The preparation of the edges of the joint members, by cutting, cleaning, plating, or other means.
Cleanliness is important in welding. The surfaces of the workpieces and the previously deposited weld metal must be cleaned of dirt, slag, and any other foreign matter that would interfere with welding. To accomplish this, the welder should have a steel wire brush, a hammer, a chisel, and a chipping hammer. These tools are used to remove dirt and rust from the base metal, remove tack welds, and chip slag from the
Edge preparation may be done by any of the thermal cutting methods or by machining. The accuracy of edge preparation is important, especially for machine
or automatic welding. For example, if a joint designed with a 6.4 mm (1/4 in.) root face were actually produced with a root face that tapered from 7.9 to 3.2 mm (5/16 to 1/8 in.) along the length of the joint, the weld might be unacceptable because of lack of penetration at the start and excessive melt-through at the end. In such a case, the capability of the cutting equipment, as well as the skill of the operator, should be checked and corrected.
Preweld and postweld cletaning are part of the welding operation. Preweld cleaning occurs by default in some types of acetylene wellding, where the pre-heating operation of the torch automatically cleans the weld site. In other instances welding and brazing fluxes aid in the cleaning. Gas welding operations rarely require postweld cleaning unless they include a corrosive type of flux, in which case the operation
includes flux removal to prevent weld or base metal corrosion.
Cleaning processes are: usually chemical or mechanical. The condition of the workpieces, the nature of any contamination, the degree of cleanliness required, and the type, shape, size, and thickness of the workpieces to be cleaned determine the choice between mechanical or chemical cleaning.
Chemical Cleaners
A chemical bath provides uniform cleaning. This uniformity is necessary, for example, to produce consistent welds in resistance welding operations. Certain
chemical cleaners require accurate timing, and the operator’s ability to control the exposure time of the material in the bath is critical to achieving a high degree of uniformity. The cleaning solution will be ineffective if the exposure time of the workpiece is insufficient. If left in the bath too long, the chemical may react with the base metal and cause a high-resistance film or other undesirable chemical reaction.
Chemical cleaning processes also present a safety hazard and require precautionary measures to prevent injury to workers.
Chemical cleaning solutions are usually one of four types: alkaline, solvent, petroleum spirits, and emulsifiable.
Alkaline Cleaners. The alkaline cleaner is probably the most popular because it will emulsify greases and oils, and because of its low cost. Since alkaline cleaners are sprayable at high pressure, the mechanical action of the spray assists in removing solid particles and dirt. Most alkaline cleaners are not caustics and therefore are less hazardous to the worker. Alkaline cleaners are effective in almost all metal cleaning applications, although they may cause corrosion in various nonferrous alloys, especially aluminum, brass, and zinc.
Solvent Cleaners. These cleaners are commonly used in resistance welding operations. In this application, the workpieces are soaked in a tank of solvent so that the cleaning penetrates rolled edges, pockets, and seams. Water-based solvents are not easily rinsed
from edges, pockets and seams. Solvent cleaners are ideal for cleaning nonferrous metal particles in applications where water and steam might allow corrosion or contamination.
Petroleum Spirit Cleaners. These cleaning agents primarily remove processing contaminants, and do not provide the chemically cleaned surface required by some finishing operations, such as plating. Petroleum spirit cleaners are highly flammable and present a fire hazard.
Emulsified Cleaners. Emulsified cleaners do not damage or attack the metal surface. This type of cleaner is effective as a spray or in a bath. Exposure time is usually short, sometimes as little as 30 seconds. After a brief drainage period, a water spray rinse removes contaminants and cleaning solution. An alternative to solvent and alkaline cleaners, emulsified cleaners are not temperature dependent, although a hot water rinse assures more satisfactory results and rapid self-drying.
Mechanical Cleaning
Mechanical cleaning requires skilled operators who must remove undesirable surface coatings and particles without roughening the surface of the material or
causing other undesirable surface conditions. Mechanical cleaning is effective for both resistance and arc welding applications. A wire brush or abrasive wheel is the most common mechanical cleaner.
The major advantage of mechanical cleaning is that it requires cleaning at the weld site only; chemical cleaning involves the entire surface of the workpiece.