Ductile iron SAE grade D4018

Details for Grade D4018, like mechanical property requirements, etc. can be found in SAE J434, Automotive Ductile (Nodular) Iron Castings.  You can order this standard from SAE at http:///www.sae/org or Global Engineering Documents at http://global.ihs.com.

Thank you both, I was able to get the mechanical properties of this material from www.matweb once I found out what type was used. Now I have my baseline to search for a suitable non ferrous alloy to replace it with. I would like possibly to substitute it with a Magnesium Alloy but I think part will have to be reengineered as no Mg Alloys are matching the strength parameters of D4018. Any ideas?  

You will DEFINITELY need to redesign/reengineer the part if you want to use Al or Mg instead of Fe.  Fe will have approximately twice the stiffness of Al and three times the stiffness of Mg, so deflections will be higher with these materials for a given force and wall thickness.  Stress levels will need to be much lower in Al or Mg, as they will not be as strong as the Fe, unless you consider the high strength heat-treatable Al & Mg grades, like A206 for Al and WE54 for Mg.  Both of these are aerospace grades, and are more expensive than typical Al & Mg alloys like 356/A356 and AZ91.

Strength at temperature is a definite concern, depending on the alloys.  Bolt load retention in Al & Mg can be quite poor at elevated temperatures (> 150 C), due to creep.  Having said that, proper design can result in good castings, with adequate strength and stiffness, that will be lower in mass than an Fe design.  The following list of websites should give you a good idea of what is available in Mg (sand casting, permanent mold casting, sand casting, etc.) and some foundries that currently make Mg castings, such as rear end housings (Lite Metals and Intermag):

http://www.magnesium-elektron.com/productsandservices/casting.asp

http://www.litemetals.com/index2.htm

http://www.intermag.qc.ca/default.htm

http://www.haley.on.ca/

http://www.intlmag.org/

http://www.uk-racing-castings.co.uk/

Aluminium axle housings are feasible, but do tend to give more axle whine than cast iron ones. There are several possible reasons for this :

1) lower stiffness housing tends to deflect more, upsetting the geometric relationship between the gears.

2) Reputedly aluminium has a lower damping coefficient than cast iron

3) The resonances of a typical aluminium housing may more closely match the excitation frequency, or they may be in a more audible part of the frequency range.

(2) is probably the main one. Magnesium (reputedly) has more internal damping than aluminium, so it may be a better bet.

Our diff regularly runs at 135 degrees C and I would regard that 150 degree C figure as achievable.

If you are in the USA you may find it difficult to get a (non-aerospace) machine shop that will handle magnesium, due to the perceived fire risk.

Cheers

Greg Locock

I think the Mg has been ruled out in favor of A206. With the strength of A206 Higher than that of the Fe D4018. I may just try molding a few parts with the current patterns and see what happens. Bean counter wants less weight with out a total retooling. After looking at the part and the mechanical properties of the two materials I think it could be achieved. Any thoughts more than welcomed. Tom

Tom,

Remember what I posted earlier, and GregLocock also mentioned, regarding stiffness considerations.  While A206 will have superior strength at ambient temperature, and possibly even at 125-150 C (I forget where it starts to degrade significantly), stiffness will be much lower, so deflection of gears, shafts, etc. needs to be considered.

Solidification for A206 will be different than cast iron, specifically the amount of shrinkage that will result.  Definitely consult with an aluminum foundry on their ability to cast this alloy-- the 2xx series Al alloys can have problems with hot tearing.

You should consider the T7 heat treatment over the T6 heat treatment.  T6 makes the alloy susceptible to stress corrosion cracking, whereas T7 is substantially better in this respect.  Strength properties are only slightly worse, and casting soundness will probably have a larger effect (grain size, inclusions, macro and micro porosity, etc.).  This alloy can develop very high strength and is used for making aerospace gearbox/transmission housings, so it is suitable for your application, just beware of using it in the exact same design as the current iron version.