Manufacturing Expertise

A practical guide to materials selection engineering for prototypes, the six decisions that drive program economics, and how the material choice shifts when prototypes move into limited run manufacturing.

Materials selection engineering is the discipline of choosing the right material for a part based on the load it will carry, the environment it will operate in, the process that will produce it, and the cost the program can absorb. For engineering prototypes, the material decision is one of the most consequential single choices in the design. It determines whether the prototype actually proves what it is meant to prove, how long it takes to build, and what the part will cost.

This guide covers what materials selection engineering means in practice, the six decisions that drive cost, performance, and lead time on every prototype, and how the material decision shifts across CNC machining, limited run manufacturing, and regulated-industry programs. It is written for engineers, R&D leads, and program managers responsible for prototype designs that will eventually move into production. For the broader picture of how materials selection fits within a manufacturing program, see our complete guide to custom manufacturing.

What is materials selection in engineering?

Materials selection in engineering is the structured process of matching a material’s properties to the functional, manufacturing, and economic requirements of a part. It sits at the intersection of mechanical engineering, materials science, and manufacturing engineering. The output of a materials selection engineering exercise is a specific material grade, condition, and certification level, captured on the drawing or in the bill of materials.

Materials selection plays three roles in any engineering program. It defines the performance envelope: how much load the part can carry, how hot it can run, how long it can resist corrosion. It constrains manufacturability: a material that cannot be machined or welded at the required tolerance will derail any design. It sets the cost floor: material is often the single largest line item in a prototype’s bill of materials.

Materials selection engineering is not a one-time decision. The material chosen at concept may be revisited as the design matures, as test data comes in, and as the program moves from prototype into limited run manufacturing and eventually production. Strong materials selection treats the decision as iterative and tied directly to the rest of the design.

What are the steps of materials selection in engineering?

The standard materials selection engineering framework follows four steps, most commonly attributed to Professor Michael Ashby at Cambridge.

• Step 1: Translate design requirements into material property requirements. Identify the functions the part performs, the loads and environments it will see, and the constraints it operates under. Convert each into a minimum or target value for a material property.
• Step 2: Screen candidate materials against property thresholds. Eliminate materials that cannot meet the minimum property requirements. This narrows a universe of thousands of materials down to a manageable shortlist of candidates.
• Step 3: Rank the surviving candidates using performance indices. Combine multiple properties into a single ranking metric (for example, strength-to-weight ratio for aerospace structural parts). Plot candidates on Ashby charts where two performance indices intersect.
• Step 4: Choose with manufacturability and cost in mind. Apply the final filters: can the material be machined, welded, or formed by the intended process? Is the supply base reliable? Does the cost fit the program budget?

The framework looks linear, but in practice it loops. New test data can drive a return to Step 1. A manufacturability constraint discovered at Step 4 can eliminate the top-ranked candidate and force a return to Step 3. Materials selection engineering is most effective when the loop is fast and the engineers running it have hands-on experience with the manufacturing processes that will produce the part.

What criteria drive materials selection?

Materials selection criteria fall into five property categories. Most engineering prototypes are evaluated against all five, though the weighting varies by application.

• Mechanical properties: yield strength, ultimate tensile strength, elongation, hardness, fatigue resistance, and elastic modulus. These determine whether the part can carry the loads the application demands.
• Thermal and environmental properties: operating temperature range, coefficient of thermal expansion, thermal conductivity, and behavior under thermal cycling. These determine whether the part performs reliably in its operating environment.
• Chemical and corrosion resistance: behavior under exposure to moisture, salt, acids, bases, solvents, and process chemicals. These determine the part’s service life in its operating environment.
• Manufacturability: machinability, weldability, formability, castability, and printability. These determine whether the part can be produced reliably and economically by the intended process.
• Cost and availability: per-pound material cost, stock form availability, lead time, and certification overhead. These determine whether the material decision fits the program’s economic and schedule constraints.

