In this tutorial, you will learn the basics of what kind of material is required for a particular design and the characteristics of some common materials used in the industry. In short, you will understand how to make an informed choice regarding selecting the material based on design requirements.
Contents:
- Criteria for Selecting a Material for the Design
- What is Cast Iron?
- Plain Carbon Steels
- What is an Alloy Steel?
- Aluminum as a Design Material
- Copper and its Alloys
- Polymers and Ceramics as Design Materials
- What is a Composite Material?
Criteria for Selecting a Material for the Design
In the life cycle of a product, when the design team selects materials for components or joints between components, many different situations will appear with new product development, cost reduction, product performance, and improved reliability, manufacturing performance, or assembly. Regardless of the situation, our goal is to find the lowest-cost material for good product performance and reliability. There are several steps in the material selection process.
- Determining as many requirements as possible is essential to increase the probability of understanding the existence of potential materials. For many products, some of these requirements do not apply, making the information-gathering process easier.
- Use material selection criteria to discard materials that do not meet all material selection criteria. When evaluating whether a material is suitable for an application, it is necessary to consider the value range of the material for the characteristic of interest.
- There may be candidates for which there is not enough data to indicate whether the material meets certain selection criteria. These materials need to be analyzed and tested to determine whether they meet the selection criteria.
- Selects materials that meet all material selection criteria at the lowest cost. Keep in mind that cost includes the cost of materials and the cost of manufacturing components or forming joints between components.
What is Cast Iron?
Cast iron is a generic term, which refers to a family of materials that differ widely in their mechanical properties. In addition to carbon, cast iron contains other elements like silicon, manganese, Sulphur, and phosphorus. Cast iron’s chemical composition differs from steel principally in its higher carbon content, being between 2 and 4.5%. A large amount of carbon present in some cast irons as graphite, makes some of these alloys easy to pour as a casting liquid and easy to machine as a solid.
- The most common means of fabrication is sand casting with subsequent machining operations. Cast irons are not easily welded, however.
- White Cast Iron is a very hard and brittle material. It is difficult to machine and has limited uses, such as in linings for cement mixers where its hardness is needed.
- Gray Cast Iron is the most used form of cast iron. Its graphite flakes give it its gray appearance and name. The graphite flakes also give it good lubricity and wear resistance. Its relatively low tensile strength recommends against its use in situations where large bending or fatigue loads are present, though it is sometimes used in low-cost engine crankshafts.
- Nodular (Ductile) Cast Iron has the highest tensile strength of the cast irons. It is tougher, stronger, more ductile, and less porous than gray cast iron.
Plain Carbon Steels
Carbon steel is an iron-carbon alloy, which contains up to 2.1 wt.% carbon. For carbon steels, there is no minimum specified content of other alloying elements, however, they often contain manganese. Depending on carbon content, Plain carbon Steel is classified into 3 major categories.
The following table details the classification of the different types of Plain Carbon Steel.
| Low Carbon Steel | Medium Carbon Steel | High Carbon Steel |
|---|---|---|
| Low-carbon consists of less than 0.30% of carbon. | Medium-carbon steel contains anywhere from 0.30% to 0.60% carbon. | High Carbon Steel has around 0.61% to 1.5% carbon. |
| Relatively weaker and softer but more easily welded and ductile | These metals can be improved via heat treatment | Lowest ductility, very wear-resistant, and almost always hardened and tempered before use |
| Used for automobile body parts, plates, and wire products. | Applications include shafts, axles, gears, rails, and railway wheels. | Used for edged tools, high-strength wires, and springs. |
What is an Alloy Steel?
Alloy steel is defined as carbon steel to which one or more alloying elements are added to obtain certain beneficial effects. The commonly added elements include silicon, manganese, nickel, chromium, molybdenum, and tungsten.
- Chromium is added to improve strength, ductility, toughness, wear resistance, and harden-ability.
- Nickel is added to improve strength without loss of ductility and enhance case hardenability.
- Molybdenum, used in combination with nickel and/or chromium, adds hardness, reduces brittleness, and increases toughness.
- Tool Steels are medium- to high-carbon alloy steels especially formulated to give very high hardness in combination with wear resistance and sufficient toughness to resist the shock loads experienced in service as cutting tools, dies, and molds.
- Stainless Steels are alloy steels containing at least 10% chromium and offer much-improved corrosion resistance over plain or alloy steels, though their name should not be taken too literally. Stainless steels will stain and corrode slowly in severe environments such as seawater.
Aluminum as a Design Material
Aluminum alloys are recent in origin compared with copper or steel. However, due to a unique
combination of certain mechanical properties, they have become the most widely used nonferrous metal.
- The principal advantages of aluminum are its low density, good strength-to-weight ratio, ductility, excellent workability, castability, and weldability, corrosion resistance, high conductivity, and reasonable cost.
- Aluminum’s principal advantages are its bright finish and good corrosion resistance.
- Aluminum alloys have a face-centered cubic crystal structure with many slip planes. This makes the material ductile and easily shaped. They can be cast, rolled, forged or extruded. Aluminum’s strength is reduced at low temperatures as well as at elevated temperatures.
