Quick Answer: A metal strength chart compares yield strength (the stress at which permanent deformation begins) and tensile strength (the stress at fracture) alongside density and relative cost. For most structural and machined parts, yield strength is the governing design parameter because parts typically fail by deformation — losing shape, tolerance, or alignment — before they fracture. Representative yield strengths for common engineering metals: mild steel A36 at ~250 MPa, aluminum 6061-T6 at ~275 MPa, 4140 alloy steel (quenched and tempered) at ~650–850 MPa, stainless steel 304 at ~215 MPa, aluminum 7075-T6 at ~500 MPa, and titanium Ti-6Al-4V (Grade 5) at ~830 MPa. The correct material is never the one with the highest strength — it is the one that meets the load requirement at the required safety factor while satisfying constraints on weight, corrosion resistance, machinability, and cost. Specifying titanium where 7075 aluminum would meet the structural requirement increases material and machining cost by 5–10× with no functional benefit.
What a Metal Strength Chart Shows and How to Read It
| Material | Yield Strength (MPa) | Tensile Strength (MPa) | Density (g/cm³) | Typical Relative Cost | Machinability |
|---|---|---|---|---|---|
| Mild steel A36 | ~250 | ~400 | 7.85 | Low | Good |
| 4140 alloy steel (Q&T) | ~650–850 | ~850–1,000 | 7.85 | Low–moderate | Moderate |
| Stainless steel 304 | ~215 | ~505 | 8.00 | Moderate | Moderate (work hardens) |
| Stainless steel 316 | ~205 | ~515 | 8.00 | Moderate–high | Moderate |
| 17-4 PH stainless (H900) | ~1,170 | ~1,310 | 7.78 | High | Moderate–difficult |
| Aluminum 6061-T6 | ~275 | ~310 | 2.70 | Low–moderate | Excellent |
| Aluminum 7075-T6 | ~500 | ~570 | 2.81 | Moderate | Good |
| Titanium Ti-6Al-4V (Grade 5) | ~830 | ~900 | 4.43 | Very high | Difficult |
| Brass C360 | ~200 | ~350 | 8.50 | Moderate | Excellent |
| Copper C110 | ~70 | ~220 | 8.94 | Moderate | Excellent |
Reading the chart correctly: The numbers in the table are typical values for the specified temper or condition. Actual mechanical properties vary with heat treatment condition, product form (bar, plate, forging), and specific alloy lot. When the design requires material at the boundary of a strength class, verify with the supplier’s certified test report (material certification) rather than relying on typical handbook values.
The strength-to-weight ratio: Dividing yield strength by density gives a normalized metric for comparing materials across different densities — relevant for weight-critical applications. Aluminum 7075-T6 at 500 MPa / 2.81 g/cm³ = 178 MPa·cm³/g. Steel 4140 at 750 MPa / 7.85 g/cm³ = 96 MPa·cm³/g. Titanium Ti-6Al-4V at 830 MPa / 4.43 g/cm³ = 187 MPa·cm³/g. The comparison shows that both 7075 aluminum and titanium substantially outperform steel on a strength-to-weight basis — which is why both are used extensively in aerospace and performance applications where structural mass must be minimized.
Yield Strength vs Tensile Strength: Which Number Controls Design
Most engineers who look at a metal strength chart focus on tensile strength because it is the more dramatic number — the stress at which the material breaks. For most structural design applications, this is the wrong metric.
Yield strength is the design limit for parts that must hold shape: Yield strength is the stress at which the material transitions from elastic (recoverable) to plastic (permanent) deformation. Once the applied stress exceeds yield strength, the material deforms permanently. A bracket that is loaded beyond its yield strength bends — even a fraction of a millimeter — and may no longer align with adjacent components, maintain its dimensional tolerances, or transmit load as designed. The part has failed functionally without fracturing.
Tensile strength governs failure modes involving fracture: Tensile strength matters when the critical failure mode is catastrophic fracture rather than deformation — high-strength fasteners under tensile preload (where yielding of the bolt produces preload loss), impact-loaded components, and safety-critical structures where fracture (not bending) is the consequence of overload. In these cases, both yield and tensile strength must be checked.
The design approach: Most mechanical components are designed with the allowable stress set as a fraction of yield strength: Allowable stress = Yield strength / Safety factor. For general mechanical design, a safety factor of 2–3 is typical; for safety-critical components or uncertain loading, 3–5 or higher. An aluminum 6061-T6 bracket with yield strength of 275 MPa and a design safety factor of 2 has an allowable stress of 137 MPa — adequate for a bracket experiencing 100 MPa peak bending stress with substantial margin.
