Aluminum is a good material for flashlights because it is strong, lightweight, conducts electricity, and transmits heat well.
Aircraft Grade Aluminum
Most flashlights that are made out of "Aircraft Grade Aluminum" are typically made of 6061-T6 alloy. This alloy is strong, hard, cheap, anodizable and weldable.
Aerospace Grade Aluminum
Flashlights that are made from "Aerospace Grade Aluminum" can be made from a wide range of alloys. Typically, these alloys are 2024-(T3 or T351), 7050-(T7451 or T6) and 7075-(T6/T651 or T7351), with 7075-T6 being the most common. These alloys are very strong and very hard. The 2024 alloys generally aren't anodizable or weldable. The 7050 and 7075 alloys generally are anodizable, but not weldable.
AnodizingWikipedia Anodizing for more information as well at this CPF thread.
Anodizing can be removed with strong alkali. People have had luck using a product called Greased Lightning, a degreaser widely available in the US (after removing switches o-rings, reflector, glass, etc.). Then they polish it on a buffing wheel with a metal polish called Rouge. The result is glossy, shiny, easily scratched aluminum. Either clear coat or a product called Sharkhide can be used to keep the shine from becoming dull. Here are some examples: DRY, Ultrafire UF-980L, and Solarforce L2i. It is much easier to remove Type II anodizing than Type III.
Flashlights with Type II anodizing can also be broiled in a kitchen oven for 15 minutes to an hour to change black anodizing to purple, brown, copper, or orange depending on the dyes that were originally used. Again, remove all glass, o-rings, circuits, switches, etc. before baking. Type III anodizing does not change colors as much. See this CPF thread and this BLF thread for results.
Stainless steel is harder than aluminum, but also much heavier. It is basically steel with a high (11% and higher) chromium content. There are a variety of types of stainless steel with different surface treatments from matte to high gloss. It is not a coating, so it will not peel or flake off. Most types of stainless steel are not magnetic but some are, so a magnet test is not definitive. Stainless steel does not transmit heat as well aluminum and therefore is not as effective in carrying heat away from a LED. See Wikipedia for more information on stainless steel.
Some flashlights are available in titanium alloy. These are usually the higher-end custom lights, but some production lights are now produced in titanium. Titanium can be polished to a very shiny finish; bead blasted, or anodized/heat treated for a variety of colors. By varying the thickness of anodizing, a number of different colors can be generated, allowing for some very artistic designs and patterns. Titanium is more resistant to corrosion than stainless steel and weighs 40% less.
Although titanium is heavier than aluminum, it is much stronger, however it is also more expensive and harder to machine. Like stainless, it does not conduct heat as well as aluminum, and this fact must be addressed when engineering high-powered flashlights. Due to its light weight and high strength, it is frequently used in aircraft, but because most titanium came from Russia and was very hard to buy during the Cold War, US defense contractors nicknamed the metal "unobtainium." See Wikipedia for more information on titanium.
Summary of Metal Properties
| Tensile Strength
| Thermal Conductivity|
Density - kilograms per cubic meter
Tensile Strength - Megapascals, millions of Newtons (a force) per square meter to cause the metal to yield (or permanently deform).
Thermal Conductivity - Watts per meter of thickness per degree Kelvin. The amount of heat (in watts) that moves through some thickness of material (in meters) given some difference in temperature between the hot side and cool side (in Kelvins).
Numbers from makeitfrom.com for 6061 aluminum alloy, yellow brass, architectural copper, 18/8 stainless steel, low carbon (mild) steel, and grade 5 titanium. Numbers for gold from Wikipedia.