Guide
MIC-6 aluminum vs 6061 plate: when should a machined fixture or base plate use cast tool plate?
MIC-6 aluminum is a cast aluminum tooling plate: the producer casts each thickness individually, stress-relieves it and machines both faces flat, so the plate arrives nearly free of residual stress and stays flat when you cut deep pockets into it. 6061-T651 is a rolled, heat-treated and stretched plate that is roughly twice as strong, anodizes cleanly and threads well, but carries more locked-in stress and arrives less flat. For a thin, heavily pocketed fixture or base plate where flatness is the requirement, specify cast tool plate; for a part that carries load, takes threads hard or needs a cosmetic anodize, specify 6061-T651.
By Saola Manufacturing engineering team · Published · Updated
The question usually comes up when a fixture plate, vacuum chuck, inspection base or machine-mounting plate is drawn with a tight flatness callout over a large area and a pattern of pockets on one face. The designer has written 6061-T651 out of habit, the first article comes back bowed after the pockets are cut, and someone suggests MIC-6. This guide compares the two materials on the five things that decide the choice: flatness as supplied, movement during machining, strength, finish and cost. It also gives a decision rule and a callout note you can copy. The temper side of the 6061 story, including what the 51 suffix means, is covered in the 6061-T6 vs T651 guide.
MIC-6 is a trade name. Other cast tooling plates are sold under names such as K100-S and ATP-5, and they are not identical: they come from different producers, use different chemistries and publish different properties. Where this page says cast tool plate it means the category; where it gives a property, it names the plate.
On this page
What is the difference between MIC-6 and 6061-T651 plate?
MIC-6 is a cast, thermally stress-relieved plate sold for flatness and stability, while 6061-T651 is a wrought plate that is solution heat-treated, stretched and aged for strength. The first is chosen for how it machines and stays flat; the second for what it can carry.
6061 is an Aluminum Association wrought alloy, an aluminum-magnesium-silicon alloy whose composition limits are published in the Association's Teal Sheets. The T651 temper means the plate was quenched from solution temperature, stretched to a controlled permanent set to relieve quench stress, then artificially aged, as defined in ANSI H35.1 and reproduced in MIL-HDBK-5J. Stretching removes most of the quench stress, but not all of it, and the plate's flatness is whatever the rolling and stretching left.
Cast tooling plate takes a different route. The MIC-6 data sheet says each thickness is cast individually on a continuous caster; the plate is then stress-relieved and both large faces are finished by the producer, so the flatness and the surface finish are created by machining at the mill rather than by rolling. MIC-6 is a proprietary cast alloy sold under its trade name, not under an Aluminum Association wrought designation; contributors on finishing.com describe it as a 7000-series (aluminum-zinc) base, so it should not be assumed to behave like any registered wrought alloy. Because the plate is not quenched or stretched on its way to you, there is very little stress left in it to release when a cutter opens it up.
How flat is MIC-6 as supplied compared with 6061 plate?
Cast tool plate is sold with machined faces held to a published flatness and thickness tolerance, so a fixture plate can often keep its as-supplied faces. Rolled 6061-T651 is sold to the much coarser commercial flatness tolerances of ANSI H35.2, so a part that needs a flat face must have that face machined.
The listing for MIC-6 in the McMaster-Carr catalog gives a thickness tolerance of plus or minus 0.005 in, a flatness tolerance of 0.015 in for the thin sizes and 0.005 in from the 3/4 in sizes up. The producer's technical data sheet gives a typical as-supplied finish of 20 microinches on both faces. Those are stock tolerances over the whole plate, not a guarantee for a machined part cut out of it, but they are tight enough that the surface a fixture sits on, or the face a vacuum chuck seals against, may need no further facing.
Rolled plate is a different starting point. Mill flatness for 6061 plate is set by the dimensional tolerance tables of ANSI H35.2, which ASTM B209 references for sheet and plate, and those tables are written for general-purpose mill product that will be machined where it matters, not for a datum face. In practice, 6061-T651 for a precision plate is bought oversize in thickness, and both faces are machined, with the part flipped and re-clamped lightly between passes so that the residual stress can come out before the final cut.
| Property as supplied | MIC-6 cast tool plate | 6061-T651 rolled plate |
|---|---|---|
| How the faces are made | Machined at the mill, both sides | Rolled, stretched; mill finish |
| Thickness tolerance | 0.005 in either way (0.127 mm) | ANSI H35.2 table by thickness and width; much wider |
| Flatness, 1/4 to 5/8 in | 0.015 in | ANSI H35.2 commercial flatness tables; much wider |
| Flatness, 3/4 in and thicker | 0.005 in | ANSI H35.2 commercial flatness tables; much wider |
| Surface finish as supplied | 20 µin typical | Mill finish; not specified for precision use |
| Residual stress | Stress-relieved casting; very low | Reduced by stretching; low but not zero |
Does MIC-6 warp less than 6061 when you pocket it?
Yes, and that is the main reason to specify it. A cast and stress-relieved plate has almost no locked-in stress to release, so removing most of one face does not bow the remainder the way it can with rolled plate.
