Processes Related to Welding

Processes related to welding are: Adhesive bonding, Arc cutting, Oxygen cutting, Other thermal cutting processes, Thermal spraying. These process are grouped are described in brief, with related letter designations, principles and application guidelines.

These process groups are shown by the Table, along with each process name and letter designations. Each process group will be briefly described.

Group Allied Process Letter Designation
Adhesive bonding Dextrin cements AB-D
Solvent or rubber cements AB-RC
Synthetic resins AB-SR
Expoxys AB-E
Arc cutting (thermal) Air carbon arc cutting AAC
Carbon arc cutting CAC
Gas tungsten arc cutting GTAC
Metal arc cutting MAC
Plasma arc cutting PAC
Oxygen cutting (thermal) Chemical flux cutting FOC
Metal powder cutting POC
Oxygen arc cutting AOC
Oxy Fuel gas cutting OFC
Oxygen lance cutting LOG
Other thermal cutting processes Electron beam cutting EEC
Laser beam cutting LBC
Thermal spraying Electric arc spraying EASP
Flame spraying FLSP
Plasma spraying PSP

Adhesive bonding

Adhesive bonding (AB) is a joining process in which an adhesive is placed between the faying surfaces which solidifies to produce an adhesive bond. The adhesive bond is the attractive force, generally physical in character, between an adhesive and the base materials.

The two principle interactions that contribute to the adhesion are the van der Waals bond and the diepole bond. The van der Waals bond is defined as a secondary bond arising from the fluctuating-diepole nature of an atom with all occupied electron shells filled. The diepole bond is a pair of equal and opposite forces that hold two atoms together and results from a decrease in energy as two atoms are brought closer to one another.

Adhesive bonding of metal-to-metal applications accounts for less than 2% of the total metal joining requirements. The bonding of metals to nonmetals, especially plastics, is very important and is the major use of adhesive bonding.

Dextrins belong to the family of starch-derived adhesives ranging in color from white to dark brown and are normally fluid filmy materials. These are glues and pastes used to bond porous materials. They are spread in a thin film.

Rubber cements or solvent cements are adhesives that contain organic solvents rather than water. They are based on nitro cellulose or polyvinyl acetate, normally elastomeric products, dispersed in solvent. They are free flowing, thin set materials that dry to hard tack free films. They are used in pressure-sensitive labeling operations and in contact bonding for the woodworking industry.

Synthetic resins are composed of synthetic organic materials and are relatively expensive. They are used when a high-quality bond is required and they are relatively heat and moisture resistant. They can be applied by automatic or semiautomatic equipment, are used for sealing cartons and for wood, and for vinyl film laminations. One of the major groups is the hot melts which are combinations of waxes and resins that form a bond by applying heat and then cooling.

Epoxy Adhesives are the newest of the adhesives and can be used to bond metal-to-metal, metal to plastics, and plastics to plastics. They are a family of materials characterized by reactive epoxy chemical groups on the ends of resin molecules. They consist of two components, a liquid resin and the hardener to convert the liquid resins to solid. They may contain other modifiers to produce specific properties for special applications. Some epoxies will bond to concrete. One of the newer advances is the oily metal epoxy that bonds directly to oily metals "as received" with normal protective films on them. The oily coating need not be removed. They achieve intimate molecular contact with the surface to be bonded and will achieve high adhesion on almost any surface. Epoxies are the most expensive of the adhesives; however, they offer more advantages.

Arc cutting

These processes utilize heat and thus differ from mechanical cutting processes such as sawing, shearing, blanking, etc.

The arc cutting processes are a group of thermal cutting processes which melt the metals to be cut with the heat of an arc between an electrode and the base metal. Within this group is air carbon arc cutting; carbon arc cutting; gas tungsten arc cutting, shielded metal arc, gas metal arc, and plasma arc cutting. Each will be briefly described.

The thermal cutting processes can be applied by means of manual, semiautomatic, machine, or automatic methods in the same manner as the arc welding processes. Air Carbon Arc Cutting(AAC) is "an arc cutting process in which metals to be cut are melted by the heat of a carbon arc and the molten metal is removed by a blast of air."

Principle of operation is the following: a high velocity air jet traveling parallel to the carbon electrode strikes the molten metal puddle just behind the arc and blows the molten metal out of the immediate area. It shows the arc between the carbon electrode and the work and the air stream parallel to the electrode coming from the special electrode holder.

The process is not recommended for weld preparation for stainless steel, titanium, zirconium, and other similar metals without subsequent cleaning.

Carbon Arc Cutting (CAC) is "an arc cutting process in which metals are severed by melting them with the heat of an arc between a carbon electrode and the base metal."

The process is identical to air carbon arc cutting except that the air blast is not employed. The process depends strictly upon the heat input of the carbon arc to cause the metal to melt. The molten metal falls away by gravity to produce the cut. The process is relatively slow, a very ragged cut results and it is used only when other cutting equipment is not available. It has little industrial significance.

Metal Arc Cutting (MAC) is "an arc cutting process which severs metals by melting them with the heat of an arc between a metal electrode and the base metal." When covered electrodes are used it is known as shielded metal arc cutting (SMAC).