When both yield and tensile strength matter: A steel shaft transmitting cyclic torque requires both to be evaluated. The yield strength determines whether the shaft will twist under torque; the tensile strength (and the fatigue endurance limit, typically approximately 40–50% of tensile strength for steel) determines whether the shaft will survive millions of load cycles without fatigue crack propagation.
Steel vs Aluminum vs Titanium: The Core Material Decision
These three material families cover the majority of structural CNC machined components. Each dominates in one primary area.
Steel: Cost-Effective Structural Strength
Mild steel (A36, 1018) provides 250 MPa yield strength at the lowest cost of any structural metal, with excellent weldability and formability. For structural frames, machine bases, shafts, and general mechanical components where weight is not a constraint, mild or low-alloy steel is the economically correct first choice.
Alloy steel (4140, 4340) heat treated and tempered provides 650–1,000 MPa yield strength with good fatigue resistance and wear properties. These steels are the standard for high-stress rotating components, gears, shafts, and tooling where the higher cost over mild steel is justified by performance requirements that mild steel cannot meet.
Steel’s limitation: Density of approximately 7.85 g/cm³ is approximately 3× aluminum and 1.75× titanium. For components in motion (robot arms, lightweight structures, portable equipment), the weight penalty of steel imposes acceleration limits, energy costs, and structural mass that becomes the constraint rather than material strength.
Aluminum: Lightweight Versatility
Aluminum 6061-T6 is the production standard for most CNC machined aluminum components: yield strength of ~275 MPa, density of 2.70 g/cm³, and machinability that allows cutting speeds 4–5× higher than steel with substantially lower tool wear. For brackets, housings, fixtures, and moderate-load structural components, 6061-T6 is the correct default material for weight-sensitive applications.
Aluminum 7075-T6 offers yield strength of ~500 MPa — approximately 80% higher than 6061-T6 at essentially the same density — at moderately higher cost and slightly lower machinability. When 6061’s yield strength is insufficient for the load condition but titanium’s cost is unjustified, 7075 fills the performance gap.
Aluminum’s limitation: Lower stiffness than steel (aluminum’s elastic modulus is approximately 69 GPa vs 200 GPa for steel) means aluminum parts deflect approximately 3× more than geometrically identical steel parts under the same load. For parts where deflection (not yield) is the failure mode — long beams, precision stages, unsupported spans — this modulus difference may require a larger cross-section that partially offsets the weight advantage. Aluminum also has lower wear resistance than steel, making it less suitable for bearing surfaces and components with repeated contact without surface treatment.
Titanium: Maximum Strength-to-Weight
Titanium Ti-6Al-4V (Grade 5) at ~830 MPa yield strength and 4.43 g/cm³ density provides the highest strength-to-weight ratio of the three material families. Its corrosion resistance is superior to both aluminum and steel in most environments, including seawater, and it is biocompatible for medical implant applications.
Titanium’s limitations are practical: Material cost is 5–10× steel; machining cost is 3–5× aluminum due to low thermal conductivity (trapping heat at the tool tip), chemical reactivity with carbide tool binders at cutting temperatures, and required cutting speeds of 30–80 m/min versus 250–500 m/min for aluminum. For an application where 7075 aluminum meets the structural requirement, the decision to use titanium must be justified by a specific performance criterion (absolute strength, corrosion environment, biocompatibility, or temperature resistance) that 7075 cannot meet — not by the material being “stronger.”
Five-Step Material Selection Process Using a Strength Chart
Step 1: Determine the applied stress and required yield strength
Calculate the maximum stress the component will experience under all anticipated load cases (static load, dynamic load, shock load). Apply the appropriate safety factor: typically 2–3 for well-characterized static loads, 3–5 for dynamic or uncertain loads. The required material yield strength is the product of the calculated peak stress and the safety factor.
Step 2: Filter materials that meet the strength requirement
Using the metal strength chart, identify all materials whose yield strength exceeds the required value. This step typically produces multiple candidates — for a 200 MPa required yield strength, mild steel, 6061 aluminum, 7075 aluminum, 4140 steel, and titanium all qualify.
Step 3: Apply weight constraints
For weight-sensitive applications (aerospace, robotics, portable equipment), calculate the structural mass for each candidate material at the minimum cross-section that meets the stress requirement. The density difference between aluminum and steel (factor of ~3) typically produces a dramatic weight difference that eliminates steel for these applications regardless of cost.
