Limits and Fits, Tolerance Dimensioning

Limits and Fits, Tolerance Dimensioning

Definitions:
nominal size: The size designation used for generalidentification. The nominal size of a shaft and a hole are thesame. This value is often expressed as a fraction.basic size: The exact theoretical size of a part. This isthe value from which limit dimensions are computed. Basic size isa four decimal place equivalent to the nominal size. The number ofsignificant digits imply the accuracy of the dimension.

example: nominal size = 1 1/4
basic size = 1.2500

design size: The ideal size for each component (shaft andhole) based upon a selected fit. The difference between the designsize of the shaft and the design size of the hole is equal to the
allowance of the fit. The design size of a part corresponds to the Maximum Material Condition (MMC). That is, the largest shaft permitted by the limits and the smallest hole. Emphasis is placed upon the design size in the writing of the actual limit dimension, so the design size is placed in the top position of the pair.

tolerance: The total amount by which a dimension is allowed to vary. For fractional linear dimensions we have assumed a bilateral tolerance of 1/64 inch. For the fit of a shaft/hole
combination, the tolerance is considered to be unilateral, that is, it is only applied in one direction from design size of the part. Standards for limits and fits state that tolerances are applied
such that the hole size can only vary larger from design size and the shaft size smaller.

basic hole system: Most common system for limit dimensions. In this system the design size of the hole is taken to be equivalent to the basic size for the pair (see above). This means that the lower (in size) limit of the hole dimension is equal to design size. The basic hole system is more frequently used since most hole generating devices are of fixed size (for example, drills, reams, etc.) When designing using purchased components with fixed outer diameters (bearings, bushings, etc.) a basic shaft system may be used.

allowance: The allowance is the intended difference in the sizes of mating parts. This allowance may be: positive (indicated with a "+" symbol), which means there is intended clearance between parts; negative("-"), for intentional interference: or "zero allowance" if the two parts are intended to be the "same size".This last case is common to selective assembly.

The extreme permissible values of a dimension are known as limits. The degree of tightness or looseness between two mating parts that are intended to act together is known as the fit of the parts. The character of the fit depends upon the use of the parts. Thus, the fit between members that move or rotate relative to each other, such as a shaft rotating in a bearing, is considerably different from the fit that is designed to prevent any relative motion between two parts, such as a wheel attached to an axle.

In selecting and specifying limits and fits for various applications, the interests of interchangeable manufacturing require that (1) standard definitions of terms relating to limits and fits be used; (2) preferred basic sizes be selected wherever possible to be reduce material and tool costs; (3) limits be based upon a series of preferred tolerances and allowances; and (4) a uniform system of applying tolerances (bilateral or unilateral) be used.

Adhesive Bonding

Adhesive Bonding

Adhesive Bonding is a modern joining process in which a liquid or semi liquid substance is applied to adjoining work pieces to provide a long lasting bond. This process is highly useful in bonding dis-similar materials that can not be welded. Materials that have the ability to be bonded together are virtually unlimited. Adhesives used in bonding can exist in many forms and be made from various natural and/or artificial compounds. A hindrance to this process is that adhesive bonds are not instantaneous such as welding or nailing. Adhesive bonds take more time to process, in order to allow the adhesives to cure.

Process Characteristics

# Uses substances in a liquid or semi-liquid state
# Allows the joining of metals with nonmetals
# Ideal process for the joining of thin materials to other materials
# Time and temperature usually must be controlled
# Helps reduce assembly costs
# Lighter than most other joining processes(Bolting, screws, etc...)
# Creates an "electrical non-conducting joint" in conductive materials
# Enhances the vibration dampening properties of a joint.

Process Schematic

The adhesive is applied to either one of both of the materials being bonded. The pieces are aligned and pressure is added to aid in adhesion and rid the bond of air bubbles. The bond is meant to be permanent, unless: insufficient time was allowed for the bond to cure; the surfaces to be bonded were not properly prepared for bonding; the temperature at which the curing took place was unfavorable for the adhesive; or insufficient adhesive was used.



Workpiece Geometry

Adhesive bonding is used a lot in joining wood pieces and light-weight material pieces together. The joint is stronger when the adhesive contact area between the two pieces is the greater. To achieve greater sheer stress resistance the bonded materials can include key, corner and shoulder joints.

Setup and Equipment

There are many different ways to apply adhesive to a workpiece. A hot glue gun or caulk gun are used to apply adhesives in a paste or semiliquid form. Liquid adhesives are applied with spray applicators, which can be automated. Adhesives also come in containers with applicator brushes built in for easy application.

Typical Tools and Geometry Produced

Adhesives come as liquids, semi-liquids(pastes), and even solids, such as a glue stick or adhesive tape. Applicators of different adhesives are designed according to the adhesive being used and the size of the area to which the adhesive will be applied. Typical geometries of this process are; thin sheets or foils can be joined to thicker workpieces; component parts can be mounted on an assembly; and fabrics can be joined to solids; etc.

Geometrical Possibilities

Adhesive bonding can be used to join materials in an infinite number of ways. More common geometries of bonded materials are flat surfaces, corners, contours ,and corrugated backing. The pressure used to bond materials usually ranges from 10 psi (pounds per square inch) to 1000 psi, which will produce a resultant sheer strength (depending on adhesive used) ranging from 900psi and 13,000psi.

Tolerance and Surface Finish

The thickness of the materials being bonded to each other can be thin or thick, as in the making of a violin. The ribs are thin enough that they are bent and then bonded using glue and smaller wood blocks to keep them together, and some violins last for hundreds of years.


Workholding Methods

There are two ways to hold workpieces, in platens or shaped dies. A specially designed fixture or vise may hold a workpiece with critical dimensions on platens. Otherwise, a shaped die, which contains guide pins, stops, and alignment blocks, may be used for bonding flat workpieces.


Process Conditions

Process conditions are measured according to tensile strength, crystallinity and Young’s modulus of elasticity. Success of bonding adhesion is critically influenced by differences of crystallinity and Young's modulus.

* Rubber : 3000 tensile strength, Low status of Crystallinity and Low Young’s modulus( 70Deg F)
* Epoxy : 6000 tensile strength, Low status of Crystallinity and Medium to High Young’s modulus( 70Deg F)
* Phenolic : 5000 tensile strength, Low status of Crystallinity and Low Young’s modulus( 70Deg F)
* Nylons : 8000 tensile strength, High status of Crystallinity and Medium to High Young’s modulus( 70Deg F)

Tool Style

* Spray gun - this is used when the productions requirements are from low to medium. It is a semiautomatic device.
* Roll coater - fully automatic device used to high production.
* Manual - used when output requirements are low and/or if the bond is critical. i.e. making a violin
* Flat honey comb panel - used to press flat surfaces during adhesion process.
* Formed Die - Used to press contour or irregular shaped objects. i.e. the upward curved tips on a snowboard.

Design Considerations

Adhesive bonding can be used no matter what the design of the two materials. As long as there is flush contact between the two materials, the bond should cure and join the two materials.

Effect on Work Materials

Adhesive bonding may increase the following:

* Tensile - the ability to hold weight
* Shear - strain in structures due to pressure
* Compression - ability to be reduced in volume
* Impact Strengths - ability to resist shock load

The effects of adhesive bonding vary depending on thew material properties of the workpiece.

* Mechanical : May increase tensile, shear, compression, and impact strengths.
* Physical : Exposure to sunlight and heat may deteriorate adherents.
* Chemical : Solvents deteriorate adherents.

