The majority of currently used additive manufacturing (AM) processes are solidification based (SAM). Another class of AM processes consists of those that rely on deformation (DAM) to place material instead of solidification. Although SAM processes are much more widely used, as research and development continues in DAM processes, they are becoming increasingly attractive, especially for the AM of metals. This article discusses some of the more widely used DAM processes, namely ultrasonic additive manufacturing, cold spray process, and friction stir welding, focusing on their applications, advantages, and limitations.

This article focuses on friction stir welding (FSW), where frictional heating and displacement of the plastic material occurs by a rapidly rotating tool traversing the weld joint. Much of the research activity early on pertained to issues related to understanding the process, such as learning about material flow, heat generation, microstructure development, and many other fundamental issues. The article summarizes the results of the research, describing the aspects of how FSW actually accomplishes sound joints in metals without melting them. It discusses the FSW process variations and the practical aspects of heat generation. The article provides information on the effect of welding on material properties and typical alloys in FSW applications. The alloys include 6061 aluminum, 5083 aluminum, 2xxx aluminum, and 7xxx aluminum alloys. The article concludes with a discussion on FSW equipment.

This article discusses the application of friction stir processing (FSP) and friction surfacing for tribological components. It describes the three critical aspects involved in the application of FSP for near-surface material modifications intended for tribological applications. These include tools, processing parameters, and machines. The article also discusses the equipment and processing parameters for friction surfacing. It describes various hybrid stir processing techniques that involve preheating of the workpiece material, especially relatively hard and high-strength ones. The article presents a partial list of surface-modification methods based on FSP. The partial list includes surface hardening, surface composites, and additive coating. The article also provides information on generation of residual stresses in metallic materials and alloys form different variants of FSP.

Resistance welding (RW) encompasses many variations on the basic theme of local Joule heating while an external pressure is applied. This article provides an overview of the most generally applied RW processes, followed by a discussion on the general design aspects of various resistance-welded joints. The various resistance-welded joints include spot welds, projection welds, seam welds, and butt welds. The article explains the relative contributions of the Joule, Peltier, and Thomson effects for typical RW scenarios. It concludes with information on the electrode “follow” behavior.

This article provides an overview of the components of a resistance welding machine. It focuses on the single-phase control system and medium-frequency direct current system of resistance welding. The article also includes information on their feedback systems, rectification systems, and power sources.

Joining is key to the manufacture of large or complex devices or assemblies; construction of large and complex structures; and repair of parts, assemblies, or structures in service. This article describes the three forces for joining: physical, chemical, and mechanical. It provides an overview of the joining processes, namely, mechanical fastening, integral attachment, adhesive bonding, welding, brazing, and soldering. The article concludes with information on the various aspects of joint design and location that determine the selection of a suitable joining method.

Arc welding is one of several fusion processes for joining metals. This article introduces the fundamentals of arc welding and provides a summary of its history and early discoveries.

Flash welding, also called flash butt welding, is a resistance welding process in which a butt joint weld is produced by a flashing action and by the application of pressure. The flash welding process consists of preweld preparation, flashing, upsetting (forging), and postweld heat treatment. This article provides an overview of both flash welding and upset welding and describes the various process and failure origins of flash welding as well as the equipment used. It also explains the characteristics and advantages of solid-state upset welding.

This article introduces the operating principles and modes of operation for high-vacuum (EBW-HV), Medium-vacuum (EBW-MV), and nonvacuum (EBW-NV) electron beam welding. Equipment, process sequence, part preparation, process control, and weld geometry are described for electron beam welding. Advantages are described in terms of welding near heat sensitive components or materials and producing deep penetration or shallow welds with the same equipment.

This article discusses the operation principles, advantages, limitations, process parameters, consumables or electrodes, the equipment used, process variations, and safety considerations of gas metal arc welding (GMAW). It reviews the important variables of the GMAW process that affect weld penetration, bead shape, arc stability, productivity, and overall weld quality. These include welding consumables, equipment settings, and gun manipulation. The major components of a GMAW installation such as a welding gun, shielding gas supply, electrode feed unit, power source, and associated controls are discussed.

The friction surfacing process enables deposition of a wide variety of high-specification materials with an ideal metallurgical bond onto a range of metal substrates. This article provides a process description and discusses the equipment used for, and the applications of, friction surfacing.

