Weldability of Metal Types

Understanding welding metals is one of the pillars of practical knowledge required for a productive welder. Each metal and metal alloy behaves in different ways to heat and in the manner they can be manipulated. Metals expand and soften whenever heated, leading to numerous uses and applications. Additionally they respond in a variety of ways to the different kinds of welding techniques utilised.

Metal Identification

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Metals need to be identified prior to welding so as to choose the appropriate electrode and method. As an illustration, magnesium and aluminium possess a similar physical appearance nonetheless require completely different welding techniques. An understanding of the various tests, physical and mechanical properties is essentially utilized to ascertain the origin of metals.

Most of the metals and alloys referred to here may be welded by several major welding techniques (Stick, TIG, MIG, Oxyfuel). Understanding of the characteristics of metals and their alloys along with particular reference to their relevance in welding procedures becomes necessary for successful welding operations.

All metals fall within two categories, ferrous or nonferrous.

Ferrous metals – are metals that incorporate iron.

Ferrous metals are available in the form of cast iron, carbon steel, and tool steel. The different alloys of iron, subsequent to undergoing certain processes, are pig iron, grey cast iron, white iron, white cast iron, malleable cast iron, wrought iron, alloy steel, and carbon steel. All these different types of iron are mixtures of iron and carbon, manganese, sulphur, silicon, and phosphorous. Additional elements can also be present, but in concentrations that do not significantly have an impact on the characteristics of the metal.

Nonferrous metals – are those which don't contain iron.

Aluminium, copper, magnesium, and titanium alloys are some of those metals that belong to this group of non-ferrous metals.

Considerations

There are generally several considerations when selecting a metal for welding. This includes:

• Melting point: temperature at which the solid and liquid forms of the metal can exist in equilibrium
• Ductility: how the metal responds to bending and stretching
• Electrical Conductivity: effects what the metal can be utilised for
• Strength: what is the breaking point of a metal

Physical Properties of Metals

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Many of the physical properties of metals and alloys determine if and how they can be welded and how they will perform in service. Physical properties several metal are shown in the tables below.

Mass Or Density

Mass or density pertains to mass with respect to volume. Typically referred to 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 illustration, the ratio of weight of one cubic meter of water to one cubic meter of cast iron is the specific gravity of cast iron. This property is measured by grams per cubic millimetre or centimetre in the metric system.

Melting Point

The melting point of a metal is extremely important with respect to welding. A metal’s fusibility relates to its melting point, the temperature at which the metal transformations from a solid state to a molten state.

Pure materials possess a sharp melting point and cross from a solid-state to a liquid state without a change in temperature. During this process, however, there exists an absorption of heat during the melting and liberation of heat during freezing. The absorption or release of thermal energy whenever a material changes state is called it's latent heat.

Mercury is the sole common metal that's in its molten state at normal room temperature. Metals possessing low melting temperatures can be welded using lower temperature heat sources. The soldering and brazing processes make use of low-temperature metals to join metals possessing higher melting temperatures.

Boiling Point

The boiling point is additionally an important factor in welding. The boiling point is the temperature when the metal changes from the liquid state to the vapour state. Some metals, when exposed to heat of an arc, will vaporise.

Conductivity

Thermal and electrical conductivity correspond with the metal’s capacity to conduct or transfer heat and electricity.

1. Thermal conductivity: Thermal conductivity is the ability of a metal to transmit heat throughout its mass, is of essential importance in welding, considering one metal might transmit heat from the welding area much more rapidly than another. The thermal conductivity of a metal indicates the requirement for preheating and the size of heat source necessary. Thermal conductivity is typically related to copper. Copper has got the highest thermal conductivity of common metals, exceeded only by silver. Aluminium possesses approximately half the thermal conductivity of copper, and steels possess abut one-tenth the conductivity of copper. Thermal conductivity is measured in calories per square centimetre per second per degree Celsius.
2. Electrical conductivity: Electrical conductivity is the capacity of metal to conduct an electric current. A measure of electrical conductivity is supplied by the capability of a metal to conduct the passage of electrical current. It's opposite is resistivity, that is measured in micro-ohms per cubic centimetre at a standardised temperature, usually 20°C. Electrical conductivity is typically considered as a percentage and is related to copper or silver. Temperature comprises an important part in this particular property. As temperature of a metal raises, its conductivity decreases. This property is particularly crucial to resistance welding and to electrical circuits.

