Welding

Welding is the process that allows the physical/chemical union of two joints through the fusion of the same, or through filler metal. This material can be the component material of the parts themselves that are joined, but can also involve foreign material to them, called filler material: in the first case we speak of autogenous welding (with or without filler material depending on the case) in the second case of heterogeneous welding or brazing (in which the fusion affects only the filler material).

It makes a permanent connection that differs from other permanent connections (e.g. nailing or gluing) that do not realize the continuity of the material. With some autogenous welding processes, if performed correctly and according to certain principles, an almost total continuity in the material characteristics of the joined parts is also guaranteed.

In its broadest sense, welding refers to the joining by heat of different materials with each other, or with similar materials, as is commonly done for example in the welding of plastics. Even glass can be “welded”, but the welding par excellence takes place between metals.

Each type of welding is carried out using different procedures and specific machinery. However, it is possible to describe a generic procedure that unites the different welding processes. To make a two-part weld, it is first necessary to prepare the two edges of the joint by what is called chamfering. Then the joint is heated to different temperatures depending on the process used.

Welding can take place by fusion (passage of solid/liquid state) or in the solid state. In the first case it is necessary to reach the melting temperature of the basic materials used; in the second case it is necessary to reach temperatures below the melting temperature and apply pressure between the edges to be joined. The heat required to carry out the process is obtained by various systems:

  • A flame produced by combustion of a gas with air or oxygen.
  • An electric arc that is formed between two electrodes (one of which may be the workpiece itself).
  • Electrical resistance obtained by the Joule effect as a current passes through the workpiece.
  • High-power lasers or other non-flame energy delivery systems.
  • Friction heating of the components by friction.

To obtain a strong, technically good weld free of imperfections, the melting zone must be protected from oxidation and the molten metal must be cleaned of slag. To avoid oxidation, therefore, welding must take place in an atmosphere as free as possible of oxygen (inert): to this end, substances such as gas, borax, silicates, and carbonates must be added to the area near the weld, creating a “protective cloud” near the melting bath and allowing the expulsion of slag. Oxyacetylene welding produces a reducing atmosphere, while arc welding is carried out in the atmosphere produced by combustion of the electrode coating or under a gas flow.

The filler metal can be in the form of rods or continuous wire, which are brought closer to the fusion zone (flame welding and TIG welding) or can be the actual electrode, which melts due to the electric arc it causes.

Manual arc welding uses the electric arc between the electrode and the pieces to be welded as a heat source. When the electrode is brought close to the workpiece, until the electrical current is allowed to flow, its end becomes very hot, causing ionization of the surrounding air and allowing current to flow, creating an electric arc, even when the electrode is brought some distance from the workpiece. Due to the temperature of the arc (greater than 4000 ºC), the electrode, handled by the operator using electrode holder pliers, gradually melts, creating a small fusion bath with the edges of the base metal.

The electrode is coated with a discouraging material, which melts with the metal core and floats on the melting bath; the purposes of this coating are many: in fact, it has a protective action, separating the molten metal from contact with the atmosphere, a purifying action, reacting with the impurities (sulfur and phosphorus) present in the melting bath, a physical action, shaping with its own weight the weld seam that is being formed, and an electrical action, stabilizing the arc by thermo-emissive salts. Arc welding may be performed with direct or alternating current.

When operating with direct current, direct polarity is used if the electrode is connected to the negative pole of the welding machine, and reverse polarity if it is connected to the positive pole. Current densities of 12-16 A per mm2 of electrode section are used; the electrodes have a diameter ranging from 1.5 to 6 mm and length from 200 to 450 mm. Welders for manual welding have a descending voltage-current characteristic, with no-load voltage of 60-80V. Welding may take place in various positions: flat, frontal, vertical, or overhead.

Types of welding and brazing

Welding

  • heterogeneous welding allows to join metallic materials with addition of filler material different from the base material.
  • homogeneous welding allows to join metallic materials with addition of filler material of the same family of the base material.
  • autogenous welding allows to join metallic materials without filler material.

Brazing

In the brazing process the base materials do not melt, the filler metal melts at a lower temperature than the base material.

  • strong brazing: temperatures over 450 °C but below the melting point of the materials to be welded; the joint must be prepared in such a way as to facilitate the penetration of the filler material by capillary action.
  • soft brazing: it is carried out with temperatures below 450 °C and below the melting point of the materials to be welded; the joint must be prepared in such a way as to facilitate the penetration of the filler material by capillary action.
  • braze welding: the filler alloys are melting alloys at even higher temperatures than those used in hard brazing but always below the melting point of the materials to be welded; the joint is prepared similarly to the preparation for autogenous welding.

Electrical welding

Electric resistance welding is an autogenous welding process by pressure, which uses heat developed by the Joule effect as a thermal source: the two elements to be welded are pressed together by copper electrodes cooled by circulating water. In the area of contact between the two elements there is an ohmic contact resistance, of a much higher value than that offered by the sheet metal, so that when the electric current passes through this area, fusion occurs. It is necessary to use very intense currents (approx. 15,000 A for overlapping sheets with a thickness of 3 mm) and very short times (in the order of tenths of a second) so that the area involved in the fusion is only that of contact between the two elements pressed together.

The compressive force used at the welding point can reach about 400 kg. The problems with this type of welding are related to the high current intensity values and the very short welding times, which require the use of complex devices for automatic current interruption and timing. Resistance welding includes spot welding (the electrodes are two opposing punches), relief welding (the electrodes are flat and on one of the two sheets to be joined protuberances are formed by drawing, so that contact between them is obtained only at certain points), roller welding (the thin sheets to be welded slide, pressed between two rollers that act as electrodes; the electrical circuit is alternately closed and reopened, so as to create an alternation of hot and cold times, resulting in the formation of a series of welding spots). The application of electric resistance welding mainly concerns the manufacture of automobile bodies, electrical appliance casings, metal furniture, etc.

Spot welding

Also called spot welding (spot welding in English) or welding nails, often carried out by means of induction welding machines, it is a type of resistance welding and consists of bringing together the parts of material to be welded and compressing the two pieces by means of a machine. Subsequently, the passage of electrical energy heats the bodies to be welded to a melting point in less than 15 seconds, thus joining the two materials by a particularly resistant internal nail that lasts over time. This type of welding is used in many pre-shaping centers to make double brackets produced in a single pass more rigid and therefore easier to handle.

Tack welding

Tack welding, unless the term is improperly used to indicate spot welding as mentioned above, does not refer to a particular welding process, but rather to a particular application of the welding processes already mentioned. This involves generating welding points on the perimeter of the pieces to be joined, that is, without creating a weld seam or a continuous weld without interruptions, but rather by creating multiple points placed at more or less regular distances from each other. This procedure is often used to join parts subject to little mechanical stress, or in any case between which the weld should not create a hermetic joint. It is typically used in non-automated welding processes as a tacking device to hold parts together for subsequent continuous welding.

Ultrasonic welding

Ultrasonic welding (USW) is a solid-state welding process that produces a weld by local application of high-frequency vibratory energy while the work pieces are held together under pressure; is one of the most popular methods for joining plastics and it is becoming an important method for welding polymeric composites. Ultrasonic welding uses a piezo-electric element to generate ultrasound (typically around 15 to 70 kHz), which is projected into the plastic parts to be joined.

The interface of the two parts is specially designed to concentrate the energy for maximum welding resistance. Ultrasonic welding can be used on almost any plastic material. It is the fastest heat sealing technology available.

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