# Aluminum vs Steel for CNC Machining: Cost and Performance > Aluminum and steel compared on density, tensile strength, specific strength, machinability and weldability, with a decision table for choosing between 6061, 7075, 1045, 4140 and 4340 on a real CNC drawing. https://www.sinbo-machining.com/blog/aluminum-vs-steel/ --- ## On This Page - Aluminum vs Steel for CNC Machining: Cost and Performance - The Short Answer - Property Comparison - Specific Strength: Where Aluminum Wins - Where Steel Wins - Machining Cost: What Actually Drives the Price - A Decision Order You Can Follow - How We Help With This Decision - Sources - Frequently Asked Questions - Related Articles - Not Sure Which Material to Specify? Materials # Aluminum vs Steel for CNC Machining: Cost and Performance October 28, 2026 | 7 min read ## Aluminum vs Steel for CNC Machining: Cost and Performance Aluminum and steel are the two default answers to "what should this part be made of", and most drawings pick one without explaining why. The short version: aluminum wins on weight, on machining cost and on anything that has to be welded; steel wins on wear, on stiffness at thickness and on anything that has to be hard. The density difference alone — 2.70 g/cm³ against 7.85 g/cm³, a factor of about 2.9 — settles a surprising number of drawings on its own. This article puts the common grades side by side and gives you the decision rules. [Bar chart comparing the density of aluminum and steel] Aluminum is 2.70 g/cm³ and steel is 7.85 g/cm³, so the same volume of steel weighs about 2.9 times as much. ## The Short Answer - Choose aluminum when the part is weight-sensitive, needs to be welded, must survive a wet or corrosive environment, or is simply a high-volume part where cycle time and tooling cost dominate. 6061-T6 is the default and covers most work. - Choose steel when the part takes impact, wears against another surface, needs to be hard or strong in a small section, or runs next to heat. 1045 for general shafts and brackets, 4140 for parts that need heat treatment, 4340 when the load is high. - Do not judge on strength alone. Divide the tensile strength by the density and the ranking changes: 6061-T6 reaches about 115 MPa per g/cm³, while even 4340 — the strongest common alloy steel in the table below — reaches about 95. Aluminum has the better strength-to-weight ratio at almost every grade. - Hardness and wear resistance is where steel has no answer. Aluminum tops out around 95–150 HB; 4140 and 4340 can be heat treated to 38 and 50 HRC, and D2 to 62 HRC. ## Property Comparison The numbers below are the values already published on the material pages of this site, so you can check any of them against its own detail page. | Property | 6061-T6 | 7075-T6 | 5052-H32 | 6082-T6 | 1045 | 4140 | 4340 | Tensile strength (MPa) | 310 | 572 | 193 | 340 | 585 (annealed) | 655 (annealed) | 745 (annealed) | Yield strength (MPa) | 275 | 503 | 145 | 290 | — | — | — | Elongation | 12% | 11% | 12% | 10% | — | — | — | Density (g/cm³) | 2.70 | 2.81 | 2.68 | 2.71 | 7.85 | 7.85 | 7.85 | Hardness | 95 HB | 150 HB | 60 HB | 100 HB | max 35 HRC (H&T) | max 38 HRC (H&T) | max 50 HRC (H&T) | Machinability | Excellent | Good | Excellent | Excellent | Good | Good | Fair | Weldable | Yes | No | Yes | Yes | Yes | Preheat required | Preheat required | Relative material cost | 1.0x | 1.8–2.5x | — | — | — | — | — > One caveat on the steel numbers: the tensile values for 1045, 4140 and 4340 are given in the annealed condition, while the aluminum values are in their working condition (T6, H32). Heat treated steel goes up substantially from the annealed figure — a 4340 quenched and tempered to 50 HRC is far stronger than the 745 MPa in the table. The comparison is still fair as a starting point, but do not use it as your design allowables. ## Specific Strength: Where Aluminum Wins The useful calculation is tensile strength divided by density. It tells you how much load a given mass of material can carry, which is what a weight-sensitive design actually cares about. | Grade | Tensile (MPa) | Density (g/cm³) | Specific tensile strength (MPa per g/cm³) | 6061-T6 | 310 | 2.70 | 114.8 | 6082-T6 | 340 | 2.71 | 125.5 | 2024-T4 | 469 | 2.78 | 168.7 | 7075-T6 | 572 | 2.81 | 203.6 | 1045 | 585 | 7.85 | 74.5 | 4140 | 655 | 7.85 | 83.4 | 4340 | 745 | 7.85 | 94.9 | [Bar chart comparing specific tensile strength of aluminum and steel grades] Specific tensile strength: 6061-T6 reaches 114.8 MPa per g/cm³, above every steel grade in the comparison. | | | Two results are worth pausing on. First, 6061-T6 beats every one of these steel grades on strength-to-weight. Second, 1045 — the most common mild steel on the shop floor — has a specific strength of about 75, barely above 5052-H32 at 72, the weakest aluminum in the table. That does not make 1045 a bad choice. It makes it a choice about stiffness, wear and cost, not about strength per kilogram. ## Where Steel Wins There are four cases where aluminum is the wrong answer, and none of them are about weight. [4140 alloy steel CNC machined parts] 4140 alloy steel parts - the usual answer when the part needs wear resistance with some toughness. 1- Wear against another surface. A bushing, a shaft, a wear plate or a seal face needs hardness. Aluminum will not hold an edge; 4140 at 38 HRC or 4340 at 50 HRC will. 1- Small sections carrying high load. Aluminum needs roughly twice the section of steel to carry the same load. If the geometry is fixed by something else — a mating part, a housing, a standard flange — there may be no room for the thicker aluminum section. 1- Impact and shock. Aluminum has much lower fracture toughness and elongates before it fails. Steel deforms and survives. 