This article discusses the fusion welding processes that are most widely used for joining titanium, namely, gas-tungsten arc welding, gas-metal arc welding, plasma arc welding, laser-beam welding, and electron-beam welding. It describes several important and interrelated aspects of welding phenomena that contribute to the overall understanding of titanium alloy welding metallurgy. These factors include alloy types, weldability, melting and solidification effects on weld microstructure, postweld heat treatment effects, structure/mechanical property/fracture relationships, and welding process application.

Carbon and low-alloy steels are the most frequently welded metallic materials, and much of the welding metallurgy research has focused on this class of materials. Key metallurgical factors of interest include an understanding of the solidification of welds, microstructure of the weld and heat-affected zone (HAZ), solid-state phase transformations during welding, control of toughness in the HAZ, the effects of preheating and postweld heat treatment, and weld discontinuities. This chapter provides information on the classification of steels and the welding characteristics of each class. It describes the issues related to corrosion of carbon steel weldments and remedial measures that have proven successful in specific cases. The major forms of environmentally assisted cracking affecting weldment corrosion are covered. The chapter concludes with a discussion of the effects of welding practice on weldment corrosion.

Nickel-base alloys are generally used in harsh environments that demand either corrosion resistance or high-temperature strength. This article first describes the general welding characteristics of nickel-base alloys. It then describes the weldability of solid-solution nickel-base alloys in terms of grain boundary precipitation, grain growth, and hot cracking in the heat-affected zone; fusion zone segregation and porosity; and postweld heat treatments. Next, the article analyzes the welding characteristics of dissimilar and clad materials. This is followed by sections summarizing the various types and general weldability of age-hardened nickel-base alloys. The article then discusses the composition, welding metallurgy, and properties of cast nickel-base superalloys. Finally, it provides information on the welding of dissimilar metals, filler metal selection for welding clad materials and for overlay cladding, service conditions during repair, and welding procedural idiosyncrasies of cobalt-base alloys.

This article describes the repair of weld defects and failed structures. It provides information on three factors that must first be considered before attempting a repair, namely material weldability, nature of the failure that prompted the repair, and involvement of any code requirements. The article discusses the processes involved in welding process selection and the methods of preparing base metal for repair welding. It presents the guidelines for weld repairs of various ferrous (carbon steels, cast irons, and stainless steels) and nonferrous (for example, titanium) base metals.

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.

This appendix provides reference tables listing weldability of cast irons, steels, and nonferrous metals. A process selection table for arc welding carbon steels is included, and recommended preheat and interpass temperature tables are also presented. This appendix includes information on qualification codes and standards.