Welding alloys, also known as filler alloys, are consumables used during a welding process to fill in the gap between two edges being joined. The filler alloy melts into the weld pool along with a portion of the base metals of the work piece and solidifies into a weld joint metal. The composition of the weld joint metal is a mixture of the filler alloy and base metal.
 

Welding Alloy Basics

Several factors should be considered when selecting a welding alloy such as the metals or alloys to be welded, the type of welding process used, and the applicable standards or references specifications. Referenced specifications are those standards, codes, or specifications called out in a part drawings, assembly plan, product document, or bill of materials. Applicable standards are the codes, standards, or specifications required for a specific product type. For example, if welding alloys are being selected for a boiler or pressure vessel design project, then the engineer should refer to the applicable portion of the American Society of Mechanical Engineers' Boiler and Pressure Vessel Code, which is ASME BVPC II SECTION II: MATERIALS - PART C - SPECIFICATION FOR WELDING RODS, ELECTRODES, AND FILLER METALS.

Base Metal and Welding Alloy Selection Factors 

The base metal refers to the composition of the workpiece being welded. Welding alloys, sometimes referred to as welding electrodes, must be compatible with specific base metals being welded. Typically, welding filler alloys are similar in composition to the base alloys or alloys of the parts being joined. Welding alloys or welding filler alloys forms melt with the base alloys of the parts being joined to fuse the parts together. The filler alloy selected should not form any brittle compounds when alloyed with base alloys of the parts. When two different alloys are being joined, additional research is required to determine the welding alloy compatible with both alloys. In some cases, the two different alloys are incompatible due to melting points, exothermic heat of mixing, insolubility, brittle phase formation, and corrosion factors. In these cases a bimetal or trimetal transition joint may be used to weld dissimilar, incompatible alloys that would normally form brittle compounds. In other cases, welding should be abandoned due to non-weldability of the base metals or due to the release of toxic fumes from lead or other alloying additions. In these cases the suitability of brazing, soldering, or an adhesive joint should be examined in place of welding. The following chart describes how various electrodes are used:

Welding Processes

Welding alloys are also designed for specific welding processes. Each welding process may require a specific alloy form; e.g. stick or wire, and may require a flux core or coated flux to isolate the weld. Common welding processes include:

Stick electrode welding or shielded metal arc welding (SMAW) is a common welding process. The flux covering the electrode melts during welding or brazing to forming a gas, which then shields the arc and molten weld pool. The flux also forms a slag that protects the cooling weld pool or braze joint. The slag must be chipped or brushed off the weld bead, however. Flux coating also provides a method of adding scavengers, deoxidizers, and alloying elements to the weld metal.

Submerged arc welding (SAW) processes use a powder flux to shield and isolate the arc and weld pool. The powdered flux is fed via a hopper while the consumable welding electrode creates an arc that is covered by the powdered flux. Both sold wire feed and flux cored electrodes may be used in SAW processes. Saw is generally used in automated process where horizontal welds with a high throughput are required.

Flux-cored arc welding (FCAW) processes primarily shield the weld pool via a slag that forms during the  decomposition and vaporization of the electrode's flux core, but may also use a gas shield. The flux core also incorporates deoxidizing and denitriding agents that improve the weld strength and durability. FCAW alloys are wire fed alloys with a flux core used for automatic and semi-automatic welding processes. FCAW is the preferred welding process for field work and when dealing with thick work pieces.

Gas metal arc welding (GMAW), also  referred to as  Metal Inert Gas (MIG), processes use a gaseous mixture opposed to a flux to shield the weld. GMAW welding alloys are available in a wide range of base materials and are supplied in as a solid core electrode or wire feed. The shielding gases used for GMAW vary by composition and bulk concentration. The shielding gases used are primarily composed of argon, carbon dioxide (CO₂), and helium. The following table illustrates the various gas mixtures used in common GMAW processes.

Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) rod welding, joins metals by melting base and filler metals with an arc struck between a tungsten electrode and the workpiece. The tungsten electrode is a non-consumable electrode and does not become part of the completed weld. The welding alloy or filler metal is a hand held rod or wire feed. In most cases Argon inert gas or inert gas mixtures are used for shielding, while autogenous welds do not require a gas shield. GTAW is the preferred welding process when welding thin sections of stainless steel and other non-ferrous metals such as aluminum, magnesium, and copper alloys.

Industrial Standards and Specifications Selection Factors

Another factor is selecting a welding alloy is the industrial, government, and OEM standards or specifications that the welding alloys are approved or conform to. The standards define forms, composition, and properties. The following is a short list of common industrial standards that pertain to welding alloys:

ASME Welding Electrode Specifications

ASME is an acronym for American Society of Mechanical Engineers (ASME). In November 2001, ASME International formed the Codes and Standards Technology Institute (CSTI) to ensure that ASME standards committees have continuing sources of research in the technologies they cover. CSTI provides the research and technology development needed to establish and maintain the technical relevance of codes and standards. Most ASME specifications are adopted from or very similar to ASTM specifications.

ASTM Welding Electrode Specifications

The American Society for Testing and Materials (ASTM) is a non-profit organization that develops and publishes voluntary standards for materials, products, systems, and services. Products that are ASTM-certified comply with design specifications for safety.

AWS Welding Alloy Specifications and Codes

The American Welding Society (AWS) establishes standards and codes for welding processes and materials. Major welding, brazing and soldering specification are contained in the AWS Core Collection.

DIN Standards for Welding and Surface Treatment

DIN is an acronym for Deutsches Institut für Normung (DIN), a German national organization for standardization.

ISO Welding Electrodes and Welding Consumables Standards

ISO is a worldwide federation of national standards organizations from over 100 countries. ISO's mission is to facilitate the international exchange of goods and services, and to foster cooperation in the spheres of intellectual, technological, and economic activity. 

References:

Avoiding Mix-ups with Shielding Gas Mixes (Fabricators & Manufacturers Association, Intl.)

GMAW Welding Guide (.pdf; Lincoln Electric)

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