Joining comprises a large number of processes used to assemble individual parts into a larger, more complex component or assembly. The selection of an appropriate design to join parts is based on several considerations related to both the product and the joining process. Many product design departments now improve the ease with which products are assembled by using design for assembly (DFA) techniques, which seek to ensure ease of assembly by developing designs that are easy to assemble. This chapter discusses the general guidelines for DFA and concurrent engineering rules before examining the various joining processes, namely fusion welding, solid-state welding, brazing, soldering, mechanical fastening, and adhesive bonding. In addition, it provides information on several design considerations related to the joining process and selection of the appropriate process for joining.

Arc welding applies to a large and diversified group of welding processes that use an electric arc as the source of heat to melt and join metals. This chapter provides a detailed overview of specific arc welding methods: shielded metal arc welding, flux cored arc welding, submerged arc welding, gas metal arc welding, gas tungsten arc welding, plasma arc welding, plasma-GMAW welding, electroslag welding, and electrogas welding. The basic characteristics of gases used for shielding during arc welding are briefly discussed.

Resistance welding is a group of processes in which the heat for welding is generated by the resistance to the flow of an electrical current through the parts being joined. This chapter discusses the processes, advantages, and limitations of specific resistance welding processes, namely resistance spot welding, resistance seam welding, projection welding, flash welding, and upset welding.

This chapter discusses the fusion welding processes, namely oxyfuel gas welding, oxyacetylene braze welding, stud welding (stud arc welding and capacitor discharge stud welding), high-frequency welding, electron beam welding, laser beam welding, hybrid laser arc welding, and thermit welding.

During fusion welding, the thermal cycles produced by the moving heat source causes physical state changes, metallurgical phase transformations, and transient thermal stresses and metal movement. This chapter begins by discussing weld metal solidification behavior and the solid-state transformations of the main classes of metals and alloys during fusion welding. The main classes include work- or strain-hardened metals and alloys, precipitation-hardened alloys, transformation-hardened steels and cast irons, stainless steels, and solid-solution and dispersion-hardened alloys. The following section provides information on the residual stresses and distortion that remain after welding. The focus then shifts to distortion control of weldments. Inclusions and cracking are discussed in detail. The chapter also discusses the causes for reduced fatigue strength of a component by a weld: stress concentration due to weld shape and joint geometry; stress concentration due to weld imperfections; and residual welding stresses. Inspection and characterization of welds are described in the final section of this chapter.

Solid-state welding processes are those that produce coalescence of the faying surfaces at temperatures below the melting point of the base metals being joined without the addition of brazing or solder filler metal. This chapter discusses solid-state welding processes such as diffusion welding, forge welding, roll welding, coextrusion welding, cold welding, friction welding, friction stir welding, explosion welding, and ultrasonic welding.

Brazing and soldering processes use a molten filler metal to wet the mating surfaces of a joint, with or without the aid of a fluxing agent, leading to the formation of a metallurgical bond between the filler and the respective components. This chapter discusses the characteristics, advantages, and disadvantages of brazing and soldering. The first part focuses on the fundamentals of the brazing process and provides information on filler metals and specific brazing methods. The soldering portion of the chapters provides information on solder alloys used, selection criteria for base metal, the processes involved in precleaning and surface preparation, types of fluxes used, solder joint design, and solder heating methods.

This chapter presents a comprehensive coverage of mechanical fastening methods. It begins with a discussion on the advantages and disadvantages of mechanical fastening followed by sections providing information on mechanically fastened joints and the selection of the correct fastener system. The chapter then describes important structural fasteners, namely bolts, screws, pins, collar fasteners, rivets, blind fasteners, machine pins, and spring clip fasteners. The following sections describe the process involved in presses, shrink fits, hole generation, and fastener installation. The chapter ends with information on miscellaneous mechanical fastening methods.

Adhesive bonding is a widely used industrial joining process in which a polymeric material is used to join two separate pieces (the adherends or substrates). This chapter begins with a discussion on the advantages and disadvantages of adhesive bonding, followed by a section providing information on the theory of adhesion. The chapter then describes the considerations for designing adhesively bonded joints and for testing or characterizing adhesive materials. The following section covers the characteristics of the most important synthetic adhesive systems and five groups of adhesives, namely structural, hot melt, pressure sensitive, water based, and ultraviolet and electron beam cured. The chapter ends with a discussion on some general guidelines for adhesive bonding and the basic steps in the adhesive bonding process.

