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Brazing - a guide to best practice

Section 4. Brazing processes


Introduction

Heat must be applied to the joint to raise the temperature of the filler metal and joint surfaces above the melting point of the filler. The joint surfaces also need to be heated otherwise the filler metal will be incapable of wetting. Where possible, the parent materials should be heated primarily which will then heat the filler metals by conduction, thus facilitating wetting.

There are two main routes to heating: local heating, where only the parts of the component in the near vicinity of the joint are heated; or, diffuse heating, where the entire assembly is raised in temperature. The primary braze processes are shown in Table 1 which illustrates which use local and which use diffuse heating techniques.

Table 1: Primary Brazing Processes

Local Heating Diffuse Heating
Torch Furnace
Induction Dip
Resistance Infrared

The various types of brazing process are outlined below.

Torch brazing

In torch brazing, the heat required to melt and flow the filler metal is supplied by a fuel gas flame. The fuel gas can be acetylene, hydrogen, or propane and is combined with oxygen or air to form a flame (Fig.1).

Torch brazing is most commonly used for repairs, one-off brazing jobs and short production runs. The process is highly flexible and can be carried out in the factory, or on location which makes it the most widely used form of brazing. Figure 2 shows an example of kettle elements which have been silver brazed.

Torch brazing (Courtesy Solbraze) Brazed electrical elements (courtesy Strix and Solbraze)

Fig. 1. Multi-jet mechanised torch brazing

Fig. 2. Torch brazed electrical elements

The braze filler metal can be in the form of wire, paste, or rod, and brazing is limited to use with a flux, or a self-fluxing filler metal. The fast heating and cooling achieved with this type of process diminishes the erosion of the substrate surfaces and therefore restricts the formation of undesirable phases, while rapid cooling ensures a fine grain size of the solidified filler and thereby superior mechanical properties.

However, rapid overheating of both base and filler metals should be avoided because rapid diffusion and 'drop through' of the base metal may result.

This process is readily automated and requires low capital investment. It is also flexible with regards to the applications and materials on which it can be used. Amongst its limitations are the requirements to post-braze clean to remove heat scale and flux residues, and the limitation in brazing temperature to below 1000C, due to the temperature constraints on the torch itself. When performed as a manual operation, torch brazing is labour intensive; however, automation through an on-line conveyor belt or turntable assembly can further increase the commercial viability of this process.

Injector assembly (Courtesy Solbraze)

Fig. 3. Copper brazed injector assembly


Most base metal systems can be brazed with a torch, the exceptions being reactive metals such as Ti and Zr since no compatible flux is available. Certain low-carbon, or stabilised stainless steels can also be torch brazed. Figure 3 shows an injector assembly which has been copper brazed.

The most commonly used filler metals are based on AG, CP and CZ. (Please note that 'AG', for example, refers to a group of Ag-based alloys and is a classification used in BS 1845. See Section 3 of this best practice guide for more information.) The AG filler metals, although higher in cost than the CZ based metals, can be used because of their low brazing temperature (600-700C) and rapid heating under a torch. This high cost can also be offset by automation, lower energy costs, ease of operator training and higher production rates. The AG fillers are most commonly used on ferrous and non-ferrous metals.

CP fillers are used primarily for copper and copper alloys, although they have also found limited use on silicon, tungsten and molybdenum. They should not be used on ferrous or nickel-based alloys due to the formation of brittle intermetallics.

CZ fillers have a higher brazing temperature (870-980C) and can be used for both ferrous and non-ferrous materials, although care must be taken not to overheat the filler thus vaporising the zinc. Torch brazing (Courtesy Solbraze)

Fig. 4. Mechanised torch brazing with braze metal being fed in


An example of an automated torch brazing operation is given in Fig.4. This shows three gas jets heating a flux coated steel member. A wire braze filler is ready on the left hand side to feed filler material into the joint once the assembly has reached brazing temperature.
Different flame settings

Fig. 5. Different flame settings for torch brazing


Figure 5 shows the different types of gas flames used during brazing:

top: neutral flame, used for maximum temperatures; (inner cone is bluish-white, with no acetylene feather)
centre: oxidising flame, not recommended for brazing, since more oxygen is present than actually required for combustion, hence oxidation of materials is likely; (sharp bluish-white inner cone, one fifth shorter than cone of neutral flame)
bottom: carburising flame, recommended for brazing (inner cone is surrounded by acetylene feather, which is white with a feathery edge)