The equipment required is identical to that required for shielded metal arc welding. When the heat input into the base metal exceeds the heat losses the molten metal pool becomes large and unmanageable. If the base metal is not too thick, the molten metal will fall away and create a hole or cut. The cut produced by the shielded metal arc cutting process is rough and is not normally used for preparing parts for welding. The metal arc cutting process using covered electrodes is used only where a small cutting job is required and other means are not available for the purpose.

Gas Tungsten Arc Cutting (GTAC) is "an arc cutting process in which metals are severed by melting them with an arc between a single tungsten (nonconsumable) electrode and the work. Shielding is obtained from a gas or gas mixture."

This process has largely been supplanted by plasma arc cutting and is of little industrial significance except for the small jobs when other equipment is not available.

Plasma Arc Cutting (PAC) is an arc cutting process which severs metal by melting a localized area with a constricted arc and removing the molten material with a high-velocity jet of hot ionized gas.

There are three major variations: (1) low-current plasma cutting which is a rather recent development, (2) the original relatively high current plasma cutting, and (3) plasma cutting with water added. The low-current plasma variation is gaining in popularity because it can be manually applied.

The principle operation of plasma cutting is almost identical with the keyhole mode of plasma welding. The difference is that the cut is maintained and the keyhole is not allowed to close as in the case of welding. Heat input at the plasma arc is so high and the heat losses cannot carry the heat away quickly enough so that the metal is melted and a hole is formed. The plasma gas at a high velocity helps cut through the metal.

The secondary gas can also assist the jet in removing molten metal and limits the formation of drops at the cutting edge. Plasma cutting is ideal for gouging and for piercing. For some operations air is used as the plasma gas. A higher arc voltage is normally used for cutting than for welding.

The plasma arc cutting process can be used to cut metals underwater.

Oxygen cutting

Oxygen Cutting (OC) is a group of thermal cutting processes used to sever or remove metals by means of the chemical reaction of oxygen with the base metal at elevated temperatures. In the case of oxidation-resistant metals the reaction is facilitated by the use of a chemical flux or metal powder. Five basic processes are involved: (1) oxy fuel gas cutting, (2) metal powder cutting, (3) chemical flux cutting, (4) oxygen lance cutting, and (5) oxygen arc cutting. Each of these processes is different and will be described.

Oxy Fuel Gas Cutting (OFC) is used to sever metals by means of the chemical reaction of oxygen with the base metal at elevated temperatures. The necessary temperature is maintained by means of gas flames obtained from the combustion of a fuel gas and oxygen.

Metal Powder Cutting (POO) is an oxygen-cutting process which severs metals through the use of powder, such as iron, to facilitate cutting. This process is used for cutting cast iron, chrome nickel stainless steels, and some high-alloy steels.

Chemical Flux Cutting (FOC) is an oxygen-cutting process in which metals are severed using a chemical flux to facilitate cutting and powdered chemicals are utilized in the same way as iron powder is used in the metal powder cutting process. This process is sometimes called flux injection cutting.

Oxygen Lance Cutting (LOC) is an oxygen-cutting process used to sever metals with oxygen supplied through a consumable tube. The preheat is obtained by other means. This is sometimes called oxygen lancing. The oxygen lance is a length of pipe or tubing used to carry oxygen to the point of cutting.

Oxygen Arc Cutting (AOC) is an oxygen-cutting process used to sever metals by means of the chemical reaction of oxygen with the base metal at elevated temperatures. The necessary temperature is maintained by means of an arc between a consumable tubular electrode and the base metal.

Other thermal cutting processes

Electron Beam Cutting (EBC) is a thermal cutting process which uses the heat obtained from a concentrated beam composed of high-velocity electrons which impinge upon the work piece to be cut. The difference between electron beam welding and cutting is the heat input-to-heat output relationship.

The electron beam generates heat in the base metal, which vaporizes the metal and allows it to penetrate deeper until the depth of the penetration, based on the power input, is achieved. In welding the electron beam actually produces a hole, known as a keyhole. The metal flows around the keyhole and fills in behind. In the case of cutting the heat input is increased so that the keyhole is not closed.

Laser Beam Cutting (LBC) is a thermal cutting process which severs materials with the heat obtained in the application of a concentrated coherent light beam impinging on the workpiece to be cut. The process can be used without an externally supplied gas.

Thermal spraying

Thermal spraying (THSP) is a group of allied processes in which finely divided metallic or nonmetallic materials are deposited in a molten or semi-molten condition to form a coating. The coating material may be in the form of powder, ceramic rod, or wire.

There are three separate processes within this group: electric arc spraying, flame spraying, and plasma spraying. These three processes differ considerably, since each uses a different source of heat. The apparatus is different and their capabilities are different.

The selection of the spraying process depends on the properties desired of the coating. Thermal spraying is utilized to provide surface coatings of different characteristics, such as coatings to reduce abrasive wear, cavitation, or erosion. The coating may be either hard or soft. It may be used to provide high temperature protection. Thermal sprayed coatings improve atmosphere and water corrosion resistance. One of the major uses is to provide coatings resistance to salt water atmospheres. Another use is to restore dimensions to worn parts.

July, 2002
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