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UNIT 12:Joining Processes- Brazing soldering and adhesives Unit 12 Copyright © MDIS. All rights reserved. 1 Manufacturing Engineering
Brazing like welding, heat is used to fuse a material for the bonding to happen, however, the filler is the only metal to be fused. Thus this is a liquid-solid-state bonding process. Once cooled, a strong joint is obtained. Typical temperatures here are of 450C and above, much lower than in welding. Materials to be brazed include brasses, bronzes, copper, glass and many non-metals can be brazed together. 2Unit 12 Copyright © MDIS. All rights reserved.
Fillers The filler material normally takes the form of a wire, rod, strips or shims. For metals the fillers are made of alloys based on the base materials to be joined. For example, to braze : aluminium, filler - aluminium -silicon alloy. Magnesium, filler - magnesium aluminium alloy ferrous and non ferrous fillers- silver and copper alloys 3Unit 12 Copyright © MDIS. All rights reserved.
Brazing Flux Flux is the material introduced into the process to enhance the quality of the joint It provides- improved joint strength, reduction of metal oxidation, and surface improvement for the bond. It would typically be borax, boric acid fluorides or chlorides. Wetting agents are introduced to enhance the filler metals ability to flow completely over the joint surfaces. Prior surface care like surface cleaning, abrasive cleaning and sand blasting are employed to obtain best results from the bond. 4Unit 12 Copyright © MDIS. All rights reserved.
Brazing tools The equipment for brazing includes a torch, grips and tongs for positioning of the job, a brazing table which is normally a stone, brick or granite The methods for brazing include torch, furnace, and induction brazing The simplest is the torch which blows a blast of very hot gases from the combustion of oxy-fuel in carburizing flame. The joint and rod are first heated to suitable high temperatures, 5Unit 12 Copyright © MDIS. All rights reserved.
then the rod is dipped into a flux, reheated to its final melting temperature on the joint the droplets of filler metal fill into the joint by surface adhesion allowing for very smooth and quality joint Soldering the temperature of the filler melting points which is relatively very low compared to welding and brazing. The solder fills the joint by capillary action as it is perfectly melted and flows onto the job surface. As the filler is a softer grade, the stronger joints need larger areas than in brazing. 6Unit 12 Copyright © MDIS. All rights reserved.
Solder methods The methods for soldering include :Torch soldering similar to brazing, furnace, iron, induction, dip, and infrared methods. The most common industrial methods are reflow and wave soldering reflow method involves the use of a solder paste that is applied directly onto the solder path, points and component leads to be soldered. Then the paste is heated to get a perfectly molten solder at the joints and soldering is accomplished in an oven over a conveyor. 7Unit 12 Copyright © MDIS. All rights reserved.
wave soldering, a solder bath which is already molten is used to pass over the PCB which is masked with a material to disallow solder to settle on unintended points. As the wave of solder passed over the PCB, it only adheres at points that are exposed thus producing the soldered joints. Adhesive Bonding Whenever heating method is not tolerable one has to rely on either the mechanical fastening or the adhesive bonds. Adhesive technology has become so advanced now that almost any material can be adhesively bonded with a joint strength almost as high as base or even stronger than base in most cases 8Unit 12 Copyright © MDIS. All rights reserved.
9Unit 12 Copyright © MDIS. All rights reserved. Materials like even PTFE which were once thought to be unbondable a few decades ago can be bonded with modern adhesives. Adhesives chemicals that have some kind of natural affinity when applied on the surfaces of the base materials. They come in all formats as liquids, solids, pastes, films, dispersions for spray on, heat applied, room temperature, pressure activated, etc. Basic types :natural adhesives- derieved from natural substances like starch, dextrin, etc
inorganic adhesive- based on inorganic chemistry synthetic organics- thermoplastics and thermosetting materials chemically reactive- which react with base materials to form very strong chemical bonding pressure sensitive- which are activated on being mechanically pressed specially in rubber compounds hot melts- which are very low temperature melting compositions that bond aggressively in the molten state electrically conductive- which are capable of basic current carrying when used between circuit components expoxy based- those that are very strong cross linked compounds but need higher temperature to cure and longer time Unit 12 Copyright © MDIS. All rights reserved. 10
Acrylics- the cyanoacrylates and acrylics are fast curing low temperature adhesives, commpnly used today anaerobic – Those that do not rely on presence of oxygen, which is important in some environments like chemical tanks, water application etc. Unit 12 Copyright © MDIS. All rights reserved. 11
Methods of applying Spray- to coat larger surfaces are normally sprayed over the surfaces. Advantages include controlled coat thicknes, larger spread and ability to mask un needed areas nozzle application- here the adhesives are made to flow out through an applicator nozzle under slight pressure for quantity control rather than velocity of flow. The nozzle can be led along the desired path by hand, by robotics or by mechanical means Squeege- the adhesive is placed onto the components to be bonded, and spread by a soft squeege plate. Large areas can be covered, but thickness is often higher Unit 12 Copyright © MDIS. All rights reserved. 12
than spray method. Dip method- components which need to be completely engulfed before bonding are dipped into the adhesive bath trickle- a droplet of fine flow adhesive is allowed to trickle into the bond area by capillary action and use of gentle movement under gravity. Unit 12 Copyright © MDIS. All rights reserved. 13
Mechanical Fastening the last resort when other methods prove to be uncapable of strength desired; unfeasible due to environment of service (temperature, humidity etc.); when disassembly is necessary; when two parts need to be separate as an intentional service element but come together during use. Bolts and screws Use of threaded components enables the disassembly and reassembly to be done quickly and Unit 2 Copyright © MDIS. All rights reserved. 14
The pressure of the joint due to the screwing together has to be controlled by the use of torque spanners, or use of spring elements like washers. Main disadvantage of this method is the failure during service caused by automatic unscrewing due to vibrations, lack of sufficient friction, and overtightened screw damages. Rivets this is a permanent joint that cannot be undone unless the rivet itself is broken. Riveting involves like bolts and screws, the insertion of a rivet into a common hole of the base materials, which is then permanently deformed to grip the two base objects in a permanent pressure based grip. Unit 2 Copyright © MDIS. All rights reserved. 15
Major advantages are of it life and ability to withstand very high forces, but disadvantage is in its failure once the contact pressure is lost. A loose bolted joint can be recovered, but a rivetted joint is useless. This process is used in pressure vessels, structural components, small and large assemblies without motion etc. Examples are : aircraft body and wings, bridges, toys and mechanical assemblies. Unit 2 Copyright © MDIS. All rights reserved. 16
Others stapling, crimping, snap-in fasteners, seams, shrink fits etc. Stapling is used extensively in packaging where metal staplers are used to join box ends snap in fasteners can be observed in snap rings of turning components like bearing retainer, swivel joints etc. Seams are used extensively in sheet metals etc. Unit 2 Copyright © MDIS. All rights reserved. 17
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