Step 4: Evaluate corrosion environment
Apply the service environment as a filter: marine, chemical, or outdoor applications where untreated carbon steel would corrode to failure disqualify carbon steel unless coatings are included in the design. Stainless steel 316, aluminum (with anodizing where wear resistance is needed), and titanium are the materials that survive aggressive environments without coatings.
Step 5: Compare manufacturing cost
Machine time varies substantially between materials. As a rough guide, aluminum 6061 machines at 1×; 7075 aluminum at 1.2–1.5×; mild steel at 1.5–2×; 4140 alloy steel at 2–3×; 304 stainless at 2.5–4×; titanium at 5–10×. These multipliers reflect cutting speed restrictions and tool wear rate differences. For high-production-volume components, machinability is a primary cost driver that may dominate the decision even when material cost differences are smaller.
Common Mistakes When Reading a Metal Strength Chart
| Mistake | Consequence | Correct Approach |
|---|---|---|
| Selecting the highest yield strength material without checking if it meets all other requirements | Overdesigned, expensive part with difficult machining and no performance benefit over a lower-cost option | Calculate actual required yield strength including safety factor; use the weakest material that meets it |
| Using tensile strength as the design parameter for parts that must hold shape | Deformation failure at loads well below tensile strength | Use yield strength for deformation-controlled designs; tensile strength for fracture-critical designs |
| Ignoring material density when weight matters | Heavy parts in motion-sensitive or portable applications | Calculate structural mass per unit load capacity; use strength-to-weight ratio as the comparison metric |
| Applying steel tolerances to aluminum designs | Aluminum’s 3× lower elastic modulus produces substantially more deflection at the same cross-section | Recalculate deflection using aluminum’s E = 69 GPa, not steel’s E = 200 GPa |
| Not applying a safety factor to the strength chart value | Part operates near yield limit; small overloads or material variation cause unexpected failure | Always design at Allowable stress = Yield / Safety factor; never at nominal yield |
| Using titanium where 7075 aluminum meets the requirement | 5–10× higher material and machining cost with no functional benefit | Identify the specific criterion titanium meets that 7075 aluminum cannot; if none, choose 7075 |
Key Takeaways
- Yield strength, not tensile strength, is the governing design parameter for most structural and machined components: parts fail by deformation (losing shape, alignment, or tolerance) at stresses above yield strength, typically well before fracture occurs at tensile strength.
- The strength-to-weight ratio reveals where aluminum and titanium outperform steel: aluminum 7075-T6 at 178 MPa·cm³/g and titanium Ti-6Al-4V at 187 MPa·cm³/g substantially exceed 4140 steel at ~96 MPa·cm³/g. For weight-critical applications, this ratio — not absolute yield strength — is the relevant comparison metric.
- Aluminum 6061-T6 is the correct default material for most CNC machined non-ferrous parts: its combination of ~275 MPa yield strength, 2.70 g/cm³ density, and machinability (cutting speeds 4–5× faster than steel) produces the lowest total manufactured part cost for the majority of structural applications where its yield strength is sufficient.
- The correct use of a metal strength chart is as a filter, not a final selection: the chart narrows the candidate materials to those meeting the strength requirement; corrosion environment, weight budget, machinability, and cost determine the final selection among the qualifying candidates.
- Machinability is a cost driver of equal or greater importance than raw material cost for high-volume CNC production: titanium at 5–10× aluminum machining time dominates the per-part cost calculation for production quantities, often making it the wrong choice economically even when the strength-to-weight ratio is the best available.
- Safety factors should be applied to yield strength, not to tensile strength, for deformation-controlled designs: a safety factor of 2 applied to a yield strength of 275 MPa produces an allowable stress of 137 MPa; the same safety factor applied to a tensile strength of 310 MPa produces 155 MPa — a 13% higher allowable stress that ignores the deformation failure mode entirely.
- For OEM procurement and design teams: when specifying material on engineering drawings, the specification should include alloy, temper, and product form standard (e.g., “Aluminum 6061-T6 per ASTM B209” for plate, or “Aluminum 7075-T6 per AMS 2770” for aerospace heat treat). A drawing note of “aluminum” or “steel” without the alloy, temper, and applicable standard allows the supplier to substitute materials that may not meet the design’s mechanical properties — particularly relevant for strength-critical aerospace, medical, and automotive components.
Frequently Asked Questions
What is the strongest metal on a metal strength chart?
