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What is a Metal Alloy?

What is a Metal Alloy

Last updated: 11/04/2023 | Author: Nick Nolan | Posted in: Materials

You’ve likely heard of the term alloy, but what exactly does that mean? Metal alloys are used everywhere. From the tiny screws that hold eyeglasses together, to the frame of a skyscraper. Parts of the phone or computer you’re reading this on are metal alloys.

Most of the metal parts and structures you see are metal alloys.

In simple terms, a metal alloy is a specific mixture of metallic and nonmetallic elements. The elements used to make the alloy are melted down into a liquid, mixed together, and then cooled to resolidify. This process is sometimes called alloyization.

Why are metal alloys used?

You’re probably familiar with most of the metallic elements in the periodic table. Metals like aluminum, iron, copper, gold, and titanium are naturally occurring elements. You might be surprised to learn that the majority of the elements are classified as metals.

Metal, Metalloid, & Non-Metal Elements

Not every element is naturally occurring, but for the ones that are, you can dig them up from the ground, and process them into metal bars or plates.

Alloys are used because in most cases, a pure, single metallic element isn’t ideal for parts or building structures.

Adding other metal or nonmetal elements improves specific properties of the alloy. The strength, durability, color, flexibility, machinability, and corrosion resistance can all be significantly improved.

For example, adding a small amount of lead or sulfur to alloys improves the machinability of metals. And adding carbon increases hardness, but decreases machinability.

Because the percentages of the elements used in alloys can be easily adjustable, there are 100s of different alloys–each having specific pros and cons.

What are the most common metal alloys?

Metal alloys usually have a base metal that makes up the majority of the composition. In addition to the base element, there are common additions. 

Numbers that are used to identify the different alloys, like 303 stainless steel, or 2011 aluminum. The Unified Numbering System (UNS) was formed in the 1970s, through a collaboration between the American Society for Testing and Materials (ASTM) and the Society of Automotive Engineers (SAE) to create clear identifying numbers for metal alloys. 

Previously, there was confusion due to the same numbers being used for different alloys, or different names used for the same alloy.

Here are a few of the common metal alloys used in precision machining.

Stainless steel alloys

Stainless steel is one of the most commonly used alloys. Stainless steel alloys have an iron base, usually around 25-35%. Another 20-30% of chromium and nickel are added to the iron to improve corrosion resistance and prevent the metal from rusting.

The remainder of stainless steel alloys are made up of 7-10 other elements. Stainless steel alloys can contain smaller amounts of sulfur, silicon, copper, carbon, nitrogen, and manganese. These elements can make the alloy harder or softer, improve machinability, and increase the corrosion resistance.

Most common stainless steel alloys: 303, 304, 316/316L, 416

Steel alloys

Steel is another commonly used metal alloy that’s incredibly diverse. Steel alloys are made with 95-99% iron. Surprisingly, pure iron isn’t useful for parts or building materials because it’s soft and brittle. It’s easily bent and malleable, until other elements are added to create a steel alloy.

Adding a small amount of carbon to the iron makes it hard, and usable for durable parts and building materials. But, adding too much carbon makes the alloy more difficult to machine. To improve the machinability of steel, elements like sulfur and lead are added to create free machining steel alloys.

Most common steel alloys: 1018, 12L14, 4140, A2, A36

Aluminum alloys

Pure aluminum is a great material, but it lacks strength and durability needed to make usable parts. To increase the tensile strength, zinc and manganese are added to aluminum alloys. Chromium can also be added to increase the corrosion resistance of the aluminum parts. 

Stronger aluminum alloys are significantly lighter than steel and stainless steel, and commonly used in the aerospace industry.

Most aluminum alloys are 85-95% aluminum. This keeps the benefits that aluminum offers–mainly the excellent strength-to-weight ratio.

Most common aluminum alloys: 2011, 2024, 5052, 6061, 7075

Copper alloys

Copper is one of the best conductors of electricity and heat. So, pure copper (99%+) is used for electrical wiring. But, copper is soft and flexible, so copper alloys are used for parts that need more strength and durability.

Copper has a wide variety of alloys that include different copper alloys, brass, and bronze. Free machining copper is 97-99% copper, and small amounts of lead, beryllium, cobalt, and/or tellurium.