A common materials selection engineering mistake is over-weighting one criterion at the expense of the others. A material that meets every mechanical property requirement but has a 16-week lead time is not the right material for a prototype on a six-week schedule. A material that is cheap and available but cannot hold the dimensional tolerances the design demands is not the right material either. Strong materials selection balances all five categories against the program’s actual constraints.

6 Decisions That Drive Cost, Performance, and Lead Time

The six materials selection engineering decisions below carry the largest combined impact on cost, performance, and lead time. Each is controlled at the drawing, before any material is purchased.

1. Material family and grade

The first materials selection engineering decision sets the broad performance envelope. The material family decision chooses between aluminum, steel, stainless steel, titanium, copper alloys, engineering plastics, and composites. The grade decision dials in the specific properties within that family: 6061 versus 7075 versus 2024 aluminum; 303 versus 304 versus 316 stainless steel; Delrin versus PEEK versus UHMW polymer. For most CNC machined prototypes, the default starting point is 6061-T6 aluminum, which balances strength, machinability, corrosion resistance, and cost. Specific applications justify specific grades, but every deviation from the default should be a deliberate choice rather than a habit.

2. Stock form

Stock form has cost and lead-time implications that are independent of alloy. Bar stock is typically about half the cost of plate per pound for the same alloy, and is the default form for round, cylindrical, or shaft-like geometry. Plate is appropriate for flat parts that require significant material removal from one face. Extrusions are economical for parts with constant cross-sections. Cast or forged near-net-shape stock minimizes material removal for complex geometry but requires sourcing relationships with foundries. Reviewing stock form during materials selection engineering is one of the highest-leverage decisions on the drawing.

3. Heat treatment and condition

Material condition determines the mechanical properties the finished part will exhibit. Aluminum 6061 alone offers six common conditions (O, T4, T6, T651, T7351, T8) with yield strength ranging from approximately 8 ksi to over 40 ksi. Stainless steels can be supplied annealed, hardened, or precipitation-hardened depending on grade. Specifying a condition that requires post-machining heat treatment adds lead time and cost. Specifying stock that arrives in the condition you need eliminates that step. Materials selection engineering best practice is to match the supplied condition to the application’s final condition where possible.

4. Material certification level

Certification level determines how much you can prove about the material that left the mill. Standard mill certifications include alloy composition, mechanical properties, and lot number. ASTM-traceable certifications meet most materials selection engineering programs. AMS-traceable (Aerospace Material Standards) certifications are required for aerospace work under FAA-approved specifications. PMI (Positive Material Identification) verification is required for some pressure vessel, nuclear, and high-stakes applications. Each certification level adds cost and lead time. Specify only the certification the program actually needs, and specify it at the materials selection engineering stage, not after the part has been quoted.

5. Material availability and lead time

Material availability is a real schedule constraint that engineers often discover too late. Common alloys like 6061 aluminum, 303 and 304 stainless, and 1018 steel are typically in stock at most metal distributors. Specialty alloys, certified aerospace material, or oversize stock can add weeks. Inconel 718 in a specific certified mill heat may have a 16-week lead time. Titanium 6Al-4V in plate form may run 8 to 12 weeks. Checking availability and lead time before committing the material on the drawing prevents schedule surprises that no design review can recover. This is the materials selection engineering decision that program managers most often wish they had escalated earlier.

6. Manufacturability characteristics

Manufacturability is the bridge between materials selection engineering and design for manufacturability. Different alloys behave differently under machining, welding, and forming. Aluminum 6061 machines at high feed rates with long tool life. Stainless 316 work-hardens during cutting, increasing machining time by 25 to 40 percent versus aluminum. Titanium machines slow and chews up tools. Inconel is among the most difficult metals to machine. Polymers split into machinable (Delrin, UHMW, PEEK) and challenging (PTFE, soft plastics that deflect under cutter pressure). Selecting a material that the production process handles well preserves the cost advantages of every other decision on the drawing.

How do you choose materials for a CNC machined prototype?