Among common engineering metals, high-strength alloy steels (4340, maraging steel, H13 tool steel) can exceed 1,700 MPa tensile strength in fully hardened condition, exceeding most other metals including titanium. However, these materials are rarely the practical choice: their very high hardness makes machining extremely difficult (requiring CBN or carbide grinding rather than standard milling), their density of ~7.85 g/cm³ produces high structural mass, and their cost is substantially above standard alloy steels. Among production-friendly materials, titanium Ti-6Al-4V (Grade 5) at ~830 MPa yield provides the best combination of high strength, lower density (4.43 g/cm³), and practical machineability. The “strongest” material is only meaningful in context — a 1,700 MPa steel is less useful than 7075 aluminum for a weight-critical part that only requires 400 MPa.
When should I use yield strength versus tensile strength as the design criterion?
Use yield strength as the design criterion when the part must maintain its shape and dimensional accuracy under load — brackets, frames, housings, shafts, and general structural components all fall in this category. If a part yields (deforms permanently), it fails functionally regardless of whether it subsequently fractures. Use tensile strength (or the fracture toughness) as the design criterion when the specific failure mode is catastrophic fracture rather than deformation — bolts under tension, safety-critical structures where the distinction between bending and fracture matters, and impact-loaded components. For fatigue-loaded components under cyclic loading, neither yield nor tensile strength directly applies; the relevant parameter is the fatigue endurance limit, typically approximately 40–50% of tensile strength for ferrous materials and requiring direct testing for non-ferrous metals.
Why is aluminum used instead of steel for many machined parts despite lower strength?
Aluminum’s combination of low density (~2.70 g/cm³ vs steel’s ~7.85 g/cm³) and excellent machinability (cutting speeds 4–5× faster than steel, substantially lower tool wear) produces lower total manufactured part cost for the majority of machined components where the structural loads are within aluminum’s capability. For a bracket experiencing 100 MPa peak stress, aluminum 6061-T6 (yield ~275 MPa) and mild steel A36 (yield ~250 MPa) both exceed the requirement with similar safety margins — but the aluminum bracket is approximately 65% lighter and costs less to machine. The stiffness disadvantage (aluminum’s elastic modulus of 69 GPa vs steel’s 200 GPa) means aluminum parts deflect approximately 3× more than geometrically identical steel parts, which may require a larger cross-section in deflection-critical applications.
What is the correct safety factor to apply to yield strength?
The safety factor selection depends on the load uncertainty, consequence of failure, and application standards. For general mechanical design with well-characterized static loads and no human safety consequence, a safety factor of 2–3 applied to yield strength is standard practice. For dynamic, shock, or vibration loads where the peak stress is uncertain, 3–4 is typical. For safety-critical applications in aerospace, pressure vessels, or structural applications where regulatory standards apply (ASME, FAA, etc.), the applicable standard specifies the required safety factors and the property basis (typically the minimum guaranteed properties rather than typical handbook values). The important point is that the safety factor is applied to yield strength for deformation-controlled designs — dividing tensile strength by the safety factor instead produces a higher allowable stress that ignores the yield failure mode.
When is titanium justified over aluminum or steel on a metal strength chart comparison?
Titanium is justified when at least one of the following specific criteria is present: (1) the application requires both high yield strength (above approximately 400–500 MPa) and low structural mass, and 7075-T6 aluminum’s yield strength is insufficient; (2) the corrosion environment is severe enough that both aluminum’s anodized surface and steel’s coatings are inadequate (seawater immersion, specific chemical environments); (3) the application is a medical implant where biocompatibility of titanium (ASTM F136, Ti-6Al-4V ELI) is a regulatory requirement; or (4) the service temperature exceeds approximately 120°C where aluminum alloys begin to lose yield strength. If none of these criteria apply, 7075-T6 aluminum typically meets structural requirements at 5–10× lower material and machining cost, and specifying titanium represents avoidable cost without performance benefit.
Written by the CMS engineering team with 15+ years of CNC machining experience producing structural components, brackets, housings, shafts, and precision parts in aluminum alloys (6061, 7075, 2024), carbon and alloy steel (A36, 4140, 4340), stainless steel (304, 316, 17-4 PH), titanium (Ti-6Al-4V), and brass and copper alloys for automotive, aerospace, medical device, industrial automation, and consumer electronics OEM manufacturing programs. Technical references: Ashby M.F. — Materials Selection in Mechanical Design (Yield and Tensile Strength selection methodology), ASTM B209 (Aluminum alloy plate and sheet), AMS 4928 (Titanium Ti-6Al-4V), ASTM A829 (Alloy steel 4140/4340), Shigley’s Mechanical Engineering Design (Material Selection and Safety Factor chapters), MatWeb Materials Property Data.
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