Warp during machining is stress release. When a plate is quenched, the skin cools first and ends up in compression, with the core in tension; the MIL-HDBK-5J temper definitions describe the stretching step that T651 adds to reduce that stress. When a cutter removes the skin from one side only, the remaining section is no longer balanced and the plate curls toward the side that was cut. Stretching makes 6061-T651 far better behaved than plain T6 plate, which is why T651 is the default for machined plate, but a thin finished section with deep pockets on one face can still move enough to fail a tight flatness callout.
The geometry that causes trouble is predictable: a large, thin plate, a high fraction of the volume removed, and the material removed asymmetrically, from one face or from one end. Vacuum fixture plates with a grid of channels, nest plates with many deep pockets, and thin base plates for optics or inspection gauges fall into that pattern. Those are the parts where cast tool plate earns its place, because the stress that would cause the movement was never put into the stock.
Cast tool plate does not remove every source of movement. Heat from aggressive cutting, heavy clamping force that springs the plate during the cut, and burrs under the part on the machine table all still produce flatness errors, so the machining strategy still matters; a good shop will rough, release the clamps, and finish with light passes on either material.
Is MIC-6 strong enough for a fixture or base plate?
For most fixture, jig and base-plate duty, yes, because those parts are designed for stiffness and flatness rather than stress. For a structural bracket, a highly loaded mount or a part that relies on heavily torqued threads, no: its yield strength is well under half that of 6061-T651.
The comparison below uses the specification minimums for 6061-T651 plate and the producer's typical values for MIC-6. Stiffness is almost the same, since the elastic modulus of aluminum alloys varies little, so a cast plate of the same thickness deflects about the same amount under the same load. What changes is how much load it can take before it yields, how much it stretches before it breaks, and how well its threads hold.
Threads are where the low strength shows first. A fixture plate that has clamps, locating pins and toe clamps bolted to it again and again will wear its tapped holes, and a soft cast plate wears them faster. The usual answer is helical-coil or key-locking thread inserts in the holes that see repeated assembly, and longer engagement in the rest; the thread engagement guide shows how engagement length scales with the strength of the internal thread.
| Property | MIC-6 cast tool plate (typical) | 6061-T651 plate |
|---|---|---|
| Tensile strength | 165 MPa (24 ksi) | 290 MPa (42 ksi) minimum |
| Yield strength | 105 MPa (15 ksi) | 241 MPa (35 ksi) minimum |
| Elongation | 3% | 6 to 10% minimum, falling with thickness |
| Brinell hardness | 65 HB | 95 HB typical |
| Elastic modulus | 71 GPa | 69 GPa typical |
| Thermal conductivity | 142 W/m·K | about 167 W/m·K typical |
| Heat treatable after delivery | No | Yes (if re-treated by the user, the temper becomes T62) |
Can you anodize MIC-6 aluminum?
You can anodize it for corrosion and wear protection, but do not expect a cosmetic finish: the cast structure tends to anodize gray, dull and blotchy, and dyed colors, black in particular, often come out uneven. If the part must look like anodized 6061, use 6061-T651 or ask the anodizer which cast plate they have had good results with.
Anodizers on the finishing.com forum report the same pattern repeatedly: cast tooling plate needs a gentle cleaning and deoxidizing sequence rather than an aggressive caustic etch, smut after dyeing and sealing is common, and a mottled gray appearance is a frequent result. Contributors in a separate finishing.com thread report that K100-S and ATP-5, described there as modified 5083 aluminum-magnesium plates, take a more even finish than MIC-6, which is another reason not to treat the trade names as interchangeable. The same thread reports that hard anodize on MIC-6 is usually achievable with careful ramping and racking, but the gray, uneven appearance remains.
For most fixture plates this does not matter, because they are left bare, which is one of the advantages of a plate that arrives with machined faces and resists general corrosion in a shop environment. If a coating is needed for wear on locating surfaces, consider whether a hardened steel insert or a pressed bushing at the wear point is better than coating the whole plate. When anodizing is specified, write the class and type on the drawing and add a note that color uniformity is not a requirement on cast plate; the anodize color guide explains why color varies even between wrought alloys.
When should a drawing call for cast tool plate instead of 6061-T651?
Specify cast tool plate when the part is a plate whose job is to be flat and stable, much of one face is cut away, and it carries only light load without a cosmetic finish. Specify 6061-T651 when strength, thread durability, welding or appearance matter more than flatness after pocketing.