For most CNC machined prototypes, the right materials selection engineering default is 6061-T6 aluminum. Deviations from that default should be driven by specific functional requirements that the default material cannot meet.

When to deviate from 6061-T6 aluminum:

• Strength requirement exceeds 6061-T6 (~40 ksi yield): move to 7075-T6 (~73 ksi yield) for aluminum, or to 4140 steel for higher strength, or to 17-4 PH stainless for strength plus corrosion resistance.
• Corrosion environment beyond 6061’s range: move to 304 stainless for general corrosion, 316 stainless for marine or chemical exposure, or to a copper-nickel alloy for specific applications.
• Operating temperature above 300 degrees F: move to stainless steel, titanium, or for very high temperatures, Inconel.
• Weight-critical aerospace or motorsport applications: 7075 aluminum for moderate gains; titanium 6Al-4V where the strength-to-weight ratio justifies the cost.
• Non-structural machined polymer prototypes: Delrin (POM) for general machinability and cost; PEEK for high-temperature or chemical resistance.

For more on the broader process of producing CNC prototype parts, see our guide to CNC machining for prototypes, which covers the production side of the same workflow.

How does materials selection differ for limited run manufacturing?

Materials selection engineering priorities shift when the program moves from one or two prototype parts into limited run manufacturing of dozens or hundreds of units. The cost structure changes, the schedule risk profile changes, and several material decisions that were minor at prototype quantity become significant at limited run manufacturing volume.

Specific shifts when designing materials for limited run manufacturing:

• Per-pound material cost matters more. A 50-unit limited run manufacturing program cannot amortize an exotic alloy across many parts. Standard alloys deliver disproportionate value at low to mid volumes.
• Material availability and lead time matter more. A limited run manufacturing program cannot absorb a 12-week material lead time on every iteration. Specifying material that is in stock at multiple distributors reduces schedule risk significantly.
• Bulk pricing leverage is limited. Limited run manufacturing quantities rarely qualify for full bulk discounts, so material per-pound cost behaves more like prototype pricing than production pricing.
• Material form choice becomes high leverage. The difference between bar stock and plate compounds when multiplied across 100 parts. Stock form decisions made carelessly at the prototype stage can lock in unnecessary cost across an entire limited run manufacturing program.
• Certification cost amortizes worse at low volume. Material test reports and certification fees do not scale with quantity. The per-part cost of certification is high at limited run manufacturing volume.
• Substitution decisions are easier to make. A limited run manufacturing program has time to test material substitutions across the run, where a single-prototype program does not. Use that flexibility to qualify standard alloys before locking the design.

The practical pattern: at limited run manufacturing volume, materials selection engineering shifts toward standard alloys, in-stock forms, and certifications matched precisely to program requirements. Exotic materials should clear a higher bar of justification, because there is no production volume across which to amortize the premium.

How do you choose materials for aerospace, medical, and food and beverage prototypes?

Regulated industries impose materials selection engineering constraints that go beyond mechanical and thermal properties. Each industry has its own standards framework, traceability requirements, and approved material lists.

Aerospace

Aerospace materials selection engineering operates under AMS (Aerospace Material Standards) specifications and FAA-approved material lists. Common aerospace alloys include 6061, 7075, and 2024 aluminum for structural parts, 17-4 PH stainless for fasteners and hardware, titanium 6Al-4V for high-strength and weight-critical parts, and Inconel for engine and high-temperature applications. Full material traceability from mill heat through final part is a contractual baseline, not a value-add. Specify AMS-traceable certifications at the materials selection stage.

Medical devices

Medical device materials selection engineering must address biocompatibility under ISO 10993. Common medical materials include titanium 6Al-4V ELI (extra low interstitial) for implants, 316LVM stainless for surgical instruments and short-term implants, PEEK for spinal and orthopedic applications, and UHMWPE for joint replacement bearings. Sterilization compatibility (autoclave, gamma, ethylene oxide) is a material constraint that must be specified at the design stage.

