---
title: "Aluminum vs Steel for Machined Parts"
description: "Steel is three times stiffer, aluminum three times lighter, and the machined part is usually cheaper in aluminum. How to decide on stiffness, fatigue and cost."
canonical: https://openspindle.com/blog/aluminum-vs-steel
author: "Tom"
datePublished: 2026-09-17T00:00:00.000Z
dateModified: 2026-09-17T00:00:00.000Z
category: "Materials"
---

# Aluminum vs Steel for Machined Parts

Steel is about three times stiffer than aluminum and about three times denser. Aluminum has roughly the same strength to weight ratio in its stronger alloys, machines several times faster, and resists corrosion without a coating. Steel wins where stiffness in a fixed envelope, surface hardness, high temperature or long fatigue life decide the part. Aluminum usually wins on machined part cost even though it costs more per kilogram, because cycle time rather than material dominates the price.

## The numbers worth comparing

Start with the properties that drive real decisions rather than the full datasheet. Density, stiffness, strength and thermal behavior explain almost every aluminum versus steel argument you will have.

The grades below are the common defaults: 6061-T6 as general purpose aluminum, 7075-T6 as the high strength aluminum, 1018 as mild steel and 4140 heat treated as the alloy steel people reach for when 1018 is not enough.

| Property | 6061-T6 aluminum | 7075-T6 aluminum | 1018 steel | 4140 heat treated |
| --- | --- | --- | --- | --- |
| Density | 2.70 g/cm3 | 2.81 g/cm3 | 7.87 g/cm3 | 7.85 g/cm3 |
| Elastic modulus | About 69 GPa (10 Msi) | About 72 GPa (10.4 Msi) | About 200 GPa (29 Msi) | About 200 GPa (29 Msi) |
| Yield strength | About 276 MPa (40 ksi) | About 503 MPa (73 ksi) | About 370 MPa (54 ksi) cold rolled | About 655 to 950 MPa depending on temper |
| Strength to weight, yield | Good | Very high | Moderate | High |
| Thermal conductivity | About 167 W/m-K | About 130 W/m-K | About 50 W/m-K | About 43 W/m-K |
| Thermal expansion | About 23.6 um/m-K | About 23.4 um/m-K | About 11.7 um/m-K | About 12.3 um/m-K |
| Corrosion without coating | Self passivating | Poorer, often anodized or clad | Rusts, requires a coating | Rusts, requires a coating |
| Relative machining time | Baseline | Slightly longer | Roughly 2 to 3x | Roughly 3 to 5x |

*Values are typical published properties for common tempers and conditions. Design to the values in the specification your material is bought to, not to a table.*

## Stiffness is not strength, and it is usually the real question

The mistake that costs the most is treating a strength number as a stiffness number. They are different properties and they fail differently.

Strength is where the material yields. Stiffness is how much it deflects before it gets anywhere near that. Steel is about three times stiffer than aluminum, and no aluminum alloy changes that: 7075 is far stronger than 6061 and has essentially the same modulus. If your part is deflecting under load, switching to a stronger aluminum will not help.

What does help is geometry. Bending stiffness scales with the cube of section thickness, so an aluminum section about 1.45 times thicker matches the bending stiffness of a steel one, at roughly half the weight. That is the whole reason aerospace and automotive structures are full of thick aluminum sections rather than thin steel ones.

So the real question is not which material is stiffer. It is whether you have the envelope to make the aluminum section deeper. Where you do, aluminum is usually lighter for the same stiffness. Where the envelope is fixed and tight, steel wins outright.

## Fatigue, and the difference that catches people out

Steel has an endurance limit. Below a certain stress amplitude, commonly around half its tensile strength, a steel part can cycle effectively indefinitely without failing. Aluminum does not have one. Its fatigue curve keeps falling, so every aluminum part under cyclic load has a finite life, however long.

For most products that distinction is academic, because the design life is far shorter than the fatigue life at working stresses. For anything that will see millions of cycles under meaningful load, it is the deciding property, and it is why steel remains standard for springs, shafts, gear teeth and highly loaded rotating parts.

Stress concentrations matter more in aluminum for the same reason. A sharp internal corner that a steel part tolerates can become the crack initiation site that ends an aluminum part early. Generous radii in aluminum are not just easier to machine, they are structurally load bearing.

## Why the cheaper metal makes the more expensive part

Aluminum bar stock costs more per kilogram than mild steel, often several times more. Machined parts in aluminum are still frequently cheaper, and the reason is that on most machined parts material is a minority of the price.

Aluminum cuts fast. Higher spindle speeds, deeper cuts, better chip evacuation, longer tool life. Roughing a pocket in 6061 takes a fraction of the time it takes in 4140, and the shop is billing machine time. Once the part has any real material removal, that difference swamps the price per kilogram.

Steel gets cheaper relative to aluminum as the part gets simpler and heavier. A thick sawn and drilled plate is mostly material and barely machined, so the cheaper metal wins. A pocketed housing with a lot of stock removed is mostly cycle time, so the faster metal wins.

And on any part in aluminum, buy weight is worth checking. A part machined from a solid block can start at four or five times its finished weight, which means the material line is bigger than the finished part suggests.

| Part character | Usually cheaper in | Why |
| --- | --- | --- |
| Pocketed housing, heavy material removal | Aluminum | Cycle time dominates and aluminum cuts several times faster |
| Simple plate, sawn and drilled | Steel | Barely machined, so the price per kilogram decides |
| Small turned part in volume | Depends on the feature set | Free machining brass and steel grades close the gap |
| Part needing surface hardness | Steel | Aluminum needs hardcoat or an insert to get there |
| Part needing a coating for corrosion | Aluminum | Steel adds a finishing operation aluminum can skip |
| Large weldment or frame | Steel | Welds without losing temper, and stock is cheap |

## The secondary properties that decide edge cases

Thermal conductivity. Aluminum moves heat roughly three times better than steel, which is why heat sinks, cold plates and enclosures for hot electronics are aluminum almost by default. It also expands roughly twice as much per degree, which matters in any assembly mixing the two materials over a temperature range.