Manual torch brazing

Advantages
- simple apparatus
- ease of operation
- moderately rapid heating
- flexible with regards to size, shape, etc., to be brazed
- minimum capital cost
- minimum maintenance

Disadvantages
- high labour costs
- training required
- low production rate
- health and safety difficult to control

Mechanical torch brazing

Advantages
- moderate equipment cost
- simple maintenance
- high production output
- flexible with regards to size, shape, etc. to be brazed
- suitable for continuous or indexing machines

Disadvantages
- heat input less rapid than induction
- local extraction of fumes not easy
- unsuitable for temperatures >1000C
- complex assemblies suffer from greater distortion than in a furnace
- more noise and heat dissipation than furnace heating

Figure 6 shows an automatic torch brazing cycle.

Automatic torch brazing cycle

Fig. 6. Automatic torch brazing cycle



Furnace brazing

The popularity of furnace brazing stems from the comparatively low cost of equipment, adaptability of the furnace and minimal jigging required. This process offers two prime advantages: protective atmosphere brazing (where high purity gases or vacuum substitute for flux) and the ability to control accurately every stage of the heating and cooling cycles. Heating is either through elements, or by gas firing. Furnaces can vary in size from 0.5 cubic metres to several cubic metres.

Advantages
- minimal distortion
- simultaneous brazing of multiple joints
- good process control

Disadvantages
- all component is heated (which can take a long time if there is a large volume of material)
- slow heating rates

There are four basic types of furnace used for brazing:

  • batch, with air or controlled atmosphere
  • continuous, with air or controlled atmosphere
  • retort, with controlled atmosphere
  • vacuum

Batch-type

The brazements are placed on the hearth and either coated with flux (if an air environment is to be used) or left fluxless in a controlled atmosphere. A positive pressure of gas flows into the chamber to flush the brazing zone and help eliminate base metal oxidation. The heat is generated either by electricity or burning of organic based materials, Fig.7.

Advantages
- usually fluxless - no post-cleaning
- components need not be as clean as for vacuum
- larger components can be processed than in continuous

Disadvantages
- longer cycle time
- limited retort life at high temperatures
- energy inefficient
Schematic of retort furnaces Schematic of continuous belt furnaces

Fig. 7. Schematic of retort furnaces

Fig. 8. Schematic of continuous belt furnaces

Continuous-type

The need to process faster than batch furnaces were capable of led to the development of semi-continuous or continuous furnaces (Fig. 8). The most common type is the conveyor with a mesh belt or roller hearth. Brazements pass through at least three zones into which a positive pressure of controlled atmosphere gas is continuously introduced. The first zone is for preheating, the second for brazing and the third for cooling.

Success with this furnace set-up is dependent upon the mass of parts, speed of the conveyor belt and the set temperature for the required braze. It is advisable to process a few assemblies in pre-determined settings and then make any necessary adjustments to speed and temperature.

Advantages
- high production throughputs
- usually fluxless - no post-cleaning
- components need not be as clean as for vacuum
- used with low cost filler

Disadvantages
- high capital cost (but less than vacuum)
- not practical for switching on and off

Retort-type

The development of higher temperature braze processes, along with the improvements to purification of hydrogen resulted in the production of this type of furnace, with an inner container (retort) made of a heat resistant alloy. The retort is sealed from outside air and products of combustion to avoid contamination of the purified hydrogen atmosphere.

Using this equipment, there are three main disadvantages: 1) the danger of explosive mixtures of hydrogen and air, 2) the cost of hydrogen and expense of indirect heating and cooling, 3) the slow cooling of stainless steel work-loads was not compatible with the physical and metallurgical properties of the base materials.

The first two disadvantages can be alleviated by using relatively inexpensive and inert nitrogen gas during purging and cooling cycles although care must be taken during the switch over to hydrogen.

Hot wall vacuum-type

A high vacuum (>10-4 bar) is created in the furnace, such that heating and brazing can take place. Back-filling with argon or nitrogen can be used to increase the cooling rate of the work load. The maximum temperature is approximately 1150C, although use of a double wall (retort-type) can extend this upper temperature limit.