Coefficient Of Linear Thermal Expansion

With few exceptions, solids expand whenever they are heated and contract whenever 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 referred to 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 additionally in breadth and thickness. This is termed volumetric expansion.

The coefficient of linear and volumetric expansion ranges over a wide range for different metals. Aluminium has the greatest coefficient of expansion, expanding almost twice as much as steel for the same temperature change. This is of importance to welding with respect to warpage, warpage control and fixturing, and for welding together dissimilar metals.

Corrosion Resistance

Corrosion resistance is the resistance to eating or wearing away by air, moisture, or other agents. i.e., the mechanical properties. The mechanical properties of metals determine the range of usefulness of the metal and establish the service required.

Types Of Welding Metals

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Cast Iron

Cast-iron is a material of preference for numerous applications because of its characteristic hardness, which originates from a high carbon content. Nevertheless, cast iron is usually challenging to work with and weld as it's hardness results in less malleability and ductility. Since it doesn't stretch or deform, cast iron is a lot more susceptible to cracking.

In terms of welding metals, low carbon steel is easier to weld than cast iron. Cast iron contains higher carbon and silicon content, and is significantly less ductile. An expert welder can help reduce the occurrence of cracks by ensuring that cast iron is appropriately preheated. When welding with cast iron the surface will have to be cleaned to eliminate any ingrained grease and oil. All cracks need to be grinded or filed.

Cast iron is welded with oxyacetylene welding.

Low Carbon Mild Steel

Steel is an alloy that contains iron and 2% of other elements. Low carbon steel alloy is common and can be found in high, low and medium varieties. Higher carbon content signifies stronger steel.

Low carbon mild steel is probably the most weldable metal. This is as a result of a number of different factors. The historic abundant use and demand for low carbon steel has resulted in scientists and engineers developing different ways to weld it. Breakthroughs include electrical arc waveform, special filler material chemical compositions, and top-of-the-line welding power sources to weld low carbon steel. All this makes it possible for welders in making satisfactory carbon steel welds with relative ease.

Another explanation why low carbon steel is extremely weldable is because it is much more ductile compared to other types of steel. This is because it has very low amounts of carbon and only trace amounts of other alloying elements. This prevents the formation of brittle micro-structures including martensite. All of this eliminates the risks of certain types of weld failures, including hydrogen cracking. As the amount of carbon increases, so does the difficulty of fabricating a satisfactory weld.

Steel is versatile which enables it to be used with any welding process. Welding areas need to be cleaned. On the downside, it can rust and flake from oxidation.

Stainless Steel

Unlike plain steel, stainless steel was designed to resist corrosion and is particularly hygienic. This is accomplished with the addition of 10% to 30% chromium to other elements such as iron. Additionally there is a nickel alloy available.

Stainless steel is a metal which can also be welded conveniently when proper technique is utilized and satisfactory expertise is applied. Many stainless steels, recognised for their corrosion resistance, can be welded inspite of their complex chemical composition. The most crucial consideration when welding stainless steel is to determine what grade is being welded. There are three major types of stainless steel: austenitic, ferritic, and martensitic. Many ferritic stainless steels are very weldable. Austenitic stainless steels is usually weldable also. Martensitic stainless steels are typically more challenging because of their high hardness and their tendency to crack.

A major concern when welding stainless steels is intergranular corrosion. When stainless steels are subjected to high temperature environments, such as those that occur during welding, the chromium may be susceptible to joining along with the carbon within the steel. This formation of chromium carbide prevents the ability of the chromium to combine with oxygen. Consequently, no chromium oxide layer is formed, and the oxygen is free to combine with the iron in stainless steel, resulting in corrosion. There are a number of techniques available which can prevent this. Using a grade stabilised with titanium or niobium such as Grade 321 can prevent intergranular corrosion as the titanium is more likely to form with the carbon atoms before the chromium. An additional method to prevent intergranular corrosion is by making use of low carbon stainless steels. These simply do not make available enough carbon to stop the formation of a protective chromium oxide layer.

Stainless is welded by means of arc welding using the MMA, Stick, Tig or Mig/Mag processes. The disadvantage is the higher cost.

