Alloy wheels

[Roger King]

Just a (possibly) amusing thought that arose out of a conversation with a customer.

 

Alloy wheels are a very trendy thing to have these days, but steel is every bit as much an alloy as the aluminium that is used for wheels. Does this mean that you could legitimately sell steel wheels as 'alloy wheels'?

 

Yes, I know that people take the word alloy to be an abbreviation of aluminium, but it actually means something else.

[Paul McKenzie]

Bog standard carbon steel isn't an alloy, though is it, Roger? I don't think you can really call it an alloy until you start to get a fair amount of alloying metal in there such as chrome or molybdenum......or have I missed the point?

Paul

[Griff]

All steels are alloys of iron, whereas pure aluminium is only an element if I remember my A-level chemistry. But to make it useable, the aluminium used in wheels is alloyed with other "ingredients" for strength, corrosion resistance etc. Literally speaking, both steel and aluminium wheels could be regarded as "alloys". One for advertising standards maybe?

 

 

[Nick M]

Excellent !! I've have just bought some new "alloy" wheels after all !! 😬

 

Giving some cheap (as in $60 each) steel wheels a go to see how they cope. 6x13" wheel weighs 11lbs or a fraction under 5kg each which is easily comparable to most alloy wheels. There are also some spun aluminium rims available but at $184 each they can wait !!

[Tony C]

Well I thought it amusing 😬

 

Anyone know what the aluminium in wheels is alloyed with? Copper, zinc?

 

Just curious

[Griff]

Don't know about alloy wheels in particular, but aluminium can be alloyed with Silicon, Iron, Copper, Magnesium, Lead, Tin, Zinc etc.

 

 

 

[R2D2]

Interesting comments, but some basics may be useful.

 

Alloys are of course any mixture of metal/non-metal that can form a solution. Simply put the constituents must be soulble in one another at least in the molten state.

 

Mild steel is an alloy of iron and carbon. The other elements that are present in this range of alloys are often added to mop up undesirable residual elements such as copper and tin that are not reduced by the steel making process.

 

It is common to add managanese which combines with sulphur and phosphorous as well as improving hardenability to some extent.

 

The subject of ferrous metallurgy is too extensive to discuss other than in very general terms but steel has a very useful and almost unique property.

 

The ability to be hardened to a very significant degree is a fundamental characterisitic of steels.

 

It is carbon combined with iron's allotropic nature that produces this ability. By heating steels with reasonably high levels of carbon to a specific temperature ( usually in the order of 850 degC) and then quenching into oil or other media depending on the application, a very hard and strong component can be produced.

 

The components made in this way are usually too brittle (notch sensitive) to use without being tempered This involves re-heating to around 550 - 600 deg C. This process does reduce strength but also increases toughness.

 

The addition of other alloying elements such as nickel. chrome, molybdenum are all chosen to increase the hardenability, or the depth to which steel can be hardened.

 

There are many other elements used for alloying depending on the specific properties needed including the need to have good impact resistance at low temperatures etc. etc etc.

 

"Mild Steel" as it is commonly called is an alloy of iron with a relatively small amount of carbon (>0.2%).

 

The carbon strengthens the iron from about 270MPa to around 390MPa. Mild steels are used for car body production and have become very sophisticated materials in the last few years and very underestimated in their importance and cost.

 

 

Aluminium alloys are equally as complex and have three basic families.

 

The first is deformation hardening alloys. These alloys gain their properties from cold rolling.

 

1000 series alloys are the most basic available and can be used in for sheet metal working. They are basically commercially pure (99% Aluminium) Itis common to buy these alloys in different cold work conditions such as 1/4 hard, half hard. fully hard. This describes the degree of cold rolling applied which also affects the ductility. They tend to have excellent corrosion resistance and are relatively low cost.

 

Floor pans of Sevens would probably be fully hard but complex shapes may well need softer material to allow more formability.

 

The "Solid Solution Strengthened Alloys" These alloys gain their properties from cold work or simple alloy additions and are not heat-treatable.

 

3000 series alloys have additions are commonly manganese or magnesium. These alloys are stronger than 1000 series and can have better formability depending on the detailed composition.