The primary disadvantages of this process are the use of indirect heating and static gas cooling of the work-load. Figure 9 shows a laboratory size hot wall vacuum, furnace, Fig.10 displays the Mo heating elements and shows the furnace insulation around the main hot zone.

Lab scale vacuum furnace Molybdenum heating elements

Fig. 9. Laboratory scale vacuum furnace

Fig. 10. Molybdenum heating elements for small scale vacuum furnace


Schematic of full scale vacuum furnace

Fig. 11. Schematic of full scale vacuum furnace

Cold wall vacuum-type

These are the most popular furnaces of the high technology era and are likely to remain the most versatile type of furnace for brazing. They are usually horizontal with side loading or vertical with top or bottom loading (Fig.11). Cold wall is a term to describe the double-wall construction and water cooling of the vessel. Heating is achieved by electrical elements which encircle the work-piece.

Unloading work from a horizontal vacuum furnace and immediately loading another batch not only provides semi-continuous operation, but also reduces open-door time and excessive contamination from room atmosphere. Vertical cold wall furnaces are ideal for large brazements to assure more uniform heating and cooling.

These types of furnace require high capital investment, however this is overshadowed by the versatility, safety and quality of the equipment.

The most commonly employed fillers for furnace brazing come from the groups AU, AG, CU, HTN.

AG can be used (with caution) to braze low carbon and stainless steels. It is important to avoid carbide precipitation or interfacial corrosion. The former attributed to the brazing temperature range and the latter due to the employment of flux.

CU can be used for carbon, low alloy and stainless steels. It is the most economical of the filler materials and comes in a wide variety of forms.

Vacuum brazed pressure transducers (Courtesy TransInstruments)

Fig. 12. Vacuum brazed pressure transducers


HTN filler metals can be used for stainless steels and corrosion and heat resistant materials, and type HTN2 is the best choice for protective atmosphere brazing for a wide variety of base materials. Figure 12 shows two pressure transducer assemblies which have been nickel brazed under vacuum.

HTN6 does not contain boron and thus finds use in nuclear assemblies. Nickel fillers are primarily selected for corrosion and heat resistant applications. Also, since they are better at filling larger gaps, they also replace CU for assemblies of carbon and low alloy, high strength steels whenever fit-up is a problem. However, due to the higher brazing temperatures required, grain growth will occur and components should be normalised to re-establish the desired metallurgical properties.

AU alloys are successful on stainless steels and corrosion and heat resistant alloys. AU5 is commonly used as a replacement for HTN in the fabrication of jet engine components due to minimal interaction and erosion of the parent metal by the filler. The AU5 alloy is compatible with most base metals, its only disadvantage being its high cost.

Advantages
- high alloy steels, Ni (with Ti and Al) and reactive metals can be brazed
- flux-free - no post-cleaning
- precise heating cycles and control

Disadvantages
- very high capital cost
- components need to be clean
- heating and cooling rates may be slower
- essential that volatile metals excluded from furnace charge

Schematic of induction coil designs

Fig. 13. Schematic of induction coil designs


Induction brazing

High frequency induction heating for brazing is clean and rapid, giving close control of temperature and location of heat. The heat for induction brazing is created by a rapidly alternating current which is induced into the workpiece by an adjacent coil. The coils, which are water cooled, are designed for individual parts and their heating efficiency relies on establishment of the best coil design and power frequency for each application. Figure 13 gives typical coil designs for heating work-pieces.

Induction brazing of an automotive component (Courtesy Solbraze)

Fig. 14. Induction brazing of an automotive component


Induction is suited to carbon and alloy steels, stainless steel, cast iron, cemented carbides, copper and copper alloys, nickel, cobalt, heat resistant alloys, titanium, zirconium and molybdenum alloys and also ceramics. Silver based filler metals are used extensively, particularly AG1, AG2 and AG9. Other silver and copper filler alloys are used occasionally.

Figure 14 shows an automotive component being inductively heated. The heat induced in the base of the sample heats the flux and filler metal to form a brazed joint.

Figures 15-18 show the production of laser flash lamp assemblies via induction brazing.