Typical Workpiece Materials

Good workpiece materials: aluminum, steel, plastic, wood

Weak workpiece materials: Brass

Difficult workpiece materials: Copper (due to fast oxidation)

Method of Application

* Manual Application - Rollers, Brush, Film or pellet
* Automatic Application - Brush, Knife coater
* Caulking gun Application - Extrusion
* Robotics Application - Film or pellet
* Spray Application - Air spray, airless spray
* Roll Coater - Bench, Pressure Roll, Dip Roll

The type of application method depends of the type of materials being bonded together, the type of adhesive being used, what is required in production, and the cost.

Types of Adhesive

* Thermoset - Quick set, High strength, Solvent-resistant
* Evaporative - Flexible and Oil-resistant
* Hot Melt - Flexible and Water-resistant
* Film - Used in honeycomb construction
* Pressure sensitive - Versatile and Inexpensive
* Delayed-tack - Short-term bond.

Curing

Curing is when the physical properties of the adhesive are changed and the bond becomes permanent. It changes the physical properties of the adhesive through a chemical reaction that is induced by the action of a catalyst, pressure, and/or heat. Curing time also includes cooling time if the adhesive needed to be heated. An example is the curing of epoxy. To get a tensile strength of 6000 psi, the epoxy is cured by clamping it at a temperature of 68 - 165 degrees Fahrenheit.

Power Requirements

Power is required when using a spray method of application. When a adhesive's temperature is raised, the viscosity of the adhesive decreases. Viscosity is the resistance of a liquid to flow. Heating the adhesive also results in the atomization energy required to be decreased. This enables a heavier film coating to be used and helps in correcting humidity related problems. (Humidity related problems occur from uneven temperatures.)

Cost and Time

It is sometimes cost effective to use adhesive bonding than other types of bonding, but it does take longer to bond with adhesive than it would be nails, welds, and other types of bonding.

Some of the cost comes from the following areas that also add time to the production process:

* Setup time
* Adhesive application time
* Load/unload time
* Bonding time
* Curing time
* Material costs
* Direct labor rates
* Overhead rate
* Amortization of equipment and tooling

Time Calculation

* Adhesive Application time (sec) = A
* Load/unload time (sec) = L
* Platen Velocity (in./sec) = P
* Bonding time (sec) = B
* Curing time (hr) = C
* Platen travel distance (in.) = D

Total Time = A + L + B + C + D/P[10]

Factors Affecting Process Results

The quality of the bond depends upon the type of work material and adhesive. Also, surface preparation, application of adhesive, curing, and equipment used to position heat and clamp/pressurize the workpieces affect the results of the process.

Safety Factors

When using adhesive bonding, one should be aware of irritations from the adhesives, the vapors and fumes produced by the adhesive bonding process, burns that can come from the heat treated adhesives and also noise during the production process. Also, one should be aware of environmental hazards, such as how material is going to be disposed, air pollution produced during the production process, fires that may occur during the heat treated adhesives or if adhesives are flammable, and also groundwater pollution is adhesive make their way into water sources.

Soldering

Soldering

Soldering is a process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a relatively low melting point. Soft soldering is characterized by the melting point of the filler metal, which is below 400 °C (752 °F). The filler metal used in the process is called solder.

Soldering is distinguished from brazing by use of a lower melting-temperature filler metal; it is distinguished from welding by the base metals not being melted during the joining process. In a soldering process, heat is applied to the parts to be joined, causing the solder to melt and be drawn into the joint by capillary action and to bond to the materials to be joined by wetting action. After the metal cools, the resulting joints are not as strong as the base metal, but have adequate strength, electrical conductivity, and water-tightness for many uses. Soldering is an ancient technique mentioned in the Bible and there is evidence that it was employed up to 5000 years ago in Mesopotamia.


Applications

One of the most frequent applications of soldering is assembling electronic components to printed circuit boards (PCBs). Another common application is making permanent but reversible connections between copper pipes in plumbing systems. Joints in sheet metal objects such as food cans, roof flashing, rain gutters and automobile radiators have also historically been soldered, and occasionally still are. Jewelry components are assembled and repaired by soldering. Small mechanical parts are often soldered as well. Soldering is also used to join lead came and copper foil in stained glass work. Soldering can also be used to effect a semi-permanent patch for a leak in a container cooking vessel.

Guidelines to consider when soldering is that since soldering temperatures are so low a soldered joint has limited service at elevated temperatures. Solders generally do not have much strength so the process should not be used for load bearing members.

Some examples of solder types and their applications are tin-lead (general purpose), tin-zinc for joining aluminium, lead-silver for strength at higher than room temperature, cadmium-silver for strength at high temperatures, zinc-aluminium for aluminium and corrosion resistance, and tin-silver and tin-bismuth for electronics.

Solders

Soldering filler materials are available in many different alloys for differing applications. In electronics assembly, the eutectic alloy of 63% tin and 37% lead (or 60/40, which is almost identical in performance to the eutectic) has been the alloy of choice. Other alloys are used for plumbing, mechanical assembly, and other applications.

A eutectic formulation has several advantages for soldering; chief among these is the coincidence of the liquidus and solidus temperatures, i.e. the absence of a plastic phase. This allows for quicker wetting out as the solder heats up, and quicker setup as the solder cools. A non-eutectic formulation must remain still as the temperature drops through the liquidus and solidus temperatures. Any differential movement during the plastic phase may result in cracks, giving an unreliable joint. Additionally, a eutectic formulation has the lowest possible melting point, which minimizes heat stress on electronic components during soldering.

Lead-free solders are suggested anywhere children may come into contact with (since children are likely to place things into their mouths), or for outdoor use where rain and other precipitation may wash the lead into the groundwater. Common solder alloys are mixtures of tin and lead, respectively:

* 63/37: melts at 183 °C (361.4 °F) (eutectic: the only mixture that melts at a point, instead of over a range)
* 60/40: melts between 183–190 °C (361–374 °F)
* 50/50: melts between 185–215 °C (365–419 °F)

Lead-free solder alloys melt around 250 °C (482 °F), depending on their composition.

For environmental reasons, 'no-lead' solders are becoming more widely used. Unfortunately most 'no-lead' solders are not eutectic formulations, making it more difficult to create reliable joints with them. See complete discussion below; see also RoHS.

Other common solders include low-temperature formulations (often containing bismuth), which are often used to join previously-soldered assemblies without un-soldering earlier connections, and high-temperature formulations (usually containing silver) which are used for high-temperature operation or for first assembly of items which must not become unsoldered during subsequent operations. Specialty alloys are available with properties such as higher strength, better electrical conductivity and higher corrosion resistance.

Flux

In high-temperature metal joining processes (welding, brazing and soldering), the primary purpose of flux is to prevent oxidation of the base and filler materials. Tin-lead solder, for example, attaches very well to copper, but poorly to the various oxides of copper, which form quickly at soldering temperatures. Flux is a substance which is nearly inert at room temperature, but which becomes strongly reducing at elevated temperatures, preventing the formation of metal oxides. Secondarily, flux acts as a wetting agent in the soldering process, reducing the surface tension of the molten solder and causing it to better wet out the parts to be joined.

Fluxes currently available include water-soluble fluxes (no VOC's required for removal) and 'no-clean' fluxes which are mild enough to not require removal at all. Performance of the flux needs to be carefully evaluated; a very mild 'no-clean' flux might be perfectly acceptable for production equipment, but not give adequate performance for a poorly-controlled hand-soldering operation.

