Welding Alloys Selection Guide

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

Welding Alloy Basics

Several factors need to be considered for selecting the right welding alloy including the metals or alloys to be welded together, the type of welding process being used, and the applicable standards or references specifications regarding the welding being performed. Referenced specifications are standards, codes, and specifications called out in part drawings, assembly plans, product documentation, and/or the bill of materials. Applicable standards are industry codes, national and/or international standards, or specifications which are required for a specific type of product. For example, if welding alloys are being selected for a boiler or pressure vessel design project, then the correct reference is to the applicable portion of the American Society of Mechanical Engineers' Boiler and Pressure Vessel Code.

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 designed for specific welding processes as per specific requirements of each welding process. Each welding process may require a specific alloy form like a stick or a wire and may require a flux core or coated flux to isolate the weld. Common welding processes include:

Shielded Metal Arc Welding (SMAW)

Shielded metal arc welding (SMAW) or stick electrode welding 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, deoxidisers, and alloying elements to the weld metal.

Submerged Arc Welding (SAW)

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 submerged arc welding processes. Saw is generally used in automated process where horizontal welds with a high throughput are required.

Flux-Cored Arc Welding (FCAW)

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

Metal Inert Gas (MIG)

Gas metal arc welding (GMAW), also referred to as Metal Inert Gas (MIG), processes uses a gaseous mixture as opposed to a flux to shield the weld. Gas metal arc welding or MIG 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 gas metal arc welding 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 Gas metal arc welding processes.

Tungsten Inert Gas (TIG)

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. Gas tungsten arc welding is the preferred welding process when welding thin sections of stainless steel and other non-ferrous metals such as aluminium, magnesium, and copper alloys.

Industrial Standards and Specifications Selection Factors

Another factor in selecting welding alloys is the industrial, government, and OEM standards or specifications that the welding alloys are approved for or conform to. These standards define the forms, composition, and properties for each welding alloy and welding process. 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). ASME International has 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 organisation 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 organisation for standardisation. These are one of the most widely used and accepted standards worldwide for a varied variety of applications

ISO Welding Electrodes and Welding Consumables Standards

International Organization for Standardization (ISO) is a worldwide federation of national standards organisations 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.