Food and beverage equipment

Food and beverage materials selection engineering operates under NSF/ANSI standards including NSF/ANSI 51 for food equipment materials and 3-A Sanitary Standards for dairy and beverage processing equipment. Common food-contact materials include 304 and 316 stainless steel, food-grade UHMW polyethylene, food-grade PEEK, and PTFE for non-stick surfaces. Bravo Team’s mechanical design practice includes NSF/ANSI food equipment as a listed capability, which carries through to materials selection on food and beverage programs.

What does a materials cost-impact table look like?

Relative material cost across common engineering alloys spans more than two orders of magnitude. The table below summarizes typical relative cost ranges using 6061-T6 aluminum as a 1x baseline. Actual prices vary with mill heat, stock form, lot quantity, and market conditions, but the ranges are representative for engineering prototype quantities.

Material	Relative Cost (6061-T6 = 1x)	Typical Use Case
6061-T6 aluminum	1x (baseline)	General machined prototypes, default starting point
1018 steel	0.5–1x	Low-cost structural parts, basic fixtures
7075-T6 aluminum	2–3x	Higher-strength aerospace and motorsport structures
4140 steel (alloy)	1–2x	Shafts, gears, structural components
303/304 stainless	3–5x	Food and beverage, mild corrosion environments
316 stainless	5–7x	Marine, chemical, and food-grade applications
17-4 PH stainless	6–9x	High-strength corrosion-resistant aerospace and medical
Delrin (POM)	1–2x	Machinable polymer for non-structural prototypes
PEEK polymer	30–80x	High-temperature, chemical-resistant, medical applications
Titanium 6Al-4V	20–50x	Weight-critical aerospace, medical implants, motorsport
Inconel 718	30–100x	Engine components, high-temperature applications

The compounding effect matters more than any single ratio. A part that combines an exotic alloy, a tight tolerance, and a low-volume lot can cost 50 to 100 times what a standard-alloy equivalent would cost, for the same design intent. Materials selection engineering is one of the highest-leverage cost levers a design team controls.

What are common materials selection mistakes (and how to avoid them)?

Materials selection engineering mistakes follow a consistent pattern across industries. Knowing the pattern is half the fix.

Over-specifying material

Choosing titanium when 6061 aluminum meets every functional requirement is the single most expensive materials selection engineering mistake. Specify only the property requirements the application actually demands, then choose the lowest-cost material that meets them.

Ignoring material availability

Selecting a specialty alloy without checking lead time is how a four-week prototype turns into a sixteen-week one. Verify availability at the materials selection stage, not after the part has been quoted.

Missing certification requirements until production

Discovering that a part needs AMS-traceable material after the parts have been built is a common, expensive mistake. Specify certification level on the drawing at the materials selection stage.

Treating materials selection separately from DFM

Materials selection and design for manufacturability are tightly coupled. A material that the production process cannot handle well undermines every other decision on the drawing. Run the two reviews together.

Specifying obsolete or discontinued grades

Standards and material distributor offerings evolve. A grade that was readily available five years ago may now be a special order. Verify current availability at the materials selection engineering stage and use current standard designations.

Forgetting the manufacturability premium

Material cost per pound is only part of the equation. A material that takes 40 percent longer to machine carries a real labor and machine-time premium that does not show up on the mill certification.

When should you bring in a materials selection engineering partner?

The right time to bring in a materials selection engineering partner is at concept, before the design has committed to a material on the drawing. A material change at concept costs nothing. A material change after the design review has approved the drawing can ripple through procurement, qualification, and testing.

What to expect from a strong materials selection engineering partner:

• Cross-material expertise. Strong partners have experience across aluminum, steel, stainless, titanium, polymers, and composites, and can compare candidates against application requirements.
• Direct connection to manufacturing. Materials selection feedback is sharpest when it comes from engineers who work with the same machinists, welders, and fabricators who will produce the part.
• Industry experience in regulated sectors. Aerospace, medical, food and beverage, and energy materials each have their own standards. Confirm the partner has shipped under those standards before.
• Quantified trade-off analysis. A materials selection engineering partner should be able to compare candidates against cost, lead time, machinability, and performance with real numbers, not just qualitative judgment.

