- SCOPE
- REFERENCES
- BRAZING AND BRAZE WELDING
- BRAZING BY GAS AND ARC PROCESSES
- RESISTANCE SPOT WELDING
- INERT-GAS ARC SPOT WELDING
- PROCESS SELECTION
- SAFETY
TABLE
- Typical Brazing Characteristics
- Guide to Selection of Joining Processes for Sheet Metals
- Comparison of Sheet Metal Gages
Brazing and Braze Welding Sheet Metal
1. Scope
This Article describes the joining of sheet metal by brazing and braze welding. It outlines certain criteria for each of these processes, and provides a guide for selection of processes for various metals. All brazing and braze welding shall be in accordance with AWS D 1.3.
2. References
Reference is made in this standard to the following documents.
American Welding Society (AWS)
D 1.3 Structural Welding Code – Sheet Steel
3. Brazing and Braze Welding
3.1 The American Welding Society defines brazing as ‘A group of welding processes wherein coalescence is produced by heating to suitable temperatures above 449 °C (840 °F) and by using a nonferrous filler metal having a melting point below that of the base metal’. The filler metal is drawn into the joint by capillary action. This process is called silver brazing, silver soldering, and hard soldering when the solder is based upon silver and copper. Since other filler metals are used (see Table I), ‘brazing’ is a more precise term. Several methods of applying heat commonly are employed, for example with a gas or carbon-arc torch, a furnace, induction coils, resistance heating, and by dipping the parts in the molten brazing alloy. The strength of a brazed joint depends upon the surface area of the joint and the clearance between the parts to be joined. The strongest joints are usually those in which the clearance is less than 0.13 mm (0.005 in); therefore, excessive clearance between component parts shall be avoided. A properly designed and executed brazed joint has a strength comparable to that of the base metal. Brazing often is the most practical method of joining dissimilar metals.
3.2 The broad classes of brazing alloys and their general applications are listed in Table I.
3.3 Special fluxes are generally required for brazing. Refer to the SES W14-F01 series of specifications for the type of flux to be used to braze various base metals. In some instances where cleanliness is paramount, as in brazing with copper-phosphorus alloys and in furnace brazing certain metals in an inert or reducing atmosphere or in a vacuum, brazed joints are produced without flux or a very minimum amount added at the joint. Often this raises the required brazing temperature.
3.4 To obtain a satisfactory brazed joint, clean the work surfaces of all oil, grease, rust, or foreign material. Even the thin film of grease left by fingerprints is sufficient to prevent a satisfactory bond. Abrasive blasting is a good method of removing adherent deposits, however, metal grit similar in composition to the base metal is recommended. Ceramic grit or sand gets embedded in the surface and interferes with the wetting and flow of the braze metal.
3.5 Align parts carefully and securely support for brazing. Arrange supports so that expansion and contraction is not restricted. Pinning, riveting, or tack-welding are methods used for holding parts in alignment.
3.6 If welding and brazing are to be performed on the same assembly, the welding shall be done first and the brazing afterward, because the temperature of brazing is lower than that of welding. Welding after brazing may remelt the braze or cause liquid metal cracking of the sheet metal. Likewise, a complicated assembly requiring sequential brazing operations shall be brazed using a high melting braze alloy for the initial braze and progressively lower melting alloys for subsequent brazes.
3.7 Braze welding differs from brazing in that tight joint tolerances are not essential, and capillary action is not relied on to draw the brazing filler metal into the joint. Braze welding is welding with a nonferrous filler metal that melts below the melting temperature of the base metal. The most common filler metal is brass (60 percent copper, 40 percent zinc) with additions of silicon, nickel, or other elements. The joint is produced by a sweating action of the brazing alloy on the basemetal surface. The strength of braze-welded joints is usually low, approximately the same as the strength of the filler metal employed. An oxyacetylene torch is used most commonly to heat the base metal and melt the filler metal, and a suitable braze welding flux is required.
