Precision stampers use many types of metals, including aluminum alloys, brass alloys, copper, nickel, steel, stainless steel, silver, and bronze. How do you know which is the right material for each project? It has to do with a variety of mechanical and chemical properties that determine how a given metal will behave during stamping and in the finished product. Designers, engineers, and stampers need to work together to find the right balance between satisfying design intent and manufacturability of a part. Metal properties also impact the manufacturing process itself, including selecting the best tool steel, stamping oils, and plating or other finishing.
Some of the most important properties to consider are discussed listed below; however, there may be additional considerations depending on your specific application.
A metal’s mechanical properties determine how it behaves when force is applied during stamping processes that cut or bend it. The goal is to use a metal that can be manufactured to customer requirements without excessive tonnage or force, while still retaining structural integrity for use in the final assembly or application.
Which of a metal’s properties or characteristics is most important can change depending on how it’s being used. For example, consider a battery contact with a 90-degree bend. Any metal considered must have acceptable conductivity, but a particularly brittle metal might crack if bent to a 90-degree angle, but another might not, even if both have comparable conductivity. So in this application bendability is the more important consideration.
Temper has to do with the strength throughout the material. These are key characteristics involved with temper:
- Percentage of elongation: In tensile strength testing, this is a measure of the amount a metal or alloy can be bent, stretched, or compressed before it breaks. It is related to formability, which is the extent to which a material’s shape can be changed without tearing, necking (localized thinning of the material, usually at a bend), or cracking. A higher percentage of elongation means higher formability, while a lower percentage of elongation means necking or other breakage is likely at lower stress and strain levels. This measurement is also related to ductility, which is a metal’s ability to be formed without breaking or drawn into a thin wire.
- Tensile and yield strength: A metal or metal alloy’s strength is related to its chemical composition. For example, some types of steel are stronger than others depending on the relative amounts of component metals (e.g. nickel, chromium, iron). Tensile strength is a measure of how much force a material can withstand before breaking. Yield strength refers to the amount of force required for the material to permanently change shape (i.e. yielding to the stress as opposed to a quantity yielded in production).
Surface hardness specifically refers to the outermost layer of the metal, not throughout the material. It’s a way to determine the amount of force needed to cut or penetrate a piece of metal, or how much force is required to permanently deform or bend it. The higher a metal or alloy’s carbon content, the harder, more brittle, and less solderable it is. Hardness also relates to a metal’s resistance to surface wear and abrasion, which can be important in some environmental conditions. It varies inversely with ductility.
Superficial hardness is a special measure of the hardness of very thin and/or small pieces of metal as well as metal plating and coatings, which often used in applications for progressive stamping.
Hardness can be increased by mechanical processes, such as rolling, heat or laser treatments, or through chemical processes such as anodizing. Manufacturing processes such as bending and stamping, which permanently deform the metal, can also cause a phenomena called work hardening.
Bendability, modulus of elasticity, and springback impact the manufacturability of a part, especially when tolerances are tight or pend ratios are large (i.e. sharp). Modulus of elasticity, also known as Young’s modulus, is a measure of how much elastic (i.e. non-permanent) recovery a material has after it has been deformed (e.g. bent or shaped). It is related to a phenomena called springback, which describes the way some materials “want to” return to their previous flat shape when force/stress is removed. This tendency is due to elastic energy stored in the metal, which is often higher in the middle of a strip of metal than on the surfaces. A higher modus of elasticity means a smaller amount of springback, and a lower modulus of elasticity means more springback.
This chemical property describes a metal’s ability to resist reacting chemically with other substances in its environment, often moisture in the air or directly in water. Rusting is a common type of corrosion and is an electrochemical process that happens in the presence of an electrolyte such as oxygen. Oxygen reacts with the metal and converts it to a more stable material, known as an oxide. Base metals, such as iron, are highly susceptible to oxidization, while noble metals, like silver, resist it. Corrosion can cover an entire surface, occur in localized spots, or form pits and cracks.
Some metals are naturally corrosion resistant, but it can also be enhanced. Treatments with electrical current can create a film that protects the underlying metal, and laser treatments can be used to change a metal’s crystal structure to one that resists corrosion. Another process, galvanization, involves coating or selectively plating or coating a metal with a second metal with different resistance. A common example is zinc plating on iron, as zinc is less noble than iron and will corrode before the iron will.
Electrical conductivity is a measure of how easily electrical charges are passed through and distributed by a material. The opposite of conductivity is resistance. Conduction happens when “free” electrons in the outermost shells of the metal’s atoms pass electrical charges between each other and cause electricity to move through the object. Conversely, materials that oppose the flow of electricity have a higher resistance.
A part’s intended use, such as an EMI shield or battery contact, determines the level of conductivity or resistance desired in the raw material used. The chart in this article shows the conductivity of different metals.
Thermal conductivity measures the ability of a material to absorb heat from high-temperature areas and transfer it to low-temperature areas. These characteristics can be used to different effect, depending on the requirements of the application. Metals with high conductivity can transfer heat efficiently away from an area or sensitive component, but lower conductivity metals can be used to protect surrounding components from heat. Metals with lower thermal conductivity include stainless steel, lead, and carbon steel, while silver, copper, and aluminum are more highly conductive.
It’s important to remember conductive metals can be used to absorb heat from surrounding parts such as PCB or engine parts, but that heat can be radiated to nearby components and cause damage like melting or warping.
Thermal conductivity impacts tooling materials too: the coefficient of thermal expansion. During press operation, tool steel becomes hot and expands and deforms. “This expansion decreases punch-to-die clearances and can result in excessive stretching, leading to tears or breaks in the stampings,” according to Metal Forming Magazine. Adjusting press and forming speed is one way to account for this and ensure tolerances can still be met.
Depending on the application, these behavioral characteristics of metals may be important considerations:
- Solderability/weldability refers to how easily a material can be soldered or welded. This is often a concern with EMI/RFI shields attached to PCBs and other parts that are permanent parts of an assembly. This chart explains the relative solderabilities of different metals.
- Density and weight are determined by the chemical composition of a metal or metal alloy. This is important in applications where the composite weight of an assembly or finished product is important such as aircraft or medical devices.
- Corrosion resistance is a result of a material’s chemical makeup. For example, stainless steel is known for its high resistance to rust, tarnish, and permanent staining. It is corrosion-resistant at the surface level, but at the atomic level, it does still corrode. Stainless steel contains a minimum of 10 percent chromium, which reacts with the surrounding oxygen to create a very thin, stable film that protects the layers beneath it. It is also resistant to spotting from mineral deposits, chemical residues, and accumulated dirt, and spots may be able to be removed through cleaning or wiping off the part. Stains due to tarnishing or chemical residues are harder to remove and tend to permanently adhere to the surface. Parts may be made of solid stainless steel alloys or plated with it, depending on requirements.
- A part’s surface finish can affect its ability to hold lubricant and stamping oils during manufacturing, as well as how easily it can be cleaned. This may be important in some consumer and medical products where easy cleaning/sterilization and/or appearance are important.
So many factors come into play when designing and manufacturing precision stamped parts. That’s why it’s helpful to have a partner who can help you strike the balance between cost, efficiency, and part performance. Contact us to talk about your next stamping project.