by PennyJo Wagner
How many times have you seen the term "aircraft aluminum" used to describe snowmobile components? Have you ever wondered what might be the difference, or is all aluminum the same?
Aluminum is known for it's light weight and corrosion resistance. It's density is about 1/3 that of steel, but by itself aluminum is very soft. It is alloyed with copper, magnesium, manganese silicon, and zinc for strength. Small additions of chromium, iron, nickel, and titanium are sometimes added to obtain desired properties. So, as you can see, all aluminum is not the same.
Why should you care ? Let's say that you are looking for parts for your snowmobile. One is more expensive than the other - but they "look" the same. Should you be settling for the cheaper component? Or, is there another difference other than price?
What no one bothers to tell you is that there are seven basic classifications of aluminum ranging from the 1000 series which is almost pure aluminum and very soft, to 7075 which is the strongest. However, it isn't safe to assume that there is a progression in strength related to a comparable progression in a higher identifying number.
The 1000 series is too soft and weak for snowmobile usage. The 2000 series has some copper and other elements added to make it stronger and resist fatigue, but has poor corrosion resistance. It is not weldable by conventional methods, but can be heat treated and is a good choice for machined parts. Some snowmobiles have suspension cross shafts made out this material identified as 2024.
The 3000 series has manganese added for good formability and is mainly used for general sheet metal work (and pots and pans). It is not heat treatable and is not good for snowmobile usage (except for type 3003, which can be formed into complex shapes such as a gas tank for a modified sled).
The 4000 series is basically welding rods containing silicon. The 5000 series has moderate to high strength and is weldable. However, it's greatest attribute is it's excellent corrosion resistance - it is used mainly for marine applications.
The 6000 series, specifically 6061, is a general purpose alloy. This is the least expensive and most versatile of the heat treatable aluminum alloys. It has very good corrosion resistance, finish-ability, excellent weld-ability, and a strength approximating that of mild steel. It anodizes nicely and is readily available. Some skis are made of 6061. It is used in tie rods, steering arms, bulkheads and tunnels, and some bumpers. If the part has been welded it should be heat treated to regain the strength that it lost in the heat affected zone. Some snowmobile parts made from 6061 seem like a good buy, but may not be strong enough.
7075 is the highest strength alloy of the commercially available aluminum, and is typically used as aircraft structures. It has zinc and copper added for ultimate strength, but because of the copper it is very difficult to weld. It anodizes beautifully. 7075 has the best machinability and results in the finest finish. 7075 is the most expensive because of it's high strength and is not readily available. This is the good stuff - snowmobile parts made from 7075 are lightweight AND strong.
Not only is the type of aluminum important to the strength, but also the process used to form the part. Clutches, cylinder heads, and motor blocks are cast aluminum. Carbs are cast from aluminum based alloys (some are cast in cheaper zinc). Casting is not as strong as an extruded or forged part. Production snowmobile pistons are cast, but some after-market pistons are forged which makes a stronger part.
The cast parts are formed by simple molding, and are not all that strong. The heated metal is poured into a mold and it cools. Extruded parts are stronger - they are pushed (with tremendous pressure) as soft (preheated) metal through a very strong steel "die" to produce the desired shape. Extruded parts are very strong - as an example, the bumper on an Arctic Cat or Polaris.
Parts that are forged are formed by heating the metal, pouring it into a form or die, and using tremendous force. Forging actually compacts the molecules in the metal making it stronger. Rotary forging is a cold forging process done by hammering, using a series of dies and immense pressure resulting in extreme strength. Generally, a rotary forged part is the strongest available.
The final step in ensuring the strength of each part is the (computer controlled) heat treating process. In this process, the metal is heated to a high temperature, followed by cooling, the metal. This process not only strengthens the metal further, but relieves the stresses caused by welding.
These differences make up for the difference in price, quality, durability, and performance of snowmobile parts. An educated buyer can make informed decisions as to what will work best for them. Just because it says "aircraft aluminum" doesnt necessarily mean its the best available.
(This article appeared in Race & Rally - Winter 1995-96 on Page 18.)
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