Brazing vs Welding

8/17/2019 Brazing vs Welding

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Brazing vs WeldingThe differences between brazing and welding

Devulapalli Kapil Siddhant

Department of Mechanical EngineeringSreenidhi Institute of Science & TechnologyHyderabad, India

[email protected]

bstract—Brazing and welding are two most important

and common metal joining process es available. In brazing

two or more materials are joined by the use of a third

dissimilar material such as braze alloy or silver solder.

Whereas in welding two metals are melted together. In this

report the major differences between brazing and welding

are analyzed.

Keywords—brazing welding metal joining proces ses

analysis

I. INTRODUCTION

Welding is a fabrication or sculptural process that joinsmaterials, usually metals or thermoplastics, bycausing fusion, which is distinct from lowertemperature metal-joining techniques suchas brazing and soldering, which do not melt the basemetal. In addition to melting the base metal, a fillermaterial is often added to the joint to form a pool of molten material (the weld pool) that cools to form a

joint that can be as strong, or even stronger, than thebase material. Pressure may also be used inconjunction with heat, or by itself, to produce a weld.Although less common, there are also solid statewelding proces ses such as friction welding or shieldedactive gas welding in which metal does not melt.Brazing is a metal-joining proces s in which two or moremetal items are joined together by melting and flowinga metal into the joint, the filler metal having a lowermelting point than the adjoining metal. Brazing differsfrom welding in that it does not involve melting the workpieces and from soldering in using higher temperaturesfor a similar proces s, while also requiring much more

closely fitted parts than when soldering. The filler metalflows into the gap between close-fitting partsby capillary action. The filler metal is brought slightlyabove its melting (liquidus) temperature while protectedby a suitable atmosphere, usually a flux. It then flowsover the base metal (known as wetting) and is then

cooled to join the work pieces together.[1]

It is similarto soldering, except for the use of higher temperatures.A major advantage of brazing is the ability to join thesame or different metals with considerable strength.

II. WORKING PRINCIPLES

A. Welding

Welding joins metals by melting and fusing themtogether, usually with the addition of a welding filler

metal. The joints produced are strong, usually as strongas the metals joined or even stronger. In order to fusethe metals, a concentrated heat is applied directly to the

joint area. This heat is high temperature. It must be – inorder to melt the "base" metals (the metals being

joined) and the filler metals as well. So weldingtemperatures start at the melting point of the basemetals. Because welding heat is intense, it isimpractical to apply it uniformly over a broad area.Welding heat is typically localized, pinpointed heat. Thishas its advantages. For example, if you want to join twosmall strips of metal at a single point, an electrical

resistance welding s etup is very practical.

This is a fast, economical way to make strong,permanent joints by the hundreds and thousands.However, if the joint is linear, ratherthan pinpointed, problems arise. The localized heat of welding tends to become a disadvantage. For example,suppose you want to butt- weld two pieces of metal –start by beveling the edges of the metal pieces to allowroom for the welding filler metal. Then weld, firstheating one end of the joint area to melting temperature,then slowly traveling the heat along the joint line,depositing filler metal in synchronization with the heat.


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The principle by which the filler metal is drawn throughthe joint to create this bond is capillary action. In abrazing operation, you apply heat broadly to the basemetals. The fillermetal is then brought into contact withthe heated parts. It is melted instantly by the heat in thebase metals and drawn by capillary action completelythrough the joint.

This, in ess ence, is how a brazed joint is made.

The joint formed by brazing is superior to that formedby soldering. It is used where mechanical strength andhigher press ure resistant joints are needed. Thestrength comes from the ability of the brazing alloys orsilver braze to flow into the porous grain s tructure of the pipe and fitting. However, this excellent bond is onlypossible if:

The surface is clean. The properflux and fillerrod are used.

The clearance gap between the outside of thepipe and the bore is only 0.003″ to 0.004″.

Filler metal for brazing is available in the followingdifferent shapes: wires, rods, sheets, and washers. Theclass ifications, each with special uses , include:

1. Aluminum-silicon—Used for brazing aluminumand aluminum alloys.

2. Copper-phosphorus—Used for joining copper,copper alloys, and othernonferrous metals.

3. Silver—Used for joining almost all ferrous andnonferrous metals. The exceptions for useinclude aluminum and other metals with lowmelting points.

4. Copper and copper-zinc—Suited for joining bothferrous and nonferrous metals. This compoundis used in a 50/ 50 mixture for brazing copper. A64% copper/ 36% zinc compound is used foriron and steel.

