April 11, 2006
This article is one of a 16-part series on the fundamentals of stamping. Descriptions of all the articles in this series, and links to them, can be found at the end of this article.Previous articles in this series focused on stamping dies and production methods. This article discusses stamping materials—both ferrous and nonferrous.
To process, design, and build a successful stamping die, it is necessary to fully understand the behavioral characteristics of the specific material to be cut and formed. For example, if you are forming 5000 series aluminum and you follow the same process you use for deep drawing steel, the operation most likely will fail—not because aluminum is bad, it's just different from steel.
Each metal has its own unique mechanical characteristics. The metal type that the die is forming and cutting often determines the tool steel that must be used, as well as how many operations are required. In addition, different metal types require different lubricants, press speeds, and capacities. Because stampers are end users of metals, this article focuses on selecting and understanding the end-product behavior only and not the metal-making process.
Although there are literally thousands of metals that can be stamped, all fall within two basic categories—ferrous and nonferrous. Ferrous metals contain iron, and nonferrous metals are those without iron. Steel is a classic ferrous metal because it is derived essentially from iron ore. Aluminum, however, contains no iron and is classified as a nonferrous metal.
With the exception of a few exotic specialty metals, ferrous metals are magnetic and nonferrous metals are nonmagnetic. Because nonferrous metals do not contain iron, they are less likely to deteriorate through oxidation or rusting. Some commonly stamped nonferrous metals are aluminum, brass, bronze, gold, silver, tin, and copper.
Aluminum is a very popular metal for applications in which strength, weight, and corrosion resistance are factors. Aluminum is approximately one-third the weight of steel. Although hundreds of alloyed steels exist, plain carbon steel is by far the most commonly stamped ferrous metal.
Carbon is a basic element of the steelmaking process. In its raw form, carbon could be described as a chunk of coal or pencil lead. A piece of coal buried a mile or so beneath the surface of the earth and subjected to intense heat and pressure for about a thousand years yields what? A diamond. A diamond is nothing more than pure, compressed carbon. (Yes, "Carbon is a girl's best friend." Just make sure that it's natural, highly compressed carbon that you are giving her.)
From this basic knowledge of carbon, it is easy to deduce that the more carbon present in the steel, typically the stronger and less formable it will be. For example, tool steel used in manufacturing dies contains far more carbon than the sheet metal being processed. Keep in mind that the carbon content of a particular metal does not fully determine the metal's mechanical properties. Carbon content is only one factor.
An alloy is a homogeneous compound or mixture of two or more metals that enhances the metal's chemical, mechanical, or physical properties. When combined, the metals must be compatible and resist separation under normal conditions. For example, two common alloys added to steel are chrome and nickel. Chrome is very hard and resists oxidation, and so does nickel. Adding chrome and nickel to steel produces stainless steel. These added alloys enable the stainless steel to resist oxidation.
If you have purchased stainless steel flatware recently, you may have noticed different grades are available. These grades usually are designated as good, better, and best. The main difference in the quality depends primarily on the alloy content. The numbers that you see on the packaging, such as 18/8 or 18/10, refer to the percentage of chromium (18 percent) and nickel (8 percent or 10 percent) in the stainless steel. Chromium is known for its stain resistance, and nickel is known for its high luster and shine. Higher alloy numbers mean higher quality and cost.
Alloys can be introduced into both ferrous and nonferrous metals. Many aluminum alloys are available today. A very common steel type used in the automotive industry is high-strength, low-alloy steel (HSLA). Alloys are combined with medium carbon steel to give the metal good load-carrying ability and reasonable formability. These mechanical properties make HSLA a good candidate for frame rails and other automotive structural parts that require strength.
The number of alloyed metals used in stamping are far too numerous to mention in this article. The thing to remember is that alloyed metals are a combination or mixture of two or more metals that create a new metal with special characteristics.
Plain carbon steel can be defined as pure steel, meaning that it contains no intentionally added alloys. Plain carbon steel—among the most popular steel types used in stamping today—usually is assigned a four-digit number, such as 1006, 1020, 1050, and 1080. To determine the steel's carbon content, simply place an imaginary decimal place between the four digits and read the last two digits as a percentage of 1 percent. For example, 1010 steel contains 10 1/100 of 1 percent carbon, or 0.10 carbon (see Figure 1).
The more carbon in the steel, the harder it will be to cut and form. Metals with increased carbon can be hardened further by heating them to a critical temperature and cooling them quickly in the proper quenching medium. Processing harder metals requires dies made from tougher, more wear-resistant tool steels. Also, greater force is needed to cut and form the metal. Knowing the metal's carbon content can help you make a better decision about the appropriate tool steel and press capacity. Figure 2 shows a few typical applications with respect to the steel's carbon content.
This article covered very basic metal types and properties only. The next article in this series will discuss the mechanical characteristics of different metals in more detail. It also will explain how the metal selection affects the die processing method and die materials.