Aluminum Castings Versus Iron Castings

Aluminum Versus Iron

Aluminum castings started making significant inroads on iron castings almost as soon as the price of aluminum began to make it economically feasible, albeit a stretch. Momentum changed dramatically with the implementation of mandatory fuel economy standards for light vehicles.

                CAFÉ legislation was passed in 1975, not to reduce what was already recognized as high levels of pollution created by automobiles, but to address the 1973 Arab oil embargo. The rules were challenged, plaintiffs were granted extensions and economic issues delayed comprehensive implementation until the mid-1980’s. The standards continue to tighten with the EPA requiring vehicles to meet a target of 63 grams of carbon dioxide equivalent per mile in model year 2025. It is most conveniently and understandable when expressed in miles per gallons but that is not the correct form.

                Taking weight out of vehicles is one of the easiest ways to meet EPA regulations. Aluminum is a very good way to take weight out. Initially, iron castings including engine blocks, heads, intake manifolds and transmission housings were converted to aluminum. In addition, steel wheels were largely replaced by cast aluminum wheels. In 1975, aluminum content in light vehicles was about 80 pounds. In  2012, aluminum content rose to an average of 343 pounds. About 85% of that weight is in the engines (130 pounds), wheels (67 pounds), transmission and drive chain(70 pounds) and radiators (32 pounds). Eighty percent of engine blocks are aluminum, 99% of cylinder heads, 100% of radiators and transmissions and 70% of road wheels are aluminum. Small percentages of aluminum castings are used in suspension arms/links (14%), bumper beams (17%), steering knuckles (27%) and hoods (30%).

                The 263 pound of aluminum added to vehicles from 1975 to 2012 came at the expense of 765 pounds of iron castings.

                It is generally accepted in the automotive industry that $2 in additional vehicle cost is reasonable to reduce weight by one pound. Ford Motor’s 2015 F150 Pickup is the first “all aluminum” vehicle. There is about $1000 in added cost due just to the use of aluminum. While the use of aluminum in light vehicles will continue to grow only a small portion will be aluminum castings. Most of the growth in the use of aluminum will be extrusions and sheet.

                Most reports compare iron and aluminum castings based either on tons (weight) or sales dollars. Neither makes any sense in the comparison. Iron weighs 491 pounds per cubic foot and aluminum weight 169 pounds per cubic foot. A comparison based on weight substantially understates the volume of aluminum cast relative to iron. In 2012, the average aluminum casting sold of $2.358 per pound compared to $.641 for ductile iron, a multiple of 3.7 times the price of ductile iron. A comparison based on sales dollars overstates the importance of aluminum.

                An “apples to apples” comparison is a comparison of the cubic feet cast.

                As the chart shows, aluminum is a larger share of the market than recognized when weight is used as a comparison. Several factors impact the volumes cast in any one year. For both iron and aluminum, automotive is the “900 pound gorilla.” In 2012, 8.9% of gray iron castings were for automotive, 12.2% for ductile iron and 30.5% for aluminum. In die casting aluminum, fully 55% of production is for automotive applications. The significant drop in aluminum production that began in 2008 and started to reverse in 2010 was due to the drop in automotive production.

                Each of the iron alloys and aluminum alloys have properties that can determine which is best in a particular application. There are times the materials are interchangeable with reasonable or no modification to the part design. As proof of this, consider the conversions of iron castings in automotive to aluminum. When the materials are substantially interchangeable, price becomes the determining factor. Aluminum is a preferable material from the standpoint of ease of processing.

                A hypothetical casting weighing one pound in iron (.34 pounds in aluminum) that can be made identically in both iron and aluminum would cost $.641 in ductile and $.802 in aluminum. The price of aluminum is trending very slowly downward. When the cost of aluminum drops another 25%, the materials will be cost competitive. Aluminum is a much more prevalent element than iron in the earth’s surface. A breakthrough in processing technology that lowers primary aluminum cost could bring a dramatic price drop. Also, aluminum is very easily recycled. As more primary aluminum is put into use and products reach the end of their life cycle and are reprocessed the supply of secondary aluminum will increase. An increased supply of secondary aluminum will put downward pressure on material costs.

                There do not appear to be any imminent breakthroughs in primary aluminum processing. The price of secondary aluminum is moving very slowly. It will be some time before aluminum, as a material in itself, is price competitive with iron.

                There are certain categories of castings where there is no comparison between iron and aluminum castings. Aluminum castings top out at 3,500 pounds with a few somewhat larger. Iron castings top out at 200,000 pounds in the United States and even larger in Europe and China.