Machine Design #54: Choosing Aluminum for Parts and Frames — Do Not Call Every Grade “Aluminum”

“Use aluminum” is not a complete material specification. A5052, A6061, A7075, plate, bar, casting, and extrusion can behave differently in strength, stiffness, machining, welding, surface treatment, corrosion, thermal expansion, cost, and supply.

Choose the material through a controlled chain:

Function → load and environment → grade and temper → process → joint and tolerance → surface → verification → maintenance.

1. Start from the function

Define whether the part is a cover, bracket, datum, load path, moving member, frame, heat path, clean surface, or sacrificial wear item. Record force, moment, vibration, cycle, stiffness, deflection, temperature, corrosion, cleanliness, and target life.

2. A5052, A6061, A7075, and extrusion are not interchangeable

A5052 is often useful for formed sheet and corrosion resistance. A6061 offers a broad balance for machined parts and structural members. A7075 can provide high strength but may have different corrosion, welding, cost, and supply constraints. Extrusions provide efficient frame geometry and modular assembly, but their slot, section, joint, and surface assumptions are part of the design.

Verify the exact alloy, temper, product form, certificate, and supplier. Do not copy a strength value from one form into another without checking the condition.

3. Stiffness is not strength

Aluminum’s modulus is lower than steel’s. A part may pass yield stress while deflecting enough to lose a sensor gap, alignment, seal, or repeatability. Check bending, torsion, joint slip, local bearing, buckling, vibration, and thermal movement.

4. Machining design

Plan datum, stock, clamp, tool reach, thin walls, pocket corners, burr, chip removal, distortion, and inspection. A heat-treated plate can move after material removal. Support the part and define the sequence if the datum must remain stable.

5. Welding changes the local condition

Heat-affected zones, distortion, residual stress, porosity, access, sequence, and post-weld treatment can reduce or change performance. Do not assume a welded A6061 part retains the same strength as the parent material. Define weld procedure, inspection, machining after welding, and acceptance.

6. Fasteners and joints

Check thread engagement, bearing, pull-out, galvanic contact, preload, slip, insert, washer, access, and repeated service. Aluminum threads may need an insert. Extrusion joints rely on slot geometry, friction, bracket stiffness, and assembly torque; test the actual joint rather than trusting a catalog load.

7. Surface treatment is part of function

Anodizing, hard coating, plating, paint, chemical film, and bare surfaces differ in thickness, tolerance, wear, electrical contact, corrosion, appearance, and repair. Define the datum before treatment, masking, edge, color, inspection, and rework.

8. Galvanic and environmental corrosion

Contact with steel, copper, stainless, carbon fiber, coolant, salt, or cleaning chemicals can create corrosion. Consider isolation washer, coating, drainage, seal, compatible fastener, and inspection. A clean-looking frame can hide a joint that is corroding inside.

9. Tolerance and thermal expansion

Aluminum moves with temperature and has a different expansion from steel, glass, plastic, and precision components. Define reference temperature, sliding or fixed joint, clearance, datum, slot, preload, and measurement condition. Do not lock both ends of a long member without checking thermal stress.

10. Extrusion frame design

Choose section by span, deflection, joint, cable path, access, guard, load, and future modification. Check feet, leveling, diagonal stiffness, panel attachment, vibration, and fastener torque. Extrusion is not automatically rigid because its catalog section is large.

11. Failure modes to review

Yield, deflection, fatigue, buckling, thread strip, joint slip, dent, corrosion, galvanic attack, weld crack, distortion, surface wear, burr, thermal drift, incorrect grade, mixed temper, wrong orientation, and wrong fastener. Record cause, effect, detection, control, and evidence.

12. Example: an aluminum sensor frame

An A6061 frame supports a vision camera and a guard. The camera needs repeatable datum; the guard needs impact and access; the frame sees temperature and vibration. Compare extrusion, machined plate, and welded construction. Define joint stiffness, thermal movement, insert, surface, cable path, adjustment, and inspection. Verify camera repeatability after assembly, temperature change, guard removal, and a maintenance cycle.

13. Maintenance and replacement

Mark grade, temper, surface, datum, fastener, torque, and replacement orientation. Provide access to joints and adjustment. Record approved alternate and certificate requirement. If a frame member is replaced, recheck alignment, grounding, guarding, and baseline—not only the part number.

14. Aluminum-selection checklist

[ ] Function, load, stiffness, environment, and life are defined.
[ ] Alloy, temper, product form, certificate, and supplier are controlled.
[ ] Strength, deflection, thermal movement, fatigue, and joint are checked.
[ ] Machining, welding, extrusion, distortion, and inspection are planned.
[ ] Fasteners, inserts, preload, galvanic isolation, and access are designed.
[ ] Surface treatment, masking, tolerance, and rework are defined.
[ ] Failure modes have controls and evidence.
[ ] Maintenance, replacement, alternate, and baseline are documented.

Aluminium is a family of materials: read the alloy and the temper together

Writing only "aluminium" on a drawing leaves three decisions to the shop that only the designer can make: which alloy, which temper, and which stock form. Those three change strength, weldability, stability after machining, and even the colour after anodising.

Alloy group	Typical grades	Choose when	Watch for
5000 series (Al-Mg)	A5052, A5083	Fixture plates, guards, formed parts, welded parts	Not heat-treatable; strength comes from the cold-worked temper (H32, H34 and similar)
6000 series (Al-Mg-Si)	A6061, A6063	Machined parts, extruded profiles, frames	Welding destroys the strengthened condition in the zone around the weld
7000 series (Al-Zn)	A7075	Parts needing the highest strength in the aluminium range	Treat as non-weldable; lower corrosion resistance; higher cost
Stress-relieved cast plate	tool plate	Table tops and base plates that must stay flat after milling	More expensive than rolled plate; not for parts that must be formed

The temper suffix matters as much as the alloy name. A6061-T6 and A6061-O behave like two different materials despite sharing an alloy number, and the same is true of A5052-H32 against A5052-O. Omitting the temper leaves a large variable open. A detailed comparison of the three common grades is in Materials #11 — Aluminium A5052, A6061 and A7075.