Traditional rosin fluxes are available in non-activated (R), mildly activated (RMA) and activated (RA) formulations. RA and RMA fluxes contain rosin combined with an activating agent, typically an acid, which increases the wettability of metals to which it is applied by removing existing oxides. The residue resulting from the use of RA flux is corrosive and must be cleaned off the piece being soldered. RMA flux is formulated to result in a residue which is not significantly corrosive, with cleaning being preferred but optional.

Basic soldering techniques

Methods

Soldering operations can be performed with hand tools, one joint at a time, or en masse on a production line. Hand soldering is typically performed with a soldering iron, soldering gun, or a torch, or occasionally a hot-air pencil. Sheetmetal work was traditionally done with "soldering coppers" directly heated by a flame, with sufficient stored heat in the mass of the soldering copper to complete a joint; torches or electrically-heated soldering irons are more convenient. All soldered joints require the same elements of cleaning of the metal parts to be joined, fitting up the joint, heating the parts, applying flux, applying the filler, removing heat and holding the assembly still until the filler metal has completely solidified. Depending on the nature of flux material used, cleaning of the joints may be required after they have cooled.

The distinction between soldering and brazing is arbitrary, based on the melting temperature of the filler material. A temperature of 450 °C is usually used as a practical cut-off. Different equipment and/or fixturing is usually required since (for instance) a soldering iron generally cannot achieve high enough temperatures for brazing. Practically speaking there is a significant difference between the two processes—brazing fillers have far more structural strength than solders, and are formulated for this as opposed to maximum electrical conductivity. Brazed connections are often as strong or nearly as strong as the parts they connect, at elevated temperatures.

"Hard soldering" or "silver soldering" (performed with high-temperature solder containing up to 40% silver) is also often a form of brazing, since it involves filler materials with melting points in the vicinity of, or in excess of, 450 °C. Although the term "silver soldering" is used much more often than "silver brazing", it may be technically incorrect depending on the exact melting point of the filler in use. In silver soldering ("hard soldering"), the goal is generally to give a beautiful, structurally sound joint, especially in the field of jewelry. Thus, the temperatures involved, and the usual use of a torch rather than an iron, would seem to indicate that the process should be referred to as "brazing" rather than "soldering", but the endurance of the "soldering" apellation serves to indicate the arbitrary nature of the distinction (and the level of confusion) between the two processes.

Induction soldering is a process which is similar to brazing. The source of heat in induction soldering is induction heating by high-frequency AC current. Generally copper coils are used for the induction heating. This induces currents in the part being soldered. The coils are usually made of copper or a copper base alloy. The copper rings can be made to fit the part needed to be soldered for precision in the work piece. Induction soldering is a process in which a filler metal (solder) is placed between the faying surfaces of (to be joined) metals. The filler metal in this process is melted at a fairly low temperature. Fluxes are a common use in induction soldering. This is a process which is particularly suitable for soldering continuously. The process is usually done with coils that wrap around a cylinder/pipe that needs to be soldered. Some metals are easier to solder than others. Copper, silver, and gold are easy. Iron and nickel are found to be more difficult. Because of their thin, strong oxide films, stainless steel and aluminium are a little more difficult. Titanium, magnesium, cast irons, steels, ceramics, and graphite can be soldered but it involves a process similar to joining carbides. They are first plated with a suitable metallic element that induces interfacial bonding.

Desoldering and resoldering

Used solder contains some of the dissolved base metals and is unsuitable for reuse in making new joints. Once the solder's capacity for the base metal has been achieved it will no longer properly bond with the base metal, usually resulting in a brittle cold solder joint with a crystalline appearance.

It is good practice to remove solder from a joint prior to resoldering—desoldering braids or vacuum desoldering equipment (solder suckers) can be used. Desoldering wicks contain plenty of flux that will lift the contamination from the copper trace and any device leads that are present. This will leave a bright, shiny, clean junction to be resoldered.

The lower melting point of solder means it can be melted away from the base metal, leaving it mostly intact though the outer layer will be "tinned" with solder. Flux will remain which can easily be removed by abrasive or chemical processes. This tinned layer will allow solder to flow into a new joint, resulting in a new joint, as well as making the new solder flow very quickly and easily.

Soldering defects

Various problems may arise in the soldering process which lead to joints which are non functional either immediately or after a period of use. The most common defect when hand-soldering results from the parts being joined not exceeding the solder's liquidus temperature, resulting in a "cold solder" joint. This is usually the result of the soldering iron being used to heat the solder directly, rather than the parts themselves. Properly done, the iron heats the parts to be connected, which in turn melt the solder, guaranteeing adequate heat in the joined parts for thorough wetting.

An improperly selected or applied flux can cause joint failure, or if not properly cleaned off the joint, may corrode the metals in the joint over time and cause eventual joint failure. Without flux the joint may not be clean, or may be oxidized, resulting in an unsound joint.

Movement of metals being soldered before the solder has cooled will cause a highly unreliable cracked joint.

Soldering is the process of a making a sound electrical and mechanical joint between certain metals by joining them with a soft solder. This is a low temperature melting point alloy of lead and tin. The joint is heated to the correct temperature by soldering iron. For most electronic work miniature mains powered soldering irons are used. These consist of a handle onto which is mounted the heating element. On the end of the heating element is what is known as the "bit", so called because it is the bit that heats the joint up. Solder melts at around 190 degrees Centigrade, and the bit reaches a temperature of over 250 degrees Centigrade. This temperature is plenty hot enough to inflict a nasty burn, consequently care should be taken.

It is also easy to burn through the PVC insulation on the soldering iron lead if you were to lay the hot bit on it. It is prudent, therefore, to use a specially designed soldering iron stand. These usually incorporate a sponge for keeping the bit clean.

Soldering irons come with various ratings from 15W to over 100W. The advantage of a high wattage iron is that heat can flow quickly into a joint, so it can be rapidly made. This is important when soldering connectors as often there is a quite a large volume of metal to be heated. A smaller iron would take a longer time to heat the joint up to the correct temperature, during which time there is a danger of the insulation becoming damaged. A small iron is used to make joints with small electronic components which are easily damaged by excess heat.

Always use a good quality multicore solder. A standard 60% tin, 40% lead alloy solder with cores of non-corrosive flux will be found easiest to use. The flux contained in the longitudinal cores of multicore solder is a chemical designed to clean the surfaces to be joined of deposited oxides, and to exclude air during the soldering process, which would otherwise prevent these metals coming together. Consequently, don't expect to be able to complete a joint by using the application of the tip of the iron loaded with molten solder alone, as this usually will not work. Having said that, there is a process called tinning where conductors are first coated in fresh, new solder prior to joining by a hot iron. Solder comes in gauges like wire. The two commonest are 18 swg, used for general work, and the thinner 22 swg, used for fine work on printed circuit boards.

Good soldering is a skill that is learnt by practice. The most important point in soldering is that both parts of the joint to be made must be at the same temperature. The solder will flow evenly and make a good electrical and mechanical joint only if both parts of the joint are at an equal high temperature. Even though it appears that there is a metal to metal contact in a joint to be made, very often there exists a film of oxide on the surface that insulates the two parts. For this reason it is no good applying the soldering iron tip to one half of the joint only and expecting this to heat the other half of the joint as well.

