Overview of Mechanical Properties of Metals
In the metal manufacturing industry, mechanical properties play a huge role when choosing the right alloy for each job. Throughout casting and machining processes, as well as during the product's useful life, the chosen material will be subject to many external forces. It’s up to the manufacturers to create products that perform as needed every step of the way. By understanding mechanical properties, manufacturing professionals can make the right choices for materials and processes.
Mechanical properties describe how a material responds to applied loads or forces. These properties are not constant; they change based on temperature and other external factors, so manufacturers must have a full understanding of the operating environment of a part before recommending an appropriate material. Key mechanical properties of metals include:
- Ductility and Malleability
- Fatigue Resistance
Physical properties are another way of distinguishing metals. You can learn more about physical properties in our blog post here.
Strength of Metals
Strength can be measured in a number of ways, but generally represents how much force an object can withstand without bending or breaking. Strength-to-weight ratio is a key property of metals, because it tells manufacturers how much material must be used to meet a specific strength requirement.
Strength is often measured in terms of a material’s response to stresses, or applied forces, in three configurations: compression, tension and shear. When a material undergoes compression, forces act on it in the direction of its centerpoint. The opposite is true of tension, which pulls the material apart. Shear forces act parallel to one another in opposite directions.
Ductility and Malleability of Metals
While strength can be a good predictor of real-world performance, manufacturers need to push materials beyond their limits to form new shapes. Ductility and malleability indicate how easily a material can be manipulated without breaking. Ductility refers to a material’s response to tensile stress, or its ability to be stretched, rolled or extruded without breaking. Malleability refers to compressive stress, as in flattening. Aluminum’s high malleability is why it’s so widely used to make thin foil.
Since a material’s behavior changes depending on the temperature, a metal might have good ductility or malleability at high temperatures, but poor ductility or malleability at room temperature. Hundreds of years ago, blacksmiths were already aware of this variable, and heated iron-based products until glowing before hammering them into shape.
Fatigue Resistance of Metals
In real-world applications, metal parts often undergo repeated stresses over a long period of time. While none of these stresses, taken individually, would challenge the part’s strength, ductility or toughness, the repetition itself can lead to failure. In fact, according to ASM International, fatigue contributes to approximately 90% of mechanical failures.
Fatigue resistance is a measure of a part’s ability to undergo repeated, cyclical stress without fracturing or permanently deforming. Amounts of stress close to the material’s limit will result in earlier fatigue failure, compared to lower amounts of stress. Like other mechanical properties, the fatigue resistance of a given alloy depends on the temperature at which the stress is applied, as well as other environmental factors.
The fatigue resistance of a final part depends not only on its material makeup, but also on the presence of defects. Issues like porosity, cracking or other flaws caused during manufacturing can drastically reduce the fatigue resistance of a part. For an overview of common casting defects and how to prevent them, read our blog post.
Hardness in materials can refer to performance across a number of parameters, including resistance to deformation, resistance to scratching or resistance to cutting. Metals show a strong correlation between tensile strength and hardness, and hardness can often be increased in steel by increasing its percentage of carbon.
Hardness can be measured in many ways, and the Brinell Hardness Test is one of the most common for metal products. The test, first proposed in the year 1900, involves pressing a sphere made of carbide or hardened steel into the surface of the part being measured. The resulting indentation on the surface is then measured using a microscope, and the Brinell Hardness Number (BHN) is given as the load divided by the surface area of the indentation.
Harder alloys are often less machinable, which can create obstacles when manufacturing products. When taking Design for Manufacturability into account, it is sometimes better overall to use a less hard alloy for a casting, especially if the product will undergo extensive machining at a later stage.
Choosing the Right Alloy Based on Mechanical Properties
In the world of manufacturing, the most important mechanical properties depend on the product's intended use. Since mechanical properties vary with the environment, the product design process must include a thorough analysis of the environment in which the product will perform, including temperature, moisture and load requirements. Manufacturers also must consider all forming processes the material will undergo before emerging as a final product. Often, engineers and metallurgists are tasked with finding a balance between performance metrics like strength, and process metrics like machinability.
An APQP process can ensure that all relevant variables have been considered before choosing an alloy, and the best foundries follow APQP procedures throughout product design. To learn more about optimal product design steps in metalcasting & CNC machining, read our blog series here.
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Written by Jason Bergman
Jason Bergman is Senior Quality Engineer and Metallurgist at Eagle Alloy. He has been with the company since 2013.