5. Nickel—Used when extreme heat and corrosionresistance is needed. Applications include foodand chemical processing equipment,automobiles, cryogenics, and vacuumequipment.

A cross -sectional enlargement of the clearance gap between apipe and a bronze pipe fitting.

Flux, an important component in the soldering process,

is even more necess ary in brazing. In addition toprotecting the surface from oxidation and aiding theflow of filler metal, brazing flux serves to indicate thetemperature of the metal. Without flux it would bealmost impossible to know when the base metalreaches the correct temperature. Fluxes are produced inpowder, pas te, and liquid form. Many different types of flux are commercially available for use with variousbase metals and filler rods. When brazing copper, it isimportant to select a flux that is compatible with thefiller metal being used. For small jobs, powdered flux isoften preferred because it will adhere to a heatedbrazing rod. Preheat the rod and stick it into thepowdered flux. When the base metal is aluminum,brazing must take place at a lower temperature.

Brazed copper joints are made at a temperature of 1400°F (760°C) or higher. An oxyacetylene torch, asshown in Figure is commonly used instead of a propanetorch because of the higher temperatures required.Correct torch tip size and the appropriate oxygen andacetylene regulator settings for various pipe and fittingdiameters are listed in Figure 9-12. For example, to


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braze 1 ⁄ 2″ or 3⁄ 4″ pipe, a No. 5 torch tip isrecommended. This tip requires an oxygen pressure of 5 psi and an acetylene pressure of 5 psi.

This table of oxyacetylene torch tip sizes and regulatorsettings is sugges ted for brazing various s ize pipes and

fittings . Pressures are not standardized for oxyacetylene units.

Following are the advantages of brazing:

1. Brazing is used to join a large variety of dissimilar metals.

2. Properly brazed joints are pressure tight.3. Pieces having great difference in cross-

sectional areas can be brazed.4. Thin sheets, pipes and gauges that can’t be

joined by welding can be joined by brazing.5. Complex assemblies can be fabricated by this

method.6. A brazed component has ability to preserve

protective metal coating.7. Brazing can be done on cast and wrought

materials.8. Corrosion resistance joints can be produced by

this method.9. Brazing preserves metallurgical characteristics

of a material better than welding.10. After brazing a component maintains more

precision tolerances than welding.11. Brazing processes can be automated for bulk

production.12. Non-metals can be joined to metals.

Following are the disadvantages of brazing:

1. It requires tightly mating parts.2. It requires propercleaning.3. Size of the job is limited.4. J oints are not successful at elevated

temperatures.5. Color of the filler metal may not match with that

of the base metal.

III. THE DIFFERENCES

When you want to make strong and permanent metal joints, your choice will generally narrow down towelding or brazing. So, which method is best? Itdepends entirely on the circumstances. The key factorsin making a decision will boil down to the size of theparts to be joined, the thickness of the metalsections, configuration of the joint, nature of the basemetals, and the number of joints to be made. Let'sconsider each of them.

A. Size of assembly

Welding is usually more s uited to the joining of largeassemblies than brazing. Because in brazing the heatmust be applied to a broad area, often to the entireassembly. And if the assembly is a large one, it's oftenhard to heat it to the flow point of the filler metal as theheat tends to dissipate faster than you build it up. Youdon't meet this limitation in welding. The intenselocalized heat of welding, sometimes a drawback,becomes an advantage in joining, a large assembly. So

does welding's ability to trace a joint. There's no way toestablish exactly the point at which size of assemblymakes one metal joining method more practical thananother. There are too many factors involved. Forexample, if the as sembly is unable to be brazed in openair (torch, induction, etc.) due to size, a furnace or dipbrazing proces s may eliminate the s ize consideration.However, you can still use this rule-of-thumb as astarting point: Large assembly-weld, if the nature of themetals permits. Small assembly-braze. Medium-sizedassembly-experiment.

B. Thickness of base metal sections

Thickness of base metal sections is an importantconsideration in selecting your metal joining method. If both sections are relatively thick – say .500" (12.7mml

– either welding or brazing can produce a strong joint.But if you want to make a T-joint, bonding a .005"(.127mm) thick sheet metal section to half-inch stockfor example, brazing is the better choice. The intenseheat of welding is likely to burn through, or at least warp,the thin section.

The broader heat and lower temperature of brazingallows you to join the sections without warp age ormetal distortion.