When the iron is hot, apply some solder to the flattened working end at the end of the bit, and wipe it on a piece of damp cloth or sponge so that the solder forms a thin film on the bit. This is tinning the bit.

Melt a little more solder on to the tip of the soldering iron, and put the tip so it contacts both parts of the joint. It is the molten solder on the tip of the iron that allows the heat to flow quickly from the iron into both parts of the joint. If the iron has the right amount of solder on it and is positioned correctly, then the two parts to be joined will reach the solder's melting temperature in a couple of seconds. Now apply the end of the solder to the point where both parts of the joint and the soldering iron are all touching one another. The solder will melt immediately and flow around all the parts that are at, or over, the melting part temperature. After a few seconds remove the iron from the joint. Make sure that no parts of the joint move after the soldering iron is removed until the solder is completely hard. This can take quite a few seconds with large joints. If the joint is disturbed during this cooling period it may become seriously weakened.

The hard cold solder on a properly made joint should have a smooth shiny appearance and if the wire is pulled it should not pull out of the joint. In a properly made joint the solder will bond the components very strongly indeed, since the process of soldering is similarly to brazing, and to a lesser degree welding, in that the solder actually forms a molecular bond with the surfaces of the joint.

It is important to use the right amount of solder, both on the iron and on the joint. Too little solder on the iron will result in poor heat transfer to the joint, too much and you will suffer from the solder forming strings as the iron is removed, causing splashes and bridges to other contacts. Too little solder applied to the joint will give the joint a half finished appearance: a good bond where the soldering iron has been, and no solder at all on the other part of the joint.

Brazing

Brazing

Brazing is a joining process whereby a filler metal or alloy is heated to melting temperature above 450 °C (840 °F)—or, by the traditional definition in the United States, above 800 °F (427 °C)—and distributed between two or more close-fitting parts by capillary action. At its liquid temperature, the molten filler metal and flux interacts with a thin layer of the base metal, cooling to form a strong, sealed joint. By definition the melting temperature of the braze alloy is lower (sometimes substantially) than the melting temperature of the materials being joined. The brazed joint becomes a sandwich of different layers, each metallurgically linked to the adjacent layers.

Brazing joins two pieces of base metal when a melted metallic filler flows across the joint and cools to form a solid bond. Similar to soldering, brazing creates an extremely strong joint, usually stronger than the base metal pieces themselves, without melting or deforming the components. Two different metals, or base metals such as silver and bronze, are perfect for brazing. Use this method to make a bond that is invisible, resilient in a wide range of temperatures, and can withstand jolting and twisting motion.


The process of brazing is the same as soldering, although metals and temperatures differ. You can braze pipes, rods, flat metals, or any other shape as long as the pieces fit neatly against each other without large gaps. Brazing handles more unusual configurations with linear joints, whereas most welding makes spot welds on simpler shapes.

First, you must clean the entire area to be joined or else the melted braze mixture will clump instead of flow, making an inconsistent joint. Wash the surface and then apply melted flux. Flux removes oxides, prevents more oxidation during brazing, and smoothes the surface so that braze "flows" evenly across the joint.

Next, you gather your torch and braze alloy. The torch uses fuels like acetylene and hydrogen to create an extremely high temperature, often between 800° F and 2000° F (430 - 1100° C). The temperature must be low enough that the base metals don't melt, yet high enough to melt the braze. Torches have sensitively controls to reach the proper temperature depending on the associated melting points.

Finally, you complete the joint by applying the braze. Braze, like solder, comes in a stick, disc, or wire, depending on your preference or the shape of the joint. After the base metals near the joint have been heated with the torch, bring the wire to the hot pieces so the braze melts, flowing around the joint. By "flow," brazers mean it penetrates the joint, working into every cavern. If the brazing was performed correctly, when the bond cools and solidifies, it is nearly unbreakable.

Brazing offers many advantages over spot welding or soldering. For instance, a brazed joint is smooth and complete, creating an airtight and watertight bond for piping that can be easily plated so the seam disappears. It also conducts electricity like the base alloys. Only brazing can join dissimilar metals, such as bronze, steel, aluminum, wrought iron, and copper, with different melting points.

Common brazements are about 1⁄3 as strong as the parent materials due either to the inherent lower yield strength of the braze alloy or to the low fracture toughness of intermetallic components. To create high-strength brazes, a brazement can be annealed to homogenize the grain structure and composition (by diffusion) with that of the parent material . On the other hand, brazed joints in automotive sheet metal are considerably stronger than the surrounding native sheet steel.

Common techniques

Furnace brazing

The furnace brazing method is accomplished by assembling the material to be brazed and the filler metal in the appropriate configurations and then placing the assembly in a furnace where it is heated uniformly.

Furnace brazing is practical when the brazing material can be in contact with the joint, and the part can survive uniform heating. This process is generally used for applications that need high volume production. When it is an applicable process, it offers the benefits of a controlled heat cycle, no post braze cleaning, and no skilled labor needed. The type of furnace used depends on whether batch or continuous operation is desired and can be designed to have a protective atmosphere to eliminate the need of protective flux in the filler metal. The type of atmosphere depends on the filler metal and the material being brazed. Common atmospheres used include hydrogen based and vacuum. In a hydrogen atmosphere, the gas cleans braze components and eliminates the need for flux. It is often mixed with inert gasses such as nitrogen, argon, or helium to lower the overall percentage of hydrogen in the furnace atmosphere. When a vacuum furnace is used, heat treating processes can be combined with the brazing process. Vacuum furnaces typically require a larger capital investment but also produce products of typically higher quality.

Silver brazing

If silver alloy is used, brazing can be referred to as 'silver brazing'. These silver alloys consist of many different percentages of silver and other compounds such as copper, zinc and cadmium. Colloquially, the inaccurate terms "silver soldering" or "hard soldering" are used, to distinguish from the process of low temperature soldering that is done with solder having a melting point below 450 °C (842 °F), or, as traditionally defined in the United States, having a melting point below 800 °F (427 °C). Silver brazing is similar to soldering but higher temperatures are used and the filler metal has a significantly different composition and higher melting point than solder. Silver brazing requires a gap not greater than a couple hundred micrometres or a few mils for proper capillary action during joining of parts. (Soldering also uses capillary action to fill small spaces, although the need for small gap distances may be less critical than in brazing.) This often requires parts to be silver brazed to be machined to close tolerances.

Brazing is widely used in the tool industry to fasten hardmetal (carbide, ceramics, cermet, and similar) tips to tools such as saw blades. “Pretinning” is often done: the braze alloy is melted onto the hardmetal tip, which is placed next to the steel and remelted. Pretinning gets around the problem that hardmetals are hard to wet.

Brazed hardmetal joints are typically two thousandths to seven thousandths of an inch thick. The braze alloy joins the materials and compensates for the difference in their expansion rates. In addition it provides a cushion between the hard carbide tip and the hard steel which softens impact and prevents tip loss and damage, much as the suspension on a vehicle helps prevent damage to both the tires and the vehicle. Finally the braze alloy joins the other two materials to create a composite structure, much as layers of wood and glue create plywood.

The standard for braze joint strength in many industries is a joint that is stronger than either base material, so that when under stress, one or other of the base materials fails before the joint.

One special silver brazing method is called Pinbrazing or Pin Brazing. It has been developed especially for connecting cables to railway track or for cathodic protection installations.