 

4000 Series alloys are norally used as welding filler rods they don't have much application as sheet materials.

 

5000 Series alloys are the highest strength materials without heat treatment. They are high in magnesium and are commonly used in military and marine applications.

 

The final category is the Precipitation hardening alloys

 

In these alloys an element that can form a solid solution at one temperature and then precipitate at another temperature must be used. This generally means that solid solubility needs to be higher at higher tempertaures.

 

The constiuent that precipitates must also be quite hard and "coherent" with the matrix of the basic alloy if it is to be effective.

 

It is common to use an Alloy of Aluminium, Magnesium and Silicon or an Alloy of Aluminium Mangnesium and Zinc top produce precipitation hardening alloys, but the classic Duralumin is an alloy with 4% Copper to form precipitates of an intermetallic compound of copper and aluminium.

 

Duralumin type alloys develop very high strength but they are not popular these days beacuse they offer poor corrosion resistance particularly in salt water environments. They also tend to overage with time. This means that precipitates tend to carry on growing slowly with time and eventually become so large ( greater than 1 micron diameter) that they become incoherent with the matrix. When this happens the alloy loses strength and becomes brittle. (This is what happens to a sixties Magnesium Minilite when it is 20/30 years old). it is also diffcult to weld these Duralumin alloys as the overage close to the weld and become brittle. The welds look great but have poor mechanical properties and lously corrosion resistance.

 

The 2000 and 7000 series alloys are the traditional alloys of this type but they are now being replaced by the 6000 Series that give much better long term stability and tend to be more weldable.

 

The heat treatment process is a "solution treatment" which takes all the alloys into a solid solution followed by water quenching to trap the alloy elements into this solution. This produses a relatively weak component. This is then followed by a precipitation stage where the component develops its strength.

 

This heat treatment process looks similar to the steel heat treatment process but the strengthening mechanisms are totally different.

 

In steel the quenched part is very strong and needs to be re-heated to temper this strength and to avoid the component being too brittle.

 

In aluminium the quencehed part is not strong and it needs re-heating to develop its strength.

 

The casting alloys used in cars are basic variations on these alloys.

 

The common alloys are things like LM4 and LM6 these are die casting alloys and they are not chosen for their strength. They have around 12% Silicon. This reduces melting point by around 100 deg C. This makes them cheap to cast. They also "freeze" very quickly (10 deg C freezing range) which means that mould can be opened as soon as the metal is poured. Again ideal for high volume die casting. The Silicon improves fluidity meaning it is easy to cast complex shapes and reduces the shrinkage so running systems can be fairly simple.

 

Alloy wheels have traditionally been cast from LM25. This is a high silicon alloy suitable for gravity die casting but also precipitation hardening.

 

Once the wheel is cast it can be heat treated and its stength will increase by a factor of around 2 from the base casting.

 

Some road wheels are not cast from this material and are relatively weak in comparison.

 

There is also a tendancy for the industry to use a material known as LM25 "Modified" but this is a bit of a throwback and not absolutely necessary.

 

When wheels were sand cast the time taken for the alloy to freeze in the mould was quite high and "coarse" grain structures were produced. This type of structure normally has poor impact resistance.

 

The solution was to modify the alloy with a sodium addition just before pouring. The sodium compound used was a hexaflouride which also removed any dissolved hydrogen from the melt and helped to reduce porosity. The sodim acted as a grain refinig compond and produced a "fine" grain structre which gave much better impact resistance.

 

Modern wheels are produce in gravity or low pressure dies, these are metal moulds rather than sand and cooling times are much shorter. This means that fine grained structure are produced without the need for modification but the practice still rolls on.

 

 

It is true that there are a great many aluminium alloy available but aluminium and lead just don't mix. If you melt tham and add them they produce two distinct layers.

 

 

Well after all that BS, I really don't think steel wheels can be described as Alloys, I do feel that we all know what the abbreviation means.

 

Sorry to bang on but metallurgy is an interest of mine.

 

 

 

 

 

Edited by - chris flavell on 21 Feb 2003 09:35:34

 

Edited by - chris flavell on 21 Feb 2003 09:42:49

[Tony C]

WOW Thanks for that Chris - just about answers my question 😬

Cheers

Tony