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C. Configuration of J oint

Is the joint a "spot"or a "line" 2 A spot joint made at onepoint can be accomplished as easily by welding as by

brazing. But a linear joint – all other things being equal – is more easily brazed than welded. Brazing needs nomanual tracing. The filler metal is drawn through the

joint area by capillary action, which works with equalease on any joint configuration.

D. Nature of Metals Suppose you’re planning a two- section metal assembly.

You want high electrical conductivity in one section,high strength and corrosion resistance in the other. Youwant to þ se copper for conductive, and stainless forstrength and corrosion resistance. Welding thisass embly will present problems. As we've seen, youhave to melt both metals to fuse them. But stainlessmelts at a much higher temperature than copper. Thecopper would completely melt and flow off before thestainless came anywhere close to its meltingtemperature. Brazing these dissimilar metals offers nosuch obstacle. All you have to do is select a brazingfiller metal that is metallurgically compatible with bothbase metals and has a melting point lower than that of the two. You get a strong joint, with minimal alterationof the properties of the metals. The point to rememberis that brazing joins metals without melting them, bymetallurgically bonding at their interfaces. The integrityand properties of each metal in the brazed ass emblyare retained with minimal change. If you plan to joindissimilar metals – thinkbrazing.

E. Number of Assemblies needed

For a single assembly, or a few ass emblies, your choicebetween welding and brazing will depend largely on thefactors discuss ed earlier – size of parts, thickness of sections, joint configurations, and nature of basemetals. Whether you braze or weld, you'll probably dothe job manually. But when your production needs runinto the hundreds, or thousands (or hundreds of thousands), production techniques and cos t factorsbecome decisive. Which method is best –for production metal joining? Both methods can beautomated. But they differ greatly in flexibility of

automation. Welding tends to be an all-or-nothingproposition. You weld manually, one-at-a-time, or youinstall expensive, sophisticated equipment to handlevery large runs of identical assemblies. There's seldoma practical in-between. Brazing is just the opposite. Youcan braze "one-at-a-time" manually, of course. But youcan easily introduce simple production techniques tospeed up the joining of several hundred assemblies. Asan example, many assemblies, pre-fluxed and bearingpre-placed lengths of filler metal, can be simultaneouslyheated and brazed in a furnace. When you get intolarger runs, it may become practical to rig up aconveyor which can run the as semblies past banks of heating torches and brazing filler metal can be appliedto the joint in a pre- measured amount. And there areendless "in-between" poss ibilities, a good many of which you can accomplish with relatively inexpensiveproduction devices . The point to keep in mind is thatbrazing is flexible. You can automate it on a step-by-step basis, at each step matching your automation

investment to your production requirements.IV. CONCLUSION

The process chosen for the joining depends upon thefactors described above. Hence a thorough study has tobe made on the required product before choosing theproces s. If a wrong choice is made then there is loss of time, manpower and money. Brazing provides good

joint strength at lower temperatures and cos ts. It alsoprovides good surface finish and maintains base metalintegrity. Also it can be easily automated. One of themain disadvantages is the lack of joint strength ascompared to a welded joint due to the softer filler

metals used. The strength of the brazed joint is likely tobe less than that of the base metal(s) but greater thanthe filler metal. Another disadvantage is that brazed

joints can be damaged under high servicetemperatures. Brazed joints require a high degree of base-metal cleanliness when done in an industrialsetting. Some brazing applications require the use of adequate fluxing agents to control cleanliness . The jointcolor is often different from that of the base metal,creating an aesthetic disadvantage. Welding providesgreater strengths at comparatively higher rigidity. It isadaptable to confined spaces and remote locations.Also it is the most versatile joining process usedeverywhere. The equipment is portable and can berelocated easily. However it is not as productive asbrazing. Costs are also relatively higher. Metal wastageis more and frequent stops are required to changeelectrodes. Each process has its advantages anddisadvantages. The right process depends on theapplication.

References


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[1] The Goodheart-Willcox Co., Inc.http:// www.maintenanceresources .com/ bookstore/ pipingplumbing/ modernplumbing.pdf

[2] Wikipedia

https:/ / en.wikipedia.org/ wiki/ Brazing

https:/ / en.wikipedia.org/ wiki/ Welding

[3] HowStuffWorks

http:// sc ience.howstuffworks.com/brazing.htm

[4] "The Brazing Book Online."http:/ / www.handyharmancanada.com/thebrazingbook/ bbook.htm

[5] Samarth Engineering Works

http:// blog.samarthenggworks.com/index.php/ advantages -disadvantages-brazing/

Brazing vs Welding

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