Flash lamps in jig (Courtesy Heraeus Noblelight) width= Jig loaded in induction brazing equipment (Courtesy Heraeus Noblelight) width=

Fig. 15. Flash lamps placed in jig ready for induction brazing

Fig. 16. Jig loaded in induction brazing equipment

Brazing of four flash lamps (Courtesy Heraeus Noblelight) width= Flash lamps after full assembly (Courtesy Heraeus Noblelight) width=

Fig. 17. Simultaneous brazing of four flash lamps

Fig. 18. Flash lamps after full assembly

Advantages
  • selective heating
- by only heating part of the assembly, metallurgical changes and distortion are minimised
  • precise heat control
- uniform joints are produced with minimum consumption of braze alloy
  • rapid heating
- normal induction heating cycles generally permit heating in air, while minimising discolouration and avoiding scale
  • adaptability for production
- this method is extremely amenable to production line assembly
  • fixture life and simplicity
- the use of induction generally reduces and simplifies holding fixtures as well as prolonging fixture life and ease of alignment
- with good equipment design, more than one assembly may be brazed at a time

Disadvantages
  • complex assemblies
- the design of inductors can make it possible to heat geometrically difficult joint areas but assemblies involving several brazed joints may be so difficult to fixture that furnace brazing is preferred
  • fit between parts
- induction brazing requires that the fit (tolerance) between brazements is good and that the interface is free from substantial burrs. This is due to the requirement for the braze filler metal to be pre-placed
  • cost of equipment
- the initial cost of setting up such equipment can be considerable. It may not be viable for a small number of parts, or when another process is equally suitable
  • specialised knowledge
- optimum system operation requires proper selection of a generator, parts-handling equipment and coil design for the heat patterns

Dip brazing

Dip brazing is divided into two techniques: immersing the parts to be brazed into a molten filler metal, or dipping the part into a molten salt. In both cases the bath temperature is below the solidification point of the parent metal, but above the melting point of the filler metal.

Molten metal bath method

The parts to be brazed are held together and immersed in a bath of molten bonding metal which flows into the joints by capillary action once the parts reach a temperature approaching that of the bath.

This method is usually limited to brazing of small assemblies, such as wire connections or metal strips. A crucible, usually made of graphite, is heated externally to the required temperature to maintain the brazing filler metal in fluid form. A cover of flux is maintained over the molten filler metal.

Batch dip brazing

Fig. 19. Molten metal batch dip brazing


The size of the molten bath (crucible) and the heating method must be such that immersion of parts in the bath will not lower the bath temperature below brazing temperature. Parts should be clean and protected with flux prior to their introduction into the bath. The ends of the supporting wires or parts must be held firmly together when they are removed from the bath until the brazing filler metal has fully solidified. Figure 19 shows the set-up of this brazing operation.

Jigging to maintain alignment is generally necessary. Because of the difficulties of heating and containing metals at high temperatures, alloys which require a brazing temperature above 1000C are rarely used. The choice of brazing filler metal is therefore restricted to brasses and silver-based alloys. However an important exception to this rule is the dip brazing of aluminium.

Advantages
- process is tolerant of a wide range of joint gaps
- good integrity joint
- rapid heat transfer

Disadvantages
- compositional drift of bath contents
- need for frequent flux replenishment
- need to preheat components
- surfaces of component are coated with braze filler metal which is wasteful

Molten chemical (flux) bath method

This brazing method requires either a metal or ceramic container for the flux and a method of heating the flux to the brazing temperature. Heat may be applied externally with a torch or internally with an electrical resistance heating unit. A third method involves electrical resistance heating of the flux itself; in that case, the flux must be initially melted by external heating.

Molten flux dip brazing

Fig. 20. Molten flux dip brazing


Suitable controls are provided to maintain the flux within the brazing temperature range. The size of the bath must be such that immersion of parts for brazing will not cool the flux below the brazing temperature. Figure 20 shows the typical set-up for this brazing process.

In the molten flux method, the brazing filler metal is located in or near the joints and is heated to the required temperature by immersion in a bath of flux. Salt bath (or flux) brazing has a greater scope than any other single brazing process; it can be used on as wide a range of parent metals as torch brazing but is not subject to the same maximum temperature limitations.

It is, unfortunately, an inflexible process. The type of salt used for a particular application depends on the ease with which the parent metal surface oxides can be removed and on the temperature required for brazing.