Electroslag welding (ESW) involves high energy input relative to other welding processes, resulting generally in inferior mechanical properties and specifically in lower toughness of the heat-affected zone. Electrogas welding (EGW) is a method of gas metal or flux cored arc welding, wherein an external gas is supplied to shield the arc, and molding shoes are used to confine the molten weld metal for vertical-position welding. This article describes the fundamentals, temperature relations, consumables, metallurgical and chemical reactions, and process development of ESW. The problems, quality control, and process applications of ESW and EGW are also discussed.

Ultrasonic metal welding is a solid-state welding process that produces coalescence through the simultaneous application of localized high-frequency vibratory energy and moderate clamping forces. This article discusses the parameters to be considered when selecting a suitable welder for ultrasonic metal welding. It details the personnel requirements, advantages, limitations, and applications, namely, wire welds, spot welds, continuous seam welds, and microelectronic welds of ultrasonic metal welding.

Plastic deformation of one or both metals is required to obtain bonding in cold welding. This article presents a theoretical model, to explain the bond strength, based on metallographic studies and continuum mechanical analysis of the local plastic deformation in the weld interface. It describes the bonding mechanisms, with illustrations. The article discusses the alternative methods of surface preparation and quality control of the weld interface of a cold weld. It concludes with a description of a variety of metal-forming processes suitable for production of cold welds, namely, rolling, indentation, butt welding, extrusion, and shear welding.

This article outlines a general approach to develop a coupled electrical-thermal-mechanical analysis for the resistance spot welding process. It provides information on the discretization of sheet-electrode geometry and distribution of contact resistivity along the sheet-sheet and electrode-sheet interfaces. The distribution can be estimated based on the discretized geometry used for the numerical modeling. The article also details the results obtained from this modeling.

This article overviews the classification of welding processes and the key process embodiments for joining by various fusion welding processes: fusion welding with chemical sources for heating; fusion welding with electrical energy sources, such as arc welding or resistance welding; and fusion welding with directed energy sources, such as laser welding, electron beam welding. The article reviews the different types of nonfusion welding processes, regardless of the particular energy source, which is usually mechanical but can be chemical, and related subprocesses of brazing and soldering.

This article focuses on the general internal state variable method, and its simplification, for single-parameter models, in which the microstructure evolution may be treated as an isokinetic reaction. It explains that isokinetic microstructure models are applied to diffusional transformations in fusion welding, covering particle dissolution, growth, and coarsening of precipitates in the heat-affected zone. The article discusses the versatility of the internal state variable approach in modeling of nonisothermal transformations for various materials and processes. It describes the process models applied to predict the microstructure evolution in Al-Mg-Si alloys during multistage thermal processing involving heat treatment and welding. The article also provides information on the microstructure models exploited in engineering design to optimize the load-bearing capacity of welded aluminum components.

Plasma arc welding (PAW) can be defined as a gas-shielded arc welding process where the coalescence of metals is achieved via the heat transferred by an arc that is created between a tungsten electrode and a workpiece. This article focuses on the operating principles and procedures, current and operating modes, advantages, disadvantages, and applications of PAW. It discusses the personnel and equipment requirements, as well as the joints used in the process. The power source, plasma control console, water cooler, welding torch, and gas supply system for the plasma and shielding gases are also reviewed.

This article describes the process applications, advantages, and limitations of resistance seam welding. The fundamentals of lap seam welding are also reviewed. The article details the types of seam welds, namely, lap seam welds and mash seam welds, and the processing equipment used for lap seam welding. The primary factors used to determine the selection of electrodes, including alloy type and wheel configuration, are reviewed. The article also describes weld quality and process control procedures.

Projection welding is a variation of resistance welding in which current flow is concentrated at the point of contact with a local geometric extension of one (or both) of the parts being welded. This article focuses on the process fundamentals, advantages, and limitations of projection welding and reviews the equipment used in the process. It discusses projection welding of copper and copper alloys, aluminum and aluminum alloys, and steels. The article provides several specifications and recommended weld schedules and practices for projection welding. It describes the embossed-projection welding of heavy-, intermediate-, and thin-gage sheet mild steel as well as the welds between dissimilar thickness joints. The article also considers the solid-projection welding of steels: annular, nut, and cross-wire projection configurations. It also details the various tests that can be used to validate projection weld quality.