Brass alloys are a combination of copper, zinc, and lead. The zinc adds hardness and durability to the parts. 

Bronze alloys are a mixture of copper and tin. Bronze is even harder than brass alloys, so it’s used when more durability is needed.

Learn about the difference between copper, brass, and bronze.

Nickel alloys

Nickel is a metal that’s added in smaller amounts to stainless steel alloys. Nickel offers some of the best corrosion resistance. It’s also more expensive than most other metals. Nickel based alloys are often considered super alloys, because of the high corrosion resistance and strength.

Monel is a nickel-copper alloy that contains around 65% nickel and 30% copper. 

Inconel is a nickel-chromium alloy that’s roughly 70% nickel, 15% chromium, and 8% iron.

Hastelloy is a nickel-chromium-molybdenum alloy. It contains 55-60% nickel, 15% chromium, and 15% molybdenum.

Classifications of alloys

Metal alloys can be classified in several ways, depending on factors such as the base metal, structural arrangement, or specific properties. 

Base metal

Ferrous Alloys: These alloys contain iron as the primary base metal. Examples include various steel and cast iron alloys. Ferrous alloys are known for their strength and durability and are widely used in construction, automotive, and other heavy-duty applications.

Non-Ferrous Alloys: These alloys do not contain iron as the primary base metal. Examples include aluminum, copper, nickel, and titanium alloys. Non-ferrous alloys are generally lightweight, corrosion-resistant, and have high thermal and electrical conductivity.

Structural arrangement

Substitutional Alloys: In these alloys, atoms of the base metal are replaced by atoms of another metal or element within the crystal lattice. This results in a uniform distribution of the alloying elements and leads to enhanced properties such as strength, hardness, and corrosion resistance. Examples include brass (copper and zinc) and stainless steel (iron, chromium, and nickel).

Interstitial Alloys: In these alloys, atoms of the alloying element are inserted into the interstices or gaps within the crystal lattice of the base metal. This can lead to significant changes in the properties of the base metal, such as increased hardness and strength. An example of an interstitial alloy is steel, where carbon atoms occupy the interstitial spaces within the iron lattice.

Properties

High-Temperature Alloys: These alloys are designed to maintain their strength and resistance to deformation at elevated temperatures. Adding Nickel and elements with very high melting points increases an alloy’s heat resistance. Examples include Inconel and Hastelloy, which are used in aerospace and high-temperature industrial applications.

Corrosion-Resistant Alloys: These alloys are designed to resist corrosion in various environments, such as exposure to water, chemicals, or high humidity. Examples include stainless steel and aluminum alloys, which are widely used in marine, chemical, and food processing industries. Chromium is particularly notable for its role in stainless steel alloys, where it creates a passive layer of chromium oxide that prevents further surface corrosion.

Secondary Processes

Metal alloys often undergo secondary processes to enhance their properties, appearance, or performance in specific applications.

Some common secondary processes include:

  1. Heat Treating: Heat treatment involves heating and cooling metal alloys to alter their microstructure and achieve desired properties such as hardness, strength, ductility, or toughness. Common heat treatment processes include annealing, quenching, and tempering. For example, steel can be heat-treated to improve its hardness and wear resistance or to make it more ductile for forming operations.
  2. Anodizing: Anodizing is an electrochemical process used to form a protective oxide layer on the surface of aluminum alloys. This layer enhances the corrosion resistance, wear resistance, and appearance of the metal. Anodized aluminum is used in applications where durability and aesthetics are essential, such as automotive trim, architectural components, and consumer electronics.
  3. Plating: Plating involves depositing a thin layer of metal or other materials onto the surface of a metal alloy, often to improve corrosion resistance, wear resistance, electrical conductivity, or appearance. Common plating methods include electroplating, electroless plating, and thermal spraying. For example, gold or silver plating is often used to enhance the appearance and conductivity of electrical connectors. Steel parts are often plated to increase corrosion resistance.
  4. Surface Finishing: Surface finishing processes are used to alter the surface properties or appearance of metal alloys. Common surface finishing techniques include polishing, grinding, sandblasting, and chemical etching.

Those are some of the most common metal alloys used for precision machined parts. But, the properties of the alloy can be changed by adding 0.2% of a certain element. This makes the combinations and applications of different alloys limitless.

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