DETROIT -- In what is expected to be the biggest new automotive application of any kind for aluminum extrusion alloys getting under way this year, General Motors Corp. will switch the bumper beams on the front and rear of all of its standard-size cars in the Buick, Oldsmobile and Pontiac lines to aluminum from steel.

If GM builds and sells as many of those cars as it thinks it will, the new bumpers will consume 8.3 million pounds or more of aluminum per year, according to GM sources here.

That's more than the big automaker is expected to use in two other important new applications for aluminum extrusion alloys starting up this year--the engine cradles in two Chevrolet car lines and the radiator enclosures in standard-size Chevy, GMC and Cadillac sports utility vehicles (AMM, Dec. 17).

Because of all these applications as well as certain others, 1999 is shaping up as a breakthrough year for extrusion grades of aluminum, which typically are magnesium-silicide alloys, in the North American auto market. Although domestic automakers have been using light-alloy extrusions in their vehicles for years, the year-to-year increase in applications is generally small and relatively difficult to measure.

GM sources said the new bumper beams, which will weigh around 26 pounds per car, will be used on the -redesigned Buick LeSabres, Olds Auroras and Pontiac Bonnevilles coming out this year as 2000 models. The new Auroras will replace the Olds 88s in GM's product stable.

More than 14 million pounds of steel per year will be replaced by the aluminum bumper beams, these sources said. The reinforcement beams will be covered by plastic fascias on all three car lines and will serve as the principal structural components in the front and rear bumper systems.

GM will buy the beams from Hydro Raufoss Automotive, Holland, Mich., which is expected to get its metal from one or two sources. The likeliest suppliers of the aluminum 6082 alloy are Hydro Goldendale, Seattle, Wash., and/or Alcan Aluminium Ltd., Montreal.

Hydro Raufoss also will make the new extruded bumper beams that GM will employ on its redesigned Cadillac DeVille luxury cars for 2000, GM sources said. Altogether, the aluminum used in the new Buick, Olds, Pontiac and Cadillac car bumpers will exceed 10 million pounds annually. In the DeVille bumpers, too, the aluminum will be replacing steel.