The method uses a silver and flux containing brazing pin which is melted down in the eye of a cable lug. The equipments are normally powered from batteries.

Braze welding

In another similar usage, brazing is the use of a bronze or brass filler rod coated with flux together with an oxyacetylene torch to join pieces of steel. The American Welding Society prefers to use the term braze welding for this process, as capillary attraction is not involved, unlike the prior silver brazing example. Braze welding takes place at the melting temperature of the filler (e.g., 870 °C to 980 °C or 1600 °F to 1800 °F for bronze alloys) which is often considerably lower than the melting point of the base material (e.g., 1600 °C (2900 °F) for mild steel).
In Braze Welding or Fillet Brazing, a bead of filler material reinforces the joint. A braze-welded tee joint is shown here.

Braze welding has many advantages over fusion welding. It allows you to join dissimilar metals, to minimize heat distortion, and to reduce extensive pre- heating. Another side effect of braze welding is the elimination of stored-up stresses that are often present in fusion welding. This is extremely important in the repair of large castings. The disadvantages are the loss of strength when subjected to high temperatures and the inability to withstand high stresses.

The equipment needed for braze welding is basically identical to the equipment used in brazing. Since braze welding usually requires more heat than brazing, an oxyacetylene or oxy-mapp torch is recommended.

‘Braze welding’ is also used to mean the joining of plated parts to another material. Carbide, cermet and ceramic tips are plated and then joined to steel to make tipped band saws. The plating acts as a braze alloy.

Cast iron "welding"

The "welding" of cast iron is usually a brazing operation, with a filler rod made chiefly of nickel being used although true welding with cast iron rods is also available.

Vacuum brazing

Vacuum brazing is a materials joining technique that offers significant advantages: extremely clean, superior, flux-free braze joints of high integrity and strength. The process can be expensive because it must be performed inside a vacuum chamber vessel. Temperature uniformity is maintained on the work piece when heating in a vacuum, greatly reducing residual stresses due to slow heating and cooling cycles. This, in turn, can significantly improve the thermal and mechanical properties of the material, thus providing unique heat treatment capabilities. One such capability is heat-treating or age-hardening the workpiece while performing a metal-joining process, all in a single furnace thermal cycle.

Vacuum brazing is often conducted in a furnace; this means that several joints can be made at once because the whole workpiece reaches the brazing temperature. The heat is transferred using radiation, as many other methods cannot be used in a vacuum.

Flux

In most cases, flux is required to prevent oxides from forming while the metal is heated and also helps to spread out the metal that is used to seal the joint. The most common fluxes for bronze brazing are borax-based. The flux can be applied in a number of ways. It can be applied as a paste with a brush directly to the parts to be brazed. Commercial pastes can be purchased or made up from powder combined with water (or in some cases, alcohol). Brazing pastes are also commercially available, combining filler metal powder, flux powder, and a non-reacting vehicle binder. Alternatively, brazing rods can be heated and then dipped into dry flux powder to coat them in flux. Brazing rods can also be purchased with a coating of flux, or a flux core. In either case, the flux flows into the joint when the rod is applied to the heated joint. Using a special torch head, special flux powders can be blown onto the workpiece using the torch flame itself. Excess flux should be removed when the joint is completed. Flux left in the joint can lead to corrosion. During the brazing process, flux may char and adhere to the work piece. Often this is removed by quenching the still-hot workpiece in water (to loosen the flux scale), followed by wire brushing the remainder.

The flux chars and adheres to the workpiece when it is used up and / or overheated. Warm flux can be extremely tenacious. Once the flux has cooled to room temperature it is much easier to remove. The goal is to use enough flux and a proper heating cycle so that the flux is not all used up.

The flux does not interact with the materials being brazed but serves as a barrier and oxygen interceptor. It often has some cleaning properties including the ability to remove oxides but should not be counted on for this.

When hot quenching, the materials are in effect heat treated. Quenching will change material properties.

Many types of brazing flux contain toxic chemicals, sometimes very toxic. Silver brazing flux often contains Cadmium, which can cause very fast onset of metal fume fever (within minutes in extreme cases), especially if brazing fumes are inhaled due to inadequate ventilation. Due care must be taken with these materials to protect persons working, and also the environment.

Strength and joint geometry

Brazing is different from welding, where higher temperatures are used, the base material melts, and the filler material (if used at all) has the same composition as the base material. Given two joints with the same geometry, brazed joints are generally not as strong as welded joints although a properly designed and executed brazed joint can be stronger than the parent metal. Careful matching of joint geometry to the forces acting on the joint and properly maintained clearance between two mating parts can lead to very strong brazed joints. The butt joint is the weakest geometry for tensile forces. The lap joint is much stronger, as it resists through shearing action rather than tensile pull and its surface area is much larger. To get braze joints roughly equivalent in strength to a weld a general rule of thumb is to make the overlap equal to 3 times the thickness of the pieces of metal being joined.

Filler materials

A variety of alloys of metals, including silver, tin, zinc, copper and others are used as filler for brazing processes. There are specific brazing alloys and fluxes recommended, depending on which metals are to be joined. Metals such as aluminum can be brazed, although aluminum requires more skill and special fluxes. It conducts heat much better than steel and is more prone to oxidation. Some metals, such as titanium, cannot be brazed because they are insoluble with other metals, or have an oxide layer that forms too quickly at high temperatures.

However Titanium can be prepared to be successfully brazed if the tendency for oxidation is allowed for. If the material is deoxidized and protected by plating, vacuum or other means then you have a chemically active surface that can make for very strong joints. This is not true with unprepared Titanium and the braze joint is a chemical join that is not dependent on the metal solubility.

Brazing filler material is commonly available as flux-coated rods, very similar to stick-welding electrodes. Typical sizes are 3 mm (0.12 in) diameter. Some widely available filler materials are:

* Nickel-Silver: Usually with blue flux coating. 600 MPa (87,000 psi) tensile strength, 680 °C (1,256 °F) - 950 °C (1,740 °F) working temperature. Used for carbon and alloy steels and most metals not including aluminum.
* Bronze: Available with white borax flux coating. 420 MPa (61,000 psi) tensile strength. 870 °C (1,600 °F) working temperature. Used for copper, steel, galvanized metal, and other metals not including aluminum.
* Brass: Uncoated plain brass brazing rod is often used, but requires the use of some type of additional flux.
* Copper Material will be workable at around 2,000 °F (1,090 °C). This has a stronger bond than some brazes.
* Gold Material will be workable at 1,800 °F (980 °C). This will also be corrosion and oxidation resistant.
* Silver Material will be workable at 1,300 °F (704 °C). This can also be mixed with Lithium to be self fluxing.

As a general rule, the braze should have a 50 °F (10 °C) to 100 °F (38 °C) space to be workable.

Flux coating colours are manufacturer specific and do not indicate specific alloy types.

Advantages

Although there is a popular belief that brazing is an inferior substitute for welding, it has advantages over welding in many situations. For example, brazing brass has a strength and hardness near that of mild steel and is much more corrosion-resistant. In some applications, brazing is highly preferred. For example, silver brazing is the customary method of joining high-reliability, controlled-strength corrosion-resistant piping such as a nuclear submarine's seawater coolant pipes. Silver brazed parts can also be precisely machined after joining, to hide the presence of the joint to all but the most discerning observers, whereas it is nearly impossible to machine welds having any residual slag present and still hide joints.