- Aluminum is among the most easily worked of engineering materials, though it tends to work harden. It casts, machines, welds, and hot and cold forms easily. It can also be extruded.
Copper and its Alloys
Pure copper is soft, weak, and malleable and is used primarily for piping, flashing, electrical conductors (wire), and motors. It cold works readily and can become brittle after forming, requiring annealing between successive draws. Many alloys are possible with copper. The most common are brasses and bronzes which themselves are families of alloys.
The table below lists the most common alloys of copper used in the industry.
| Alloy Name | Composition | Applications |
|---|---|---|
| High-copper alloys | >96% Copper | Electrical conductors & connectors, springs, fasteners. |
| Brasses | Cu – Zn | Deep drawn containers, tanks, heat exchangers, architectural panels, coins. |
| Leaded brasses | Cu – Zn – Pb | Cylinders, builder’s hardware, wear plates, fasteners. |
| Tin brasses | Cu – Zn – Sn – (Pb) | Electrical switches, springs, terminals, bearings. |
| Phosphor bronzes | Cu – Sn – P | Fasteners, springs, chemical hardware, wear plates. |
| Leaded phosphor bronzes | Cu – Sn – Pb – P | Bearings, bushings, gears, valves. |
| Aluminum bronzes | Cu – Al – Ni – Fe – Si – Sn | Heat exchangers, pump parts, machine parts, structural members. |
| Silicon bronzes | Cu – Si – Sn | Fasteners, springs, electrical connectors. |
| Copper – nickels | Cu – Ni – Fe | Condensers, heat exchangers, brake lines, saltwater pipes. |
| Nickel silvers | Cu – Ni –Zn | Silverplate (EPNS), nameplates, hollowware |
| Red brasses | Cu – Zn – Sn –(Pb) (75 – 89% Cu) | Valves, pump parts, plumbing hardware |
| Yellow brasses | Cu – Zn – Sn – (Pb) (57 – 74% Cu) | Fittings, trim, builder’s hardware |
| Tin bronzes | Cu – Sn – Zn – (Pb) | Gears, bearings, bushings, pump parts |
Polymers and Ceramics as Design Materials
The use of nonmetallic materials has increased greatly in the last 50 years. The usual advantages sought are lightweight, corrosion resistance, temperature resistance, dielectric strength, and ease of manufacture. There are three general categories of nonmetals of general engineering interest: polymers (plastics), ceramics, and composites.
- Polymers have a wide variety of properties, principally low weight, relatively low strength, and stiffness, good corrosion and electrical resistance, and relatively low cost per unit volume.
- Because of their variety, it is difficult to generalize about the mechanical properties of polymers, but compared to metals they have low density, low strength, low stiffness, nonlinear elastic stress-strain curves, low hardness, excellent electrical and corrosion resistance, and ease of fabrication.
- Polymers are divided into two classes, thermoplastic, and thermosets. Thermoplastic polymers can be melted and solidified repeatedly, although their attributes can decline given the high melt temperatures. When heated during the first time, thermosetting polymers cross-link and will burn instead of melt when reheated.
- Ceramic materials are finding increasing application in engineering, and a great deal of effort is being devoted to the development of new ceramic compounds. Ceramics are among the oldest known engineering materials; clay bricks are ceramic materials.
- The principal properties of ceramic materials are high hardness and brittleness, high temperature and chemical resistance, high compressive strength, high dielectric strength, and potentially low cost and weight.
- Ceramic materials are too hard to be machined by conventional techniques and are usually formed by compaction (usually using hydrostatic pressure) of powder, then fired or sintered to form bonds between particles and increase their strength.
What is a Composite Material?
Composites can have almost any combination of properties you want to build into them, including the highest specific strengths obtainable from any materials. Composites can be low or very high in cost.
- Man-made composites are typically a combination of some strong, fibrous material such as glass, carbon, or boron fibers glued together in a matrix of resin such as epoxy or polyester.
- Custom composites are finding increased use in highly stressed applications such as airframes due to their superior strength-to-weight ratios compared to the common structural metals.
- Small-diameter fibers of carbon and boron exhibit even higher tensile strengths than glass fiber, which explains their use in composites for spacecraft and military aircraft applications, where their relatively high cost is not a barrier.
- Wood is a composite of long cellulose fibers held together in a resinous matrix of lignin.
The following diagram classifies the different types of Composite Materials
Key Points to Remember
Here is the list of key points we need to remember about “Materials Selection and Design”.
- The selection of proper material is important when designing a product.
- Carbon Steel is one of the most common materials prevalent in the industry.
- Steel can be alloyed with multiple elements to enhance certain characteristics.
- Aluminum is a material widely used in light-weight designs, and can be alloyed with elements to increase its strength
- Copper is amongst the oldest metals used in the production industry and has fathered a large set of alloys depending on the application
- Ceramics are typically hard and chemically non-reactive and can be formed or densified with heat.
- Polymers have unique properties, depending on the type of molecules being bonded and how they are bonded.
- A composite material is a combination of two materials with different physical and chemical properties, when combined, specialized to do a certain job.
