave/images/ass_bapws_imgi.gif

http://memim.com/images/wave-soldering-01.jpg


Adhesive Bonding

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• Adhesive bonding has gained increased acceptance in manufacturing. • Adhesives are available in various forms: liquid, paste, solution,

emulsion, powder, tape, and film. When applied, adhesives typically are about 0.1 mm thick.


Overview of Adhesive Types

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• Adhesives come in many forms and chemistries, formulated to solve different problems with different features.

• Epoxy adhesives are the most widely used structural adhesive, because of their excellent adhesion to most substrates, high strength, and light weight.

• Urethane adhesives are environmentally tough and flexible without sacrificing much bond strength, especially in two-component versions.

• Methacrylate adhesives are toughened structural adhesives designed to bond plastics, metals, composites, and dissimilar substrates.



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• Instant adhesives (cyanoacrylates) are incredibly strong and quick-setting, more than run-of-the-mill "super glues.“

• Contact adhesives are fast-acting and ideal for attaching large surfaces together, with the ability to provide water and heat resistant bonds.

• IQ Glue combines the best qualities of a sealant in a stronger-than-usual environmentally friendly adhesive.

• Wood, paper & fibre Adhesives are high concentration Polyvinyl Acetate (PVA) based adhesives formulated for versatility in labelling, packaging, insulating, and wood working



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• PermaTack adhesives deliver the critical qualities of pressure sensitive adhesives (PSAs): permanent tackiness, instant holding power, and your choice of permanent or removable application.

• Threadlockers (anaerobic adhesives) seal the threads of screws into metal objects, especially securing nuts to bolts.

http://www.adhesive.com/types


Adhesive Bonding

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• Because of their strength, synthetic organic adhesives are the most important adhesives in manufacturing processes, particularly for load-bearing applications. They are classified as follows:

• Chemically reactive: polyurethanes, silicones, epoxies, cyanoacrylates, modified acrylics, phenolics, and polyimides. Also included are anaerobics, which cure in the absence of oxygen, such as Loctite® for threaded fasteners.

• Pressure sensitive: natural rubber, styrene-butadiene rubber, butyl rubber, nitrile rubber, and polyacrylates. Hot melt: thermoplastics (such as ethylene-vinyl acetate copolymers, polyolefins, polyamides, and polyester) and thermoplastic elastomers.

• Reactive hot melt: a thermoset portion (based on urethane’s chemistry) with improved properties.


Adhesive Bonding

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• Evaporative or diffusion: vinyls, acrylics, phenolics, polyurethanes, synthetic rubbers, and natural rubbers.

• Film and tape: nylon epoxies, elastomer epoxies, nitrile phenolics, vinyl phenolics, and polyimides.

• Delayed tack: styrene-butadiene copolymers, polyvinyl acetates, polystyrenes, and polyamides.

• Electrically and thermally conductive: epoxies, polyurethanes, silicones, and polyimides. Electrical conductivity is obtained by the addition of fillers, such as silver (used most commonly), copper, aluminium, and gold. Fillers that improve the electrical conductivity of adhesives generally also improve their thermal conductivity.


Adhesive Bonding

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• Adhesive bonding has gained increased acceptance in manufacturing. • Adhesives are available in various forms: liquid, paste, solution,

emulsion, powder, tape, and film. When applied, adhesives typically are about 0.1 mm thick.


Adhesives

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• Reactive hot melt: a thermoset portion (based on urethane’s chemistry) with improved properties.

• Evaporative or diffusion: vinyls, acrylics, phenolics, polyurethanes, synthetic rubbers, and natural rubbers.

• Film and tape: nylon epoxies, elastomer epoxies, nitrile phenolics, vinyl phenolics, and polyimides.

• Delayed tack: styrene-butadiene copolymers, polyvinyl acetates, polystyrenes, and polyamides.

• Electrically and thermally conductive: epoxies, polyurethanes, silicones, and polyimides. Electrical conductivity is obtained by the addition of fillers, such as silver (used most commonly), copper, aluminium, and gold. Fillers that improve the electrical conductivity of adhesives generally also improve their thermal conductivity.


Mechanical Fastening

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• Two or more components may have to be joined or fastened in such a way that they can be taken apart sometime during the product’s service life or life cycle. Numerous products, such as mechanical pencils, watches, computers, appliances, engines, and bicycles, have components that are fastened mechanically. Mechanical fastening may be preferred over other methods for the following reasons:


Mechanical Fastening

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• Ease of manufacturing.• Ease of assembly and transportation.• Ease of disassembly, maintenance, parts replacement, or repair.• Ease in creating designs that require movable joints such as hinges,

sliding mechanisms, and adjustable components and fixtures.• Lower overall cost of manufacturing the product.

Documents

UNIT 8: Joining Processes and Equipment Unit 8 Copyright © 2014. MDIS. All rights reserved. 1 Modern Manufacturing