A stronger alloy does not make a frame stiffer

This is the most expensive misunderstanding in aluminium frame and support design. Aluminium alloys differ considerably in strength, but their elastic modulus is practically the same. Beam deflection depends on the elastic modulus and on the second moment of area, so moving from A5052 to A7075 barely reduces deflection at all.

Stiffness has to come from geometry: increase section height, add ribs, shorten the unsupported span, or move to a closed box section. Choosing a stronger alloy only helps when the problem is yielding or fatigue, not when the problem is deflection. The same reasoning applies when weighing an extruded profile frame against a welded frame, covered in Machine Design #06.

Residual stress: a part that bows after milling is not a shop error

Rolled plate carries residual stress distributed through its thickness. Milling one face breaks that balance and the plate bows on the machine table. The effect is strongest on thin, long parts where most of the material is removed from one side.

Four remedies, in the order worth trying:

Remove material symmetrically from both faces, leaving stock for a final pass once

the part has settled.

Split into two setups: rough, release the clamps so the stress relaxes, then finish.
Move to stress-relieved cast plate for table tops and base plates that must be flat.
Review the clamping. Clamping a part into a bowed shape and then milling it flat gives

a part that bows the other way once released.

The equivalent mechanism in steel is described in Materials #09 — SS400 and SS400-D.

Welding and anodising are decided with the alloy, not after it

Welding. The 5000 series welds well. The 6000 series welds, but the heat-affected zone loses its strengthened condition, so the weld area is no longer the strong part. The design has to keep welds away from highly stressed regions, or be calculated in the annealed condition. The 7000 series is not used for welded structures.

Anodising. The anodic layer grows both into and out of the base surface, so it eats into the tolerance of precision holes and shafts. Three things must be stated: whether the dimensions apply before or after anodising, which surfaces must be masked (electrical contact faces, measuring datums), and how threads are handled, masked during the process or re-tapped afterwards. On appearance, different alloys take colour differently, so parts sitting side by side must share both the alloy and the batch.

Frequently asked questions

Is writing "aluminium" on the drawing enough?

No. State the alloy with its temper, for example in the form A6061-T6 or A5052-H32, and state the stock form where it affects the result, rolled plate or stress-relieved cast plate. Without those three, each batch can turn out differently while the drawing stays unchanged.

Will a stronger alloy reduce deflection in a machine table?

Almost not at all. The elastic modulus of aluminium alloys is nearly identical, so deflection barely changes. Increase the section height, add ribs, or shorten the span instead. A stronger alloy helps only against yielding or fatigue.

An aluminium part bows after milling. How is that handled?

The usual cause is residual stress in the stock released as material is removed. Machine symmetrically from both faces, split the work into two setups with a clamp-release step, leave stock for a final pass, or move to stress-relieved cast plate. This is a design decision and belongs on the drawing.

Which alloy should a welded part use?

Prefer the 5000 series. If the 6000 series is unavoidable, accept that the zone around the weld no longer holds its strengthened condition and place welds away from highly stressed regions. The 7000 series is not used for welded structures.

How does anodising affect tolerances?

The anodic layer grows both inward and outward from the base surface, so holes come out smaller and shafts larger. The drawing has to say whether dimensions apply before or after anodising, which faces are masked, and whether threads are masked or re-tapped after the process.

Conclusion

Aluminum selection is a function-and-process decision, not a generic material label. Compare grade, temper, form, joint, surface, environment, tolerance, and maintenance, then verify the real assembly. The result is a machine part or frame that is light for a reason and stable for the life it must serve.

15. Drawings and material certificates

State alloy, temper, product form, thickness, surface, heat treatment, supplier, certificate, inspection, and traceability on the drawing or controlled specification. “Aluminum” in a title block is not enough for a replacement or audit.

16. Design joints before choosing the section

The frame is a system of members, brackets, fasteners, feet, panels, and loads. Model joint stiffness, slip, preload, access, repeated service, cable routing, grounding, and guard attachment. A larger extrusion cannot compensate for a flexible or poorly torqued joint.

17. Machined versus welded versus extruded

Machined plate may provide datum and compact stiffness but can distort or waste material. Welded construction can be efficient but needs procedure, distortion control, inspection, and local-strength review. Extrusion supports modularity but may need gussets, inserts, and controlled slot joints. Choose the process that the supplier and maintainer can repeat.

18. Thermal and environmental test

Test the assembled datum, sensor, guard, seal, and moving interface at reference and worst temperature, after cleaning, after guard removal, and after maintenance. Record measurement instrument, condition, sample, and acceptance. A room-temperature measurement is not enough for a machine that heats during production.

19. Aluminum failure review

Review fatigue, joint loosening, corrosion, galvanic contact, burr, wrong temper, surface damage, thermal drift, and replacement mix-up in FMEA. Link controls to drawing, assembly instruction, inspection, spare identity, and baseline.

20. Material change control

If a supplier proposes a new alloy, temper, extrusion source, coating, or fastener, compare technical data and recheck critical load, deflection, corrosion, joint, surface, and life. Approve the exact asset and scope; do not silently substitute a commercial label.

References

Aluminum Association — Aluminum standards and properties: https://www.aluminum.org/
ISO 6892-1 — Metallic materials tensile testing: https://www.iso.org/standard/78322.html

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