Aluminium

Similar to stainless steel, aluminium likewise isn’t as corrosive as other metals. It is lighter than stainless steel. In welding, pure aluminium and alloys are generally used.

Aluminium alloys include:

• copper aluminium alloy
• manganese aluminium alloy
• zinc aluminium alloy

While it is typically more difficult than low carbon steel, aluminium can be welded without much difficulty provided that appropriate skills and techniques are utilized. A particular concern when welding aluminium is ensuring that the suitable grade is selected. Grades in the 1XXX series can be welded without significant additional effort. Grades in the 6XXX series can be welded, however , proper filler material and welding operation should be used to assist in crack prevention. Aluminium in the 2XXX series is usually not weldable ın any way, nevertheless some grades in this series can be welded with proper filler material and technique.

One other consideration when welding aluminium is material strength. Some aluminium alloys, including those with a T6 designation, have been artificially aged to increase their strength. Consequently, they've been heated to a certain temperature for a specific amount of time to ensure intermetallic precipitates are the proper size and shape to increase the strength of the aluminium. When these grades of aluminium are welded, the intermetallic precipitates change their form, and it is typical for the strength of the aluminium to get reduced significantly. To return these welded aluminium alloys back to their original strength, they must be artificially aged again by using a heat treatment process.

Tig welding (GTAW) is the process of preference for welding aluminium. Other welding methods which are used include GMAW (gas metal arc welding or Mig). Stick aluminium welding is only utilized for smaller projects. The process commences by selecting a joint design for the base metals (tee, lap, edge, corner or butt).

Copper

Among the welding metals, copper is widely used because of its electrical conductivity, heat conductivity, corrosion resistance, appearance and wear resistance. To be identified as copper, it requires to be 99.3% minimum copper content.

Copper and copper alloys provide a unique combination of material properties which makes them advantageous for numerous manufacturing environments. They are widely used for their excellent electrical and thermal conductivity, outstanding resistance to corrosion, ease of fabrication, and good strength and fatigue resistance. Additional advantageous characteristics comprise of spark resistance, metal-to-metal wear resistance, low-permeability properties, and distinctive colour.

There are multiple types of copper alloys:

• copper-nickel-zinc alloy called nickel silver
• copper-nickel alloy
• copper-silicon called silicon bronze
• copper-aluminium alloy called aluminium bronze
• copper-tin alloy
• copper-zinc alloy also known as brass
• high copper alloys (up to 5% alloy)

Processes utilised in welding include welding, brazing and soldering. Copper is normally welded by using Gas Tungsten Arc Welding (TIG) and Gas Metal Arc Welding. Some welders use manually operated metal arc welding, but it can result in low quality. When welding copper, the joint designs are generally wider when compared to those recommended for steel. The shielding gas for copper is welding grade argon.

Weld areas are cleaned using a wire bronze brush thereafter degreased. Oxides which form must be removed after welding. Copper is preheated, however, copper alloys don't need to be preheated as a consequence of high levels of thermal conductivity.

Nickel Alloys

Many applications for Nickel-base alloys necessitate the utilization of a welding or joining process before the component will be installed into service or for a repair procedure after the component has already experienced service. Fabrication or repair can be accomplished using several welding processes and alloys provided the process, process parameters, and metallurgy of the alloy are considered. The capability of Ni-base alloys to be joined by welding processes and also to perform as intended in their service environment describes their ‘weldability.’ Testing procedures are available which can analyze weldability ın order that proper material or process selection will be implemented during fabrication and repair.

Nickel welding metals are available in the form of several alloys. These include:

• Nickel Alloy 141: Utilized for welding cast and wrought pure nickel (nickel 200 and 201). It is additionally used to join nickel to steel.
• Nickel Alloy 61: Same as previously mentioned.
• Nickel-Copper Alloy 190: For welding to itself as well as to steel.
• Nickel-Copper Alloy 60: Used for welding to itself.

