Alnico production standards, then & now

[OldSoundGuy]

I'm doing a bit of forensic research into Alnico 2 & 5. What I am interested to know is how changes in processing technology would have changed the characteristics of Alnico magnets over the years. The question I am trying to resolve relates to variations in sound from one manufacturer to another (today) and vintage vs modern materials with magnets of the same grade and size (old vs new materials).

The application I am involved in is the building of transducers. Microphones and instrument pickups.

While my current background is acoustics and computers, my early career was as a process tech for a powdered metallurgy start up(worked on neodymium powders in the mid 80's) and for a vacuum furnace manufacturer. I have a pretty good handle on chemistry and have done extensive searches and reading on the processing of magnetic materials but there seems to be a void when it comes to Alnico as far as this kind of information is concerned.

I've scoured various metallurgical reference materials and a few books on magnetic materials but I'm not getting a historical or technical perspective on the subject.

Anyone have any ideas that would point me in the right direction?

Thanks,

The Old Sound Guy

 

[IRstuff]

Isn't this a bit like the tail wagging the dog? Wouldn't the cone materials, crossover filters, and enclosures swamp out any effect from the magnets?

TTFN

FAQ731-376

 

[OldSoundGuy]

I would agree with you most certainly if I were building loudspeakers. In the case of instrument pickups the main variable I am talking about are the magnets.

In the case of single coil pickups there are 3 main magnetic components. The magnet, coil and steel string. The coil bobbin is fiberboard and the cover is plastic and as such would have the least (read none) impact.

Test are conducted with the same components with the variable being the magnets. (same wire type and number of wire turns installed in the same instrument.

I'm trying to figure out if there would be differences in chemistry, mass or magnetic properties from solution treating or tempering? I could assess chemistry via testing but know of no way to determine if method of processing is having an effect.

Microphones and pickups are a fairly complex electro-magnetic system that is affected by magnetic field, eddy currents, self inductance and capacitance of the coil and so forth. I have a good handle on that, just want to investigate the possible changes in how magnets are made today vs yesteryear.

 

[MagMike]

Here is my 2 cents:
Even if you are dealing with reputable suppliers, you will see variation in the chemistry and density (in cast parts). It's only natural since there is no official definition for chemical composition for Alnico materials. Magnetic performance is the primary criteria. As such, suppliers have leeway in the ratio of Aluminum/Cobalt/Nickel, etc. as long as the magnetic performance is adequate.

Alnico 2 & 5 produced nowadays (US and non-US) from quality suppliers performs just as well as it did 10+ years ago, when I started working with permanent magnets.

Back when I started, some of the less-reputable suppliers would try to pass along parts with huge porosity voids, causing much frustration. Fortunately they are out of business now.

Since your application is particularly sensitive to density, you may want to use sintered parts. They'll be more consistent because there won't be any significant voids.

Finally, here is a link to an article by Paul Studebaker, editor of Plant Services Magazine reminiscencing about his days making Alnico:

http://www.plantservices.com/articles/2009/016.html

 

[OldSoundGuy]

Thanks for the input MagMike

I did go through a variety of patents starting at the mid 50's through present regarding Alnico processing as part of my search. There was some valuable information there that I found helpful in a forensic way. What I found that was helpful was the current common method of making powder and also the nominal size particle. Having worked with spherical powders for metal injection molding, I have some insight into compaction density. Less so with the plasma/spinning wheel method where the powder is flake that is further crushed to size. With spherical powders, it is easier to calculate density from particle size distribution analysis.

The other thing besides mass that I'm looking into relates to something I read in a publication regarding the magnetic properties of alloys.

“An impurity-based model cannot explain the dependencies of magnetic properties on technology, the concentrations of hydrogen or oxygen, the isomeric composition, crystallinity and the hybridization of carbon atoms. We note immediately that magnetic properties can be greatly affected by a mechanical effect without any change in the elemental composition.”

Having once worked in material science research some 25 years ago, I am aware that there is constant change in the manor things are processed. Improvement in efficiency and material quality are always a goal. While alnico is outdated by modern materials, it is still commonly used in audio. Some have likened my search to seeking the "holy grail" of audio. Others insist that it is all variations in chemistry. Based on all I've read of late, I am further convinced that minor variations in chemistry alone cannot account for the differences. As the secondary processing of alnico is heat treating to achieve the proper magnetic properties, this seems natural to look at. Carbon, depending on its state, also has much to do with magnetic properties. Hence the reason some add Si to the chemistry. It helps draw the carbon out as graphite which is fairly magnetically neutral.

Right now I've collected various data points but none are pointing in one direction. Life was much easier when I only had to deal with the mechanical properties of materials or acoustic issues.

 

[EdStainless]

I am pretty sure that your old stuff was cast ALNICO 5, not PM. It would have significantly different properties.
If this is a transducer then you can measure the exact working field levels and how they change with applied field. If you match B-H curves then the response has to be identical (physics works). 'Sounds' like you are dealing with changes in properties.
I wouldn't overlook changes in the steel also. We used to anneal pole pieces for test rigs for 2 or three days in dry hydrogen at very high temp. It was the only way to get consistent results.

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Plymouth Tube

 

[OldSoundGuy]

Thanks EdStainless for the input. I've been researching through a number of materials including old patents on Alnico. There have been a variety of changes over the years in processing that all could affect density and magnetic properties. The article MagMike pointed to also illustrates how many manufacturers had issues even from batch to batch.

As the powder research work I did in the 80's was in a tightly controlled environment, it is not something we had much issue with.

I am glad you pointed to the steel itself. I'm not getting much traction on this idea with others. I had read a paper on the relationship between mechanical and magnetic properties in cold rolled steel. Here they were looking to use non-destructive methods to perform material tests via magnetic testing. One thing that was clear from the data, the more cold rolled is worked, the more the magnetic properties are changed. Thinner sheet stock of the same grade will have different properties than bar stock due to just the mechanical stress alone introduced by processing. This was something I just wasn't previously aware of (magnetic's are not my field).

I do however have some experience with heat treatment of metals as I once worked for a company that builds custom vacuum furnaces. I've sent some steel back plate samples to a friend that has access to a vacuum furnace. I don't think I need to go the dry hydrogen route (unless I decide I need to draw out more carbon) but a long anneal and cool down is a test I am currently running to see if there is a sonic difference. He is going to program the run at the end of the day with an overnight cool down for me sometime this week.

 

[EdStainless]

Yes we were after more C reduction.
When we made magnets for transducers (geo type) we used a select alloy and then sorted the magnets into classes. And the testing wasn't open circuit buy at a load line that was similar to the application.
We used to cast 5 different Alnico 5's and 7 different Alnico 8's each with their own properties. (that list doesn't include the 5DG, 6, or 9)

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[cermag]

I have had plenty of discussions with Guitar pickup device manufacturers regarding "sound" created from different Alnico materials used, with supposedly Alnico 2 giving a "softer" sound and Alnico 5 giving a "harsher" sound. Dad tells me stories of how he was involved in the design of then current now "vintage" pickups in the 60's when he worked for Balfour Darwins magnet manufacturers in Sheffield UK. I have a copy of the chemical composition of all the alnico alloys as printed in the 1967 permanent magnet association catalogue if thats any help.