* The lower temperature of brazing and brass-welding is less likely to distort the work piece, significantly change the crystalline structure (create a heat affected zone) or induce thermal stresses. For example, when large iron castings crack, it is almost always impractical to repair them with welding. In order to weld cast-iron without recracking it from thermal stress, the work piece must be hot-soaked to 870 °C (1,600 °F). When a large (more than 50 kg/110 lb) casting cracks in an industrial setting, heat-soaking it for welding is almost always impractical. Often the casting only needs to be watertight, or take mild mechanical stress. Brazing is the preferred repair method in these cases.
* The lower temperature associated with brazing vs. welding can increase joining speed and reduce fuel gas consumption.
* Brazing can be easier for beginners to learn than welding.
* For thin workpieces (e.g., sheet metal or thin-walled pipe) brazing is less likely to result in burn-through.
* Brazing can also be a cheap and effective technique for mass production. Components can be assembled with preformed plugs of filler material positioned at joints and then heated in a furnace or passed through heating stations on an assembly line. The heated filler then flows into the joints by capillary action.
* Braze-welded joints generally have smooth attractive beads that do not require additional grinding or finishing. The most common filler materials are gold in colour, but fillers that more closely match the color of the base materials can be used if appearance is important.

Processes

* Pinbrazing
* Block Brazing
* Diffusion Brazing
* Dip Brazing
* Exothermic Brazing
* Flow Brazing
* Furnace Brazing
* Induction Brazing
* Infrared Brazing
* Resistance Brazing
* Torch Brazing
* Twin Carbon Arc Brazing
* Vacuum Brazing

Alternatives to brazing include the use of a connector that does not require heat similar to Lokring connectors used by most of the auto makers and larger appliance manufacturers.

design of welding joints

design of welding joints

Welds are made at the junction of the various pieces that make up the weldment. The junctions of parts, or joints, are defined as the location where two or more nembers are to be joined. Parts being joined to produce the weldment may be in the form of rolled plate, sheet, shapes, pipes, castings, forgings, or billets.

a. B, Butt Joint. A joint between two members lying approximately in the same plane.

b. C, Corner Joint. A joint between two members located approximately at right angles to each other in the form of an angle.

c. E, Edge Joint. A joint between the edges of two or more parallel or mainly parallel members.

d. L, Lap Joint. A joint between two overlapping members.

e. T, Tee Joint. A joint between two members located approximately at right angles to each other in the form of a T.

WELD JOINTS

In order to produce weldments , it is necessary to combine the joint types with weld types to produce weld joints for joining the separate members. Each weld type cannot always be combined with each joint type to make a weld joint.


WELD JOINT DESIGN AND PREPARATION

a. Purpose. Weld joints are designed to transfer the stresses between the members of the joint and throughout the weldment. Forces and loads are introduced at different points and are transmitted to different areas throughout the weldment. The type of loading and service of the weldment have a great bearing on the joint design required.

b. Categories. All weld joints can be classified into two basic categories: full penetration joints and partial penetration joints.

(1) A full penetration joint has weld metal throughout the entire cross section of the weld joint.

(2) A partial penetration joint has an unfused area and the weld does not completely penetrate the joint. The rating of the joint is based on the percentage of weld metal depth to the total joint; i. e., a 50 percent partial penetration joint would have weld metal halfway through the joint.

NOTE

When joints are subjected to dynamic loading, reversing loads, and impact leads, the weld joint must be very efficient. This is more important if the weldment is sub jetted to cold-temperature service. Such services require full-penetration welds. Designs that increase stresses by the use of partial-penetration joints are not acceptable for this type of service.

c. Strength. The strength of weld joints depends not only on the size of the weld, but also on the strength of the weld metal.

(1) Mild and low alloy steels are generally stronger than the materials being joined.

(2) When welding high-alloy or heat-treated materials, special precautions must be taken to ensure the welding heat does not cancel the heat treatment of the base metal, causing it to revert to its lower strength adjacent to the weld.

d. Design. The weld joint must be designed so that its cross-sectional area is the minimum possible. The cross-sectional area is a measurement of the amount or weight of weld metal that must be used to make the joint. Joints may be prepared by shearing, thermal cutting, or machining.

(1) Carbon and low alloy joint design and preparation. These weld joints are prepared either by flame cutting or mechanically by machining or grinding, depending on the joint details. Before welding, the joint surfaces must be cleared of all foreign materials such as paint, dirt, scale, or must. Suitable solvents or light grinding can be used for cleaning. The joint surface should not be nicked or gouged since nicks and gouges may interfere with the welding operation.

CAUTION

Aluminum and aluminum alloys should not be cleaned with caustic soda or strong cleaner with a pH above 10. The aluminum or aluminum alloy will react chemically with these types of cleaners. Other nonferrous metals and alloys should be investigated prior to using these cleaners to determine their reactivity.

(2) Aluminum and aluminum alloy joint design and preparation. Weld joint designs often unintentionally require welds that cannot be made. Check your design to avoid these and similar errors. Before welding, the joint surfaces must be cleared of all foreign materials such as paint, dirt, scale, or oxide; solvent cleaning, light grinding, or etching can be used. The joint surfaces should not be nicked or gouged since nicks and gouges may interfere with welding operations.

(3) Stainless steel alloy joint design and preparation. These weld joints are prepared either by plasma arc cutting or by machining or grinding, depending on the alloy. Before welding, the joint surfaces must be cleaned of all foreign material, such as paint, dirt, scale, or oxides. Cleaning may be done with suitable solvents (e. g., acetone or alcohol) or light grinding. Care should be taken to avoid nicking or gouging the joint surface since such flaws can interfere with the welding operation.

WELD ACCESSIBILITY

The weld joint must be accessible to the welder using the process that is employed. Weld joints are often designed for welds that cannot be made.

Welding

Welding

Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material (the weld pool) that cools to become a strong joint, with pressure sometimes used in conjunction with heat, or by itself, to produce the weld. This is in contrast with soldering and brazing, which involve melting a lower-melting-point material between the workpieces to form a bond between them, without melting the workpieces.

Many different energy sources can be used for welding, including a gas flame, an electric arc, a laser, an electron beam, friction, and ultrasound. While often an industrial process, welding can be done in many different environments, including open air, under water and in outer space. Regardless of location, however, welding remains dangerous, and precautions must be taken to avoid burns, electric shock, eye damage, poisonous fumes, and overexposure to ultraviolet light.

Until the end of the 19th century, the only welding process was forge welding, which blacksmiths had used for centuries to join metals by heating and pounding them. Arc welding and oxyfuel welding were among the first processes to develop late in the century, and resistance welding followed soon after. Welding technology advanced quickly during the early 20th century as World War I and World War II drove the demand for reliable and inexpensive joining methods. Following the wars, several modern welding techniques were developed, including manual methods like shielded metal arc welding, now one of the most popular welding methods, as well as semi-automatic and automatic processes such as gas metal arc welding, submerged arc welding, flux-cored arc welding and electroslag welding. Developments continued with the invention of laser beam welding and electron beam welding in the latter half of the century. Today, the science continues to advance. Robot welding is becoming more commonplace in industrial settings, and researchers continue to develop new welding methods and gain greater understanding of weld quality and properties.

Processes

Arc

These processes use a welding power supply to create and maintain an electric arc between an electrode and the base material to melt metals at the welding point. They can use either direct (DC) or alternating (AC) current, and consumable or non-consumable electrodes. The welding region is sometimes protected by some type of inert or semi-inert gas, known as a shielding gas, and filler material is sometimes used as well.