Compared to other alloys, Ni-base alloys demonstrate both gish welding and shallow penetration characteristics, that is typically as a consequence of low viscosity of molten nickel. Accordingly, joint design and weld bead placement should be meticulously considered to make certain that accurate weld bead fusion is accomplished. Nickel alloys in addition have a predisposition to crater crack, so grinding of starts and stops is advisable. An additional important factor to achieve good welds is cleanliness of the weld joint region. Contamination by grease, oil, corrosion product, lead, sulfur, as well as other low-melting-point elements can result in severe cracking problems.

The three most common welding processes which are used to join Ni-base alloys are gas tungsten arc welding (GTAW), gas metal arc welding (GMAW), and shielded metal arc welding (SMAW). Besides these common welding processes, other welding processes including flux-cored arc welding (FCAW), submerged arc welding (SAW), plasma arc welding, resistance spot welding, laser beam welding (LBW), and electron beam welding (EBW) are used. The plasma arc cutting process is often used to cut alloy plate into required shapes and prepare the weld geometry. The use of oxyacetylene welding and cutting is not recommended due to carbon pickup from the flame. For all welding processes, weld parameter selection, since it relates to heat input, is extremely important because of the greater possibility of hot cracking as heat input increases.

Magnesium

Magnesium alloys are lightweight (2/3 of aluminium), they absorb vibration and are easy to cast. They have a melting temperature comparable to aluminium and are welded similarly. When grinding magnesium, be aware that the shavings are flammable.)

Currently magnesium alloys have not typically been welded with the exception of some repaired structures due to the occurrence of defects including oxide films, cracks, and cavities. Magnesium alloy components can be joined making use of mechanical clasps in addition to a variation of welding methods including tungsten arc inert gas (TIG), plasma arc welding, electron beam welding (EBW), laser beam welding (LBW), friction stir welding (FSW), explosion, electromagnetic welding, ultrasonic welding, and resistance spot welding (RSW).

Titanium

Titanium is commonly utilized in applications that require high stress without a lot of added weight. It is merely half as heavy as steel but just as strong, accompanied by a high corrosion resistance. Titanium additionally retains good mechanical properties at high temperatures. The most crucial precaution that needs to be taken in order to successfully weld titanium is to use a strong shielding gas. The reason being, titanium quickly changes to titanium dioxide when exposed to oxygen.

Lead

Lead is usually identified by the colour associated with unfinished surface (white to grey), the colour and structure of a newly fractured surface (light grey, crystalline), the colour of a freshly filed surface (white).

The welding of lead is identical to welding of other metals with the exception that no flux is required. Processes besides gas welding are not in general use.

Three combinations of gases are typically used for lead welding:

• Oxyacetylene
• Oxyhydrogen
• Oxygen-natural gas

The oxyacetylene and oxyhydrogen processes are generally sufficient for all positions. The oxygen-natural gas is not used for overhead welding.

The filler rods should be belonging to the same composition as the lead to be welded. The smaller sizes are utilized for lightweight lead and the larger sizes for heavier lead.

Zinc Alloys

The zinc metal is brittle at room temperatures, necessitating elevated temperatures for mechanical working. Sheet surfaces are frequently mechanically abraded for the purpose of cleaning before spot welding begins. Zinc alloys possess 2x the electrical conductivity of steel, lower melting ranges, higher thermal conductivity and approximately the equivalent specific heats.

Zinc alloys are slightly lighter as compared to iron. There are two types used in welding:

• rolled zinc
• cast zinc

Typical uses include cast forms including die-casting, sheets utilized as roofing and in plants that do chemical processing.

The oxyacetylene and oxyhydrogen processes are generally suitable for all positions. The welding rod can be of pure zinc or a die-casting alloy of the same type as that to be welded. Metal flux of 50 percent zinc chloride and 50 percent ammonium chloride, may be used, although is not mandatory.

Resistance welding is a process employed to weld rolled zinc alloys. It makes use of the least heat input in comparison to arc or gas welding and provides good welds. Seam or spot welding is carried out on flange or lap joints. The overlap size is determined by the thickness of the metal sheet.

Weldability and Joinability

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Weldability is the ability of any material, typically metals and its alloys, to weld with comparable materials.The weldability and joinability of common materials is an important factor for designing real systems, generally very complex if not large. Many metals and thermoplastics could be welded together to fabricate a final material that is required further in the fabrication process or any other industrial process.

Metal Welding

Different welding processes work for different types of metal. Each type of welding process comes with advantages and disadvantages.