The decision table turns that rule into checks a designer can run against a drawing. If any row in the right-hand column applies, 6061-T651 is the safer choice even when flatness is important, and the flatness is then achieved through the machining sequence and, if needed, an intermediate stress relief. If every row in the left-hand column applies, cast tool plate will usually give a flatter part for less machining effort.
| Check on the drawing | Points to cast tool plate (MIC-6, K100-S, ATP-5) | Points to 6061-T651 |
|---|---|---|
| Part form | Flat plate of uniform thickness | Bracket, block, housing, or plate with tall bosses |
| Material removed | Deep pockets, channels or a grid on one face | Light features, or material removed evenly from both faces |
| Flatness callout | Tight flatness over most of the plate area | Flatness only on small local pads |
| Load | Locating, supporting, vacuum holding; stiffness-driven | Structural load, shock, fatigue, or a strength calculation on the drawing |
| Threads | Light-duty, or inserts acceptable | Repeatedly torqued threads without inserts |
| Finish | Bare, or functional coating with no appearance requirement | Cosmetic or dyed anodize, color match |
| Joining | Bolted | Welded |
| Material certification | Producer certificate with lot traceability is enough | An ASTM or AMS material specification is required by contract |
Cost needs a sentence of its own, because the per-pound price is the wrong comparison. Cast tool plate is often priced at or above 6061 plate by weight, but it can arrive at final thickness with finished faces, while a precision 6061 plate is bought thicker and faced on both sides with a flip in between. For a large, thin plate that machining time and the scrap risk from a bowed part usually outweigh the difference in stock price; for a small block that will be machined on every face anyway, the advantage disappears.
How should you call out MIC-6 on a drawing and an RFQ?
Name the specific plate and state whether substitutes are allowed, mark which faces may stay as supplied, and put the flatness requirement on the part as a geometric tolerance. A bare note that says "cast aluminum" or "tool plate" leaves the supplier to choose a material with different strength and finish.
A workable material note reads: "MATERIAL: CAST ALUMINUM TOOLING PLATE, MIC-6. EQUIVALENT CAST PLATE (K100-S, ATP-5) ONLY WITH WRITTEN APPROVAL. FURNISH PRODUCER CERTIFICATE WITH LOT NUMBER. FACES MARKED ‘AS SUPPLIED’ MAY REMAIN UNMACHINED." Then apply the flatness tolerance to the functional face with a feature control frame per ASME Y14.5-2018, so it is inspected on the finished part rather than assumed from the stock tolerance. If the as-supplied face is the datum, say so, because a shop will otherwise face it by default. The difference between a flatness and a parallelism requirement on a mounting plate is covered in the parallelism vs flatness guide.
When we review a plate drawing for CNC milling, the things we check are the ones in the decision table: how much of one face is removed, where the flatness callout applies, whether the threads are working threads, and whether a finish is specified. If the drawing calls for 6061-T651 on a part that fits the cast-plate profile, we raise it as a question rather than changing the material, because the material is the designer's decision. In the RFQ, send the STEP model and the PDF drawing, the material note with its substitution rule, any coating, and the inspection evidence you want for the flatness callout. Send the drawing for a quote with those points on it.
Frequently asked
Is MIC-6 the same alloy as 6061?
No. 6061 is a wrought aluminum-magnesium-silicon alloy that gets its strength from heat treatment. MIC-6 is a proprietary cast tooling plate sold under a trade name; it is stress-relieved, not heat-treated to a T6 temper, and it is much weaker.
Are MIC-6, K100-S and ATP-5 interchangeable?
They fill the same role, but they come from different producers with different chemistries, strengths and anodizing behavior. Treat them as approved equivalents only after checking each data sheet against your loads and finish, and say on the drawing whether a substitute is allowed.
Can MIC-6 be welded?
Published guidance is inconsistent between sources, and welding heat would distort the flatness that is the reason for using the plate. Design cast tool plate assemblies to be bolted and pinned; if a welded frame is needed, make the frame from a weldable wrought alloy and bolt the flat plate to it.
Is MIC-6 more expensive than 6061?
Per pound it is often similar or higher, depending on size and the distributor. The total part cost can still be lower for a large, thin, flat plate, because the faces may not need machining and the risk of scrapping a bowed part is lower.
Does MIC-6 stay flat after machining?
It stays much flatter than rolled plate when material is removed from one face, because it carries very little residual stress. Clamping force, cutting heat and the machining sequence can still distort it, so the flatness callout should be checked on the finished part.
Sources
- MIL-HDBK-5J, Metallic Materials and Elements for Aerospace Vehicle Structures, Department of Defense, 31 January 2003 (cancelled and superseded by MMPDS; cited for temper definitions and Section 3.6.2, 6061 plate)
- ASTM B209/B209M-21a, Aluminum and Aluminum-Alloy Sheet and Plate (scope), ASTM International, 2021
- Standards: ANSI Accredited Standards Committee H35, covering H35.1 and H35.2, The Aluminum Association, accessed October 2026
- International Alloy Designations and Chemical Composition Limits for Wrought Aluminum and Wrought Aluminum Alloys (Teal Sheets), The Aluminum Association, accessed October 2026
- MIC-6 aluminum cast plate, technical data sheet, producer listing, accessed October 2026
- MIC6 aluminum plate: thickness and flatness tolerances, McMaster-Carr catalog, accessed October 2026
- MIC-6 vs K100 tooling plate for anodizing, practitioner Q&A, finishing.com, accessed October 2026
- ASME Y14.5-2018 (reaffirmed 2024), Dimensioning and Tolerancing, ASME, 2018
- Anodizing cast aluminum tooling plate, practitioner Q&A, finishing.com, accessed October 2026