expanding world of metals: Not just steel and cast iron, The

Manufacturing Engineering, Sep 2001 by Aronson, Robert B

Not just steel and cast iron

With materials, as with much of manufacturing, the drive for change frequently comes from the automotive and aerospace industries. Car makers, in particular, are looking for ways to reduce weight without compromising safety, yet maintain high production volume. This effort has led to a greater interest in lighter weight materials and higher-strength steel. Aerospace too wants lighter weight materials, but at low volume. They are looking at new aluminum alloys, titanium, and composites.

Materials researchers note that many of the materials now presented to meet current demands are not new alloys. Instead, they are often materials that have been around for some time and their value is just being realized, or pressures on the industry have only recently begun to force their acceptance. A case in point is aluminum taking over from steel in some automotive applications, chiefly because its lighter weight and recyclability give it an edge.

To the shop manager, these changes in material present new challenges, which are not always welcome. Manufacturers want a stable, simple process. But sometimes standardization is not the best idea. Using the same speeds and the same tools with a new material will probably work, but not always effectively, "The decision is compromise versus optimize," says Thomas Carlberg of Sandvik Coromant (Fair Lawn, NJ). The answers are rarely just a matter of changing inserts, they require a review of the entire process. For example, when high-speed machining is an issue, do you take a machine designed for 1000 rpm and run it at its highest limit, or buy a machine that can easily handle 10,000 rpm? In many cases, newer, harder materials may also mean a slower process or require greater rigidity in the machine tool to achieve higher precision.

Here is a sampling of today's materials and the challenges they impose.

Magnesium

Manufacturers are looking for greater use of magnesium. It's tough to work with, and fires caused by the heat of cutting action are an issue. But if it is used in volume, ways will be found to avoid the problem. Magnesium use in commercial products is growing at a 6% annual rate, chiefly for automotive applications.

About 2.2 to 3.5 kg of magnesium are used in every American-made vehicle, but if manufacturing problems can be overcome, researchers say that 100 kg or more might be used.

Magnesium has the advantage of having about a quarter the weight of steel and twothirds the weight of aluminum. But it is expensive, costly to manufacture, and there are concerns about recycling magnesium, its tensile strength, corrosion resistance, and supply for high-volume operations. Its automotive uses include instrument panels, car seats, steering wheels, and valve covers.

Composites and Ceramics

"Although they offer many strength and weight advantages, the cost of composites limits them chiefly to aerospace applications," says Dr. A.T. Santhanam, Consulting Engineer, Carbide Products Development, Kennametal Inc. (Latrobe, PA). "Advanced technologies and materials move from military and space applications to commercial aviation and other industries. For example, both Boeing and Airbus already use composites for the empennage-the vertical tail sectionof some of their airplanes."

"With greater emphasis on weight reduction, composites and ceramics are gaining more importance particularly in the aerospace industry," explains Mike Sess, process development engineer, Makino (Mason, OH).

Compacted Graphite Iron

Initially, the dominant cast iron was gray cast iron, which has graphite particles in a lamellar or flake shape. It has good castability, vibration damping, and thermal conductivity. Although the material is relatively weak and brittle, it has long been the standard material for engine blocks.

Researchers found that adding magnesium to the cast iron caused the graphite to grow in a spherical form which increased strength significantly, but reduced thermal conductive ity and castability. Called nodular or ductile cast iron, this material is used where mechanical strength is needed, but it does not have sufficient castablity to produce complex shapes such as engine blocks and heads.

Then an "intermediate" type of graphite or vermicular graphite was found to form under some conditions. It is a form of cast iron in which the growth of graphite particles has been controlled to improve the metal's performance, and became known as compacted graphite iron (CGI). Similar to gray iron, CGI has elongated graphite particles, however, the particles are shorter and thicker, and have rounded edges and a rough surface. The result is an iron with good castability and thermal conductivity, yet sufficient strength, which is ideal for high compression gasoline and dieselengine cylinder blocks. It is 70% stronger, exhibits 35% greater bending strength, and has double the fatigue life of normal gray iron. The improved properties allow designers to reduce engine weight or increase engine power.

Although CGI has been around since the 1960s, only recently have developers been able to control the process so that a quality material could be produced in volume. It arrives at an ideal time, when engine builders are tryng to increase performance without increasing engine size or weight.