Metals need to be identified before welding in order to pick the correct electrode and method.
For example, magnesium and aluminum have a similar appearance but require different welding processes.
The following pages will walk you through the various tests, physical and mechanical properties that are used to determine the origin of metals.
Most of the metals and alloys described in this section of the site can be welded by one or more major welding processes (Stick, TIG, MIG, Oxyfuel).
This section describes the characteristics of metals and their alloys, with particular reference to their significance in welding operations.
All metals fall within two categories, ferrous or nonferrous.
- Ferrous metals – are metals that contain iron.
Ferrous metals appear in the form of cast iron, carbon steel, and tool steel. The various alloys of iron, after undergoing certain processes, are pig iron, gray cast iron, white iron, white cast iron, malleable cast iron, wrought iron, alloy steel, and carbon steel. All these types of iron are mixtures of iron and carbon, manganese, sulfur, silicon, and phosphorous. Other elements are also present, but in amounts that do not appreciably affect the characteristics of the metal.
- Nonferrous metals – are those which do not contain iron.
Aluminum, copper, magnesium, and titanium alloys are among those metals which belong to this group.
Physical Properties of Metals
Many of the physical properties of metals determine if and how they can be welded and how they will perform in service.
Physical properties which is comprised of several metal ID methods, are shown in table 7-1 a&b below.
Physical Properties of Metals – Table 7-1a and 7-1b
Color relates to the quality of light reflected from the metal.
Mass or Density
Mass or density relates to mass with respect to volume.
Commonly known as specific gravity, this property is the ratio of the mass of a given volume of the metal to the mass of the same volume of water at a specified temperature, usually 39°F (4°C).
For example, the ratio of weight of one cubic foot of water to one cubic foot of cast iron is the specific gravity of cast iron. This property is measured by grams per cubic millimeter or centimeter in the metric system.
The melting point of a metal is important with regard to welding.
A metal’s fusibility is related to its melting point, the temperature at which the metal changes from a solid to a molten state.
Pure substances have a sharp melting point and pass from a solid-state to a liquid without a change in temperature.
During this process, however, there is an absorption of heat during the melting and liberation of heat during freezing.
The absorption or release of thermal energy when a substance changes state is called its latent heat.
Mercury is the only common metal that is in its molten state at normal room temperature. Metals having low melting temperatures can be welded with lower temperature heat sources. The soldering and brazing processes utilize low-temperature metals to join metals having higher melting temperatures.
The boiling point is also an important factor in welding.
The boiling point is the temperature at which the metal changes from the liquid state to the vapor state. Some metals, when exposed to the heat of an arc, will vaporize.
Thermal and electrical conductivity relate to the metal’s ability to conduct or transfer heat and electricity.
- Thermal conductivity: the ability of a metal to transmit heat throughout its mass, is of vital importance in welding, since one metal may transmit heat from the welding area much more quickly than another. The thermal conductivity of a metal indicates the need for preheating and the size of heat source required. Thermal conductivity is usually related to copper. Copper has the highest thermal conductivity of the common metals, exceeded only by silver. Aluminum has approximately half the thermal conductivity of copper, and steels have abut one-tenth the conductivity of copper. Thermal conductivity is measured in calories per square centimeter per second per degree Celsius.
- Electrical conductivity: is the capacity of metal to conduct an electric current. A measure of electrical conductivity is provided by the ability of a metal to conduct the passage of electrical current. Its opposite is resistivity, which is measured in micro-ohms per cubic centimeter at a standardize temperature, usually 20°C. Electrical conductivity is usually considered as a percentage and is related to copper or silver. Temperature bears an important part in this property. As temperature of a metal increases, its conductivity decreases. This property is particularly important to resistance welding and to electrical circuits.
Coefficient of Linear Thermal Expansion
With few exceptions, solids expand when they are heated and contract when they are cooled. The coefficient of linear thermal expansion is a measure of the linear increase per unit length based on the change in temperature of the metal.
Expansion is the increase in the dimension of a metal caused by heat. The expansion of a metal in a longitudinal direction is known as the linear expansion. The coefficient of linear expansion is expressed as the linear expansion per unit length for one degree of temperature increase. When metals increase in size, they increase not only in length but also in breadth and thickness. This is called volumetric expansion.
The coefficient of linear and volumetric expansion varies over a wide range for different metals. Aluminum has the greatest coefficient of expansion, expanding almost twice as much as steel for the same temperature change. This is important for welding with respect to warpage, wapage control and fixturing, and for welding together dissimilar metals.
Corrosion resistance is the resistance to eating or wearing away by air, moisture, or other agents. c. Mechanical Properties.
The mechanical properties of metals determine the range of usefulness of the metal and establish the service needed.