Parts should be cleaned, assembled, and preferably held in jigs prior to immersion into the bath. Brazing filler metal is pre-placed as rings, washers, slugs, paste or as a cladding on the base metal. Preheat may be necessary to ensure dryness of the parts and to prevent the flux from freezing on the parts thus causing selective flux melting and braze flowing.

The molten flux method generally causes less distortion than torch brazing; however, it may require complex tooling and is therefore best suited to medium or high production runs. The process is extremely well suited for small and medium sized parts with multiple hidden joints.

Filler metal, in the form of wire, washers, foil, powder or paste should be placed in close proximity with the joint. If filler is placed external to the joint, it should only be placed on one side and in sufficient quantity to produce visible fillets at the edges of the joint's faying surfaces. All binders and flux compounds used must not leave any form of residue.

Advantages
- precise and even heating of complex assemblies
- simultaneous heat treatment may be carried out

Disadvantages
- careful pre-heating required
- strict control of flux bath composition
- requires provision of flux drainage and access
- post-braze flux removal required
- parts close to their melting temperature are prone to distort

Resistance brazing

Resistance brazing is a process in which heat is generated from resistance to an electrical current flowing in a circuit which includes the workpieces. The process is most applicable to relatively simple joints in metals which have high electrical conductively.

In general, the heating current (normally ac), is passed through the joint itself. The joint becomes part of the electrical circuit and brazing heat is generated by resistance at the joint. The pressure required to establish electrical contact is normally applied through electrodes. Portable resistance brazing equipment is shown in Fig.21.

Portable resistance brazing equipment (Courtesy Solbraze) Clamping of brazed assembly parts (Courtesy Solbraze)

Fig. 21. Portable resistance brazing equipment

Fig. 22. Clamping of brazed assembly until parts are cool

The brazing filler metal (wire, shims, washers, powder, paste) is pre-placed or fed into the joint area. Fluxing is done with due attention to the conductivity of the flux material. After brazing, the pressure of the two electrodes clamping on the brazement is maintained until the joint has solidified, Fig.22.

Resistance brazing is most useful for low-volume parts where heating must be: localised, flameless, non-contaminating, rapid and closely controlled. It is economic for joining of a large number of small bonds, since it does not require heating up of the full component. The localised heating reduces distortion and excessive heating of delicate components, it is also extremely rapid.

Resistance brazing (Courtesy Solbraze)

Fig. 23. Resistance brazing of non-uniform cross-sectional thicknesses


However, it is not useful for : non-conducting workpieces, large workpieces, non-uniform cross-sections and fragile components, Fig.23.

This process is applicable to many alloys, but is most commonly used for high electrical conductivity metals such as silver and copper. High conductivity metals heat slowly due to their low electrical resistance and hence the electrodes used are required to have high resistance. The flow of electrical current develops the necessary temperature in the electrodes which in turn heat the base metals by conduction.

When resistance brazing low conductivity alloys, high conductivity electrodes should be used so that the resistance of the alloys generates sufficient internal heat to melt the braze filler metal.

Since the process is normally carried out in air, excessive oxidation can occur. To minimise this, brazing should be carried out at the lowest practical temperature and hence low melting point filler metals are preferred. For this reason, nickel based fillers are rarely used. Most frequently used are: AG1, AG2, AG7, CP1, CP6 whose brazing solidus/liquidus temperatures are in the range 605-925C.

The forms of the brazing filler metal should be selected in order to optimise the brazing process. For example, large flat areas should be brazed using strips or preformed shims. If the workpiece is irregularly shaped, then a paste or powder may be more appropriate.

Advantages
- extremely rapid localised heating
- low running costs
- lower capital costs than induction
- closely controlled, reproducible heating

Disadvantages
- higher running costs than manual torch
- severe limitations on shape and size
- high wear of electrodes
- potentially poor repeatability if flux used
- not suitable for high temperature parent materials

Other brazing processes

Brazing processes described above are the most commonly found industrial techniques; however alternative processes have also been investigated. Some of these are described below.

