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Brazing Filler metals typically melt above 450 o C (840 o F); Below MP of host metals. Term brazing derived from brass, to harden Process originated 3000-2000 B.C. Typically, a filler (braze) metal wire is placed along periphery of metals to be joined; heat applied; by capillary action, the filler metal flows along faying surfaces Braze Welding Filler metal (typically brass) is deposited at the joint in a manner similar to oxyfuelgas welding (below MP of host metal). Common application is for repairs of cast irons and cast steels Dissimilar metals can be assembled with good joint strength 5
Brazing Intricate, lightweight shapes can be joined rapidly and with little distortion 6
Filler Metals (braze metals) Several metals available with a range of brazing temperatures Filler metals for brazing generally have a composition that is significantly different than the composition of the metals to be joined. This is different than for fusion welding. Filler metals are available in a variety of shapes, such as wire, rod, ring, shim stock, filings 7
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Brazed Joint Strength 10
Brazed Joint Strength Strength depends on: -Joint clearance -Joint area -Nature of bond at the interfaces With small clearances, roughness of mating surfaces becomes important Must clean the surfaces (mechanically or chemically) to ensure full capillary action Therefore, use of a flux is important 11
TB First heat joint with torch; then deposit brazing rod or wire in joint; Repair work or automated production FB Complete components heated uniformly, therefore appropriate for complex shapes; Batch or continuous; IB Parts preloaded with filler metal and placed near coil for rapid heating; Use flux or a neutral atmosphere; continuous RB Parts either preloaded with filler metal or metal supplied externally during brazing; Rapid, narrow, controlled heat zones; Automate for reliable and uniform quality DB Bath at temperature just above MP of filler metal; Coat all surfaces, therefore good for small parts; Dip many parts simultaneously IRB Particularly suitable for very thin section components; radiant energy focused on the joint DFB Brazing time 30 min to 24 hr.; Strong lap joints; Thin foil to 2 BW Filler metal deposited at the joint (as in fusion welding) rather than drawn in by capillary action (for normal brazing); Temperature below MP of part; less distortion than fusion welding; Flux essential; Principal application in maintenance & repair, especially CI & steel 12
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Strong joints require a larger contact area for brazing, than for welding 14
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Tin-lead solder is general purpose (wide application) but the lead is toxic and has adverse effects on the environment Many lead-free solders being developed Many of new solders are various metals in some combination with tin; however none of these is suitable for every soldering application (as Tin-Lead) 17
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Soldering used extensively in electronics industry Soldered joint has limited use at elevated temperatures (low MP of solder) Solders have low strength; therefore avoid load bearing applications Can improve joint by mechanical interlocking see (e) to (j) above Can be used to join metals of different thicknesses Design guidelines are similar to those for brazing Important to employ large contact surfaces (because of low strength of solders) Avoid butt joints (small faying surfaces & low strength) 24
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All adhesives do not need to high in all areas on list Relative importance of particular property depends on the application See table 32.3 (next slide) 26
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Synthetic Organic Adhesives are the most important group of adhesives for manufacturing applications. This is because of their strength, making them particularly attractive for load bearing applications 28
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Adhesive joints are designed to withstand shear, compressive and tensile forces They should not be subject to peeling 33
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Case Study 32.1 39
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Riveting The most common method of permanent or semi-permanent mechanical joining Riveting may be performed at room or elevated temperature Rivets may be solid or tubular Solid riveting: two steps insert, then deform shank by upsetting (Hole is usually punched or drilled) Hollow rivet is installed by flaring its smaller end Riveting may be performed by hand, mechanically, or automatically using programmable robots 41
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Metal Stitching & Stapling Operation is fast Particularly suitable for joining thin metallic & non-metallic materials Clinching: fastener material needs to be thin & ductile to withstand local deformation during sharp bending 43
Seaming Materials need to be capable of undergoing bending and folding at very small radii Can combine with other processes to improve strength, durability -Make impermeable by combining with adhesives, seals, soldering 44
Crimping A method of joining without using fasteners Can be done with beads or dimples -Created by shrinking or swaging operations Can be performed on both tubular and flat parts -Materials need to be sufficiently thin and ductile to withstand large localized deformations 45
Spring & Snap-in Fasteners Economical Permit easy & rapid assembly (disassembly is not nearly so easy or rapid!) 46
Shrink Fitting -Based on thermal contraction of two components Press Fitting -One component forced over another -Can achieve high joint strength when joints are well designed Shape-Memory Alloys -These materials recover their shape upon heating 47
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