Power supplies

To supply the electrical energy necessary for arc welding processes, a number of different power supplies can be used. The most common welding power supplies are constant current power supplies and constant voltage power supplies. In arc welding, the length of the arc is directly related to the voltage, and the amount of heat input is related to the current. Constant current power supplies are most often used for manual welding processes such as gas tungsten arc welding and shielded metal arc welding, because they maintain a relatively constant current even as the voltage varies. This is important because in manual welding, it can be difficult to hold the electrode perfectly steady, and as a result, the arc length and thus voltage tend to fluctuate. Constant voltage power supplies hold the voltage constant and vary the current, and as a result, are most often used for automated welding processes such as gas metal arc welding, flux cored arc welding, and submerged arc welding. In these processes, arc length is kept constant, since any fluctuation in the distance between the wire and the base material is quickly rectified by a large change in current. For example, if the wire and the base material get too close, the current will rapidly increase, which in turn causes the heat to increase and the tip of the wire to melt, returning it to its original separation distance.


The type of current used in arc welding also plays an important role in welding. Consumable electrode processes such as shielded metal arc welding and gas metal arc welding generally use direct current, but the electrode can be charged either positively or negatively. In welding, the positively charged anode will have a greater heat concentration, and as a result, changing the polarity of the electrode has an impact on weld properties. If the electrode is positively charged, the base metal will be hotter, increasing weld penetration and welding speed. Alternatively, a negatively charged electrode results in more shallow welds. Nonconsumable electrode processes, such as gas tungsten arc welding, can use either type of direct current, as well as alternating current. However, with direct current, because the electrode only creates the arc and does not provide filler material, a positively charged electrode causes shallow welds, while a negatively charged electrode makes deeper welds. Alternating current rapidly moves between these two, resulting in medium-penetration welds. One disadvantage of AC, the fact that the arc must be re-ignited after every zero crossing, has been addressed with the invention of special power units that produce a square wave pattern instead of the normal sine wave, making rapid zero crossings possible and minimizing the effects of the problem.

Processes

One of the most common types of arc welding is shielded metal arc welding (SMAW), which is also known as manual metal arc welding (MMA) or stick welding. Electric current is used to strike an arc between the base material and consumable electrode rod, which is made of steel and is covered with a flux that protects the weld area from oxidation and contamination by producing CO2 gas during the welding process. The electrode core itself acts as filler material, making a separate filler unnecessary.

The process is versatile and can be performed with relatively inexpensive equipment, making it well suited to shop jobs and field work. An operator can become reasonably proficient with a modest amount of training and can achieve mastery with experience. Weld times are rather slow, since the consumable electrodes must be frequently replaced and because slag, the residue from the flux, must be chipped away after welding. Furthermore, the process is generally limited to welding ferrous materials, though special electrodes have made possible the welding of cast iron, nickel, aluminium, copper, and other metals. Inexperienced operators may find it difficult to make good out-of-position welds with this process.

Gas metal arc welding (GMAW), also known as metal inert gas or MIG welding, is a semi-automatic or automatic process that uses a continuous wire feed as an electrode and an inert or semi-inert gas mixture to protect the weld from contamination. As with SMAW, reasonable operator proficiency can be achieved with modest training. Since the electrode is continuous, welding speeds are greater for GMAW than for SMAW. Also, the smaller arc size compared to the shielded metal arc welding process makes it easier to make out-of-position welds (e.g., overhead joints, as would be welded underneath a structure).

The equipment required to perform the GMAW process is more complex and expensive than that required for SMAW, and requires a more complex setup procedure. Therefore, GMAW is less portable and versatile, and due to the use of a separate shielding gas, is not particularly suitable for outdoor work. However, owing to the higher average rate at which welds can be completed, GMAW is well suited to production welding. The process can be applied to a wide variety of metals, both ferrous and non-ferrous.

A related process, flux-cored arc welding (FCAW), uses similar equipment but uses wire consisting of a steel electrode surrounding a powder fill material. This cored wire is more expensive than the standard solid wire and can generate fumes and/or slag, but it permits even higher welding speed and greater metal penetration.

Gas tungsten arc welding (GTAW), or tungsten inert gas (TIG) welding (also sometimes erroneously referred to as heliarc welding), is a manual welding process that uses a nonconsumable tungsten electrode, an inert or semi-inert gas mixture, and a separate filler material. Especially useful for welding thin materials, this method is characterized by a stable arc and high quality welds, but it requires significant operator skill and can only be accomplished at relatively low speeds.

GTAW can be used on nearly all weldable metals, though it is most often applied to stainless steel and light metals. It is often used when quality welds are extremely important, such as in bicycle, aircraft and naval applications. A related process, plasma arc welding, also uses a tungsten electrode but uses plasma gas to make the arc. The arc is more concentrated than the GTAW arc, making transverse control more critical and thus generally restricting the technique to a mechanized process. Because of its stable current, the method can be used on a wider range of material thicknesses than can the GTAW process, and furthermore, it is much faster. It can be applied to all of the same materials as GTAW except magnesium, and automated welding of stainless steel is one important application of the process. A variation of the process is plasma cutting, an efficient steel cutting process.

Submerged arc welding (SAW) is a high-productivity welding method in which the arc is struck beneath a covering layer of flux. This increases arc quality, since contaminants in the atmosphere are blocked by the flux. The slag that forms on the weld generally comes off by itself, and combined with the use of a continuous wire feed, the weld deposition rate is high. Working conditions are much improved over other arc welding processes, since the flux hides the arc and almost no smoke is produced. The process is commonly used in industry, especially for large products and in the manufacture of welded pressure vessels. Other arc welding processes include atomic hydrogen welding, carbon arc welding, electroslag welding, electrogas welding, and stud arc welding.

Gas

The most common gas welding process is oxyfuel welding, also known as oxyacetylene welding. It is one of the oldest and most versatile welding processes, but in recent years it has become less popular in industrial applications. It is still widely used for welding pipes and tubes, as well as repair work. It is also frequently well-suited, and favored, for fabricating some types of metal-based artwork. Oxyfuel equipment is versatile, lending itself not only to some sorts of iron or steel welding but also to brazing, braze-welding, metal heating (for bending and forming), and also oxyfuel cutting.

The equipment is relatively inexpensive and simple, generally employing the combustion of acetylene in oxygen to produce a welding flame temperature of about 3100 °C. The flame, since it is less concentrated than an electric arc, causes slower weld cooling, which can lead to greater residual stresses and weld distortion, though it eases the welding of high alloy steels. A similar process, generally called oxyfuel cutting, is used to cut metals. Other gas welding methods, such as air acetylene welding, oxygen hydrogen welding, and pressure gas welding are quite similar, generally differing only in the type of gases used. A water torch is sometimes used for precision welding of small items such as jewelry. Gas welding is also used in plastic welding, though the heated substance is air, and the temperatures are much lower.

Resistance

Resistance welding involves the generation of heat by passing current through the resistance caused by the contact between two or more metal surfaces. Small pools of molten metal are formed at the weld area as high current (1000–100,000 A) is passed through the metal. In general, resistance welding methods are efficient and cause little pollution, but their applications are somewhat limited and the equipment cost can be high.
Spot welder

Spot welding is a popular resistance welding method used to join overlapping metal sheets of up to 3 mm thick. Two electrodes are simultaneously used to clamp the metal sheets together and to pass current through the sheets. The advantages of the method include efficient energy use, limited workpiece deformation, high production rates, easy automation, and no required filler materials. Weld strength is significantly lower than with other welding methods, making the process suitable for only certain applications. It is used extensively in the automotive industry—ordinary cars can have several thousand spot welds made by industrial robots. A specialized process, called shot welding, can be used to spot weld stainless steel.