Bravo Team’s materials selection engineering practice works because engineering and machining sit side by side. The team’s machinists are not merely operators but advisors with 112 collective years of hands-on machining experience. When an engineer is weighing two alloys or considering an exotic material, a machinist with decades of cutting that material can walk over, talk through what the feeds, toolpaths, and finishes will actually look like, and surface trade-offs that do not show up in a property table. For the production-side perspective on how materials selection plays out, see our guide to build-to-print manufacturing.

Frequently Asked Questions

What is materials selection in engineering?

Materials selection in engineering is the structured process of matching a material’s properties to the functional, manufacturing, and economic requirements of a part. It determines the specific material grade, condition, and certification level that appears on the drawing.

What are the steps in the materials selection process?

The standard four-step framework, derived from Professor Michael Ashby’s methodology, is: (1) translate design requirements into material property requirements, (2) screen candidate materials against thresholds, (3) rank surviving candidates using performance indices, and (4) choose with manufacturability and cost in mind.

What is the Ashby method for materials selection?

The Ashby method is a structured materials selection engineering approach developed by Professor Michael Ashby at Cambridge. It plots two performance indices (such as strength versus density) on logarithmic axes, allowing engineers to visually screen thousands of candidate materials against application requirements.

What are the 5 criteria for selecting materials?

The five primary categories are: (1) mechanical properties (strength, fatigue, hardness), (2) thermal and environmental properties (temperature, thermal expansion), (3) chemical and corrosion resistance, (4) manufacturability (machinability, weldability), and (5) cost and availability.

What is the best material for a CNC machined prototype?

For most CNC machined prototypes, 6061-T6 aluminum is the right starting point. It balances strength, machinability, corrosion resistance, and cost. Deviate by exception when a specific functional requirement demands a different material.

What is the difference between 6061 and 7075 aluminum?

6061 aluminum is the general-purpose aluminum alloy of choice for most machined parts, with a yield strength around 40 ksi in the T6 condition and excellent machinability. 7075 aluminum has a yield strength around 73 ksi in the T6 condition and is the aerospace-grade choice for high-strength applications. 7075 costs 2 to 3 times more than 6061 and machines somewhat harder.

What is the difference between ASTM and AMS standards?

ASTM (American Society for Testing and Materials) standards apply broadly across industrial materials. AMS (Aerospace Material Standards), maintained by SAE International, are a specialized subset of standards for aerospace materials. AMS specifications carry stricter traceability, testing, and certification requirements than equivalent ASTM standards.

How does material selection affect manufacturing cost?

Material selection affects cost in four ways: per-pound material cost, stock form cost (bar versus plate), manufacturability premium (machining time varies by alloy), and certification overhead. The compounding effect of these four factors can span more than two orders of magnitude between a standard and an exotic alloy.

Does material selection affect lead time?

Yes. Common alloys are typically in stock and available within days. Specialty alloys, certified aerospace material, or oversize stock can add weeks. Inconel 718 in a specific certified mill heat can require 16 weeks. Verify material lead time at the materials selection stage.

What software is used for materials selection?

Common materials selection software includes Granta MI (the commercial successor to Cambridge’s CES materials database), Ansys GRANTA, and Solidworks Sustainability. Software supports screening and ranking but does not replace experienced engineering judgment, particularly for manufacturability and lead-time considerations.

Working with Bravo Team for Materials Selection Engineering

Materials selection engineering pays back across every dimension of a prototype and limited run manufacturing program: faster lead time, lower cost, more reliable performance, and a cleaner handoff from design to production. The discipline is most effective when it lives inside the design process, integrated with manufacturability review, rather than bolted on after the drawing is committed.

If your next prototype needs a partner that can deliver materials selection engineering feedback before the drawing is committed, talk to Bravo Team about your prototype.