Fusion welding can be defined as the technique of joining metal pieces in which materials are melted locally at the place of joint by the application of heat. Filler materials may or may not be added to the weld area during welding. The source of heat, generally, is a gas flame or an electric arc. Fusion welds made without the addition of any filler material are known as autogenous welds.

Fusion welding processes that use gas as the source of heat are termed as gas or oxy-fuel gas welding processes. Some important oxy-fuel gas welding processes will be described in this chapter. The subject matter will cover the basic principles of each process, the equipment used, their relative advantages and limitations, their capabilities and applications.

Extrusion is a process in which high compressive pressure is applied on a billet of ductile metal to force it through an orifice of required shape in a steel die. The extrusion process is analogous to squeezing toothpaste from its plastic tube. It may be carried out in either hot or cold state, that is, above or below recrystallization temperature. Extrusion is mostly carried out on ductile non-ferrous metals such as aluminium, magnesium, zinc and copper alloys. Other metals can also be extruded; the force required is high even when they are hot extruded. Because the orifice geometry is unchanged throughout the operation, extruded products have a constant cross-section.

The mechanical working of metal is defined as the plastic deformation of metals under the action of externally applied forces. Depending upon whether the metal is worked above or below the recrystallization temperature, the product quality differs in terms of surface finish and precision, grain structure and residual stresses in it. Metal working processes can be classified according to the shape and size of the products they produce. Common metal working processes are rolling, forging, swaging, coining, extrusion and drawing. These processes are generally carried out in hot state of metals mainly because of the several advantages in hot working, as explained in Chapter 8. These processes will be described in this and the following chapters.

High energy rate forming (HERF) and high velocity forming (HVF) processes can be differentiated from conventional metal forming processes by their higher speeds of forming. The range of speeds for conventional forming is 0.3 to 5 m/s whereas this range is 30 to 300 m/s in HERF and HVF processes. HVF processes were developed from the principle of the proportionality of kinetic energy of hammers to the square of the velocity. That means high kinetic (mechanical) energy can be delivered to the metal to deform it by using a small weight ram or die at high velocity. This has led to the development of high speed hammers of smaller size (resulting in reduced cost) and shorter stroke (giving higher rate of production). The velocity achievable from these high speed hammers is in the range of 5 to 60 m/s, only limited by the inertia of the moving parts.

Introduction

Students of English will find a wealth of textbooks on the history of the language, as well as a substantial number of textbooks introducing them to the principles of historical linguistics in general. In the last ten years we have also seen a spate of “handbooks” on the history of English or English historical linguistics published. How do all of these approaches to the history of English differ, and how is the following textbook distinguished?

In broad outline, textbooks on historical linguistics tend to be organized around linguistic levels of change – phonological change, morphological change, syntactic change, and semantic change. Of course, they also cover a variety of other topics, such as internal and external reconstruction, causes of change, language birth and death, language contact, language classification, and so on. In contrast, histories of the English language – with very rare exceptions – are organized chronologically, following the different “periods” of English (see below on “periodization”). Principles of linguistic change, if discussed explicitly at all in these textbooks, are subsumed to the overall presentation of a “narrative” of change from Old English to the present day. The more recent handbooks of English – all impressive works in their own right, collecting work by many of the best scholars in the field today – are typically organized by period (like histories of English) or by linguistic level (like introductions to historical linguistics), though again they may treat a myriad of other topics.

The linguistic study of the English language has a long history, as will be described briefly in the next section, and over time scholars have made different assumptions about the nature of language and language change, have adopted different theoretical perspectives, and have utilized different methodologies in studying the history of English. There is not one monolithic, coherent approach to the history of English. Some of the recent handbooks of English present discussions of these different approaches and perspectives, but these handbooks are generally addressed to the scholarly researcher, not the student of English, and often focus on the “state of the art” in research rather than providing descriptive information on methodology and approach.

A majority of the products that we use and see around us every day, whether they are industrial products, home appliances or even products of personal use, are created by assembling a number of component parts. Depending upon the need, the component parts are joined together either permanently or in such a way that they can be subsequently dissembled for maintenance, repair or other purposes. It is very important that engineers should be aware of the large variety of available joining processes and understand the fundamental principles involved in each process. They should also appreciate the advantages and limitations of each process so that better products can be produced at low cost and made available to society.

As in all manufacturing processes, in welding too it is important to establish the quality of the welded joint by inspection and testing. For this, the finished weld needs to be inspected for undercut, overlap, surface checks, cracks or other defects. Moreover, the degree of penetration and side wall fusion, extent of reinforcement, and size and position of the welds are important factors in the determination of the quality of the weld. There are several standardized tests and test procedures established by organizations like the Indian Welding Society, American Welding Society, Indian Standards Institution, etc. There are two basic categories of tests for welded joints: Destructive techniques and non-destructive techniques. Each of these techniques has its capabilities, limitations, reliability, sensitivity and requirement of operator’s skill and equipment.

Powder metallurgy (PM) is a metal working process that forms precision metal components from metal powders. The process is simple to understand, but not so easy to carry out because in order to get consistent product quality, specialized equipment, thorough knowledge of the process, and an extensive amount of experience is required. In simple words, the process has three steps. In the first step, fine metallic and/or non-metallic powder(s) required to give the end product its desired properties are taken and mixed together. The powder mix is introduced into a metal die. In the second step, the powder mix is compressed with high pressure in the die (the operation is called compaction) to produce what is called a pre-form. In the third step, this pre-form is heated to a high temperature (the operation is called sintering) in an oven having vacuum or a controlled atmosphere for a finite period to get the final product. A detailed description of the process follows.

Since these operations are time consuming and expensive, it is essential to try to eliminate or at least minimize their need when designing the product and choosing the specific casting method. Besides, possibilities should also be explored for automation of these operations.

Sand cores are removed from castings usually by mechanical shaking, hydro blasting or at times by chemical dissolution depending upon the size, complexity and hardness of the core. Gates and risers are either knocked off or cut-off by an abrasive wheel or a power saw. Alternatively, they are removed by melting them away with an oxy-acetylene flame. The casting surface, from where the gates and risers have been removed, is generally rather rough. Small projections may easily be chipped off with the help of either hand tools or pneumatic chisels. To remove fins, flash and sand that might be sticking to the surface of castings, a process called tumbling is employed. In the tumbling process, castings together with abrasive material (in the form of broken grinding wheels), cleansing fluid and metal shots are loaded in a horizontal barrel and given a slow rotary motion. The castings rub and strike against each other as well as against the abrasive grinding wheel pieces. Alternatively, castings may be subjected to sand blasting, shot blasting or vibratory finishing process.