Like spot welding, seam welding relies on two electrodes to apply pressure and current to join metal sheets. However, instead of pointed electrodes, wheel-shaped electrodes roll along and often feed the workpiece, making it possible to make long continuous welds. In the past, this process was used in the manufacture of beverage cans, but now its uses are more limited. Other resistance welding methods include flash welding, projection welding, and upset welding.

Energy beam

Energy beam welding methods, namely laser beam welding and electron beam welding, are relatively new processes that have become quite popular in high production applications. The two processes are quite similar, differing most notably in their source of power. Laser beam welding employs a highly focused laser beam, while electron beam welding is done in a vacuum and uses an electron beam. Both have a very high energy density, making deep weld penetration possible and minimizing the size of the weld area. Both processes are extremely fast, and are easily automated, making them highly productive. The primary disadvantages are their very high equipment costs (though these are decreasing) and a susceptibility to thermal cracking. Developments in this area include laser-hybrid welding, which uses principles from both laser beam welding and arc welding for even better weld properties.

Solid-state

Like the first welding process, forge welding, some modern welding methods do not involve the melting of the materials being joined. One of the most popular, ultrasonic welding, is used to connect thin sheets or wires made of metal or thermoplastic by vibrating them at high frequency and under high pressure. The equipment and methods involved are similar to that of resistance welding, but instead of electric current, vibration provides energy input. Welding metals with this process does not involve melting the materials; instead, the weld is formed by introducing mechanical vibrations horizontally under pressure. When welding plastics, the materials should have similar melting temperatures, and the vibrations are introduced vertically. Ultrasonic welding is commonly used for making electrical connections out of aluminum or copper, and it is also a very common polymer welding process.

Another common process, explosion welding, involves the joining of materials by pushing them together under extremely high pressure. The energy from the impact plasticizes the materials, forming a weld, even though only a limited amount of heat is generated. The process is commonly used for welding dissimilar materials, such as the welding of aluminum with steel in ship hulls or compound plates. Other solid-state welding processes include co-extrusion welding, cold welding, diffusion welding, friction welding (including friction stir welding), high frequency welding, hot pressure welding, induction welding, and roll welding.

Geometry

Welds can be geometrically prepared in many different ways. The five basic types of weld joints are the butt joint, lap joint, corner joint, edge joint, and T-joint. Other variations exist as well—for example, double-V preparation joints are characterized by the two pieces of material each tapering to a single center point at one-half their height. Single-U and double-U preparation joints are also fairly common—instead of having straight edges like the single-V and double-V preparation joints, they are curved, forming the shape of a U. Lap joints are also commonly more than two pieces thick—depending on the process used and the thickness of the material, many pieces can be welded together in a lap joint geometry.

Often, particular joint designs are used exclusively or almost exclusively by certain welding processes. For example, resistance spot welding, laser beam welding, and electron beam welding are most frequently performed on lap joints. However, some welding methods, like shielded metal arc welding, are extremely versatile and can weld virtually any type of joint. Additionally, some processes can be used to make multipass welds, in which one weld is allowed to cool, and then another weld is performed on top of it. This allows for the welding of thick sections arranged in a single-V preparation joint, for example.

After welding, a number of distinct regions can be identified in the weld area. The weld itself is called the fusion zone—more specifically, it is where the filler metal was laid during the welding process. The properties of the fusion zone depend primarily on the filler metal used, and its compatibility with the base materials. It is surrounded by the heat-affected zone, the area that had its microstructure and properties altered by the weld. These properties depend on the base material's behavior when subjected to heat. The metal in this area is often weaker than both the base material and the fusion zone, and is also where residual stresses are found.

Distortion and cracking

Welding methods that involve the melting of metal at the site of the joint necessarily are prone to shrinkage as the heated metal cools. Shrinkage, in turn, can introduce residual stresses and both longitudinal and rotational distortion. Distortion can pose a major problem, since the final product is not the desired shape. To alleviate rotational distortion, the workpieces can be offset, so that the welding results in a correctly shaped piece. Other methods of limiting distortion, such as clamping the workpieces in place, cause the buildup of residual stress in the heat-affected zone of the base material. These stresses can reduce the strength of the base material, and can lead to catastrophic failure through cold cracking, as in the case of several of the Liberty ships. Cold cracking is limited to steels, and is associated with the formation of martensite as the weld cools. The cracking occurs in the heat-affected zone of the base material. To reduce the amount of distortion and residual stresses, the amount of heat input should be limited, and the welding sequence used should not be from one end directly to the other, but rather in segments. The other type of cracking, hot cracking or solidification cracking, can occur with all metals, and happens in the fusion zone of a weld. To diminish the probability of this type of cracking, excess material restraint should be avoided, and a proper filler material should be utilized.

Weldability

The quality of a weld is also dependent on the combination of materials used for the base material and the filler material. Not all metals are suitable for welding, and not all filler metals work well with acceptable base materials.

Steels

The weldability of steels is inversely proportional to a property known as the hardenability of the steel, which measures the probability of forming martensite during welding or heat treatment. The hardenability of steel depends on its chemical composition, with greater quantities of carbon and other alloying elements resulting in a higher hardenability and thus a lower weldability. In order to be able to judge alloys made up of many distinct materials, a measure known as the equivalent carbon content is used to compare the relative weldabilities of different alloys by comparing their properties to a plain carbon steel. The effect on weldability of elements like chromium and vanadium, while not as great as carbon, is more significant than that of copper and nickel, for example. As the equivalent carbon content rises, the weldability of the alloy decreases. The disadvantage to using plain carbon and low-alloy steels is their lower strength—there is a trade-off between material strength and weldability. High strength, low-alloy steels were developed especially for welding applications during the 1970s, and these generally easy to weld materials have good strength, making them ideal for many welding applications.

Stainless steels, because of their high chromium content, tend to behave differently with respect to weldability than other steels. Austenitic grades of stainless steels tend to be the most weldable, but they are especially susceptible to distortion due to their high coefficient of thermal expansion. Some alloys of this type are prone to cracking and reduced corrosion resistance as well. Hot cracking is possible if the amount of ferrite in the weld is not controlled—to alleviate the problem, an electrode is used that deposits a weld metal containing a small amount of ferrite. Other types of stainless steels, such as ferritic and martensitic stainless steels, are not as easily welded, and must often be preheated and welded with special electrodes.

Aluminum

The weldability of aluminum alloys varies significantly, depending on the chemical composition of the alloy used. Aluminum alloys are susceptible to hot cracking, and to combat the problem, welders increase the welding speed to lower the heat input. Preheating reduces the temperature gradient across the weld zone and thus helps reduce hot cracking, but it can reduce the mechanical properties of the base material and should not be used when the base material is restrained. The design of the joint can be changed as well, and a more compatible filler alloy can be selected to decrease the likelihood of hot cracking. Aluminum alloys should also be cleaned prior to welding, with the goal of removing all oxides, oils, and loose particles from the surface to be welded. This is especially important because of an aluminum weld's susceptibility to porosity due to hydrogen and dross due to oxygen.