Casting Materials

Casting Materials. Their use and choice

In recent years there has been a move to new types of materials being used for castings in mechanical applications.  The following should provide some clarity in the types of materials used in castings and why we use the metals we do for our specific application of model engineering operating with the general public. All of these materials have specific usage in mind however we only use what we feel comfortable in supplying.

Gun Metal

Alloys of copper with tin, zinc and lead have been used for at least 2000 years due to their ease of casting and good strength and corrosion resistance. Early uses were for brooches, mirror cases, church doors, fonts and statues. The use for cannons in Mediaeval times led to the term 'gun metal' being adopted; this use is now obsolete with the manufacture of steel ordnance.

Today, in the UK and Australia, the term 'gunmetal' is applied to a family of copper-based casting alloys containing between 2-11% tin and 1-10% zinc. Modified forms may contain, in addition, such elements as lead (up to 7%) and nickel (up to 6%) when the alloys are classified as 'leaded gunmetal' and 'nickel gunmetal'.

Gunmetals are noted for the manufacture of intricate castings required to be pressure tight such as valves, pipe fittings and pumps. This makes it an ideal material for use in steam fittings and boilers and for casting boiler components.  This is used for all our small parts and by request for cylinder blocks for locomotives.

They are also frequently used for bearings where loads and speeds are moderate.

 

Cast Iron

 

Grey Cast Iron

Grey flake cast irons are the most used of the general purpose engineering irons. They are relatively inexpensive and easy to produce. They are readily machined and the machined surfaces are resistant to sliding wear. They have high thermal conductivity, low modulus of elasticity and an ability to withstand thermal shock. This makes the material suitable for castings subjected to local or repeated stressing.  Grey Cast Iron supplied by us in generally grade 10/12 although a higher grade is available by request

Spheroidal Graphite ( SG) Iron

These irons have higher mechanical properties than a comparable grey cast iron with the same composition, because the carbon is in the shape of spheroidal graphite. This is achieved by inoculating low-sulphur molten iron having low silicon content with magnesium or cerium or both, followed by addition of silicon. Subsequent cooling can produce a variety of matrix structures with ferrite and pearlite being the most common. Compared to grey cast iron, spheroidal graphite irons have higher ductility, tensile strength, modulus of elasticity and resistance to elevated temperature oxidation. Machinability and corrosion resistance are comparable to grey cast iron, though damping capacity is lower. Fluidity is lower than grey cast iron but better than steel.  The SG irons are widely used in automobile and farming industry: axle housings, brake calipers, brake cylinders, camshafts, connecting rods, crankshafts, gears, pistons and yokes. They are also used to make bulldozer parts, conveyor frames, couplers, crawler sprockets, elevator buckets, railway wheels and hoist drums. Other general engineering applications include boiler segments, coal crushers, hammers, die blocks, frames and jigs, nuclear fuel containers, tank covers, tunnels segments and turret heads.

We use this grade of cast iron in our bogie castings.

 

Aluminium

AA601

AA601 is an aluminium alloy that is particularly suitable for use in high quality castings where strength, definition and good corrosion resistance is important. Alloys with similar chemical composition are used throughout the world With a maximum BHN of 65 ( equivalent to brass) it is most suitable for applications with similar metals or stationary applications. We use this aluminium in our castings where the same material is touching or for static application such as wheel splashers, dummy springs etc.  We cannot recommend its use with other metals such as mild steel.

Zinc Alloys

ZA alloys represent a new family of zinc-based die casting materials that contain higher aluminum content than standard zinc alloys. These alloys provide high strength characteristics plus high hardness and good bearing properties (Table 2). Thin wall castability characteristics and die life are similar to zinc alloys. ZA-8 is recommended for hot chamber die casting. ZA-12 and ZA-27 must be cast by the cold chamber die casting process. All ZA alloys offer similar creep properties and are superior to standard zinc alloys.


ZA-8 -- Provides strength, hardness and creep properties.
ZA-12 -- Provides excellent bearing properties with strength and hardness characteristics between ZA-8 and ZA-27. Good dimensional stability properties and somewhat better castability than ZA-27.
ZA-27 -- Offers the highest mechanical properties of the ZA family and is therefore recommended when maximum performance is required.

 

With ZA-27 containing between 25-28% Aluminium 3% of other materials you are left with a zinc content between a minimum of 69% to a maximum of 72%. In order to achieve a higher strength the amount of zinc was decreased and replaced by Aluminium. In the standard die casting process the final casting will have a small amount of porosity. This prevents any heat treating or welding, because the heat causes the gas in the pores to expand, which causes micro-cracks inside the part and exfoliation of the surface.  It is for these reason we do not recommend the use of ZA-27 in any operational part. All of this information is easily verifiable on the internet.  All of our casting are cast here in Australia. Why.  We want to be able to support Australian manufacturers.  It is not cost effective just what we consider the right thing to do

ADC12 Die Cast.

 

This is a relatively new product introduced onto the Australian market.  The properties of these castings are as follows.  This material is used in the automotive industry for static non load bearing applications such as rocker covers and watet pump covers.

Silicon  9.5-12%, Iron 1.3% max, 1.5-3.5% copper, maganese 0.5%, Magnesium 0.3%, Nickle 0.5%, Zinc 1%max, Tin 0.3%, The rest is aluminium, So you have between 5.5% to 8.5% of metal 9.5% to 12% silicon and between 79.5% to 85% aluminium.  Basically ADC12 is made of 85% Aluminium, 12% Silicon and a whopping 3% metal.  The wearing properties for wheels on steel rail is indicative in the  Brinell scale of hardness. ADC12 has a score of 75 which is close to soft brass (60). Mild Steel is 130 and grey cast iron is170-207.  This is the grade of cast iron we use for all of our wheels.

 

 

 Physical Properties of various casting materials used in our hobby

 

AA601  Tensile Strength 200 Mpa BH 65 ( http://www.alumascprecision.co.uk/AlloyLM25.php)

ZA 27 Tensile Strength 345 Mpa  BH 105-125 (http://www.kenwalt.com/DiecastingAlloys.pdf)

ADC12 Tensile Strength 310Mpa  BH 75 (http://www.makeitfrom.com/material-data/?for=383-ADC12-LM2-A03830-Die-Cast-Aluminum)

Grey Cast Iron Grade 10/12  Tensile Strength 448Mpa  BH 170-207 (http://www.mid-atlanticcasting.com/ductile-iron_casting-guide_FEB05.pdf)

Cast Steel grade 1020 Tensile Strength 397Mpa  BH 111 (http://www.azom.com/article.aspx?ArticleID=6114#3)

Therefore in terms of hardness and yield strength the best material for our applications are

1 Grey Cast Iron

2 Cast Steel Grade 1020

3 ZA 27 Aluminium

4 Pressure Cast Aluminium ADC12.

5 AA601 Aluminimum

 

It should be noted that the first two are sand cast are are substantially higher in physical properties than the  pressure cast aluminimum and therefore it cannot be said that Pressure Cast Aluminium wheels from ADC12 are stronger than 'other sand cast wheels currently on the market'.  E and J Winter - Bolton Scale Models stands by its products and subscribes to truth in advertising.  We do not obfuscate in our advertising and support our statements with readily available facts.  The choice is yours.

 

I hope that helps with any questions you might have.  E and  J Winter - Bolton Scale Models