Would a steel, instead of an aluminium, plate be reasonable?
Whether you should use steel or aluminum depends on the construction of your print bed stack. Either will work, but there are trade-offs involved.
Various considerations that may come into play:
- A flat sheet of aluminum has better stiffness/weight ratio than a flat sheet of steel. If weight/mass is a concern, such as with a Mendel style moving Y-bed, aluminum may be superior to an equivalent stiffness steel sheet. On the other hand, steel has the superior stiffness/thickness ratio, so if the total thickness of the plate is constrained, such as by the printer's mounting hardware, steel may be higher performance. In short, for sheets of equivalent stiffness, aluminum will be thicker but lighter. That may or may not matter for your specific printer.
- Differential thermal expansion can be an issue, depending on what the plate is attached to. For example, rigidly bolting a steel sheet to an aluminum sub-frame will cause the structure to warp when its temperature changes. Likewise for bolting an aluminum sheet to a steel sub-frame. Bed mounts that "float" will not cause warping in either case.
- All kinds of rolled sheet metal may have a tendency to warp when heated/cooled, due to residual stresses and grain alignment effects from manufacture. When people want extreme dimensional stability for the flattest and lowest-warp print bed possible, an aluminum cast tooling plate material such as MIC 6 is usually used. The MIC 6 sheet is then precision ground to be flat. Of course, that adds cost.
- Neither aluminum nor steel is a particularly good bare print surface. Either will need some kind of adhesion layer, like Kapton tape or gluestick. The adhesion layer matters far more than the type of metal underneath.
- Aluminum sheets are often used as heat-spreaders to even out the bed temperature of heated beds. This is very important when heaters are smaller than the total bed size, or have hot spots. (Most heaters do have hot spots.) Steel is a relatively poor conductor of heat, so the surface of the bed will take longer to heat up and will be less even. That may or may not be an issue, depending on the printer and desired materials.
So, it's really a holistic design decision. Aluminum is far more common because of its thermal properties and lower weight, but steel print beds (particularly with permanent adhesion coatings) are often used too.
+1, this answer has helped me in my design. However, en lieu of the Kapton tape, or gluestick, a PEI sheet could be used, as used in the Prusa i3 MK2. 12"x12" PEI sheet is available for around $15 (Amazon) to $20 (eBay). It can be secured using either bulldog clips or 3M 468MP adhesive sheet.