Surface hardness. A bare aluminum surface galls, wears and dents easily. Type III hardcoat anodize helps a great deal on wear faces, and threaded holes that will be assembled and disassembled repeatedly want a steel insert rather than bare aluminum threads. Steel takes hardening, nitriding and case treatments that aluminum has no equivalent for.

Welding. Steel welds predictably. Welding 6061-T6 locally anneals the heat affected zone, so a welded aluminum structure is weaker at exactly the joints, and recovering that means re-solution treating and aging the whole assembly. Design welded aluminum around that or use fasteners.

Galvanic corrosion. Aluminum in contact with stainless steel in a wet environment corrodes preferentially, which is why marine and outdoor assemblies isolate the two or use coated fasteners. It is easy to design in without noticing.

## A selector by what the part has to do

Name the property that would fail first, then pick the material that answers it. Almost every clean decision comes from one dominant requirement rather than a weighted comparison.

| Requirement | Material | Notes |
| --- | --- | --- |
| Light structure with envelope to spare | 6061-T6 | Make the section deeper to recover stiffness |
| High strength to weight in a tight envelope | 7075-T6 | Poorer corrosion resistance and weldability than 6061 |
| Stiffness in a fixed thin envelope | Steel | Three times the modulus, and no aluminum alloy changes it |
| Millions of load cycles | Steel | Aluminum has no endurance limit |
| Wear surface or repeated thread assembly | Steel, or aluminum with hardcoat and inserts | Bare aluminum galls |
| Heat dissipation | Aluminum | Roughly three times the thermal conductivity |
| Service above about 150 C | Steel | Aluminum tempers over age and lose strength |
| Corrosion exposure without a coating | Aluminum | Steel needs plating, painting or powder coating |
| Welded frame or weldment | Steel | Welding 6061-T6 anneals the joint |
| Prototype where cost and speed matter most | 6061-T6 | Fastest to cut, widest stock availability |

## What to send the shop

State the alloy and temper, not just the metal. 6061-T6 and 6061-T651 behave differently in a thin part, and 7075 in place of 6061 changes the price and the finishing options. The same goes for steel: 1018, 1045 and 4140 pre hard are three different quoting conversations.

Say whether the drawing dimensions are before or after any coating, since anodize and plating both move surfaces.

And ask one question that regularly moves the price more than the material choice does. Given this geometry, is there a stock size that avoids most of the roughing? A part designed around plate thickness or bar diameter that a shop keeps can cost noticeably less than the same part a few millimeters larger in a section that has to be cut down from the next size up.

## Related reading

- [Aluminum material guide](https://openspindle.com/materials/aluminum.md)
- [Carbon steel material guide](https://openspindle.com/materials/carbon-steel.md)
- [Alloy steel material guide](https://openspindle.com/materials/alloy-steel.md)
- [Metal properties comparison chart](https://openspindle.com/reference/metal-properties-comparison.md)
- [Stainless vs aluminum](https://openspindle.com/blog/stainless-vs-aluminum.md)
- [6061 vs 7075 aluminum](https://openspindle.com/blog/6061-vs-7075-aluminum.md)

## Frequently asked questions

### Is steel stronger than aluminum?

Some steels are stronger than some aluminums, but the more useful comparison is per unit weight, where the gap largely closes. 7075-T6 has a higher yield strength than mild steel at about a third of the density. Where steel is unambiguously ahead is stiffness: it is about three times stiffer, and no aluminum alloy changes that, because modulus barely varies across aluminum alloys.

### Why are aluminum parts often cheaper than steel parts?

Because machine time usually costs more than material. Aluminum cuts several times faster than steel, with higher speeds, deeper cuts and longer tool life, so a part with significant material removal is quicker and therefore cheaper even though the stock costs more per kilogram. Steel regains the advantage on simple parts that are barely machined, where the price per kilogram is most of the cost.

### Can a stronger aluminum alloy fix a part that flexes too much?

No. Deflection is governed by elastic modulus, and every common aluminum alloy has almost the same modulus at about 69 to 72 GPa. Switching 6061 for 7075 raises the yield strength substantially and changes the stiffness barely at all. The fixes are geometric: a deeper section, a rib, or a change in how the part is supported.

### Does aluminum have a fatigue limit?

No true one. Steel has an endurance limit below which it can cycle effectively indefinitely; aluminum fatigue curves keep falling, so every aluminum part under cyclic load has a finite life. For most products the design life is far shorter than that and it does not matter. For parts seeing millions of meaningful load cycles it is the deciding property, which is why shafts, springs and gear teeth stay steel.

### When should I use steel instead of aluminum?

When stiffness is needed in an envelope you cannot make thicker, when the part will see very high cycle fatigue, when it needs a hard wear surface or repeated thread assembly, when service temperature is above roughly 150 degrees Celsius, or when the part is a welded frame. Also on simple, barely machined parts where material cost dominates and steel stock is cheaper.

### Can aluminum and steel be used together in an assembly?

Yes, with two things watched. Aluminum expands roughly twice as much per degree as steel, so a mixed assembly over a wide temperature range needs clearance or compliance designed in. And aluminum in contact with stainless in a wet environment corrodes galvanically, so marine and outdoor assemblies isolate the joint or use coated fasteners.