Schematic of infrared brazing assembly

Fig. 24. Schematic of infrared brazing assembly


Infrared brazing

Infrared brazing may be considered a form of furnace brazing with heat being supplied by long-wave light radiation (typically from high intensity quartz lamps capable of delivering up to 5kW of radiant energy), with concentrating reflectors focusing the radiation onto the parts. The process is generally not as fast as induction, but the equipment is less expensive. This process is shown in Fig.24.

Advantages
- can be used in situ
- heat can be focused
- large areas can be brazed
- heating rates are quicker than furnace, but slower than induction
- simple temperature control and measurement

Disadvantages
- normally custom built
- requires a controlled atmosphere
- lamp terminals require cooling
- delicate equipment

Joint designs for braze welding

Fig. 25. Joint designs for braze welding


Braze welding

Unlike standard brazing, the filler metal in braze welding does not feed into the joint by capillary action. Bonding is obtained by wetting and is often accompanied by some degree of diffusion with the base metals. Stringent fit-up is not critical because the filler is deposited into grooves and spaces. Figure 25 shows typical joint designs for braze welding.

This process is normally used for joining steels and grey cast irons, but is also applicable for copper, nickel and nickel alloys. Weldability of the braze weld joint can be facilitated by pre-coating (buttering) one of the faces. For example, when joining a copper-based alloy to nickel, the copper base metal is usually buttered with a nickel filler metal.

In braze welding, the filler metal may be melted by an oxyfuel gas flume or the TIG and MIG arc processes. With oxyfuel gas heating, brass filler metals can be employed but these are unsuitable for the arc processes which use other copper alloys such as Cu-Al or Cu-Mn-Si.

Diffusion brazing

The process forms a liquid braze metal by diffusion between dissimilar base metals or between base metal and filler metal pre-placed at the faying surfaces. The filler metal diffuses with the base metal to the extent that the joint properties approach those of the base metal.

Pressure may, or may not be applied. The desired diffusion can be obtained and controlled by holding the brazement at a temperature near the liquidus of the filler metal for an extended time.

Electron beam brazing

This process is generally performed under high vacuum, (>10-4 bar) whereby the beam is defocused to a large spot size to avoid melting of the base metal. Since brazing is performed under vacuum, no flux is required, although the filler metal must be selected such that there is little or no vaporisation during brazing. The process is particularly suitable for small assemblies and where components require an internal vacuum.

Advantages
- localised heat source
- can permeate hard-to-reach locations
- low residual stress ceramic-metal joints are possible

Disadvantages
- high capital and running costs (higher than vacuum brazing)
- not always reproducible
- components need to be under vacuum
- complex sample manipulators are required to keep the sample in the path of the electron beam

Exothermic brazing

Exothermic brazing uses an exothermic compound as a heat source to melt the braze filler metal. These processes may use a highly exothermic material, such as compounds based on zirconium, aluminium and oxides of chromium and iron. When ignited, this material heats both the base and filler metals to a temperature where the filler will flow into the joint area.

The exothermic material may also contain the braze filler as part of its composition. The process uses only localised heat (thereby reducing distortion) is fast (20-30 seconds) and the equipment is inexpensive and portable. However experience of the process is required to use it effectively, safety is a major consideration and choice of filler metal is limited.

Laser brazing

This process is only used for specialised applications, where the thermal energy is created by laser beams to make localised brazed joints on thin-wall critical components. Its major advantage is the ability to heat a precise localised area, rather than the entire component. Fluxes may be required dependent on whether it is feasible to provide atmospheric protection in the form of argon, or vacuum. The process is expensive, but highly applicable in some niche markets.

Advantages
- localised heat source
- suitable for sealing joints where uniform heating of entire joint is impractical

Disadvantages
- high capital cost (lower than electron beam brazing)
- cannot be used for extensive capillary flow

Schematic of microwave applicator

Fig. 26. Schematic of microwave applicator for brazing


Microwave brazing

The technique is being expanded to include ceramics for high-temperature, corrosion-resistant applications.

The technique uses a single-mode cavity whereby the iris (Fig.26) controls the percentage of microwaves reflected into the cavity and the plunger adjusts the frequency. Together, they focus microwaves on to the joint. The major advantage is that the entire part does not require heating, only the interface.

It is faster than conventional heating and joints can be made in a fraction of the time and hence at reduced cost. One disadvantage is the current lack of commercially available microwave equipment. At present, most applicators are produced only for research and development purposes.


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