Hi All:

I am a material geek of sorts and just found something that might interest some of you as well.


When JBL introduced Mg diaphragm drivers, I was baffled about how they could use such an inherently brittle material in a bending application.

In the automotive racing world, we are familiar with magnesium and its properties. Formed parts are wrought at elevated temperatures of about 200 - 400° C. and virtually all other parts are castings.

Initially I assumed that JBL must use a surround of different material, something similar to what Materion has employed in the TruExtent diaphragms. I also assumed the domes must be formed while hot.

A couple of years ago on a visit to JBL Northridge I had the privilege to speak with J Morro for a while. My interest in materials led him to show me some unassembled diaphragms. I was surprised to see that the Be and Mg diaphragms had standard looking diamond surround forms like the 2445-2450.

He showed me a hard anodized aluminum diaphragm that was very stiff and light but he told me they fail at the surround as the anodizing makes them brittle. I mentioned that I was surprised that they did not mask that area where the bending would occur. I didn't prod as I was grateful for the time he spent with me and questioning him about it didn't seem appropriate.

Back to Mg:

I found an article on an relatively new process developed at Monash University in Melbourne Australia that makes it possible to shape pure magnesium at ambient temperature.

Professor of material science engineering Nick Birbilis states "The recipe is relatively simple: Pure magnesium is pushed through a die at 80°C, then cold rolled."

"This process changes the microstructure of the magnesium so that it is no longer brittle."

"By refining the microstructure, we have changed the deformation mechanism from intra granular (brittle) to inter granular (formable)."

Pretty cool! I wondered for a long time how they did it.

Oh by the way, "Duralumin" as used in JBL Al diaphragms is 2024 series aluminum, a very high strength aluminum alloy with good elongation properties whos major alloying metal is copper.

All the best!
Barry.