Why are car bodies not made from a low density metal like magnesium?

Answer 1

The main reason is cost and the ability to mass produce bodies with existing manufacturing equipment. Safety is a major concern for magnesium and its alloys since it can ignite at temperatures you see in typical car crashes (imagine a body panel skidding across asphalt). Once ignited, it can burn in the presence of oxygen, CO2, or water making it almost impossible to extinguish in emergency situations.

The raw material cost is only part of the total cost differential. Most car manufacturers are equipped to efficiently handle giant rolls of sheet steel. Aluminum is a good alternative in that it can be formed similarly to steel, however it is still not as easy to work with. These differences incur additional tooling and die costs even with economies of scale. The added costs means that aluminum is usually only used on more luxury car bodies such as the Audi A8 as the consumer is less likely to pay more for aluminum on less expensive cars. It is expected that aluminum will see increased use in car bodies as gas mileage regulations force auto makers to reduce weight.

Other low density high strength metals like beryllium and titanium are prohibitively expensive to use in their pure form. As an alloy, they are usually combined with aluminum, thus still becoming more expensive than aluminum, but without providing as drastic of a weight reduction as compared to steel. These alloys are even harder to work with thus adding more tooling costs over aluminum.

Carbon-fiber is not a metal, however its low density and high strength make it desirable as a raw material for car bodies. Despite the high availability of carbon, the supply of carbon fibers is low, thus making it an expensive commodity. Furthermore, the molding of carbon fiber into panels and structural members can take hours compared to the few seconds needed to stamp a sheet metal part. Thus carbon fiber is not used for any mass produced bodies.

Plastic is also not a metal, but because of its light weight and ease of manufacture, it is often used for nonstructural components like body panels, bumpers, and trims.

Answer 2

Many low density metals are not strong enough to withstand the forces exerted on a car's frame. Magnesium is also fairly unstable, as the racing world found out after the magnesium frames of their cars began catching on fire under extreme conditions.