4. Brazing by Gas and Arc Processes
4.1 Gas and arc welding processes are used to produce joints having properties comparable to those of the base metal. Welding is generally employed in preference to soldering or brazing where considerations of strength, ductility, corrosion resistance, and elevated temperature strength are important.
4.2 In general, welding filler metal for sheet-metal joints shall have approximately the same analysis as the base metal. However, satisfactory welded joints are produced between various combinations of dissimilar metals, using the appropriate filler metal. See SES W14-F01.
4.3 In gas welding, coalescence at the joint is produced by heating with a gas flame, with or without the addition of filler metal. The oxyacetylene flame produces a temperature of 3200 °C (5792 °F) which is sufficient to melt the metals most commonly welded. Flanged joints in sheet metals can be welded by fusing together the joint edges without adding filler metal. Where it is necessary to add filler metal to produce the weld, the process consists of heating an area of the work and the end of the filler rod to the melting temperature, and then adding metal from the filler rod to the molten puddle on the work. Fuel gas welding (FGW) is a relatively slow process, and the heat input into the work is high. Consequently, distortion is frequently a problem in welding sheet metal. Sheet metal thicknesses of 18 gage and thicker are readily welded by this method. Thinner material may also be welded but it requires a much higher degree of welding skill. This process is quite flexible and equipment costs are relatively low.
4.4 In shielded metal arc welding (SMAW), coalescence at the joint is produced by heating with an arc established between the work and a straight length of flux-coated electrode, which is consumed in the arc. When the arc is established, a molten puddle is formed at the joint, and molten metal from the electrode is projected into this puddle. The flux coating on the rod produces a protective atmosphere for the transfer of metal from the electrode, and also produces a slag covering over the weld puddle which protects it from atmospheric contamination. Several varieties of coated electrodes are available, and choice of electrode depends upon the desired chemical analysis and properties of the deposit, the type of welding power supply, the joint design, and the position in which welding is to be performed. Higher welding speeds are more possible with SMAW than with FGW, and the heat input is lower, thus decreasing the tendency for distortion. The SMAW process is relatively flexible; however, high skill is required to produce consistently good-quality welds, particularly in sheet thinner than 16 gage.
4.5 The gas tungsten arc welding (GTAW) process produces heat for welding by means of an arc between the work and a nonconsumable tungsten or tungsten-alloy electrode in a surrounding envelope of inert (argon or helium) gas. The inert gas protects the electrode and the weld puddle from contamination by atmospheric oxygen and nitrogen. Depending upon the joint design, welds may be made with or without the addition of filler metal. Where the addition of filler metal is necessary, bare filler rods are used, and the technique of adding filler metal is similar to that employed in FGW. The inert-gas shield eliminates the need for a flux. GTAW is performed on practically all metals and alloys, and it is an ideal process for sheet metal welding. This process generally affords a lower heat input to the work than does FGW, resulting in less distortion in sheet metal weld. Sheet-metal thicknesses of 18 gage and thicker are readily welded by this method. Thinner material may also be welded, but requires a much higher degree of welding skill.
4.5.1 Use of the GTAW process is growing because of the high quality of welds which can be produced. Use of silicon-bronze filler rods produces economical results when joining galvanized steel, and requires no flux. Removal of galvanizing at the joint is not necessary with that filler. The arc flame shall be directed towards the puddle so that the puddle progresses more by a brazing action than by fusion. This process is called braze welding.
4.5.2 Silicon-bronze filler wire is also used when joining aluminized steel. While direct current straight polarity electric power (DCSP) is usually satisfactory for this application, alternating current high frequency power (ACHF) has been found to produce superior results, particularly where the aluminized sheet has been exposed to the elements for some time, or cannot be cleaned to bright metal.
