Welding Processes

One of the most common types of arc welding is shielded metal arc welding (SMAW), which is also known as manual metal arc welding (MMA) or stick welding. Electric current is used to strike an arc between the base material and consumable electrode rod, which is made of steel and is covered with a flux that protects the weld area from oxidation and contamination by producing CO2 gas during the welding process. The electrode core itself acts as filler material, making a separate filler unnecessary.

The process is versatile and can be performed with relatively inexpensive equipment, making it well suited to shop jobs and field work. Weld times are rather slow, since the consumable electrodes must be frequently replaced and because slag, the residue from the flux, must be chipped away after welding. Furthermore, the process is generally limited to welding ferrous materials, though special electrodes have made possible the welding offcast iron, nickel, aluminium, copper, and other metals.

Gas metal arс welding (GMAW), also known as metal inert gas or MIG welding, is a semi-automatic or automatic process that uses a continuous wire feed as an alectrode and an inert or semi-inert gas mixture must protect the weld from contamination. The equipment required to perform the GMAW process is more complex and expensive than that required for SMAW, and requires a more complex setup procedure. Therefore, GMAW is less portable and versatile, and due to the use of a separate shielding gas is not particularly suitable for outdoor work. The process can be applied to a wide variety of metals, both ferrous and non-ferrous.

A related process, flux-cored arc welding (FCAW), uses similar equipment but uses wire consisting of a steel electrode surrounding a powder fill material.

Gas tungsten arc welding (GTAW) uses a nonconsumable tungsten electrode, an inert or semi-inert gas mixture, and a separate filler material. GTAW can be used on nearly all weldable metals, though it is most often applied to stainless steel and light metals. It is often used when quality welds are extremely important, such as in bicycle, aircraft and naval applications. A variation of the process is plasma cutting, an efficient steel cutting process.

Submerged arc welding (SAW) is a high-productivity welding method in which the arc is stuck beneath a covering layer of flux. This increases arc quality since contaminants in the atmosphere are blocked by the flux. Working conditions are much improved over other arc welding processes since the flux hides the arc and almost no smoke is produced.

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Text IV

The concept of Nanotechnology

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subsequently	[′sʌbsɪkwəntlɪ]	впоследствии, потом, позже
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to digest	[dɪ′dʒest]	переваривать, усваивать

The concept of Nanotechnology

When the term “nanotechnology” was coined and popularized by Eric Drexler, it referred to a future manufacturing technology based on molecular machine systems. The premise was that molecular-scale biological analogies of traditional machine components demonstrated that molecular machines were possible, and that manufacturing technology based on the mechanical functionality of such components as gears, bearings, motors, and structural members would enable programmable positional assembly to atomic specification.

The term “nanotechnology” was subsequently applied to other uses, new terms evolved to refer to this distinct usage: “molecular nanotechnology”, “molecular manufacturing”, and most recently “productive nanosystems”. Advanced nanotechnology, called molecular manufacturing, is a term given to the concept of engineered nanosystems (nanoscale machines) operating on the molecular scale.

Nanotechnology covers a wide range of industries, and therefore the potential benefits, are also widespread. Telecommunications and information technology could benefit in terms of faster computers and advanced data storage. Healthcare could see improvements in skin care and protection, advanced pharmaceuticals, drug delivery systems, biocompatible materials, nerve and tissue repair, and cancer treatments. Other industries benefits include catalysts, sensors and magnetic materials and devices.

One of the problems facing nanotechnology concerns how to assemble atoms and molecules into smart materials and working devices. A very important tool here is supramolecular chemistry. The definition of nano-composite material has broadened significantly to encompass a large variety of systems such as one-dimensional, two-dimensional, three-dimensional and amorphous materials, made of distinctly dissimilar components and mixed on the nanometer scale.

However, in these new technologies there exist the potential risks. Critics of nanotechnology point to the potential toxicity of new classes of nanosubstances that could adversely affect the stability of cell membranes or disturb the immune system when inhailed, digested or absorbed through the skin. Nanoparticles in the environment could potentially accumulate in the food chain.