Strength is a key attribute to assess when evaluating materials for a CNC machining project. Stronger metals can withstand higher mechanical loads, resist deformation, and maintain structural integrity under stress — making them ideal for components that must perform reliably in demanding environments.
Selecting the right metal for your machined component isn’t as simple as choosing an alloy with the highest tensile, yield, and/or compressive strength. You also need to consider what factors are determining that metal’s strength, the potential modes of failure, that specific alloy’s impact on machining, cost, and availability
This blog post will address these key considerations in more detail, as well as give you a strategy for how to choose the right metal for strength applications.
What Determines a Metal's Strength
Higher strength doesn’t always translate to better performance across the board; it often comes with trade-offs like reduced ductility, increased brittleness, or poor machinability.
A metal's degree of strength, and corresponding trade-offs, is dictated by a number of important factors, including composition, grain size, and work hardening.
Composition
A metal’s strength is largely determined by its chemical makeup. Key elements like carbon, chromium, nickel, and molybdenum directly impact mechanical properties.
For example, higher carbon in steel increases tensile strength and hardness but reduces ductility and machinability. Alloying with nickel and chromium improves toughness and corrosion resistance, while impurities can cause brittleness or weakness.
Grain Size
Grain size significantly influences a metal’s strength. Finer grains introduce more boundaries, which hinder dislocation movement—the main mechanism of plastic deformation—resulting in increased strength, as described by the Hall-Petch relationship. Larger grains may enhance ductility, but generally lower overall strength.
Work Hardening
Work hardening is the strengthening of metal through plastic deformation. Cold working increases dislocation density within the metal, making additional deformation more difficult and producing a higher yield and tensile strength (though often at the expense of ductility).
Heat Treatment
Heat treatment methods like annealing and tempering alter a metal’s strength characteristics. For example, while annealing softens metal for improved ductility, tempering increases hardness and tensile strength.
Modes of Failure
While it can mean catastrophic breakage, failure can also refer to permanent deformation, cracking, or loss of function.
The most common modes of failure in metals, especially in the context of strength applications, include yielding, fracture, fatigue, impact, and creep.
- Yielding – the onset of permanent deformation in a metal when the applied stress exceeds its elastic limit
- Fracture – the complete separation of a material into two or more pieces due to stress exceeding its ultimate strength
- Fatigue – progressive structural damage caused by cyclic loading, often occurring well below the material’s yield strength
- Impact failure – sudden breakage resulting from a high-rate load or shock, typically involving low ductility and high strain rates
- Creep – time-dependent, permanent deformation under constant stress at elevated temperatures
|
Failure Mode |
Visual Cue |
Typical Scenario |
Key Characteristics |
|
Yielding |
Metal bends or stretches but doesn’t break |
Overloaded beam slowly sagging under weight |
Permanent deformation begins once stress exceeds yield strength |
|
Fracture |
Clean or jagged break in the material |
Brittle crack in a cold steel rod |
Sudden failure after reaching ultimate tensile strength |
|
Fatigue |
Microscopic cracks growing over time |
Aircraft wing experiencing repeated stress cycles |
Initiated by cyclic loading, often below yield strength |
|
Impact |
Shattered or splintered break |
Hammer strike causing a brittle fracture |
High strain rate, low ductility, minimal warning |
|
Creep |
Slowly sagging or elongating part |
Turbine blade deforming under high heat and stress |
Time-dependent deformation at elevated temperatures |
Understanding how and why metals fail is essential for engineers who design parts for safety, durability, and performance.
Material Selection Strategy for Strength Applications
Selecting a strong metal for a machined part begins with defining the component’s main functional requirements and mechanical stresses, such as static loads, impacts, or cyclic forces.
Once those requirements are defined, engineers should then take machinability, cost, and material availability into account when determining which metal will best suit their component.
Machinability is a critical factor in material selection. Stronger metals are generally harder to machine, leading to increased tooling costs, longer cycle times, and greater complexity. Assess if the chosen material can be efficiently machined with current equipment. In some cases, selecting a more machinable alloy and then enhancing its strength through heat treatments—such as quenching, tempering, or precipitation hardening—can be more practical. These processes can significantly improve the mechanical properties of materials like 4140 steel or aluminum 7075.
Cost and availability are key factors in material selection. High-performance alloys such as Inconel or titanium provide excellent strength but are costlier and more complex to machine. When budget or lead time is a concern, engineers often choose accessible options like high-strength aluminum or alloy steels, which offer a balance of performance and manufacturability.
Early collaboration with manufacturing partners allows machinists to recommend material and design adjustments that improve manufacturability while maintaining required strength.
Amanda White
