are these two metals in parallel or in a series? flux will follow the easiest (least permeability) path. if two metals are in parallel, the majority of flux will go through the one with higher permeability. however, keep in mind that the permeability changes drastically with applied field. if the material is saturated, it acts like air. Generally speaking, mu-metal has highest permeability, but least Ms, Hiperco has highest Ms, but least permeability. If your design allows to apply a high current, you may use hiperco, if low current is allowed and/or sensitivity is critical, use mu-metal.
recommend a trade off material, say permealloy 49Ni, offers high permeability while still high Ms. for a high flux, you can simply increase the size of the core.
Two-metal idea is not a bad one in theory though.

in the attachment you can see, in section, how the magnetic motor of a field coil loudspeaker is made.
you see a central body, an empty red space where the copper wire winding and a double outer ring are placed to close and channel the magnetic field.
the idea is to use the central core in the shape of a cylinder wound by the coil with hiperco 50 to obtain maximum magnetic saturation, then the outer ring with another metal, also because hiperco 50 is expensive and difficult to lathe.
I also thought of using mu-metal but it has a very low magnetic saturation. in reality it is not clear to me how the two quantities (saturation and permeability) of the outer ring must be in order for everything to work at its best.
why do you say that it takes so much current for hiperco 50 ? looking at the BH curve it doesn't seem like it to me.
I searched the net for the magnetic characteristics of permalloy 49ni but I didn't find anything, where should I look? .
Could using low carbon steel and then annealing it to increase the magnetic properties be a good idea? .

• https://files.engineering.com/getfile.aspx?folder=44f1b279-ec1b-4fe7-acd3-c

I have now seen that there is a variant called hypermco 50 with better magnetic properties but apart from a simple graph, nothing can be found on the net.

"hypermco 50" is 99.99999% certain a misspelling of Hiperco® 50.

https://www.researchgate.net/figure/6-B-H-curves-o...

https://core.ac.uk/download/pdf/42630611.pdf

Same advisor. Doesn't mention Hiperco® 50.

Contact the advisor on that paper for more information.

Download of full text for that diagram says they got the information from https://iopscience.iop.org/article/10.1088/1742-65... which calls for materials "Radiometal 4550, Armco, Hyperm 0, Hyperm Co50" which was written by S. H. Khan.

There is always V Permandure if you need high sat and high field.
48-49% Fe and Co with 2%V.
We used to use it for tapered pole pieces on electromagnets where we went to 24kG.
Adn I have seen it used for laminations (0.004" thick) for very compact transformers.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

Hiperco 50 is often used for aerospace and thin film chips where the limited space is key. if your speaker has enough space, no bother to use FeCo. low carbon steel (electical iron) and low si core iron have as high as B at 21kG, normal Si Steels at 16-18kGs, Hiperm 49 (commercial name for 49Ni) 16kG, Mu-metal (other commerical name Hymu 80) 7.5kG.
Hymu 80, max DC permeability can be as high as 600,000! it also owns some degree of corrosion resistance. if corrosion is key, you could also think about ferrite stainless with B up to 18kG.

among all these soft magentic materials, hiperco has loweset permeability, which means you need a much higher current to get the same B level. note, flux = B.S, for a same flux, you can either increase B, or simply increase cross section area S if your applications allow.

thanks everyone for the replies

one person made me wonder.
with the same level in magnetic saturation B (Tesla) comparing steel with low carbon content and all iron-cobalt alloys, which of the two consumes less electric current H (amp/m) ?

depending on quality of the steel, it varies a lot. if you use "soft" iron with super low carbon, you need a field of 1,000 A/M to reach 2T, while Hiperco needs 5,000 A/M to get this point. If you apply a field of about 20,000 A/M, you can get 2.4T from Hiperco, however no gain on iron after saturation at 1000a/m.

DC or AC? The hyst losses would be very different.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

in reality there are two coils, the mobile one connected to the cone which produces the sound and the fixed one which generates the magnetic field.
so the current is continuous

2T may be enough for me, plus with 1,000 A/M I can save energy. for the central body I therefore choose low carbon steel. now I have to understand which steel I have to choose, there are already two candidates who are:
- 1001ELI (EDDS) .
- ARMCO purified.
which of the two is better in your opinion? .
it remains only to understand what material can I use for the outer ring and with what characteristics, I think it must have a saturation at least equal to that of the central body, but the permeability instead?.
however, other problems arise later, the two formats of semi-finished product that I need are the full round with a diameter of about 10 cm and the tube with an internal diameter sufficient to contain the full round, we need to see if the two steels I mentioned are marketed in this way. alternatively I could find a small forge that melts the metal, and also use it to do the annealing.
in the end I also have to go to the turner.
to give me an idea, how much do you think all these steps could cost me?

I don't know if it is better to buy steel with a low carbon content, work it on the lathe and finally do the annealing to increase the purity or instead, buy iron that is already pure (it can be found in powder or granular with a degree of purity 4N 99.98% but without certification from china), cast it in molds and finally work it on a lathe. if I proceed with the second method, are there any difficulties with the lathe machining? Do I still have to carry out the annealing or can I do without it?.
I need to have your opinion.

Which part are you talking about?
The flux density will be different in each section so the demands on the material will be also.
What non-magnetic material are you using for the back plate?
You don't want powder.
Unless you have dies, high pressure presses, and hydrogen sintering furnaces you will never get close to full density.
You need to find steel bar a size that works and with less than 0.02% C.
Machine and anneal (conventional).
Then test. If you need a bit more then try a purification treatment.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

high purity 4N or even 5N does NOT mean much here. it often only indicates total metal impurities tested by GDMS. The metallic impurity does not do much bad to magnetics, the gas impurities(C, S, O) do, which are normally tested LECO. what you really need is to get as low as poosible low level of gas impurities to boost the soft magnetic properties. anneal after machining can remove stress, which can also be helpful.