1- Heat. Steel handles temperature far better. For a part sitting next to a furnace, a die or an engine, aluminum is the wrong family. ## Machining Cost: What Actually Drives the Price The material price is usually the smaller half of the quotation. These four items are usually the larger half. [6061-T651 aluminum CNC machined part] 6061-T651 machined part: rated Excellent for machinability, which is where most of the aluminium saving comes from. - Cycle time. Aluminum is rated "Excellent" for machinability on this site's material pages, and 1045 and 4140 only "Good". In practice, an aluminum part with the same geometry runs faster and can often run at higher feed rates. - Tool life. Aluminum is far kinder to inserts. 7075-T6, while still only rated "Good", cuts harder and wears tooling noticeably faster. This is the most common reason a 7075 part ends up costing more than the 6061 quote suggested. - Secondary operations. Aluminum welds and anodizes; steel often needs preheat before welding and pickling or passivation after. Those are separate setup charges. - Deburring and finishing. Softer and more ductile material burrs differently and often needs more hand work or a different deburring strategy. Steel is not more expensive to machine in every case. 4340 and D2 are rated "Fair" and "Poor" for machinability — D2 specifically because of its abrasive carbides — and those grades will cost more in cycle time than 6061 will. The expensive combination is not "steel"; it is "hard, abrasive tool steel with tight tolerances." ## A Decision Order You Can Follow When a drawing lands on your desk, ask these in order: [1045 steel CNC machined shaft components] 1045 steel components - the default mild steel for shafts, brackets and general machined parts. 1- Does the part rub against another surface, or need a hard edge? Yes → steel, and pick the temper from the wear requirement rather than from the price. 1- Is the part weight-critical or handled a lot? Yes → aluminum, and check that the thickness is still manufacturable. 1- Is there welding? If yes and the part is aluminum, 6061 or 6082 are the answers. If the part must be steel, 1045 welds without preheat; 4140 and 4340 need it. 1- Does it sit near heat or need to survive impact? Yes → steel. 1- [5052-H32 aluminum CNC machined parts] None of the above? → 6061-T6. It is the cheapest path to a finished part in most cases. 5052-H32 parts: the aluminium grade to reach for when weldability and a marine environment matter. ## How We Help With This Decision Material selection is a fixed step in the DFM review we run between receiving a drawing and returning a quotation. Our engineers check: [Multi-port CNC machined aluminum housing with bores and threaded features] Multi-port machined housing: a geometry that is comfortable in 6061 can be a tooling problem in a harder alloy. - Whether the drawing states alloy and condition. "Steel part" is not a specification; 1045 and 4340 are different products with different prices and different lead times. - Whether the surface finish and the material agree. Anodizing only applies to aluminum, and hard anodizing on 7075 adds residual stress that must be accounted for. - Whether heat treatment is actually required, or whether it was copied from an older drawing. It is one of the most expensive line items in a steel quotation and often unnecessary. - The tooling and cycle time plan, which is usually what turns "aluminum is cheaper" into a number you can compare against the steel alternative. If you have a part where the material choice is still open, send the file over and we will return a quotation for both options so you can compare them on the same basis. Property values are typical figures from public material handbooks (ASM/MatWeb) and from the material pages of this site. Actual values vary with product form, condition and lot. Base engineering decisions on the material specification you hold and on measured data, not on this table alone. ## Sources Every number on this page comes from our engineering wiki. - [Aluminum Alloys ](https://www.sinbo-machining.com/wiki/materials/aluminum/) - [Steel ](https://www.sinbo-machining.com/wiki/materials/steel/) ## Frequently Asked Questions On tensile strength, 4340 steel reaches 745 MPa against 310 MPa for 6061-T6, so the steel is stronger in absolute terms. Divide by density, though, and the ranking reverses: 6061-T6 gives about 114.8 MPa per g/cm³, 4340 about 94.9. On strength per unit of weight, aluminum wins. Steel wins on hardness and wear resistance, where aluminum cannot compete at all. No. Aluminum is rated Excellent for machinability and 1045 and 4140 only Good, so the same geometry usually runs faster in aluminum. The expensive combination is hard abrasive tool steel: 4340 is rated Fair and D2 Poor because of its abrasive carbides. Steel becomes the costlier option when you also need heat treatment, which adds a separate setup charge. Yes, if the grade is weldable. 6061-T6, 5052-H32 and 6082-T6 are the standard weldable aluminum grades and need no special preparation beyond cleaning. 7075-T6 and 2024-T4 crack when welded and should not be. On the steel side, 1045 welds without preheat, while 4140 and 4340 need preheat because of their higher hardenability. Match the grade to the wear mechanism, not to the price. 1045 quenched and tempered reaches about 35 HRC and is fine for shafts and general brackets. 4140 reaches about 38 HRC and is the usual answer for parts that need wear resistance with some toughness. 4340 reaches about 50 HRC for higher load and impact, and D2 reaches about 62 HRC when the part is a tool rather than a structural component. ## Related Articles [6061 vs 7075 Aluminum: How to Choose for CNC Parts Read more → ](https://www.sinbo-machining.com/blog/6061-vs-7075-aluminum/)[CNC Machining Services Read more → ](https://www.sinbo-machining.com/services/cnc-machining/)#aluminum vs steel #material selection #CNC machining cost #6061 #4140 #machinability [Back to Blog ](https://www.sinbo-machining.com/blog/) ## Not Sure Which Material to Specify? Send us the drawing and our engineers will confirm the alloy, the condition and the process against your load, wear and environment requirements. 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