4.6 Gas metal arc welding (GMAW), in the short circuiting transfer mode, wherein welding heat is produced by an arc between the workpiece and a consumable bare wire fed from a spool or reel through the torch (‘gun’), can be used with excellent results on sheet metal as shown in Table II. Rates of speed are considerably higher than with other manual methods, resulting in economy as well as greatly decreased heat input and consequent distortion. Metals of 16 gage can be readily welded. Accurate fitting and tacking is required, and tacks shall be made with either the GTAW or GMAW method. Metals as light as 18 and 20 gage have been successfully welded but, as thickness decreases below 16 gage, the degree of welder skill required for good results is considerably greater than average. This consideration may rule out use of the process in thicknesses under 16 gage for many sheet-metal fabricating shops.
5. Resistance Spot Welding
5.1 Resistance spot welding is a joining process which consists of placing an opposing pair of copper alloy electrodes in contact with two or more overlapping pieces of metal to be welded, applying pressure, and discharging an electrical impulse of relatively low voltage and high amperage between the electrodes. Due to the high electrical resistance at the point of contact between the sheets, the passage of current generates sufficient heat to produce fusion of the sheets at their point of contact. Combined sheet-metal thickness up to 6.4 mm (1/4 in) can be welded readily in this manner. Spot welding is rapid and economical, and produces almost flush surfaces without distortion. However, the equipment costs are relatively high, and the equipment is not ordinarily portable. The process has rather limited versatility, and it is generally better suited to repetitive production-type operations than to repair or maintenance work.
5.2 Design spot-welded joints primarily to transmit shear or compression rather than tension loads. When assemblies of light-gage sheets contain a long row of spot welds which are subject to tension due to bending under load, insert a rivet at each end of the row and, also, intermittently along the row of spot welds at some extended pitch.
6. Inert-Gas Arc Spot Welding
6.1 There are two types of inert-gas-shielded arc spot welding processes. One is the nonconsumable tungsten electrode process, and the other is the consumable metal wire electrode process. Both processes employ an automatically timed, short-duration arc in an inert atmosphere which produces a fusion-type spot weld between overlapping pieces of metal. The gas tungsten arc spot welding process uses either argon or helium shielding gas. The gas metal arc spot welding process uses argon, helium, or carbon dioxide shielding gas; the latter gas for welding carbon steels. These processes are useful for welding sheet materials to each other, or for welding sheets to heavy thicknesses of plate. Gas tungsten arc spot welding can be applied to low-carbon and alloy steel, stainless steels, copper, and nickel alloys. The maximum thickness of sheet materials which can be attached to another sheet or to thicker plate by this process is 3.2 mm (1/8 in), if the sheets are clean and are in tight contact, or less than 3.2 mm (1/8 in) with poorer fit-up. Gas metal arc spot welding is employed on low-carbon and alloy steels, copper, nickel, and aluminum. Sheet materials and plates up to 9.5 mm (3/8 in) thick are attached to other sheets or plates by this process. Distortion is held to relatively low levels.
6.2 Equipment for inert-gas arc spot welding is moderately portable. Cable and hoses for the tungsten arc spot welding equipment can be longer than those for the metal arc equipment, since the latter process is limited by the length of wire which can be fed from a spool to the welding gun. Equipment for both processes is moderately expensive, and its use is limited to producing spot-weld type joints. However, the process appears to be relatively economical and considerably more versatile than resistance spot welding for maintenance work, and for construction work where it is necessary to bring the welding equipment to the work.
7. Process Selection
A general guide for selection of welding and brazing processes for sheet metals is given in Table II. It shall be recognized that in a table of this sort, it is impossible to include all of the factors involved in selection of a joining process, or to list all of the advantages and limitations of processes applied to specific materials for specific applications. Information about standard gage thicknesses, comparison of manufacturer’s standard gage, and U.S. standard gage are given in Table III.
8. Safety
Safe practices prescribed in SES W02-F01 shall be followed on plant sites.
9. Inspection
The amount and type of inspection required depends upon the service conditions and hazards to personnel and property. It is the responsibility of the originator to define the inspection requirements. Inspection shall be in accordance with applicable code(s).
TABLE I – Typical Brazing Characteristics
TABLE II – Guide to Selection of Joining Processes for Sheet Metals
TABLE III – Comparison of Sheet Metal Gages