This chapter reviews materials issues encountered in joining, including challenges involved in welding of dissimilar metal combinations; joining of plastics by mechanical fastening, solvent and adhesive bonding, and welding; joining of thermoset and thermoplastic composite materials by mechanical fastening, adhesive bonding, and, for thermoplastic composites, welding; the making of glass-to-metal seals; and joining of oxide and nonoxide ceramics to themselves and to metals by solid-state processes and by brazing. The classification, types, applications, and the mechanism of each of these methods are covered. The factors influencing joint integrity and the main considerations in welding dissimilar metal combinations are also discussed.

It is well established that solidification behavior in the fusion zone controls the size and shape of grains, the extent of segregation, and the distribution of inclusions and defects such as porosity and hot cracks. Since the properties and integrity of the weld metal depend on the solidification behavior and the resulting microstructural characteristics, understanding weld pool solidification behavior is essential. This article provides a general introduction of key welding variables including solidification of the weld metal or fusion zone and microstructure of the weld and heat-affected zone. It discusses the effects of welding on microstructure and the causes and remedies of common welding flaws.

This article describes the weldability tests that are used to evaluate the effects of welding on such properties and characteristics as base-metal and weld-metal cracking; base-metal and weld-metal ductility; weld penetration; and weld pool shape and fluid flow. It also describes several weldability tests for evaluating cracking susceptibility, classified as self-restraint or externally loaded tests. The article discusses the processes, advantages, and disadvantages of the weld pool shape tests, the weld penetration tests, and the Gleeble test.

This article reviews nondestructive and destructive test methods used to characterize welds. The first process of characterization discussed involves information that may be obtained by direct visual inspection and measurement of the weld. An overview of nondestructive evaluation is included that encompasses techniques used to characterize the locations and structure of internal and surface defects, including radiography, ultrasonic testing, and liquid penetrant inspection. The next group of characterization procedures discussed is destructive tests, requiring the removal of specimens from the weld. The third component of weld characterization is the measurement of mechanical and corrosion properties. Following the discussion on the characterization procedures, the second part of this article provides examples of how two particular welds were characterized according to these procedures.

Discontinuities are interruptions in the desirable physical structure of a weld. This article describes the types of weld discontinuities that are characteristic of the principal welding processes. Discontinuities covered are metallurgical discontinuities, discontinuities associated with specialized welding processes, and base metal discontinuities. In addition, information on the common inspection methods used to detect these discontinuities is provided.

The formation of defects in materials that have been fusion welded is a major concern in the design of welded assemblies. This article describes four types of defects that, in particular, have been the focus of much attention because of the magnitude of their impact on product quality. Colloquially, these four defect types are known as hot cracks, heat-affected zone microfissures, cold cracks, and lamellar tearing.

Welded joints in any component or structure require a thorough inspection. The role of nondestructive evaluation (NDE) in the inspection of welds is very important, and the technology has become highly developed as a result. This article describes the applications, methods, evaluation procedures, performance, and limitations of NDE. It provides information on the training and certification of NDE operators, evaluation of test results, and guidance to method selection. Typical examples of various NDE methods for welds are also described.

This article discusses the various options for controlling fatigue and fracture in welded steel structures, the factors that influence them the most, and some of the leading codes and standards for designing against these failure mechanisms. The two most widely used approaches discussed for fatigue control in welded joints are the S-N curve approach and the fracture mechanics assessment methods.

This article is intended to help engineers understand why the fatigue behavior of weldments can be such a confusing and seemingly contradictory topic and hopefully to clarify this complex subject. It first reexamines the factors influencing the fatigue behavior of an individual weldment using extensive experimental data and a computer model that simulates the fatigue resistance of weldments. Next, the process of fatigue in weldments is discussed in general terms, and the service conditions that favor long crack growth and the conditions that favor crack nucleation are contrasted. The article then presents experimental data that show the effect of weldment geometry on fatigue resistance. Several useful geometry classification systems are compared. Finally, a computer model is employed to investigate the behavior of two hypothetical weldments: a discontinuity-containing ("Nominal") weldment and a discontinuity-free ("Ideal") weldment.

Depending on the operating environment and the nature of the applied loading, a structure can fail by a number of different modes, including brittle fracture, ductile fracture, plastic collapse, fatigue, creep, corrosion, and buckling. These failure modes can be broken down into the categories of fracture, fatigue, environmental cracking, and high-temperature creep. This article discusses each of these categories, as well as the benefits of a fitness-for-service approach.